Chapter 14: The Autonomic Nervous System

Introduction to the Autonomic Nervous System (ANS)

• Definition and General Function:     * The Autonomic Nervous System (ANS) is a system of motor neurons that innervates smooth muscles, cardiac muscle, and glands.     * Its primary role is to make adjustments to ensure optimal support for complex body activities.     * The system operates via subconscious control, meaning it is not under voluntary oversight.     * Alternative names for the ANS include the involuntary nervous system or the visceral (autonomic) motor system.

Comparative Anatomy: Somatic vs. Autonomic Nervous Systems

• Structural and Functional Differences:     * Both the somatic and autonomic systems utilize motor fibers, but they differ significantly in three specific areas: the effectors they target, the number of neurons used to reach those effectors, and the neurotransmitters released at the effector site.

• Target Effectors:     * Somatic Nervous System (SNS): This system specifically innervates skeletal muscles.     * Autonomic Nervous System (ANS): This system targets cardiac muscle, smooth muscle, and various glands.

• Efferent Pathways and Ganglia:     * Somatic System Structure: Identifiable by a single-neuron pathway. The cell body resides in the Central Nervous System (CNS), and a single, thick, heavily myelinated Group A axon extends through spinal or cranial nerves directly to the skeletal muscle.     * Autonomic System Structure: Utilizes a two-neuron chain to reach effectors:         * Preganglionic Neuron: The cell body is located within the CNS. It possesses a thin, lightly myelinated preganglionic axon that extends into a ganglion.         * Postganglionic (Ganglionic) Neuron: The cell body is located outside the CNS in an autonomic ganglion. It synapses with the preganglionic axon and utilizes a nonmyelinated postganglionic axon to reach the target effectors.

• Neurotransmitter Effects:     * Somatic Motor Neurons: All somatic motor neurons release acetylcholine ($ACh$). The effect on the muscle is always excitatory.     * Autonomic Motor Neurons:         * Preganglionic fibers always release $ACh$.         * Postganglionic fibers vary: most sympathetic fibers release norepinephrine ($NE$), while most parasympathetic fibers release $ACh$.         * The effect on the effector can be either excitatory or inhibitory, depending on the specific type of receptors present on the target organ.

• Overlap of Function:     * Higher brain centers regulate and coordinate both systems simultaneously.     * Most spinal nerves and many cranial nerves are mixed, containing both somatic and autonomic fibers.     * Adaptations often require both systems to work together. For instance, when skeletal muscles are active and require more oxygen and glucose, the ANS increases heart rate and opens the airways to facilitate this somatic demand.

Divisions of the Autonomic Nervous System

• The Two Arms of the ANS:     * Parasympathetic Division: Primarily promotes maintenance functions and energy conservation.     * Sympathetic Division: Responsible for mobilizing the body during periods of high activity.

• Dual Innervation:     * Almost all visceral organs are served by both divisions. These divisions generally cause opposite effects to maintain homeostasis through a process called dynamic antagonism.

Functional Roles and Anatomical Origins

• Parasympathetic Division ("Rest-and-Digest"):     * Functions to keep body energy use as low as possible during maintenance activities.     * Directs processes such as digestion, diuresis (urination), and defecation.     * Anatomical Origin: Called the craniosacral division. Fibers originate in the brain stem (cranial nerves) and the sacral region of the spinal cord ($S_2 - S_4$).     * Fiber Lengths: Long preganglionic axons and short postganglionic axons.     * Ganglia Location: Located in or near the visceral effector organs (terminal or intramural ganglia).

• Sympathetic Division ("Fight-or-Flight"):     * Mobilizes the body during exercise, excitement, emergency, or embarrassment.     * Physical Physiological Responses: Increased heart rate, dry mouth, cold and sweaty skin, dilated pupils.     * Blood Redirection: Shunts blood to skeletal muscles and the heart, dilates bronchioles, and triggers the liver to release glucose into the blood.     * Anatomical Origin: Called the thoracolumbar division. Fibers originate from the thoracic and lumbar regions of the spinal cord ($T_1 - L_2$). The cell bodies of these preganglionic neurons form the lateral horns of the spinal cord.     * Fiber Lengths: Short preganglionic axons and long postganglionic axons.     * Ganglia Location: Located close to the spinal cord (sympathetic trunk/chain ganglia or collateral/prevertebral ganglia).

• Unique Sympathetic Innervation:     * Some structures are unique to the sympathetic division: sweat glands (eccrine and apocrine), arrector pili muscles, and the smooth muscle of all blood vessels.

Anatomical Specifics of the Parasympathetic Division

• Cranial Part:     * Oculomotor Nerves (III): Control eye smooth muscle, causing pupil constriction and lens bulging for focus.     * Facial Nerves (VII): Stimulate large glands in the head (nasal, lacrimal, salivary).     * Glossopharyngeal Nerves (IX): Activate the parotid salivary glands.     * Vagus Nerves (X): Accounts for approximately $75\%$ of all preganglionic parasympathetic fibers in the body. These serve all thoracic and abdominal viscera. Axons arise from the medulla and synapse in intramural ganglia within target organ walls.         * Cardiac Plexus: Slows the heart rate.         * Pulmonary Plexus: Serves the lungs and bronchi.         * Esophageal Plexus: Forms vagal trunks that serve the stomach, liver, gallbladder, pancreas, and intestines.

• Sacral Part:     * Arises from neurons in $S_2 - S_4$. Serves the pelvic organs and the distal half of the large intestine.     * Axons form pelvic splanchnic nerves that synapse with ganglia in the pelvic floor (inferior hypogastric plexus) or intramural ganglia.

Anatomical Specifics of the Sympathetic Division

• Sympathetic Trunks and Pathways:     * Preganglionic fibers leave the spinal cord and pass through white rami communicantes (myelinated) to enter the sympathetic trunk ganglia (chain ganglia).     * Three Possible Synapse Pathways:         1. Synapse at the same level in the trunk ganglion.         2. Synapse at a higher or lower level in the trunk.         3. Pass through the trunk without synapsing to enter a collateral (prevertebral) ganglion.     * Gray Rami Communicantes: Nonmyelinated postganglionic axons that exit the trunk to enter the ventral ramus of spinal nerves heading to effectors.

• Pathways to the Adrenal Medulla:     * Some preganglionic fibers pass directly to the adrenal medulla without synapsing. Upon stimulation, medullary chromaffin cells secrete hormones into the blood: $80\%$ epinephrine (adrenaline) and $20\%$ norepinephrine (noradrenaline).

Neurotransmitters and Receptors in the ANS

• Neurotransmitter Types:     * Cholinergic Fibers: Release $ACh$. Includes all ANS preganglionic axons and all parasympathetic postganglionic axons.     * Adrenergic Fibers: Release $NE$. Includes almost all sympathetic postganglionic axons (exception: those serving sweat glands release $ACh$).

• Cholinergic Receptors (Binding $ACh$):     * Nicotinic Receptors: Found on all postganglionic neurons, adrenal medulla cells, and skeletal muscle sarcolemma. The effect is always excitatory.     * Muscarinic Receptors: Found on all parasympathetic target organs and some sympathetic targets like eccrine sweat glands. Effect can be excitatory or inhibitory (e.g., slows heart, increases gut motility).

• Adrenergic Receptors (Binding $NE$/Epi):     * Beta 1 ($\beta_1$): Specifically located in the heart, kidneys, and adipose tissue. Increases heart rate and force; stimulates renin release.     * Beta 2 ($\beta_2$): Found in lungs and blood vessels serving the heart/liver/skeletal muscle. Generally inhibitory: dilates bronchioles and blood vessels; relaxes smooth muscle in the digestive/urinary tracts.     * Beta 3 ($\beta_3$): Found in adipose tissue; stimulates lipolysis.     * Alpha 1 ($\alpha_1$): Found in blood vessels of skin, mucosae, and abdominal viscera. Constricts blood vessels and visceral sphincters; dilates pupils.     * Alpha 2 ($\alpha_2$): Located on adrenergic axon terminals and the pancreas. Inhibits $NE$ release and insulin secretion.

Pharmacology and the ANS

Dynamic Interactions and Autonomic Tone

• Antagonistic Interaction: Both divisions are partially active at all times. Dynamic antagonism allows for precise control (e.g., sympathetic increases heart rate, parasympathetic decreases it).

• Sympathetic (Vasomotor) Tone: The sympathetic division entirely controls blood vessel smooth muscle. This results in a continual state of partial constriction. If BP drops, vasomotor fibers fire faster to constrict vessels; if BP rises, they fire slower to dilate vessels.

• Parasympathetic Tone: Dominates the heart and smooth muscle of digestive/urinary tracts. It maintains normal activity levels but can be overridden by the sympathetic system during stress.

• Cooperative Effects: Seen in external genitalia control. Parasympathetic causes vasodilation (erection), and Sympathetic causes ejaculation or vaginal contraction.

Unique Roles and Effects of the Sympathetic Division

• Exclusive Sympathetic Control: Adrenal medulla, sweat glands, arrector pili muscles, kidneys, and most blood vessels.

• Thermoregulation: Sympathetic nerves dilate skin vessels and activate sweat glands when body temperature rises; they constrict vessels when temperature drops.

• Metabolic Effects: Increases metabolic rates, raises blood glucose levels, and mobilizes fats for fuel.

• Duration of Effects: Parasympathetic effects are short-lived/localized due to rapid destruction of $ACh$ by acetylcholinesterase. Sympathetic effects are longer-lasting and bodywide because $NE$/Epi hormones from the adrenal medulla persist in the blood.

Central Nervous System Control of the ANS

• Brain Stem and Spinal Cord: The reticular formation exerts the most direct influence. Medullary centers regulate heart and BP; the midbrain controls pupil/lens; the spinal cord handles defecation and micturition (subject to conscious override).

• Hypothalamus: The main integrative "boss" of the ANS. Anterior regions handle parasympathetic functions, while posterior handles sympathetic functions. It coordinates heart activity, temperature, water balance, and emotional responses (via the limbic system).

• Cerebral Cortex: Influence is usually subconscious via the limbic system, but voluntary control is possible through biofeedback training (e.g., managing migraines).

Homeostatic Imbalances

• Autonomic Neuropathy: Damage to autonomic nerves, often a complication of diabetes mellitus. Symptoms include sexual dysfunction, dizziness (poor BP control), urinary incontinence, and sluggish pupils.

• Hypertension (High Blood Pressure): Result of an overactive sympathetic vasoconstrictor response to stress; treated with adrenergic blockers.

• Raynaud's Disease: Exaggerated vasoconstriction in digits, causing them to turn pale or cyanotic. Treated with vasodilators.