Page 243 - Anatomy and Physiology of Farm Animals, 8th Edition

228 / Anatomy and Physiology of Farm Animals

Table 11-4. Location of Adrenergic Receptors and the Effects of Stimulation by
Neurotransmitters of the Autonomic Nerves
VetBooks.ir Receptor subtype Locations Effect
α Vascular smooth muscle Contracts muscle to constrict vessel
1
Smooth muscle sphincters in Contracts muscle to constrict sphincters
gastrointestinal tract
Radial muscle of iris of eye Contracts muscle to enlarge pupil
Smooth muscle sphincter of urinary Contracts muscle to reduce opening into urethra
bladder
β Heart: sinoatrial node Increase heart rate
1
Heart: atrioventricular node Increase impulse conduction velocity
Heart: ventricular muscle Increase force of contraction
β Arterial vessels supplying blood to Relaxes smooth muscle to permit dilation of
2
skeletal muscle vessels
Smooth muscle of lung airways Relaxes muscle to permit airways to open
(bronchiolar)
Smooth muscle in wall of Relaxes muscle to reduce motility
gastrointestinal tract
Liver Increases glycogenolysis, gluconeogenesis in
some species

Axons in the peripheral nervous sys­ Horses that are anesthetized or
tem (i.e., nerves) of animals are frequently restrained in lateral recumbency should
injured by cutting (e.g., wire cuts) or have their halter removed to avoid injury
crushing (as in forced extraction of a calf to the facial nerve. The buckles of
during dystocia). Unlike the situation in halters are often just ventral to the ear,
the CNS, peripheral axons are capable of where branches of the facial nerve are
considerable repair. The likelihood that a close to the skin. Facial paralysis resulting
given nerve injury will undergo func­ from compression of the nerve by a hal-
tional recovery is correlated with (1) how ter buckle may or may not be reversible,
close to the nerve cell body the injury is depending on the severity and duration
and (2) whether or not the connective tis­ of the compressive injury.
sue nerve sheath is disrupted. The more Dystocia, or difficult birth, is com-
proximal a nerve injury, the less likely it is mon among first‐calf heifers bred to
to recover. In fact, axonal injury that large bulls. The young cow may have dif-
occurs very close to the cell body may ficulty passing a very large calf through
result in death of the neuron. Axonal the pelvic canal. Because the obturator
injuries that preserve the supportive tis­ nerve passes on the medial aspect of the
sues of a nerve (e.g., a crushing injury body of the ilium next to the reproduc-
with minimal disruption of the myelin tive tract, it is at risk for compression by
and connective tissue sheath) have a bet­ the oversized calf. Overzealous pulling
ter chance of functional healing than on a large calf can crush the obturator
those in which the sheath is disturbed nerves bilaterally, paralyzing the muscles
(e.g., a cut nerve). Minor injuries to that adduct the pelvic limbs. The cow
peripheral nerves may regenerate at 2 to so affected may become recumbent,
4 mm a day, whereas more severe injuries with pelvic limbs splayed out laterally.
may take many months to recover, if they Recovery depends on the severity of the
recover at all. injury.