ANS

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HLTH115 Anatomy & Physiology 1

  • Instructor: Michael Porter, PhD, LAT, ATC, Lock Haven University

Autonomic Nervous System Overview

Functional Organization

  • Somatic Nervous System (SNS)
    • Involves consciously perceived or controlled processes.
    • Somatic Sensory Portion: Detects signals from special senses:
    • Vision
    • Hearing
    • Equilibrium
    • Smell
    • Taste
    • Signals from skin and proprioceptors.
    • Somatic Motor Portion:
    • Sends signals from Central Nervous System (CNS) to skeletal muscles.
    • Voluntary Movements: Involve the cerebrum.
    • Reflexive Movements: Involve the brainstem and spinal cord.

Autonomic Nervous System (ANS)

  • Also known as autonomic motor or visceral motor system.
  • Regulates processes below the conscious level.
  • Transmits signals from CNS to:
    • Heart
    • Smooth muscle
    • Glands
  • Responds to visceral sensory inputs (e.g., from blood vessels).
    • Note: Those sensory neurons are not part of ANS.
  • Functions to maintain homeostasis, keeping internal conditions within optimal ranges.

Lower Motor Neurons of the Somatic vs. Autonomic Nervous System

Lower Motor Neurons in Somatic Nervous System

  • A single lower motor neuron extends from the CNS to skeletal muscle fibers.
    • Cell body: Within the brainstem or spinal cord.
    • Axon: Myelinated, large diameter, releases acetylcholine (ACh) from the synaptic knob to excite muscle fibers.

Lower Motor Neurons in Autonomic Nervous System

  • Two Motor Neurons: The ANS uses a chain of two motor neurons to reach the effector.
    • First Motor Neuron (Preganglionic):
    • Cell body within the brainstem or spinal cord.
    • Has a thin, myelinated axon projecting to an autonomic ganglion in the peripheral nervous system.
    • Releases ACh from the synaptic knob to excite the second motor neuron.
    • Second Motor Neuron (Ganglionic / Postganglionic):
    • Cell body within an autonomic ganglion.
    • Very thin, unmyelinated axon projects to cardiac or smooth muscle or glands.
    • Releases either ACh or norepinephrine (NE) from the synaptic knob to either excite or inhibit the effector.
      • Effectors possess various receptor types.

Advantages of the ANS Chain of Neurons

  • Neuronal Convergence: Multiple preganglionic neurons synapse on one ganglionic neuron.
  • Neuronal Divergence: Branches of axon from one preganglionic neuron synapse with numerous ganglionic neurons.

CNS Control of the Autonomic Nervous System

  • The ANS is regulated by various parts of the brain and spinal cord:
    • Hypothalamus: Critical for controlling both divisions of the ANS.
    • Plays a vital role in emotional ANS processes such as fight-or-flight.
    • Influenced by cortex, thalamus, and limbic system.
    • Brainstem Nuclei: Mediate visceral reflexes (e.g., maintenance of blood pressure).
    • Spinal Cord: Governs some ANS reflexes (e.g., defecation and urination controlled by the parasympathetic nervous system).

Functional Differences in ANS Divisions

  • Parasympathetic Division:
    • Designation of “rest-and-digest.”
    • Aims to conserve energy and replenish nutrients.
  • Sympathetic Division:
    • Designation of “fight-or-flight.”
    • Activated during exercise, excitement, and emergencies.

Degree of Response in ANS Activity

Parasympathetic Activity

  • Fairly localized effects due to long preganglionic axons with limited branching influencing only one or a few effectors simultaneously.

Sympathetic Activity

  • Often leads to mass activation:
    • Short preganglionic axons with many branches influence several effectors simultaneously (e.g., during exercise or fear).
    • Also involves the release of adrenal medulla hormones to facilitate effects.
  • Sometimes a single effector can be engaged (e.g., pupil dilation in low light).

Parasympathetic Division Components

Cranial Components

  • Four Cranial Nerves convey parasympathetic signals:
    • Oculomotor (CN III)
    • Preganglionic axons extend to ciliary ganglion; control pupil constriction and ciliary muscle for lens focusing.
    • Facial (CN VII)
    • Extend to pterygopalatine/submandibular ganglia; influence lacrimal glands, nose, mouth, and salivary glands.
    • Glossopharyngeal (CN IX)
    • Preganglionic axons control parotid salivary gland via otic ganglion.
    • Vagus (CN X)
    • Preganglionic axons project extensively to ganglia in thorax and abdomen; regulate heart rate, lung bronchioles, digestive secretions, and carbohydrate storage.

Summary of Sympathetic Division

  • Sympathetic Function: Primarily engaged during exercise and emergencies.

Organization and Anatomy

  • Sympathetic preganglionic neurons originate from the lateral horn of the spinal cord (from T1 to L2).
    • Myelinated axons traverse anterior roots of spinal nerves and branch off into sympathetic trunks and ganglia.
    • Sympathetic Trunks and Ganglia: Resemble a string of pearls; "string" composed of axons,