The Autonomic Nervous System: Structure, Function, and Neurotransmission

Autonomic Nervous System (ANS)

Overview of the Autonomic Nervous System

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 sexual arousal. It operates largely below the level of consciousness and is essential for maintaining homeostasis.

• Somatic Nervous System: Controls voluntary movements of skeletal muscles.

• Autonomic Nervous System: Controls involuntary actions of smooth muscle, cardiac muscle, and glands.

General Anatomic Organization of the ANS

The ANS consists of a two-neuron chain: a preganglionic neuron (cell body in the CNS) and a postganglionic neuron (cell body in an autonomic ganglion). The postganglionic neuron innervates the target organ.

• Preganglionic neuron: Cell body in the CNS; axon projects to an autonomic ganglion. Type B neuron = myelinated

• Postganglionic neuron: Cell body in the autonomic ganglion; axon projects to the effector (smooth muscle, cardiac muscle, or gland). Type C neuron = unmyelinated

Divisions of the Autonomic Nervous System

Sympathetic Division

The sympathetic division is responsible for the "fight or flight" response, preparing the body for stressful or emergency situations. It increases alertness, metabolic rate, and energy availability.

• Active during high stress or emergency situations.

• Allows for maximum energy expenditure.

• Originates from the thoracolumbar region (T1–L2) of the spinal cord.

Layout of the Sympathetic Division

• Short, branching preganglionic neurons = Type B

• Long postganglionic neurons = Type C

• Autonomic ganglia are close to the spinal cord

Types of Sympathetic Ganglia

• Sympathetic Chain Ganglia: Lateral to the spinal cord; innervate head, body wall, limbs, and thoracic cavity.

  • Eyes = contracts dialator pupillae muscle, relaxes ciliary muscles for distant vision

  • Heart = how your heart feels when your are stressed, increased heart rate, force of contraction

  • Lungs = bronchodilation, increase in rate of respiration

  • Other effects:

    • increased metabolic rate and energy use

    • increased mental alterness

    • increased sweating

    • decreased blood flow to the skin

    • increased blood flow to skeletal muscles

• Collateral Ganglia: Anterior to the vertebral column; innervate abdominopelvic organs.

  1. Celiac ganglion = stomach, gallbladder, liver, pancreas, and spleen

  2. Superior mesenteric ganglion = small intestine and proximal 2/3 large interstine

  3. Inferior mesenteric ganglion = distal 1/3 of large intestine, kidney, bladder, and reproductive organs

  • Digestive system

    • decreased digestive secretions

    • increased glycogen breakdown

    • contract sphincters in GI tract

    • relax smooth muscle in GI tract

  • Urinary system

    • decreased blood flow to urinary organs

    • contract urinary sphincters

    • relax smooth muscle

  • Reproductive system

    • ejaculation in males

    • uterine contraction in females

• Adrenal Medulla: Center of the adrenal gland; releases hormones into the blood for a widespread effect.

  • Chromaffin cells = take the place of the postganglionic neuron and release epinephrine and norepinesphrine (as hormones)

    • neurotransmitters released to the blood -> provide sympathetic innervation to the entire body all at once, has more generalized effect

Parasympathetic Division

The parasympathetic division is responsible for the "rest and digest" response, promoting maintenance activities and conserving body energy. It is dominant during resting conditions.

• Promotes digestion, energy storage, and tissue repair.

• Originates from the craniosacral region (brainstem and S2–S4 spinal segments).

Layout of the Parasympathetic Division

• Long, unbranched preganglionic neurons

• Short postganglionic neurons

• Autonomic ganglia are close to or within the target tissue

Types of Parasympathetic Ganglia

• Terminal Ganglia: Located near the target organ

• Intramural Ganglia: Located within the wall of the target organ

Ciliary Ganlgion

• oculomoter nerve is preganglionic

• innervates the smooth muscle in the eyes

  • sphrincter pupillae

  • contract ciliary muscles = visual accommodation

Pterygopalatine Ganglion

• facial nerve is preganglionic

• innervates nasal and lacrimal glands

  • increased secretion in secretion in both types. ofgland

Submandibular Ganglion

• facial nerve is preganglionic

• innervates the submanidbular and sublingual salivary gland

  • increases salivary secretion

Optic Ganglion

• glossopharyngeal nerve is preganglionic

• innervates parotid salivary glands

  • increases salivary secretion

intramural Ganglia

• vagus nerve is preganglioninc

• cardiac muscle cells = decreases heart rate and force of contraction

• smooth muscle in airways = bronchoconstriction and decreased rate of respiration

• digestive organs

  • smooth muscle in GI tract = increases motility

  • increases secretion from digestive glands

  • relaxes sphincters in GI tract

Intramural Ganglia

• pelvic nerves are preganlgionic

• urinary tract

  • increases urine production

  • relaxes urinary sphincters

• rectum = relaxes sphincters

• reproductive organs

  • increases gametogenesis = makes more sperm/eggs

  • causes erections in males

• other parasympathetic effects

  • decreased sweating

  • decreased mental altertness

Enteric Division

The enteric division is a network of neurons within the walls of the digestive tract that regulates gastrointestinal function independently but is modulated by the sympathetic and parasympathetic divisions.

• Provides localized control of digestive activity

• Coordinates reflexes for peristalsis and secretion

Neurotransmitters and Receptors in the ANS

Sympathetic Neurotransmitters and Receptors

• Preganglionic neurons release acetylcholine (ACh), which binds to nicotinic receptors on postganglionic neurons.

• Postganglionic neurons release norepinephrine (N.Epi) or epinephrine (Epi), which bind to adrenergic receptors (alpha and beta types) on target cells.

Receptor Type	Main Effect	Location
α1	Excitatory (smooth muscle contraction, gland secretion)	Blood vessels, glands
α2	Inhibitory (decreased cAMP, closes GI sphincters)	GI tract sphincters
β1	Stimulates cardiac muscle, increases metabolism	Heart
β2	Relaxes smooth muscle (bronchodilation, vasodilation)	Lungs, blood vessels
β3	Increases lipolysis	Adipose tissue

Parasympathetic Neurotransmitters and Receptors

• Preganglionic neurons release acetylcholine (ACh), which binds to nicotinic receptors on postganglionic neurons.

• Postganglionic neurons release acetylcholine (ACh), which binds to muscarinic receptors on target cells.

Receptor Type	Main Effect	Location
Nicotinic	Excitatory (depolarization)	All postganglionic neurons
Muscarinic	Excitatory or inhibitory (depending on target)	Effector organs (heart, smooth muscle, glands)

Medications and Adrenergic Receptors

• many medications act by binding to adrenergic receptors

• Blockers = block the receptor site and prevent N.epi binding

• Agonists = Mimic effects of N. Epi binding

Dual Innervation and Autonomic Tone

Dual Innervation

Most organs receive input from both sympathetic and parasympathetic divisions, which typically have opposing effects. This allows for precise regulation of organ function.

Autonomic Tone

Both divisions of the ANS are partially active at all times, creating a baseline level of activity known as autonomic tone. The balance between sympathetic and parasympathetic tone determines the overall effect on an organ.

• Sympathetic tone: Maintains partial constriction of blood vessels, helping to regulate blood pressure.

• Parasympathetic tone: Maintains resting heart rate at about 72 bpm.

Summary Table: Comparison of Somatic and Autonomic Nervous Systems

Feature	Somatic Nervous System	Autonomic Nervous System
Control	Voluntary	Involuntary
Effector Organs	Skeletal muscle	Cardiac muscle, smooth muscle, glands
Number of Neurons	One (CNS to effector)	Two (preganglionic and postganglionic)
Neurotransmitter	Acetylcholine (ACh)	ACh, norepinephrine (N.Epi), epinephrine (Epi)
Effect	Always excitatory	Excitatory or inhibitory