Autonomic Nervous System: Structure, Function, and Physiology

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Autonomic Nervous System (ANS) Overview

Definition and Divisions

The autonomic nervous system (ANS) is the involuntary arm of the peripheral nervous system (PNS), responsible for regulating vital bodily functions without conscious control. It is also known as the visceral motor division.

Divisions: The ANS is divided into two main branches:
- Sympathetic nervous system: Prepares the body for 'fight or flight' responses.
- Parasympathetic nervous system: Promotes 'rest and digest' activities.
Main functions: Regulation of heart rate, blood pressure, digestive and urinary processes.

Functions of the ANS and Visceral Reflex Arcs

Visceral Reflexes

The ANS manages vital processes through a series of events called visceral reflex arcs, which are sensory stimuli leading to a predictable motor response.

Sensory signals from the viscera and skin are sent by afferent sensory neurons to the brain or spinal cord, where they are integrated by the CNS.
Neural motor impulses from the CNS are sent out via efferent neurons in the cranial and spinal nerves, which usually lead to autonomic ganglia in the PNS.
Autonomic ganglia send impulses via other efferent neurons to various target organs where they trigger a motor response.

Comparison of Somatic and Autonomic Nervous Systems

Main Differences

The somatic and autonomic nervous systems differ in their effectors, neural pathways, and neurotransmitters.

Somatic motor division: Innervates skeletal muscle, causing voluntary muscle contractions and conscious movement.
Autonomic motor division: Innervates smooth muscle, cardiac muscle, and glands, producing involuntary actions.
Neural pathway:
- Preganglionic neuron: The first neuron in the autonomic pathway, with its cell body in the CNS.
- Postganglionic neuron: The second neuron, with its cell body in the autonomic ganglion in the PNS.

Sympathetic and Parasympathetic Nervous Systems

Sympathetic Nervous System

The sympathetic division prepares the body for emergency situations and physical activity. It is also called the thoracolumbar division because its preganglionic cell bodies originate in the thoracic and upper lumbar spinal cord.

Sympathetic ganglia: Located near the spinal cord; postganglionic axons proceed to the target.
"Fight or flight" response: Increases heart rate, blood pressure, and energy mobilization.

Parasympathetic Nervous System

The parasympathetic division maintains homeostasis at rest and is called the craniosacral division because its preganglionic cell bodies are located in the nuclei of several cranial nerves and the sacral region of the spinal cord.

Cranial nerves: Innervate structures of the head, neck, thoracic viscera, and most abdominal viscera.
Sacral nerves: Innervate structures within the pelvic cavity.
Postganglionic neurons: Usually located near the target organ, requiring only a short axon.

Balance Between Divisions

The sympathetic and parasympathetic systems work together to maintain homeostasis, with each division exerting opposite effects on target organs.

Gross and Microscopic Anatomy of the Sympathetic Nervous System

Sympathetic Chain Ganglia

Most postganglionic cell bodies are found in the sympathetic chain ganglia, which run parallel to the vertebral column.

Chain appearance: Extends from the superior cervical ganglion to the inferior ganglion.
Preganglionic axons: Originate in the thoracic and lumbar spinal cord, exit with motor neurons via the anterior root.
Synapse options:
- Synapse with postganglionic cell bodies in the sympathetic chain ganglion at the same spinal level.
- Ascend or descend to synapse at a different spinal level.
- Pass through the chain ganglion to synapse in collateral ganglia near target organs.
Gray (unmyelinated) rami communicantes: Postganglionic axons travel with spinal nerves to reach target cells.

Neurotransmitters and Receptors in the ANS

Classes of Sympathetic Neurotransmitters

Acetylcholine (ACh): Released at synapses between preganglionic axons and postganglionic neurons.
Norepinephrine (noradrenaline): Most frequently released neurotransmitter at synapses between postganglionic axons and target cells.
Epinephrine (adrenaline): Released by the adrenal medulla into the bloodstream.

Adrenergic Receptors

Adrenergic receptors bind to norepinephrine and epinephrine. There are two major types: alpha and beta, each with subtypes.

Alpha-1 receptors: Located in the plasma membranes of smooth muscle cells of many organs, including blood vessels, GI tract, kidneys, arrector pili muscles, and certain organs of the genitourinary tract.
Alpha-2 receptors: Located in the plasma membranes of preganglionic sympathetic neurons and in peripheral target cells.
Beta-1 receptors: Found in cardiac muscle cells, certain kidney cells, and adipose cells.
Beta-2 receptors: Found in the plasma membranes of smooth muscle cells in the airways, urinary bladder, skeletal muscle fibers, and some glands.
Beta-3 receptors: Found primarily in adipose cells and smooth muscle cells in the wall of the digestive tract.

Muscarinic Receptors

Muscarinic receptors respond to acetylcholine and are found on sweat glands in the skin.

Effects of the Sympathetic Nervous System on Target Cells

Cardiac Muscle Cells

Norepinephrine binding to β-1 receptors:
- Opens ion channels, increasing heart rate and force of contraction.
- Increases blood flow to tissues and blood pressure during physical activity.

Effects on Smooth Muscle Cells

Constriction of blood vessels: Occurs when norepinephrine binds to α-1 receptors, decreasing blood flow to digestive, urinary, and integumentary systems.
Dilation of bronchioles: Occurs when norepinephrine binds to β-2 receptors, increasing oxygen intake.
Dilation of blood vessels serving skeletal and cardiac muscle: Occurs when norepinephrine binds to β-2 receptors, increasing blood flow during physical activity.
Contraction of urinary and digestive sphincters: Occurs when norepinephrine binds to β-2 and β-3 receptors, making urination and defecation more difficult during stress.
Relaxation of smooth muscle of the digestive tract: Occurs when norepinephrine binds to β-2 receptors, slowing digestion.
Constriction of blood vessels serving most exocrine glands: Occurs when norepinephrine binds to α-1 receptors, decreasing secretion of saliva (except sweat glands).

Effects on Cellular Metabolic Rate

Binds to β-3 receptors on adipocytes: Triggers breakdown of lipids, releasing fatty acids into the bloodstream.
Binds to β-2 receptors on liver cells: Triggers release of glucose from glycogen and synthesis of glucose from other resources.
Binds to β-2 receptors on pancreatic cells: Triggers release of glucagon, increasing blood glucose levels.

Effects on Secretion from Sweat Glands

Postganglionic sympathetic neurons release ACh: Stimulates sweat gland cells in the skin.
This is part of a negative feedback loop that corrects elevated body temperature.

Effects on Cells of the Adrenal Medulla

Adrenal medulla: Sits on top of each kidney and is composed of modified sympathetic postganglionic neurons.
Functions:
- ACh released from preganglionic neurons binds to nicotinic receptors on the adrenal medulla cells.
- Adrenal medulla cells release norepinephrine and epinephrine into the bloodstream.
- These hormones act as chemical messengers and are considered hormones rather than neurotransmitters.

Pharmacology and Sympathetic Nervous System Receptors

Different subtypes of sympathetic nervous system receptors are targets for medication therapy for diseases such as hypertension and asthma.

Summary Table: Adrenergic Receptor Types and Locations

Receptor Type	Location	Main Effect
Alpha-1	Smooth muscle cells of blood vessels, GI tract, kidneys, arrector pili, genitourinary tract	Vasoconstriction, increased blood pressure
Alpha-2	Preganglionic sympathetic neurons, peripheral target cells	Inhibits neurotransmitter release, negative feedback
Beta-1	Cardiac muscle cells, kidney cells, adipose cells	Increased heart rate and contractility
Beta-2	Airway smooth muscle, urinary bladder, skeletal muscle, glands	Bronchodilation, vasodilation
Beta-3	Adipose cells, digestive tract smooth muscle	Lipolysis, relaxation of digestive tract

Key Equations

Blood Pressure Regulation:
Heart Rate Regulation:

Additional info:

Some context and terminology were inferred from standard Anatomy & Physiology textbooks to clarify fragmented or abbreviated points.
Table entries and receptor effects were expanded for completeness and clarity.