Chapter 14: The Autonomic Nervous System and Homeostasis – Study Notes

Autonomic Nervous System (ANS) Overview

Definition and Function

The autonomic nervous system (ANS) is the involuntary branch of the peripheral nervous system (PNS), also called the visceral motor division. It regulates vital body functions without conscious control.

• Divisions: The ANS consists of the sympathetic and parasympathetic nervous systems, which work together to maintain homeostasis.

• Vital Functions: Oversees heart rate, blood pressure, digestion, and urinary processes.

Visceral Reflex Arcs

The ANS manages vital processes through visceral reflex arcs, which are predictable motor responses to sensory stimuli.

• Afferent sensory neurons carry signals from viscera and skin to the CNS.

• Efferent neurons send motor impulses from the CNS to target organs via autonomic ganglia.

• Autonomic ganglia relay signals to various target organs, triggering motor responses.

• Example: Regulation of digestive tract motility.

Comparison of Somatic and Autonomic Nervous Systems

Main Differences

The motor divisions of the PNS differ in their structure, targets, and control mechanisms.

• Somatic motor division: Innervates skeletal muscle; produces voluntary muscle contractions.

• Autonomic motor division: Innervates smooth muscle, cardiac muscle, and glands; produces involuntary actions.

Neuronal Pathways

• Somatic motor neurons: Directly innervate target muscle fibers.

• ANS motor neurons: Use a two-neuron circuit:

  • Preganglionic neuron: Cell body in CNS; axon releases acetylcholine.

  • Postganglionic neuron: Cell body in autonomic ganglion; axon travels to target cells, releasing acetylcholine or norepinephrine.

Table: Comparison of Somatic and Autonomic Nervous Systems

Feature	Somatic Nervous System	Autonomic Nervous System
Structure	Single neuron from CNS to muscle	Two-neuron chain (preganglionic and postganglionic)
Target	Skeletal muscle fibers	Cardiac muscle, smooth muscle, glands
Control	Voluntary	Involuntary

Divisions of the Autonomic Nervous System

Sympathetic Nervous System

The sympathetic nervous system prepares the body for emergency situations and physical activity ("fight or flight").

• Preganglionic axons: Usually short; originate in thoracic and upper lumbar spinal cord (thoracolumbar division).

• Postganglionic axons: Usually long; synapse in ganglia near the spinal cord.

• Functions: Increases heart rate, blood pressure, and mediates responses to emotion.

Parasympathetic Nervous System

The parasympathetic nervous system maintains body functions at rest ("rest and digest").

• Preganglionic axons: Usually long; originate in brainstem and sacral spinal cord (craniosacral division).

• Postganglionic axons: Usually short; synapse near or within target organs.

• Functions: Promotes digestion, urinary function, and homeostasis at rest.

Balance and Homeostasis

• Actions of parasympathetic division antagonize those of sympathetic division.

• Together, they maintain a delicate balance to ensure homeostasis.

Gross and Microscopic Anatomy of the Sympathetic Nervous System

Sympathetic Chain Ganglia

Sympathetic chain ganglia run parallel to the vertebral column and are the site of synapse for most postganglionic cell bodies.

• Sections above thoracic spinal cord terminate in superior cervical ganglion.

• Sections below lumbar spinal cord terminate in inferior sacral ganglion.

Preganglionic Neurons and Rami Communicantes

• Preganglionic neurons originate in thoracic and lumbar spinal cord.

• Preganglionic axons exit via white (myelinated) rami communicantes to reach sympathetic chain ganglia.

• Postganglionic axons exit ganglia via gray (unmyelinated) rami communicantes and travel with spinal nerves to target cells.

Synapse Locations

• Preganglionic axons may synapse at the same level, ascend/descend to other levels, or pass through to collateral ganglia near target organs.

• Axons synapsing in collateral ganglia near abdominopelvic organs are part of splanchnic nerves.

Sympathetic Neurotransmitters and Receptors

Neurotransmitters

• Acetylcholine (ACh): Released by preganglionic axons; excitatory at synapses with postganglionic neurons.

• Norepinephrine (NE) and Epinephrine (adrenaline): Released by postganglionic axons at target cells; mediate most sympathetic effects.

Receptor Types

• Adrenergic receptors: Bind epinephrine and norepinephrine; subdivided into alpha and beta types.

• Alpha-1 receptors: Found on smooth muscle cells; cause contraction (e.g., blood vessels).

• Alpha-2 receptors: Found on plasma membranes of preganglionic sympathetic neurons; involved in negative feedback.

• Beta-1 receptors: Found on cardiac muscle cells; increase heart rate and force.

• Beta-2 receptors: Found on smooth muscle cells of respiratory tract, urinary bladder, and some glands.

• Beta-3 receptors: Found on adipose cells and smooth muscle cells in digestive tract.

• Cholinergic receptors: Bind acetylcholine; include muscarinic (sweat glands) and nicotinic (postganglionic neurons, adrenal medullae).

Table: Sympathetic Receptor Types and Locations

Receptor Type	Location	Main Effect
Alpha-1	Smooth muscle (blood vessels, GI tract)	Contraction
Alpha-2	Preganglionic neuron membranes	Negative feedback, inhibits excess output
Beta-1	Cardiac muscle	Increases heart rate and force
Beta-2	Respiratory tract, urinary bladder, glands	Relaxation, dilation
Beta-3	Adipose tissue, digestive tract	Lipid breakdown, smooth muscle relaxation
Muscarinic	Sweat glands	Secretion
Nicotinic	Postganglionic neurons, adrenal medullae	Excitation

Effects of Sympathetic Nervous System on Target Cells

Cardiac Muscle Cells

• Norepinephrine binds to beta-1 receptors, increasing heart rate and force of contraction.

• Blood pressure and blood delivery to tissues increase during physical activity.

Additional info:

• Sympathetic activation also affects smooth muscle (vasoconstriction/vasodilation), glandular secretion, metabolism, and other organs to support "fight or flight" responses.