CHAPTER 24: PERIPHERAL NEUROPATHIES
Introduction
The most common peripheral neuropathies affecting the horse are the paralysis of lower motor neurons caused by trauma to the motor neuron axon. Paralysis commonly affects many cranial & spinal nerves. Other conditions affecting the peripheral nerves of the horse include cauda equina neuritis, equine motor neuron disease, recurrent laryngeal nerve hemiplasia and carpal and tarsal tunnel syndrome.
Nerve Degeneration & Regeneration
To fully understand peripheral neuropathies, it is important to understand how nerves respond to axonal damage. There are 3 types:
1)
(Wallerian) – degeneration distal to the lesion
2)
– degeneration proximal to the lesion
3) Transneuronal (transsynaptic) – degeneration in neurons not directly affected by the lesion, mostly seen in the optic nerves Wallerian degeneration occurs in the peripheral nervous system following an injury that transects the axon of a nerve. Transection of the axon results in the formation of a proximal axon stump and a distal axon stump. The proximal stump remains attached to the , and is therefore viable and functional. The distal stump is no longer attached to the soma and is therefore considered unviable even though the Schwann cells that make up the neurolemma are undamaged distal to the site of injury. The distal axon stump is degenerated through the action of and phagocytic cells, but the neurolemma of that stump is left intact to form a hollow tube distal to the site of injury. Approximately four days after injury begin to form off of the distal portion of the proximal axon stump. These sprouts are attracted to the neurolemma tube by chemotaxic factors released by the cells.
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