Neurons

Neurons (Figure 41.1) are the primary components of the nervous system. Functioning alone, or as units, neurons detect internal and external changes and initiate body responses needed to maintain homeostasis. Each neuron is composed of a cell body, with projections forming dendrites, and one long axon. The dendrites are short-branched fibres, which receive impulses and conduct them towards the cell body of a neuron. The axon, which may vary in length from miniscule to over a metre, conducts impulses away from the cell body of a neuron. Generally, a neuron has only one axon but many dendrites. Axons leave the grey matter and become the fibres of the white matter. Each axon has a covering called a neurilemma, and most have a fatty sheath, the myelin sheath. The myelin sheath protects and insulates the axon and increases the transmission rate of nervous impulses. Neurons are bound together by a special type of connective tissue called neuroglia.

Figure 41.1 A typical neuron

Neuroglia

The neuroglia includes many types of cells that support and protect the neurons. They play a role in regulating neuronal activity, and in providing neurons with nutrients. Neuroglia differ from neurons in that they are not capable of transmitting nerve impulses and never lose their ability to divide. The neuroglia are comprised mainly of two types of cells: astrocytes, cells with small cell bodies and processes like dendrites, which protect the neurons from harmful substances that may be in the blood by forming a living barrier between the capillary blood supply and the neurons; and oligodendrocytes, which have few processes and produce the myelin sheath around the processes of the neurons.

Functions of neurons

Neurons have two major functional properties: irritability and conductivity. Irritability is the ability to respond to a stimulus and convert it into a nerve impulse. Conductivity is the ability to transmit the impulse to other neurons, muscles, or glands. An impulse is a complex electrical and chemical signal transmitted along a nerve pathway in response to a stimulus. The speed of transmission varies with the size of the nerve fibre, and may be as much as 120 metres per second.

A synapse is the space between the terminal axon of one neuron and the dendrites of another. By means of a chemical substance (neurotransmitter) released by the axons, impulses are transmitted through this space from one neuron to another. Examples of neurotransmitters are acetylcholine and noradrenaline. Many different types of stimuli can excite neurons so that they become active and generate an impulse. Most neurons are excited by the neurotransmitters released by other neurons, but other stimuli can excite neurons. For example, sound excites some of the neuronal receptors of the ear, and pressure excites some cutaneous receptors of the skin. Receptors, or sensory nerve terminals, act as transducers, converting the energy of a stimulus into impulses that pass to the brain.

The nervous system can be divided into two primary divisions: the central nervous system, consisting of the brain and spinal cord, and the peripheral nervous system, consisting of nerves that connect the central nervous system with the body tissues.

The brain

The brain is a large organ weighing about 1.4 kg in the adult, held in position within the skull by membranes called the meninges. In most parts of the brain the outer portion, or cortex, consists of grey matter, while white matter forms the inner portion. The grey matter is convoluted to provide a greater surface area. The brain is divided into the:

The cerebrum

The cerebrum (Figure 41.2) is the largest part of the brain, filling the vault of the cranium from front to back. It is divided by fissures into the left and right hemispheres, and each hemisphere is further divided by fissures into four lobes:

Figure 41.2 The brain — cerebral hemispheres (left) and major structures (right)

The left hemisphere is usually associated with language, mathematical skills and reasoning. The right hemisphere is generally associated with skills such as artistic awareness and imagination. Within each hemisphere is a cavity called the lateral ventricle, which is concerned with the formation of cerebrospinal fluid.

The cerebrum is divided into several areas, some of which are sensory and some of which are motor areas. The sensory areas of each hemisphere receive and interpret sensations from the opposite side of the body, including touch, temperature, pain, pressure and an awareness of the position of the body in its environment. The motor areas of each hemisphere control all voluntary movement on the opposite side of the body. The centres of special sense are located in the various lobes, including the centres for hearing, speech, smell, taste and sight (Figure 41.3).

Figure 41.3 The sensory and motor areas of the cerebrum

The functions of the cerebrum are therefore to receive and interpret impulses from the sensory organs, to initiate and control the movements of skeletal muscles, and to perform the higher levels of mental activity such as thinking, reasoning, intelligence, learning and memory.

The diencephalon

The thalami are two oval masses of grey matter that form the lateral walls of the third ventricle. Each thalamus is subdivided into a number of nuclei. Most sensory pathways (except smell) synapse here. The thalamus plays a role in the control of somatic motor activity and also influences mood and strong emotions.

The hypothalamus lies beneath the thalami, and the pituitary gland is closely connected to it. The hypothalamus controls all the activities of the autonomic nervous system, which is described later in this chapter. The hypothalamus is important in controlling the endocrine system, as it regulates pituitary gland function.

The brainstem

The midbrain. The midbrain is a short narrow segment connecting the cerebrum with the pons varolii. It is composed primarily of ascending and descending fibre tracts. Its functions are to provide a pathway for impulses passing between the cerebrum and spinal cord, and to receive stimuli that initiate eye and postural movements.

The pons varolii. The pons varolii is about 2.5 cm long, lying anterior to the cerebellum and above the medulla oblongata. It contains two respiratory centres, the pneumotaxic centre and the apneustic centre. Its functions are to act as a relay station from the cerebrum to the cerebellum, and to modify the activity of the medullary respiratory centres through the pneumotaxic and apneustic centres.

The medulla oblongata. The medulla oblongata is about 2.5–3.0 cm long, lying between the pons and the spinal cord. It provides the link between the brain and the spinal cord and contains the cardiac, respiratory, vasomotor and reflex centres. Its functions are:

The cerebellum

The cerebellum lies behind the pons and medulla and below the occipital lobes of the cerebrum. Like the cerebrum, the cerebellum is divided into two hemispheres that have shallow convolutions in their surface of grey matter. Its functions are coordination of muscular activity and regulation of muscle tone, and maintenance of balance and posture.

Blood supply to the brain

The carotid and the vertebral arteries supply blood to the brain. These arteries branch and join up again, forming a circle of arteries at the base of the brain called the circle of Willis. From here smaller cerebral arteries branch off to supply each region of the brain. Blood returns from the brain via the jugular veins to the superior vena cava.

The blood–brain barrier is a barrier that prevents or delays the entry of certain substances into brain tissue. The relatively low permeability of the capillaries supplying the brain means that some substances are either completely or partially prevented from gaining access to brain tissue. The blood–brain barrier thus acts as a protective mechanism, preventing substances such as bilirubin, which could disrupt brain function, from crossing the barrier.

The spinal cord

The spinal cord is a cylindrical structure that lies within a canal inside the vertebral column. It extends from an opening on the underside of the skull (the foramen magnum) to the level of the first or second lumbar vertebra. Below this level the vertebral canal is occupied by nerves from the lumbar and sacral segments of the cord; these constitute the cauda equina (‘horse’s tail’). The spinal cord, which is about 46 cm in length, consists of nervous tissue, with the white matter on the outside and the grey matter arranged roughly in an ‘H’ formation in the centre (Figures 41.4 and 41.5). The two anterior projections of grey matter are called the anterior horns, and the posterior projections are called the posterior horns. Sensory nerve fibres enter the posterior horns, and motor nerve fibres leave the anterior horns.

Figure 41.4 Vertical section of the meninges and spinal cord

Figure 41.5 Cross-section of the spinal cord

Leaving the spinal cord at intervals throughout its length are 31 pairs of spinal nerves. The functions of the spinal cord are:

A reflex action, or arc, is an automatic motor response to a sensory stimulus without conscious involvement (Figure 41.6). Most reflex actions are protective in nature and take place more quickly than voluntary actions. The structures involved in a reflex action are:

Figure 41.6 A reflex arc

An example of a reflex action is when the hand comes into contact with a very hot object. The skin on the hand receives the stimulus of heat, and an impulse travels from the sensory nerve endings in the skin to the posterior horn of the spinal cord. From there, the impulse is transmitted to the anterior horn, then passed along the motor nerves to the muscles of the shoulder, arm and hand. As a result, the hand is pulled rapidly away from the source of heat before the brain has even processed the information. The brain may inhibit or exaggerate reflexes.

The meninges

The meninges are the three membranes that cover the brain and the spinal cord. The individual membranes are:

The subarachnoid space is the space between the arachnoid mater and pia mater, filled with cerebrospinal fluid in circulation. The functions of the meninges are to form a protective covering against physical injury around the brain and spinal cord, and to help secure the brain to the cranial vault.

Cerebrospinal fluid

Cerebrospinal fluid (CSF) is a clear watery fluid with a composition similar to plasma. It contains substances including water, glucose, sodium, chloride, potassium, protein and waste products such as urea. The CSF is formed from the blood and is produced by a combination of filtration and active secretory processes by the choroid plexus in the ventricles of the brain. CSF circulates in the subarachnoid space surrounding the brain and spinal cord. The total volume of the CSF is about 150 mL. The fluid is formed continuously in the ventricles at a rate of about 600–700 mL/day, and is reabsorbed into the blood at about the same rate. The normal CSF pressure when the body is horizontal is 5–10 mmHg. The functions of CSF are to:

The cranial nerves

Twelve pairs of cranial nerves leave the brainstem. The numbers (Roman numerals are the convention), names and functions of the cranial nerves are:

The spinal nerves

The spinal nerves project out of the vertebral canal, one pair emerging below each vertebra, and one pair emerging between the cranium and the first cervical vertebra. The spinal nerves are mixed nerves, containing both sensory and motor fibres. They allow for sensation and movement in peripheral parts of the body not supplied by the cranial nerves, such as skin, muscles, bones and joints of the trunk and limbs. The spinal nerves are arranged in groups according to their region of origin in the cord. There are:

In some regions the nerves divide immediately after leaving the cord. These then branch and unite with each other to form what is called a plexus (meaning braid). The major plexuses are:

The sciatic nerve is the largest nerve in the body, running over the hip posteriorly down the back of the thigh to the knee where it divides into the peroneal nerve and tibial nerve.

The sympathetic nervous system

The sympathetic nervous system arises from grey matter in the spinal cord from T1 through to L2. Sympathetic nerves then synapse in a chain of ganglia that lies on either side of the vertebral column, before reaching organs or tissues. A ganglion (plural ganglia) is a knot-like mass of cell bodies. Plexuses are formed by fibres from these ganglia; for example, the solar plexus lying behind the stomach and supplying the abdominal organs, and the cardiac plexus supplying the heart and lungs.

The parasympathetic nervous system

The parasympathetic nervous system consists of cranial nerves III, VII, IX and X and nerves that emerge from the sacral region of the spinal cord. The vagus nerve (cranial nerve X) is the largest autonomic nerve.

Functions of the autonomic nervous system

The functions of the autonomic nervous system are to control the movements of internal organs and the secretions of glands. The system provides dual control: the activity of an organ is stimulated by one set of nerves, and inhibited by the other set of nerves. This dual control achieves smooth rhythmic action of involuntary muscles and internal organs, maintaining a balance between activity and rest.

The sympathetic nerves are called adrenergic nerve fibres and release the neurotransmitter noradrenaline. These nerves can be affected by strong emotions such as anger, fear, or excitement, and have a stimulating effect on most organs. The effect resembles that produced by adrenaline, a hormone secreted by the adrenal glands. This effect is called the ‘fright, fight or flight’ effect, in which the body responds to a fright either by preparing to fight or by running away. The response of the body includes:

The parasympathetic nerves are called cholinergic fibres and release the neurotransmitter acetylcholine. These nerves tend to slow down body processes, so that the end result of the antagonistic action of each division of the autonomic nervous system is a balance between acceleration and retardation. After the ‘fright’ or stressful situation is over, the parasympathetic nervous system returns things to normal. The digestive organs receive more blood, the glands increase their secretions, the heartbeat is decreased and the blood pressure falls. The effects of sympathetic and parasympathetic stimulation on various body organs are compared in Table 41.1.

TABLE 41.1 EFFECTS OF SYMPATHETIC AND PARASYMPATHETIC STIMULATION

Congenital disorders

The central nervous system of the developing fetus is very vulnerable to damage. Factors that may cause nervous system damage include the passage of microorganisms and drugs across the placental barrier into the fetal circulation. Other factors that may cause nervous system defects and result in physical and intellectual deterioration include chromosomal abnormalities, metabolic disorders, cranial malformations and structural abnormalities. The central nervous system may also be damaged during the birth process; for example, by cerebral anoxia or cerebral haemorrhage.

Inflammatory and infectious conditions

Bacterial or viral infective processes affecting the central nervous system may result in the destruction of nervous tissue through the action of toxins released by the living microorganisms and from the material released from dead microorganisms, which stimulates the inflammatory process. Infection and inflammation of nervous tissue may result in altered behaviour, altered consciousness, and sensory or motor deficits.

Trauma

Trauma to the nervous system may result from elements within the system or from external forces. Trauma occurring from elements within the nervous system includes bleeding from an aneurysm or ruptured intracranial vessel; transient interruption of the cerebral blood flow, causing ischaemia; and occlusion of a cerebral blood vessel by a thrombus or embolus. The most common causative factors in these conditions are hypertension and atherosclerosis.

Trauma from external forces may be caused by a direct or an indirect injury. A direct acceleration brain injury occurs when the head is struck by a moving object, and a direct deceleration injury occurs when the head in motion strikes a stationary object. In an indirect brain injury, the traumatic force is transmitted to the head through an impact to another part of the body, such as the neck or buttocks.

In open head trauma, a penetrating injury damages the integrity of the skull and/or meninges and brain. Infection may occur, as the injury allows the entry of microorganisms. A closed head injury is non-penetrating, with no disruption to the integrity of the cerebral meninges. Closed head injuries can result in jarring, bruising or tearing of brain tissue, which can cause haemorrhage, cranial nerve damage and cerebral oedema. An important event in closed head injury is known as coup and contrecoup. The coup injury is cerebral bruising resulting from impact to the skull, and contrecoup refers to the rebound effect of the injury, the movement of the brain opposite to the site of impact.

The nervous system response to trauma, which may cause more damage than the actual injury, results in oedema, bleeding and increased intracranial pressure. These factors destroy nervous tissue by compression or restriction of the circulation.

The spinal cord may be damaged as a result of a crushing or penetrating injury, dislocation of the spinal column, prolapsed intravertebral discs or neoplasia. In addition to tearing of, and pressure on, the spinal cord tissues, damage may be caused by haemorrhage, oedema or disruption of the blood supply to the spinal cord.

Damage to the peripheral nerves may result in loss of sensory and/or motor function.

Neoplasia

Tumours of the nervous system, which may be either benign or malignant, cause symptoms related to pressure, destruction of nervous tissue, oedema and disruption of the blood supply. The neurological manifestations of a tumour affecting the nervous system depend on the location of the tumour and on its rate of growth.

Degenerative conditions

The causes of degenerative disorders of the nervous system are varied and involve atrophy of neurons and nerve fibres. The course of a degenerative disorder is generally gradual and progressive over many years. The effects of degenerative disorders include progressive muscular atrophy; impaired speech, chewing, swallowing and breathing; deterioration of intellectual capacity; impaired motor function; and dementia.

Metabolic and endocrine disorders

Nervous system dysfunction may result from the effects of certain metabolic and endocrine disorders. Some nutritional deficiencies may affect nerve cells, resulting in their damage or death. For example, degeneration of the posterior and lateral columns of the spinal cord may occur from the vitamin B12 deficiency of pernicious anaemia. Disorders of cortical function, leading to confusion or coma, may result from a deficiency of thiamine (vitamin B1).

Specific endocrine disorders such as hypothyroidism result in decreased metabolic rate, and hypothermia can develop. If the body temperature falls below 30°C, unconsciousness will result. Myxoedema coma is characterised by exaggeration of the signs and symptoms of hypothyroidism, with neurological impairment leading to loss of consciousness.

Headaches, or cephalalgia

Headaches are common in a variety of disorders and situations ranging from functional disturbances of blood vessels to tension and stress. In neurological disorders, headaches are one of the most common symptoms. Headaches can result from compression, traction, displacement or inflammation of the cranial periosteum, the dura mater, cerebral arteries or branches of the cranial nerves. Headaches may also occur as a result of tension within extracranial structures such as muscles, air sinuses and blood vessels. Headaches are commonly classified as vascular, tension or traction–inflammatory.

Sensory changes

Sensory changes, which can result from disorders of the brain, spinal cord or peripheral nerves, include alterations in the sense of touch, pain, temperature sensitivity and the loss of a sense of position. The loss of these sensations may be partial or complete. Common sensory disturbances include neuritis and neuralgia. Neuritis, characterised by pain and tenderness along the path of a nerve, can progress to complete loss of sensory and motor function. Neuralgia is characterised by severe stabbing pain, and can be caused by a variety of disorders affecting the nervous system. Other sensory changes that may accompany nervous system disorders include a loss of taste or smell, visual changes and hearing loss.

Motor changes

Alterations in motor function include localised or generalised weakness, with difficulty in moving normally. Muscle tone may be abnormally increased or decreased. A pronounced increase in tone is referred to as rigidity. Spasticity of muscles is an increased resistance to passive stretch, with rapid flexion of a joint. Abnormal movements include:

Symptoms of ataxia, a condition characterised by impaired ability to coordinate movement, may be caused by a lesion in the spinal cord or cerebellum. Dizziness or vertigo, when the individual is unable to maintain normal balance in a standing or seated position, may also be related to a disorder of the nervous system. Unusual gait or stance may result from motor or sensory deficits caused by a disorder of the nervous system, such as Parkinson’s disease. Paralysis, a symptom of motor disturbances, can occur in varying degrees with many nervous system disorders. Upper motor neuron lesions, in which the reflex area remains intact, generally cause spastic paralysis. Flaccid paralysis generally occurs in lower motor neuron lesions, which disrupt the reflex area.

Reflex changes

Reflex changes can provide evidence of damage to the nervous system. The absences of normal reflexes, or the presence of abnormal reflexes, generally indicate nervous system dysfunction. Reflexes are classed as either superficial (cutaneous) or deep tendon (muscle stretch). Superficial reflexes are elicited when a stimulus is applied to the skin surface or to mucous membrane. Deep tendon reflexes are elicited when a stimulus is applied to a tendon, bone or joint.

Altered awareness, personality or level of consciousness

A neurological disorder that results in altered brain structure may cause impairment of a person’s cognitive functions. They may experience difficulty in being able to think, remember, reason or understand. The person may also be confused in that orientation to time, place and person is impaired. Signs of reduced alertness or responsiveness may also be shown. Brain damage, for example as a result of a head injury, can cause a confused state characterised by fluctuating disorientation and incoherence.

Cerebral impairment may also cause mood changes and/or inappropriate emotional responses. Diffuse brain damage such as that caused by a large cerebral infarction may result in emotional instability or lability (a tendency to show alternating states of happiness and sadness that seem to be inappropriate). The person may also show signs of emotional flatness or apathy, demonstrated by a reactive absence of emotions. Alternatively, the client may become euphoric.

Damage to the brain can also result in altered states of consciousness, ranging from drowsiness and difficulty in being aroused by normal stimuli, to coma. Further information on assessment of level of consciousness is provided later in this chapter.

Seizures

Seizures, which are paroxysmal events associated with sudden abnormal discharges of electrical energy in the neurons of the brain, may be secondary to central nervous system disease. Seizure disorders are described later in this chapter.

Anencephaly

Anencephaly is the failure of normal development of the cranium and scalp. The precise cause is unknown, and babies with the disorder do not live.

Spinal cord defects

These include spina bifida, meningocele and myelomeningocele. These conditions are the result of incomplete closure of the neural tube during the first 3 months of embryonic development. Causes are thought to include maternal exposure to viruses, radiation and other environmental factors.

In severe forms, spina bifida involves incomplete closure of one or more of the vertebrae, causing protrusion of the spinal contents in an external sac. In spina bifida with meningocele, the sac contains meninges and CSF. In spina bifida with myelomeningocele the sac contains meninges, CSF and a portion of the spinal cord or nerve roots. Manifestations of congenital spinal cord defects vary and include a depression, dimple or tuft of hair on the skin over the spinal defect. The more severe defects cause neurological dysfunction such as paralysis of the legs, bowel and bladder incontinence and hydrocephalus.

Hydrocephalus

Hydrocephalus is an excessive accumulation of CSF within the ventricles of the brain. It may result from an obstruction in CSF flow or from faulty absorption of CSF. The condition can also occur after birth as a result of cerebral injury or disease. In infants, the obvious manifestation of hydrocephalus is abnormal enlargement of the head. Other characteristics include distended scalp veins, thin and fragile scalp skin, downward displacement of the eyes, a shrill high-pitched cry, irritability and abnormal muscle tone of the legs.

Muscular dystrophy

Muscular dystrophy is a group of congenital disorders characterised by progressive wasting and weakness of muscles. Duchenne muscular dystrophy, which begins to manifest between the ages of 3 and 5 years, is the most common and severe form. Initially it affects the leg and pelvic muscles but there is progressive involvement of all voluntary muscles. Later in the disease, progressive weakening of cardiac and respiratory muscles results in heart or respiratory failure. Early manifestations of Duchenne’s muscular dystrophy include a waddling gait, lordosis (increased curvature of the lumbar spine) and marked difficulty rising from a supine to a standing position. As the disease progresses, facial, oropharyngeal and respiratory muscles become involved.

Huntington’s chorea

Huntington’s chorea is a disorder in which degeneration of the cerebral cortex and basal ganglia causes chronic progressive choreiform movements and mental deterioration. Onset is generally in early middle age, and the individual gradually develops progressively severe choreiform movements and dementia. The movements usually begin slowly, with facial grimacing and jerking arm actions. Over time, the movements become frequent, erratic and violent, affecting the trunk and lower limbs. Dementia may be mild at first but eventually severely disrupts the personality.

Neurofibromatosis

Neurofibromatosis is characterised by a variety of congenital abnormalities and the condition is usually classified according to which parts of the nervous system are affected. In the peripheral form, multiple cutaneous and subcutaneous nodules of varying size occur. Subcutaneous nodules may attach to the peripheral portion of the nerve, causing pain or pressure and, rarely, sensory loss in the distribution of the affected nerve. Neuromas, which are an overgrowth of subcutaneous tissue, may reach enormous sizes and commonly affect the face, scalp, neck and chest. Neurological symptoms may appear if the tumours cause pressure on the brain or spinal cord.

Cerebral palsy

Cerebral palsy comprises a group of neuromotor disorders resulting from prenatal, perinatal or postnatal cerebral hypoxia or damage. The incidence of cerebral palsy is highest in premature infants or in infants who have experienced a difficult birth resulting in cerebral damage. Causative factors include chromosomal abnormalities; prenatal factors such as maternal infections, exposure to harmful chemicals or malnutrition; perinatal factors such as premature birth or instrumental delivery causing cerebral anoxia; and postnatal factors such as trauma, infection or malnutrition causing cerebral damage.

The manifestations of cerebral palsy range from mild muscle incoordination to severe spasticity. The spastic form of the disorder is characterised by rapid alternating muscle contraction and relaxation, muscle weakness and underdevelopment, and muscle contraction in response to manipulation. The athetoid form of cerebral palsy is characterised by grimacing, writhing and jerking involuntary movements, which become more severe during stress. Ataxic cerebral palsy is characterised by disturbed balance, incoordination, muscle weakness and tremor. In addition to the range of motor deficits, the individual may experience sensory deficits such as speech, visual or hearing impairment. Intellectual disability accompanies cerebral palsy in about 40% of cases.

Cerebral aneurysm

A cerebral aneurysm is an abnormality of the wall of a cerebral artery that results in a localised dilation. If the aneurysm ruptures, blood enters the subarachnoid space or cerebral tissue. Causative factors include congenital defects in the arterial walls, sclerotic changes in blood vessels, hypertension and cerebral trauma. Manifestations of a cerebral aneurysm do not generally appear until the aneurysm ruptures. The most common symptom of rupture is the sudden onset of a severe headache, which may be accompanied by nausea and vomiting, motor deficits, visual disturbances and loss of consciousness. A cerebral aneurysm may be detected before it ruptures if the individual shows signs of oculomotor nerve compression, eyelid ptosis and a pupil that is sluggish or non-reactive.

Transient ischaemic attacks

Transient ischaemic attacks (TIAs) are recurrent episodes of neurologic deficit. The attacks, which may last from seconds to hours, are generally considered to be warning signs of an impending thrombotic cerebrovascular accident (CVA). The characteristics of a TIA, which may be caused by micro-emboli or arteriole spasm, are various symptoms of neurological dysfunction followed by a return of normal function. Symptoms include double vision, slurred or thick speech, unilateral loss of vision, staggering or uncoordinated gait, unilateral weakness or numbness, dizziness and falling because of leg weakness.

Trigeminal neuralgia

Trigeminal neuralgia is a painful disorder of one or more branches of the trigeminal nerve that produces paroxysmal attacks of excruciating facial pain. While the cause is often unknown, the disorder may be associated with other neurological conditions such as aneurysms, cerebral tumours or multiple sclerosis. The individual experiences excruciating burning pain, which generally occurs suddenly in response to a stimulus, such as a draft of cold air, drinking hot or cold fluids, brushing the teeth, or speaking or laughing. The frequency of attacks varies from many times a day to several times a month or year.

Intellectual disability

Intellectual disability is a syndrome of incomplete intellectual development associated with impaired learning and social adjustment. The causes include:

Manifestations include poor motor development, impaired concepts of space and time, learning difficulties, inappropriate behaviour and difficulty with social interactions.

Peripheral neuritis

Peripheral neuritis (polyneuritis) is the degeneration of peripheral nerves, resulting in muscle weakness and atrophy, sensory loss and decreased or absent tendon reflexes. Causes include chronic intoxication (alcohol, arsenic, lead), metabolic and inflammatory disorders (diabetes mellitus, rheumatoid arthritis), nutrient deficiencies (thiamine), and infectious diseases (meningitis or Guillain–Barré syndrome). Manifestations usually develop slowly, beginning with leg pains and numbness or tingling in the feet and hands. As the disease progresses the individual experiences flaccid paralysis, muscle wasting, pain of varying intensity, and loss of reflexes in the legs and arms. Footdrop, ataxic gait and inability to walk will eventually occur.

Guillain–Barré syndrome

Guillain–Barré syndrome is characterised by an acute onset of ascending motor and sensory deficits, which are rapidly progressive. Although the precise cause is unclear, there is suggestion that the disorder may be viral or immunological in origin. About 50% of people with the condition experience a mild respiratory tract or gastrointestinal infection 1–3 weeks before the onset of polyneuritis. Manifestations include paraesthesia and weakness of the leg muscles, which extend to the upper body within 24–72 hours. Respiratory distress can result from diaphragm and intercostal muscle weakness, and the cranial nerves may become involved.

Bell’s palsy

Bell’s palsy is a disorder of the seventh cranial nerve that produces unilateral facial weakness or paralysis. An inflammatory reaction occurs in or around cranial nerve VII, resulting in nerve compression and the onset of flaccid facial paralysis. Factors responsible for the inflammatory reaction include infection, prolonged exposure to cold temperature, and local trauma. The seventh cranial nerve can also be affected by other conditions such as a cerebral tumour, meningitis or a middle-ear infection. Manifestations are unilateral facial weakness, which is sometimes associated with pain around the angle of the jaw or behind the ear. On the affected side the mouth droops and the individual is unable to wrinkle the forehead, close the eyelid, or smile. There may be excessive watering from the affected eye and drooling of saliva from the affected side of the mouth.

Parkinson’s disease

Parkinson’s disease is a degenerative process of nerve cells in the basal ganglia and substantia nigra. The substantia nigra is an area in the basal ganglia considered necessary for motor control. Although the precise cause of the disorder is unknown, research has demonstrated that a dopamine deficiency prevents affected brain cells from functioning normally. Dopamine is a neurotransmitter that plays a role in the transmission of nerve impulses between synapses. Factors implicated in the development of Parkinson’s disease include cerebral atherosclerosis and long-term therapy with drugs such as haloperidol and phenothiazine.

Manifestations of Parkinson’s disease are related to disturbances of movement: tremor, muscle rigidity and dyskinesia. The most common initial symptom is tremor; for example, ‘pill-rolling’ movements of the fingers, and to-and-fro head tremors. Tremors are aggravated by fatigue and stress, and decrease when the individual performs a purposeful activity or is asleep. The person experiences difficulty in initiating voluntary movement and loss of posture control, so that walking is with the body bent forwards. The gait consists of short shuffling steps that are slowly initiated. Facial expression becomes mask-like, and the voice is commonly high-pitched and monotone. The person’s intelligence is not affected.