chapter 14
Autonomic Nervous System (ANS)
Definition: The Autonomic Nervous System (ANS) is a part of the nervous system that consists of motor neurons that innervate smooth muscles, cardiac muscles, and glands.
Functions of ANS:
Ensures optimal support for body activities such as:
Shunting blood to areas that need it
Adjusting heart rate, blood pressure, and digestive processes
Operates via subconscious control.
Also known as the involuntary nervous system or general visceral motor system.
Structure of the ANS
Place of the ANS in the Structural Organization of the Nervous System
The ANS is integrated within the broader framework of the nervous system as visualized in Figure 14.1.
Comparison with the Somatic Nervous System (SNS)
Similarities: Both systems have motor fibers.
Key Differences:
Effectors:
SNS: Innervates skeletal muscles.
ANS: Innervates cardiac muscle, smooth muscle, and glands.
Efferent pathways and ganglia:
SNS: The cell body is in the central nervous system (CNS), with a single, thick myelinated group A axon extending directly to skeletal muscle via spinal or cranial nerves.
ANS: Utilizes a two-neuron chain.
Preganglionic neuron has its cell body in the CNS with a thin, lightly myelinated axon extending to a ganglion.
Postganglionic (ganglionic) neuron is outside the CNS; the body synapses with the preganglionic axon in an autonomic ganglion and has a nonmyelinated axon extending to the effector organ.
Target organ responses to neurotransmitters:
SNS: All somatic motor neurons release acetylcholine (ACh) with a stimulatory effect.
ANS:
Preganglionic fibers release ACh.
Postganglionic fibers release norepinephrine or ACh at effectors, leading to either stimulatory or inhibitory effects depending on the type of receptors present.
Overlap between Somatic and Autonomic Function
Higher brain centers regulate and coordinate both systems.
Most spinal and many cranial nerves contain both somatic and autonomic fibers.
Adaptations often involve both skeletal muscles and visceral organs.
Example: Active muscles require more oxygen and glucose, so ANS nerves increase heart rate and open airways.
Divisions of the Autonomic Nervous System
Two main arms of the ANS:
Parasympathetic Division:
Promotes maintenance functions and conserves energy.
Known as the 'rest-and-digest' system.
Directs activities such as digestion, diuresis, and defecation.
Example: After a meal, blood pressure, heart rate, and respiratory rates decrease while gastrointestinal activity increases; pupils constrict.
Sympathetic Division:
Mobilizes the body during activity, referred to as the 'fight-or-flight' system.
Activated during exercise, excitement, emergencies, and embarrassment, causing increased heart rate, dry mouth, cold, sweaty skin, and dilated pupils.
During vigorous physical activity, shunts blood to skeletal muscles and heart, dilates bronchioles, and prompts the liver to release glucose.
Dual Innervation:
All visceral organs receive input from both divisions; they produce opposite effects on maintain homeostasis through dynamic antagonism.
Key Anatomical Differences Between ANS Divisions
Differences in anatomy:
Sites of Origin:
Parasympathetic fibers: Craniosacral, originating in the brain and sacral spinal cord.
Sympathetic fibers: Thoracolumbar, originating in thoracic and lumbar regions of the spinal cord.
Relative Lengths of Fibers:
Parasympathetic: Long preganglionic and short postganglionic fibers.
Sympathetic: Short preganglionic and long postganglionic fibers.
Location of Ganglia:
Parasympathetic ganglia: Located in or near their visceral effector organ.
Sympathetic ganglia: Lie close to the spinal cord.
Parasympathetic Division Anatomy
Basic Structure
Called the craniosacral division because fibers originate from the brain stem and sacral regions of the spinal cord.
Preganglionic Fibers: Long preganglionic fibers extend from CNS almost to target organs, synapsing with postganglionic neurons in terminal ganglia located near or within target organs.
Postganglionic Fibers: Short postganglionic fibers synapse with effectors.
Cranial Part of Parasympathetic Division
Locations of Cell Bodies: Located in the brain stem.
Preganglionic Fibers Run In:
Oculomotor Nerves (III): Control smooth muscle of the eye, causing pupils to constrict and lenses to bulge for focusing.
Postganglionic cell bodies are located in ciliary ganglia within eye orbitals.
Facial Nerves (VII): Stimulate large glands in the head.
Fibers activating nasal and lacrimal glands synapse in pterygopalatine ganglia.
Fibers activating submandibular and sublingual salivary glands synapse in submandibular ganglia.
Glossopharyngeal Nerves (IX): Stimulate parotid salivary glands, with fibers synapsing in otic ganglia.
Vagus Nerves (X): Account for approximately 90% of all preganglionic parasympathetic fibers in the body, serving all thoracic and abdominal viscera.
Preganglionic fibers arise from the medulla and synapse in terminal ganglia within the walls of target organs.
Plexuses:
Cardiac Plexus: Slows heart rate.
Pulmonary Plexus: Serves lungs and bronchi.
Esophageal Plexus: Forms anterior and posterior vagal trunks with branches supplying the stomach, liver, gallbladder, pancreas, small intestine, and parts of the large intestine.
Sacral Part of Parasympathetic Division
Originates from neurons in S2-S4, serving pelvic organs and distal half of the large intestine.
Axons travel in the ventral root of spinal nerves, branching to form pelvic splanchnic nerves.
Synapses occur with ganglia in the pelvic floor (inferior hypogastric plexus) and in the walls of the distal half of the large intestine, urinary bladder, ureters, and reproductive organs.
Sympathetic Division Anatomy
Complexity: The sympathetic system innervates more organs compared to the parasympathetic division.
Unique Structures:
Some structures receive only sympathetic innervation, including sweat glands, arrector pili muscles of hair follicles, and smooth muscles of all blood vessels.
Referred to as thoracolumbar division, with preganglionic neurons located in spinal cord segments T1-L2, forming lateral horns of the spinal cord.
Ganglia and Pathways
Preganglionic fibers pass through white rami communicantes to enter sympathetic trunk (chain or paravertebral) ganglia.
There are 23 paravertebral ganglia in the sympathetic trunk with specific subdivisions:
3 cervical
11 thoracic
4 lumbar
4 sacral
1 coccygeal
Sympathetic Pathways
Upon entering sympathetic trunk ganglion, short preganglionic fibers may synapse in one of three pathways:
Synapse in Trunk Ganglia:
Synapse with ganglionic neuron in the same trunk ganglion.
Ascend or descend sympathetic trunk to synapse in another trunk ganglion.
Synapse in Collateral Ganglia:
Pass through trunk ganglion and emerge without synapsing (only in abdomen and pelvis).
Synapse Outside of Trunk:
In collateral ganglia.
Autonomic Control Mechanisms
Visceral Reflexes
Definition: Reflex arcs involving autonomic responses to stimuli, consisting of:
Receptor
Sensory Neuron
Integration Center
Motor Neuron
Effector
Differences from Somatic Reflex Arcs:
Two consecutive neurons in the motor pathway.
Afferent fibers are visceral sensory neurons.
Effectors are smooth muscle, cardiac muscle, and glands, rather than skeletal muscles.
Examples of Visceral Reflexes:
Reflexes that empty the rectum and bladder are examples of visceral reflexes involving enteric nervous system components.
Neurotransmitters in the ANS
Main Neurotransmitters:
Acetylcholine (ACh):
Released by cholinergic fibers at all ANS preganglionic axons and all parasympathetic postganglionic axons.
Norepinephrine (NE):
Released by adrenergic fibers at almost all sympathetic postganglionic axons, except those at sweat glands (which release ACh).
Effects of Neurotransmitters:
ACh effect depends on binding to cholinergic or adrenergic receptors.
Cholinergic Receptors
Types:
Nicotinic Receptors:
Found on:
All postganglionic neurons (sympathetic and parasympathetic).
Hormone-producing cells of adrenal medulla.
Sarcolemma of skeletal muscle cells.
Effect of ACh: Always stimulatory, opening ion channels in postsynaptic cells.
Muscarinic Receptors:
Found on all effector cells stimulated by postganglionic cholinergic fibers.
Effects depend on the type (inhibitory or excitatory) depending on the target organ.
Example: ACh binding to cardiac muscle slows heart rate, while binding to intestinal smooth muscle increases motility.
Adrenergic Receptors
Classes:
Alpha (α) receptors (subclasses α1, α2)
Beta (β) receptors (subclasses β1, β2, β3)
Effects:
Actions depend on the receptor subclass predominant on the target organ.
Example: NE binding to β1 receptors in cardiac muscle increases heart rate; epinephrine binding to β2 receptors leads to bronchial relaxation.
ANS Interactions
Dual Innervation of Organs
Most visceral organs receive dual innervation from both ANS divisions, with antagonistic and cooperative effects.
Dynamic Antagonism:
Allows precise control of visceral activity:
Sympathetic division increases heart and respiratory rates while inhibiting digestion and elimination.
Parasympathetic division decreases heart and respiratory rates while promoting digestion and waste disposal.
Tone of the Divisions
Sympathetic Tone (Vasomotor Tone):
Maintains a continual state of partial arterial constriction, crucial for blood pressure regulation.
If BP drops, sympathetic fibers increase firing rates leading to increased vessel constriction and BP elevation.
Inversely, if BP rises, sympathetic fibers decrease firing to dilate vessels, decreasing BP.
Parasympathetic Tone:
Dominates heart and smooth muscle of digestive and urinary tract organs, always slightly activated.
Controls normal digestive and urinary activity levels.
Can be overridden during stress by sympathetic division.
Unique Roles of the Sympathetic Division
Exclusively Innervated Structures:
Includes adrenal medulla, sweat glands, arrector pili muscles, kidneys, and most blood vessels.
Specific Functions:
Thermoregulatory Responses to Heat:
When body temperature rises, sympathetic nerves dilate skin blood vessels for heat dissipation and activate sweat glands.
When temperature drops, sympathetic fibers constrict blood vessels to retain body heat.
Release of Renin from Kidneys:
Sympathetic activation causes renin release from kidneys, triggering pathways to increase blood pressure.
Metabolic Effects:
Increases metabolic rates of cells, raises blood glucose levels, and mobilizes fats for energy use.
Localized vs. Diffuse Effects
Parasympathetic Division:
Elicits short-lived, highly localized control over effectors; ACh is rapidly inactivated by acetylcholinesterase.
Sympathetic Division:
Produces longer-lasting effects with a broader influence, as NE is inactivated more slowly than ACh; NE and epinephrine from adrenal medulla can extend effects beyond direct sympathetic signals.
Clinical Aspects of the ANS
Autonomic Neuropathy:
Damage to autonomic nerves, often a complication of diabetes mellitus, leading to symptoms like sexual dysfunction, dizziness after standing, urinary incontinence, slow pupil responses, and impaired sweating.
Maintenance of good blood glucose levels is essential for prevention.
Control of ANS Function
Brain Centers Involved
CNS Centers:
Brain stem and spinal cord, hypothalamus, and cerebral cortex control ANS function.
The hypothalamus acts as the main integrative center for ANS activity, although cerebral inputs can modify operations subconsciously via linkage to the limbic system.
Specific Brain Regions:
Brain Stem Regulates:
Heart rate, blood vessel diameter, and gastrointestinal activity through the medullary centers.
Muscles controlling pupil shape via midbrain regions.
Spinal cord manages defecation and urination, under voluntary control.
Hypothalamic Control:
Anterior regions direct parasympathetic functions, while posterior directs sympathetic actions.
Controls various autonomic functions such as heart activity, blood pressure, temperature balance, water balance, endocrine activity, and emotional responses.
Cortical Controls:
Hypothalamic connections to the limbic lobe facilitate cortical influence on the ANS.
Some voluntary control is achievable via biofeedback, which allows some individuals to consciously influence physiological conditions such as stress levels and migraines.
Disorders of the ANS
Common Disorders
Hypertension:
Characterized by an overactive sympathetic response leading to elevated blood pressure and increased risk of cardiovascular issues.
Can be managed with adrenergic receptor-blocking drugs.
Raynaud's Disease:
Symptoms include painful vasoconstriction in fingers and toes, which become pale and then cyanotic.
Treatable with vasodilators.
Autonomic Dysreflexia:
A life-threatening condition in quadriplegics that causes uncontrolled activation of autonomic neurons, particularly in those with spinal cord injuries above T6, leading to dangerous blood pressure spikes.
Developmental Aspects
ANS preganglionic neurons develop from the neural tube; structures in the peripheral nervous system (PNS) derive from the neural crest.
Age-Related Effects:
Efficiency of the ANS declines with age due to structural changes at preganglionic axon terminals, leading to issues like constipation, dry eyes, frequent eye infections, and orthostatic hypotension existence due to less responsive pressure receptors.