Autonomic Nervous System Notes

The Autonomic Nervous System (ANS)

• Consists of motor neurons that innervate smooth and cardiac muscle, and glands.
• Effectors are the targets, where electrical signals are converted to chemical signals.
• Utilizes neurotransmitters acetylcholine (ACh) or norepinephrine (NE).
• Supplies glands.

Neurons

• Preganglionic neurons: Cell bodies are located in the CNS, with lightly myelinated preganglionic axons extending to the ganglion. They always release acetylcholine.
• Postganglionic neurons: Are non-myelinated and have acetylcholine receptors. The neurotransmitter released changes at this stage.

Neurotransmitters

• Norepinephrine (NE) is primarily used by the sympathetic division (99.9%).
• Acetylcholine (ACh) is used by the parasympathetic division (100%).
• Only norepinephrine and acetylcholine are used as neurotransmitters in the ANS.

Parasympathetic Division

• Known as the "Rest and Digest" division.
• Conserves body energy by keeping energy use as low as possible, even during maintenance activities.

Axon Fiber Length

• Depends on the division.
• Sympathetic: Short pre-ganglionic fibers and long post-ganglionic fibers.
• Parasympathetic: Long pre-ganglionic fibers and short post-ganglionic fibers.

Dual Innervation

• Parasympathetic fibers originate from cranial nerves (III, VII, IX, X).
• Sympathetic fibers originate from $T1$ to $L2$ (visceral motor neurons in the lateral horns of the spinal cord).

Specific Cranial Nerves and Ganglia (Parasympathetic)

• Oculomotor Nerve (III): Innervates the iris and part of the ciliary muscle.
  • Postganglionic cell bodies are located in the ciliary ganglia.
  • Causes pupillary constriction.
• Facial Nerve (VII): Stimulates large glands.
  • Submandibular ganglia (saliva).
  • Pterygopalatine ganglia (lacrimal glands).
• Vagus Nerve (X): Carries 90% of preganglionic parasympathetic fibers in the body.
  • Intermural ganglia.
  • Forms plexuses: cardiac, pulmonary, and esophageal.
• $S2$-$S4$: Innervates the distal part of the digestive tract.

Blood Vessels

• Blood vessels primarily have epinephrine receptors.
• Parasympathetic innervation to blood vessels is mainly to external genitalia.
• Proximal vagus and hypogastric ($S2$-$S4$).

Sympathetic Division

• "Flight or Fight" response.
• Thoracolumbar division ($T1$-$L2$).
• Cell bodies originate from the lateral ganglia of the spinal cord.

Sympathetic Trunk Ganglia

• Axons can track up, down, or synapse at the same level to reach the target organ.
• 23 sympathetic trunk ganglia.
• White and gray rami communicantes facilitate movement of sympathetic fibers.
  • White rami: preganglionic fibers (myelinated).
  • Gray rami: postganglionic fibers (non-myelinated).
• Postganglionic fibers communicate to the effector.

Collateral Ganglia

• Fibers do not synapse at the trunk ganglia.
• Located anterior to the vertebral column.
• Include superior, inferior, and middle ganglia.
  • $T1$-$T2$: Eyes and lacrimal glands.
  • $T1$-$T6$: Heart.
    • Lungs: Inferior cervical ganglia.
      • Increases heart rate.
    • Coronary blood vessels: Dilates blood vessels.
    • Dilates bronchioles.

Autonomic Nervous System Overview

• The ANS innervates smooth muscle, cardiac muscle, and glands, controlling visceral activity.
• Controls both conscious and unconscious processes.
• "Autonomic" = "Self-govern".

ANS Divisions

• Two divisions: Sympathetic and Parasympathetic.
• Effectors: Cardiac muscle, smooth muscle, and glands.
• Differs from the somatic nervous system, which only innervates skeletal muscle.

Two-Neuron Chain

• The ANS uses a two-neuron chain to reach effectors.
  • Preganglionic neuron: Resides in the brain stem or spinal cord; lightly myelinated axon synapses with the second motor neuron.
  • Postganglionic neuron: Cell body is in the autonomic ganglion; non-myelinated axon extends to the effector organ.
• Preganglionic axons: thin, myelinated fibers.
• Postganglionic axons: even thinner, non-myelinated fibers.

Neurotransmitters in ANS

• Acetylcholine (ACh): involved with parasympathetic fibers (100%).
• Norepinephrine (NE): involved with sympathetic fibers (99.9%).

Functional Differences: Parasympathetic vs. Sympathetic

• Parasympathetic: Maintenance functions and conserves body energy.
• Sympathetic: Mobilizes the body during activity.
• One stimulates smooth muscle or glands, while the other inhibits the action; dual innervation allows counterbalance.

Parasympathetic

• "Rest and Digest".
• Keeps body energy low.
• Low blood pressure and heart rate.
• Gastrointestinal tract digests food, pupils are constricted, and vision accommodates close-up images.

Sympathetic

• "Flight or Fight".
• Evident during excitement or emergencies.
• Rapid heartbeat, deep breathing, dry mouth, cold, sweaty skin, and dilated pupils.
• Constricts visceral blood vessels to shunt blood to active skeletal muscles.
• Dilates bronchioles in the lungs to increase airflow.
• Stimulates the liver to release more glucose into the blood.

Anatomical Divisions

Parasympathetic

• Origin: Cranial-Sacral (Brain & Sacral).
• Cranial Nerves: III (Oculomotor), VII (Facial), IX (Glossopharyngeal), X (Vagus).
• Fiber Length: Long preganglionic fibers & short postganglionic fibers.
• Ganglia Location: Near viscera/effector organs.

Sympathetic

• Origin: Thoracolumbar ($T1$-$L2$).
• Fiber Length: Short preganglionic & long postganglionic.
• Ganglia Location: Close to the spinal cord.
• Gray rami contain non-myelinated postganglionic fibers.
• White rami contain myelinated preganglionic fibers.

Detailed Look at the Parasympathetic Division

• Also known as the Cranial Sacral division.
• Preganglionic fibers extend from the CNS nearly all the way to the structures they innervate.
• Axons synapse with postganglionic neurons in terminal ganglia close to the organ.
• Short postganglionic axons synapse with effector cells in the immediate area.
• Preganglionic fibers run in oculomotor (III), facial (VII), glossopharyngeal (IX), and vagus (X) nerves.

Oculomotor Nerve (III)

• Innervates smooth muscle in the eye, causing pupillary constriction and lens bulging.
• Postganglionic neurons are in the ciliary ganglia within the eye orbits.

Facial Nerve (VII)

• Stimulates large glands in the head (originate from pons).
• Nasal and lacrimal glands synapse in the pterygopalatine ganglia.
• Submandibular and sublingual salivary glands synapse in the submandibular ganglia.

Glossopharyngeal Nerve (IX)

• Originates from the inferior salivatory nuclei of the medulla.
• Activates the parotid salivary glands; synapses in the otic ganglia.

Vagus Nerve (X)

• Accounts for 90% of parasympathetic fibers in the body, serving all thoracic and abdominal viscera.
• Preganglionic axons arise from the dorsal motor nuclei of the medulla and synapse within terminal ganglia.

Vagus Nerve Plexuses

• Cardiac plexus: Slows heart rate.
• Pulmonary plexus: Gives airways to lungs.
• Esophageal plexus: Serves the esophagus and part of the stomach.
• Anterior & Posterior vagal trunks: Fibers extend to the abdominal aortic plexus, innervating the liver, gallbladder, stomach, small intestine, pancreas, and proximal half of the large intestine.

Sacral Part of Parasympathetic Division

• Serves pelvic organs and the distal half of the large intestine.
• Originates from neurons in the lateral gray matter of the spinal cord ($S2$-$S4$).
• Axons run in the ventral root of the spinal nerve and branch off to form the pelvic splanchnic nerve, passing through the inferior hypogastric plexus.
• Synapses with intermural ganglia in walls of the large intestine, urinary bladder, gallbladder, uterus, and reproductive organs.

Sympathetic Division (Detailed)

• More complex due to innervation of more organs.
• Sweat glands, pili muscles of hair follicles, and smooth muscle of arteries and veins are only innervated by the sympathetic division.
• Preganglionic neurons originate in the spinal cord ($T1$-$L2$), known as the "Thoracolumbar" division.
• Preganglionic sympathetic neurons are in the gray matter, forming lateral horns.
• Lateral horns are not found in the sacral cord.
• Preganglionic fibers leave the ventral root, passing through the white ramus communicantes to enter the sympathetic trunk ganglion, forming part of the sympathetic trunk.
• The sympathetic trunk consists of sympathetic ganglia (chain ganglia).
• Sympathetic fibers only arise from thoracic and lumbar cord segments.

23 Sympathetic Ganglia

• 3 cervical.
• 11 thoracic.
• 4 lumbar.
• 4 sacral.
• 1 coccygeal.

Ways a Preganglionic Axon can synapse

• Synapse at the same level: synapse is in the same trunk ganglion.
• Synapse at a higher or lower level: axon ascends or descends the trunk to another trunk ganglion.
• Synapse at a collateral ganglion: passes through the trunk ganglion and emerges without synapsing, synapsing in the collateral ganglia, which only occurs in the abdomen & pelvis, forming splanchnic nerves.

Post-Synapse Route

• After synapse occurs in sympathetic trunk ganglia, postganglionic axons enter via the ventral ramus.

Rami Communicantes

• Grey communicants: Nonmyelinated postganglionic axons.
• White communicants: Myelinated preganglionic axons.
• They only contain sympathetic neurons ($T1$-$L2$).
• Rami communicant only carry sympathetic nerves; no parasympathetic involvement.

Pathway to the Head (Sympathetic)

• Sympathetic preganglionic fibers emerge and synapse in the superior cervical region, innervating skin, blood vessels, stimulating dilator muscles of the iris, inhibiting nasal and salivary glands, and innervating smooth muscles of the upper eyelid.

Pathway to Thorax (Sympathetic)

• Originate from $T1$-$T6$, synapse to the nearest sympathetic trunk ganglion.
• Preganglionic axons ascend in the sympathetic trunk to synapse in the middle and inferior cervical ganglia.
• From there, postganglionic fibers descend through the cardiac plexus and into the heart wall.
• Postganglionic fibers pass through pulmonary & esophagus.

Collateral Ganglia (Sympathetic)

• Most preganglionic fibers from $T5$-$L2$ synapse in collateral ganglia outside of the trunk, forming splanchnic nerves.
• Greater, lesser splanchnic nerves are associated with
  • Celiac ganglion which effects the Liver, gall bladder, stomach, spleen, kidney
  • Superior mesenteric ganglion for the large intestine and samll intestine.
  • Inferior Mesenteric ganglion Lumbar Splanchnic nerve which effect the Large intestine and rectus.
• Splanchnic nerves interweave, forming the abdominal aortic plexus that contains several important ganglia.

Pelvic Pathway (Sympathetic)

• Preganglionic fibers originate from $T{10}$-$L2$ and travel to the trunk to lumbar and sacral ganglia.
• Synapse happens with collateral ganglia, such as the inferior mesenteric ganglion with the lumbar splanchnic nerves.
• Postganglionic fibers serve the distal half of the large intestine, urinary bladder, large intestine, & reproductive organs.
• Sacral splanchnic nerves do not synapse.

Adrenal Medulla (Sympathetic)

• Preganglionic fibers pass directly without synapsing and synapse directly at the adrenal medulla.
• Upon stimulation, chromaffin cells secrete epinephrine and norepinephrine into the blood (80% epinephrine, 20% norepinephrine).

Visceral Sensory Neurons

• Send information about chemical changes, stretch, temperature, and irritation of viscera to the brain, which interprets the info as hunger, fullness, pain, or nausea.
• Receptors are free nerve endings located in the dorsal root ganglia & sensory ganglia of cranial nerves (Vagus nerve).
• Axons travel the same as autonomic fibers.

Visceral Reflex

• Receptor -> sensory neuron -> integration center -> motor neuron -> effector.
• Afferent fibers are visceral sensory neurons.
• Effectors are muscle, cardiac, and glands.
• Example: Empty rectum & bladder.
• Neurons composing reflex are in walls of the gastrointestinal tract, which is the enteric nervous system.

Acetylcholine and Norepinephrine

• Major neurotransmitters released are norepinephrine and acetylcholine.
• Cholinergic fibers release acetylcholine: all preganglionic axons and parasympathetic postganglionic axons.
• Adrenergic fibers release norepinephrine: majority of sympathetic postganglionic axons.
• Effects of neurotransmitters depend on the receptor.

Cholinergic Receptors

• Two types bind with acetylcholine: nicotinic and muscarinic.

Nicotinic Receptors

• Found on all postganglionic neurons, hormone-producing cells of the adrenal medulla, and sarcolemma of skeletal muscle.
• Effects of ACh at nicotinic receptors are always excitatory.
• Opens channels, depolarizing the postsynaptic cell.

Muscarinic Receptors

• Stimulated by postganglionic cholinergic fibers.
• Effect can be inhibitory or excitatory, depending on the receptor.

Adrenergic Receptors

• Alpha ($α$) and beta ($β$) receptors ($α1$ & $α2$, $β1$, $β2$, $β_3$ ).
• Organs that respond to norepinephrine have these receptors.

Effects

• NE binding to $β_1$ receptors of cardiac muscle increases heart rate and force of contraction.
• $β_2$ with NE dilates blood vessels & bronchioles, relaxes smooth muscle wall of the digestive organs, and relaxes the bladder.
• $β_3$ stimulates lipolysis by fat cells.

Dual Innervation

• Antagonistic interaction: The two divisions produce opposite effects that precisely control visceral organs.
• Sympathetic division increases heart rate and respiratory rate and inhibits digestion & elimination.
• Parasympathetic division decreases heart rate and respiratory rate, allowing for digestion to occur and eliminate waste.

Sympathetic Tone (Vasomotor Tone)

• Continual state of partial constriction of blood vessels.

Cooperative Effects

• Parasympathetic stimulation causes erection of the penis & clitoris.
• Sympathetic stimulation causes ejaculation of semen & orgasm of the female vagina.

Unique Roles of the Sympathetic Division

• Adrenal medulla, sweat glands, arrector pili muscles, kidneys, & blood vessels receive only sympathetic fibers.

Thermoregulatory Response to Heat

• Sympathetic fibers dilate skin blood vessels, allowing heat to escape, and activate sweat glands.

Thermoregulatory Response to Cold

• When the temperature drops, blood vessels constrict.

Effects of Kidney

• Release of renin from the kidney, causing an increase in blood pressure.

Metabolic Effects

• Raise glucose levels and mobilize fats for use as fuel.
• The sympathetic division is long-lasting.

ANS Control

• Brain stem, spinal cord, hypothalamus, and cerebral cortex.
• Hypothalamus is the main integrative center.
• Medulla regulates heart rate, blood vessel diameter, and gastrointestinal activities.
• Midbrain controls pupil and lens.

Hypothalamic Control

• Mediates reactions of fear (emotional response) heart activity, blood pressure, water balance, and endocrine activity.

Disorders

• Hypertension.
• Raynaud's disease: Painful; digits turn pale due to exposure to stress or cold.
• Autonomic dysreflexia: Spinal cord injuries above T6; life-threatening arterial blood pressure skyrockets, which can rupture blood vessels in the brain & cause a stroke.

Summary of Parasympathetic and Sympathetic Effects on Various Organs

Eye (Iris)

• Parasympathetic: Constricts pupils.
• Sympathetic: Dilates pupils.

Eye (Ciliary Muscle)

• Parasympathetic: Makes the lens bulge for close vision.
• Sympathetic: Flattens the lens for far vision.

Glands

• Parasympathetic: Stimulates secretory activity.
• Sympathetic: Inhibits secretory activity.

Salivary Glands

• Parasympathetic: Stimulates secretion of watery saliva.
• Sympathetic: Stimulates secretion of thick saliva.

Adrenal Medulla

• Parasympathetic: No effect.
• Sympathetic: Stimulates medulla cells to secrete epinephrine and norepinephrine.

Arrector Pili

• Parasympathetic: No effect.
• Sympathetic: Causes "goosebumps".

Heart (Muscle)

• Parasympathetic: Decreases heart rate.
• Sympathetic: Increases heart rate.

Heart (Blood Vessels)

• Parasympathetic: Dilates blood vessels only to genitalia
• Sympathetic: No effect.

Urinary Bladder

• Parasympathetic: Contracts bladder wall; promotes voiding.
• Sympathetic: Relaxes bladder wall

Lungs

• Parasympathetic: Constricts bronchioles.
• Sympathetic: Dilates bronchioles.

Digestive Tract

• Parasympathetic: Increases activity of digestive, Increases motility and allows food to move, promotes activity.
• Sympathetic: Decrease activity of digestive.

Liver

• Parasympathetic: excites, increase glucose intake from blood.
• Sympathetic: Release glucose to blood, inhibits,

Kidney

• Parasympathetic: No effect.
• Sympathetic: Promotes renin release, decrease Urine output.

Penis/Vagina

• Parasympathetic: Cause erection.
• Sympathetic: Ejaculation/Contraction.

Blood Vessels (Systemic)

• Parasympathetic: No effect (only to genitalia).
• Sympathetic: Constricts blood vessels, causing blood pressure to increase.