Autonomic Nervous System: Structure, Function, and Neurotransmitters

Autonomic Nervous System (ANS): Overview

The Autonomic Nervous System (ANS) is a division of the peripheral nervous system that regulates involuntary physiological processes, including heart rate, blood pressure, respiration, digestion, and glandular activity. It operates subconsciously to maintain homeostasis and respond to stress.

• Innervates smooth and cardiac muscle and glands

• Maintains optimal support for body activities

• Operates via subconscious control

• Has viscera as most of its effectors

Somatic vs. Autonomic Nervous Systems

Key Differences

• Effectors:

  • Somatic Nervous System (SNS): Skeletal muscles

  • ANS: Cardiac muscle, smooth muscle, glands

• Efferent Pathways:

  • SNS: Heavily myelinated axons extend from the CNS to the effector

  • ANS: Two-neuron chain:

    1. Preganglionic neuron: Lightly myelinated axon from CNS to ganglion

    2. Postganglionic neuron: Unmyelinated axon from ganglion to effector

• Neurotransmitter Response:

  • SNS: All somatic motor neurons release acetylcholine (ACh), which has an excitatory effect

  • ANS: Preganglionic fibers release ACh; postganglionic fibers release ACh or norepinephrine (NE), with effects that may be excitatory or inhibitory depending on the receptor

Table: Comparison of Somatic and Autonomic Nervous Systems

Feature	Somatic Nervous System	Autonomic Nervous System
Effectors	Skeletal muscle	Cardiac muscle, smooth muscle, glands
Efferent Pathways	Heavily myelinated axons from CNS to effector	2-neuron chain: preganglionic (lightly myelinated) and postganglionic (unmyelinated)
Response to Neurotransmitters	ACh has excitatory effect	Response depends on neurotransmitter released and receptor type

Divisions of the Autonomic Nervous System

Sympathetic vs. Parasympathetic

The ANS is divided into two main parts: the sympathetic and parasympathetic divisions. These divisions generally have opposing effects and maintain homeostasis through dynamic antagonism.

• Sympathetic Division: Mobilizes the body during emergency situations ("fight-or-flight" response)

  • Increases heart rate, dilates pupils, inhibits digestion

  • Promotes blood flow to muscles, increases respiratory rate

  • Skin is cold and sweaty

• Parasympathetic Division: Conserves energy and promotes housekeeping functions ("rest-and-digest")

  • Decreases heart rate, constricts pupils, stimulates digestion

  • Promotes elimination and absorption activities

  • Skin is warm, pupils are constricted

Anatomy of the ANS: Sympathetic and Parasympathetic Pathways

Origin and Ganglia

• Sympathetic: Thoracolumbar origin (spinal cord segments T1-L2)

  • Short preganglionic and long postganglionic fibers

  • Ganglia close to spinal cord (sympathetic trunk or chain ganglia)

• Parasympathetic: Craniosacral origin (brainstem and sacral spinal cord)

  • Long preganglionic and short postganglionic fibers

  • Ganglia in or near effector organs (terminal ganglia)

Table: Sympathetic vs. Parasympathetic Pathways

Division	Origin	Preganglionic Fiber	Postganglionic Fiber	Ganglia Location
Sympathetic	Thoracolumbar (T1-L2)	Short	Long	Near spinal cord (sympathetic chain, collateral ganglia)
Parasympathetic	Craniosacral (brainstem, sacral spinal cord)	Long	Short	In/near effector organs (terminal ganglia)

Types of Ganglia

• Sympathetic Chain Ganglia: Paired, run alongside the vertebral column; allow preganglionic fibers to synapse at multiple levels

• Collateral Ganglia: Unpaired, located anterior to the vertebral column; mainly supply abdominal and pelvic organs

• Terminal Ganglia: Located near or within the walls of target organs; characteristic of the parasympathetic division

Neurotransmitters and Receptors in the ANS

Major Neurotransmitters

• Acetylcholine (ACh): Released by all preganglionic axons and all parasympathetic postganglionic axons

• Norepinephrine (NE): Released by most sympathetic postganglionic axons

Cholinergic and Adrenergic Fibers

• Cholinergic fibers: Release ACh

• Adrenergic fibers: Release NE

Receptor Types

• Cholinergic Receptors:

  • Nicotinic receptors: Found on motor end plates, all ganglionic neurons, and adrenal medulla; always excitatory

  • Muscarinic receptors: Found on all effector cells stimulated by postganglionic cholinergic fibers; can be excitatory or inhibitory depending on the target organ

• Adrenergic Receptors:

  • Two main types: alpha and beta

  • Effects depend on the receptor subtype and the tissue

Referred Pain

Referred pain occurs when pain stimuli arising from the viscera are perceived as originating from somatic (body wall) structures. This phenomenon is likely due to the fact that visceral pain afferents travel along the same pathways as somatic pain fibers.

Key Terms and Definitions

• Ganglion: Cluster of nerve cell bodies in the peripheral nervous system (PNS)

• Preganglionic neuron: First neuron in the ANS pathway; cell body in CNS, axon synapses in ganglion

• Postganglionic neuron: Second neuron in the ANS pathway; cell body in ganglion, axon extends to effector

• Effector: Target tissue (muscle or gland) innervated by a neuron

Summary Table: Neurotransmitters and Receptors

Fiber Type	Neurotransmitter Released	Receptor Type	Effect
All preganglionic (sympathetic & parasympathetic)	ACh	Nicotinic	Excitatory
Postganglionic (parasympathetic)	ACh	Muscarinic	Excitatory or inhibitory
Postganglionic (sympathetic)	NE (most), ACh (some)	Alpha, Beta (adrenergic), Muscarinic (sweat glands)	Excitatory or inhibitory

Formulas and Equations

• Heart Rate (HR) and Cardiac Output (CO):

$CO = HR \times SV$

Where $CO$ is cardiac output, $HR$ is heart rate, and $SV$ is stroke volume.

Additional info:

• Dynamic antagonism between sympathetic and parasympathetic divisions allows precise control of visceral activity.

• Some organs receive only sympathetic innervation (e.g., sweat glands, arrector pili muscles).