Autonomic Nervous System Notes

The Autonomic Nervous System

Overview of the Autonomic Nervous System (ANS)

• ANS Definition: Controls body functions without conscious thought (outside our awareness).
• Function: Fast-acting regulator of homeostasis.
• Effect: Short-lived.

Somatic Nervous System

• Components:
  • Upper motor neurons in the primary motor cortex.
  • Somatic motor nuclei of the brain stem.
  • Lower motor neurons.
  • Skeletal muscles.
• Process:
  • Signals travel from the upper motor neurons to the somatic motor nuclei.
  • Lower motor neurons then innervate skeletal muscles, leading to voluntary movement.

Autonomic Nervous System

• Components:
  • Visceral motor nuclei in the hypothalamus.
  • Preganglionic neurons.
  • Autonomic ganglia.
  • Ganglionic neurons.
  • Visceral effectors (smooth muscle, glands, cardiac muscle, adipocytes).
• Process:
  • The hypothalamus sends signals via motor neurons to autonomic centers.
  • A series of two motor neurons carries information to visceral effectors.

Neurons in the Autonomic Nervous System

• Preganglionic Neurons:
  • Carry information from the hypothalamus to autonomic centers.
• Ganglionic Neurons:
  • Cell bodies located in peripheral autonomic ganglia.
  • Ganglia contain hundreds to thousands of ganglionic neurons.
  • Innervate visceral effectors such as cardiac muscle, smooth muscle, glands, and adipose tissue.

Divisions of the Autonomic Nervous System

• Two Major Divisions:
  • Sympathetic division.
  • Parasympathetic division.
• Enteric Nervous System (ENS):
  • Influenced by both sympathetic and parasympathetic divisions.

Sympathetic Division

• Activity: Predominantly active during exertion, stress, or emergencies.
• Alternative Name: Thoracolumbar division.
• Ganglia Location: Axons innervate ganglia relatively close to the spinal cord.

Parasympathetic Division

• Activity: Dominant under resting conditions.
• Alternative Name: Craniosacral division.
• Ganglia Location: Axons innervate ganglia very close to (or within) target organs.

Anatomical Organization of Sympathetic Division

• Synapses: Preganglionic neurons synapse on ganglionic neurons.
• Ganglionic Neuron Locations:
  • Within the sympathetic chain.
  • In collateral ganglia.
  • Within modified ganglion cells in the adrenal medullae.
• Fiber Length:
  • Preganglionic fibers (axons of preganglionic neurons) are shorter.
  • Postganglionic fibers (axons of ganglionic neurons) are longer.

Sympathetic Chain Ganglia

• Function: Innervate visceral organs in the thoracic cavity via sympathetic nerves.
• Communication: Connect to spinal nerves via gray and white rami.

Collateral Ganglia

• Function: Innervate visceral organs in the abdominopelvic cavity via splanchnic nerves.

Suprarenal Medullae

• Function: Release neurotransmitters (hormones) into general circulation.
• Mechanism: Preganglionic fibers stimulate endocrine cells (specialized ganglionic neurons) to secrete hormones.

Sympathetic Innervation of Visceral Organs in the Abdominopelvic Cavity

• Splanchnic Nerves: Innervate three collateral ganglia.
  • Celiac ganglion: Postganglionic fibers innervate the stomach, liver, gallbladder, pancreas, and spleen.
  • Superior mesenteric ganglion: Postganglionic fibers innervate the small intestine and proximal two-thirds of the large intestine.
  • Inferior mesenteric ganglion: Postganglionic fibers innervate the kidneys, urinary bladder, terminal segments of the large intestine, and sex organs.

Responses to Increased Sympathetic Activity

• Fight or Flight Responses:
  • Heightened mental alertness.
  • Increased metabolic rate.
  • Decreased digestive and urinary functions.
  • Mobilization of energy reserves.
  • Dilation of respiratory passageways and increased respiratory rate.
  • Increased heart rate and blood pressure.
  • Activation of sweat glands.

Organization of Parasympathetic Division

• Anatomical Characteristics:
  • Long preganglionic fibers.
  • Ganglionic neurons (postganglionic) are adjacent to or inside target organs.
    • Terminal ganglia.
    • Intramural ganglia.
  • Short postganglionic fibers (in or near target organs).
• Effects: Specific and localized stimulation due to its structure.

Parasympathetic Nuclei

• Location: Midbrain, pons, and medulla oblongata; lateral gray horns of sacral spinal cord segments S2-S4.
• Cranial Nerves: Associated with cranial nerves III, VII, IX, and X.
• Pelvic Nerves: Preganglionic fibers in sacral spinal cord segments form distinct pelvic nerves.
  • Innervate intramural ganglia in kidneys, bladder, terminal portions of the large intestine, and sex organs.

Parasympathetic Innervation

• Vagus Nerve: Cranial nerve X provides about 75% of all parasympathetic outflow.
• Nerve Plexuses: Branches intermingle with preganglionic and postganglionic fibers of the sympathetic division.

Responses to Increased Parasympathetic Activity

• Rest and Digest Responses:
  • Decreased metabolic rate.
  • Decreased heart rate and blood pressure.
  • Increased secretion by salivary and digestive glands.
  • Increased motility and blood flow to the digestive tract.
  • Stimulation of urination and defecation.
  • Constriction of pupils.

Sympathetic Neurotransmitters and Receptors

• Receptors: Adrenergic receptors located in the plasma membrane of target cells.
  • Alpha receptors.
  • Beta receptors.
• Neurotransmitters:
  • Norepinephrine (NE).
  • Epinephrine (E).

Neurotransmitter Release

• Localized Sympathetic Activity:
  • Involves release of NE at sympathetic terminals.
  • Primarily affects alpha receptors near those terminals.
  • Effects persist for a few seconds.
• Generalized Sympathetic Activity:
  • Involves release of NE and E by adrenal medullae.
  • Affects alpha and beta receptors throughout the body.
  • Three times as much E released, resulting in greater effects on beta receptors.
  • Levels of NE and E elevated for up to 30 seconds and metabolic effects persist for several minutes.

Sympathetic Receptors

• Alpha ($\alpha$) Receptors:
  • Generally stimulated by NE and E.
  • Stimulation activates associated G proteins on the cytoplasmic side of the plasma membrane.
    • Alpha-1 receptors: Generally excitatory.
    • Alpha-2 receptors: Generally inhibitory.
• Beta ($\beta$) Receptors:
  • Generally stimulated by E.
  • Located in many organs (skeletal muscles, lungs, heart, liver).
  • Stimulation of beta receptors and G protein activation triggers changes in metabolic activity of the target cell.
    • Beta-1 (\beta1): Stimulation of cardiac muscle.
    • Beta-2 (\beta2): Relaxation of smooth muscle in respiratory passages and blood vessels of skeletal muscle.
    • Beta-3 (\beta3): Release of fatty acids by adipose tissue.

Parasympathetic Neurotransmitters and Receptors

• Neurotransmitter: Acetylcholine (ACh) for all parasympathetic activity.
• Cholinergic Receptors:
  • Nicotinic receptors.
  • Muscarinic receptors.

Cholinergic Receptors

• Nicotinic Receptors:
  • Always excitatory.
  • Also stimulated by nicotine.
• Muscarinic Receptors:
  • G protein-coupled receptors.
  • Occur at cholinergic neuromuscular or neuroglandular junctions in the parasympathetic division and few cholinergic junctions in the sympathetic division.
  • Activation produces longer-lasting effects than at nicotinic receptors.
  • Response can be excitatory or inhibitory.
  • Also stimulated by muscarine (toxin produced by poisonous mushrooms).

Functional Characteristics of the Sympathetic Division

• Neurotransmitters: Acetylcholine (ACh), Norepinephrine (NE), Epinephrine (E).
• Preganglionic neuron releases Ach, while postganglionic neuron releases NE, and the adrenal medulla releases E.

Functional Characteristics of the Parasympathetic Division

• Effects:
  • Center on relaxation, food processing, and energy absorption.
  • Also called the anabolic division because stimulation leads to increase in nutrients in the blood.
  • Cells use these nutrients for growth and development and excess is stored as lipids or glycogen for energy reserve.

Dual Innervation

• ANS Control: Autonomic tone (continuous level of spontaneous activity maintained even in the absence of stimuli).
• Dual Innervation: Receiving instructions from both sympathetic and parasympathetic divisions.
  • Occurs in many vital organs.
  • Effects may be complementary or opposite.
  • In organs with only sympathetic innervation, the responses vary depending on the type of receptor stimulated.

Dual Innervation Example: Effects on the Heart

• Heart: Composed of cardiac muscle tissue, contractions triggered by pacemaker cells.
• ANS Divisions: Have opposing effects on pacemaker function.
  • ACh release by parasympathetic division reduces heart rate.
  • NE release by sympathetic division accelerates heart rate.
  • Small amounts of both neurotransmitters released continuously.
  • Under resting conditions, parasympathetic effects dominate.

Visceral Reflexes

• Visceral Reflex Arc Components:
  • Receptor.
  • Sensory neuron.
  • Interneuron(s).
  • Motor neuron.
  • Effector.
• Types of Reflexes:
  • Short reflexes.
  • Long reflexes.

Short Reflexes

• Description: Afferent (sensory) fibers directly synapse with ganglionic neurons in autonomic ganglia, leading to a response in a peripheral effector.

Long Reflexes

• Description: Involve processing centers in the spinal cord and brain, with preganglionic neurons influencing ganglionic neurons to affect peripheral effectors.

Common Visceral Reflexes

• Examples:

  • cardioacceleratory reflex
  • Vasomotor reflexes.
  • Pupillary reflex.
  • Ejaculation (in males).
  • Gastric and intestinal reflexes.
  • Defecation.
  • Urination.
  • Direct light and consensual light reflexes.
  • Swallowing reflex.
  • Coughing reflex.
  • Baroreceptor reflex.
  • Sexual arousal.

  Each reflex involves specific stimuli and responses coordinated in various centers such as the medulla oblongata.

Interoceptors

• Function: Provide visceral sensory information.
• Location: Mouth, palate, pharynx, larynx, trachea, esophagus, and associated vessels and glands; visceral organs between the diaphragm and pelvic cavity; organs in the inferior portion of the pelvic cavity.
• Solitary Nucleus: Located on each side of the medulla oblongata and receives visceral sensory information.

Baroreceptors

• Function: Provide information on blood pressure, lung stretching, and volume within the digestive tract segments and bladder wall.
• Location: Carotid sinus, aortic sinus, lungs, colon, digestive tract, bladder wall.

Chemoreceptors

• Function: Sensitive to changes in pH, PCO2, and PO2 in blood and cerebrospinal fluid.
• Location: Respiratory centers in the medulla oblongata, carotid bodies, aortic bodies.

Levels of Control in the ANS

• Basic Visceral Reflexes: Controlled by nuclei in spinal cord and brain stem (rapid and automatic responses).
• More Complex Reflexes: Coordinated by processing centers in the medulla oblongata (cardiovascular and respiratory centers, salivating, swallowing, digestive secretions, peristalsis, urinary function).
• Centers in Medulla Oblongata: Regulated by the hypothalamus, which acts as the headquarters of both sympathetic and parasympathetic divisions.
• Continual Feedback: Occurs between higher brain centers, hypothalamus, and brain stem.

Motor Control in the Autonomic Nervous System

• Hierarchy: Cerebral cortex influences the limbic system, thalamus, and hypothalamus.
• Hypothalamus: Acts as the headquarters for sympathetic and parasympathetic divisions.
• Medulla Oblongata: Contains processing centers for complex visceral reflexes (cardiac, vasomotor, swallowing, coughing, respiratory centers).
• Spinal Cord: Controls sympathetic visceral reflexes (T1-L2) and parasympathetic visceral reflexes (S2-S4).