Chapter 14 Nervous System
Nervous System – Anatomy and Physiology
Considerations of anatomy and physiology have important applications to diseases of the central nervous system (CNS), particularly their effects and spread.
The anatomy of the various coverings is important.
The skull and vertebrae form a rigid compartment protecting the delicate CNS tissues.
Neuronal Damage
NEURONES are sensitive to damage by a wide variety of agents including anoxia, hypoglycaemia, virus infections and intracellular metabolic disturbances (e.g. associated with vitamin B deficiencies).
There are two main types, depending on the rapidity of the changes.
Note: There is no regeneration of destroyed neurones.
Glial Reactions
The glial cells react vigorously in many diseases of the CNS.
Increased Intracranial Pressure
INCREASED INTRACRANIAL PRESSURE (ICP) occurs in two main circumstances:
Intracranial Expanding Lesions
These lesions may occur within the brain substance or in the meninges. Important examples are:
The situation is often aggravated by cerebral OEDEMA.
The severity of the effects is modified by two important factors:
There are three stages in the progress of increased intracranial pressure (ICP).
Cerebral Oedema
Swelling of the brain, of which oedema is the major component, is an important complication of many brain diseases because the enlargement either initiates or aggravates increased intracranial pressure.
The process may be localised or generalised depending on the type of initiating disorder.
| Localised conditions | Generalised conditions |
|---|---|
| Examples: | Examples: Intoxications |
| Infarcts, and local ischaemia | Metabolic disturbances, e.g. hypoglycaemia |
| Haematomas (due to vessel rupture and injury) | Generalised hypoxia |
| Tumours | Severe head trauma |
| Malignant hypertension |
The pathological mechanism is as follows:
Increased Intracranial Pressure
Secondary Complications
Note: Axonal flow in the optic nerve is reduced, contributing to swelling of the optic disc.
Circulatory Disturbances
(1) Hypoxia and ischaemia and (2) intracranial haemorrhage are the important and common mechanisms causing brain damage.
Cerebral Infarction
This condition, the commoner of the two main types of stroke (the other is spontaneous intracerebral haemorrhage), is caused by failure of the supply of oxygen (and glucose) to maintain the viability of the tissues in the territory of a cerebral arterial branch. This is not always due to simple local arterial occlusion, and very often a component of central circulatory deficiency is contributory. The lesion is essentially necrosis of all the tissues in the affected territory.
Sites
While infarcts may occur anywhere in the brain, depending on the vagaries of the precipitating arterial lesions, certain sites are more commonly affected.
Brain Damage Due to Cardiac Arrest
| Affected areas – | total cortical necrosis | |
| or | ||
| most sensitive zones | – hippocampus | |
| – layers III, V, VI of cortex | ||
| – within sulci | ||
| – Purkinje cells of cerebellum. |
Cerebral Infarction
The diagram below illustrates the evolution of an infarct, e.g. in the territory of the middle cerebral artery. Up to 24 hours there is virtually no visible change.
Cerebral Haemorrhage
Spontaneous intracranial bleeding is the second main type of stroke. In the great majority of cases there is localised arterial disease aggravated by hypertension. A small number are associated with cerebral tumours, systemic bleeding diathesis or arteriovenous malformations.
Subarachnoid Haemorrhage
This is commonly but not exclusively the result of rupture of a ‘berry’ aneurysm at or near the circle of Willis. The basic abnormality is a congenital weakness of the elastic tissues in the arterial wall; only rarely is an aneurysm present at birth, and while subarachnoid haemorrhage does occur in young people, the incidence increases with age. Hypertension is an important contributing factor.
Sites Often multiple, near arterial junctions.
Not all aneurysms rupture; they are found incidentally at autopsy.
Progress
Note: The aneurysm may rupture directly into the brain and mimic an intracerebral haemorrhage.
Head Injury
Head injuries of varying severity are common nowadays, particularly as a consequence of road traffic accidents. Immediate damage is caused by two main mechanisms which overcome the protection of the vulnerable cerebral tissues provided by the skull and the CSF ‘water cushion’.
Delayed Complications
In addition to damage sustained immediately at the time of impact, certain serious complications may supervene over the next hours or few days.
Ageing and Dementia
Alzheimer’s Disease
This disease accounts for around 70% of cases of dementia. While typically a disease of the elderly, especially females, it is also seen in patients under 60 years, in whom there is often a family history. Almost all patients with Down’s syndrome who survive to 50 years develop Alzheimer’s (suggesting that chromosome 21 is important).
