The majority of veterinary patients with neurological disease display some abnormality of posture and locomotion.

The abnormalities range from weakness or paralysis to spasticity» rigidity» and convulsions. The goal of the diagnostic process for such patients is to determine the location, the extent, and the cause of the lesion.

This chapter defines lower and upper motor neurons because these concepts are useful in understanding the physiology of posture and locomotion and arc essential in locating pathological processes in the nervous system. Malfunctions of these two neuron populations are also described briefly. central nervous system (CNS) and whose axon extends out through the peripheral nerves to synapse with the extrafusal skeletal muscle fibers (Figure 9-1). The cell bodies of these neurons are located either in the ventral horn of spinal cord gray matter or in cranial nerve nuclei of the brainstem. This is the “final common pathway” through which the CNS chan­nels commands to respective skeletal muscles, or combina­tions of those muscles, to produce movement. This definition predates the discovery of gamma (γ) motor neurons, which innervate muscle spindles, and some authors would include γ motor neurons in the definition of lower motor neurons. Some also consider postganglionic autonomic neurons to be lower motor neurons (see Chapter 13). Virtually all the clinical signs caused by lower motor neuron disease, however, can currently be explained by the loss or malfunction of the α motor neuron.