The Nervous System: Words to Know

Arachnoid (ah-RAK-noid):
Weblike middle layer of the three meninges covering the brain and spinal cord.
Autonomic nervous system (aw-toh-NOM-ik NERV-us SIS-tem):
Part of the peripheral nervous system that controls involuntary actions, such as the heartbeat, gland secretions, and digestion.
Axon (AK-son):
Taillike projection extending out a neuron that carries impulses away from the cell body.
Basal ganglia (BAY-zul GANG-lee-ah):
Paired masses of gray matter within the white matter of the cerebrum that help coordinate subconscious skeletal muscular movement.
Central controlling and coordinating organ of the nervous system.
Cauda equina (KAW-da ee-KWHY-nah):
Spinal nerves that hang below the end of the spinal cord.
Central nervous system:
Part of the nervous system consisting of the brain and spinal cord.
Cerebral cortex (se-REE-bral KOR-tex):
Outermost layer of the cerebrum made entirely of gray matter.
Cerebrum (se-REE-brum):
Largest part of the brain, involved with conscious perception, voluntary actions, memory, thought, and personality.
Corpus callosum (KOR-pus ka-LOW-sum):
Large band of neurons connecting the two cerebral hemispheres.
Dendrites (DEN-drites):
Branchlike extensions of neurons that carry impulses toward the cell body.
Diencephalon (die-en-SEF-ah-lon):
Rear part of the forebrain that connects the midbrain to the cerebrum and that contains the thalamus and hypothalamus.
Dura mater (DUR-ah MAY-tur):
Outermost and toughest of the three meninges covering the brain and spinal cord.
Ganglion (GANG-glee-on):
Any collection of nerve cell bodies forming a nerve center in the peripheral nervous system.
Gray matter:
Grayish nerve tissue of the central nervous system containing neuron cell bodies, neuroglia, and unmyelinated axons.
Gyri (JYE-rye):
Outward folds on the surface of the cerebral cortex.
Hippocampus (hip-ah-CAM-pes):
Structure in the limbic system necessary for the formation of long-term memory.
Hypothalamus (hi-po-THAL-ah-mus):
Region of the brain containing many control centers for body functions and emotions; also regulates the pituitary gland's secretions.
Limbic system (LIM-bik SIS-tem):
Group of structures in the cerebrum and diencephalon that are involved with emotional states and memory.
Medulla oblongata (mi-DUL-ah ob-long-GAH-tah):
Part of the brain located at the top end of the spinal cord that controls breathing and other involuntary functions.
Meninges (meh-NIN-jeez):
Membranes that cover the brain and spinal cord.
Part of the brain between the hypothalamus and the pons that regulates visual, auditory, and rightening reflexes.
Myelin (MY-ah-lin):
Soft, white, fatty material that forms a sheath around the axons of most neurons.
Bundle of axons in the peripheral nervous system.
Neuroglia (new-ROGUE-lee-ah):
Also known as glial cells, cells that support and protect neurons in the central nervous system.
Neuron (NUR-on):
Nerve cell.
Neurotransmitter (nur-oh-TRANS-mi-ter):
Chemical released by the axon of a neuron that travels across a synapse and binds to receptors on the dendrites of other neurons or body cells.
Node of Ranvier (NODE OF rahn-VEEAY):
Small area between Schwann cells on an axon that is unmyelinated or uncovered.
Oligodendrocyte (o-li-go-DEN-dro-site):
Cell that produces the myelin sheath around the axons of neurons in the central nervous system.
Parasympathetic nervous system (pair-ah-simpuh-THET-ik NERV-us SIS-tem):
Division of the autonomic nervous system that controls involuntary activities that keep the body running smoothly under normal, everyday conditions.
Peripheral nervous system (peh-RIFF-uh-ruhl NERV-us SIS-tem):
Part of the nervous system consisting of the cranial and spinal nerves.
Pia mater (PIE-ah MAY-tur):
Delicate innermost layer of the three meninges covering the brain and spinal cord.
Part of the brain connecting the medulla oblongata with the midbrain.
Reflex (REE-flex):
Involuntary and rapid response to a stimulus.
Schwann cell (SHWAHN SELL):
Cell that forms the myelin sheath around axons of neurons in the peripheral nervous system.
Somatic nervous system (so-MAT-ik NERV-us SIS-tem):
Part of the peripheral nervous system that controls the voluntary movements of the skeletal muscles.
Spinal cord:
Long cord of nerve tissue running through the spine or backbone that transmits impulses to and from the brain and controls some reflex actions.
Sulci (SUL-sye):
Shallow grooves on the surface of the cerebral cortex.
Sympathetic nervous system (sim-puh-THET-ik NERV-us SIS-tem):
Division of the autonomic nervous system that controls involuntary activities that help the body respond to stressful situations.
Synapse (SIN-aps):
Small space or gap where a nerve impulse passes between the axon of one neuron and a dendrite of the next neuron.
Thalamus (THAL-ah-mus):
Part of the brain behind the hypothalamus that acts as the brain's main relay station, sending information to the cerebral cortex and other parts of the brain.
White matter:
Whitish nerve tissue of the central nervous system containing bundles of myelinated axons.

The Human Nervous System

- Human Neurology


The nervous system is essentially a biological information highway, and is responsible for controlling all the biological processes and movement in the body, and can also receive information and interpret it via electrical signals which are used in this nervous system

It consists of the Central Nervous System (CNS), essentially the processing area and the Peripheral Nervous System which detects and sends electrical impulses that are used in the nervous system

The Central Nervous System (CNS)

The Central Nervous System is effectively the centre of the nervous system, the part of it that processes the information received from the peripheral nervous system. The CNS consists of the brain and spinal cord. It is responsible for receiving and interpreting signals from the peripheral nervous system and also sends out signals to it, either consciously or unconsciously. This information highway called the nervous system consists of many nerve cells, also known as neurones, as seen below.

The Nerve Cell

Diagram of a Neurone - With the Axon and Dendrites projecting from the Cell Body

Each neurone consists of a nucleus situated in the cell body, where outgrowths called processes originate from. The main one of these processes is the axon, which is responsible for carrying outgoing messages from the cell. This axon can originate from the CNS and extend all the way to the body's extremities, effectively providing a highway for messages to go to and from the CNS to these body extremities.

Dendrites are smaller secondary processes that grow from the cell body and axon. On the end of these dendrites lie the axon terminals, which 'plug' into a cell where the electrical signal from a nerve cell to the target cell can be made. This 'plug' (the axon terminal) connects into a receptor on the target cell and can transmit information between cells

The Way Nerve Cells Communicate

The "All-Or-None-Law" applies to nerve cell communication as they use an on / off signal (like an digital signal) so that the message can remain clear and effective from its travel from the CNS to the target cell or vice versa. This is a factor because just like electricity signals, the signal fades out and must be boosted along its journey. But if the message is either 1 or 0 (i.e.) on or off the messages are absolute.

Classification of Neurones

Interneurones - Neurones lying entirely within the CNS

Afferent Neurones - Also known as sensory neurones, these are specialised to send impulses towards the CNS away from the peripheral system

Efferent Neurones - These nerve cells carry signals from the CNS to the cells in the peripheral system

The next page elaborates on how the nervous system works...

The Conscious & Unconscious Nervous System

- Human Neurology


The Central Nervous System is arguably the most important part of the body because of the way it controls the biological processes of our body and all conscious thought. Due to their importance, they are safely encased within bones, namely the cranium protecting the brain and the spine protecting the spinal cord

Brain Divisions

There are three main components of the brain, namely the brainstem, cerebellum and the forebrain. These are elaborated upon below

  • The Brainstem - The brainstem is the connection between the rest of the brain and the rest of the central nervous system. This part of the brain was the first to be found in the evolutionary chain, though has developed over time and via evolution to develop into the two other components. It is primarily concerned with life support and basic functions such as movement, thus meaning that more advanced processes are left to the more evolved areas of the brain, as explained below.
  • The Cerebellum - Consisting of two hemispheres, the cerebellum is primarily concerned with movement and works in partnership with the brainstem area of the brain and focuses on the well being and functionality of muscles. The structure can be found below the occipital lobe and adjacent to the brainstem
  • The Forebrain - The forebrain lies above the brainstem and cerebellum and is the most advanced in evolutionary terms. Due to its complexity, more info is divulged about this part of the brain below

The Forebrain

The forebrain has many activities that it is responsible for and is divided into many component parts. The below list elaborates on the localised areas of the forebrain and their functions.

  • The Hypothalamus - A section of the brain found next to the thalamus that is involved in many regulatory functions such as osmoregulation and thermoregulation. The hypothalamus has a degree of control over the pituitary gland, another part of the brain situated next to it, and also controls sleeping patterns, eating and drinking and speech. The hypothalamus is also responsible for the secretion of ADH (Anti-Diuretic Hormone) via its neurosecretory cells
  • The Cerebrum - The cerebrum is the largest part of the human brain, and the part responsible for intelligence and creativity, and also involved in memory. The 'grey matter' of the cerebrum is the cerebral cortex, the centre that receives information from the thalamus and all the other lower centres in the brain.
  • The Cerebral Cortex - Part of the cerebrum, this part of the brain deals with almost all of the higher functions of an intelligent being. It is this part of brain that deals with the masses of information incoming from the periphery nervous system, furiously instructing the brain of what is going on inside its body and the external environment. It is this part that translates our nervous impulses into understandable quantifiable feelings and thoughts. So important is the cerebral cortex that it is sub-divided into 4 parts, explained below
  1. Frontal Lobe - Found at the front of the head, near the temples and forehead, the frontal lobe is essential to many of the advanced functions of an evolved brain. It deals with voluntary muscle movements and deals with more intricate matters such as thought and speech
  2. Parietal Lobe - Situated behind the frontal lobe, this section deals with spatial awareness in the external environment and acts as a receptor area to deal with signals associated with tough.
  3. Temporal Lobe - The temporal lobes are situated in parallel with the ears, they serve the ears by interpreting audio signals received from the auditory canal
  4. Occipital Lobe - This is the smallest of the four lobe components of the cerebrum, and is responsible in interpreting nerve signals from the eye at the back of the brain

The above components of the brain work in tandem in a healthy brain. However, in some cases the brain can be injured in some way, causing brain damage. The next page looks at how brain damage can affect the way we operate.

The Central Nervous System

- Human Neurology


Myelin Sheath

Myelin is a substance that forms the myelin sheath associated with nerve cells. This sheath is a layer of phospholipids that increases the conductivity of the electrical messages that are sent through the cell. Diseases such as multiple sclerosis are a result in a lack of this myelin sheath, with the resultant effect being that the conductivity of signals is much slower severely decreasing the effectiveness of the nervous system in sufferers.

In total, there are 43 main nerves that branch of the CNS to the peripheral nervous system (the peripheral system is the nervous system outside the CNS. These are the efferent neurones that carry signals away from the CNS to the peripheral system.

Somatic Nervous System

These efferent fibres are divided into the somatic nervous system and the autonomic nervous system. The somatic fibres are responsible for the voluntary movement of our body, i.e. movement that you consciously thought about doing.

The Autonomic Nervous System

The autonomic nervous system incorporates all the impulses that are done involuntarily, and are usually associated with essential functions such as breathing, heartbeat etc. However this type of system can further be broken down into the sympathetic and parasympathetic systems which keep one another in check in a form of negative feedback such as the release of insulin and glucagon in sugar control of the blood. 

All of the actions executed by the autonomic nervous system are unconsciously done.

These informational pulses executed in our nervous system allow us to do our daily functions. The processing of this information is done in the CNS, the brain, a highly developed mass of nerve cells. The inner workings of the brain are investigated on the next page.

Types and Causes of Brain Damage

- Human Neurology


Causes of Brain Damage

The brain is a highly specialised tissue, far more complex than today's 21st century supercomputers. Due to this magnificent complexity, even the slightest damage can have extreme consequences

The brain can be damaged in a variety of ways, and depending on the areas damaged and the severity of the damage, it can prove relatively harmless to fatal. Some causes of brain damage are below

  • Genetics - A dysfunctional hereditary gene could have been passed on to the offspring which prevented the full development of a healthy brain
  • Blow - A sufficient blow to the head can supercede the skulls defences (particularly at the temple) and can therefore allow structural damage to occur.
  • Lack of Blood - Lack of blood to the brain can cause severe problems for the cells associated with the brain. A human can survive for four minutes without oxygen before the brain damage becomes so severe there is no realistic chance of survival. A stroke is an event where there is a blood shortage to the brain, which is caused by a blood clot
  • Tumours - Cancer has been a major non-infectious disease more recognised over the last decade, and more cases of brain tumours are detected nowadays due to more sophisticated techniques. The continued growth of these cancerous cells puts pressure on the brain, which can cause a blood clot or directly cause brain damage due to the pressure of the tumour pressing against it.

Types of Brain Damage

  • Aphasia - A type of brain damage affecting communication capabilities in the organism. This can range from the inability to construct a sentence either in voice or on paper, to the inability to recognise speech itself. This sort of damage focuses on the frontal lobe area of the brain
  • Visual Neglect - This is where the information collated on one half of the brain is rejected and therefore the sufferer can only operate with one eye, because the part of the brain receiving visual information from the other eye is not functioning properly. In some cases, sufferers may only be able to paint half a painting or eat one half of a plate of food as they are unaware of the information about the other half of the environment.
  • Amnesia - Or retrograde amnesia, this sort of damage affects the memory, caused by degeneration / damage in the frontal lobe. Sufferers have memory blanks when relating to past experiences in their life
  • Agnosia - This unusual sort of brain damage is where sufferers still have the complete ability to see around them (unlike visual neglect), though cannot relate their surroundings in a quantifiable way, i.e. they fail to recognise a familiar surrounding, person or object, due to a malfunction in recalling past events involving the surrounding, person or object

The next page of this neurology tutorial takes a further look at the brain, and the capabilities of it to be used to our advantage in daily life.

IQ, Creativity and Learning

- Human Neurology


Evolution of Human Intelligence

Human's, as evolved as we are, are the species most capable of exhibiting intelligence and creativity due to our capacity to learn. It is nothing short of remarkable how we, intelligent beings, came to exist.

  • Humans evolved from similar primates millions of years ago, who were better equipped to survive in their environment (more info in the Evolution of Species tutorial)
  • This knowledge gained has been passed on (greatly accelerated by advances in the way we communicate)
  • As a consequence, offspring of our species have harnessed previously accumulated (and written) knowledge to our advantage.
  • Our technological and intellectual powers has allowed us to exist in huge numbers, and take advantage of our environment in remarkable ways - continuing to attempt to make best use of what we know or could possibly know.

In light of this snowball effect, and as a continuation of the last bulleted point; we have been able to sustain a rising human population over time. In turn, in accordance with natural selection, more intelligent people may be favoured by our gene pool over the long term, thus making the species as a whole more intelligent as a collective.

Ability to Learn

Humans continually learn from one another and share their information over generations. This is what makes our species a cut above the rest. Our ability to understand the value of learning and to do so gives us the tool to understand more and more about ourselves and our environment.


Intelligence offers us the means to utilise abstract ideas and implement reasoning in our arguments to justify the things we do. The degree of intelligence in people is variable to a number of factors, like genetics, the local environment and even diet.

It is important to note the following

  • Knowledge is the accumulation and retention of information
  • Intelligence is the ability to analyse this information to the persons advantages, i.e. answering correctly in the exam by making best use of the information you know.


It does not take an intelligent person to be creative. It is a popular belief that technically minded people tend to be less creative as others, who, in turn, are not very technically minded.

It is believed that creativity is made possible in the right brain hemisphere while the technical information is processed in the left hemisphere. It is worth noting that many of the famous creative individuals, all the famous writers, artists etc were generally intelligent.

Creativity can rely on a number of factors, some of which are named below

  • Motivation - If the person has no desire to utilise their creativeness, they will not be creative.
  • Personality - Peoples' unique inclinations and differences in decision making makes our choice of creativity unique and thus the decisions made in creating something will be different with each person.
  • Parental Guidance - Parents provide the crucial link for learning between birth and maturity, therefore their learning, and partly their creativity and intelligence will rub off on those they learn, as will the people that you communicate with.

Moreover to the last factor, it is worth considering that any factor in the external environment will be a factor in your creativity. If someone offered you a million euros to write a good poem, you may instantaneously feel more creative!

Moving one step on from the conscious learning mind, we look at the unconscious mind on the next page; via sleep and dreams.

Sleep and Dreams - Neurology

- Human Neurology


The Falling Asleep Process

During the day when we are a awake, our body and brain are working tirelessly to operate our body, and as they do so they slowly degrade at a cellular level. A person will get progressively tired from this bodily breakdown, because sleep gives us a chance to build and replace the cells and resolve our end of day homeostatic imbalances.

If you have not slept for a while, the decrease in the efficiency and effectiveness of the body begins to tell, and you will begin to feel sleepy as less energy is available to you. The longer we stay up the more likely we will fall asleep.

If certain conditions prevail, like a state of inactivity or relaxing in a warm dry place, there is a higher chance of us falling asleep due to the preferable conditions for us to do so.


When we fall asleep, our metabolic rate slows down, as does almost every other function across the board, we effectively go into hibernation mode. The amount of adrenaline in our body promoting awareness decreases and somatotrophin, controlling the repair of tissue is more abundant. This is effectively the healing process of sleep that revitalises us.

The synaptic nerve connections containing recollections about the last day are also strengthened, hence when you wake up the more you realised you did yesterday. This localised area of memory is what many of our dreams consist of, our past recollections of the day. You may have dreamt something twice, and on the second time it was only because you thought of that first dream the day before you dreamt the second. When looking at it like this, it confirms the reason why you have the same dream, your conscious thought about it accesses that part of the brain thus 'remembers' it at night.

Dreams Telling the Future?

Some people believe that dreams tell the future. But, when 6 billion people dream every night, there is bound to be a coincidence when there are trillions of dreams every year. Those people who have dreamed of winning the lottery are one of many.

I, personally don't believe they tell the future, though could be a sign of intelligence, the brain interpreting possibilities in the future from the knowledge of past events. This would be perfectly viable, as it would be a case of the brain 'adapting' to its future environment, and preparing you for the possible future.


REM stands for rapid eye movement and is the points in time during sleep where dreams occur. They occur after periods of deep sleep.

As suggested, rapid eye movement occurs in REM, while the body is under a state of paralysis.

In effect, our brain takes us on a virtual reality of our thoughts while it steadily repairs itself for the next day. The most vivid and deepest dreams will occur in the periods between REM while drowsy, almost conscious dreams occur in the REM stages.

Our Environment Outside Sleep

Have you ever had a dream where someone next door is playing music, and the music is conveniently woven into your dream? This is your body trying to lessen the chances of you awakening while it is repairing itself.

However, sleep deprived people go into much deeper sleep, and may not detect such noises. The overriding point here is, that sleep is essential to the body, and that there are compensations made to our usual behaviour (like paralysis) that enables our body to do what is required for itself.

Sleep Troubles

The older we get, the less sleep we require. Teenagers buck the trends in needing the most sleep of us all, due to the growth spurt occurring at puberty that involves a larger turnover of materials and energy.

  • Newborn babies can sleep up to 60% of the day
  • Adults require around 7 hours minimum
  • With aging, the amount required is less due to the gradual degeneration of parts of the body that are not getting repaired.

Certain drugs are available to induce sleeping, but most are addictive and require controlled and responsible use. The next page looks at the works of famous past neurologists like Carl Gustav Jung and Sigmund Freud, who both actively pursued the way in which we dream as a career in neurology.

Sigmund Freud and Carl Gustav Jung

- Human Neurology


Sigmund Freud

Sigmund Freud was a famous Austrian neurologist (1856 - 1939), who stated that dreams were the manifestation of the unconscious. Himself and another neurologist, Carl Gustav Jung (1875 - 1961), believed that conscious behaviour derived from unconscious instinct which exists in all of us.

These unconscious thoughts were linked to suppressed sexual desires. Freud identified three key stages in the life cycle where the child's tendency to focus on sexual areas of the body changes over time.

  • The first year of a babies life they focus on the mothers mammarian gland for feeding (the breast).
  • This state is succeeded from age 1 to 3 where the toddler is learning how to control their bowel and concentrates on their anal region.
  • This is in turn succeeded by attention towards the reproductive organs at age 3 to 4.

Freud argued that in these stages of unconscious repression, male children are attracted to their mother and become instinctively aggressive towards the father. The father reciprocally injects fear into the child by his male superiority, thus insinuating an essence of competition and games theory. Either way, the prime fact is that the child must grow to become sexually active and mature.

Differences Between Jung and Freud

Jung believed that a persons' brain consisted of the forgotten conscious and a cluster of memories of past experiences. He came to this hypothesis by studying humans suffering a mental disorder, who had hallucinations that were not a past recollection, thus Jung deduced there was another component of the brain adding to this illusion, i.e. the unconscious.

Freud on the other hand believed that the brain was divided into three parts

  • The ID - Inherited natural instincts
  • The Ego - The sense of self and attitude towards the external environment
  • The Superego - Superimposed values deriving from society and parental guidance

Essentially, this method of thinking, and approaching the brain from a self-realizing approach, neurology has been able to develop since these initial theories by Jung and Freud.

It also paved advance in psychiatry, and methods of psycho therapy to combat mental disorders, which are investigated upon in the next page.

Psychiatry & Mental Disorders

- Human Neurology


The Definition of Mad

When someone says 'mental disorder' many people associate it with madness. This is truly not the case. There are many states of mental disorder where the sufferer is not clinically insane.

Madness essentially means psychosis, being out of touch of reality and not being capable of rational and controlled thought. A person in psychosis may have irrational delusions and hallucinations that illustrate this imbalance in conscious and unconscious mind.


Schizophrenia can effect mind and personality. In severe cases, the sufferer believes that 'something' is in control of them, and that they have lost control of themselves.

Affective Mood Disorder

  • Mania - The sufferer is overly cheerful and can possibly appear as if they are under the influence of alcohol
  • Depression - Where beliefs and perceptions are unclear & unproductive

Obsessive & Compulsive Disorder

A mental disorder where the sufferer must undergo meticulous rituals to live their normal lives, such as excessive washing of their skin and hair. If the sufferer cannot do this, anxiety kicks in as a "withdrawal symptom" until they allow themselves to repeat the ritual once again.


A preconception about a given situation or object, such as a fear of snakes or being in high places. A huge diversity of phobias have been discovered by psychologists.

Depressive Neurosis

The classic case of depression where depression is the primary emotion in the sufferer, resulting in a lack of motivation and self-esteem to be functional in society and to themselves.

Physical Disease

Not only can the way the brain works be affected by disorders, the physical components of the brain can also be infected by pathogens. Dementia is such a physical disease, where the long term memory of the sufferer is broken down due to the physical components of the brain and nervous synapses degrading over time.

Drugs for Mental Disorders

A wide range of drugs are now available for those suffering mental disorders, though many people face a psychological barrier when it comes to taking medication to cure their 'soul'. Many of the drugs used prove addictive which in turn can also lead to further psychiatric problems.

However, psychotherapy is an alternative communicative treatment designed to get the patient to understand themselves better. This can be combined with drugtherapy, and eventually develop the patients' self realisation into a moreproductive and positive state. Medicinal neurology is a fairly new area of medicine.

The next page investigates perception and two people can interpret the same thing differently.

Human Perception - Neurology

- Human Neurology


A better understanding of human perception unlocks the key to how the mind works, an advantage when working with people with mental disorders.

Visual Perception

The below diagram is an illustration as to how we all perceive things in our own way, as suggested by the theories of Jung and Freud.

Human Perception in Action

What do you see? Some of you may see 2 green faces, other may see a white chalice. This all depends on your initial perception of the diagram. You may find that when you look again, you may see the alternate picture within the diagram.

The retina is responsible for interpreting visual stimuli such as this, where it picks up photons of light via the 130 000 000 rods and cones situated on it. In pre-modern times it was considered that visual perception simply encompassed what was seen by the eye on the outside. This external stimuli would in turn produce a perception in the brain caused by the stimulus.

However this is not the case. Modern medicine now knows that information from the eye is simply a physiological process that does not actually process the signals it receives. This job is left to the brain.

The senses simply act as a messenger to a particular stimulus that is seen, the brain is the place where external stimuli is actually perceived.

Spatial Awareness

The environment we live in is 3 dimensional, thus needs a 3 dimensional approach to understand it. Therefore height, width and depth must be measured by the eyes

This is possible by the way the eyes are situated on the head. Positioned either side of the nose, the right eye picks up vision on the left hemisphere and the left eye picks up vision in the right hemisphere.

The images picked up by the eyes are projected upside down on each of the eyes retina. This in turn will be perceived the right way up by the brain, which will interpret the three dimensional values of the external environment at a very fast and effective rate


It is possible for the physiological state of the brain to deviate from the norm and trigger of a mental disorder. Illusions are a symptom of such mental disorders.

However it is also possible to trick the senses of a perfectly functional healthy brain. Illusions, such as the mirages that appear in the desert are caused by trickery of the sense, leading us to believe there is something out there when this is not the case.

More information on illusions and mind trickery are looked upon in the next page

Neurology of Illusions

- Human Neurology


As mentioned in the previous page looking at perception, illusions can be caused by mental disorders or misreading of the sensory data obtained from the external environment. For now we will look at the latter.

Visual Illusions

  • Ambiguities
    These types of illusions are perceptual changes, leaving the brain to second guess the actual position of the elusive object
  • Distortions
    Caused by sensory misreadings in regards to spatial awareness, where the illusion can be distorted from its actual location and outline
  • Paradoxes
    Illusions that appear to be logically impossible, and therefore makes the brain unsure if it is real or really an illusion
  • Fictions
    This is caused by the brain 'assuming' the presence of hard surfaces where there may be not, therefore creating the potential of an illusion if the brains assumption about the external object in incorrect

Auditory Illusions

One of the most famous of these is the Doppler Effect, where a noise situated close to you has a higher pitch of sound to that of a sound further away. This is the case if you should and get an echo, your voice will always sound more deeper in the echo when it is not. This is effectively an illusion.

The Study of Illusions

As mentioned previously, the study of illusions in sufferers of a mental disorder provide a key into a deeper understand of what is going on in their mind. This is also the case of a healthy brain, where the study of illusions can work out the parameters at which it compensates for its own lack of ability

It is worth noting that the trial and error the sensory organs function have, they are as just as foolproof as any other conscious human thought. The chances of your brain not being able to guess the spatial distance of a fuzzy moving object is the same lack in ability that people have in typing an error free document

In this sense, illusions is studying the perceptions and sensory data obtained from situations where human error prevents us from seeing the real deal.

Another interesting fact is that the retina is read by the brain every 0.1 seconds, meaning that you are not actually seeing anything in the present, but something that just happened a fraction of a second ago.


Central Nervous System

The central nervous system is divided into two major parts: the brain and the spinal cord. In the average adult human, the brain weighs about 3 pounds. The brain contains about 100 billion nerve cells (neurons) and trillons of "support cells" called glia. The spinal cord is about 43 cm long in adult women and 45 cm long in adult men and weighs about 35-40 gm. The vertebral column or spine, the collection of bones (back bone) that houses the spinal cord, is about 70 cm long. So the spinal cord is much shorter than the vertebral column.

Centrally located in the human organism are accumulations of nerve cells forming the brain or cerebrum and the spinal cord or medulla.

Originating from these central groups, bundled nerve cell processes run out to form the peripheral nervous system.

The signals received from the
peripheral nervous system are analyzed, can be stored, and motor signals are constructed. These functions, which are predominantly accomplished by the brain, are extremely complicated and require the cooperation of thousands of nerve cells.

The most important activity of the nervous system is performed in t
he brain or cerebral cortex. Here is the center of all bodily functions.

A large vascular net supplies the central nervous system with oxygen and nutrients. The central nervous system itself is embedded in bone cavities formed in the area of the head by the skull or cranium and in the area of the neck and trunk by vertebral arches.


These bony cavities protection to the brain and the spinal medulla. Within these cavities, the protection is reinforced by three layers of connective tissue called the meninges, (see figure to left), which, in the head, envelope the brain, and in the vertebral column, the spinal cord or medulla.

Between the middle and the inner connective tissue layers are spaces filled with brain and spinal fluid called the cerebrospinal fluid. This fluid protects the central nervous system largely from mechanical damage.


Peripheral Nervous System

The peripheral nervous system is a channel for the relay of sensory and motor impulses between the central nervous system on the one hand and the body surface, skeletal muscles, and internal organs on the other hand. It is composed of (1) spinal nerves, (2) cranial nerves, and (3) certain parts of the autonomic nervous system. As in the central nervous system, peripheral nervous pathways are made up of neurons (that is, nerve cell bodies and their axons and dendrites) as well as the points at which one neuron communicates with the next (that is, the synapse). The structures commonly known as nerves (or by such names as roots, rami, trunks, and branches) are actually composed of orderly arrangements of the axonal and dendritic processes of many nerve cell bodies.

Spinal Cord SectionThe cell bodies of peripheral neurons are often found grouped into clusters called ganglia. Based on the type of nerve cell bodies found in ganglia, they may be classified as either sensory or motor. Sensory ganglia are found as oval swellings on the dorsal roots of spinal nerves, and they are also found on the roots of certain cranial nerves. The sensory neurons making up these ganglia are unipolar. Shaped much like a golf ball on a tee, they have round or slightly oval cell bodies with concentrically located nuclei, and they give rise to a single fiber that undergoes a T-shaped bifurcation, one branch going to the periphery and the other entering the brain or spinal cord. There are no synaptic contacts between neurons in a sensory ganglion.

Motor ganglia are associated with neurons of the autonomic nervous system. Many of these are found in the sympathetic trunks, two long chains of ganglia lying along each side of the vertebral column from the base of the skull to the coccyx; these are referred to as paravertebral. Other motor ganglia (called prevertebral) are found near internal organs innervated by their projecting fibers, while still others (called terminal ganglia) are found on the surfaces or within the walls of the target organs themselves. Motor ganglia contain multipolar cell bodies, which have irregular shapes and eccentrically located nuclei and which project several dendritic and axonal processes. Preganglionic fibers originating from the brain or spinal cord enter motor (autonomic) ganglia, where they synapse on multipolar cell bodies. These postganglionic cells, in turn, send their processes to visceral structures.

Sensory input from the body surface, from joint, tendon, and muscle receptors, and from internal organs passes centrally through the dorsal roots of the spinal cord. fibers from motor cells in the spinal cord exit via the ventral roots and course to their peripheral targets (autonomic ganglia or skeletal muscle). The spinal nerve is formed by the joining of dorsal and ventral roots, and it is the basic structural and functional unit on which the peripheral nervous system is built.

In humans there are 31 pairs of spinal nerves. In descending order from the most rostral end of the spinal cord, there are 8 cervical (designated C1C8), 12 thoracic (T1T12), 5 lumbar (L1L5), 5 sacral (S1S5), and 1 coccygeal (Coc1). Each spinal nerve exits the vertebral canal through an opening called the intervertebral foramen. The first spinal nerve (C1) exits between the skull and the first cervical vertebra; consequently, spinal nerves C1C7 exit above the correspondingly numbered vertebrae. Spinal nerve C8, however, exits between the 7th cervical and first thoracic vertebrae, so that, beginning with T1, all other spinal nerves exit below their corresponding vertebrae.

Just outside the intervertebral foramen, two branches, known as the gray and white rami communicates, connect the spinal nerve with the sympathetic trunk. These rami, along with the sympathetic trunk and more distal ganglia, are concerned with the innervation of organs. In addition, small meningeal branches leave the spinal nerve and gray ramus and reenter the vertebral canal, where they innervate dura and blood vessels.

More peripherally, the spinal nerve divides into ventral and dorsal rami. All dorsal rami (with the exception of those from C1, S4, S5, and Coc1) have medial and lateral branches, which innervate deep back muscles and overlying skin. The medial and lateral branches of the dorsal rami of spinal nerves C2C8 supply both muscles and skin of the neck. Those of T1T6 are mostly cutaneous (that is, supplying only the skin), while those from T7T12 are mainly muscular. Dorsal rami from L1L3 have both sensory and motor fibers, while those from L4L5 are only muscular. Dorsal rami of S1S3 may also be divided into medial and lateral branches, serving deep muscles of the lower back as well as cutaneous areas of the lower buttocks and perianal area. Undivided dorsal rami from S4, S5, and Coc1 also send cutaneous branches to the gluteal and perianal regions.

Ventral rami of the spinal nerves carry sensory and motor fibers for the innervation of the muscles, joints, and skin of the lateral and ventral body walls and the extremities (see below Plexuses of the ventral rami). Both dorsal and ventral rami also contain autonomic fibers.

Because spinal nerves contain both sensory fibers (from the dorsal roots) and motor fibers (from the ventral roots), they are known as mixed nerves. When individual fibers of a spinal nerve are identified by their specific function, they may be categorized as one of four types: (1) general somatic afferent, (2) general visceral afferent, (3) general somatic efferent, and (4) general visceral efferent. The term somatic refers to the body wall (broadly defined to include skeletal muscles as well as the surface of the skin), and visceral refers to structures composed of smooth muscle, cardiac muscle, or glandular epithelium (or a combination of these). Efferent fibers carry motor information to skeletal muscle and to autonomic ganglia (and then to visceral structures), and afferent fibers carry sensory information from them.

General somatic afferent receptors are sensitive to pain, thermal sensation, touch and pressure, and changes in the position of the body. (Pain and temperature sensation coming from the surface of the body are called exteroceptive, while sensory information arising from tendons, muscles, or joint capsules are called proprioceptive.) General visceral afferent receptors are found in organs of the thorax, abdomen, and pelvis; their fibers convey, for example, pain information from the digestive tract. Both types of afferent fiber project centrally from cell bodies in dorsal-root ganglia.

General somatic efferent fibers originate from large ventral-horn cells and distribute to skeletal muscles in the body wall and in the extremities. General visceral efferent fibers also arise from cell bodies located within the spinal cord, but they exit only at thoracic and upper lumbar levels or at sacral levels (more specifically, at levels T1L2 and S2S4). fibers from T1L2 enter the sympathetic trunk, where they either form synaptic contacts within a ganglion, ascend or descend within the trunk, or exit the trunk and proceed to ganglia situated closer to their target organs. fibers from S2S4, on the other hand, leave the cord as the pelvic nerve and proceed to terminal ganglia located in the target organs. Postganglionic fibers arising from ganglia in the sympathetic trunk rejoin the spinal nerves and distribute to blood vessels, sweat glands, and arrector pili muscles, while postganglionic fibers arising from prevertebral and terminal ganglia innervate viscera of the thorax, abdomen, and pelvis.

All plexuses arising from the ventral rami of spinal nerves contain sensory, motor, and autonomic fibers. The plexuses are the cervical, brachial, lumbar, sacral, and coccygeal.

Superficial Veins & Cutaneous Nerves of the NeckCervical levels C1C4 are the main contributors to the cervical plexus; in addition, small branches link C1 and C2 with the vagus nerve, C1 and C2 with the hypoglossal nerve, and C2C4 with the accessory nerve. Sensory branches of the cervical plexus are the lesser occipital nerve (to scalp behind the ear), the great auricular nerve (to the ear and to the skin over the mastoid and parotid areas), transverse cervical cutaneous nerves (to lateral and ventral neck surfaces), and supraclavicular nerves (along the clavicle, shoulder, and upper chest). Motor branches serve muscles that stabilize and flex the neck, muscles that stabilize the hyoid bone (to assist in actions like swallowing), and muscles that elevate the upper ribs.

Originating from C4, with small contributions from C3 and C5, are the phrenic nerves, which carry sensory information from parts of the pleura and pericardium and motor impulses to muscles of the diaphragm. Injury to the phrenic nerves would paralyze the diaphragm and render breathing difficult or impossible.

Cervical levels C5C8 and thoracic level T1 contribute to the formation of the brachial plexus; small fascicles also arrive from C4 and T2. Spinal nerves from these levels converge to form superior (C5 and C6), middle (C7), and inferior (C8 and T1) trunks, which in turn split into anterior and posterior divisions. The divisions then form cords (posterior, lateral, and medial), which provide motor, sensory, and autonomic fibers to the shoulder and upper extremity.

Nerves to shoulder and pectoral muscles include the dorsal scapular (to the rhomboid muscles), suprascapular (to supraspinatus and infraspinatus), medial and lateral pectoral (to pectoralis minor and major), long thoracic (to serratus anterior), thoracodorsal (to latissimus dorsi), and subscapular (to teres major and subscapular). The axillary nerve carries motor fibers to the deltoid and teres minor muscles as well as sensory fibers to the lateral surface of the shoulder and upper arm. The biceps, brachialis, and coracobrachialis muscles, as well as the lateral surface of the forearm, are served by the musculocutaneous nerve.

The three major nerves of the arm, forearm, and hand are the radial, median, and ulnar. The radial nerve innervates the triceps, anconeus, and brachioradialis muscles, eight extensors of the wrist and digits, and one abductor of the hand; it is also sensory to part of the hand. The median nerve branches in the forearm to serve the palmaris longus, two pronator muscles, four flexor muscles, thenar muscles, and lumbrical muscles; most of these serve the wrist and hand. The ulnar nerve serves two flexor muscles and a variety of small muscles of the wrist and hand.

Cutaneous innervation of the upper extremity originates, via the brachial plexus, from spinal cord levels C3T2. The shoulder is served by supraclavicular branches (C3, C4) of the cervical plexus, while the anterior and lateral aspects of the arm and forearm have sensory innervation via the axillary (C5, C6), nerve as well as the dorsal (C5, C6), lateral (C5, C6), and medial (C8, T1) antebrachial cutaneous nerves. These same nerves have branches that wrap around to serve portions of the posterior and medial surfaces of the extremity. The palm of the hand is served by the median (C6C8) and ulnar (C8, T1) nerves. The ulnar nerve also wraps around to serve medial areas of the dorsum, or back, of the hand. A line drawn down the midline of the ring finger represents the junction of the ulnar-radial distribution on the back of the hand and the ulnar-median distribution on the palm. A small part of the thumb and the distal thirds of the index, middle, and lateral surface of the ring finger are served by the median nerve. The inner aspect of the arm and adjacent armpit is served by intercostobrachial and posterior and medial brachial cutaneous nerves (T1T2).

Spinal nerves from lumbar levels L1L4 contribute to the formation of the lumbar plexus, which, along with the sacral plexus, provides motor, sensory, and autonomic fibers to gluteal and inguinal regions and to the lower extremity. Lumbar roots are organized into dorsal and ventral divisions.

Minor cutaneous and muscular branches of the lumbar plexus include the iliohypogastric, genitofemoral, and ilioinguinal (projecting to the lower abdomen and to inguinal and genital regions) and the lateral femoral cutaneous nerve (to skin on the lateral thigh). Two major branches are the obturator and femoral nerves. The obturator enters the thigh through the obturator foramen; motor branches proceed to the obturator internus and gracilis muscles as well as the adductor muscles, while sensory branches supply the articular capsule of the knee joint. An accessory obturator nerve supplies the pectineus muscle of the thigh and is sensory to the hip joint.

The sartorius muscle and medial and anterior surfaces of the thigh are served by branches of the anterior division of the femoral nerve. The posterior division of the femoral nerve provides sensory fibers to the inner surface of the leg (saphenous nerve), to the quadriceps muscles (muscular branches), the hip and knee joints, and the articularis genu muscle.

The Sacral PlexusThe ventral rami of L5 and S1S3 form the sacral plexus, with contributions from L4 and S4. Branches from this plexus innervate gluteal muscles, muscles forming the internal surface of the pelvic basin (including those forming the levator ani), and muscles that run between the femur and pelvis to stabilize the hip joint (such as the obturator, piriformis, and quadratus femoris muscles). These muscles lend their names to the nerves that innervate them. Cutaneous branches from the plexus serve the buttocks, perineum, and posterior surface of the thigh.

The major nerve of the sacral plexus, and the largest in the body, is the sciatic. Formed by the joining of ventral and dorsal divisions of the plexus, it passes through the greater sciatic foramen and descends in back of the thigh. There, sciatic branches innervate the biceps femoris, semitendinosus and semimembranosus muscles, and part of the adductor magnus muscle. In the popliteal fossa (just above the knee) the sciatic nerve divides into the tibial nerve and the common fibular (or peroneal) nerve. The tibial nerve (from the dorsal division) continues distally in the calf and innervates the gastrocnemius muscle, deep leg muscles such as the popliteus, soleus, and tibialis posterior, and the flexor muscles, lumbrical muscles, and other muscles of the ankle and plantar aspects of the foot. The peroneal nerve, from the ventral division, passes to the anterior surface of the leg and innervates the tibialis anterior, the fibularis muscles, and extensor muscles that elevate the foot and fan the toes. Cutaneous branches from the tibial and common fibular nerves serve the outer sides of the leg and the top and bottom of the foot and toes.

The ventral rami of S4, S5, and the 1st coccygeal nerve form the coccygeal plexus, from which small anococcygeal nerves arise to innervate the skin over the coccyx (tailbone) and around the anus.


Autonomic Nervous System

The autonomic nervous system is a part of the peripheral nervous system that functions to regulate the basic visceral (organ) processes needed for the maintenance of normal bodily functions. It operates independently of voluntary control, although certain events, such as emotional stress, fear, sexual excitement, and alterations in the sleep-wakefulness cycle, change the level of autonomic activity.

The autonomic system is usually defined as a motor system that innervates three major types of tissue: cardiac muscle, smooth muscle, and glands. However, this definition needs to be expanded to encompass the fact that it also relays visceral sensory information into the central nervous system and processes it in such a way as to make alterations in the activity of specific autonomic motor outflows, such as those that control the heart, blood vessels, and other visceral organs. It also causes the release of certain hormones involved in energy metabolism (e.g., insulin, glucagon, epinephrine) or cardiovascular functions (e.g., renin, vasopressin). These integrated responses maintain the normal internal environment of the body in an equilibrium state called homeostasis.

The autonomic system consists of two major divisions: the sympathetic nervous system and the parasympathetic nervous system. These often function in antagonistic ways. The motor outflow of both systems is formed by two serially connected sets of neurons. The first set, called preganglionic neurons, originates in the brain stem or the spinal cord, and the second set, called ganglion cells or postganglionic neurons, lies outside the central nervous system in collections of nerve cells called autonomic ganglia. Parasympathetic ganglia tend to lie close to or within the organs or tissues that their neurons innervate, whereas sympathetic ganglia lie at a more distant site from their target organs. Both systems have associated sensory fibers that send feedback information into the central nervous system regarding the functional condition of target tissues. To view a figure depicting the difference in function between the sympathetic and parasympathetic nervous system click here.

A third division of the autonomic system, termed the enteric nervous system, consists of a collection of neurons embedded within the wall of the entire gastrointestinal tract and its derivatives. This system controls gastrointestinal motility and secretions.

Sympathetic nervous system

The Sympathetic Nervous SystemSympathetic preganglionic neurons originate in the lateral horns of the 12 thoracic and the first 2 or 3 lumbar segments of the spinal cord. (For this reason the sympathetic system is sometimes referred to as the thoracolumbar outflow. The diagram to the left depicts this.) The axons of these neurons exit the spinal cord in the ventral roots and then synapse on either sympathetic ganglion cells or specialized cells in the adrenal gland called chromaffin cells.

Sympathetic ganglia

Sympathetic ganglia can be divided into two major groups, paravertebral and prevertebral (or preaortic), on the basis of their location within the body. Paravertebral ganglia generally lie on each side of the vertebrae and are connected to form the sympathetic chain or trunk. There are usually 21 or 22 pairs of these ganglia: 3 in the cervical region, 10 to 11 in the thoracic region, 4 in the lumbar region, 4 in the sacral region, and a single, unpaired ganglion lying in front of the coccyx called the ganglion impar. The three cervical sympathetic ganglia are the superior cervical ganglion, the middle cervical ganglion, and the cervicothoracic ganglion (also called the stellate ganglion). The superior ganglion innervates viscera of the head; the middle and stellate ganglia innervate viscera of the neck, thorax (i.e., the bronchi and heart), and upper limb. The thoracic sympathetic ganglia innervate the trunk region, and the lumbar and sacral sympathetic ganglia innervate the pelvic floor and lower limb. All the paravertebral ganglia provide sympathetic innervation to blood vessels in muscle and skin, arrector pili muscles attached to hairs, and sweat glands.

Intrinsic Autonomic Plexus of the IntestineThe three preaortic ganglia are the celiac, superior mesenteric, and inferior mesenteric. Lying on the anterior surface of the aorta, they provide axons that are distributed with the three major gastrointestinal arteries arising from the aorta. The three ganglia retain a pattern of innervation that originates in the embryo. Thus, the celiac ganglion innervates structures derived from the embryonic foregut, including the stomach, liver, pancreas, duodenum, and the first part of the small intestine; the superior mesenteric ganglion innervates the small intestine, which is derived from the embryonic midgut; and the inferior mesenteric ganglion innervates embryonic hindgut derivatives, which include the descending colon, sigmoid colon, rectum, urinary bladder, and sexual organs.

Neurotransmitters and receptors

Upon reaching their target organs by traveling with the blood vessels that supply them, sympathetic fibers terminate as a series of varicosities close to the end organ. Because of this anatomical arrangement, autonomic transmission takes place across a junction rather than a synapse. Presynaptic sites can be identified because they contain aggregations of synaptic vesicles and membrane thickenings; postjunctional membranes, on the other hand, rarely possess morphological specializations, but they do contain specific receptors for various neurotransmitters. The distance between pre- and postsynaptic elements can be quite large as compared to typical synapses. For instance, the gap between cell membranes of a typical chemical synapse is 3050 nanometres, while in blood vessels the distance is often greater than 100 nanometres and, in some cases, 12 micrometres (1,0002,000 nanometres). Owing to these relatively large gaps between autonomic nerve terminals and their effector cells, transmitters tend to act slowly; they become inactivated rather slowly as well. To compensate for this apparent inefficiency, many effector cells, such as those in smooth and cardiac muscle, are connected by low-resistance pathways that allow for electrotonic coupling of the cells. In this way, if only one cell is activated, multiple cells will respond and work as a group.

At a first approximation, chemical transmission in the sympathetic system appears simple: preganglionic neurons use acetylcholine as a neurotransmitter, whereas most postganglionic neurons utilize norepinephrine (noradrenaline)with the major exception that postganglionic neurons innervating sweat glands use acetylcholine. On closer inspection, however, neurotransmission is seen to be more complex, because multiple chemicals are released, and each functions as a specific chemical code affecting different receptors on the target cell. In addition, these chemical codes are self-regulatory, in that they act on presynaptic receptors located on their own axon terminals.

The chemical codes are specific to certain tissues. For example, most sympathetic neurons that innervate blood vessels secrete both norepinephrine and neuropeptide Y, sympathetic neurons that innervate the submucosal neural plexus of the gut contain both norepinephrine and somatostatin, and sympathetic neurons that innervate sweat glands contain calcitonin gene-related peptide, vasoactive intestinal polypeptide, and acetylcholine. In addition, other chemicals besides the neuropeptides mentioned above are released from autonomic neurons along with the so-called classical neurotransmitters, norepinephrine and acetylcholine. For instance, some neurons synthesize a gas, nitric oxide, that functions as a novel type of neuronal messenger molecule. Thus, neural transmission in the autonomic nervous systems involves the release of combinations of different neuroactive agents that affect both pre- and postsynaptic receptors.

Neurotransmitters released from nerve terminals bind to specific receptors, which are specialized macromolecules embedded in the cell membrane. The binding action initiates a series of specific biochemical reactions in the target cell that produce a physiological response. These effects can be modified by various drugs that act as agonists or antagonists. In the sympathetic nervous system, for example, there are five types of adrenergic receptors (receptors binding epinephrine): a 1, a 2, b 1, b 2, and b 3. These are found in different combinations in various cells throughout the body. Activation of a 1 receptors in arterioles causes blood-vessel constriction, whereas stimulation of a 2 autoreceptors (receptors located in sympathetic presynaptic nerve endings) function to inhibit the release of norepinephrine. Other types of tissue have unique adrenergic receptors. Heart rate and myocardial contractility, for example, is controlled by b 1 receptors, bronchial smooth muscle relaxation is mediated by b 2 receptors, and lipolysis is controlled by b 3 receptors.

Cholinergic receptors (receptors binding acetylcholine) also are found in the sympathetic system (as well as the parasympathetic system). Nicotinic cholinergic receptors cause sympathetic postganglionic neurons, adrenal chromaffin cells, and parasympathetic postganglionic neurons to fire and release their chemicals. Muscarinic receptors are associated mainly with parasympathetic functions and are located in peripheral tissues (e.g., glands, smooth muscle). Peptidergic receptors exist in target cells as well.

The length of time that each type of chemical acts on its target cell is variable. As a rule, peptides cause slowly developing, long-lasting effects (one or more minutes), whereas the classical transmitters produce short-term effects (about 25 milliseconds).

The sympathetic nervous system normally functions to produce localized adjustments (such as sweating) and reflex adjustments of the cardiovascular system. Under conditions of stress, however, the entire sympathetic nervous system is activated, producing an immediate, widespread response that has been called the fight or flight response. This is characterized by the release of large quantities of epinephrine from the adrenal gland, an increase in heart rate, an increase in cardiac output, skeletal muscle vasodilation, cutaneous and gastrointestinal vasoconstriction, pupillary dilation, bronchial dilation, and piloerection. The overall effect is to prepare the individual for imminent danger.

Functions of the Nervous System

The complex activities of the body are controlled jointly by the Endocrine and the Nervous systems. As opposed to the Endocrine system the Nervous system has a more or less instant effect on the body via a complex network of nerves and control centres. The Central Nervous System (CNS) includes the brain and spinal cord, while Peripheral Nervous System (PNS) includes nerves connected to the spinal cord. The nervous system can be further divided into sub-systems, all of which are composed of neurons and connective tissue:


About Neurons

Neurons are specialised cells of the nervous system, they vary greatly in appearance and length, but contain a similar structure:
  • Axons are long nerve processes which carry nerve impulses from the Soma to other neurons, they vary in length but can become almost as long as half of the human body.
  • The soma (body) of the neuron contains the nucleus which acts as the cell's control centre, these contain many small neurofibrils which project from the nucleus into the dendrites.
  • Dendrites are short, thick processes which branch out of the soma in a tree like manor. They conduct nerve impulses to the soma.
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The three categories of neurons:

  • Afferent (Sensory) Neurons have the dendrites connected to receptors such as the eyes, ears etc. These receptors change the information they receive into electrical impulses that are transmitted to other neurons. In sensory neurons the axons are connected to other neurons.
  • Efferent (Motor) Neurons have the dendrites connected to other neurons, the axons are connected to effectors. Effectors are either glands or a muscle cell that is the receiving end of the nerve impulse. The nerve, when excited will cause the effector to react (move, contract, or secrete etc).
  • Internuncial Neurons have both the dendrites and the axons are connected to other neurons. They are sometimes referred to as connector neurons. Internuncial neurons are found throughout the body, but especially in the spinal cord and brain.

Properties and characteristics of Neurons:

  • Normally the electrical impulses (messages) travel through a neuron in only one direction.
  • The axon may be surrounded by a 'coat' of lipids (fats) and proteins known as the myelin sheath which acts as an insulator.
  • Neurons are specialist cells that have lost the ability to reproduce themselves. Once the soma of a neuron has died the entire neuron dies, and can never be replaced.
  • Repair of damaged neurons only occurs in myelinated neurons.
  • white matter are coloured by myelin, consisting of many neurons supported by neuroglia.
  • grey matter is soma and dendrites or bundles of unmyelinated axons and neuralgia.


Nerves, Neuroglia, and Ganglia

A nerve is a bundle of fibres (axons and/or dendrites) outside the CNS.

Neuroglia are cells of the nervous system that help protect and support it.

Ganglia are groups of nerve cell bodies lying outside the CNS.


The Spinal Chord

A spinal tract is a bundle of fibres in the CNS that travel long distances up or down the spinal chord. Ascending tracts carry impulses up the chord to the brain, while descending tracts carry impulses down the chord from the brain. Tracts run along the spinal canal inside the protective spinal column, conveying sensory and motor (movement ) information to and from the brain. Spinal meninges are tough tubes of tissue which protect the chord.


The Brian

The brain is highly complex, it contains about 1000 billion neurons, and weighs about 3 lbs in adults.

There are four main areas of the brain:

  1. The brain stem is at the base of the brain where it joins the spinal chord (contains the medulla, pons, and mid brain)
  2. The diencephalon is above the brain stem (contains the thalamus and hypothalamus)
  3. The cerebrum is above the diencephalon and forms the majority of the brain
  4. The cerebellum is the lower back of the brain

The brain has two hemispheres , there are functional differences, for example the left had side of the brain controls the right hand side of the body and visa versa (lateralisation).

Neurotransmitters are substances which excite or inhibit the neurons of the brain, facilitating communication between brain cells. These include endorphins, neuropeptides.

Cerebrospinal fluid circulates around the brain and spinal tracts to provide protection in addition to that provided by the meninges and protective bones of the spine and skull. A lumbar puncture (spinal tap) is where a needle is placed between the vertebra in the lower back. A sample of cerebrospinal fluid might be taken to see if cancerous cells have entered the CNS, or chemotherapy might be administrated to prevent or combat CNS involvement.


The Sensory Systems

A receptor or sense organ picks up stimulus and converts it into a nerve impulse. This impulse is then conducted along a neural pathway to the brain, where the signal is converted into a sensation. There are various receptors:
  • Cutaneous (skin) senses. The skin contains specialist receptors for touch, pressure, vibration, hot, cold, and pain.
  • Proprioceptive (muscle) sensations inform us of the activities and current posture of the muscles.
  • Integrative sensations are not well defined but include memory, sleep, emotions etc.
  • Olfactory sensations (smell). Olfactory neurons have dendrites that are connected to fine hairs in the nose that react to odours.
  • Gustatory sensations (taste) there are about 2000 taste buds, mostly on the tongue and a few in the throat. Taste buds contain gustatory cells which contain sensitive hair like processes.
  • Ophthalmic sensations (sight). The retina of the eye converts light into nerve impulses which are transmitted to the optic nerve. Retinoblastoma is a rare tumour in the cells of the retina.
  • Auditory senses (hearing). Sound waves cause fine hairs in the inner ear to vibrate generating nerve impulses.
  • Equilibrium (sense of balance) the ear also contains receptors that give a sense of static equilibrium (position of the head) and dynamic equilibrium (sudden movements).


The Autonomic Nervous system (ANS)

The nerves of the ANS activate the involuntary smooth muscles, cardiac muscles, and some glands.


Roots, suffixes, and prefixes

component meaning example
ASTRO- star
astrocyte = star shaped brain cell.
CRANI- skull
cranial radiation = radiation to the head.
CEPHAL- head
encephal = the brain, en(in) cephal (the head).
MENING- membrane
meningitis = inflammation of the membranes of the spinal chord.
NEUR- nerve
neuroblast = an immature nerve cell.
ventricles are small cavities in the brain & spinal cord. Ventriculscopy = examination of ventricles.
-MALACIA softening
neuromalacia = morbid softening of the nerves.
-GRAM record
electroencephalogram (EEG) = brain scan.


Cancer Focus


Overview of CNS Tumours
Internet Resources for Childhood Brain Tumours


Adult CNS Tumours


Childhood CNS Tumours
Childhood Brain tumours are the second most common type of childhood cancer. They are however a diverse group of different types of tumours. Classification of brain tumours is based on both histopathology and location in the brain. For example, undifferentiated neuroectodermal tumours of the cerebellum are referred to as medulloblastomas, while tumours with similar histology in the pineal region would be diagnosed as pineoblastomas. Patients may present with headaches, drowsiness, weakness, or vomiting caused by the pressure inside the skull caused by the growing tumour.

Medulloblastoma is nearly always found in children or young adults, 80% are found in children aged under 15. It can spread from the medulla (part of the brain stem) to the spine or to other parts of the body. Prognosis will depend on the child's age, how much of the tumour remains following surgery, and whether the cancer has metastasised.

Cerebellar astrocytoma arise in brain cells called astrocytes. Cerebellar astrocytoma is usually low grade (slow growing and non metastatic cells), while Cerebral astrocytoma can be malignant.

Brainstem gliomas are tumours arising in the mid brain, pons or medulla. They may grow rapidly or slowly, depending on the grade of the tumour, but overall have a less favourable prognosis compared to other tumours such as Cerebellar astrocytoma.

Other brain tumours include: Primitive neuroectodermal tumour (PNET), craniopharyngioma, intracranial germ cell tumour, pineal parenchymal tumour, and optic tract glioma.


Cancers of the Eye


Retinoblastoma is a rare tumour of the eye which develops in the cells of the retina, most patients are under 5 years old. Sometimes only one eye is affected (unilateral-retinoblastoma ), but in about two fifths of patients both eyes have the disease (bilateral-retinoblastoma ). Some cases are known to be hereditary.
Internet Resources for Retinoblastoma


Intraocular Melanoma
Intraocular melanoma is a rare cancer, in which malignant cells are found in the uvea (this is the part of the eye which contains the iris and other tissues). The uvea contains melanocytes which are cells that contain colour, intraocular melanoma occurs when these cells become cancerous.
Internet Resources for IntraOcular Melanoma


Neuroblastoma occurs most often in babies, very young children. It is a disease in which cancer cells are found in certain nerve cells in the body, it originates in the adrenal medulla or other sites of sympathetic nervous system tissue. The most common site is the abdomen, either in the adrenal glands or around the spinal cord. The majority of patients present with metastatic disease. Age and stage are the main prognostic factors. Patients aged under one year at diagnosis have a more favourable prognosis. Stage 4S are a special group of patients aged under one year whose neuroblastoma may undergo spontaneous regression (tumour disappears without treatment). Also patients aged under one a higher proportion of low stage patients compared to older patients. There is an excess of males compared to females, there are a higher proportion of males in patients with less favourable sites and stage.
Internet Resources for Neuroblastoma

Related Abbreviations and Acronyms:


ABTA American Brain Tumour Association
BAER Brainstem Auditory Evoked Responce
CNS Central nervous system - the brain and spine
CSF Cerebro spinal fluid
EANO European Association for NeuroOncology
EEG Electroencephalogram - brain scan
ENSG European Neuroblastoma Study Group
INFA International Neurofibromatosis Association
INSS International Neuroblastoma Staging System
LP Lumbar puncture
NNFF National Neurofibromatosis Foundation (USA)
NSE Neuron-Specific Enolase - a neural marker
PNET Primitive neuroectodermal tumour Context: CNS tumours
PNS Peripheral nervous system - nervous system outside the brain and spine.

More Cancer Related Abbreviations