Introduction to the Autonomic Nervous System

Comparing the Somatic and Autonomic Nervous Systems

The peripheral nervous system includes both a voluntary, somatic branch and an involuntary branch that regulates visceral functions.

Learning Objectives

Identify the differences between the somatic and autonomic nervous systems

Key Takeaways

Key Points

  • The somatic nervous system (SoNS) is the part of the peripheral nervous system associated with the voluntary control of body movements through the skeletal muscles and mediation of involuntary reflex arcs.
  • The autonomic nervous system (ANS) is the part of the peripheral nervous system that controls visceral functions that occur below the level of consciousness.
  • The ANS can be subdivided into the parasympathetic nervous system (PSNS) and the sympathetic nervous system (SNS).

Key Terms

  • peripheral nervous system: Consists of the nerves and ganglia outside of the brain and spinal cord.
  • autonomic: Acting or occurring involuntarily, without conscious control.
  • somatic nervous system: The part of the peripheral nervous system that transmits signals from the central nervous system to skeletal muscles, and from receptors of external stimuli, thereby mediating sight, hearing, and touch.

Examples

Examples of body processes controlled by the ANS include heart rate, digestion, respiratory rate, salivation, perspiration, pupillary dilation, urination, and sexual arousal.

The peripheral nervous system (PNS) is divided into the somatic nervous system and the autonomic nervous system. The somatic nervous system (SoNS) is the part of the peripheral nervous system associated with the voluntary control of body movements via skeletal muscles.

The SoNS consists of efferent nerves responsible for stimulating muscle contraction, including all the non-sensory neurons connected with skeletal muscles and skin. The somatic nervous system controls all voluntary muscular systems within the body, and also mediates involuntary reflex arcs. The somatic nervous system consists of three parts:

This is a full body view of the human nervous system The major organs (brain, cerebellum, and spinal column) and nerves of the human nervous system are shown.

The human nervous system: The major organs and nerves of the human nervous system.

  1. Spinal nerves are peripheral nerves that carry motor commands and sensory information into the spinal cord.
  2. Cranial nerves are the nerve fibers that carry information into and out of the brain stem. They include information related to smell, vision, eyes, eye muscles, the mouth, taste, ears, the neck, shoulders, and the tongue.
  3. Association nerves integrate sensory input and motor output; these nerves number in the thousands.

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. The ANS affects heart rate, digestion, respiratory rate, salivation, perspiration, pupillary dilation, micturition (urination), and sexual arousal.

Whereas most of its actions are involuntary, some, such as breathing, work in tandem with the conscious mind. The ANS is classically divided into two subsystems: the parasympathetic nervous system (PSNS) and sympathetic nervous system (SNS).

The enteric nervous system is sometimes considered part of the autonomic nervous system, and sometimes considered an independent system.

Divisions of the Autonomic Nervous System

The autonomic nervous system (ANS) contains two subdivisions: the parasympathetic (PSNS) and sympathetic (SNS) nervous systems.

Learning Objectives

Distinguish between the parasympathetic and sympathetic subsystems of the autonomic nervous system

Key Takeaways

Key Points

  • The enteric nervous system is sometimes considered part of the autonomic nervous system, and sometimes considered an independent system.
  • Sympathetic and parasympathetic divisions have complementary roles: the sympathetic division functions in actions requiring quick responses (fight or flight) and the parasympathetic division regulates actions that do not require rapid responsiveness (rest and digest).
  • The SNS and PSNS can be seen as constantly modulating vital functions, in usually antagonistic fashion, to achieve homeostasis. This includes both cardiovascular and respiratory functions.

Key Terms

  • autonomic: Acting or occurring involuntarily, without conscious control.
  • fight or flight: This theory states that animals react to threats with a general discharge of the sympathetic nervous system, priming the animal for fighting or fleeing.
  • vasoconstriction: The constriction (narrowing) of a blood vessel.

Examples

Example functions of the SNS include diverting blood flow away from the gastrointestinal (GI) tract and increasing heart rate. Example functions of the PSNS include dilating the blood vessels that lead to the GI tract and stimulating salivary gland secretion.

The autonomic nervous system (ANS) is classically divided into two subsystems: the parasympathetic nervous system (PSNS) and sympathetic nervous system (SNS). The enteric nervous system is sometimes considered part of the autonomic nervous system, and sometimes considered an independent system.

This is a diagram of the CNS and the subdivisions of the autonomic nervous system. In the autonomic nervous system, preganglionic neurons connect the CNS to the ganglion.

The subdivisions of the autonomic nervous system: In the autonomic nervous system, preganglionic neurons connect the CNS to the ganglion.

Sympathetic and parasympathetic divisions typically function in opposition to each other. This opposition is often viewed as complementary in nature rather than antagonistic. For an analogy, one may think of the sympathetic division as the accelerator and the parasympathetic division as the brake.

The sympathetic division typically functions in actions requiring quick responses. The parasympathetic division functions with actions that do not require immediate reaction. Many think of sympathetic as fight or flight and parasympathetic as rest and digest or feed and breed.

However, many instances of sympathetic and parasympathetic activity cannot be ascribed to fight or rest situations. 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.

Another example is the constant, second-to-second modulation of heart rate by sympathetic and parasympathetic influences as a function of the respiratory cycles. More generally, these two systems should be seen as permanently modulating vital functions, in usually antagonistic fashion, to achieve homeostasis.

Some functions of the SNS include diverting blood flow away from the gastrointestinal (GI) tract and skin via vasoconstriction, enhancing blood flow to skeletal muscles and the lungs, dilating the bronchioles of the lung to allow for greater oxygen exchange, and increasing heart rate.

The PSNS typically functions in contrast to the SNS by dilating the blood vessels leading to the GI tract, causing constriction of the pupil and contraction of the ciliary muscle to the lens to enable closer vision, and stimulating salivary gland secretion, in keeping with the rest and digest functions.