CH10: Autonomic and Somatic Nervous Systems

Autonomic and Somatic Nervous Systems and Homeostasis

The autonomic nervous system (ANS) and somatic nervous system (SNS) are essential components of the peripheral nervous system that play crucial roles in maintaining homeostasis within the body by conveying motor output from the central nervous system (CNS) to various effectors, such as muscles and glands. This ensures that appropriate physiological responses are generated in reaction to integrated sensory information.

Objectives

  • Describe the functions and pathways of the autonomic nervous system.

  • Explain the role of the autonomic nervous system.

  • Identify the components of an autonomic motor pathway.

  • Discuss the functional operation of the neuroeffector junction.

  • Describe the neurotransmitters and receptors of the ANS.

  • Compare physiological responses of the parasympathetic and sympathetic branches of the ANS.

  • List components of an autonomic reflex arc.

  • Provide examples of autonomic control centers.

  • State the importance of biofeedback for health management.

Introduction to the Nervous System

The nervous system's efferent division is divided further into the somatic nervous system (SNS) and the autonomic nervous system (ANS).

  • The SNS is responsible for carrying voluntary motor output to skeletal muscles, enabling conscious movement control.

  • The ANS, on the other hand, handles involuntary control, relaying motor output to smooth muscle, cardiac muscle, and glands, thereby managing essential functions like heart rate, digestion, and respiratory rate without conscious effort.

Autonomic Nervous System (ANS)

Purpose

The primary purpose of the ANS is to regulate the activities of visceral effectors, which include smooth and cardiac muscles as well as glands. While it operates autonomously, the ANS is intricately regulated by centric areas of the CNS, including the hypothalamus and brainstem, facilitating a balance between the autonomic responses and homeostasis in the body.

Branches of the ANS

  • Parasympathetic Nervous System (PNS): This branch promotes rest-and-digest functions, helping to conserve and restore energy by stimulating activities such as increased salivation, digestion, and relaxation.

  • Sympathetic Nervous System (SNS): Often referred to as the fight-or-flight system, the SNS facilitates the body's response to stress and emergencies, increasing heart rate, redirecting blood flow away from non-essential functions, and enhancing alertness.

  • Enteric Nervous System (ENS): This system governs the complex processes of the gastrointestinal (GI) tract. Although it operates independently, it also communicates with the CNS to coordinate digestive activities.

Autonomic Motor Pathways

Autonomic motor pathways comprise two neurons:

  1. Preganglionic neuron: Origins in the CNS and synapses in an autonomic ganglion; generally long in the parasympathetic system and short in the sympathetic system.

  2. Postganglionic neuron: Located in the PNS, this neuron extends to visceral effectors and is typically short in the parasympathetic division and long in the sympathetic division.

Neuroeffector Junction (NEJ)

The NEJ refers to the specialized synapse between a postganglionic neuron and a visceral effector, distinguishing it from typical synapses.

Key differences include:

  • Axon terminals contain varicosities that release neurotransmitters in a more diffuse manner, rather than from discrete synaptic bulbs.

  • Receptors for neurotransmitters are distributed throughout the entire effector surface, facilitating broader and more effective signaling.

Signal Transmission at NEJ

The steps involved in signal transmission at the neuroeffector junction comprise:

  1. Arrival of an action potential at the varicosity.

  2. Opening of voltage-gated Ca2+ channels, leading to calcium influx.

  3. Release of neurotransmitters into the synaptic cleft.

  4. Neurotransmitter binding with receptors on the effector cell.

  5. Activation of G proteins, causing either excitation or inhibition of the effector cell, depending on the type of neurotransmitter and receptor involved.

Neurotransmitters and Receptors in the ANS

  • Cholinergic neurons release acetylcholine (ACh), which is found in all preganglionic neurons and most postganglionic neurons of the parasympathetic division.

  • Adrenergic neurons release norepinephrine (NE), predominantly in postganglionic sympathetic neurons.

Receptors are categorized as:

  • Cholinergic: Nicotinic (excitatory) and muscarinic (can be either depending on subtype).

  • Adrenergic: Alpha (typically excitatory) and beta (can be excitatory or inhibitory depending on subtype).

Responses of ANS

Responses driven by the ANS can be delineated into:

  • Parasympathetic responses (rest-and-digest): Enhance bodily functions such as salivation, lacrimation, urination, digestion, and defecation (collectively known as SLUDD).

  • Sympathetic responses (fight-or-flight): Activate a state of alertness, with physiological changes including increased heart rate, pupil dilation, and dilated airways, which prepare the body for quick reactions and energy mobilization.

Autonomic Reflex Arcs

An autonomic reflex arc involves five primary components:

  1. Sensory receptor: Detects changes in the internal environment.

  2. Sensory neuron: Transmits information to an integrating center (usually in the brain or spinal cord).

  3. Integrating center: Processes the information and issues commands through autonomic motor neurons.

  4. Autonomic motor neurons (preganglionic and postganglionic): Carry commands to visceral effectors.

  5. Visceral effectors: Execute the commanded reactions, thus maintaining internal homeostasis.

These reflex arcs are instrumental in regulating vital processes including cardiovascular activities, digestion, and micturition.

Autonomic Control Centers

Control centers for autonomic functions are primarily located in the brain stem and hypothalamus. These regions are reactive to incoming sensory stimuli and modulate visceral responses to maintain homeostasis, often without conscious awareness of the individual.

Biofeedback

Biofeedback is a technique that leverages electronic monitoring devices to enable individuals to gain some level of conscious control over certain autonomic responses. By practicing this technique, individuals can improve stress management, enhance relaxation, and promote healthful adaptations in bodily functions.

Summary of Key Points

The autonomic and somatic nervous systems work synergistically to sustain homeostasis through both automatic and voluntary actions. Additionally, biofeedback represents a valuable approach for individuals seeking to gain greater control over their autonomic responses, thereby fostering overall health and well-being.