ANS

Nervous System: Autonomic Nervous System (ANS)

Introduction

  • Instructor: Dr. Jay Ferguson, DC

Outline of the Autonomic Nervous System (ANS)

  • Components:
    ➤ Autonomic Nervous System (ANS)
    ▶ Sympathetic Nervous System
    ▶ Parasympathetic Nervous System

Learning Objectives

  1. Explain how autonomic and somatic nervous systems differ.

  2. Identify the anatomical components of the parasympathetic and sympathetic systems.

  3. Identify the cranial nerves (CN) with General Visceral Efferent (GVE) function and trace the path from nuclei to target organ.

Anatomical Components of the Nervous System

  • Spinal Nerve Structure:

    • Spinal nerves consist of:

    • Sensory axon and cell body

    • Dorsal Root: Responsible for sensory input.

    • Dorsal Root Ganglion: Contains sensory neuron cell bodies.

    • Dorsal Ramus: Carries sensory and motor signals.

    • Ventral Ramus: Carries signals to limbs and trunk.

    • Ventral Root: Contains motor neuron axons.

  • Neuromuscular Junction: Connection between a motor neuron and skeletal muscle.

  • Sensory Receptors in Skin: e.g., free nerve endings of sensory neurons.

Somatic Nervous System

  • Refers to voluntary control of skeletal muscles.

  • Neural Chain:

    • One neuron runs from the spinal cord to the skeletal muscle neuroeffector tissue (one-neuron chain).

    • This neuron is a multipolar, General Somatic Efferent (GSE) neuron located in the ventral horn of spinal cord, with an axon present in a peripheral nerve.

Visceral Nervous System (Autonomic Nervous System)

  • Definition of Viscera: Greek term meaning "guts."

  • The Visceral Nervous System detects and responds to information from the internal environment.

  • The autonomic nervous system (ANS) is synonymous with the visceral nervous system and includes:

    • Sympathetic Branch

    • Parasympathetic Branch

  • The arrangement of neurons in the visceral nervous system differs from that of the somatic nervous system.

Neuronal Chains in the ANS

  • Two-Neuron Chain:

    • There are two neurons from the spinal cord to the neuroeffector tissue considered a two-neuron chain:

    • Preganglionic Neuron: Multipolar, General Visceral Efferent (GVE) neuron located in the lateral horn of the spinal cord (T1-L2 segments for sympathetic NS).

    • Postganglionic Neuron: Located in a ganglion (either paravertebral or prevertebral).

    • Neuroeffector tissues include smooth muscle, cardiac muscle, and glands.

    • The primary neurotransmitter used is norepinephrine.

  • Parasympathetic Chain:

    • Multipolar GVE neuron cell bodies are located in nuclei in the brain stem or sacral segments (S2,3,4).

    • Follows the same two-neuron chain but different ganglionic locations (terminal ganglion).

    • The axon is present in a cranial nerve or pelvic splanchnic nerve (not a spinal nerve). Neuroeffector tissues: smooth muscle, cardiac muscle, and glands.

Organization of the Viscera

  • The visceral nervous system is segmented like the somatic division and has two components:

    • General Visceral Afferent (GVA): Develops from neural crest cells and is responsible for chemoreception, mechanoreception, and stretch reception.

    • General Visceral Efferent (GVE): Develops from lateral regions of the neural tube, forming the ANS. It serves as the motor pathway to smooth muscles, cardiac muscles, and glands, classifying them as neuroeffector tissues.

Functions of the ANS

  • Major Functions:

    • Regulates involuntary effector tissues such as:

    • Smooth muscles

    • Cardiac muscles

    • Glands

    • Clinical Functions:

    • Heart rate and force of contraction

    • Smooth muscle contraction

    • Regulation of blood vessels, bronchioles, and glandular secretion.

Division of the ANS

  • Efferent Component:

    • Comprises of two divisions:

    • Sympathetic Nervous System

    • Parasympathetic Nervous System

Basic Components of the ANS

  • Autonomic Nerves Include:

    • Preganglionic neurons

    • Postganglionic neurons

    • Ganglia: Collections of neuronal cell bodies.

    • Plexuses: Networks of interacting nerves.

    • GVE neurons conduct impulses away from the CNS (brainstem and spinal cord) to effectors such as smooth muscle, cardiac muscle, and glands.

Comparison of Autonomic and Somatic Motor Systems

  • Autonomic Nervous System (ANS):

    • Cell bodies are located in the central nervous system (CNS).

    • Comprises a two-neuron chain from CNS to effector organs.

    • Uses neurotransmitters such as acetylcholine (ACh) for preganglionic neurons and norepinephrine (NE) or ACh at postganglionic effector sites.

    • Effects can be stimulatory or inhibitory depending on neurotransmitter and receptor types.

  • Somatic Nervous System:

    • Contains a single neuron from CNS to effector organs (skeletal muscle).

    • Uses ACh for neuronal transmission to skeletal muscles.

    • Effects are invariably stimulatory on skeletal muscles.

ANS and Its Pathways

  • Sympathetic Pathway Features:

    • Dilates pupil.

    • Accelerates heartbeat.

    • Dilates bronchi.

    • Inhibits peristalsis and secretion.

    • Conversion of glycogen to glucose.

    • Secretion of adrenaline and norepinephrine.

    • Inhibits bladder contraction.

  • Parasympathetic Pathway Features:

    • Constricts pupil.

    • Stimulates flow of saliva.

    • Slows heartbeat.

    • Stimulates peristalsis and secretion.

    • Stimulates release of bile.

    • Contracts bladder.