General Characteristics and Classification
The primary function of the autonomic nervous system is the innervation of internal organs. Under its continuous regulation are blood vessel tone, bronchial tree diameter, cardiac activity, and thermoregulation (including sweating and heat production). Additionally, it controls the smooth muscle of the gastrointestinal tract, urinary tract, intrinsic eye muscles, and lacrimation.
The system is traditionally divided into three divisions:
- Sympathetic — has centers located in the CNS.
- Parasympathetic — also has representations in the brain and spinal cord.
- Metasympathetic (Enteric) — located exclusively in the periphery, directly within the walls of internal organs.
Many organs receive triple innervation. The sympathetic and parasympathetic branches most often act as functional antagonists (exerting opposing effects), but in some cases, their influence may be synergistic.
Comparative Physiology of the Divisions
The sympathetic system is responsible for the body's adaptation to acute stress (fight-or-flight response). Its activation increases blood pressure (via vasoconstriction of cutaneous and visceral vessels), accelerates heart rate, and dilates bronchi to enhance gas exchange. Energy production rises, body temperature increases, and sweating begins. Meanwhile, digestion, gastrointestinal motility, and bladder emptying are inhibited. The pupils dilate (mydriasis).
Conversely, the parasympathetic system promotes restorative processes (trophotropic function). It decreases vascular tone, slows heart rate, and constricts bronchi. Digestion and GI motility are enhanced, and bladder function is stimulated. The pupils constrict (miosis), and the ciliary muscle contracts for accommodation.
The metasympathetic (enteric) system acts as a local coordinator. It integrates external sympathetic and parasympathetic signals and can regulate organ function even in their complete absence. Because of this system, an isolated segment of the intestine retains peristaltic and secretory capacity, and an isolated heart responds to mechanical stretch with increased contraction.
Morphology of Autonomic Reflex Neurons
The structural unit of the autonomic nervous system is the reflex arc. It consists of three links, each with distinct morphological features:
- Sensory (afferent) neuron. This is a pseudounipolair cell with a long dendrite. The cell bodies of these neurons lie outside the CNS in sensory ganglia of spinal or cranial nerves. Their processes run from internal organs within mixed nerves (e.g., the vagus nerve, nervus vagus, consists of up to 90% sensory fibers).
- Associative (interneuron) neuron. Their cell bodies are clustered in specific nuclei of the spinal cord and brainstem. The axons of these cells (preganglionic fibers) project to autonomic ganglia. They are myelinated, but thinner than somatic fibers, resulting in slower impulse conduction.
- Effector (efferent) neuron. The cell bodies of these neurons are always located outside the CNS (within autonomic ganglia). Their postganglionic axons are unmyelinated and project directly to target organs. Signal conduction velocity here is the lowest.
Receptor Apparatus and Neurotransmitters
To provide feedback, the ANS utilizes an extensive sensory network. The most numerous group is mechanoreceptors, which respond to stretching, contraction, or displacement of organs. They are divided into rapidly adapting (registering dynamic phases) and slowly adapting (monitoring static parameters such as volume or geometry). Chemoreceptors act as blood composition monitors, responding to partial pressures of oxygen and carbon dioxide. There are also specific receptors: thermoreceptors in the GI tract and osmoreceptors in the liver. As for visceral pain, its exact source remains complex: it may arise from specialized nociceptors or via excessive stimulation of polymodal sensors.
Chemical neurotransmission is mediated by more than 10 types of neurotransmitters. The primary ones are:
- Acetylcholine — the primary neurotransmitter in all ganglia (transmitting from preganglionic to effector neurons) and at target organs of most parasympathetic fibers.
- Norepinephrine — released at target organs by sympathetic postganglionic fibers.
Other compounds such as serotonin, ATP, and various amino acids may also participate in neurotransmission.