Browse the corpus

The autonomic nervous system (ANS), a component of the peripheral nervous system, regulates involuntary physiologic processes, including heart rate, blood pressure, respiration, digestion, and sexual arousal. The ANS consists of 3 anatomically distinct divisions: sympathetic, parasympathetic, and enteric (see Image. Autonomic Nervous System Pathways). The sympathetic (SNS) and parasympathetic (PNS) nervous systems contain afferent fibers that transmit sensory input to the central nervous system (CNS), as well as efferent fibers that convey motor output from the CNS. Motor pathways of the SNS and PNS generally follow a 2-neuron sequence: a preganglionic neuron with a cell body in the CNS and a postganglionic neuron with a cell body in peripheral ganglia that innervates target tissues. The enteric nervous system (ENS) forms an extensive, web-like network capable of autonomous function independent of the CNS.[1] The ENS contains over 100 million neurons of more than 15 morphological types, exceeding the total of all other peripheral ganglia, and primarily regulates digestive processes.[2] Activation of the SNS induces a state of heightened activity and alertness, commonly referred to as the “fight or flight” response. Physiologic effects include increases in heart rate and blood pressure, stimulation of glycogenolysis, and inhibition of gastrointestinal peristalsis.[3] Sympathetic innervation extends broadly to most organs and tissues, but structures such as cartilage and avascular epithelia lack direct sympathetic input. The PNS facilitates “rest and digest” processes, promoting homeostatic functions, such as modulating heart rate and atrioventricular conduction and restoring gastrointestinal peristalsis and digestion.[4] Attenuation of arterial pressure primarily occurs indirectly via cardiac output reduction and reflex mechanisms, as systemic vascular tone is predominantly maintained by sympathetic activity. Parasympathetic innervation is largely confined to the head, viscera, and external genitalia, with minimal presence in musculoskeletal tissue and skin, rendering the PNS anatomically smaller than the SNS.[5] The ENS comprises reflex pathways that regulate digestive functions, including smooth muscle contraction and relaxation, secretion and absorption, and local blood flow.[6]

Attenuation of arterial pressure primarily occurs indirectly via cardiac output reduction and reflex mechanisms, as systemic vascular tone is predominantly maintained by sympathetic activity. Parasympathetic innervation is largely confined to the head, viscera, and external genitalia, with minimal presence in musculoskeletal tissue and skin, rendering the PNS anatomically smaller than the SNS.[5] The ENS comprises reflex pathways that regulate digestive functions, including smooth muscle contraction and relaxation, secretion and absorption, and local blood flow.[6] Presynaptic neurons of both the SNS and PNS utilize acetylcholine (ACh) as their neurotransmitter. Postsynaptic sympathetic neurons primarily release norepinephrine to act on target tissues, whereas postsynaptic parasympathetic neurons release ACh exclusively.[7] Enteric neurons employ multiple neurotransmitters, including ACh, nitric oxide, and serotonin.[8]