The nervous system consists of the brain, spinal cord and nerves. It is a complex, sophisticated system that detects and interprets changes inside and outside the body and responds to them by sending electrochemical messages through nerves to effector organs such as muscles and glands.
The nervous system controls and coordinates all the body’s functions and activities – every movement, thought, breath and heartbeat.
In humans the sophistication of the nervous system makes it possible to have language, culture and other features of society that would not exist without the highly developed human brain.
Neurones – the basic functional unit of the nervous system
Nervous tissue in the nervous system is distinguished by the presence of two main types of cells: neurones (also called nerve cells or neurons) and glial cells (also called neuroglia or glia).
Neurones vary in size and shape in different parts of the nervous system but share common features including:
- a cell body which contains the nucleus and other essential parts.
- dendrites which are hair-like branching processes that emerge from the cell body and receive information from other nerve cells.
- an axon or nerve fibre which is a long hair-like process that carries information away from the cell body. Axons frequently occur in bundles which make up nerves
There are three main classes of neurone:
- Afferent neurones – these carry information from peripheral receptors in the body to the central nervous system. If the information they carry reaches a conscious level they are also called sensory neurones.
- Efferent neurones – these carry impulses away from the central nervous system and if they innervate skeletal muscle to cause movement they are called motor neurones.
- Interneurones – these are found only within the central nervous system where they connect neurone to neurone (and are sometimes called relay neurones).
Information is coded within neurones and travels along the axon by changes in electrical energy. When the information has travelled to the end of the nerve axon in order for it to pass to the next cell it must cross the gap that exists between the cells. This gap is called the synaptic gap or cleft.
Arrival of the message at the axon terminals causes synaptic vesicles located at the end of the axon to release their chemical contents (called neurotransmitters) into the synaptic cleft. The released neurotransmitters diffuse across the cleft and bind to receptors on the other cell inducing changes in its membrane so passing on the intended message. The entire process takes a fraction of a millisecond.
Neurotransmitters released into the synaptic cleft are only active for a short time. They are inactivated by enzymes and then taken back into the axon and recycled. There are many molecules that act as neurotransmitters.
Some neurological conditions involve deficiencies in neurotransmitters, e.g., in Parkinson’s disease there is a shortage of the neurotransmitter dopamine. The progressive death of nerve cells in the brain in Parkinson’s disease leads to a dopamine deficiency which produces the characteristic symptoms including tremors, rigidity, poor balance and falls.
Glial cells are non-neuronal cells and constitute the other major component of the nervous system. Glial cells do not have a direct role in information processing but fulfil other roles essential for the normal functioning of nerve cells, e.g., support and nutrition.
There are three main types of glial cell:
- oligodendrocytes (oligodendroglia). These secrete a fatty substance (myelin) that forms the ‘myelin sheath’ around many neuronal axons which insulates the axon permitting faster conduction of the electrical messaging. Schwann cells perform this role in the peripheral nervous system.
- astrocytes. These are thought to be involved in the exchange of chemicals between the circulatory system and nervous tissue and to form the selectively permeable protective ‘blood brain barrier’ which restricts the access of circulating chemicals to the brain and spinal cord.
- microglia. These are involved in the destruction of pathogens and removal of dead neurones.
Structure of the nervous system
The nervous system is divided into two parts: the central nervous system and the peripheral nervous system.
The central nervous system
The central nervous system (CNS) includes the brain and spinal cord.
The CNS is the most complex part of the nervous system and contains the majority of nerve cell bodies and synaptic connections.
The brain is located within the cranial cavity of the skull and the spinal cord occupies the vertebral or spinal canal, within the vertebral column or spine.
The outermost, the dura mater, is a tough fibrous loose-fitting membrane.
The middle membrane, the arachnoid mater, is a soft translucent loose fitting membrane
The inner membrane, the pia mater, is a microscopically thin delicate and highly vascular membrane that closely adheres to the surface of the brain.
Between the pia and arachnoid mater lies the subarachnoid space.
This contains the cerebrospinal fluid (CSF) which plays an important role in helping to cushion the brain from sudden movements of the head.
The brain has billions of neurones that receive, analyze, and store information about internal and external conditions. It is the source of conscious and unconscious thoughts, moods, and emotions and essentially makes us what we are.
The brain is composed of three parts; the cerebrum (‘seat of consciousness’), brain stem and cerebellum. The latter two are part of the ‘unconscious’ brain.
The cerebrum (front of brain) is the largest part of the brain (comprising 85% of the total weight of the brain) and governs conscious awareness and thought, memory, intellect, language, personality development and vision. A deep central groove (the great longitudinal fissure) incompletely separates the cerebrum into two hemispheres (halves), a right hemisphere and a left hemisphere, which communicate with each other through the corpus callosum (a bundle of nerve fibres running between them). Each hemisphere controls the muscles and glands on the opposite side of the body.
The cerebrum is covered with an outer layer of grey matter (un-myelinated nerve cell bodies) known as the cerebral cortex beneath which lies an inner core of white matter. In addition, buried within the white matter lie several large masses of cell bodies collectively referred to as the basal ganglia. The surface of the cerebral hemispheres are wrinkled and folded to form ridges and grooves. The folds are called gyri and the grooves are called sulci. This folding serves to increase the surface area so that more grey matter can exist within the skull cavity.
- The frontal lobe (front of head) is concerned with motor activity and integration of muscle activity, speech, intellect, thought processes and the planning of behaviour.
The parietal lobe (middle of head) processes information about touch, taste, pressure, pain and heat and cold.
The occipital lobe (back of head) receives and processes visual information.
The temporal lobe(side of head) receives auditory signals, processes language and the meaning of words.
The cerebrum and diencephalon together constitute the forebrain.
The diencephalon consists of four parts, the epithalamus, thalamus, subthalamus and hypothalamus.
The thalamus is the largest part of the diencephalon and plays an important role in sensory, motor and cognitive functions and has extensive connections with the cerebral cortex.
The hypothalamus regulates homeostasis (body’s steady state) and controls thirst, hunger, body temperature, water balance and links the nervous system to the endocrine system. The endocrine system is composed of glands (the pituitary, pancreas, ovaries, testes, thyroid, parathyroid and the adrenal glands) that control the rate at which we grow and many other functions.
The epithalamus is the most posterior part of the diencephalon. It contains a vascular network involved in the production of cerebrospinal fluid. Extending from the epithalamus is the pineal gland. Its role is not yet fully understood but it is thought to be involved in the control of body rhythms.
The cerebellum is located at the back of the head. It is the second largest part of the brain. Like the cerebrum it has two hemispheres that control the opposite sides of the body and is covered by an outer layer of grey matter, the cerebellar cortex, over a central core of white matter. The cortical surface is highly convoluted to form a regular pattern of narrow, parallel folds or folia. The cerebellum is concerned with the co-ordination of movement and operates at an entirely unconscious level.
The brain stem lies deep in the brain and connects the cerebrum and cerebellum to the spinal cord. It includes the midbrain, pons, and the medulla oblongata. The brain stem is the smallest part of the brain but vitally important. Through it pass ascending and descending nerve fibre tracts linking the brain and spinal cord. These carry sensory information from and permit movement of the trunk and limbs. The brain stem also contains the sites of origin and termination of most of the cranial nerves through which the brain innervates the head region. Within the brain stem lie centres controlling the vital functions such as respiration and the cardiovascular system.
The midbrain is the centre for visual and auditory reflexes e.g., blinking and adjusting the ear to sound volume. The pons, in the middle section, bridges the cerebellum hemispheres and higher brain centres with the spinal cord. The medulla oblongata, the lowest part of the brain stem and closest to the spinal cord, is involved with regulating heartbeat, breathing and the reflexes for vomiting, coughing, swallowing and sneezing.
The spinal cord is a continuation of the brain stem. It is long and cylindrical and passes through a tunnel in the vertebrae called the vertebral canal. The spinal cord has many spinal segments from which the 31 pairs (one pair per segment) of spinal nerves arise. Like the cerebrum and cerebellum, the spinal cord has grey and white matter but in the spinal cord the grey matter is on the inside. The spinal cord carries messages between the CNS and the rest of the body and mediates numerous spinal reflexes e.g., the knee-jerk reflex.
It is the distribution of nerve cell bodies and their processes within the CNS that gives rise to grey and white matter.
Regions that are relatively rich in nerve cell bodies e.g., the cerebral cortex, are referred to as grey matter whereas other regions containing mostly nerve processes (usually axons) which are often myelinated making them appear paler, are termed white matter.
Conventionally, a cluster of nerve cell bodies in the brain or spinal cord is called a ‘nucleus’ but a cluster of nerve cell bodies in the periphery is called a ‘ganglion’. However, there are a few exceptions to this rule, e.g., in the forebrain there are collections of grey matter called the ‘basal ganglia’.
The peripheral nervous system
It consists of nerves joined to the brain and spinal cord (twelve pairs of cranial nerves and thirty one pairs of spinal nerves), their ramifications within the body, nerve endings, and groups of peripherally located nerve cells bodies that are aggregated in structures called ganglia.
The cell bodies of neurones that give rise to nerves in the PNS do not lie within the nerves themselves but lie centrally within the brain and spinal cord or peripherally in the ganglia.
The cranial nerves link the brain with the sense organs and muscles of the head and neck.
The first two cranial nerves attach directly to the forebrain while the rest attach to the brain stem and are associated with various nuclei (collections of nerve cell bodies) within the brain stem.
Cranial nerves are individually named and numbered in Roman numerals. During a complete neurological examination, most of these nerves will be evaluated to help determine functioning of the brain.
I olfactory nerve - the nerve of smell.
II optic nerve -one of the nerves controlling vision.
III oculomotor nerve - controls pupil size and movement of the eyeball.
IV trochlear nerve - involved with movement of the eyeball.
V trigeminal nerve – many functions e.g., detecting sensations on the face and inside the nose and mouth; control of muscles used in chewing.
VI abducens nerve - helps with movement of the eyeball.
VII facial nerve - various functions including movement of face, taste, salivation and tear production.
VIII vestibulocochlear (acoustic) nerve - involved with hearing and head movement.
IX glossopharyngeal nerve - involved with taste, salivation and swallowing.
X vagus nerve - controls the ability to swallow, the gag reflex, some aspects of taste and some aspects of speech.
XI accessory nerve – movement of the head and shoulders.
XII hypoglossal nerve - movement of the tongue.
Each spinal nerve has a dorsal root and a ventral root. The dorsal roots contain afferent neurones that transmit information to the spinal cord from sensory receptors; their cell bodies are located in dorsal root ganglia. The ventral roots carry efferent neurones that transmit messages from the spinal cord to muscles and organs in the body; their cell bodies lie within the spinal cord grey matter.
Spinal nerves are grouped according to the level of the spine from which they arise and are numbered in sequence. Each spinal nerve carries the sensory innervations for a part of the body surface. The area of skin supplied by a particular spinal nerve is celled a dermatome. The group of skeletal muscles innervated by a particular spinal nerve are called a myotome.
There are eight pairs of ‘cervical’ nerves (numbered C1 – C8) serving mainly the arms; twelve pairs of ‘thoracic’ (T1 – T12) leading to the sternum, internal organs and muscles of the chest; five pairs of ‘lumbar’ (L1 – L5) serving the abdominal wall and legs; five pairs of ‘sacral’ (S1 – S5) and one pair of ‘coccygeal’ nerves leading mainly to the legs.
The nerves coming from the upper part of the spinal cord leave the vertebral canal almost horizontally while those from the lower regions descend at sharp angles. This is a consequence of growth. In early life, the spinal cord extends the entire length of the vertebral column but with age, the column grows faster than the cord. As a result, the adult spinal cord ends at the level between the first and second lumbar vertebrae, so the lumbar, sacral, and coccygeal nerves descend to reach their exit point beyond the end of the spinal cord.
Some peripheral nerves are myelinated, others are not. Within peripheral nerves the nerve fibres are arranged in bundles and enveloped by sheaths of connective tissue. Between individual fibres is a delicate connective tissue called endoneurium. Bundles of fibres are surrounded by perineurium and the entire nerve is covered by a tough coat called epineurium. This arrangement provides support and strength for the nerve. The cranial and spinal membranes (meninges – dura mater, arachnoid mater, pia mater) are continuous with the connective tissue sheaths of spinal and cranial nerves. Hence, the dura mater is continuous with the epineurium while the arachnoid and pia are continuous with the perineurium and endoneurium.
Nerve endings are either sensory or effector. Sensory endings respond to mechanical (pressure/touch), thermal (temperature) or chemical stimulation and the peripheral nerve fibre to which they belong conducts an impulse to the CNS. Effector endings contact muscle or secretory cells and under control from the CNS result in movement or in the secretion of substances.
The sensory systems are divided into the special senses (vision, hearing, taste, smell (olfaction), and balance) and the general senses (touch, pain, pressure, temperature).
For the special senses receptors in the eyes (i.e., photoreceptors called rods and cones), ears (receptors for sound and balance), nose (receptors in the lining of the nose that respond to chemicals dissolved from the air) and mouth (taste buds) detect changes and relay this information via peripheral nerves to the CNS.
For the general senses there are three types of sensory ending:
- Exteroceptors – found in the skin which respond to painful stimuli, temperature, touch and pressure.
- Interoceptors – found in the viscera (internal organs).
- Proprioceptors – found in muscles, joints and tendons which provide awareness of posture and movement.
Sensory nerve endings may be either unencapsulated or encapsulated.
Unencapsulated or free nerve endings consist of the terminal branches of sensory nerve fibres lying freely in the innervated tissue and appear to respond to thermal and painful sensations.
Encapsulated nerve endings are ‘encapsulated’ by non-neural tissue to form a corpuscle. Pacinian corpuscles are found in the skin and in deep tissues such as around joints, and respond to mechanical distortion. Meissner’s corpuscles occur in the skin, especially in the fingertips, and are thought to be responsible for fine touch.
Peripheral nerves tend to lie deep under the skin except in a few places, e.g., the elbow joint, but are still vulnerable to physical damage. If a peripheral nerve is completely cut through it will often regenerate but this can take many months. Besides physical damage, nerves can be damaged by inflammation (e.g., Guillain-Barre syndrome), metabolic disorders (e.g., diabetes), infectious diseases (e.g., leprosy, shingles), or by toxins (e.g., heavy metals). Peripheral nerves may also lose function temporarily resulting in numbness or stiffness when brought about by mechanical pressure, low temperature or chemical interaction with local anaesthetic drugs.
Physical damage to the spinal cord may result in loss of sensation or movement. If the injury produces swelling only then the symptoms will be transient. If however, nerve fibres in the spine are destroyed, the loss of function will be permanent. Studies have shown that spinal nerve fibres attempt to re-grow in the same way as peripheral nerve fibres but in the spinal cord the scar tissue produced acts as a barrier to the re-growing nerves.
The autonomic (involuntary) nervous system
The term autonomic nervous system is used for those nerve cells located within both the central and peripheral nervous systems that are concerned with innervating and controlling the visceral organs (e.g., heart, bladder, liver, lungs) smooth muscle and secretory glands.
The main function of the autonomic nervous system is to maintain a ‘steady state’ in the body by regulating cardiovascular, respiratory, digestive, excretory and thermoregulatory mechanisms which occurs automatically and with little volitional control. Changes in the internal and external environment and emotional factors influence autonomic activity.
The autonomic system comprises two parts – the sympathetic nervous system and the parasympathetic nervous system which have opposite effects on the structures they innervate.
The sympathetic nervous system speeds things up in response to a stressful situation or danger and is involved in the ‘fight or flight’ syndrome, e.g., the heart beats faster pumping more blood to supply energy for muscles, the salivary glands produce less saliva so the mouth goes dry, the adrenal glands secrete the hormone adrenaline which prepares the body to fight or run away, breathing becomes faster to provide more oxygen to the body, the liver releases glucose providing extra energy for muscles, and the stomach and intestines have their blood diverted to the heart, CNS and muscles and the wave like movements of the intestinal walls stop and the various sphincters close.
The parasympathetic nervous system is concerned with restoring the body to normal peaceful rates after an emergency, e.g., the heart and breathing rates are slowed down.
Interesting facts about the nervous system
The human brain contains about 100 billion neurons. If all of these neurons were lined up it would form a 600 mile long line. Each neuron is connected to around 10,000 other cells which equates to around 1000 trillion connections in the brain.
The human brain contains around 400 miles of blood vessels.
The diameter of the neurones can range between 4 to 100 microns (there are 1000 microns in a millimetre).
Neurones which are the largest cells in the human body do not undergo the process of mitosis (cell division).
The average adult’s brain weighs 3 – 4 lbs (1.5kg) but consumes about 20% of the body’s oxygen supply. The brain needs constant blood flow in order to keep up with the heavy metabolic demands of the neurones.
At any one time only 4% of cells in the brain are active.
Male humans have about a 10% larger brain than females. But differences in brain weight and size do not equal differences in mental ability.
There is evidence of a gradual increase in average brain size over the last centuries, estimated to have been around 0.5% per decade.
The biggest part of the brain is the cerebrum which represents up to 85% of the brain’s weight. The cerebrum is the thinking part of the brain and controls our voluntary muscles.
As we age our brains lose about a gram each year.
In humans the right side of the brain controls the left side of the body and the left side of the brain controls the right side.
A newborn baby’s brain triples in size during the course of the first year.
The human spinal cord consists of around 13,500,000 neurones.
A landmark study in 1998 by researchers from Sweden and the Salk Institute in La Jolla, California, showed for the first time that some brain cells in mature humans may regenerate under certain circumstances.
The brain can stay alive for 4 to 6 minutes without oxygen. After that cells begin die. Loss of oxygen for 5 to 10 minutes can cause serious brain damage.
A 10 second loss of blood supply to the brain will result in unconsciousness.
The brain generates ~25 watts of power while you are awake – enough to power a light bulb.
Every time you have a new thought, or recall a memory, a new brain connection is made between two or more brain cells.
Your brain changes in response to what you do.
Alcohol does not kill brain cells but damages the dendrites which interferes with the way cells in the brain communicate. The damage is reversible and not permanent. However, years of alcohol abuse can cause serious neurological damage including Wernicke-Korsakoff syndrome.
A living brain is soft – you could cut it with a table knife, and is not grey but a deep red in colour.
There is no sense of pain within the brain itself which explains why brain surgeons can probe areas of the brain even when the patient is awake.
In myelinated nerve axons, impulses are transmitted at between 10 and 100 metres per second. In unmylinated nerve axons impulses are transmitted more slowly, at about 1 metre per second.
We do not know how memory is stored and recalled years later nor do we know why we dream.
Brain death is the irreversible end of all brain activity (including involuntary activity necessary to sustain life) due to total necrosis of the cerebral neurones following loss of blood flow and oxygenation.
A brain-dead person has no evidence of brain activity upon physical examination. This includes no response to pain and no cranial nerve reflexes e.g., no pupillary response. The diagnosis of brain death needs to be rigorous to determine whether the condition is irreversible. If tests show brain activity, the patient may be in a coma or vegetative state. It is important to distinguish between brain death and states that may resemble brain death – as many comatose patients can recover.