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 central nervous system is composed of nervous tissue, which is commonly described as grey and white matter. Examination of a section of the brain reveals that it is grey on the outside and white on the inside. Microscopic examination reveals that the grey matter is composed of neuron cell bodies, and the white matter is made up of myelinated fibres.

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:

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).

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 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.

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:

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 peripheral nervous system consists of the 12 pairs of cranial nerves that leave the brainstem, and 31 pairs of spinal nerves that leave the spinal cord. The peripheral nerves may be sensory, motor or mixed. Sensory (afferent) nerves carry impulses to the brain and spinal cord. Motor (efferent) nerves carry impulses from the brain and spinal cord to the muscles, organs and tissues. Mixed nerves are composed of both sensory and motor fibres and transmit impulses in both directions. The motor peripheral nervous system has two functional divisions:

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 autonomic nervous system is the division of the peripheral nervous system concerned with involuntary activity of the body. It supplies nerves to all the structures in the body that are not under conscious control. The autonomic nervous system consists of two divisions: the sympathetic and the parasympathetic nervous systems.

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.


Organ Sympathetic stimulation Parasympathetic stimulation
Heart Increases rate/strength of heartbeat Decreases rate/strength of heartbeat
  Dilates coronary arteries to increase blood supply to the heart muscle Constricts coronary arteries to decrease supply of blood to the heart muscle
Bronchi Dilates bronchi, allowing more air to enter the lungs Constricts bronchi, limiting air intake
Digestive system

Urinary bladder Relaxes bladder wall. Contracts internal sphincter muscle Contracts bladder wall. Relaxes internal sphincter muscle
Eye Dilates the pupil. Retracts the eyelids Constricts the pupil. Closes the eyelids


The pathophysiological changes that can disrupt normal function of part or all of the nervous system can be due to congenital or developmental disorders, infectious or inflammatory conditions, trauma, neoplasia, degenerative conditions, and metabolic or endocrine disorders. Any pathophysiological change is capable of causing various types and degrees of dysfunction.



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.


The manifestations of a nervous system disorder will vary depending on the type and severity of the disorder.

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.

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.


Disorders of the nervous system may be congenital or genetic, due to multiple causes, degenerative, infectious or inflammatory, immunological, neoplastic, obstructive or traumatic.


Structural congenital abnormalities include anencephaly, spinal cord defects and hydrocephalus.


Hereditary genetic defects include muscular dystrophy, Huntington’s chorea and neurofibromatosis.



Seizure disorders may be primary and idiopathic, or secondary and symptomatic of a central nervous system disorder. The most common type of seizure disorder is epilepsy. Seizures may be focal or partial, absence or generalised. Focal or partial seizures generally affect a specific body part, and the symptoms of an attack depend on the location of the cerebral focus. For example, the focal motor, or Jacksonian, seizure occurs from a lesion in the motor cortex or strip. Typically it causes stiffening or jerking in one extremity that is accompanied by numbness or tingling.

Absence, or petit mal, seizures last only a few seconds but may progress to generalised tonic–clonic seizures. They generally begin with a brief change in the level of consciousness, which is indicated by a blank stare, eyelid fluttering or head nodding, or a pause in conversation. The individual generally retains posture and returns to pre-seizure activity without difficulty. This type of epilepsy usually occurs in childhood and may continue into early adolescence.

Although seizures may result from a nervous system disorder, they may be caused by many other factors and are often idiopathic. The international classification of seizures is given in Table 41.2.


I Focal or partial seizures
  Simple (general, without an impairment of level of consciousness):

II Generalised seizures (without a local onset, bilateral, symmetric)

III Unilateral seizures
IV Unclassified seizures (when complete data are not available)
V Classification of paroxysmal forms

A generalised seizure is commonly referred to as a tonic–clonic, or grand mal, seizure or convulsion. This type of seizure usually consists of three phases:

Status epilepticus is a condition in which the individual experiences continuous seizures without regaining consciousness in between. It is generally accompanied by respiratory distress.

Figure 41.7 shows a sample nursing critical pathway for managing a client after a seizure, and Clinical Interest Box 41.1 provides an outline of teaching for home care of clients in relation to seizures.

CLINICAL INTEREST BOX 41.1 Teaching for home care of clients affected by generalised seizures

Teaching must be planned around a systematic assessment of the needs of both the client and their significant others. Significant others need to be included so that they can learn seizure management, care and observations. The importance of safety and maintenance of a patent airway should be stressed. The following recommendations assist the client and significant others to adjust:


Feb 12, 2017 | Posted by in NURSING | Comments Off on NEUROLOGICAL HEALTH
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