B Neundörfer (Erlangen):
Neurocontrol of bladder, bowel and sexual functions 1. Anatomy and function of the bladder: The bladder is a muscular cartilaginous hollow organ for collecting and excreting urine. The muscles of the bladder wall, called detrusor vesicae, form a three-dimensional network whose contraction causes the excretion of urine. In the lower part, the bladder narrows into the bladder neck through which urine is excreted. Normally, however, it is closed by the internal sphincter muscle, which is regulated via the autonomic nervous system and it consists of smooth muscles as does the detrusor. The bladder neck is closed in order to maintain continence and avoid the reflux of ejaculation. The sympathetic system, the parasympathetic system and pudendal nerve innervate the bladder. Stimulation of alphaadrenergic sympathetic fibers causes contraction of the internal sphincter muscle, while stimulation of beta-adrenergic fibers causes relaxation of the detrusor. Stimulation of the parasympathetic system causes relaxation of the bladder neck and contraction of the detrusor, being predominantly innervated by the parasympathetic nerves. In the bladder wall, there is a close network of sympathetic and parasympathetic fibers with direct synaptic contacts and mutual convergence at the postganglionic neurons. The pudendal nerve is responsible for the motor supply of the external sphincter muscle as well as the pelvic floor muscles and also for the sensory supply of the intestinal area, the outer genitals and urethra. The sensory afferences of the bladder run via the parasympathetic system mediating stretch stimuli for information as to the content of the bladder. Sympathetic afferences mediate pain stimuli, which are conveyed in the same way. Micturition occurs as follows: The normal bladder has a capacity of 400ml corresponding to an intravesical pressure of 10cm H2O. The micturition reflex is triggered off when the volume has increased to 450 - 500ml and the pressure to approximately 15cm H2O. Impulses of the stretch receptors lead to contractions of the detrusor and relaxation of the bladder neck as well as of the internal and external sphincter muscles. 2. Male sexual system The innervation of the male sexual organs runs – like in the bladder – through the sympathetic, parasympathetic and pudendal nerves. Stimulation of these sympathetic fibers leads to contraction of the smooth muscles in the testicles, deferent duct, seminal vesicles and prostate which, in turn, allows for the ejaculation of semen into the urethra. The resulting erection is called “sympathogenic erection”. The function of the parasympathetic fibres is to allow for vasodilation with resulting erection, i.e. “parasympathogenic erection”. The pudendal nerve is responsible for the somatic motor and sensory innervation of the sexual organs. The stimulation leads to contraction of the external cavernous, ischiocavernous and bulbocavernous sphincter muscles, leading to excretion of semen from the urethra. Partly the sensory sympathetic and parasympathetic as well as the somatic afferences project even on the intermediolateral nucleus of the lumbar and sacral part of the spinal cord so that spinal reflexes can take place. We have to distinguish between three mechanisms for an erection: a) Reflectory erections: The reflex arc runs via sensory stimuli at the glans penis via the parasympathetic erection center S2 –S4. b) Sleeping erections: While sleeping they occur predominantly during the REM-phase. They are probably induced via the sympathetic system. C) Psychogenically induced erections: They can probably be caused either via the sympathetic or the parasympathetic system. Orgasm, which is a purely psychic event with
regard to lust fulfillment and which causes ejaculation, only occurs if the sensory afferences of the spinothalamic to the hypothalamus are intact. 3. Female sexual system The innervation of the female genital organs is basically the same as the male. The stimulation of sympathetic fibers lead to vasoconstriction and to contractions of the uterine and vaginal muscle which is important regarding labor and orgasm. Stimulation of the parasympathetic fibres leads to vasodilation with filling of the cavernous bodies (clitoris, vagina and labia minora) and secretion of mucous glands. The pudendal nerve supplies the pelvic floor muscles and the mucous membranes, as well as the outer skin of the female genitals. The sympathetic and parasympathetic efferences partly project onto the corresponding sympathetic and parasympathetic spinal centers or they go up to the CNS – as in males – and have connections to the reticular formation, hypothalamus, limbic system and association cortex. We can distinguish between the spinal level and the central nervous level as a superordinated center, which has an influence via downgoing sympathetic and parasympathetic pathways. Tactile stimuli of the outer genitals lead to stimulation of the spinal cord center of the parasympathetic system in the sacral part of the spinal cord via sensory afferences of the pudendal nerve. As a result, the cavernous bodies are filled and erection of the clitoris as well as lubrication are present. Sensory impulses to the diencephalon cause rhythmic contractions of the abdominal, gluteal and femur muscles. Via the reticular formation they cause sympathetic coreaction with an increase in pulse and respiratory rate, in blood pressure and perspiration. In the further course there is a general sympathetic dominance in orgasm, which leads to rhythmic contractions of the vagina, cervix and uterus, as well as the pelvic floor muscles via the pudendal nerve. 4. Rectum The rectum is also innervated by the sympathetic, parasympathetic and the pudendal nerve. The sympathetic nerves cause inhibition of the peristalsis and contraction of the external sphincter ani muscle and hence obstipation. Stimulation of the parasympathetic fibers leads to contraction of the rectum with following defecation. The pudendal nerve supplies the external sphincter ani muscle and the anal and perianal region. Regarding the sensory afferences, on the one hand sympathetic and parasympathetic afferences conduct impulses from the intestinal wall and excretory ducts to the corresponding spinal sympathetic and parasympathetic centers via the dorsal root ganglia. On the other hand they conduct them to the defecation centers of the hypothalamus and the region of the parasagittal cortex without spinal switching via the posterior columns. The somatic afferences of the pudendal nerve also transmit their impulses to the parasagittal cortex region. Defecation takes place by dilation of sigmoid and rectum, which leads to impulse of defecating via previously mentioned sensory afferences. If the voluntary, centrally controlled inhibition fails, defecation is cased by activation of the parasympathetic nerves and inhibition of the sympathetic nerves and the voluntary innervation. Keywords: micturition, erection, ejaculation, orgasm, defecation. Reference list: 1. Camilleri MC (1997) Autonomic Regulation of Gastrointestinal Motility. In: Low PhA (ed). Clinical Autonomic Disorder. Lippincott-Raven, Philadelphia, New York, 135-45. 2. Fowler CJ (1999) Neurological disorders of micturition and their treatment. Brain 122: 1213-31. 3. Kirby RS, Carson CC, Webster GD (eds) (1991) Impotence: Diagnosis and Management of Male Erectile Dysfunction. Botterworth-Heinemann Ltd., Oxford.
B. Neundรถrfer, Neurological Department of the University of Erlangen-Nuremberg, Schwabachanlage 6, 91054 Erlangen, Germany. Tel.: ++49-9131-8534563(4), Fax: ++499131-8536597.