More generally, these two systems should be seen as permanently modulating vital functions, in usually antagonistic fashion, to achieve homeostasis. For example, standing up from a reclining or sitting position would entail an unsustainable drop in blood pressure if not for a compensatory increase in the arterial sympathetic tonus.Īnother example is the constant, second-to-second modulation of heart rate by sympathetic and parasympathetic influences as a function of the respiratory cycles. However, many instances of sympathetic and parasympathetic activity cannot be ascribed to fight or rest situations. Many think of sympathetic as fight or flight and parasympathetic as rest and digest or feed and breed. The parasympathetic division functions with actions that do not require immediate reaction. The sympathetic division typically functions in actions requiring quick responses. For an analogy, one may think of the sympathetic division as the accelerator and the parasympathetic division as the brake. This opposition is often viewed as complementary in nature rather than antagonistic. Sympathetic and parasympathetic divisions typically function in opposition to each other. The subdivisions of the autonomic nervous system: In the autonomic nervous system, preganglionic neurons connect the CNS to the ganglion. The enteric nervous system is sometimes considered part of the autonomic nervous system, and sometimes considered an independent system. The ANS is classically divided into two subsystems: the parasympathetic nervous system (PSNS) and sympathetic nervous system (SNS). Whereas most of its actions are involuntary, some, such as breathing, work in tandem with the conscious mind. The ANS affects heart rate, digestion, respiratory rate, salivation, perspiration, pupillary dilation, micturition (urination), and sexual arousal. The autonomic nervous system (ANS) is the part of the peripheral nervous system that acts as a control system, functioning largely below the level of consciousness and controlling visceral functions. Association nerves integrate sensory input and motor output these nerves number in the thousands.They include information related to smell, vision, eyes, eye muscles, the mouth, taste, ears, the neck, shoulders, and the tongue. Cranial nerves are the nerve fibers that carry information into and out of the brain stem.

Spinal nerves are peripheral nerves that carry motor commands and sensory information into the spinal cord.The human nervous system: The major organs and nerves of the human nervous system.

The somatic nervous system is