Autonomic Nervous System

Autonomic Nervous System (ANS) Overview

  • Function: Regulates involuntary control of body functions including:

    • Blood pressure

    • Heart rate

    • Digestive and urinary function

  • Components:

    • Central Nervous System (CNS): Brain and spinal cord

    • Peripheral Nervous System (PNS): Somatic and autonomic divisions

Efferent Innervation

  • Somatic vs Visceral: Major differences in pathways

    • Somatic has one neuron pathway, voluntary control over skeletal muscles.

    • Visceral has two neurons in its pathway (preganglionic and postganglionic) for involuntary control over cardiac muscles, smooth muscles, and glands.

Structure of the ANS

  • Preganglionic Neuron:

    • Myelinated fibers

    • Release Acetylcholine (ACh)

  • Postganglionic Neuron:

    • Unmyelinated fibers

    • Release ACh or Norepinephrine (NE) based on the system (Sympathetic or Parasympathetic)

Divisions of the ANS

  • Sympathetic Division (Fight or Flight)

    • Preganglionic neurons originate in the lateral horns of the thoracic spinal cord.

    • Functions:

    • Arousal, stress, danger response

    • Increases physical activity, heart rate, blood flow to muscles

  • Parasympathetic Division (Rest and Digest)

    • Preganglionic neurons in brainstem (CN III, VII, IX, and X) and S2-S4 spinal cord.

    • Functions:

    • Conservation of energy, nutrient replenishment

    • Decreases heart rate, increases digestive activity

Neurotransmitters and Receptors

  • Sympathetic System:

    • Preganglionic: ACh (Nicotinic receptors)

    • Postganglionic: NE (Adrenergic receptors: alpha and beta) or ACh in some cases -> Cholinergic fibers (muscarinic receptors)

  • Parasympathetic System:

    • Preganglionic: ACh

    • Postganglionic: ACh (specific effects on target organs)

    • Receptors: Nicotinic and Muscarinic

ANS Comparison

Feature

Sympathetic

Parasympathetic

Type of control

Involuntary

Involuntary

Preganglionic Neurons

Short, T1-L2 spinal cord

Long, brainstem and S2-S4

Postganglionic Neurons

Long, to target organs

Short, near or in target organs

Neurotransmitter

ACh (preganglionic), NE

ACh (both pre- and post-ganglionic)

Function

Fight or flight

Rest and digest

Specific Effects of ANS

  • Parasympathetic Effects:

    • Eye accommodation (Ciliary muscle)

    • Decreased heart rate

    • Increased GI motility and secretion

    • Urinary bladder contraction for micturition

  • Sympathetic Effects:

    • Pupil dilation, increased heart rate, bronchodilation, decreased GI activity, and glycogen breakdown in the liver

Control by Higher Brain Centers

  • Regulation:

    • Integration of sensory input and modification of autonomic activity occurs mainly in:

    • Cerebral cortex

    • Medulla oblongata

    • Hypothalamus (hungry/thirst responses)

    • Limbic system (emotional responses like blushing)

Disorders of the ANS

  • Horner’s Syndrome: Unilateral pupillary constriction, sagging eyelid, flushing due to sympathetic lesions.

  • Raynaud Disease: Excessive vasoconstriction leading to pallor and pain in digits, often triggered by cold stress, common in young women.

Aging Effects on ANS

  • Increased sympathetic activity.

  • Higher risk for hypertension and cardiovascular diseases due to increased sympathetic tone and decreased parasympathetic response.