Regeneration and Repair in the Nervous System
In mammals most neurons are fully differentiated at birth, although division of glial cells (including those that myelinate axons) continues postnatally. With very few exceptions, neurons are incapable of mitosis, and therefore nerve cells lost to injury or disease are not replaced.
Axons, however, may regenerate following injury if the neuronal cell body is healthy.Axonal regeneration in the CNs does not usually result in recovery of function. it is believed that mature oligodendrocytes prevent regrowth and reestablishment of meaningful neuronal connections. This is why spinal cord injuries are generally irreversible. The partial recovery of function seen over time with some CNs injuries is mostly attributable to recruitment of remaining uninjured connections and to the individual’s ability to learn to use those remaining connections.
Axons in the peripheral nervous system (nerves) are frequently injured by crushing (as in forced extraction of a calf during dystocia) or cutting (e.g., wire cuts). Unlike the situation in the CNS, peripheral axons are capable of considerable repair. The likelihood that a given nerve injury will undergo functional recovery is correlated with (1) how close to the nerve cell body the injury is and (2) whether or not the nerve sheath is disrupted. The more proximal a nerve injury, the less likely it is to recover. in fact, axonal injury that occurs very close to the cell body may result in death of the neuron. Axonal injuries that preserve the supportive tissues of a nerve (e.g., a crushing injury with minimal disruption of the myelin and connective tissue sheath) have a better chance of healing than those in which the epineurium is disturbed (e.g., a cut nerve). Minor injuries to peripheral nerves may regenerate at 2-4 mm a day, whereas more severe injuries may take many months to recover, if they recover at all.
Horses that are anesthetized or restrained in lateral recumbency should have their halter removed to avoid injury to the facial nerve. The buckles of halters are often ventral to the ear, where branches of the facial nerve are close to the skin. Facial paralysis resulting from compression of the nerve by a halter buckle may or may not be reversible, depending on the severity and duration of the compressive injury.
Dystocia, or difficult birth, is common among first-calf heifers bred to large bulls. The young cow may have difficulty passing a very large calf through the pelvic canal. Because the obturator nerve passes on the medial aspect of the body of the ilium next to the reproductive tract, it is at risk for compression by the oversized calf. Over- zealous pulling on a large calf can crush the obturator nerves bilaterally, paralyzing the muscles that adduct the pelvic limbs. The cow so affected may become recumbent, with pelvic limbs splayed out laterally. Recovery depends on the severity of the injury.