Autonomic Nervous System: Structure, Function, and Regulation in Anatomy & Physiology

Autonomic Nervous System (ANS)

Maintains homeostasis by regulating involuntary activities such as heart rate, breathing rate, body temperature, and digestion.

Motor Division of Nervous System

Divided into somatic motor (skeletal muscle) and autonomic motor (smooth muscle, cardiac muscle, glands).

Sympathetic Division

Part of the ANS with preganglionic cell bodies located in the lateral horns of the spinal cord from T1 to L2.

Preganglionic Axons

Pass through ventral roots to white rami communicantes to the sympathetic chain ganglia or collateral ganglia.

Somatic Nervous System

Innervates skeletal muscles and involves conscious and unconscious movement.

Autonomic Nervous System

Innervates smooth and cardiac muscle and glands, and is regulated unconsciously.

Neurons in Somatic System

One neuron; myelinated.

Neurons in Autonomic System

Two neurons; first is myelinated, second is unmyelinated.

Acetylcholine in Somatic System

Acts as the neurotransmitter at the neuromuscular junction.

Receptor Molecules in Somatic System

Nicotinic receptors are present.

Acetylcholine and Norepinephrine in Autonomic System

Released by preganglionic neurons and ACh or norepinephrine by postganglionic neurons.

Receptor Molecules in Autonomic System

Varies with synapse and neurotransmitter.

Routes of Sympathetic Axons

Include spinal nerves, sympathetic nerves, splanchnic nerves, and innervation to the adrenal gland.

Spinal Nerves in Sympathetic Pathway

Preganglionic axons synapse with postganglionic neurons within the sympathetic chain.

Sympathetic Nerves

Preganglionic axons synapse with postganglionic neurons, which exit the ganglia through sympathetic nerves.

Splanchnic Nerves

Preganglionic axons pass through the chain ganglia without synapsing to form splanchnic nerves.

Collateral Ganglia

Preganglionic axons synapse with postganglionic neurons in these ganglia, which send fibers to target organs.

Adrenal Medulla

Derived from the same cells as postganglionic ANS cells and secretes epinephrine and norepinephrine.

Epinephrine and Norepinephrine

Hormones secreted by the adrenal medulla that promote physical activity.

Spinal nerve route

A pathway taken by sympathetic axons.

Sympathetic nerve route

A pathway taken by sympathetic axons.

Collateral ganglia via splanchnic nerve

A pathway taken by sympathetic axons.

Adrenal medulla via splanchnic nerve

A pathway taken by sympathetic axons.

Preganglionic cell bodies

Located in nuclei of brainstem or lateral parts of spinal cord gray matter from S2 to S4 in the parasympathetic division.

Preganglionic axons from brain

Pass to terminal ganglia through cranial nerves III, VII, IX, and X.

Preganglionic axons from sacral region

Pass through pelvic splanchnic nerves to terminal ganglia.

Terminal ganglia

Located near the organ innervated or embedded in the wall of the organ.

Sympathetic axons

Reach organs through spinal nerves, head and neck nerve plexuses, thoracic nerve plexuses, and abdominopelvic nerve plexuses.

Parasympathetic axons

Reach organs through cranial nerves and pelvic splanchnic nerves.

Oculomotor nerve (III)

Innervates ciliary muscles and iris through the ciliary ganglion.

Facial nerve (VII)

Innervates lacrimal glands, mucosal glands of nasal cavity and palate, and some salivary glands through pterygopalatine and submandibular ganglia.

Glossopharyngeal nerve (IX)

Innervates the parotid salivary gland through the otic ganglion.

Vagus nerve

Supplies heart, lungs, and esophagus through the esophageal plexus.

Abdominopelvic nerve plexuses

Parts of vagus nerve supply stomach and other viscera.

Pelvic splanchnic nerves

Innervate colon, urinary bladder, and reproductive organs.

Sensory neurons in autonomic nerve plexuses

Parts of reflex arcs regulating organ activities, transmitting pain and pressure sensations from organs to the CNS.

Enteric Nervous System

Consists of nerve plexuses within the wall of the digestive tract.

Dual innervation

Innervation by both sympathetic and parasympathetic divisions; most organs are dually innervated.

Sympathetic influences during exercise

Includes increased heart rate, vasodilation of blood vessels in skeletal and cardiac muscle, dilation of air passages, increased glucose production, increased body temperature, and decreased digestion and urine production.

Parasympathetic influences during resting conditions

Includes simulation of digestion and urine production, lowering heart rate, and constricting air passages.

Neurotransmitters

Primary substances produced by neurons of the ANS.

Acetylcholine

Released by cholinergic neurons.

Norepinephrine

Released by adrenergic neurons.

Cholinergic

Receptors that bind acetylcholine, with two forms: nicotinic and muscarinic.

Nicotinic

All receptors on postganglionic neurons, all skeletal muscles, adrenal glands.

Muscarinic

All receptors on parasympathetic effectors and some sweat glands; may be excitatory or inhibitory.

Adrenergic receptors

Bind norepinephrine/epinephrine, further subdivided into alpha and beta receptors.

α1 and β1

Usually have opposite effects than α2 and β2.

Nicotinic receptors location

Located on the cell bodies of both sympathetic and parasympathetic postganglionic neurons in autonomic ganglia.

Muscarinic receptors location

Located on the cells of all parasympathetic effectors and some sympathetic effectors, such as sweat glands.

Adrenergic receptors location

Located on most sympathetic effectors.

Autonomic reflexes

Control most of the activity of visceral organs, glands, and blood vessels.

Hypothalamus

Has overall control of the ANS and influences autonomic reflex activity.

Local reflex

Does not involve CNS; produces involuntary, unconscious, stereotypical response to stimulus.

Parasympathetic reflex

Via vagus lowers heart rate.

Sympathetic reflex

Via cardiac accelerator nerves causes heart rate to increase.

Coordinated activities

Both divisions may act together to coordinate the activities of different structures.

Sympathetic effects

Usually more widespread and general.

Parasympathetic effects

Usually more local.