The involvement of neuroendocrine reflex mechanisms in avariety of reproductive functions is implied by a number of observations. Inreflex ovulators such as the rabbit, cat, and ferret, stimuli applied to theexternal genitalia result in pituitary gonadotropin release and ovulation,whereas in some spontaneous ovulators such as the rat and mouse, this samestimulus causes pseudopregnancy.
Olfactory stimuli supplied by strange malemice block pregnancy. The degree of dependence of reproductive activity uponexternal stimuli, however, shows considerable species variation.· Despiteobvious gross anatomical differences between the genders, neuroanatomicalpathways within the pelvis, spinal cord, and the brain also show a great dealof resemblance· Afferentterminations of the penis are free nerve endings (FNE) of thin myelinated Ag, and unmyelinated C fiber type.· Theypenetrate the epidermis and end in the stratum granulosum.· Paciniancorpuscles, Ruffini’s corpuscles, and so-called genital end bulbs are alsoobserved.· Thebrain and spinal cord also receive sensory information from mechanoreceptorslocated in the urethra, muscle spindles/golgi tendons of ischiocavernosus(ICM), bulbospongiosus (BCM) and external urethral sphincter (EUS) muscles,tunica albuginea (TA), and corpora· Thissensory input is crucial in the central regulation of micturition, erection,and ejaculation.
· Afferentreceptors converge and form bundles that enter the main branch of the pudendalnerve (PN) via dorsal (DNP) (dorsal surface of the penis) and perineal (PerN)nerves (ventral surface and frenulum of the penis). · Lateralarborizing fibers of the DNP innervate the distal urethral lumen, and the PerNsupplies the proximal bulbar and membranous urethra· Thefemale equivalent of the DNP is the dorsal nerve of the clitoris, and the PerNbranches out to become the posterior nerves of labia majora and labia minora.These nerves join to become the PN.· Boththe DNP and PerN join and become the PN nerve at the ICM and BCM junction. Thelatter enters the spinal cord through S2-S4 roots to terminate on spinalneurons and interneurons in the gray commissure and central region of thelumbosacral segment. · Spinothalamicand spinoreticular pathways relay sensory information (pain, touch,temperature, etc.
) from the spinal cord to the thalamus, hypothalamus, medialreticular formation (MRF), sensory cerebral cortex, and other centers.· Inaddition, penile afferent nerves interact with autonomic centers located in thesacral intermediolateral cell columns to modulate reflexogenic erection andmicturition. The Role of Peripheral Nerves in Reproductive Processes inthe Male· Theglans penis are important sources of sensory information to the CNS forinduction and maintenance of sexual and micturition reflexes. · Infact, desensitization of the glans of rats severely impairs erectile abilityand successful penile intromission. · Themost numerous genital receptors of both sexes are free nerve endings (FNE)· FNEare known to express polymodality, which implies their ability to perceivedifferent stimuli, such as temperature, mechanical (touch, pressure, stretch,vibration), and pain. · Thisphysiologic capacity is called “dissociated sensibility”· FNEreceptors can translate multiple sensory inputs to excitatory signals. Thisinformation is rapidly transmitted by genital afferents to the spinal cord andhigher centers to produce sexual arousal, erection, ejaculation, and pleasure.· Externalurethral sphincter activity after PN stimulation appears to be frequencydependent.
· Ina nonspinal male cat study, high frequency (1–10 kHz) electrical stimulation(ES) of PN causes reversible block of External Urethral Sphincter contractions(EUS). · Inanother cat study, ES of the dorsal nerve of penis (DNP) either inhibits (lowfrequency) or activates (high frequency) detrusor contractions, promotingstorage or emptying, respectively. · Evokedelectromyographic (EMG) activity of the EUS after DNP electrical stimulationwas also observed. · Cansensory stimulation of a nerve with both sensory and motor function lead tocontraction of striated muscles supplied by it? o ThePN appears to have this special characteristic. This functional coupling wasstudied in the cat. ES of the PN lead to vaginal dilation and anal sphinctercontraction. The Role of Peripheral Nerves in Reproductive Processes inthe Female· Abundantevidence indicating an important role of afferent stimuli on reproductiveprocesses in the female of many species· Forexample, ovulation in the rabbit is induced by the act of coitus or vaginalstimulation with a glass rod.
· Ovulationin response to coitus, however, appears not to be dependent solely upon genitalreceptors, since local anesthesia of the vulva and vagina, denervation of thegenital region, abdominal sympathectomy, hysterectomy, and extirpation of theproximal half of the vagina does not affect the response.· Itseems likely that a variety of receptors and afferent pathways capable ofactivating the release of a pituitary ovulating hormone exist.· Inrat and mouse, pseudopregnancy can be induced by mechanical or electricalstimulation of the cervix or by sterile mating.· Itcould be due to convergence of afferent impulses on the hypothalamus, causingthe release of prolactin which activates the corpora lutea to secreteprogesterone.· Thelength of the pseudopregnant period is determined by the functional life of thecorpora lutea and is approximately 12-13 days or about one-half the length ofthe normal gestation period.Modification of Gonadal Function by Manipulation of the GenitalTract· Effectsof Uterine Distention:o Uterinedistention is accompanied by an alteration of the estrous cycle length in somespecies. o Thiseffect has been attributed to a neurogenic stimulus from the distended hornacting by way of the hypothalamopituitary pathway to influence the release ofhormones controlling the persistence or regression of corpora lutea.
o Insheep the insertion of a plastic beads in utero during the early luteal phaseshortened the cycle by several days, similar effect has been reported in cowand guinea pig§ due to afferent impulses from uterinereceptors, since denervation of the segment containing the bead results inessentially normal cycles.o Presenceof a mechanical irritant in the magnum portion of the oviduct of laying henssuppresses ovulation.o Alterationsin the secretion of anterior pituitary gonadotropic hormones by uterinedistention have not been reported, but the secretion of oxytocin followinggenital stimulation is well documented, particularly in ruminants.o Injectionof oxytocin early in the estrous cycle inhibits the development of the corpusluteum and produces a precocious estrus. o Onthe other hand, when a similar treatment was given to hysterectomized animals,the induction of estrus did not occur. o Itis that the stimulation of the uterus by injections of oxytocin or distentionresults either in an inhibition of a pituitary luteotropic factor by way ofneural pathways or alternatively by the production of a uterine luteolyticfactor.
· IntrauterineForeign Bodies:o Plasticdevices Inserted in utero for contraceptive purposes in humans to preventpregnancy.o Studieson the effect of loosely placed sutures in the uterine horn of laboratoryrodents – implantation fails to occur in the operated horn · Effectsof Removal of Uteruso Removalof the uterus causes a significant prolongation of the life span of corporalutea in the guinea pig, cow, sheep, pig, rat, mouse and rabbit but not inferret, monkey or human.§ Luteolysis by PGF2? is secreted byendometricum Exteroceptive Stimuli Influencing Reproductive Processes· OlfactoryStimuli:o Whenmale domestic animals are exposed to a receptive female, most display asomewhat stereotyped behavioral pattern. o Thisincludes smelling and licking the vulva, tasting the urine, and an occasionallifting of the head, curling the lip, and expanding the nares (Flehmanresponse).
o Therole of olfactory stimuli in mating behavior due to the odor emanating from theurine or vaginal secretions of the female in estrus.o Inmost laboratory rodents, olfaction does not appear to be important forcopulation as long as other sensory capacities remain active.o Rabbitsand rats show no alteration in sexual behavior after removal of the olfactorybulbs as long as the neocortex is intact.o Completeablation of the olfactory bulbs of sexually mature sows causes a permanentanestrus, a reduction in uterine weight, ovarian atrophy.· VisualStimuli:o Bullsshow an increased interest in the nonestrous heifer if first an estrous cow hasbeen led past the stanchion.o Anincreased uterine motility can be demonstrated in heifers at the sight of abull.o Inyoung female pigs the presence of a male apparently triggers a release ofpituitary gonadotropins which precipitates first estrus.
o Visualstimuli have marked effects on reproduction in birds. § An example of a purely visualstimulus affecting reproductive processes is the observation that the pigeonwill ovulate when she sees another pigeon or an image of herself in a mirror.§ Many birds display definite courtshipperformances such as a type of dance. o Ithas been proposed that these sexual displays and the presence of other animalsserve to cause afferent stimulation of the hypothalamus and in turn activationof the pituitary.· AuditoryStimuli:o Ringingof bell 1 out of every 10 minutes persistently – after 1 week female ratsdevelop prolonged and more or less persistent estrus.o Rabbitsreact to auditory stimuli by forming large hemorrhagic ovarian folliclessimilar to those found in a positive pregnancy test.
o Intrials where estrous sows were exposed to combined auditory and olfactorystimuli from a boar, 90% responded to manual pressure on the back with animmobilization reflex in contrast to 50% in the absence of these stimuli and81% of those exposed to olfactory stimuli alone.o Dairybulls who have refused to mount a cow for a long time have been induced tomount it again when stimulated by specific calls of cows recorded and playedSummaryEvidence for the role of a variety ofexteroceptive stimuli in reproductive processes seems irrefutable. It isdifficult to prove that one sensory modality or one afferent pathway is ofprimary importance over another; rather, it would seem that all the senses areinvolved and to some degree interrelated, no one sense being absolutelyessential. The impulses apparently impinge upon neurons of the hypothalamus andthereby affect pituitary gonadotropin secretion.