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Structure and Function of Nerves in Dogs

By: Virginia Wells

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What Are Nerves?

Nerves are bundles of fibers made up of specialized cells that send and receive information to and from the brain. Your dog's muscles, stomach, and even heart would not function if a nerve did not relay instructions from the brain. Nerves can be sensory (relay information on sensations), motor (cause activation of muscles) or mixed (both sensory and motor). Together, the nerves of the body make up the peripheral nervous system.

Where Are Nerves Located?

Most nerves extend from the brain or spinal cord to carry information to the muscles and organs. Twelve pairs of major nerves in the head arise from the brain and emerge through holes in the skull to reach their target areas such as the muzzle, eyeball, teeth and tongue. These nerves of the head are called cranial nerves. Nerves to the rest of the body arise in pairs from the spinal cord and exit from the vertebral column through spaces between the vertebrae and are called peripheral nerves.

What Is the General Structure of a Nerve?

The basic unit of the nervous system is the neuron, which is a highly specialized cell. Neurons have two characteristic properties. The first is irritability, which means that they are capable of responding to stimulation. The second is conductivity, which means they are able to conduct impulses. A neuron consists of the following parts:

  • A cell body. The cell body contains the nucleus of the cell.

  • Dendrites. These are thread-like extensions that typically branch into tree-like processes. They receive stimuli from other nerves or from a receptor organ such as the skin or ear and transmit them to the axon.

  • An axon. The axon is a long outgrowth of the cell body that carries impulses away from the cell body to another neuron, thereby stimulating an action.

  • Nerve terminals. These are the tips of the axon that join with other cells.

    What Are the Functions of Nerves?

    Depending on their function, nerves are classified as sensory, motor or mixed. Sensory, or afferent nerves carry information from the periphery of the body to the brain and spinal cord. Motor nerves or efferent nerves transmit impulses from the brain and spinal cord to the periphery. Mixed nerves are composed of both motor and sensory fibers and transmit messages in both directions.

    What Are the Common Disease of Nerves?

  • Acute polyradiculoneuritis (Coonhound Paralysis) is a widespread inflammation of the nerve roots of the peripheral nervous system. It is believed to develop in association with certain immunologic reactions against myelin, the fatty coating around nerves. It was originally diagnosed in coonhounds after they were bitten by raccoons, but it can be seen in a wide variety of dogs (even if they have no contact with raccoons). The disease results in progressive paralysis that is often temporary.

  • Brachial plexus avulsion is the stretching and tearing of the brachial plexus, a collection of nerves that arise from the spinal cord and travel to the front leg. The nerves are usually torn from their root of attachment to the spinal cord by some sort of trauma that results in significant traction on the forelimb with stretching of soft tissues of the armpit/shoulder area. The avulsion results in complete or partial paralysis of the affected forelimb.

  • Weakness or paralysis of the facial nerve (7th cranial nerve) results in improper function or paralysis of the muscles associated with facial expression. These include the muscles of the ears, lips, eyelids and nose. The eyelids on the affected side may not blink and protect the eye properly, and the lip may droop on that side. In many cases the cause of facial nerve paralysis is unknown. The condition may also occur with certain ear diseases and surgery of the ear.

  • Myasthenia gravis is a disorder characterized by muscular weakness that worsens with activity. The weakness improves with rest. Myasthenia is caused by impairment in the transmission of nerve impulses to the muscles. The acquired form of the disease is due to a defect in the immune system whereby antibodies are made against the animal's own neuromuscular receptors. As a result the normal transmission of signals from the nerves to the muscles does not occur properly.

  • Polyneuropathy is the name given to a group of diseases that affect multiple nerves around the body. Polyneuropathies may be inherited and present soon after birth or may be acquired later in life. Dogs with inherited polyneuropathies generally show signs by six months of age, while acquired neuropathies (depending on the specific disorder) may be seen in all ages.

  • Trigeminal neuropathy is also known as mandibular paralysis. It is characterized by paralysis of the muscles of mastication, or the chewing muscles. The cause of the disorder is unknown, but an inflammation of the trigeminal nerves prevents them from transmitting information to the muscles of mastication.

  • Certain toxins such as botulism can affect the peripheral nerves. Botulinum toxin binds with the nerve cells, preventing them from transmitting information to the muscles. Paralysis often begins first in the hindlegs and may progress to involve the whole body. Dogs are most often exposed to the toxin from eating decayed animals or meat containing the bacterium, Clostridium botulinum.

  • Injuries to the peripheral nerves can occur in a variety of ways. Nerves may be stretched or lacerated from their close proximity to bite wounds, bony fractures, and other soft tissue injuries. The clinical signs that develop from such injuries vary widely depending upon the specific function of the injured nerve.

    What Types of Diagnostic Tests Are Used to Evaluate Nerves?

  • A complete neurologic examination with the evaluation of numerous neurogenic reflexes is very helpful in identifying dysfunction of nerves.

  • Routine laboratory tests, such as a complete blood count, biochemistry profile, and urinalysis may detect evidence of infection and other organ abnormalities.

  • Although nerves do not show up on X-rays, damage or abnormalities to the surrounding tissues may be discovered.

  • Certain serologic tests may be indicated to test for viruses, parasites, and toxins that affect the peripheral nerves.

  • Electrodiagnostic testing is available for some nerves. Such tests include electromyelography (EMG) and measurement of nerve conduction velocity (NCV).

  • Certain immunologic tests are available to detect antibodies against components of the nerves.

  • Occasionally advanced imaging techniques, such as a CAT scan or MRI, and the analysis of cerebrospinal fluid may be helpful.

  • Biopsy of a nerve or the muscles activated by the nerve is sometimes performed.

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