Autonomic Nervous System -

AUTONOMIC NERVOUS SYSTEM

Overview

  • Specialized subdivision of the peripheral nervous system.

  • Functions as a neurological negative feedback mechanism.

  • Two Main Divisions: Sympathetic (panic) and Parasympathetic (calm down).

Functions of the ANS

  • Regulates unconscious physiological processes:

    • Heart rate

    • Blood pressure

    • Respiration

    • Digestion

    • Sexual arousal

  • Motor only.

Stimulation vs. Inhibition

  • Stimulation (Excitation): ON/Yes.

  • Inhibition: OFF/No.

  • ANS is regulated by alternating stimulation/inhibition.

  • Examples:

    • Sympathetic excitation: dilating pupils in response to danger.

    • Parasympathetic inhibition: slowing heart rate when falling asleep.

Pre vs. Post Ganglionic Fibers

  • Ganglion: cluster of cell bodies in PNS, site of synapse. ANS has ganglia as extra stops along motor pathways.

  • Pre-Ganglionic Fibers: (before ganglion)

    • Short in SNS, long in PSNS.

    • Lightly myelinated.

  • Post-Ganglionic Fibers: (after ganglion)

    • Long in SNS, short in PSNS.

    • Unmyelinated.

Major Differences Between SNS and PSNS

  • SNS (Sympathetic):

    • Goal: panic (fight or flight)

    • Fiber origin: cranial nerves, brain stem, lumbar nerves.

    • Long pre-ganglionic, short post-ganglionic.

    • Near effector organs.

    • Neurotransmitters: ACh (ganglia), NE (effector).

  • PSNS (Parasympathetic):

    • Goal: calm down.

    • Fiber origin: thoracic and upper lumbar nerves (T1-L2).

    • Short pre-ganglionic, long post-ganglionic.

    • Near the spine.

    • Neurotransmitter: ACh only.

Organs Affected by SNS and PSNS

  • SNS:

    • Eyes, skin (sweat glands), salivary glands, lungs, heart, stomach, pancreas, liver, adrenal glands, bladder, genitals.

  • PSNS:

    • Eyes, salivary glands, lungs, heart, stomach, pancreas, bladder, genitals.

Parasympathetic Cranial Nerves

  • CN III (Oculomotor): Constricts pupils.

  • CN VII (Facial): Stimulates tears, nasal secretions, saliva.

  • CN IX (Glossopharyngeal): Stimulates saliva production.

  • CN X (Vagus): Slows heart and respiration rate, increases digestion.

Sympathetic Nervous System

  • Paired ganglion near the spinal cord.

  • Runs from T1-L2, fibers reaching the entire body.

  • White vs. Grey Rami Communicans:

    • White: pre-ganglionic (myelinated).

    • Grey: post-ganglionic (unmyelinated).

Sexual Arousal and the ANS

  • Parasympathetic: Increases blood flow and lubricating secretions.

  • Sympathetic: Closes off rectum, allows for orgasm.

  • Physiological arousal is involuntary; can be overridden by the cerebral cortex (conscious brain).

SNS and PSNS Imbalance

  • Both systems are meant to offset each other; excessive sympathetic state can lead to:

    • Anxiety, hyperventilation, insomnia, muscle cramps, gut issues.

  • Parasympathetic hyperactivity is rare; usually presents after head injury with low blood pressure and fatigue.