Thepupillarylightreflexissometimesabsentinpatientswithtertiary(advanced)syphilis,whichaffectstheCNS(i.e.,intheformoftabesdorsalis).Althoughthepupilfailstorespondtolight,ithasanormalaccommodationresponse.ThisconditionisknownastheArgyll Robertson pupil. Theexactmechanismiscontroversial.Oneexplanationrestsonthefactthatsomeoptictractfibersprojecttothepretectalareainthemidbrain.Thesefiberscanbedamagedinsyphiliticmeningitis,possiblybythepresenceofspirochetesinthesubarachnoidspace.NotethatthepretectalareaprojectstotheEdinger-Westphalnucleus,alsointhemidbrain,whosecellsoriginatetheparasympatheticinnervationoftheeye,whichcontrolsthepupillarysphinctermuscle.Althoughinputtotheolivarypretectalnucleusisinterrupted,theoptictractfibersprojectingtothelateralgeniculatenucleusarenotdestroyed,andthusvisionismaintained,asispupillaryconstrictionduringaccommodation.
The urinary bladder is controlled by reflex pathways in the spinal cord and also by a supraspinal center (
Fig. 11.3 ). The sympathetic innervation originates from preganglionic sympathetic neurons in the upper lumbar segments of the spinal cord. Postganglionic sympathetic axons act to inhibit the smooth muscle (detrusor muscle) throughout the body of the bladder, and they also act to excite the smooth muscle of the trigone region and the internal urethral sphincter.
•Fig. 11.3 Illustration of Descending and Efferent Pathways for Reflexes That Control the Urinary Bladder. Forclarity,onlysomeofthemajorinvolvedpathwaysareshown.(RedrawnfromdeGroatWC,BoothAM.InDyckPJ,etal[eds].Peripheral Neuropathy. 2nded.Philadelphia:WBSaunders;1984.)
The detrusor muscle is tonically inhibited during filling of the bladder, and such inhibition prevents urine from being voided. Inhibition of the detrusor muscle is mediated by the action of norepinephrine on β receptors, whereas excitation of the trigone and internal urethral sphincter is elicited by the action of norepinephrine on α receptors.
The external sphincter of the urethra also helps control voiding. This sphincter is a striated muscle, and it is innervated by motor axons in the pudendal nerves, which are somatic nerves. The motor neurons are located in the Onuf nucleus, in the ventral horn of the sacral spinal cord.
The parasympathetic preganglionic neurons that control the bladder are located in the sacral spinal cord (the S2 and S3 or S3 and S4 segments). These cholinergic neurons project through the pelvic nerves and are distributed to ganglia in the pelvic plexus and the bladder wall. Post-ganglionic parasympathetic neurons in the bladder wall innervate the detrusor muscle, as well as the trigone and sphincter. The parasympathetic activity contracts the detrusor muscle and relaxes the trigone and internal sphincter. These actions result in micturition, or urination. Some of the postganglionic neurons are cholinergic and others are purinergic (they release ATP).
Micturition is normally controlled by the micturition reflex (see