1a afferents also inhibit the corresponding antagonist muscle group. Activation of flexor motor neurons is typically widespread so that there is withdrawal at the ankle, knee and hip. 2.2 Control of Sphincters Urine is held in the bladder as a result of the physiological sphincter of the detrusor muscle and the external striated muscle sphincter. Emptying of both bladder and rectum is brought about by motor impulses also carried in parasympathetic fibers. In the bladder there is detrusor contraction and opening of the physiological internal sphincter at the time of micturition.
Sympathetic activity acts to inhibit parasympathetic activity and increases urine storage.
This is the usual reason for the patient seeking help. If of gradual onset the complaints are usually of dragging the leg or stiffness of the limbs. Patients may also complain about flexor spasms and may present with urgency, urge incontinence or urinary retention with overflow. Impotence and ejaculatory impairment or difficulty attaining orgasm are frequent complaints if asked for. Damage to anterior root or anterior horn cell produces atrophy, flaccid weakness and fasciculations in a myotomal distribution.