There are three stages involved in the deposition of spermatozoa into the genital tract of the mare: erection, emission and ejaculation. Each will be considered in turn.
Erection is the first reaction observed after sexual stimulation. It is the result of both physical stimulation, via nerve-end stimulation in the highly innervated area of the glans penis, and psychological stimulation from the
sight or smell of an oestrous mare or an area associated with sexual contact. The initial stage of erection is the relaxation of the penile muscles normally responsible for holding the penis retracted within its sheath. This is accomplished via parasympathetic stimulation of the splanchnic nerves to the penis, overriding the normal sympathetic stimulation and partly constricting the arterioles of the penis. This allows engorgement of the penile arterioles, increasing the volume of blood within the erectile tissue. The initial effect is evident in the corpus carvenosus penis, followed by the corpus carvenosus urethra. At the same time the sympathetic stimulation of the retractor muscle is overridden, allowing relaxation of this muscle and extrusion of the penis. The subsequent penile relaxation and engorgement of the erectile tissue result in the lengthening and gradual stiffening of the penis. Erection leads initially to a state of turgid pressure within the air spaces of the penis. However, it is not adequate for intromission to occur successfully; for this, intromission pressure must be achieved. As a development of, and partly concurrent with, the above changes, continual central nervous system (CNS) stimulation activates the muscles in the root of the penis, namely the ischiocarvenosus and bulbo-spongiosus, and the urethralis muscles surrounding the urethra. As a result of stimulation, these muscles contract, drawing the base of the body of penis up against the ischial arch of the pelvis, which largely prevents the venous return of blood from the penis via the deep dorsal veins. This retention of blood within the penis increases the blood pressure further, achieving intromission pressure. At the same time the stallion's heart rate, and hence the cardiac output, increases, further enhancing the effect (Beckett et al., 1973, 1975).
The penis returns to its normal flaccid state once sexual stimulation has been removed. The penile muscles relax and the arteries return to their normal constricted state as dominance of the sympathetic stimulation recurs (Pearson and Weaver, 1978).
Emission is the mixing of spermatozoa with seminal plasma and the passage of both to the urethra penis. This is achieved by a series of pulses. These pulses are achieved by nervous system stimulation, which causes strong peristaltic contractions of the walls of the epididymis, vas deferens and accessory glands, followed by contraction of the bulbospongiosus and the ure-thralis muscle. As a result, semen is ejaculated in a series of jets.
Ejaculation occurs in association with and as a result of emission. Both emission and ejaculation result from muscle contraction, primarily of the walls of the vas deferens and the urethra. During ejaculation there is additional help from contraction of the penile muscle fibres. As already noted, ejaculation occurs in a series of jets and is accompanied by a series of pelvic thrusts. There are normally six to nine jets in total per ejaculate, lasting on average
6.15 ± 2.98 s and with a decrease in volume with each successive jet. These jets can be grouped into those that make up the pre-sperm fraction, the sperm-rich fraction and the post-sperm fraction. The pre-sperm fraction is a minor secretion often evident before intromission. The major fraction is the sperm-rich fraction, normally the first three jets, in which 70% of the biochemical components of the sample are contained (Tischner et al., 1974; Kosiniak, 1975; Weber and Woods, 1993). These initial three jets are produced at high pressure, when engorgement of the penis is at its maximum and with the stallion thrusting hard. At this time the stallion's urethral process is pressed in close to the mare's cervix, which, as a result, is forced open by the fully engorged glans penis, which also acts as a seal within the vagina. These three jets are, therefore, deposited directly into the uterus. The remaining jets may be deposited in the vagina as detumescence begins (Boyle, 1992).
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