chapter 14
Chapter 14: The Autonomic Nervous System and Homeostasis
14.1 Functions of the Autonomic Nervous System and Reflex Arcs (1 of 2)
Autonomic Nervous System (ANS):
Oversees vital functions such as:
Heart rate
Blood pressure
Digestive and urinary functions
Operates without conscious control through Visceral Reflex Arcs:
Sensory signals from viscera and skin are sent by afferent sensory neurons to the brain or spinal cord.
Integration of stimuli by the CNS (central nervous system).
Motor impulses from the CNS are sent via efferent motor neurons in cranial and spinal nerves to autonomic ganglia.
Autonomic ganglia send impulses via other efferent motor neurons to target organs, triggering motor responses in target cells.
14.1 Functions of the Autonomic Nervous System and Reflex Arcs (2 of 2)
Figure 14.1: Visceral reflex arcs.
14.1 Comparison of Somatic and Autonomic Nervous Systems (1 of 2)
PNS Motor Division:
Somatic Motor Division:
Somatic motor neurons directly innervate skeletal muscle fibers.
Voluntary control.
Utilizes Acetylcholine (ACh) as an excitatory neurotransmitter.
Visceral Motor Division (ANS):
Autonomic motor neurons innervate smooth and cardiac muscle cells and glands.
Involuntary control.
Involves two neurons:
Preganglionic Neuron:
Cell body located in the CNS; synapses on the postganglionic neuron's cell body.
Postganglionic Neuron:
Cell body located in the PNS autonomic ganglion; axon synapses on the target cell; utilizes ACh and norepinephrine (NE) which can be excitatory or inhibitory.
14.1 Comparison of Somatic and Autonomic Nervous Systems (2 of 2)
Figure 14.2: Comparison of the somatic and autonomic nervous systems.
14.1 Divisions of the ANS (1 of 4)
Autonomic Nervous System Divisions:
Sympathetic Nervous System:
Preganglionic axons are typically short; postganglionic axons are long.
Parasympathetic Nervous System:
Opposite configuration (long preganglionic, short postganglionic).
Figure 14.3: Overview of the structure of ANS divisions.
14.1 Divisions of the ANS (2 of 4)
Sympathetic Nervous System:
Referred to as the “Fight or Flight” system.
Cell bodies of preganglionic neurons originate in the thoracic and upper lumbar spinal cord (Thoracolumbar Division).
Sympathetic ganglia are generally located near the spinal cord.
Parasympathetic Nervous System:
Referred to as the “Rest and Digest” system.
Cell bodies of preganglionic neurons located within nuclei of cranial nerves in the brainstem and in the sacral region of the spinal cord (Craniosacral Division).
Parasympathetic ganglia tend to be near or within target organs.
Cranial nerves innervate head & neck structures and thoracic/most abdominal viscera; sacral nerves innervate pelvic cavity structures.
14.1 Divisions of the ANS (3 of 4)
Figure 14.3: Overview of the structure of ANS divisions.
14.1 Divisions of the ANS (4 of 4)
Balance Between Parasympathetic and Sympathetic Nervous Systems:
Actions of the parasympathetic nervous system directly antagonize those of the sympathetic nervous system.
Example of antagonism: Parasympathetic neurons decrease heart contraction rates while sympathetic neurons increase it.
The two divisions work in balance to ensure homeostasis is maintained.
14.2 The Sympathetic Nervous System
Function: Prepares the body for emergency situations and maintains homeostasis during physical activity.
Adjustments included:
Temporary changes in heart rate and blood vessel diameter to counteract gravity effects on blood pressure (orthostatic hypotension).
Mediates visceral responses to strong emotions (fear, excitement, rage, embarrassment).
14.2 Gross and Microscopic Anatomy of the Sympathetic Nervous System (1 of 8)
Organization:
Preganglionic neurons originate in the lateral horns of the thoracic and lumbar spinal cord.
Axons exit along with somatic lower motor neurons via the anterior root.
They travel with the spinal nerve and anterior ramus, branching into White Rami Communicantes (myelinated).
Axons enter the sympathetic chain ganglion housing postganglionic sympathetic neuron cell bodies.
14.2 Gross and Microscopic Anatomy of the Sympathetic Nervous System (2 of 8)
Postganglionic Neurons:
Majority of cell bodies reside in a series along the vertebral column called Sympathetic Chain Ganglia.
Extend from the Superior Cervical Ganglion to the Inferior Sacral Ganglion.
Some preganglionic neurons synapse on cell bodies in different chain ganglia or on Collateral Ganglia near target organs (e.g., pre-aortic ganglia attached to aorta).
14.2 Gross and Microscopic Anatomy of the Sympathetic Nervous System (3 of 8)
Collateral Ganglia:
Preganglionic axons synapsing near organs of the abdominopelvic cavity via Splanchnic Nerves (including Celiac, Superior Mesenteric, and Inferior Mesenteric Ganglion).
14.2 Gross and Microscopic Anatomy of the Sympathetic Nervous System (4 of 8)
Pathways of Preganglionic Neuron Synapsing:
A preganglionic neuron can synapse with a postganglionic neuron in three ways:
Axon ascends or descends to synapse in a different chain ganglion.
Axon synapses with a postganglionic neuron in the same sympathetic chain ganglion.
Axon passes through the chain ganglion and enters the abdominopelvic cavity to synapse in a collateral ganglion.
14.2 Gross and Microscopic Anatomy of the Sympathetic Nervous System (5 of 8)
Postganglionic Innervation:
Axons exiting ganglia via Gray Rami Communicantes (unmyelinated) rejoin spinal nerves to reach target cells.
Axons from the third pathway travel directly to target cells, includes splanchnic nerves.
14.2 Gross and Microscopic Anatomy of the Sympathetic Nervous System (6 of 8)
Figure 14.4: Organization of the sympathetic nervous system.
14.2 Gross and Microscopic Anatomy of the Sympathetic Nervous System (7 of 8)
Role of White and Gray Rami:
White rami communicantes allow the passage of myelinated preganglionic fibers.
Gray rami communicantes consist of unmyelinated postganglionic fibers that reach targets.
14.2 Gross and Microscopic Anatomy of the Sympathetic Nervous System (8 of 8)
Integration of Neuronal Pathways:
Preganglionic cells synapse with postganglionic neurons affecting major physiological pathways to prepare for “fight or flight” situations.
14.2 Sympathetic Neurotransmitters and Receptors (1 of 6)
Classes of Sympathetic Neurotransmitters:
Preganglionic axon uses Acetylcholine (ACh) for communication at excitatory synapse with postganglionic neuron.
Postganglionic neuron releases either:
Norepinephrine (approximately 80% of postganglionic sympathetic neurons).
Epinephrine (less common).
Acetylcholine (ACh) may also be released.
14.2 Sympathetic Neurotransmitters and Receptors (2 of 6)
Classes of Sympathetic Receptors:
Adrenergic Receptors:
Alpha (α) Receptors and Beta Receptors: Bind to norepinephrine or epinephrine.
Alpha-1 Receptors: Located on smooth muscle cell membranes including those in blood vessels, gastrointestinal organs, kidneys, arrector pili muscles, pupils, uterus, and certain genitourinary organs.
14.2 Sympathetic Neurotransmitters and Receptors (3 of 6)
Classes of Sympathetic Receptors (continued):
Alpha-2 Receptors:
Located on select sympathetic target cells in pancreas and adipose tissue, influencing hyperpolarization and dampening sympathetic responses.
14.2 Sympathetic Neurotransmitters and Receptors (4 of 6)
Classes of Sympathetic Receptors (continued):
Beta-1 Receptors: Present in cardiac muscle cells and certain kidney and adipose tissue cells.
Beta-2 Receptors: Present in smooth muscle cells of bronchioles, skeletal muscle fibers, urinary bladder, blood vessels of skeletal muscle, liver, pancreas, and salivary glands.
Beta-3 Receptors: Found in adipose tissue and smooth muscle cells within the digestive tract.
14.2 Sympathetic Neurotransmitters and Receptors (5 of 6)
Cholinergic Receptors:
Bind to Acetylcholine (ACh).
Muscarinic Receptors: Located in sweat glands.
Nicotinic Receptors: Found on membranes of all postganglionic neurons and adrenal medulla cells (modified postganglionic neurons).
14.2 Sympathetic Neurotransmitters and Receptors (6 of 6)
Figure 14.6: The effect of receptors on preganglionic neurons.
Key: N.E. = norepinephrine.
14.2 Effects of the Sympathetic Nervous System on Target Cells (1 of 11)
Basic Function: Ensure survival and maintain homeostasis during physical or emotional stress.
Example Scenario: Running a race:
Heart rate increases;
Sweating is stimulated;
Blood pressure goes up;
Reactions also occur when feeling nervous or excited (sympathetic activation triggers norepinephrine and epinephrine release).
14.2 Effects of the Sympathetic Nervous System on Target Cells (2 of 11)
Figure 14.7: Main effects of the sympathetic nervous system on target cells.
Key: NT = neurotransmitter, NE = norepinephrine, ACh = acetylcholine.
14.2 Effects of the Sympathetic Nervous System on Target Cells (3 of 11)
Effects on Cardiac Muscle Cells:
Increased heart rate and blood pressure due to norepinephrine binding to receptors on cardiac muscle cells, opening ion channels to enhance contraction rates and blood flow.
Effects on Smooth Muscle Cells:
Vasoconstriction of blood vessels in the digestive, urinary, and integumentary systems occurs to redirect blood towards skeletal and cardiac muscles, reducing flow to other areas (resulting in paler skin).
14.2 Effects of the Sympathetic Nervous System on Target Cells (4 of 11)
Continued Effects on Smooth Muscle Cells:
Bronchodilation occurs due to norepinephrine binding to receptors on bronchioles, increasing airway diameter and oxygen intake.
Vasodilation of vessels serving skeletal and cardiac muscles enhances blood flow.
14.2 Effects of the Sympathetic Nervous System on Target Cells (5 of 11)
Constriction of urinary and digestive sphincters:
Norepinephrine binds to sphincter smooth muscle cells, complicating bowel and bladder emptying.
Relaxation of the digestive tract smooth muscle: Reduces food movement and digestion.
Pupil dilation: Occurs allowing more light entry via norepinephrine on the dilator pupillae muscles.
14.2 Effects of the Sympathetic Nervous System on Target Cells (6 of 11)
Continued Effects on Smooth Muscle Cells:
Constriction of blood vessels in most exocrine glands; generally decreases secretion (except for sweat glands).
14.2 Effects of the Sympathetic Nervous System on Target Cells (7 of 11)
Effects on Cellular Metabolic Rate:
Norepinephrine drives increased ATP need:
Triggers lipid breakdown in adipocytes, releasing free fatty acids into the bloodstream.
Stimulates glucose release from glycogen in liver cells and promotes glucose synthesis from other precursors.
Increases glucagon release from pancreatic cells, elevating blood glucose levels.
14.2 Effects of the Sympathetic Nervous System on Target Cells (8 of 11)
Effects on Sweat Glands:
Postganglionic sympathetic neurons release ACh to sweat gland cells; this promotes secretion through muscarinic receptor binding.
14.2 Pseudoscience Exposed: The Sympathetic Nervous System and Weight Loss Supplements
Overview: Dietary supplements like Yohimbine claim to promote weight loss by acting on adipocytes.
Real Effects:
Yohimbine blocks receptors in blood vessels and the spinal cord causing vasodilation and excessive sympathetic activity, which can dangerously elevate heart rates and lead to serious health issues such as seizures, hypertension, kidney failure, and anxiety disorders.
14.2 Effects of the Sympathetic Nervous System on Target Cells (9 of 11)
Effects on Cells of the Adrenal Medulla:
Comprises modified postganglionic sympathetic neurons acting as a ganglion:
Preganglionic neuron releases ACh.
Binds to nicotinic receptors on adrenal medulla cells.
ACh stimulates cells to release additional epinephrine (80%) and norepinephrine (20%) into the bloodstream, prolonging sympathetic effects.
14.2 Effects of the Sympathetic Nervous System on Target Cells (10 of 11)
Figure 14.8: Sympathetic nervous system stimulation of the adrenal medulla shows release processes.
14.2 Effects of the Sympathetic Nervous System on Target Cells (11 of 11)
Other Effects:
Boosts mental alertness by increasing neuronal activity in cerebral cortex association areas.
Enhances blood clotting, which is beneficial during injury.
Raises skeletal muscle tension, allowing for stronger contractions under stress (“adrenaline rush”).
Triggers contraction of arrector pili muscles causing goosebumps and facilitates male ejaculation via effects on reproductive duct smooth muscle cells.
14.2 Pharmacology and Sympathetic Nervous System Receptors (1 of 2)
Drug Design Implications: Different sympathetic receptor subtypes facilitate the creation of target-specific drugs, minimizing side effects.
Types of Drugs:
Antagonists: Block receptors, preventing norepinephrine from binding, effectively lowering blood pressure or relaxing prostate smooth muscle in benign prostatic hyperplasia.
Agonists: Mimic norepinephrine's effects, used in treating hypertension and opiate withdrawal.
14.2 Pharmacology and Sympathetic Nervous System Receptors (2 of 2)
Various Effects of Blockers (Antagonists):
Decrease heart contraction rate; applicable in hypertension treatments.
Bind smooth muscle cells in bronchioles causing bronchodilation beneficial for asthma management.
14.3 The Parasympathetic Nervous System
Function Overview: Crucial for body maintenance, including digestion and urine formation, generally dominant during rest but not entirely inactive.
14.3 Gross and Microscopic Anatomy of the Parasympathetic Nervous System (1 of 4)
Organization:
Preganglionic neuron cell bodies found in cranial nerves:
Oculomotor, Facial, Glossopharyngeal, and Vagus.
Sacral nerves including S2-S4 also contain preganglionic neuron bodies.
Preganglionic axons synapse with postganglionic neurons within Terminal Ganglia near target cells; these axons are relatively short.
14.3 Gross and Microscopic Anatomy of the Parasympathetic Nervous System (2 of 4)
Figure 14.9: Organization of the parasympathetic nervous system visually represented.
14.3 Gross and Microscopic Anatomy of the Parasympathetic Nervous System (3 of 4)
Parasympathetic Cranial Nerves Detailed:
Vagus Nerves: Supply 90% of parasympathetic innervation body-wide; form plexuses innervating various organs (e.g., cardiac plexuses).
Oculomotor Nerves: Synapse on Ciliary Ganglia.
Facial Nerves: Synapse on Submandibular and Pterygopalatine Ganglia.
Glossopharyngeal Nerves: Synapse on Otic Ganglia.
14.3 Gross and Microscopic Anatomy of the Parasympathetic Nervous System (4 of 4)
Parasympathetic Sacral Neurons:
Supply the terminal segments of large intestine, urinary bladder, and reproductive organs.
Branches form Pelvic Splanchnic Nerves, creating pelvic plexuses.
Most neurons synapse in Terminal Ganglia located in organ walls.
14.3 Parasympathetic Neurotransmitters and Receptors
Nature of Neurotransmitters: Both pre- and postganglionic parasympathetic neurons release ACh at synapses; generally excitatory effects.
Types of Receptors:
Nicotinic Receptors: Located in membranes of all postganglionic neurons.
Muscarinic Receptors: Found in membranes of all parasympathetic target cells.
14.3 Effects of the Parasympathetic Nervous System on Target Cells (1 of 9)
Functional Role: Maintains homeostasis when the body is at rest.
Example: Post-meal effects such as:
Slowed heart rate;
Lower blood pressure;
Promoted digestion;
Adjusted eye focus for reading (near vision).
14.3 Effects of the Parasympathetic Nervous System on Target Cells (2 of 9)
Figure 14.10: Main effects of the parasympathetic nervous system illustrated regarding target cell interactions.
14.3 Effects of the Parasympathetic Nervous System on Target Cells (3 of 9)
Effects on Cardiac Muscle Cells:
Heart rate and blood pressure lowering:
Preganglionic parasympathetic neurons innervate the heart via vagus nerves, reducing contraction rates.
14.3 Effects of the Parasympathetic Nervous System on Target Cells (4 of 9)
Effects on Smooth Muscle Cells:
Pupil Constriction: Controlled by the oculomotor nerve innervating sphincter pupillae muscle, allowing less light.
Accommodation for Near Vision: Smoothed muscle cells contract, modifying lens shape for clearer focus.
Bronchoconstriction: Vagal neurons induce mild contraction of airflow linings, regulating air passages.
14.3 Effects of the Parasympathetic Nervous System on Target Cells (5 of 9)
Continued Effects on Smooth Muscle:
Peristalsis: Smooth muscle contraction in the digestive tract is triggered by vagal neurons to facilitate food movement.
Sphincter Relaxation: Promotes urination and bowel movement through relaxation of smooth muscle in sphincters.
14.3 Effects of the Parasympathetic Nervous System on Target Cells (6 of 9)
Genital Blood Vessel Effects:
In males and females, parasympathetic innervation promotes engorgement of the penis/clitoris via pelvic splanchnic neurons triggering smooth muscle relaxation and vasodilation.
Most vessels dilate due to reduced sympathetic activity and diminished epinephrine impact.
14.3 Effects of the Parasympathetic Nervous System on Target Cells (7 of 9)
Effects on Glandular Cells:
Minimal to no impact on sweat glands (controlled by sympathetic).
Increased tear production from lacrimal glands, mucus secretion from mucosa, salivation from salivary glands, and enzyme secretion from digestive system due to cranial and vagal nerve activity.
14.3 Effects of the Parasympathetic Nervous System on Target Cells (8 of 9)
Other Physiological Effects:
No direct effect on metabolic rate, mental alertness, skeletal muscle contraction strength, blood clotting, adipocyte activity, or most endocrine functions.
Each returns to resting states during parasympathetic activity, helping in fuel storage for future sympathetic activities.
14.3 Effects of the Parasympathetic Nervous System on Target Cells (9 of 9)
Memory Strategy: Use a memory card game to distinguish ANS terms like thoracolumbar vs. craniosacral or preganglionic vs. postganglionic, enhancing recall of definitions and functions for both divisions.
14.3 Pharmacology of the Parasympathetic Nervous System
Muscarinic Agonists:
Activate muscarinic receptors, mimicking parasympathetic actions to stimulate gastrointestinal activity or bladder emptying, especially after surgery.
Muscarinic Antagonists:
Block muscarinic receptors, preventing parasympathetic effects; e.g., Atropine can increase slow heart rates and dilate pupils; Scopolamine is applied to treat nausea and related symptoms.
14.3 Drugs with Anticholinergic Side Effects
Common Medications can disrupt ACh action, leading to:
Dry Mouth: Reduced saliva production.
Constipation: Slowed digestive contractions, obstructing bowel movements.
Urinary Retention: Hindered urinary sphincter relaxation.
Blurred Vision: Dilation of pupils and ciliary muscle inability to contract, obstructing near vision focus.
14.4 Interactions of Autonomic Divisions
General Functionality: Sympathetic and parasympathetic systems generally function antagonistically.
Dual Innervation: Most organs receive input from both systems.
Sympathetic predominates during exercise or crises.
Parasympathetic regulates post-exercise recovery.
14.4 Concept Boost: Differences in Sympathetic and Parasympathetic Functions (1 of 2)
Sympathetic: Active in a dangerous situation, promoting escape mechanisms and mobilizing energy.
Parasympathetic: Restores normal conditions post-crisis, enhancing digestion and nutrient absorption.
14.4 Concept Boost: Differences in Sympathetic and Parasympathetic Functions (2 of 2)
Example Scenarios:
Sympathetic Activation: Increased heart rate, blood flow rerouted from digestive organs to skeletal muscle.
Parasympathetic Recovery: Promotion of digestive processes, slower heart rate after an emergency.
14.4 Autonomic Tone
Definition: Represents the constant activity level from both systems; varies across different organs.
Sympathetic Tone: Dominant in blood vessels, maintaining partial constriction.
Parasympathetic Tone: Dominant in heart function, averaging 72 beats per minute, stronger in athletic individuals.
14.4 Postural Orthostatic Tachycardia Syndrome (POTS)
Overview: Characterized by an abnormal heart rate increase upon standing, often leading to symptoms like dizziness, lightheadedness, or fatigue due to excessive sympathetic activity.
Treatment includes dietary modifications and medications blocking sympathetic receptors.
14.4 Summary of Nervous System Control of Homeostasis (1 of 3)
Both ANS divisions contribute to central homeostasis regulation.
The hypothalamus and brainstem reticular formation play essential roles in regulating physiological variables.
Signals from the hypothalamus influence Autonomic Centers in the reticular formation controlling preganglionic neurons.
14.4 Summary of Nervous System Control of Homeostasis (2 of 3)
Higher brain input affects autonomic responses:
Emotional states from the amygdala and cerebral cortex influence autonomic activities.
14.4 Summary of Nervous System Control of Homeostasis (3 of 3)
Figure 14.12: Illustrates the control mechanisms of the nervous system over homeostasis, linking various brain regions to autonomic functions.