The Autonomic Nervous System

Neural Control of Involuntary Effectors

• Autonomic Motor Circuit:
  • Utilizes a two-neuron pathway from the central nervous system (CNS) to involuntary visceral effectors (cardiac muscle, smooth muscle, and glands).
  • Preganglionic neuron: Cell body originates in the midbrain, hindbrain, or the thoracic, lumbar, or sacral regions of the spinal cord.
  • Postganglionic neuron: Cell body resides within an autonomic ganglion in the peripheral nervous system (PNS) and projects to the target organ.

• Somatic vs. Autonomic Motor Comparison:
  • Effector Organs: Somatic innervates skeletal muscle; Autonomic innervates cardiac muscle, smooth muscle, and glands.
  • Effector Neurons: Somatic uses one neuron; Autonomic uses two neurons (preganglionic and postganglionic).
  • Nerve Fiber Characteristics:
  • Somatic: Fast-conducting, thick ($9\text{ to }13\,\mu\text{m}$), myelinated.
  • Autonomic: Preganglionic fibers are lightly myelinated and thin ($3\,\mu\text{m}$); postganglionic fibers are unmyelinated and very thin (about $1.0\,\mu\text{m}$).
  • Synaptic Structure: Somatic forms a specialized motor end plate; Autonomic lacks specialized postsynaptic membranes, releasing neurotransmitters via varicosities (synapses en passant).
  • Effect of Denervation: Somatic loss leads to flaccid paralysis and atrophy; Autonomic loss preserves muscle tone/function and induces denervation hypersensitivity.
  • Action: Somatic is excitatory only; Autonomic is either excitatory or inhibitory.

Divisions of the Autonomic Nervous System

• Sympathetic (Thoracolumbar) Division:

  • Preganglionic neurons originate in the thoracic and lumbar spinal cord ($\text{T1}$ to $\text{L2}$).
  • Synapse in parallel paravertebral ganglia (forming the sympathetic chain) or in collateral (prevertebral) ganglia (celiac, superior mesenteric, and inferior mesenteric ganglia via splanchnic nerves).
  • Exhibits specific neural circuitry:
  • Divergence: One preganglionic neuron synapses onto several postganglionic neurons.
  • Convergence: Several preganglionic neurons synapse onto a single postganglionic neuron.
  • Facilitates mass activation as a single unit during emergency or "fight or flight" responses.
  • Adrenal Medulla: Modified sympathetic ganglion directly innervated by preganglionic neurons; secretes epinephrine and norepinephrine into blood circulation as part of the sympathoadrenal system.

• Parasympathetic (Craniosacral) Division:

  • Preganglionic neurons originate in the brainstem or sacral spinal cord ($\text{S2}$ to $\text{S4}$) and synapse in terminal ganglia near or within target organs.
  • Cranial Pathways:
  • Oculomotor Nerve (CN III): Synapses in ciliary ganglion; innervates ciliary muscle and iris.
  • Facial Nerve (CN VII): Synapses in pterygopalatine and submandibular ganglia; innervates lacrimal, mucous, and salivary glands.
  • Glossopharyngeal Nerve (CN IX): Synapses in otic ganglion; innervates parotid gland.
  • Vagus Nerve (CN X): Branches to terminal ganglia in heart, lungs, esophagus, stomach, liver, pancreas, and intestines.
  • Sacral Pathways: Pelvic spinal nerves ($\text{S2}$ to $\text{S4}$) innervate the lower large intestine, rectum, urinary, and reproductive organs.

Neurotransmitters and Receptor Mechanisms

• Neurotransmitter Classification:
  • Cholinergic: Acetylcholine ($\text{ACh}$) is released by all preganglionic neurons (sympathetic and parasympathetic), most parasympathetic postganglionic neurons, and sympathetic postganglionic neurons innervating sweat glands and skeletal muscle blood vessels.
  • Adrenergic: Norepinephrine ($\text{NE}$) is released by most sympathetic postganglionic neurons.

• Adrenergic Receptors:

  • Operate via G-proteins and second messenger systems.
  • $\alpha_1$ Receptors: Mediate smooth muscle contraction, vasoconstriction in skin/viscera, radial muscle contraction (pupil dilation), and digestive sphincter contraction; operate via a $\text{Ca}^{2+}$ second messenger.
  • $\alpha_2$ Receptors: Located on presynaptic axon terminals; inhibit $\text{NE}$ release via negative feedback.
  • $\beta_1$ Receptors: Located primarily in the heart; increase heart rate and contractility; operate via $\text{cAMP}$.
  • $\beta_2$ Receptors: Mediate smooth muscle relaxation, bronchodilation, and skeletal muscle arteriole dilation; operate via $\text{cAMP}$.
  • $\beta_3$ Receptors: Located in adipose tissue; stimulate lipolysis and heat production.

• Cholinergic Receptors:

  • Nicotinic Receptors:
  • Located in autonomic ganglia and neuromuscular junctions.
  • Function as ligand-gated cation channels ($\text{Na}^+/\text{K}^+$); always excitatory.
  • Stimulated by nicotine; blocked by curare or $\text{D-tubocurarine}$.
  • Muscarinic Receptors:
  • Located in visceral effector organs ($\text{M}_1$ through $\text{M}_5$).
  • G-protein-coupled receptors; can be excitatory or inhibitory.
  • $\text{M}_3$/$\text{M}_5$ cause smooth muscle contraction and gland secretion via opening $\text{Ca}^{2+}$ channels; $\text{M}_2$ causes hyperpolarization and slows heart rate via opening $\text{K}^+$ channels.
  • Blocked by atropine.

Autonomic Regulation and Higher Control

• Dual Innervation Actions:

  • Antagonistic: Sympathetic and parasympathetic divisions exert opposing effects (e.g., heart rate, gastrointestinal motility, pupil constriction vs. dilation).
  • Complementary: Both divisions produce similar functional outcomes (e.g., salivary secretion where parasympathetic stimulates watery fluid and sympathetic induces thick secretion).
  • Cooperative: Distinct effects work together toward a single outcome (e.g., parasympathetic causes erection, sympathetic causes ejaculation).

• Single Innervation (Sympathetic Only):

  • Adrenal medulla, arrector pili muscles, sweat glands, and most blood vessels receive sympathetic innervation only.
  • Regulated strictly through increases or decreases in sympathetic firing rates.

• Central Control:

  • Medulla Oblongata: Primary center integrating autonomic reflexes for cardiovascular, pulmonary, digestive, and reproductive systems.
  • Hypothalamus: Major regulatory center governing body temperature, hunger, thirst, and pituitary activity.
  • Limbic System: Directs autonomic responses associated with emotional states.
  • Aging: Correlated with elevated sympathetic activity and tone, increasing risks for hypertension and cardiovascular diseases.