Lecture 5 Autonomic Nervous System Overview

Autonomic Nervous System (ANS)

• Controls body functions automatically and involuntarily
• Maintains homeostasis through reflexes
• Controls smooth muscle, cardiac muscle, and glands
• Influences various bodily functions including respiration, heart rate, blood pressure, digestion, sleep, arousal, and body temperature

Structure and Function of ANS

• Sensory cells: Specialized receptors for specific stimuli (e.g., baroreceptors for pressure, thermosensors for temperature)
• Sensory nerve fibers: Similar to somatic nervous system
• Control centers:
  • Hypothalamus: Master command center
  • Brain stem: Specific control functions (e.g., respiratory center)
  • Spinal cord: Local integration in thoracic and sacral segments
• Motor nerve fibers:
  • Differ from somatic motor system, target organ (smooth muscle, cardiac muscle, glands)
  • Two neurons involved in peripheral circuit:
    • Preganglionic neuron: Cell body in CNS, axon innervates postganglionic neuron
    • Postganglionic neuron: Cell Body in ganglion, axon innervates effector organ

Major Divisions of the ANS

• Sympathetic (thoracolumbar): Activated during stress, preganglionic neurons originate in thoracic or lumbar spinal area
• Parasympathetic (cranio-sacral): Most active during rest, preganglionic neurons originate in brain or sacral spinal cord
• Enteric Nervous System: Associated with parasympathetic

Sympathetic System

• Preganglionic axons enter the paravertebral sympathetic ganglion chain
• Postganglionic fibers are long, innervate hollow organs, blood vessels, etc.
• Some preganglionic fibers extend to the adrenal medulla, stimulating secretion of epinephrine and norepinephrine into the blood

Parasympathetic System

• Long preganglionic fibers: Cranial nerves III, VII, IX, and X supply parasympathetic fibers to glands and organs, Vagus nerve being a major supplier
• Ganglia located in the wall or adjacent to target organs, no interconnection
• Short postganglionic fibers, direct connection with specific organs, no enhancement by hormones

Communication Within ANS

• Neurotransmitters:
  • Acetylcholine: Released by preganglionic neurons of both sympathetic and parasympathetic divisions
  • Norepinephrine: Released by postganglionic neurons of the sympathetic division
  • Epinephrine: Released by adrenal medulla (sympathetic)

Receptors to ANS Neurotransmitters

• Acetylcholine receptors:
  • Nicotinic: Located at preganglionic synapses, ionotropic
  • Muscarinic: Located in target organs of parasympathetic, metabotropic
• (Nor)epinephrine receptors (adrenergic): Metabotropic
  • Alpha-adrenergic:
    • Alpha 1: Increases intracellular calcium, usually smooth muscle constriction
    • Alpha 2: Decreases cAMP, usually smooth muscle constriction
  • Beta-adrenergic:
    • Beta 1 and Beta 2: Increases cAMP, main form in heart, relaxation of smooth muscles

Pharmacological Significance

• Agonists: Drugs that activate neurotransmitter receptors
• Antagonists: Drugs that block neurotransmitter receptors
• Hypertension treatment: Beta-adrenergic blockers (Propranolol) reduce heart rate
• Asthma treatment: Beta-adrenergic agonists (salbutamol) dilate bronchioles
• Atropine: Muscarinic receptor antagonist, dilates pupils, inhibits contraction of lower GI tract, reduces mucus production in the respiratory tract

Reciprocal Effects of Sympathetic and Parasympathetic Systems

• Effectors:
  • Eye: Sympathetic dilation, Parasympathetic constriction
  • Salivary gland: Sympathetic stimulation, Parasympathetic inhibition
  • Heart rate: Sympathetic increased, Parasympathetic decreased
  • Blood vessels: Sympathetic constriction (most), Parasympathetic dilation (few)
  • Lungs (bronchiole): Sympathetic dilation, Parasympathetic constriction
  • Intestinal mobility: Sympathetic inhibition, Parasympathetic stimulation
  • Sphincters: Sympathetic closing, Parasympathetic opening
  • Urinary bladder: Sympathetic relaxation, Parasympathetic contraction
  • Penis: Sympathetic ejaculation, Parasympathetic erection

### Autonomic Nervous System

• Controls bodily functions without conscious thought (automatically, involuntarily), a key role in homeostasis
• Achieved via reflexes - autonomic reflexes
• Works on smooth muscle, cardiac muscle, and glands
• Responsible for respiration, heart rate, blood pressure, digestion, rumination, sleep, arousal, and body temperature

Autonomic Reflexes

• Sensory cells (baroreceptors, cold and warm receptors, etc...) - dependent on the system
• Sensory nerve fibers: similar to somatic arc reflexes
• Control centers: hypothalamus, brain stem, spinal cord
  • Hypothalamus: master command
  • Brain stem: special control function (respiratory center)
  • Spinal cord: local integration
• Receive continuous information from sensory nerves and compare it to a set reference, if different, alter frequency on the motor fiber.

Motor Nerve Fibers

• Differ from somatic motor systems:
  • Target organ (Smooth muscle, cardiac muscle, glands)
  • Number of neurons in its peripheral circuit: two peripheral nerves (vs. 1 in somatic arc reflex)
    • Preganglionic neuron: cell body in CNS, axon innervates second neuron (postganglionic neuron)
    • Postganglionic neuron: cell body in a ganglion

Major Divisions of the ANS

• Sympathetic (thoraco-lumbar): activated during stress, preganglionic neurons originate in thoracic or lumbar spinal area
• Parasympathetic (cranio-sacral): most active during rest, preganglionic neurons originate in the brain or sacral part of the spinal cord
• Enteric Nervous System: associated with the parasympathetic

Sympathetic System

• Outflow from thoracolumbar spinal cord, preganglionic axons enter the paravertebral sympathetic ganglion chain
• Postganglionic fibers are very long, innervate hollow organs, blood vessels, etc.
• Some preganglionic fibers extend to the adrenal medulla, stimulating the secretion of epinephrine and norepinephrine into the bloodstream

Parasympathetic System

• Very long preganglionic fibers, cranial nerves III, VII, IX, and X supply parasympathetic fibers, X (vagus nerve) is the major supplier
• Ganglia: located in the wall or adjacent to target organs, no interconnection between ganglia
• Postganglionic fibers: short axons, direct connection with specific organs with no branching off or enhancement by hormones

Receptors to ANS Neurotransmitters

• Acetylcholine:

  • Nicotinic: preganglionic synapses, ionotropic, part of an ion channel (here: Na+)
  • Muscarinic: in target organs of parasympathetic, separate from ion channel, requires intracellular second messengers, slower, and more prolonged effect than nicotinic

• (Nor)Adrenalin (Adrenergic): separate receptors associated with G-proteins, activate intracellular second messengers

  • α-adrenergic:
    • α1 =  Ca2+
    • α2 =  cAMP
    • Usually constriction of smooth muscles (vascular)
  • β-adrenergic:
    • β1 and β2 = cAMP
    • Main form in the heart   contraction strength & rate
    • Relaxation of smooth muscles (GI tract, uterus, bronchioles)

Pharmacological Significance

• Use of agonists and antagonists to selectively activate or block receptors
• Hypertension:
  • β-adrenergic blocker (Propranolol) =  heart rate
• Asthma:
  • β2-adrenergic agonist (salbutamol) = bronchodilation
• Atropine:
  • Muscarinic receptor blocker
  • Drop in eye = dilation of pupils
  • Inhibits contraction of lower GI tract
  • Reduces mucus production in respiratory tract

Reciprocal Effects

• Effectors:
  • Eye:
    • Sympathetic: Pupil dilation
    • Parasympathetic: Pupil constriction
  • Salivary gland:
    • Sympathetic: Stimulation
    • Parasympathetic: Inhibition
  • Heart Rate:
    • Sympathetic: Increased
    • Parasympathetic: Decreased
  • Blood Vessels:
    • Sympathetic: Constriction (most)
    • Parasympathetic: Dilation (few)
  • Lungs (Bronchiole):
    • Sympathetic: Dilation
    • Parasympathetic: Constriction
  • Intestinal Mobility:
    • Sympathetic: Inhibition
    • Parasympathetic: Stimulation
  • Sphincters:
    • Sympathetic: Stimulate closing
    • Parasympathetic: Inhibit closing
  • Urinary Bladder:
    • Sympathetic: Muscle tone relaxed
    • Parasympathetic: Contraction
  • Penis:
    • Sympathetic: Ejaculation
    • Parasympathetic: Erection