Autonomic Nervous System (ANS): Study Notes for CH 14

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 divided into the sympathetic and parasympathetic divisions.

Somatic vs. Autonomic Nervous Systems

• Effectors: The somatic nervous system controls skeletal muscles, while the ANS controls smooth muscle, cardiac muscle, and glands.

• Efferent Pathways: Somatic pathways consist of a single neuron from the CNS to the effector. ANS pathways involve a two-neuron chain: a preganglionic neuron (CNS to ganglion) and a postganglionic neuron (ganglion to effector).

• Neurotransmitters: Somatic neurons release acetylcholine (ACh), always excitatory. ANS neurons release ACh or norepinephrine (NE), which can be excitatory or inhibitory depending on the receptor type.

Sympathetic vs. Parasympathetic Divisions

• Sympathetic Division: Prepares the body for 'fight or flight' responses. Increases heart rate, dilates pupils, inhibits digestion, and mobilizes energy stores.

• Parasympathetic Division: Promotes 'rest and digest' activities. Decreases heart rate, constricts pupils, stimulates digestion, and conserves energy.

Sympathetic Division: Structure and Pathways

• CNS Origin: Thoracolumbar region (T1-L2) of the spinal cord.

• Ganglia Locations: Close to the spinal cord in the sympathetic trunk (chain) or collateral ganglia.

• Fiber Pathways: Short preganglionic fibers, long postganglionic fibers. Preganglionic neurons release ACh; postganglionic neurons typically release NE.

Parasympathetic Division: Structure and Pathways

• CNS Origin: Brainstem (cranial nerves III, VII, IX, X) and sacral spinal cord (S2-S4).

• Ganglia Locations: Near or within the target organs (terminal or intramural ganglia).

• Fiber Pathways: Long preganglionic fibers, short postganglionic fibers. Both pre- and postganglionic neurons release ACh.

Cholinergic and Adrenergic Fibers and Receptors

• Cholinergic Fibers: Release acetylcholine (ACh). Includes all ANS preganglionic neurons and all parasympathetic postganglionic neurons.

• Adrenergic Fibers: Release norepinephrine (NE). Most sympathetic postganglionic neurons are adrenergic.

• Receptors:

  • Cholinergic Receptors: Nicotinic (always excitatory) and muscarinic (excitatory or inhibitory).

  • Adrenergic Receptors: Alpha (α) and beta (β) receptors, with various subtypes mediating different effects.

Visceral vs. Somatic Reflexes

• Visceral Reflexes: Involve internal organs, use two-neuron efferent pathways, and are generally slower. Example: baroreceptor reflex regulating blood pressure.

• Somatic Reflexes: Involve skeletal muscles, use a single motor neuron, and are typically faster. Example: knee-jerk reflex.

Effects of Parasympathetic and Sympathetic Divisions on Organs

Organ	Parasympathetic Effect	Sympathetic Effect
Eyes (Pupils)	Constricts pupils	Dilates pupils
Heart	Decreases heart rate	Increases heart rate and force
Blood Vessels	Little effect	Constriction (most vessels), dilation in skeletal muscle
Gastrointestinal Tract	Stimulates digestion	Inhibits digestion
Lungs	Constricts bronchioles	Dilates bronchioles
Adrenal Medulla	No effect	Stimulates secretion of epinephrine and norepinephrine
External Genitalia	Promotes erection	Promotes ejaculation

Autonomic Nervous System Controls

• The ANS is regulated by the hypothalamus, brainstem, spinal cord, and higher cortical centers.

• Hypothalamus is the main integration center, coordinating heart rate, blood pressure, body temperature, water balance, and endocrine activity.

• Brainstem and spinal cord mediate reflexes such as swallowing, coughing, and urination.

Example: During exercise, the sympathetic division increases heart rate and dilates airways, while the parasympathetic division is suppressed to reduce digestive activity.