Caring for the patient with a disorder of the nervous system

9 Caring for the patient with a disorder of the nervous system



Anatomy at a glance 1

Physiology you need to know

Cerebrovascular accident (CVA)

Head injury

Brain tumours

Degenerative disorders of the nervous system

Ten top tips


ANATOMY AT A GLANCE (P653)


The nervous system is structurally composed of two parts, the central nervous system (brain, cranial nerves and spinal cord) and the peripheral nervous system, which consists of the spinal nerves connecting to the rest of the body.


If the nervous system is considered in terms of its function, it can again be split into two parts:


image The somatic (voluntary) nervous system which transmits messages to and from the non-visceral part of the body (skeletal, skin, etc.), and is generally under conscious control.

image The autonomic nervous system which is concerned with the regulation of visceral muscles and glands and as such is not usually under conscious control.

The nervous system consists of two main groups of cells, neurones and neuroglia.




Neurones


These are the functional cells responsible for initiating or transmitting messages (see Figure 9.1). Each neurone has a nucleus, one or more projections known as dendrites, which conduct the nerve impulse into the cell body, and a single long projection known as an axon, which conducts the nerve impulse away from the cell body to the next cell. Cell bodies are generally known as grey matter, while axons make up the white matter. Where the axon of one neurone meets the dendrite of the next, a synapse is said to occur. Although the axon and dendrite of the two neurones are in close proximity, they are not connected. The nerve impulse is transmitted via neurotransmitter molecules which cross the synapse. Neurones can have a sensory, motor or connecting function (interneurones).


image

Figure 9.1 Structure of a typical neurone.



Neuroglia


These are supportive and protective cells. They make up approximately 50% of the volume of the CNS, and are divided into different types depending upon their specialist functions. Unlike neurones, they can divide and multiply but they cannot pass messages.



The Central Nervous System


The brain is what makes us human through its ability for thought, consciousness and intelligence. It is also responsible for controlling motor function, receiving and interpreting sensory input and regulation of the whole body environment through the autonomic nervous system and by interacting with the endocrine system. The higher functions of the brain are concentrated in the cerebrum. Other important parts of the brain are summarized below:


image Basal ganglia are groups of nerve cells deeply imbedded within the white matter whose function is not clearly understood.

image Thalami (singular thalamus) are relay stations for sensory messages incoming to the brain.

image Hypothalamus contains important centres to do with temperature regulation, hunger, thirst and emotion. It is also responsible for controlling much endocrine function via its association with the pituitary gland.

image Brainstem consists of three portions
• The medulla oblongata consists of ascending and descending nerve pathways, and contains vital autonomic centres controlling breathing, heart rate, and vasomotor tone as well as centres controlling the following reflexes: vomiting, coughing, sneezing and gagging.

• The pons Varolii consists of many nerve tracts linking the medulla with the upper portion of the brainstem, the midbrain (pons means bridge in Latin).

• The midbrain contains grey matter (nucleii) and white matter (axons) and is the site of the cerebral aqueduct which connects the 3rd and 4th ventricles.

image Ventricles. These consist of four cavities through which the cerebrospinal fluid circulates.

image Cerebrospinal fluid (CSF). This fluid bathes the whole brain and spinal cord. The average adult has 125–150 ml of CSF which is a clear water-like liquid. It circulates via the subarachnoid space and cerebral aqueducts acting as a shock absorber for the brain.

image Meninges. These are the three outer layers that cover the brain and spinal cord. The tough fibrous outer layer is known as the dura mater, the middle layer is the arachnoid mater and the very delicate inner layer is the pia mater. The space between the arachnoid and pia mater (subarachnoid space) is full of CSF.

image Spinal cord. This originates at the medulla oblongata, running down within the vertebral column to the level of the first or second lumbar vertebra. Spinal nerves exit and join at each space between the vertebrae. The remaining nerves exit the vertebral column at the level of the intervertebral disc which separates L1 and L2 to form a delicate fan-like structure known as the cauda equina. Within the spinal cord are both grey matter (neuronal cell bodies) and major nerve trunks (axons) conveying messages to and from the brain via sensory and motor pathways.


Blood Supply to the Brain


The brain receives approximately 20% of cardiac output and consumes 20% of all the oxygen that the body uses. As the brain has no reserve stores of glucose or oxygen, any reduction in blood supply will have immediate and serious effects. Brain cells suffer irreversible damage within 2–4 minutes of losing their blood supply.


The blood supply to the brain is derived from the two internal carotid arteries and two vertebral arteries which unite to form the basilar artery. Various small connecting arteries link these two arterial systems to form what is known collectively as the circle of Willis.



Cranial Nerves


The 12 pairs of cranial nerves are conventionally numbered in Roman numerals from I to XII. Cranial nerves III through XII emerge from the brainstem, whilst I and II are purely sensory and are directly connected to the appropriate sensory centre in the brain (I is olfactory and II is the optic nerve). The Xth cranial nerve is the vagus nerve, and its motor branch is part of the autonomic nervous system which is responsible for regulating smooth muscle function in a wide range of visceral organs.



Spinal Nerves


There are 31 pairs of spinal nerves arising from the spinal cord. They are described as mixed because they contain both motor and sensory nerve fibres. The motor fibres exit the spinal cord at the front (anterior root), whilst sensory input to the spinal cord is via the posterior root.



The Autonomic Nervous System


This contains both sensory and motor nerves and controls the visceral function of the body affecting everything from heart rate, diameter of blood vessels (and hence blood pressure) through to gastrointestinal function. There are two divisions of the autonomic nervous system:


image The Sympathetic Nervous System: this originates in the lumbar and thoracic regions of the spinal cord and produces generalized responses that help the body cope with threat and stress known as the ‘fight or flight response’. These include, for example, bronchodilation and increasing heart rate.

image The Parasympathetic Nervous System: approximately 75% of all parasympathetic fibres travel in the vagus nerve (Xth cranial). Parasympathetic nerves promote a state of resting and conservation of body resources whilst eliminating waste.

Generally speaking, both the sympathetic and parasympathetic systems innervate the same organs but have opposing effects. As a simplification, it may help to think of the sympathetic division as acting like the accelerator pedal of a car, whilst the parasympathetic division acts as the brake.



PHYSIOLOGY YOU NEED TO KNOW


Nerve impulse transmission depends upon the following series of events:


1. A resting neurone is in a polarized state. As a result the outside of the neurone carries a positive charge and the inside a negative charge.

2. A stimulus causes a localized alteration in the permeability of the nerve cell membrane resulting in the movement of large numbers of positively charged sodium ions into the cell.

3. The outer part of the membrane now has a negative charge and the inside of the neurone acquires a positive charge. Depolarization of this portion of the cell membrane has occurred and this triggers voltage-gated ion channels in the next section of the axon to open allowing more positive sodium ions to rush into the axon triggering further depolarization.

4. This zone of depolarization now sweeps down the length of the axon, followed by repolarization of the cell membrane as it reverts to its original state. This change in polarity is referred to as an action potential.

5. When the action potential reaches the end of the nerve axon, it stimulates the release of various molecules known as neurotransmitters.

6. Neurotransmitter molecules move from the end of the axon across the gap to the next neurone in the chain. This gap is known as a synapse. The next cell may be a muscle cell, which is being stimulated by a motor neurone to contract, in which case this is known as a neuromuscular junction.

7. The neurotransmitter molecules lock onto specific receptor sites on the membrane of the next nerve or muscle cell (postsynaptic membrane) where their presence may initiate a new wave of depolarization (action potential) to spread through the next neurone and on to the next axon in the chain.
• The speed with which nerve impulses are conducted in this way increases with the width of the axon conducting the impulse and also is greatly increased by the presence of a myelin sheath surrounding the axon.

• Nerves are not in contact with each other at a synapse. The gap is referred to as the synaptic cleft.

• Some neurotransmitters act to block the transmission of an impulse (inhibitory neurotransmitters).

• Some molecules can act as hormones and neurotransmitters such as epinephrine (adrenaline).

• Many drugs work by blocking the effect of neurotransmitters (such as beta blockers) or by imitating this effect in a controlled therapeutic way in order to achieve an enhanced effect (such as salbutamol).


CEREBROVASCULAR ACCIDENT (CVA) [P672]



PATHOLOGY: Key facts


A CVA involves an interruption of the blood supply to a part of the brain and the consequent development of neurological deficits which can be mild to severe and in extreme cases fatal. The cause of the interruption can be:


image Atherosclerosis which narrows and gradually occludes the lumen of one of the cerebral arteries.

image A thrombosis associated with atherosclerosis of the carotid, vertebral or other cerebral artery.

image An embolism, usually derived from a cerebral thrombosis, may lodge in an artery and block it.

image A rupture of the cerebral artery wall (associated with atherosclerosis and/or hypertension).

Additionally, many patients suffer transient ischaemic attacks (TIA) before a major CVA. These TIAs occur when disease of the arterial wall leads to a temporary reduction in blood supply, and they are an early warning sign of serious problems ahead. They typically last only a few minutes or a few hours and the neurological deficit they produce resolves spontaneously (e.g. slurred speech, visual disturbances, paresis, dizziness, confusion). Eighty per cent of people who have a thrombotic stroke have had preceding TIAs.


Most CVAs occur in people aged over 60. However, a significant number occur in young adults when the cause is a congenital weakness in a cerebral artery known as a cerebral aneurysm. The person is born with a weak area lacking muscle and elastic tissue in one of their cerebral arteries. Gradually over the years, this protrudes as a swelling from the arterial wall (hence the common name berry aneurysm), which may then start to leak or in worst cases, rupture completely, which is usually fatal. The bleeding may be into the subarachnoid space (subarachnoid haemorrhage) or into the brain substance itself.


Commonly there is a volume of oedematous tissue around the damaged region and as a result of raised intracranial pressure, neuronal function decreases. The extent of this oedema peaks at around 72 hours. It is only when this oedema has resolved over the next 2 weeks or so that the full extent of the patient’s functional loss can be estimated.



WHAT TO LOOK OUT FOR



image Level of consciousness is the key observation for all neurological patients whether they have had a CVA, head injury or any other neurological disorder. Any deterioration in their condition will usually show itself by a decrease in the level of consciousness. This is assessed objectively by the Glasgow Coma Scale (GCS) (see Table 9.1), and these observations may be carried out as frequently as every 15 minutes in an unstable patient. The GCS is based upon the best responses to objective measures of how awake the patient is (eye opening), motor ability and how oriented they are (verbal responses).

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Feb 3, 2017 | Posted by in NURSING | Comments Off on Caring for the patient with a disorder of the nervous system

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