From  Horse and Rider - UK Jan. 2016

THE  EQUINE  NERVOUS  SYSTEM  CONTROLS  YOUR  HORSE'A  BODY  INSIDE  AND  OUT. VET  ADAM  JACKSON  FROM  LIPHOOK  EQUINE  HOSPITAL  EXPLAINS  HOW  IT  WORKS  AND  WHAT  HAPPENS  WHEN  THINGS  GO  WRONG.

ON  YOUR  HORSE'S  NERVES

The horses's nervous system consists of nervous tissue that is made up of nerve cells called neurones. He is born with as full complement of neurones and produces no more during his life. He possesses billions of neurones that connect with each other to form neurological circuits - some nerve cells may have contact with more than 100,000 other nerve cells…..

components  of  the  equine  nervous  system

The equine nervous system, which is extremely complex, consists of the brain, spinal cord, and motor  and  sensory  nerves.  There are three main groups  to  the  horses's  nervous  system….

* Central nervous system

* Peripheral nervous system

* Autonomic nervous system

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Essential functions

The horse's nervous system has three main functions...

Sensory function The ability to sense changes in stimuli that occur both inside and outside of his body. Sensory neurons carry information from the body, such as the sensation of temperature,pain,body position, touch, smell, sight, hearing, taste and balance.

Integrative function This is the processing of information that has been received from sense organs, so that he responds to the stimuli appropriately. This information is analysed and stored as memory.

Motor function Where various body organs respond to stimuli, such as the contraction of muscles or glands secreting. Motor neurons carry responses to the rest of the body, and are important for voluntary movements and muscle impulses.

|Central nervous system

The central nervous system is made up of the brain and spinal cord. The horse's brain is similar in shape and function to that of other mammals and weighs 400-700 grammes. The brain tissue is extremely active and, even when the horse is resting, it uses up 20% of the body's oxygen.

The brain is composed of three main sections, the brain stem (controls basic life functions), the cerebrum (centre of conscious decision-making) and the cerebellum (involved in movement and motor control). The brain is found in the protective cranial cavity of the skull. Additional protection is provided by surrounding membranes called meninges and a clear, colourless fluid called cerebrospinal fluid.

The spinal cord, which is the thickness of a thumb and around two metres long, is a cable of nerve tissue that passes down the channel in the vertebra, from the brain stem to the end of the tail. It consists of white matter on the outside and grey matter in the shape of a butterfly on the inside. The grey matter contains large motor neurons that innervate the skeletal muscles. All of the nerve tracts that link the limbs and horse's body with the brain are found in the white matter.

Peripheral nervous system

The peripheral nervous system consists of the nerves that are found throughout the rest of the body. Peripheral nerves may be classified by their function - nerves that carry nerve impulses into the central nervous system are called afferent nerves and those that carry nerve impulses out of the central nervous system are called efferent nerves.

Some peripheral nerves are connected directly to the brain and are referred to as cranial nerves, and those connected to the spinal cord are referred to as spinal nerves. The cranial nerves are 12 pairs of nerves that can be seen on the bottom surface of the brain.

The nervous system, extremely complex, consists of the brain, spinal cord, and

motor and sensory nerves

Some of the cranial nerves bring information from the sense organs to the brain, such as smell, hearing, sight and balance. Others control muscles involved in eye movement, pupil constriction, facial expressions, chewing, swallowing, and movement of the head and tongue. Finally, some are connected with glands and internal organs, such as the heart and lungs.

Autonomic nervous system

The autonomic nervous system is part of the peripheral nervous system. It consists of a specialized set of neurons that control and regulate basic, unconscious bodily functions that support life, such as the pumping of the heart and the digestion of food.

There are two main parts to the autonomic nervous system - the sympathetic and parasympathetic nervous system. The sympathetic nervous system governs the flight or fight reaction in a horse. It alerts him to things such as danger or stress, and stimulates responses that protect from these adverse stimuli. An example would be the increase in heart rate, respiratory rate and blood flow to skeletal muscles, while reducing blood flow to less critical regions, such as the skin or digestive system, in preparation to get away from a predator.

The parasympathetic nervous system is in action when the horse is relaxed, such as when he is resting, sleeping or digesting. When the horse is in a parasympathetic state, it allows basic functions to be initiated, such as effective, regular heart and breathing rates, as well as the stimulation of urination and defecation.

Signs of a problem

The horse can acquire disease or injury in either his central or peripheral nervous system, resulting in the alteration in his ability to sense his surrounding environment or react to stimuli. The effects on sensory and motor functions depend on the location of the injury and its severity.

The main signs of a nervous system disorder include...

* behavioural changes

* tremors

* lack of co-ordination

* weakness

* paralysis of one or more legs

* numbness

* seizures

* pain

Inherited and congenital disorders

Normally, a foal is born with a well-developed nervous system. However, some disorders of the nervous system may be congenital defects (present at birth), hereditary (inherited from his parents) or caused by abnormal environmental factors occurring in the womb, such as viral infections and nutritional deficiencies. Here are some of the more common disorders.

Juvenile epilepsy is a forebrain disorder occurring in young foals, especially Arabians, which presents as a foal having seizures that can lead to head injury and blindness.

Narcolepsy is another forebrain disorder that can occur in otherwise healthy horses, but presents as excessive sleepiness or sudden episodes of paralysis with rapid eye movement while conscious. It is observed in several breeds, but especially Shetlands.

Equine degenerative myeloencephalopathy is the degeneration of the nervous connections between the spine and cerebellum that leads to the loss of co-ordination or paralysis of all four legs. This condition may start as early as one week to one month after birth and is believed to be related to vitamin E deficiency.

Wobbler syndrome (cervical stenotic myelopathy) may have a genetic component and is often seen in young, rapidly-growing horses such as Thoroughbreds. This condition results from injuries to the vertebrae in the neck and can appear between six months and three years of age.

Non-inherited disorders

There are several non-inherited disorders and injuries to parts of the nervous system that horses may acquire, including...

Stringhalt is demonstrated in horses who have brisk, involuntary flexing of one or both hind limbs when the hind limb is lengthened. This condition may be mild, with jerking of the leg, to severe, where the leg contracts to the point where the horse can hardly walk. This condition can lead to muscle wasting and the cause is unknown.

Polyneuritis equi is due to inflammation of the nerves located around the sacral vertebrae (pelvis region), but may also affect cranial nerves. The cause is unknown, but it is thought to be a result of an immune response from a viral infection and is found in adult horses in Europe and North America. Horses with this condition may present with tail paralysis, urinary and faecal incontinence, loss of co-ordination of hind limbs and loss of feeling around the perineum (area between anus and scrotum or vulva).

The effects of a disease or injury to the system depend on the the problem and the severity

Botulism is a result of horses who have been exposed to a neurotoxin produced by a micro-organism called Clostridium botulinum. This disease may develop as a result of the horse ingesting the toxin arising from rotting vegetation or carcasses, or from the infection of a wound. The symptoms include rapid paralysis of all four legs, and loss of motor control of the face and head.

Organophosphate poisoning from herbicides, pesticides or other industrial toxins. The severity of the condition depends on the amount of exposure to the toxin and symptoms include excess salivation, diarrhoea, vomiting, muscle tremors, seizures and even coma.

Injuries to the suprascapular, femoral, sciatic or facial nerves can lead to nervous disorders of the peripheral nervous system.

Inflammation and infection of the spinal cord due to fungal, viral, bacterial, parasitic and protozoal infections.

Equine protozoal myeloencephalitis is an infectious disorder caused by protozoa (single-celled organisms) such as Sarcocystis neurona or Neospora hughesi. The symptoms depend on the location of the infection, but may include loss of motor control, partial paralysis of the legs, weakness, muscle wastage of the legs and cranial nerve dysfunction.

Equine herpesvirus-I (EHV-1) can infect horses of any age, but horses older than four are more susceptible. This virus infects blood vessels found in the central nervous tissue, resulting in bleeding in the brain and spinal cord, which causes neurological signs. Nasal infection and abortion may also result from infection.

Equine infectious anaemia may cause brain disorders, resulting in lack of co-ordination and weakness of the hind limbs.

Tetanus is caused by Clostridium tetani, which produces toxins when entering a wound. Neurological signs such as muscle stiffness, rigid legs, lockjaw, paralysis of facial muscles, protruding eyelids and the inability to swallow develop 5-10 days following infection.

Encephalitis is the inflammation of the brain. Meningitis is the inflammation of the meninges, which is the membrane that covers the brain and spinal cord. These two conditions are often seen together and are referred to as meningoencephalitis. It may be caused by bacteria, fungi, viruses, protozoa or parasites. 

Getting to the bottom of the problem

Diagnosis of these conditions can be difficult, and requires a detailed and sometimes a lengthy investigation that includes looking at the horse's history and may involve many diagnostic tests. If a neurological condition is suspected, your vet will attempt to determine the cause of the condition and the location of the lesion in your horse's nervous system.

Often your vet will start their examination at your horse's head and work their way down to the tail. The head will be examined in order to evaluate the function of the cranial nerves. Attention is then moved to the neck, thorax, fore limbs and hind limbs, where bone and muscle symmetry is assessed. Vets will often feel horses to determine whether normal reflexes are present, as well as to ascertain the presence of feeling and pain. The rectum, bladder, anus and tail are also evaluated to determine whether reflexes and normal muscle tone are present.

Finally, your vet will evaluate gait and posture for any abnormalities suggestive of a neurological problem. After the examination, further tests may be necessary, such as X-rays, ultrasonography, blood tests, spinal fluid evaluation and CT or MRI scans, to help confirm if and where the problem is. ■