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Diseases of the nervous system

The functions of the nervous system are directed at the maintenance of the body's spatial relation to its environment.
These functions are performed by the several divisions of the nervous system including:
The sensorimotor system, responsible for the maintenance of normal posture and gait.
The autonomic nervous system, controlling the activity of smooth muscle and endocrine glands, and thereby the internal environment of the body.
The largely sensory system of special senses.
The psychic system, which controls the animal's mental state.

It is often difficult to determine in a sick animal ,whether abnormalities are present in the nervous system, the musculoskeletal system or acid-base and electrolyte status. Accordingly, the first step when examining an animal with apparent abnormalities in the nervous system is to determine whether other relevant systems are functioning normally.

The nervous system itself is not independent of other organs and its functional

capacity is regulated to a large extent by the function of other systems, particularly
the cardiovascular system.

Hypoxia due to cardiovascular disease commonly leads to altered cerebral function because of the dependence of the brain on an adequate oxygen supply.
Principles of nervous dysfunction

Nervous dysfunction can thus be broadly divided into two forms, depressed activity and exaggerated activity. These can be further subdivided into four common modes of nervous dysfunction:


Excitation (irritation) signs.
Release of inhibition signs.
Paresis or paralysis due to tissue damage.
Nervous shock.

MODES OF NERVOUS DYSFUNCTION

Excitation (irritation) signs

The increased activity of the reactor organ occurs when there is an increase in the

number of nerve impulses received either because of excitation of neurons or because of facilitation of passage of stimuli.

The excitability of nerve cells can be increased by many factors, including:

1-Stimulant drugs.
2-Inflammation .

Thus, early or mild hypoxia may result in increased excitability while constant or severe hypoxia will cause depression of function or even death of the nerve cell.

Irritation phenomena

May result from many causes, including inflammation of nervous tissue associated with bacteria or viruses, certain nerve poisons, hypoxia and edema.

In those diseases that cause an increase in intracranial pressure, irritation phenomena result from interference with circulation and the development of local anemic hypoxia.

The major manifestations of irritation of nervous tissue are tetany, local muscle tremor, and whole body convulsions in the motor system and hyperesthesia and paresthesia in the sensory system .

Release of inhibition signs

Exaggeration of normal nervous system activity occurs when lower nervous centers are released from the inhibitory effects of the higher centers.

Cerebellar ataxia is another example of inhibitory release.

In the absence of cerebellar control combined limb movements are exaggerated in all modes of action including rate, range, force, and direction.

Paresis or paralysis due to tissue damage

Depression of activity can result from depression of metabolic activity of nerve
cells, the terminal stage being complete paralysis when nervous tissue is destroyed.
Such depression of activity may result from failure of supply of oxygen and other
essential nutrients, either directly from their general absence or indirectly because
of failure of the local circulation.


Infection of the nerve cell itself may cause initial excitation, then depression of function and finally complete paralysis when the nerve cell dies.

Nervous shock

An acute lesion in the nervous system causes damage to nerve cells in the
immediate vicinity of the lesion .

Clinical manifestations of the nervous disease :

The major clinical signs of nervous system dysfunction include:
Altered mentation.
Involuntary movements
Abnormal posture and gait
Paresis or paralysis
Altered sensation
Blindness
Abnormalities of the autonomic nervous system.

ALTERED MENTATION

Excitation states:


Excitation states include : mania, frenzy, and aggressive behavior, which are
manifestations of general excitation of the cerebral cortex.

Mania

In mania the animal acts in a bizarre way and appears to be unaware of its surroundings.

Maniacal actions include licking, chewing of foreign material, sometimes

themselves, abnormal voice , constant bellowing, apparent blindness, walking
into strange surroundings, drunken gait and aggressiveness in normally docile
animals.

Diseases characterized by mania include:

a. Encephalitis, e.g. the furious form of rabies.
b.Degenerative diseases of the brain, e.g poisoning by Astragalus spp.
c.Toxic and metabolic diseases of brain, e.g. nervous ketosis, pregnancy
toxemia, acute lead poisoning.

Frenzy

Frenzy is characterized by violent activity and with little regard for surroundings.
The animal's movements are uncontrolled and dangerous to other animals in the
group and to human attendants, and are often accompanied by aggressive physical
attacks.


Examples of frenzy in diseases of the nervous system include:

o Encephalomyelitides, e.g. Aujeszky's disease

o Toxic and metabolic brain disease, e.g. hypomagnesemic tetany .

Examples of frenzy in diseases of other body systems are:

o Acute pain of colic in horses
o Extreme cutaneous irritation, e.g. photosensitization in cattle.

Depressive states

Depressive mental states include somnolence, lassitude, narcolepsy/catalepsy,
syncope and coma.

Depression leading to coma In all species this may result from:

Encephalomyelitis and encephalomalacia
Toxic and metabolic diseases of the brain such as uremia, hypoglycemia,
hepatic insufficiency, toxemia, septicemia and most toxins that damage
tissues generally
Hypoxia of the brain, as in peripheral circulatory failure of milk fever
Heat stroke.


Syncope

The sudden onset of fainting (syncope) may occur as a result of:

Acute circulatory and heart failure leading to acute cerebral hypoxia
Spontaneous cerebral hemorrhage.
Narcolepsy (catalepsy)
Affected animals experience episodes of uncontrollable sleep and literally 'fall'
asleep.

Compulsive walking or headpressing Head -pressing is a syndrome characterized

by the animal pushing its head against fixed objects, into a corner of a pen, leaning into a stanchion or between fence posts.

INVOLU NTARY MOVEMENTS

Involuntary movements are due to involuntary muscle contractions, which include

Tremor

This is a continuous, repetitive twitching of skeletal muscles, which is usually visible
and palpable.
The muscle units involved may be small and cause only local skin movement, in which case the tremor is described as fasciculations; or the muscle units may be extensive, the movement much coarser and sufficient to move the extremities, eyes or parts of the trunk.


True tremor is often sufficiently severe to cause incoordination and severe disability in
gait.
Examples of causes of tremor include:
1-Diffuse diseases of the cerebrum, cerebellum, spinal cord
2-Degenerative nervous system disease, e.g. hypomyelinogenesis of the newborn
3- Toxic nervous system disease caused by a large number of poisons, Clostridium
botulinum toxin ,early stages of hypocalcemia in the cow (fasciculations of the
eyelids and ears).

Tics

Tics are spasmodic twitching movements made at much longer intervals than in
tremor, the intervals being usually at least several seconds in duration and often
much longer. The movements are sufficiently widespread to be easily visible and
are caused by muscles that are ordinarily under voluntary control. They are rare in
large animals but may occur after traumatic injury to a spinal nerve.

Tetany

Tetanus is a constant contraction of the muscles without tremor. The most common
cause is Clostridium tetani intoxication following localized infection with the organism.


Convulsions

Convulsions, seizures are violent muscular contractions affecting part or all of the body and occurring for relatively short periods as a rule, although in the later stages of encephalitis they may recur with such rapidity as to give the impression of being continuous.

The convulsion may be clonic, the typical 'paddling' involuntary movement

in which repeated muscle spasms alternate with periods of relaxation.

Tetanic or tonic convulsions are less common and are manifested by prolonged muscular spasm without intervening periods of relaxation.