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Read this article to get study notes on Central and Peripheral Nervous System.
Central Nervous System:
A. Brain:
The brain constitutes about one-fiftieth of the body weight and lies within the cranial cavity.
The parts of the brain are:
1. Cerebrum
2. Midbrain
3. Pons
4. Medulla and brain stem
5. Cerebellum.
Blood Supply to the Brain:
The circulus arteriosus and its contributing arteries play a vital role in maintaining a constant supply of oxygen and glucose to the brain even when a contributing artery is narrowed or the head is moved. The brain receives about 15% of the cardiac output, approximately 750 ml of blood per minute.
Auto regulation keeps blood flow to the brain constant by adjusting the diameter of the arterioles across a wide range of arterial blood pressure (about 65-140 mmHg) with changes occurring only outside these limits.
I. Cerebrum:
This is the largest part of the brain and it occupies the anterior and middle cranial fossae. It is divided by a deep cleft, the longitudinal cerebral fissure, into right and left cerebral hemispheres, each containing one of the lateral ventricles. Deep within the brain the hemispheres are connected by a mass of white matter (nerve fibres) called the corpus callosum.
The falx cerebri is formed by the dura mater. It separates the two hemispheres and penetrates to the depth of the corpus callosum. The superficial (peripheral) part of the cerebrum is composed of nerve cell bodies or grey matter, forming the cerebral cortex, and the deeper layers consist of nerve fibres or white matter.
The cerebral cortex shows many infoldings or furrows of varying depth. The exposed areas of the folds are the gyri or convolutions and these are separated by sulci or fissures. These convolutions greatly increase the surface area of the cerebrum.
For descriptive purposes each hemisphere of the cerebrum is divided into lobes which take the names of the bones of the cranium under which they lie:
1. Frontal
2. Parietal
3. Temporal
4. Occipital.
The boundaries of the lobes are marked by deep sulci (fissures). These are the central, lateral and parietooccipital sulci.
Interior of the Cerebrum:
The surface of the cerebral cortex is composed of grey matter (nerve cell bodies). Within the cerebrum the lobes are connected by masses of nerve fibres, or tracts, which make up the white matter of the brain.
The afferent and efferent fibres linking the different parts of the brain and spinal cord are as follows:
1. Association (arcuate) fibres:
These connect different parts of a cerebral hemisphere by extending from one gyrus to another, some of which are adjacent and some distant.
2. Commissural fibres:
They connect corresponding areas of the two cerebral hemispheres; the largest and most important commissure is the corpus callosum.
3. Projection fibres:
They connect the cerebral cortex with grey matter of lower parts of the brain and with the spinal cord, e.g. the internal capsule. The internal capsule is an important area consisting of projection fibres. It lies deep within the brain between the basal nuclei (ganglia) and the thalamus.
Many nerve impulses passing to and from the cerebral cortex are carried by fibres that form the internal capsule. Motor fibres within the internal capsule form the pyramidal tracts (corticospinal tracts) that cross over (decussate) at the medulla oblongata.
Functions of the Cerebrum:
There are three main varieties of activity associated with the cerebral cortex:
1. Mental activities involved in memory, intelligence, sense of responsibility, thinking, reasoning, moral sense and learning are attributed to the higher centres.
2. Sensory perception, including the perception of pain, temperature, touch, sight, hearing, taste and smell.
3. Initiation and control of skeletal (voluntary) muscle contraction.
Functional Areas of the Cerebrum:
The main areas of the cerebrum associated with sensory perception and voluntary motor activity are known but it is unlikely that any area is associated exclusively with only one function. Except where specially mentioned, the different areas are active in both hemispheres.
Motor Areas of the Cerebrum:
The pre-central (motor) area:
This lies in the frontal lobe immediately anterior to the central sulcus. The cell bodies are pyramid-shaped (Betz cells) and they initiate the contraction of skeletal muscles. A nerve fibre from a Betz cell passes downwards through the internal capsule to the medulla oblongata where it crosses to the opposite side then descends in the spinal cord.
At the appropriate level in the spinal cord the nerve impulse crosses a synapse to stimulate a second neuron which terminates at the motor end-plate of a muscle fibre. This means that the motor area of the right hemisphere of the cerebrum controls voluntary muscle movement on the left side of the body and vice versa.
The neuron with its cell body in the cerebrum is the upper motor neuron and the other, with its cell body in the spinal cord, is the lower motor neuron. Damage to either of these neurons may result in paralysis.
In the motor area of the cerebrum the body is represented upside down, i.e. the cells nearest the vertex control the feet and those in the lowest part control the head, neck, face and fingers. The sizes of the areas of cortex representing different parts of the body are proportional to the complexity of movement of the body part, not to its size.
The premotor area:
This lies in the frontal lobe immediately anterior to the motor area. The cells are thought to exert a controlling influence over the motor area, ensuring an orderly series of movements. For example, in tying a shoelace or writing, many muscles contract but the movements must be coordinated and carried out in a particular sequence. Such a pattern of movement, when established, is described as manual dexterity.
In the lower part of this area just above the lateral sulcus there is a group of nerve cells known as the motor speech (Broca’s) area which controls the movements necessary for speech. It is dominant in the left hemisphere in right-handed people and vice versa.
The frontal area:
This extends anteriorly from the premotor area to include the remainder of the frontal lobe. It is a large area and is more highly developed in humans than in other animals.
It is thought that communications between this and the other regions in the cerebrum are responsible for the behaviour, character and emotional state of the individual. No particular behaviour, character or intellectual trait has so far been attributed to the activity of any one group of cells.
Sensory Areas of the Cerebrum:
1. The post-central (sensory) area:
This is the area behind the central sulcus. Here sensations of pain, temperature, pressure and touch, knowledge of muscular movement and the position of joints are perceived. The sensory area of the right hemisphere receives impulses from the left side of the body and vice versa.
The size of the areas representing different parts of the body is proportional to the extent of sensory innervation, e.g. the large area for the face is consistent with the extensive sensory nerve supply by the three branches of the trigeminal nerves (5th cranial nerves).
2. The parietal area:
This lies behind the post-central area and includes the greater part of the parietal lobe of the cerebrum. Its functions are believed to be associated with obtaining and retaining accurate knowledge of objects. It has been suggested that objects can be recognized by touch alone because of the knowledge from past experience (memory) retained in this area.
3. The sensory speech area:
This is situated in the lower part of the parietal lobe and extends into the temporal lobe. It is heard that the spoken word is perceived. There is a dominant area in the left hemisphere in right-handed people and vice versa.
4. The auditory (hearing) area:
This lies immediately below the lateral sulcus within the temporal lobe. The cells receive and interpret impulses transmitted from the inner ear by the cochlear (auditory) part of the vestibulocochlear nerves (8th cranial nerves).
5. The olfactory (smell) area:
This lies deep within the temporal lobe where impulses from the nose via the olfactory nerves (1st cranial nerves) are received and interpreted.
6. The taste area:
This is thought to lie just above the lateral sulcus in the deep layers of the sensory area. This is the area where impulses from special nerve endings in taste buds in the tongue and in the lining of the cheeks, palate and pharynx are perceived as taste.
7. The visual area:
This lies behind the parietooccipital sulcus and includes the greater part of the occipital lobe. The optic nerves (2nd cranial nerves) pass from the eye to this area which receives and interprets the impulses as visual impressions.
8. Other Areas of the Cerebrum:
Deep within the cerebral hemispheres there are groups of cell bodies called nuclei (previously called ganglia) which act as relay stations where impulses are passed from one neuron to the next in a chain.
Important masses of grey matter include:
1. Basal nuclei
2. Thalamus
3. Hypothalamus.
1. Basal nuclei:
These are areas of grey matter, lying deep within the cerebral hemispheres, with connections to the cerebral cortex and thalamus. The basal nuclei form part of the extrapyramidal tracts and are thought to be involved in initiating muscle tone in slow and coordinated activities. If control is inadequate or absent, movements are jerky, clumsy and uncoordinated.
2. Thalamus:
The thalamus consists of two masses of nerve cells and fibres situated within the cerebral hemispheres just below the corpus callosum, one on each side of the third ventricle. Sensory input from the skin, viscera and special sense organs is transmitted to the thalamus before redistribution to the cerebrum.
3. Hypothalamus:
The hypothalamus is composed of a number of groups of nerve cells. It is situated below and in front of the thalamus, immediately above the pituitary gland. The hypothalamus is linked to the posterior lobe of the pituitary gland by nerve fibres and to the anterior lobe by a complex system of blood vessels. Through these connections, the hypothalamus controls the output of hormones from both lobes of the gland.
Other functions with which the hypothalamus is concerned include control of:
1. The autonomic nervous system
2. Appetite and satiety
3. Thirst and water balance
4. Body temperature
5. Emotional reactions, e.g. pleasure, fear, rage
6. Sexual behaviour including mating and child rearing
7. Biological clocks or circadian rhythms, e.g. sleeping and waking cycles, body temperature and secretion of some hormones.
II. Midbrain:
The midbrain is the area of the brain situated around the cerebral aqueduct between the cerebrum above and the pons below. It consists of groups of cell bodies and nerve fibres (tracts) which connect the cerebrum with lower parts of the brain and with the spinal cord. The cell bodies act as relay stations for the ascending and descending nerve fibres.
III. Pons:
The pons is situated in front of the cerebellum, below the midbrain and above the medulla oblongata. It mainly consists of nerve fibres which form a bridge between two hemispheres of the cerebellum, and of fibres passing between higher levels of the brain and spinal cord.
There are groups of cells within the pons which act as relay stations and some of these are associated with the cranial nerves. The anatomical structure of the pons differs from that of the cerebrum in that the cell bodies (grey matter) lie deeply and the nerve fibres are on the surface.
IV. Medulla:
The medulla extends from the pons above and is continuous with the spinal cord below. It is about 2.5 cm long and it just lies within the cranium above the foramen magnum. Its anterior and posterior surfaces are marked by central fissures.
The outer aspect is composed of white matter which passes between the brain and the spinal cord, and grey matter lies centrally. Some cells constitute relay stations for sensory nerves passing from the spinal cord to the cerebrum.
The medulla has several special features:
(i) Decussation (crossing) of the pyramids:
In the medulla motor nerves descending from the motor area in the cerebrum to the spinal cord in the pyramidal (corticospinal) tracts cross from one side to the other. This means that the left hemisphere of the cerebrum controls the right half of the body and vice versa. These tracts are the main pathway for impulses to skeletal (voluntary) muscles.
(ii) Sensory decussation:
Some of the sensory nerves ascending to the cerebrum from the spinal cord cross from one side to the other in the medulla. Others decussate at lower levels, i.e. in the spinal cord.
The vital centres consisting of groups of cells associated with autonomic reflex activity, lie in its deeper structure.
There are the:
1. Cardiovascular centre
2. Respiratory centre
3. Vasomotor centre
4. Reflex centres of vomiting, coughing, sneezing and swallowing.
1. The cardiovascular centre:
The cardiovascular centre controls the rate and force of cardiac contraction. Sympathetic and parasympathetic nerve fibres originating in the medulla pass to the heart. Sympathetic stimulation increases the rate and force of the heartbeat and parasympathetic stimulation has the opposite effect.
2. The respiratory centre:
The respiratory centre controls the rate and depth of respiration. From this centre, nerve impulses pass to the phrenic and intercostal nerves which stimulate contraction of the diaphragm and intercostal muscles, thus initiating inspiration. The respiratory centre is stimulated by excess carbon dioxide and to a lesser extent by deficiency of oxygen in its blood supply and by nerve impulses from the chemoreceptors in the carotid bodies.
3. The vasomotor centre:
The vasomotor centre controls the diameter of the blood vessels, especially the small arteries and arterioles which have a large proportion of smooth muscle fibres in their walls. Vasomotor impulses reach the blood vessels through the autonomic nervous system. Stimulation may cause either constriction or dilatation of blood vessels depending on the site.
The sources of stimulation of the vasomotor centre are the arterial baroreceptors, body temperature and emotions such as sexual excitement and anger. Pain usually causes vasoconstriction although severe pain may cause vasodilatation, a fall in blood pressure and fainting.
4. Reflex centres:
Reflex centres when irritating substances are present in the stomach or respiratory tract, nerve impulses pass to the medulla oblongata, stimulating the reflex centres which initiate the reflex actions of vomiting, coughing and sneezing to expel the irritant.
5. Reticular Formation:
The reticular formation is a collection of neurons in the core of the brain stem, surrounded by neural pathways which conduct ascending and descending nerve impulses between the brain and the spinal cord. It has a vast number of synaptic links with other parts of the brain, and is therefore, constantly receiving ‘information’ being transmitted in ascending and descending tracts.
Functions:
The reticular formation is involved in:
1. Coordination of skeletal muscle activity associated with voluntary motor movement and the maintenance of balance.
2. Coordination of activity controlled by the autonomic nervous system, e.g. cardiovascular, respiratory and gastrointestinal activities.
3. Selective awareness that functions through the reticular activating stem (RAS) which selectively blocks or passes sensory information to the cerebral cortex, e.g. the slight sound made by a sick child moving in bed may arise his mother but the noise of regularly passing trains may be suppressed.
V. Cerebellum:
The cerebellum is situated behind the pons and immediately below the posterior portion of the cerebrum occupying the posterior cranial fossa.
Functions:
The cerebellum is concerned with the coordination of voluntary muscular movement, posture and balance. Its activities are not under voluntary control. The cerebellum controls and coordinates the movements of muscles ensuring smooth, and precise actions. It coordinates activities associated with the maintenance of the balance and equilibrium of the body.
The sensory input for these functions is derived from the muscles and joints, the eyes and the ears. Proprioceptor impulses from the muscles and joints indicate their position in relation to the body as a whole and those impulses from the eyes and the semicircular canals in the ears provide information about the position of the head in space.
Impulses from the cerebellum influence the contraction of skeletal muscle so that balance and posture are maintained. Damage to the cerebellum results in clumsy uncoordinated muscular movement, staggering gait and inability to carry out smooth, steady, precise movements.
Membranes Covering the Brain and Spinal Cord (the Meninges):
The brain and spinal cord are completely surrounded by three membranes, the meninge, lying between the skull and brain and between the vertebrae and spinal cord.
Named from outside inwards they are:
1. Dura mater
2. Arachnoid mater
3. Pia mater
The dura and arachnoid maters are separated by a potential space, the subdural space. The arachnoid and pia maters are separated by the subarachnoid space, containing cerebrospinal fluid.
1. Dura Mater:
The cerebral dura mater consists of two layers of dense fibrous tissue. The outer layer takes the place of the periosteum on the inner surface of the skull bones and the inner layer provides a protective covering for the brain.
There is only a potential space between the two layers except where the inner layer sweeps inwards between the cerebral hemispheres to form the falx cerebri; between the cerebellar hemispheres to form the false cerebelli and between the cerebrum and cerebellum to form the tentorium cerebelli.
Venous blood from the brain drains into venous sinuses between the layers of dura mater. The superior sagittal sinus is formed by the falx cerebri, and the tentorium cerebelli forms the straight and transverse sinuses.
Spinal dura mater forms a loose sheath round the spinal cord, extending from the foramen magnum to the second sacral vertebra. Thereafter it encloses the filum terminale and fuses with the periosteum of the coccyx.
It is an extension of the inner layer of cerebral dura mater and is separated from the periosteum of the vertebrae and ligaments within the neural canal by the epidural or extradural space, containing blood vessels and areolar tissue.
It is attached to the foramen magnum and, by a number of fibrous slips, to the posterior longitudinal ligament at intervals along its length. Nerves entering and leaving the spinal cord pass through the epidural space. These attachments stabilize the spinal cord in the neural canal. Dyes, used for diagnostic purposes, and local anaesthetics or analgesics to relieve pain, may be injected into the epidural space.
2. Arachnoid Mater:
This delicate serous membrane lies between the dura and pia maters. It is separated from the dura mater by the subdural space, and from the pia mater by the subarachnoid space, containing cerebrospinal fluid.
The arachnoid mater passes over the convolutions of the brain and accompanies the inner layer of dura mater in the formation of the falx cerebri, tentorium cerebelli and falx cerebelli. It continues downwards to envelop the spinal cord and ends by merging with the dura mater at the level of the 2nd sacral vertebra.
3. Pia Mater:
This is a fine connective tissue containing many minute blood vessels. It adheres to the brain, completely covering the convolutions and dipping into each fissure. It continues downwards surrounding the spinal cord. Beyond the end of the cord it continues as the filum terminale, pierces the arachnoid tube and goes on with the dura mater to fuse with the periosteum of the coccyx.
Ventricles of Brain:
Within the brain there are four irregular-shaped cavities or ventricles, containing cerebrospinal fluid (CSF).
They are:
1. Right and left lateral ventricles
2. Third ventricle
3. Fourth ventricle.
1. Lateral Ventricles:
These cavities lie within the cerebral hemispheres, one on each side of the median plane just below the corpus callosum. They are separated from each other by a thin membrane, the septum pellucidum, and are lined with ciliated epithelium. They communicate with the third ventricle by inter-ventricular foramina.
2. Third Ventricle:
The third ventricle is a cavity situated below the lateral ventricles between the two parts of the thalamus. It communicates with the fourth ventricle by a canal, the cerebral aqueduct or aqueduct of the midbrain.
3. Fourth Ventricle:
The fourth ventricle is a diamond-shaped cavity situated below and behind the third ventricle, between the cerebellum and pons. It is continuous below with the central canal of the spinal cord and communicates with the subarachnoid space by foramina in its roof. Cerebrospinal fluid enters the subarachnoid space through these openings and through the open distal end of the central canal of the spinal cord.
Cerebrospinal Fluid (CSF):
Cerebrospinal fluid (CSF) is secreted into each ventricle of the brain by choroid plexuses. These are vascular areas where there is a proliferation of blood vessels surrounded by ependymal cells in the lining of ventricle walls. CSF passes back into the blood through tiny diverticula at arachnoid mater, called arachnoid villi (arachnoid granulations), that project into the venous sinuses.
The movement of CSF from the subarachnoid space to venous sinuses depends upon the difference in pressure on each side of the walls of the arachnoid villi, which act as one-way valves.
When CSF pressure is higher than venous pressure, CSF passes into the blood and when the venous pressure is higher, the arachnoid villi collapse, preventing the passage of blood constituents into the CSF. There may be also some reabsorption of CSF by cells in the walls of the ventricles.
From the roof of the fourth ventricle CSF flows through foramina into the subarachnoid space and completely surrounds the brain and spinal cord. There is no intrinsic system of CSF circulation but its movement is aided by pulsating blood vessels, respiration and changes of posture.
CSF is secreted continuously at a rate of about 0.5 ml per minute, i.e. 720 ml per day. The amount around the brain and spinal cord remains fairly constant at about 120 ml, which means that absorption keeps pace with secretion. CSF pressure may be measured using a vertical tube attached to a lumbar puncture needle. It remains fairly constant at about 10 cm H2O when sitting up.
If the brain is enlarged by haemorrhage or tumour, some compensation is made by a reduction in the amount of CSF. When the volume of brain tissue is reduced such as in degeneration or atrophy, the volume of CSF is increased.
CSF is a clear, slightly alkaline fluid with a specific gravity of 1.005 consisting of:
1. Water
2. Mineral salts
3. Glucose
4. Plasma proteins: Small amounts of albumin and globulin
5. Creatinine
6. Urea in small amounts
7. A few leukocytes.
Functions of Cerebrospinal Fluid:
1. It supports and protects the brain and spinal cord.
2. It maintains a uniform pressure around these delicate structures.
3. It acts as a cushion and shock absorber between the brain and cranial bones.
4. It keeps the brain and spinal cord moist and there may be interchange of substances between CSF and nerve cells, such as nutrients and waste products.
B. Spinal Cord:
The spinal cord is the elongated, almost cylindrical part of the central nervous system, which is suspended in the vertebral canal surrounded by the meninges and cerebrospinal fluid. It is continuous above with the medulla oblongata and extends from the upper border of the atlas to the lower border of the 1st lumbar vertebra.
It is approximately 45 cm long in an adult Caucasian male, and is about the thickness of the little finger. When a specimen of cerebrospinal fluid is required it is taken from a point below the end of the cord, i.e. below the level of the 2nd lumbar vertebra. This procedure is called lumbar puncture.
Except for the cranial nerves, the spinal cord is the nervous tissue between the brain and the rest of the body. Nerves conveying impulses from the brain to the various organs and tissues descend through the spinal cord. At the appropriate level they leave the cord and pass to the structure they supply. Similarly, sensory nerves from organs and tissues enter and pass upwards in the spinal cord to the brain.
Some activities of the spinal cord are independent of the brain, i.e. spinal reflexes. To facilitate these there are extensive neuron connections between sensory and motor neurons at the same or different levels in the cord.
The spinal cord is divided into two equal parts, anteriorly by a short, shallow median fissure and posteriorly by a deep narrow septum, the posterior median septum. A cross-section of the spinal cord shows that it is composed of grey matter in the centre surrounded by white matter supported by neuroglia.
Grey Matter:
The arrangement of grey matter in the spinal cord resembles the shape of the letter H, having two posterior, two anterior and two lateral columns. The area of grey matter lying transversely is the transverse commissure and it is pierced by the central canal, an extension from the fourth ventricle, containing cerebrospinal fluid.
The cell bodies may be:
1. Sensory cells which receive impulses from the periphery of the body.
2. Lower motor neurons which transmit impulses to the skeletal muscles.
3. Connector neurons linking sensory and motor neurons, at the same or different levels which form spinal reflex arcs.
At each point where nerve impulses are passed from one neuron to another there is a synaptic cleft and a neurotransmitter.
Posterior Columns of Grey Matter:
These are composed of cell bodies which are stimulated by sensory impulses from the periphery of the body. The nerve fibres of these cells contribute to the formation of the white matter of the cord and transmit the sensory impulses upwards to the brain.
Anterior Columns of Grey Matter:
These are composed of the cell bodies of the lower motor neurons which are stimulated by the axons of the upper motor neurons or by the cell bodies of connector neurons linking the anterior and posterior columns to form reflex arcs.
The posterior root (spinal) ganglia are composed of cell bodies which lie just outside the spinal cord on the pathway of the sensory nerves. All sensory nerve fibres pass through these ganglia. The only function of the cells is to promote the onward movement of nerve impulses.
White Matter:
The white matter of the spinal cord is arranged in three columns or tracts; anterior, posterior and lateral. These tracts are formed by sensory nerve fibres ascending to the brain, motor nerve fibres descending from the brain and fibres of connector neurons. Tracts are often named according to their points of origin and destination, e.g. spinothalamic, corticospinal.
Sensory Nerve Tracts (Afferent or Ascending) in the Spinal Cord:
There are two main sources of sensation transmitted to the brain via the spinal cord:
1. The skin:
Sensory receptors (nerve endings) in the skin, called cutaneous receptors, are stimulated by pain, heat, cold and t ouch, including pressure. Nerve impulses generated are conducted by three neurons to the sensory area in the opposite hemisphere of the cerebrum where the sensation and its location are perceived. Crossing to the other side or decussation occurs either at the level of entry into the cord or in the medulla.
2. The tendons, muscles and joints:
Sensory receptors are nerve endings in these structures, called proprioceptors, and they are stimulated by stretch. Together with impulses from the eyes and the ears they are associated with the maintenance of balance and posture and with perception of the position of the body in space.
These nerve impulses have two destinations:
1. By a three-neuron system the impulses reach the sensory area of the opposite hemisphere of the cerebrum.
2. By a two-neuron system the nerve impulses reach the cerebellar hemisphere on the same side.
The following table provides further information about the origins, routes of transmission and the destinations of sensory nerve impulses.
Motor Nerve Tracts (Efferent or Descending) in the Spinal Cord:
Neurons which transmit nerve impulses away from the brain are motor (efferent or descending) neurons.
Motor neuron stimulation results in:
1. Contraction of skeletal (striated, voluntary) muscle.
2. Contraction of smooth (involuntary) muscle, cardiac muscle and the secretion glands controlled by nerves of the autonomic nervous system.
Voluntary Muscle Movement:
The contraction of the muscles which move the joints is in the main under conscious (voluntary) control, which means that the stimulus to contract originates at the level of consciousness in the cerebrum. However, some nerve impulses which affect skeletal muscle contraction are initiated in the midbrain, brain stem and cerebellum.
This involuntary activity is associated with coordination of muscle activity, e.g. when very fine movement is required and in the maintenance of posture and balance. Efferent nerve impulses are transmitted from the brain to the body via bundles of nerve fibres or tracts in the spinal cord. The motor pathways from the brain to the muscles are made up of two neurons. These tracts are either.
1. Pyramidal (corticospinal)
2. Extrapyramidal.
The motor fibres that form the pyramidal tracts travel through the internal capsule and are the main pathway for impulses to voluntary (skeletal) muscles. Those motor fibres that do not pass through the internal capsule form the extrapyramidal tracts and have connections with many parts of the brain including the basal nuclei and the thalamus.
The upper motor neuron:
This has its cell body (Betz cell) in the pre-central sulcus of the cerebrum. The axons pass through the internal capsule, pons and medulla. In the spinal cord they form the lateral corticospinal tracts of white matter and the fibres terminate in close association with the cell bodies of the lower motor neurons in the anterior columns of grey matter. The axons of these upper motor neurons made up the pyramidal tracts and decussate in the medulla oblongata, forming the pyramids.
The lower motor neuron:
This has its cell body in the anterior horn of grey matter in the spinal cord. Its axon emerges from the spinal cord by the anterior root, joins with the incoming sensory fibres and forms the mixed spinal nerve which passes through the intervertebral foramen.
Near its termination in muscle the axon branches into a variable number of tiny fibres which form motor end-plates, each of which is in close association with a sensitive area on the wall of a muscle fibre.
The motor end-plates of each nerve and the muscle fibres they supply form a motor unit. The neurotransmitter that conveys the nerve impulse across the synapse to stimulate the muscle fibre is acetylcholine. Motor units contract as a whole and the strength of contraction of a muscle depends on the number of motor units in action at a time.
The lower motor neuron has been described as the final common pathway for the transmission of nerve impulses to skeletal muscles. The cell body of this neuron is influenced by a number of upper motor neurons originating from various sites in the brain and by some neurons which begin and end in the spinal cord.
Some of these neurons stimulate the cell bodies of the lower motor neuron while others have an inhibiting effect. The outcome of these influences is smooth, coordinated muscle movement, some of which are voluntary and some involuntary.
Involuntary Muscle Movement:
Upper motor neurons:
These have their cell bodies in the brain at a level below the cerebrum, i.e. in the midbrain, brain stem, cerebellum or spinal cord. They influence muscle activity in relation to the maintenance of posture and balance, the coordination of muscle movement and the control of muscle tone. The following table gives the details of the area of origin of these neurons and the tracts which their axons form before reaching the cell of the lower motor neuron in the spinal cord.
Spinal reflexes:
These consist of three elements:
1. Sensory neurons
2. Connector neurons in the spinal cord
3. Lower motor neurons.
In the simplest reflex arc there is only one of each. A reflex action is an immediate motor response to a sensory stimulus. Many connector and motor neurons may be stimulated by afferent impulses from a small area of skin, e.g. the pain impulses initiated by touching a very hot surface with the finger are transmitted to the spinal cord by sensory nerves.
These stimulate many connector and lower motor neurons in the cord which results in the contraction of many skeletal muscles of the hand, arm and shoulder, and the removal of the finger. Reflex action takes place very quickly, in fact, the motor response may have occurred simultaneously with the perception of the pain in the cerebrum.
Reflexes of this type are invariably protective but they can on occasion be inhibited. For example, if it is a precious plate that is very hot when lifted every effort will be made to overcome the pain to prevent dropping it.
Stretch reflexes:
Only two neurons are involved. The cell body of the lower motor neuron is stimulated by the sensory neuron. There is no connector neuron involved. The knee jerk is one example, but this type of reflex can be demonstrated at any point where a stretched tendon crosses a joint.
By tapping the tendon just below the knee when it is bent, the sensory nerve endings in the tendon and in the thigh muscles are stretched. This initiates a nerve impulse which passes into the spinal cord to the cell body of the lower motor neuron in the anterior column of grey matter on the same side.
As a result, the thigh muscles suddenly contract and the foot kicks forward. This is used as a test of the integrity of the reflex arc. This type of reflex has a protective function—it prevents excessive joint movement that may damage tendons, ligaments and muscles.
Peripheral Nervous System:
This part of the nervous system consists of:
1. 31 pairs of spinal nerves
2. 12 pairs of cranial nerves
3. The autonomic part of the nervous system.
Most of the nerves of the peripheral nervous system are composed of sensory nerve fibres conveying afferent impulses from sensory end organs to the brain, and motor nerve fibres conveying efferent impulses from the brain through the spinal cord to the effector organs, e.g. skeletal muscles, smooth muscle and glands.
Each nerve consists of numerous nerve fibres collected into bundles. Each bundle has several coverings of protective connective tissue.
1. Endoneurium is a delicate tissue, surrounding each individual fibre, which is continuous with the septa that pass inwards from the perineurium.
2. Perineurium is a smooth connective tissue, surrounding each bundle of fibres.
3. Epineurium is the fibrous tissue which surrounds and encloses a number of bundles of nerve fibres. Most large nerves are covered by epineurium.
Spinal Nerves:
There are 31 pairs of spinal nerves that leave the vertebral canal by passing through the intervertebral foramina formed by adjacent vertebrae. They are named and grouped according to the vertebrae with which they are associated.
1. 8—cervical
2. 12—thoracic
3. 5—lumbar
4. 5—sacral
5. 1—coccygeal.
Although there are only seven cervical vertebrae, there are eight nerves because the first pair leaves the vertebral canal between the occipital bone and the atlas and the eight pairs leave below the last cervical vertebra. Thereafter the nerves are given the name and number of the vertebra immediately as mentioned above.
The lumbar, sacral and coccygeal nerves leave the spinal cord near its termination at the level of the first lumbar vertebra, and extend downwards inside the vertebral canal in the subarachnoid space, forming a sheaf of nerves which resembles a horse’s tail, the cauda equina. These nerves leave the vertebral canal at the appropriate lumbar, sacral or coccygeal level, depending on their destination.
The spinal nerves arise from both sides of the spinal cord and emerge through the intervertebral foramina. Each nerve is formed by the union of a motor and a sensory nerve root and is, therefore, a mixed nerve. Each spinal nerve has a contribution from the sympathetic part of the autonomic nervous system in the form of a preganglionic fibre.
Nerve Roots:
The anterior nerve root consists of motor nerve fibres which are the axons of the nerve cells in the anterior column of grey matter in the spinal cord and, in the thoracic and lumbar regions, sympathetic nerve fibres which are the axons of cells in the lateral columns of grey matter.
The posterior nerve root consists of sensory nerve fibres. Just outside the spinal cord there is a spinal ganglion (posterior root ganglion), consisting of a little cluster of cell bodies. Sensory nerve fibres pass through these ganglia before entering the spinal cord. The area of skin supplied by each nerve is called a dermatome.
For a very short distance after leaving the spinal cord, the nerve roots have a covering of dura and arachnoid maters. These terminate before the two roots join to form the mixed spinal nerve. The nerve roots have no covering of pia mater.
Immediately after emerging from the intervertebral foramen each spinal nerve divides into a ramus communicans, a posterior ramus and an anterior ramus.
The rami communicans are part of preganglionic sympathetic neurons of the autonomic nervous system.
The posterior rami pass backwards and divide into medial and lateral branches to supply skin and muscles or relatively small areas of the posterior aspect of the head, neck and trunk.
The anterior rami supply the anterior and lateral aspects of the neck, trunk and the upper and lower limbs.
In the cervical, lumbar and sacral regions the anterior rami unite near their origins to form large masses of nerves or plexuses, where nerve fibres are regrouped and rearranged before proceeding to supply skin, bones, muscles and joints of a particular area.
This means that these structures have a nerve supply from more than one spinal nerve, and therefore, damage to one spinal nerve does not cause loss of function of a region.
In the thoracic region the anterior rami do not form plexuses.
There are five large plexuses of mixed nerves formed on each side of the vertebral column.
They are the:
1. Cervical plexuses
2. Brachial plexuses
3. Lumbar plexuses
4. Sacral plexuses
5. Coccygeal plexuses.
1. Cervical Plexus:
This is formed by the anterior rami of the first four cervical nerves. It lies opposite the 1st, 2nd, 3rd and 4th cervical vertebrae under the protection of the sternocleidomastoid muscle.
The superficial branches supply the structures at the back and side of the head and the skin of the front neck to the level of the sternum.
The deep branches supply muscles of the neck, e.g. the sternocleidomastoid and the trapezius.
The phrenic nerve originates from cervical roots 3, 4 and 5 and passes downwards through the thoracic cavity in front of the root of the lung to supply the muscle of the diaphragm with impulses which stimulate contraction.
2. Brachial Plexus:
The anterior rami of the lower four cervical nerves and a large part of the first thoracic nerve form the brachial plexus. The plexus is situated in the neck and shoulder above and behind the subclavian vessels and in the axilla.
The branches of the brachial plexus supply the skin and muscles of the upper limbs and some of the chest muscles. Five large nerves and a number of smaller ones emerge from this plexus, each with a contribution from more than one nerve root containing sensory, motor and autonomic fibres.
1. Axillary (circumflex)
2. Radial nerve
3. Musculocutaneous nerve
4. Median nerve
5. Ulnar nerve
6. Medial cutaneous nerve.
The axillary (circumflex) nerve winds round the humerus at the level of the surgical neck. It then breaks up into minute branches to supply the deltoid muscle, shoulder joint and overlying skin. The radial nerve is the largest branch of the brachial plexus.
It supplies the triceps muscle behind the humerus, crosses in front of the elbow joint then winds round to the back of the forearm to supply extensors of the wrist and finger joints. It continues into the back of the hand to supply the skin of the thumb, the first two fingers and the lateral half of the third finger.
The musculocutaneous nerve passes downwards to the lateral aspect of the forearm. It supplies the muscles of the upper arm and the skin of the forearm. The median nerve passes down the midline of the arm in close association with the brachial artery.
It passes in front of the elbow joint then down to supply the muscles of the front of the forearm. It continues into the hand where it supplies small muscles and the skin of the front of the thumb, the first two fingers and the lateral half of the third finger. It gives off no branches above the elbow.
The ulnar nerve descends through the upper arm lying medial to the brachial artery. It passes behind the medial epicondyle of the humerus to supply the muscles on the ulnar aspect of the forearm. It continues downwards to supply the muscles in the palm of the hand and the whole skin of the little finger and the medial half of the third finger. It gives off no branches above the elbow.
3. Lumbar Plexus:
The lumbar plexus is formed by the anterior rami of the first three and part of the fourth lumbar nerves. The plexus is situated in front of the transverse processes of the lumbar vertebrae and behind the psoas muscle.
The main branches, and their nerve roots are:
1. Iliohypogastric nerve
2. Ilioinguinal nerve
3. Genitofemoral nerve
4. Lateral cutaneous nerve of thigh
5. Femoral nerve
6. Obturator nerve
7. Lumbosacral trunk.
The iliohypogastric, ilioinguinal and genitofemoral nerves supply muscles and the skin in the area of the lower abdomen, upper and medial aspects of the thigh and the inguinal region. The lateral cutaneous nerve of the thigh supplies the skin of the lateral aspect of the thigh including part of the anterior and posterior surfaces.
The femoral nerve is one of the largest branches. It passes behind the inguinal ligament to enter the thigh in close association with the femoral artery. It divides into cutaneous and muscular branches to supply the skin and the muscles of the front of the thigh. One branch, the saphenous nerve, supplies the medial aspect of the leg, ankle and foot.
The obturator nerve supplies the adductor muscles of the thigh and skin of the medial aspect of the thigh. It ends just above the level of the knee joint. The lumbosacral trunk descends into the pelvis and makes a contribution to the sacral plexus.
4. Sacral Plexus:
The sacral plexus is formed by the anterior rami of the lumbosacral trunk and the first, second and third sacral nerves. The lumbosacral trunk is formed by the fifth and part of the fourth lumbar nerves. It lies in the posterior wall of the pelvic cavity.
The sacral plexus divides into a number of branches supplying the muscles and skin of the pelvic organs. In addition to these it provides the sciatic nerve which contains fibres from L4, 5; and S1, 2, 3. The sciatic nerve is the largest nerve in the body. It is about 2 cm wide at its origin.
It passes through the greater sciatic foramen into the buttock then descends through the posterior aspect of the thigh supplying the hamstring muscles. At the level of the middle of the femur it divides to form the tibial and the common peroneal nerves.
The tibial nerve descends through the popliteal fossa to the posterior aspect of the leg where it supplies muscles and skin. It passes under the medial malleolus to supply muscles and skin of the sole of the foot and toes. One of the main branches is the sural nerve which supplies the tissues in the area of the heel, the lateral aspect of the ankle and a part of the dorsum of the foot.
The common peroneal nerve descends obliquely along the lateral aspect of the popliteal fossa, winds round the neck of the fibula into the front of the leg where it divides into the deep peroneal (anterior tibial) and the superficial peroneal (musculocutaneous) nerves. These nerves supply the skin and muscles of the anterior aspect of the leg and the dorsum of the foot and toes.
The pudendal nerve:
The perineal branch supplies the external anal sphincter, the external urethral sphincter and adjacent skin.
5. Coccygeal Plexus:
The coccygeal plexus is a very small plexus formed by part of the fourth and fifth sacral and the coccygeal nerves. The nerves from this plexus supply the skin in the area of the coccyx and the levators ani and coccygeus muscles of the pelvic floor and the external anal sphincter.
The thoracic nerves do not intermingle to some plexuses, there are 12 pairs and the first 11 are the intercostal nerves, they pass between the ribs supplying them the intercostal muscles and overlying skin. The 12th pair are the subcostal nerves, the 7th to the 12th thoracic nerves also supply the muscles and the skin of the posterior and anterior abdominal walls.
Thoracic Nerves:
There are 12 cranial nerves originating from nuclei in the inferior surface of the brain, some are sensory, some motor and some mixed.
Their names and numbers are:
I. Olfactory—sensory
II. Optic—sensory
III. Oculomotor—motor
IV. Trochlear—motor
V. Trigeminal—mixed
VI. Abducent—motor
VII. Facial—mixed
VIII. Vestibulocochlear (auditory)—sensory 1
IX. Glossopharyngeal—mixed
X. Vagus—mixed
XI. Accessory—motor
XII. Hypoglossal—motor.
A summary of the cranial nerves is as follows:
Autonomic Nervous System:
The autonomic or involuntary part of the nervous system controls the function of the body carried out ‘automatically’, i.e. initiated in the brain below the level of the cerebrum. Although stimulation does not occur voluntarily the individual may be conscious of its effects, e.g. an increase in the heart rate. The effects of autonomic control are rapid and essential for homeostasis.
The effector organs are:
1. Smooth muscle
2. Cardiac muscle
3. Glands.
Effects of autonomic stimulation include:
1. Changes in rate and force of the heartbeat
2. Stimulation or depression of secretion of glands
3. Vasoconstriction or vasodilatation
4. Bronchoconstriction or bronchodilator
5. Changes in size of the pupils or the eyes.
The efferent (motor) nerves of the autonomic nervous system arise from nerve cells in the brain and emerge at various levels between the midbrain and the sacral region of the spinal cord. Many of them travel within the same nerve sheath as the peripheral nerves of the central nervous system to reach the organs which they innervate.
The autonomic nervous system is divided into two divisions:
1. Sympathetic (thoracolumbar outflow)
2. Parasympathetic (craniosacral outflow).
The two divisions have both structural and functional differences. They normally work in an opposing manner, enabling or restoring balance of involuntary functions, maintaining homeostasis. Sympathetic activity tends to predominate in stressful situations and parasympathetic activity during rest. Each division has two efferent neurons in its peripheral pathways between the central nervous system and effector organs.
These are:
1. The preganglionic neuron
2. The postganglionic neuron.
The cell body of the preganglionic neuron is in the brain or spinal cord. Its axon terminals synapse with the cell body of the postganglionic neuron in an autonomic ganglion outside the central nervous system. The postganglionic neuron conducts impulses to the effector organ.
1. Sympathetic Nervous System:
Neurons convey impulses from their origin in the hypothalamus reticular formation and medulla oblongata to effector organs and tissues. The first neuron has its cell body in the brain and its fibre extends into the spinal cord.
Preganglionic neuron:
This has its cell body in the lateral column of grey matter in the spinal cord between the levels of the 1st thoracic and 2nd or 3rd lumbar vertebrae. The nerve fibre of this cell leaves the cord by the anterior root and terminates in one of the ganglia either in the lateral chain of sympathetic ganglia or passes through it to one of the pre-vertebral ganglia. Acetylcholine is the neurotransmitter.
Postganglionic neuron:
This has its cell body in a ganglion and terminates in the organ or tissue supplied. Noradrenaline is usually the neurotransmitter.
Sympathetic Ganglia:
Lateral chains of sympathetic ganglia:
These are chains of ganglia which extend from the upper cervical level to the sacrum, one chain lying on each side of the bodies of the vertebrae. The ganglia are attached to each other by nerve fibres, preganglionic neurons that emerge from the cord may synapse with the cell body of the postganglionic neuron at the same level or they may pass up or down the chain through one or more ganglia before synapsing.
For example, the nerve which dilates the pupil of the eye leaves the cord at the level of the 1st thoracic vertebra and passes up the chain to the superior cervical ganglion before it synapses with the cell body of the postsynaptic neuron. The postganglionic neurons then pass to the eyes.
The major exception is that there is no parasympathetic supply to the sweat glands, skin and blood vessels of skeletal muscles. These structures are supplied by only sympathetic fibres, some of which have acetylcholine and some are adrenaline and noradrenaline as their neurotransmitter. They have, therefore, the effects of both sympathetic and parasympathetic nerve supplies.
Pre-vertebral ganglia:
There are three pre-vertebral ganglia situated in the abdominal cavity close to the origins of arteries of the same names:
1. Coeliac ganglion
2. Superior mesenteric ganglion
3. Inferiomesenteric ganglion.
The ganglia consist of nerve cell bodies rather diffusely distributed among a network of nerve fibres which form plexuses. Preganglionic sympathetic fibres pass through the lateral chain to reach these ganglia.
2. Parasympathetic Nervous System:
Two neurons (preganglionic and postganglionic) are involved in the transmission of impulses from their source to the effector organ. The neurotransmitter at both synapses is acetylcholine.
Preganglionic neuron:
This has its cell body either in the brain or spinal cord. Those originating in the brain are cranial nerves III, VII, IX and X, arising from nuclei in the midbrain and brain stem, and their nerve fibres terminate outside the brain.
The cell bodies of the sacral outflow are in the lateral columns of grey matter at the distal end of the spinal cord. Their fibres leave the cord in sacral segments 2, 3 and 4 and synapse with postganglionic neurons in the walls of pelvic organs.
Postganglionic neuron:
This has its cell body either in a ganglion or in the wall of the organ supplied.
Functions of Autonomic Nervous System:
Autonomic nervous system is involved in a complex of reflex activities which, like the reflexes described previously, depend on sensory input to the brain or spinal cord, and on motor output. In this case the reflex action is rapid contraction or inhibition of contraction of involuntary (smooth and cardiac) muscle or glandular secretion.
These reflexes are coordinated subconsciously in the brain, i.e. below the level of the cerebrum. Some sensory inputs reach consciousness and may result in temporary inhibition of the reflex action, can be inhibited temporarily.
The majority of the organs of the body are supplied by both sympathetic and parasympathetic nerves which have opposite effects that are finely balanced to ensure the optimum functioning of the organ.
Sympathetic stimulation prepares the body to deal with exciting and stressful situations, e.g. strengthening its defenses in danger and in extremes of environmental temperature.
The adrenal glands are stimulated to secrete the hormones adrenaline and noradrenaline into the bloodstream. These hormones potentiate and sustain the effects of sympathetic stimulation. It is sometimes said that sympathetic stimulation mobilizes the body for “fight or flight”.
Parasympathetic stimulation has a tendency to slow down body processes except digestion and absorption of food and the functions of the genitourinary systems. Its general effects is that of a ‘peace maker’ allowing restoration processes to occur quietly and peacefully. Normally, the two systems function together by maintaining a regular heartbeat, normal temperature and an internal environment compatible with the immediate external surroundings.
Effects of Autonomic Stimulation:
1. Cardiovascular System:
Sympathetic Stimulation:
1. Exerts an accelerating effect upon the sinoatrial node in the heart, increasing the rate and force of the heartbeat.
2. Causes dilatation of the coronary arteries, increasing the blood supply to cardiac muscle.
3. Causes dilatation of the blood vessels supplying skeletal muscle, increasing the supply of oxygen and nutritional materials and the removal or metabolic waste- products, thus increasing the capacity of the muscle to work.
4. Raises peripheral resistance and blood pressure by constricting the small arteries and arterioles in the skin. In this way an increased blood supply is available for highly active tissue such as skeletal muscle, heart and brain.
5. Constricts the blood vessels in the secretory glands of the digestive system, reducing the flow of digestive juices. This raises the volume of blood available for circulation in dilated blood vessels.
6. Blood coagulation occurs more quickly because of vasoconstriction.
Parasympathetic Stimulation:
1. Increases the rate and force of the heartbeat.
2. Causes constriction of the coronary arteries reducing the blood supply to cardiac muscle.
The parasympathetic nervous system exerts little or no effect on blood vessels except the coronary arteries.
2. Respiratory System Sympathetic Stimulation:
This causes dilatation of the airways, especially the bronchioles, allowing a greater amount of air to enter the lungs at each inspiration and increases the respiratory rate. In conjunction with the increased heart rate, the oxygen intake and carbon dioxide output of the body are increased.
Parasympathetic Stimulation:
Produces constriction of the bronchi.
3. Digestive and Urinary Systems Sympathetic Stimulation:
1. The liver converts an increased amount of glycogen to glucose, making more carbohydrate immediately available to provide energy.
2. The adrenal (suprarenal) glands are stimulated to secrete adrenaline and noradrenaline which potentiate and sustain the effects of sympathetic stimulation.
3. Stomach and small intestine: Smooth muscle contraction and secretion of digestive juices are inhibited, delaying digestion, onward movement and absorption of food and the tone of sphincter muscles increases.
4. Urethral and anal sphincters: The muscle tone of the sphincters is increased, inhibiting micturition and defecation.
5. The bladder wall relaxes.
6. The metabolic rate is greatly increased.
Parasympathetic Stimulation:
1. Stomach and small intestine:
The rate of digestion and absorption of food is increased.
2. Pancreas:
There is an increase in the internal urethral sphincter accompanied by contraction of the muscle of the bladder wall and micturition occurs. Similar relaxation of the internal anal sphincter is accompanied by contraction of the muscle of the rectum and defecation occurs. In both cases there is voluntary relaxation of the external sphincters.
Eye:
Sympathetic stimulation:
This causes contraction of the radiating muscle fibres of the iris, dilating the pupil. Retraction of the levator palpebral muscles occurs, opening the eyes wide and giving the appearance of alertness and excitement. The ciliary muscle that adjusts the thickness of the lens is slightly relaxed.
Parasympathetic stimulation:
This causes contraction of the circular muscle fibres of the iris, constricting the pupil. The eyelids tend to close, giving the appearance of sleepiness.
Skin:
Sympathetic stimulation:
1. Causes increased secretion of sweat, leading to increased heat loss from the body.
2. Produces contraction of the arrectores pilorum (the muscles in the hair follicles of the skin), giving the appearance of goose flesh.
3. Causes constriction of the blood vessels preventing heat loss.
There is no parasympathetic nerve supply to the skin. Some sympathetic fibres are adrenergic, causing vasoconstriction; and some are cholinergic, causing vasodilatation.
Afferent Impulses from Viscera:
Sensory fibres from the viscera travel through autonomic fibres and are sometimes called autonomic afferents.
The impulses they transmit are associated with:
1. Visceral reflexes, usually at an unconscious level, e.g. cough, blood pressure.
2. Sensation of hunger, thirst, nausea, sexual sensation, rectal and bladder distension.
3. Visceral pain.
Visceral Pain:
Normally, the viscera are insensitive to cutting, burning and crushing.
However, a sensation of dull, poorly located pain is experienced when:
1. Visceral nerves are stretched.
2. A large number of fibres are stimulated.
3. There is ischaemia and local accumulation of metabolites.
4. The sensitivity of nerve endings to painful stimuli is increased, e.g. during inflammation.
If the cause of the pain, e.g. inflammation affects the parietal layer of a serous membrane (pleura, peritoneum) the pain is acute and easily located over the site of inflammation. This is because the peripheral spina (somatic) nerves supplying the superficial tissues also supply the parietal layer of serous membrane. They transmit the impulses to the cerebral cortex where somatic pain is perceived and accurately located.
Appendicitis is an example of this type of pain. Initially, it is dull and vaguely located around the midline of the abdomen. As the condition progresses the parietal peritoneum becomes involved and acute pain is clearly located in the right iliac fossa, i.e. over the appendix.
Referred Pain:
In some cases of visceral disease, pain may be perceived to occur in superficial tissues remote from the viscus, i.e. referred pain. This occurs when sensory fibres from the viscus enter the same segment of the spinal cord as somatic nerves, i.e. those from the superficial tissues.
It is believed that the sensory nerve from the viscus stimulates the closely associated nerve in the spinal cord and it transmits the impulses to the sensory area in the cerebral cortex where the pain is perceived as originating in the area supplied by the somatic nerve.
Effects of Poisons on the Central Nervous System:
Many chemical substances encountered either as drugs or in the environment may damage the nervous system. Neuron metabolism may be disturbed directly or result from damage to other organs, e.g. liver, kidneys. The outcome depends on the toxicity of the substance, the dose and the duration of exposure, ranging from short- term neurological disturbance to encephalopathy which may cause coma and death.