Answer
The autonomic nervous system (ANS) is not completely independent of the influences of the central nervous system (CNS). The cerebral cortex, the hypothalamus, the brainstem, and the spinal cord all generate messages that modify the the results of ANS activities.:
The cerebral cortex generates emotions and retrieves memories.--fear, and anger are generated in the limbic system. Through the limbic system the cerebral cortex is connected with the ANS. This facilitates the modulation intrinsic ANS activities by conscious CNS influences. Thus heart rate can be increased by fear and blood pressure can be raised by anger. Furthermore, memories and thoughts of good food-- occurring in parts of the cerebral cortex-- can work through the ANS to cause increased gastrointestinal tract motility.
The hypothalamus can also modify effects of ANS activity. Thus hunger, thirst, thermoregulation, emotions, and sexuality-- well-known ANS functions-- are all modifiable by hypothalamic nuclei. the hypothalamus is involved in the "fight or flight" response generated by sympathetic nervous system activity, as well as in the "rest-digest" calming response generated by parasympathetic activity. Neural stimuli from the hypothalamus travel caudally to brainstem nuclei, thence to the sympathetic neurons of the spinal cord (SC) by way of cranial nerves .
Work Step by Step
There are several nuclei in the brainstem (midbrain, pons and medulla) which generate neural outputs that modify ANS activities and effects. These include nucleus gracilis, nucleus cuneatus, hypoglossal, vagal and olivary nuclei; also in this group are centers for sneezing, and vomiting and for the control of vascular tone, salivation, swallowing, urination, defecation and pupillary constriction. Output impulses leave the brainstem by way of certain cranial nerves; by CN III for pupillary constriction and dilation; by CN IX to modify salivation and swallowing; by the vagus motor fibers to modify heart rate, pulmonary activity, and gastrointestinal motility.
Urination, defecation, and sexual activities (erection, ejaculation, orgasm) are ANS activities. However, the cerebrum is able to inhibit urination, and defecation --at least for variable periods . If the spinal cord connection with the brain is severed, conscious regulation of urination and of defecation are lost, and these two functions are then regulated entirely by autonomic spinal reflexes.
