Читать книгу Bovine Reproduction - Группа авторов - Страница 244

When mounting an estrus female the bull must position himself to make searching motions and use the penis to locate the vulva in order to make intromission. Sensory innervation to the glans and free portion of the penis is necessary for the bull to align the penis, achieve intromission, and successfully complete the breeding act. Without sufficient sensory innervation of the distal penis the bull is unable to locate the vagina and the coital act cannot be completed [41].

Sensory input from the distal penis is transmitted by branches of the paired dorsal nerves of the penis through the pudendal nerves to reach the spinal cord and brain. Although disruption of any portion of the neurologic pathway could result in loss of sensation, damage to the dorsal nerves of the penis is the most likely etiology of penile desensitization. Denervation injury does not interfere with erection, but affected bulls are unable to breed by natural service and usually cannot be successfully collected with an AV. Semen collection with an electroejaculator for artificial insemination is possible.

An observed test mating utilizing an estrus female in a confined area with adequate footing remains the best method to evaluate function of the dorsal nerves of the penis. Successful intromission and ejaculation definitively demonstrate normal nerve function [17]. In bulls with impairment of sensation to the distal penis, erection is not impaired but the searching motions made with the penis are ineffective and the bull will be unable to achieve intromission and complete the breeding act. Affected bulls often place the penis to the side of the tail head, on the cow's hip, or below the vulva near the udder (Figure 15.24). Observation of typical signs at the time of test mating coupled with a history of reproductive failure is usually sufficient for diagnosis, but definitive electrodiagnostic testing of nerve function [39, 42] is available at some veterinary teaching hospitals (Figure 15.25).

Figure 15.24 Failed attempt at copulation due to denervation injury of the penis. During repeated breeding attempts during test mating, the bull was unsuccessful in locating the vulva with the penis. Note the location of the penis beside the tail head of the cow.

Figure 15.25 Electrodes and stimulator in place for nerve conduction study of dorsal penile nerves. Electrical stimulation applied through the electrode on the free portion of the penis is recorded as the signal passes the proximally placed electrodes.