Muscle Mass and Tone
Normal mass and tone of the musculature depends on an intact nerve supply. Primary diseases of muscle and loss of use of a limb secondary to orthopedic disease are often associated with mild to moderate muscle atrophy that develops over weeks to months.
Atrophy caused by denervation, however, is more severe and rapid in onset. Visible loss of mass of specific muscles or groups of muscles is most likely caused by damage to the nerve supply to those muscles, either by direct injury to peripheral nerves or by injury to the cell bodies of those nerves in the ventral horn of the gray matter of the spinal cord (Fig. 8.7). Knowledge of the central origins of the nerves to the limbs and the course of those nerves in the periphery can be used to specifically localize neurologic lesions (see Tables 8.4 and 8.5). Electromyography and nerve conduction testing can further be used to help identify muscle denervation and peripheral neuropathies (see Chapter 35).Muscle tone can be evaluated in the recumbent animal by passively flexing the limbs. Evaluation is not accurate in the standing animal or in animals supported in slings because of resistance from taut bands of connective tissue. In normal animals repeated flexion is accompanied by an increase in the tone in the flexed limb. The limbs of animals with a lower motor neuron deficit remain flaccid.
The tone of the thoracic limbs is controlled through spinal cord segments C6 to T2 and the radial, musculocutaneous, median, ulnar, axillary, and long thoracic nerves. The motor tone of the pelvic limbs is controlled through spinal cord segments L3 to S2 and the femoral, cranial and caudal gluteal, and sciatic nerves. The lower motor neurons to the anus originate in spinal cord segments S1 to S5, via the pudendal nerve. The tail is innervated by the coccygeal segmental spinal nerves.