Miscellaneous Neurologic or Neuromuscular Disorders in Horses

Miscellaneous Neurologic or Neuromuscular Disorders in Horses: Successful locomotion depends on the initiation and coordination of movement controlled by the nervous system over skeletal muscles. The peripheral nervous system can be classified into 2 major functional systems: afferent or sensory and efferent or motor systems. The efferent system is comprised of somatic and visceral components. This chapter will focus on the general somatic efferent system, specifically the neuromuscular (NM) region. The NM system is composed of motor units. The motor unit is defined as one lower motor neuron (or motoneuron), its axon and axonal terminal(s) or telodendron, NM junction, and all the skeletal muscle fibers (myofibers or myocytes) innervated by it (). Briefly, there are 2 main types of motor neurons in the ventral horn of the spinal cord; A alpha (skeletomotor) and A gamma (fusimotor) motor neurons. The alpha or extrafusal skeletomotor neurons have large cell bodies in the ventral horn of the spinal cord and fast myelinated conducting axons of large diameter located in the ventral root and spinal nerves. Their axons terminate at the NM junction and only innervate extrafusal skeletal muscle fibers. The gamma or fusimotor neurons have smaller cell bodies and thinner, slower myelinated conducting axons that innervate intrafusal muscle fibers (muscle fibers contained within muscle spindle stretch receptors). One-third and two-thirds of the motor outflow from the ventral horn are from gamma and alpha motor neurons, respectively. There are also lower motor neurons within cranial nerve nuclei (III-VII, IX-XII) but will not be discussed here. Cranial nerve nuclei III, IV, and VI are discussed in the neuro-ophthalmic article.