Exam 3 Final

Chapter 14: The Autonomic Nervous System

Peripheral Nervous System

  • Branches of the Peripheral Nervous System:

    • 1) Somatic Nervous System (SNS):

      • Responsible for voluntary muscle movements and reflex arcs involving skeletal muscle tissue.

      • Mostly voluntary control.

    • 2) Autonomic Nervous System (ANS):

      • An efferent branch of PNS.

      • Mostly involuntary control; innervates visceral organs such as cardiac muscle, smooth muscle, and glands.

      • General function helps maintain a stable internal environment (homeostasis).

        • Examples: Regulation of heart rate, blood vessel diameter, pupil size, body temperature, and stomach secretions.

The ANS vs The SNS

  • Differences between the two systems:

    • 1) Effector organs:

      • SNS: innervates skeletal muscle.

      • ANS: innervates visceral organs, cardiac and smooth muscles, and glands.

    • 2) Efferent pathways and ganglia:

      • SNS: consists of a single motor neuron from CNS to the effector. Thick, heavily myelinated fibers.

      • ANS: consists of a two-neuron chain (preganglionic and postganglionic neurons). Preganglionic cell bodies located in CNS; axons extend to ganglia.

        • Preganglionic fibers are thin, lightly myelinated; postganglionic fibers are thin and unmyelinated.

    • 3) Neurotransmitter effects:

      • SNS: releases acetylcholine (ACh) at synapses, always has an excitatory effect on muscle.

      • ANS: releases norepinephrine or ACh, with excitatory or inhibitory effects depending on the effector organ and receptor type bound.

Divisions of the ANS

  • Parasympathetic Division:

    • Known as “Rest and Digest.”

    • Function: directs housekeeping activities; lowers energy use when relaxed.

      • Effects: Digestion, waste elimination, reduced blood pressure and heart rate, constricted pupils.

    • Origin of Fibers: brain and sacral spinal cord. Preganglionic fibers are long; postganglionic fibers are short.

      • Ganglia located in or near the effector organs.

    • Cranial Portions of the Parasympathetic Division:

      • Supplies fibers to the head, neck, thoracic & abdominal region.

        • Nerves involved: Oculomotor, Facial, Glossopharyngeal, and Vagus nerves.

        • Cardiac Plexus: innervates heart to slow heart rate.

        • Pulmonary Plexus: controls lung functions including respiratory rate.

  • Sacral Portion of Parasympathetic Division: - Forms pelvic splanchnic nerves serving pelvic organs and the distal portion of the large intestine.

  • Sympathetic Division:

    • Known as “Fight or Flight.”

    • Activated during emergencies; mobilizes the body.

      • Effects: Constricts visceral blood vessels, dilates bronchioles, increases blood glucose.

    • Origin of Fibers: thoracolumbar region of spinal cord (T1-L2). Preganglionic fibers short; postganglionic fibers long.

    • Ganglia Location: close to the spinal cord in the sympathetic trunk.

    • Complex innervation patterns with multiple synapse options - at the same level, higher or lower, or at distant collateral ganglia.

Visceral Reflex Arcs

  • Visceral reflex arcs consist of 5 components:

    • 1) Receptor: Detects stimulus in the viscera.

    • 2) Sensory Neurons: Send afferent messages to CNS.

    • 3) Integration Center: Interprets info and decides on a response; may include preganglionic neurons or interneurons.

    • 4) Motor Neurons: Form a two-chain system with pre- and postganglionic neurons.

    • 5) Visceral Effector: Smooth or cardiac muscle or glands that carry out the response.

ANS Neurotransmitters

  • Acetylcholine (ACh):

    • Effects vary depending on receptor type bound.

    • Released by all ANS preganglionic axons and all parasympathetic postganglionic axons.

  • Cholinergic Receptors:

    • 1) Nicotinic Receptors: Always stimulatory; found on postganglionic neurons and neuromuscular junctions.

    • 2) Muscarinic Receptors: Can be either stimulatory or inhibitory; found on parasympathetic and some sympathetic effectors.

  • Norepinephrine (NE):

    • Released by sympathetic postganglionic axons; effects depend on receptor type.

    • Adrenergic Receptors:

      • 1) Alpha receptors: Located on sympathetic target organs.

      • 2) Beta receptors: Located on heart (increase activity) and lungs (dilation).

Key Functions of Autonomic Divisions

  • Dually Innervated Organs: Most have both sympathetic and parasympathetic fibers. The balance affects organ function and activity.

  • Tonal Control: Sympathetic tone maintains blood vessel constriction; parasympathetic tone maintains organ function.

  • Homeostatic Imbalances: Conditions like hypertension, Raynaud’s disease, and autonomic dysreflexia relate to uncontrolled autonomic response.