Chapter 16- Lecture 1

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The 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.

Divisions: The ANS is divided into the sympathetic and parasympathetic divisions, each with distinct anatomical and functional characteristics.
Comparison with Somatic Nervous System: The somatic nervous system controls voluntary movements of skeletal muscles, while the ANS controls involuntary actions of smooth muscle, cardiac muscle, and glands.

Diagram showing the sympathetic and parasympathetic divisions of the ANS and their target organs

Comparison of Somatic and Autonomic Nervous Systems

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

Somatic Nervous System: Controls voluntary movements via a single neuron pathway from the CNS to skeletal muscle. Neurotransmitter: acetylcholine (ACh).
Autonomic Nervous System: Controls involuntary functions via a two-neuron chain (preganglionic and postganglionic neurons). Neurotransmitters: acetylcholine (ACh) and norepinephrine (NE).

Comparison of somatic and autonomic nervous systems: sensory receptors, effectors, and pathways

Visceral Effectors in the Skin

Visceral effectors in the skin include blood vessels, sweat glands, and arrector pili muscles. These structures are innervated by the autonomic nervous system and are responsible for thermoregulation, piloerection, and sweat production.

Blood vessels: Vasoconstriction and vasodilation regulate blood flow and temperature.
Sweat glands: Control sweating for thermoregulation.
Arrector pili muscles: Cause hair to stand up (goosebumps) in response to cold or emotional stimuli.

Diagram of skin showing innervation of hair follicles, arrector pili muscles, blood vessels, and sweat glands

Organization of the Autonomic Nervous System

Neural Pathways: Somatic vs. Autonomic

The neural pathways of the somatic and autonomic nervous systems differ in the number of neurons and the location of synapses.

Somatic Pathway: Single motor neuron extends from the CNS to the effector muscle.
Autonomic Pathway: Involves a preganglionic neuron (from CNS to autonomic ganglion) and a postganglionic neuron (from ganglion to effector organ).

Diagram comparing somatic and autonomic motor pathways

Sites of Ganglia in the Autonomic Nervous System

Autonomic ganglia are sites where preganglionic neurons synapse with postganglionic neurons. There are three main locations:

Paravertebral (Sympathetic Chain) Ganglia: Located alongside the vertebral column.
Prevertebral (Collateral) Ganglia: Located anterior to the vertebral column, near major abdominal arteries.
Adrenal Medulla: Modified sympathetic ganglion where preganglionic fibers synapse directly on chromaffin cells, which release hormones into the bloodstream.

Diagram showing the distribution of sympathetic and parasympathetic fibers to various organs

Divisions of the Autonomic Nervous System

Sympathetic (Thoracolumbar) Division

The sympathetic division prepares the body for 'fight or flight' responses. Its preganglionic neurons originate from the thoracic and lumbar regions (T1–L2) of the spinal cord.

Functions: Increases heart rate, dilates pupils, inhibits digestion, stimulates glucose release, and increases blood flow to muscles.
Pathways: Preganglionic fibers may synapse in sympathetic chain ganglia, collateral ganglia, or the adrenal medulla.

Sympathetic nervous system effects on target organs Diagram showing sympathetic pathways from spinal cord to target organs

Parasympathetic (Craniosacral) Division

The parasympathetic division promotes 'rest and digest' activities. Its preganglionic neurons originate from the brainstem and sacral spinal cord (craniosacral outflow).

Functions: Decreases heart rate, constricts pupils, stimulates digestion, and promotes energy storage.
Pathways: Long preganglionic fibers synapse in ganglia close to or within target organs.

Parasympathetic nervous system effects on target organs

Functional Comparison: Sympathetic vs. Parasympathetic

Major Effects on Organs

The sympathetic and parasympathetic divisions often have opposing effects on the same organs, maintaining homeostasis through dynamic balance.

Organ	Sympathetic Effect	Parasympathetic Effect
Pupil (Eye)	Dilates pupil	Constricts pupil
Heart	Increases rate and force	Decreases rate
Blood Vessels	Constriction (most)	Dilation (some)
Lungs	Dilates bronchi	Constricts bronchi
GI Tract	Inhibits motility/secretion	Stimulates motility/secretion
Liver	Stimulates glucose release	Promotes glycogen storage
Adrenal Gland	Stimulates hormone release	Minimal effect
Bladder	Relaxes wall	Contracts wall

List of organs affected by the autonomic nervous system

Effects on Skin Structures

The ANS regulates sweat glands and arrector pili muscles in the skin, with distinct effects from each division.

Sympathetic: Increases sweating, causes piloerection (hair stands up).
Parasympathetic: Decreases sweating, relaxes hair follicles.

Sympathetic effects on sweat glands and arrector pili muscle Parasympathetic effects on sweat glands and arrector pili muscle

Neurotransmitters and Receptors

Somatic vs. Autonomic Neurotransmission

The somatic nervous system uses acetylcholine (ACh) at the neuromuscular junction, while the ANS uses both ACh and norepinephrine (NE) depending on the division and target organ.

Somatic: ACh stimulates skeletal muscle contraction.
Sympathetic: Preganglionic neurons release ACh; postganglionic neurons typically release NE (except sweat glands, which use ACh).
Parasympathetic: Both pre- and postganglionic neurons release ACh.

Somatic nervous system neurotransmission Autonomic nervous system neurotransmission

Sympathetic Pathways and Ganglia

Sympathetic Chain and Collateral Ganglia

Sympathetic preganglionic fibers may synapse in the sympathetic chain ganglia (paravertebral), collateral ganglia (prevertebral), or directly on the adrenal medulla. These pathways allow for widespread and coordinated responses during stress.

Sympathetic Chain Ganglia: Innervate organs in the thoracic cavity, head, body wall, and limbs.
Collateral Ganglia: Innervate organs in the abdominopelvic cavity.
Adrenal Medulla: Secretes epinephrine and norepinephrine into the bloodstream for systemic effects.

Sympathetic pathways from spinal cord to target organs Sympathetic chain ganglia and target organs Collateral ganglia and target organs Preganglionic and postganglionic neuron pathways to target organs

Adrenal Medulla as a Modified Ganglion

The adrenal medulla acts as a modified sympathetic ganglion. Preganglionic sympathetic fibers synapse directly on chromaffin cells, which release epinephrine and norepinephrine into the bloodstream, producing prolonged and widespread sympathetic effects.

Hormones released: Epinephrine (adrenaline) and norepinephrine (noradrenaline).
Effect: Amplifies and sustains the fight-or-flight response.

Adrenal medulla and sympathetic stimulation

Clinical and Functional Considerations

Effects of Sympathetic Trunk Damage

Damage to different regions of the sympathetic trunk can affect various organ systems:

Cervical/Thoracic Damage: May impair vision, salivation, thermoregulation, and stress responses.
Lumbar Damage: May affect bladder control, genital function, and blood flow to lower limbs.

Sample Clinical Questions

Pupil Dilation During Stress: Sympathetic fibers from T1–T2 via the superior cervical ganglion innervate the dilator pupillae muscle.
Sympathetic Trunk Lesions: May cause warmer, redder skin due to reduced vasoconstrictor activity.
Adrenal Medulla Innervation: Preganglionic sympathetic fibers synapse directly on adrenal medulla cells (not the cortex).

Summary Table: Somatic vs. Autonomic Nervous System

Feature	Somatic	Autonomic
Effectors	Skeletal muscle	Cardiac muscle, smooth muscle, glands
Control	Voluntary	Involuntary
Neural Pathway	Single neuron from CNS	Two-neuron chain (pre- and postganglionic)
Neurotransmitter	Acetylcholine (ACh)	ACh, norepinephrine (NE)
Effect	Always excitatory	Excitatory or inhibitory