Autonomics – Visceral Motor System

Introduction

The autonomic (visceral motor) system is a critical component of the peripheral nervous system (PNS) responsible for regulating involuntary bodily functions. This system works alongside the somatic motor system, which controls voluntary movements, to maintain homeostasis and respond to internal and external stimuli.

Somatic vs. Autonomic Nervous Systems

Overview and Definitions

• Somatic Nervous System: Controls voluntary movements by innervating skeletal muscles. It consists of a single motor neuron extending from the central nervous system (CNS) directly to the target muscle.

• Autonomic Nervous System (ANS): Regulates involuntary functions by innervating smooth muscle, cardiac muscle, and glands. It uses a two-neuron chain: a preganglionic neuron (originating in the CNS) and a postganglionic neuron (located in a peripheral ganglion).

Key Differences:

• Somatic motor pathway: One neuron from CNS to muscle.

• Autonomic motor pathway: Two neurons (preganglionic and postganglionic) from CNS to effector.

• Somatic system uses acetylcholine as its neurotransmitter; autonomic system uses acetylcholine and norepinephrine, depending on the division.

Somatic Motor System

• Targets skeletal muscle for voluntary movement.

• Cell body of the motor neuron is located in the ventral horn of the spinal cord.

• Neurotransmitter: Acetylcholine (ACh).

• Pathway: One long, myelinated axon extends from the CNS to the muscle.

• Example: The brachial plexus innervates the upper limb muscles via nerves such as the axillary, radial, and ulnar nerves.

Autonomic (Visceral Motor) System

• Targets smooth muscle, cardiac muscle, and glands for involuntary control.

• Consists of a two-neuron chain: preganglionic neuron (cell body in CNS) and postganglionic neuron (cell body in peripheral ganglion).

• Neurotransmitters: Acetylcholine (ACh) and Norepinephrine (NE).

• Pathway: Preganglionic axon (myelinated) synapses in a ganglion; postganglionic axon (unmyelinated) extends to the effector organ.

Divisions of the Autonomic Nervous System

Sympathetic Division

The sympathetic division prepares the body for stressful or emergency situations, often referred to as the "fight, flight, or fright" response.

• Originates from thoracic and lumbar regions of the spinal cord (T1–L2).

• Preganglionic neurons are short; postganglionic neurons are long.

• Functions:

  • Increases heart rate and breathing rate.

  • Dilates pupils and airways.

  • Inhibits motility of digestive and urinary tracts.

  • Increases blood flow to skeletal muscles.

• Neurotransmitters: Preganglionic neurons release acetylcholine; postganglionic neurons typically release norepinephrine.

Parasympathetic Division

The parasympathetic division is active during restful periods and is responsible for conserving energy and overseeing "housekeeping" functions.

• Originates from cranial (brainstem) and sacral regions of the CNS.

• Preganglionic neurons are long; postganglionic neurons are short and located near or within the target organ.

• Functions:

  • Decreases heart rate and breathing rate.

  • Constricts pupils.

  • Stimulates digestion and urinary activity.

• Neurotransmitter: Both pre- and postganglionic neurons release acetylcholine.

Autonomic Pathways and Ganglia

Parasympathetic Pathways

• Cranial Outflow: Preganglionic fibers originate in the brain and travel via cranial nerves III, VII, IX, and X to synapse in ganglia near or within target organs (e.g., ciliary, pterygopalatine, submandibular, otic ganglia).

• Sacral Outflow: Preganglionic fibers arise from sacral spinal cord segments (S2–S4) and form pelvic splanchnic nerves, which innervate lower abdominal and pelvic organs.

• Parasympathetic ganglia are located in or near the target organs (intramural ganglia).

Sympathetic Pathways

• Sympathetic Trunk (Chain) Ganglia: Paired ganglia running alongside the vertebral column; connected by short nerves to form sympathetic trunks.

• Prevertebral (Collateral) Ganglia: Unpaired ganglia located anterior to the vertebral column, usually near the aorta (e.g., celiac, superior mesenteric, inferior mesenteric ganglia).

• White Rami Communicantes: Carry preganglionic fibers into the sympathetic trunk ganglia.

• Gray Rami Communicantes: Carry postganglionic fibers to peripheral structures.

Sympathetic Pathways: Three Routes

1. Synapse at the same level in the sympathetic trunk ganglion and exit via a spinal nerve.

2. Ascend or descend in the sympathetic trunk to synapse at a different level.

3. Pass through the sympathetic trunk, exit via a splanchnic nerve, and synapse in a collateral ganglion.

Note: Pelvic splanchnic nerves are parasympathetic, while thoracic and lumbar splanchnic nerves are sympathetic.

Visceral Reflexes

General Features

• Visceral reflexes are similar to somatic reflexes but involve autonomic pathways.

• They regulate involuntary functions such as heart rate, blood pressure, and digestion.

• Components: Sensory receptor, afferent neuron, integration center, efferent neuron, and effector.

Summary Table: Somatic vs. Autonomic Nervous System

Key Terms and Concepts

• Preganglionic neuron: The first neuron in the autonomic pathway, originating in the CNS.

• Postganglionic neuron: The second neuron, located in a peripheral ganglion, innervates the target organ.

• Ganglion: A cluster of neuron cell bodies outside the CNS.

• Splanchnic nerves: Nerves that carry autonomic fibers to visceral organs.

• Acetylcholine (ACh): Neurotransmitter used by all somatic motor neurons and parasympathetic postganglionic neurons.

• Norepinephrine (NE): Neurotransmitter used by most sympathetic postganglionic neurons.

Relevant Equations

• Nerve impulse transmission: (Ohm's Law, where is voltage, is current, and is resistance)

• Neurotransmitter release:

Additional info: The study notes have expanded upon the brief points and images in the slides to provide a comprehensive overview suitable for college-level Anatomy & Physiology students. All major topics and pathways have been included for exam preparation.