Autonomic Nervous System Flashcards

Overview of the Autonomic Nervous System

• Innervates smooth muscle, cardiac muscle, and glands

• Adjusts their activities to ensure optimal support for bodily functions

• Described as involuntary

Divisions of the ANS

• Two competing parts of the ANS

• They serve the same organs (dual innervation) but have opposite effects on them

• Sympathetic nervous system

• Parasympathetic nervous system

Sympathetic nervous system

• Mobilizes the body for activity; “fight or flight”

• Activated by the 4 E’s: exercise, excitement, emergency, embarrassment

• Physiological effects include: ext{increased heart rate}, ext{ dry mouth}, ext{ increased sweat production}, ext{ pupil dilation}

• During vigorous physical activity:

  • Sends blood to skeletal muscles and heart

  • Dilates bronchioles (airway)

  • Causes liver to release glucose

Parasympathetic nervous system

• Conserves energy; responsible for maintenance of bodily systems

• Described as “rest & digest”

• Calming effect on organs

• Directs the 3 D’s: digestion, diuresis, defecation

• Blood pressure, heart rate, and respiratory rates are low; GI tract activity is high

• Pupils constricted

• The Vagus nerve (CN X) comprises about 90% of parasympathetic nerve fibers; i.e. ext{CN X} ext{ contains } rac{9}{10} ext{ of parasympathetic fibers}

Overview/Major Effects of Divisions

• Parasympathetic effects:

  • Constricts pupils

  • Stimulates salivation

  • Slows heart rate

  • Constricts bronchi

  • Stimulates digestion

  • Stimulates bile release

  • Contracts bladder

  • Mediated largely by the Vagus nerve (CN X)

• Sympathetic effects:

  • Dilates pupils

  • Inhibits salivation

  • Accelerates heart rate

  • Dilates bronchi

  • Inhibits digestion

  • Converts glycogen to glucose

  • Inhibits bladder constriction

• Note on pathways: a chain of sympathetic ganglia is involved; parasympathetic pathways often involve the brainstem and sacral regions

Origins of neurons

• Neurons originate in spinal cord or brainstem

• Sympathetic: thoracolumbar

  • Originate in thoracic & lumbar regions of the vertebral column

• Parasympathetic: craniosacral

  • Originate in the base of the brain and the sacral region of the spine

Ganglia

• Ganglia = clusters of neurosomas; synaptic connection points between neurons

• Two neurons are needed for each pathway in the ANS (for both divisions):

  • Neuron 1: spinal cord/brainstem → ganglion (preganglionic neuron)

  • Neuron 2: ganglion → effector (postganglionic neuron)

ANS Signaling

• Involves neurotransmitters and hormones

• The same chemical can have different effects depending on where it acts in the body

• The same chemical can be both a neurotransmitter and a hormone

• Neurotransmitter: transmitted via nerve fibers

• Hormones: transmitted in the bloodstream

Sympathetic Neurotransmitters

• Acetylcholine (ACh)

  • Sent from preganglionic fiber to the ganglion

  • ACh is released from cholinergic fibers

• Norepinephrine (NE)

  • Sent from postganglionic nerve fiber to the effector

  • NE is released from adrenergic fibers

Sympathetic Hormones

• There is a set of fibers that run from the spinal cord to the adrenal glands

• Preganglionic fibers release ACh

• ACh triggers chromaffin cells in the adrenal gland to release epinephrine and norepinephrine into the bloodstream

• Result: circulating hormones EPI ext{ and } NE act on target organs

Parasympathetic Neurotransmitter

• ACh = acetylcholine (cholinergic)

• ACh is released from both preganglionic and postganglionic fibers

• Cranial parasympathetic neurons and sacral parasympathetic neurons use ACh on target organs

• Sympathetic neurons typically use NE on target organs, though some sympathetic postganglionic fibers (e.g., to sweat glands) use ACh (cholinergic)

• Diagrammatic summary (from slide):

  • Brain: ACh = acetylcholine (cholinergic)

  • ACh released from both preganglionic AND postganglionic fibers

  • Cranial parasympathetic neurons; Sacral parasympathetic neurons use ACh on visceral effectors

  • Sympathetic neurons: postganglionic release NE on visceral effectors; some exceptions use ACh

Dual Innervation

• Most visceral organs receive innervation from both divisions

• Both systems are usually active simultaneously

• Sympathetic tone = rate of action potentials moving through the sympathetic nervous system

• Parasympathetic tone = rate of action potentials moving through the parasympathetic nervous system

• Usually one system is dominant at rest to keep sympathetic activity from going haywire

Antagonistic Control

• Antagonism: regulators have opposite effects on a system

• Can both stimulate AND inhibit the same system at the same time (in different tissues)

• Allows for precise control of visceral activity

• Example: running from a burning house

  • Increase in blood flow to leg muscles

  • Decrease in blood flow to digestive system

• How this is achieved: receptor differences on target tissue cells; organs/tissues have different receptor types and numbers which change the effect of the neurotransmitter

Visceral (Autonomic) Reflexes

• Similar to somatic reflexes but visceral (not skeletal) effects

• Unconscious, automatic, predictable responses to stimulation

• Pathway: receptor → afferent neuron → integrating center (interneuron) → efferent neuron → effector

• Example: blood pressure regulation

• Integrating centers of the ANS: brainstem; hypothalamus; spinal cord

Chronic stress and physiological sigh

• Chronic stress: overactivity of the sympathetic nervous system can lead to health issues, including

  • High blood pressure

  • Digestive problems

  • Suppression of immune system

• Activating the parasympathetic nervous system decreases the negative effects of chronic stress

• Try the “physiological sigh”:

  • Double inhale: deep breath in + a tiny bit more at the top of the breath

  • Slow breath out

  • Repeat 1–3 times

• The sympathetic nervous system decreases activity of these “less necessary” functions when danger is perceived

• Reference/idea in video: https://www.youtube.com/watch?v=kSZKIupBUuc

Overview of the Autonomic Nervous System

• Innervates smooth muscle, cardiac muscle, and glands found throughout the body, including those of the viscera and blood vessels.

• Adjusts their activities largely unconsciously and involuntarily to ensure optimal support for bodily functions and maintain homeostasis.

• Described as involuntary because its control is sub-conscious, meaning it operates without conscious thought or effort.

Divisions of the ANS

• The ANS is divided into two functionally opposing, yet complementary parts:

• They serve the same organs (dual innervation) but typically have opposite or complementary effects on them, allowing for fine-tuned control of organ function.

• Sympathetic nervous system

• Parasympathetic nervous system

Sympathetic nervous system

• Often referred to as the “fight or flight” system, it mobilizes the body for activity and prepares it to cope with perceived threats or stressful situations.

• Activated by the 4 E’s: exercise, excitement, emergency, embarrassment, demanding situations requiring quick action.

• Physiological effects include: \text{increased heart rate and contractility}, \text{ dry mouth (due to inhibited salivary gland secretion)}, \text{ increased sweat production (especially from eccrine glands)}, \text{ pupil dilation (mydriasis) to allow more light in}, \text{ bronchodilation (widening of airways)}

• During vigorous physical activity or stress:

  • Sends blood away from digestive and excretory organs and primarily to skeletal muscles and heart to optimize oxygen and nutrient delivery.

  • Dilates bronchioles (airway) to increase air intake and oxygen absorption.

  • Causes the liver to release glucose into the bloodstream (glycogenolysis) to provide energy for muscle activity.

  • Inhibits non-essential functions like digestion and urine formation.

Parasympathetic nervous system

• Often described as the “rest & digest” or “feed and breed” system, it conserves energy and is responsible for maintenance of bodily systems during quiescent periods.

• Has a calming and restorative effect on organs, promoting relaxation and vegetative functions.

• Directs the 3 D’s: digestion (nutrient absorption), diuresis (urination), defecation (elimination of waste).

• Blood pressure, heart rate, and respiratory rates are typically low and stable; GI tract activity is high (increased motility and glandular secretion).

• Pupils are constricted (miosis) to regulate light intake and focus on near objects.

• The Vagus nerve (CN X) comprises about 90% of all parasympathetic preganglionic nerve fibers emerging from the brainstem, innervating most thoracic and abdominal viscera; i.e. \text{CN X contains } \frac{9}{10} \text{ of parasympathetic fibers}. This highlights its critical role in parasympathetic control.

Overview/Major Effects of Divisions

• Parasympathetic effects:

  • Constricts pupils (miosis).

  • Stimulates salivation (watery saliva).

  • Slows heart rate.

  • Constricts bronchi.

  • Stimulates digestion (increases GI motility and secretions).

  • Stimulates bile release and pancreatic enzyme secretion.

  • Contracts bladder (promotes urination).

  • Promotes erection (primarily through nitric oxide release).

  • Mediated largely by the Vagus nerve (CN X) for most visceral organs.

• Sympathetic effects:

  • Dilates pupils (mydriasis).

  • Inhibits salivation (thick, viscous saliva).

  • Accelerates heart rate and increases force of contraction.

  • Dilates bronchi.

  • Inhibits digestion (slows GI motility and secretions).

  • Converts glycogen to glucose (glycogenolysis) in the liver.

  • Inhibits bladder constriction (promotes urine retention).

  • Stimulates ejaculation in males and orgasm in females.

• Note on pathways: Sympathetic pathways typically involve a short preganglionic neuron originating in the thoracolumbar region and synapsing in a chain of sympathetic ganglia (paravertebral ganglia) or collateral ganglia. Parasympathetic pathways often involve long preganglionic neurons originating in the brainstem (cranial nerves) and sacral regions, synapsing with short postganglionic neurons in terminal ganglia located near or within target organs.

Origins of neurons

• Autonomic neurons originate in specific regions of the spinal cord or brainstem.

• Sympathetic: thoracolumbar outflow

  • Preganglionic neurons originate in the lateral horns of the grey matter of the thoracic and first few lumbar regions (T1-L2) of the vertebral column.

  • Fibers exit the spinal cord via ventral roots.

• Parasympathetic: craniosacral outflow

  • Preganglionic neurons originate from nuclei of specific cranial nerves (III, VII, IX, X) in the brainstem and from the lateral grey matter of the sacral region of the spine (S2-S4).

  • Cranial nerve fibers innervate organs in the head and most of the torso (via Vagus nerve).

  • Sacral fibers innervate pelvic organs.

Ganglia

• Ganglia are clusters of neurosomas (cell bodies of neurons) in the peripheral nervous system; they serve as synaptic connection points where preganglionic neurons synapse with postganglionic neurons.

• Two neurons are needed for each pathway in the ANS to transmit signals from the CNS to the effector organ (for both divisions):

  • Neuron 1 (Preganglionic neuron): Extends from the spinal cord/brainstem to the ganglion. These fibers are typically myelinated.

  • Neuron 2 (Postganglionic neuron): Extends from the ganglion to the effector organ. These fibers are typically unmyelinated.

• Sympathetic ganglia: Often located close to the spinal cord (e.g., sympathetic trunk ganglia) or slightly further out (e.g., collateral ganglia).

• Parasympathetic ganglia: Generally located within or very close to the wall of the effector organs (terminal or intramural ganglia).

ANS Signaling

• Involves the release of specific neurotransmitters and, in some cases, hormones.

• The same chemical can have different effects depending on the specific receptor types present on the target cell and where it acts in the body (e.g., norepinephrine can be excitatory or inhibitory).

• The same chemical can function as both a neurotransmitter (acting locally at a synapse) and a hormone (traveling via the bloodstream to exert widespread effects).

• Neurotransmitter: Chemical messenger released from a presynaptic neuron terminal into a synapse, acting on a post-synaptic neuron or effector cell, transmitted via nerve fibers over short distances.

• Hormones: Chemical messengers secreted by endocrine glands directly into the bloodstream, traveling longer distances to act on distant target cells.

Sympathetic Neurotransmitters

• Acetylcholine (ACh):

  • The neurotransmitter released from all sympathetic preganglionic fibers to the postganglionic neuron within the ganglion.

  • ACh acts on nicotinic receptors (cholinergic receptors) on the postganglionic cell membrane.

  • ACh is released from cholinergic fibers.

• Norepinephrine (NE):

  • Primarily the neurotransmitter released from most sympathetic postganglionic nerve fibers to the effector organ (e.g., heart, smooth muscle, glands).

  • NE acts on various adrenergic receptors (alpha and beta types) on the target cell membrane, leading to diverse effects based on receptor subtype.

  • NE is released from adrenergic fibers.

  • Exception: Some sympathetic postganglionic fibers, such as those innervating sweat glands and certain blood vessels in skeletal muscle during exercise, release ACh (cholinergic).

Sympathetic Hormones

• There is a specialized set of sympathetic preganglionic fibers that run directly from the spinal cord to the adrenal medulla.

• These preganglionic fibers release ACh, which acts on nicotinic receptors on chromaffin cells within the adrenal medulla, a modified sympathetic ganglion.

• ACh triggers these chromaffin cells to release a mixture of approximately 80% epinephrine (adrenaline) and 20% norepinephrine (noradrenaline) directly into the bloodstream.

• Result: Circulating hormones \text{EPI (epinephrine) and NE (norepinephrine)} act on distant target organs throughout the body, reinforcing and prolonging the effects initiated by sympathetic nerve fibers.

Parasympathetic Neurotransmitter

• ACh = acetylcholine (cholinergic) is the primary neurotransmitter in the parasympathetic nervous system.

• ACh is released from both preganglionic and postganglionic fibers in the parasympathetic division.

• Parasympathetic preganglionic fibers release ACh onto nicotinic receptors in the ganglion.

• Parasympathetic postganglionic fibers release ACh onto muscarinic receptors (M1, M2, M3, M4, M5, M.5) on target organs, leading to various effects depending on the specific muscarinic receptor subtype present.

• Cranial parasympathetic neurons and sacral parasympathetic neurons principally use ACh on their target organs.

• Sympathetic neurons typically use NE on target organs, though some sympathetic postganglionic fibers (e.g., to sweat glands) use ACh (cholinergic) acting on muscarinic receptors.

• Diagrammatic summary (from slide):

  • Parasympathetic: ACh released from both preganglionic AND postganglionic fibers (cholinergic).

    • Preganglionic ACh acts on nicotinic receptors in ganglia.

    • Postganglionic ACh acts on muscarinic receptors on visceral effectors (e.g., heart, smooth muscle, glands).

  • Sympathetic:

    • Preganglionic ACh acts on nicotinic receptors in ganglia.

    • Most postganglionic fibers release NE on visceral effectors (adrenergic).

    • Some exceptions (e.g., sweat glands) use ACh on muscarinic receptors.

    • Specialized preganglionic fibers release ACh on adrenal medulla chromaffin cells, leading to EPI/NE hormone release.

Dual Innervation

• Most visceral organs receive innervation from both sympathetic and parasympathetic divisions. This provides a precise and