M06 | Autonomic Nervous System | ANS

Autonomic Nervous System (ANS)

ANS consists of motor neurons that innervate smooth and cardiac muscle, and glands, make adjustments to ensure optimal support for body activities, and operate via subconscious control.

Somatic Nervous System

The somatic nervous system has motor fibers that control skeletal muscles.

Effectors

Effectors are the organs or tissues that respond to motor neuron signals; in the somatic system, they are skeletal muscles, while in the ANS, they include cardiac muscle, smooth muscle, and glands.

Efferent Pathways

Efferent pathways are the routes taken by motor neurons to reach effectors; they differ between the somatic and autonomic systems.

Preganglionic Neuron

A preganglionic neuron is located in the CNS and has a thin, lightly myelinated axon.

Postganglionic Neuron

A postganglionic neuron is located in an autonomic ganglion outside the CNS and has a nonmyelinated axon that extends to the effector organ.

Sympathetic Division

The sympathetic division mobilizes the body during activity and is known as the 'fight-or-flight' system.

Parasympathetic Division

The parasympathetic division promotes maintenance activities and conserves body energy, often referred to as 'rest and digest.'

Dual Innervation

Dual innervation refers to most visceral organs being served by both sympathetic and parasympathetic divisions, causing opposite effects.

Dynamic Antagonism

Dynamic antagonism between the sympathetic and parasympathetic divisions maintains homeostasis.

Neurotransmitters in Somatic System

All somatic motor neurons release acetylcholine (ACh), and effects are always stimulatory.

Neurotransmitters in ANS

Preganglionic fibers in the ANS release ACh, while postganglionic fibers release norepinephrine or ACh at effectors, with effects being either stimulatory or inhibitory.

Sympathetic Pathway

The sympathetic pathway has a short preganglionic fiber that synapses in ganglia and releases ACh, followed by a long postganglionic fiber that releases norepinephrine (NE).

Parasympathetic Pathway

The parasympathetic pathway has a long preganglionic fiber that synapses in ganglia and releases ACh, followed by a short postganglionic fiber that also releases ACh.

Role of Parasympathetic Division

The parasympathetic division directs digestion, diuresis, and defecation, promoting maintenance activities and conserving body energy.

Role of Sympathetic Division

The sympathetic division increases heart rate, causes dry mouth, cold sweaty skin, and dilates pupils during exercise, excitement, emergency, or embarrassment.

Ganglia Location in Sympathetic Division

Ganglia in the sympathetic division are located close to the spinal cord.

Craniosacral Origin

The parasympathetic system is sometimes referred to as 'craniosacral' because that is where the cell bodies originate.

Varicosities

Varicosities are neural swellings used by the ANS to terminate at involuntary targets, as opposed to neuromuscular junctions (NMJs).

Homeostasis

Homeostasis is maintained through the dynamic antagonism between the sympathetic and parasympathetic divisions.

Cardiac Muscle

Cardiac muscle is an effector of the autonomic nervous system.

Smooth Muscle

Smooth muscle is an effector of the autonomic nervous system.

Glands

Glands are effectors of the autonomic nervous system.

Rest and Digest

A term referring to the parasympathetic division of the autonomic nervous system, which conserves energy and promotes maintenance activities.

Ganglia

Clusters of nerve cell bodies located close to effectors in the autonomic nervous system.

Cranial Part of Parasympathetic Division

Includes cell bodies in the brain stem and preganglionic fibers from cranial nerves III, VII, IX, and X.

Cranial Nerve III

Oculomotor nerve, which controls smooth muscle of the eye.

Cranial Nerve VII

Facial nerve, which stimulates large glands in the head.

Cranial Nerve IX

Glossopharyngeal nerve, which innervates the parotid salivary glands.

Cranial Nerve X

Vagus nerve, which innervates neck and nerve plexuses for most thoracic and abdominal viscera.

Sacral Part of Parasympathetic Division

Serves pelvic organs and distal half of the large intestine, originating from neurons in lateral gray matter of S2-S4.

Preganglionic Neurons

Neurons located in spinal cord segments T1 - L2 that form the lateral horns of the spinal cord in the sympathetic division.

Paravertebral Ganglia

Ganglia in the sympathetic trunk that vary in size, position, and number, totaling 23 in the sympathetic trunk.

Dorsal Root Ganglion

Housing for sensory neuron cell bodies.

Ventral Root

Motor fibers for the somatic and autonomic nervous systems travel through this root.

White Ramus

Pathway into ganglia for preganglionic fibers, which are myelinated.

Gray Ramus

Pathway out of ganglia for postganglionic fibers, which are non-myelinated.

Synapsing at Same Level

Occurs when a preganglionic fiber synapses with a ganglionic neuron in the same trunk ganglion.

Ascending or Descending Synapse

Occurs when a preganglionic fiber ascends or descends the sympathetic trunk to synapse in another trunk ganglion.

Involuntary Effector

The target organ or tissue that receives signals from postganglionic fibers in the sympathetic system.

Thoracolumbar Regions

The regions of the spinal cord where sympathetic cell bodies are located.

Preganglionic cell body

Located in lateral horn

Axon travels through ventral root

Pathway for sympathetic preganglionic fibers

Axon travels through spinal nerve

Pathway for sympathetic preganglionic fibers

Axon travels through white rami

Pathway for sympathetic preganglionic fibers

Paravertebral ganglion

Where post-ganglionic fiber leaves through gray rami

Post-ganglionic axon

Runs to involuntary effector

Sympathetic cell bodies

Found only in the thoracolumbar regions

Sympathetic trunk ganglion

Short preganglionic fiber may pass through without synapsing, only in abdomen and pelvis

Splanchnic nerve

Leaves through this nerve and synapses later in a ganglion closer to the effector

Adrenal gland

PRE-ganglionic fiber continues all the way to this gland

Adrenal medulla

Acts as a ganglion, and cells within release norepinephrine (NE)

Visceral reflex arcs

Have the same components as somatic reflex arcs but with two neurons in the motor pathway

Visceral pain afferents

Travel along the same pathways as somatic pain fibers, contributing to referred pain

Visceral reflex arc

All are polysynaptic

Cholinergic fibers

Release neurotransmitter Acetylcholine (ACh)

Adrenergic fibers

Release neurotransmitter Norepinephrine (NE)

Sympathetic postganglionic fibers

Secrete ACh at sweat glands, exception to the rule

Adrenaline

Older term for Norepinephrine, comes from the adrenal gland

Fight or flight

Refers to the sympathetic system

Cholinergic receptors

Receptors for Acetylcholine (ACh)

Adrenergic receptors

Receptors for Norepinephrine (NE)

Cholinergic fiber

Releases acetylcholine.

Cholinergic receptor

Binds to acetylcholine.

Adrenergic fiber

Releases norepinephrine.

Adrenergic receptor

Binds to norepinephrine.

Nicotinic receptors

Binds Ach. Stimulates/depolarizes.

Effect of ACh at nicotinic receptors

Always stimulatory.

Locations of nicotinic receptors

Found on skeletal muscle cells at Neuromuscular Junction, all postganglionic neurons (sympathetic and parasympathetic), and hormone-producing cells of adrenal medulla.

Parasympathetic system effects

Allows the system to 'rest' (slow the heart with ACh) and 'digest' (activate the digestive tract, also with ACh).

Postganglionic neurons

Neurons that bind acetylcholine at cholinergic receptors.

Muscarinic

Found on all effector cells stimulated by postganglionic cholinergic fibers.

Effect of ACh at muscarinic receptors

Can be either inhibitory or excitatory, depending on receptor type of target organ.

Effects of NE

Depend on which subclass of receptor predominates on target organ.

Somatic

Cholinergic fiber ACh.

Parasympathetic

Cholinergic fiber ACh.

Sympathetic

Cholinergic fiber ACh and NE.

Nicotinic Receptor locations

Binds with ACh; these targets can also have muscarinic receptors.

Adrenergic Receptor locations

Binds with NE.

Interactions of the Autonomic Divisions

Most visceral organs have dual innervation; dynamic antagonism allows for precise control of visceral activity.

Sympathetic division effects

Increases heart rate, increases respiratory rates, inhibits digestion and elimination.

Parasympathetic division effects

Decreases heart rate, decreases respiratory rates, encourages digestion and discarding of wastes.

Sympathetic Tone

Controls blood pressure, even at rest; keeps blood vessels in continual state of partial constriction.

To Elevate blood pressure

Sympathetic fibers fire more rapidly to constrict blood vessels.

To decrease blood pressure

Sympathetic fibers fire less rapidly to prompt vessels to dilate.

Parasympathetic Tone

Normally dominates heart, slows the heart, encourages function of smooth muscle of digestive and urinary tract organs.

Cooperative Effects

Best seen in control of external genitalia; parasympathetic fibers cause vasodilation responsible for erection, while sympathetic fibers cause ejaculation.

Unique Roles of the Sympathetic Division

Influenced only by sympathetic fibers; controls thermoregulatory responses to heat, release of renin from kidneys, and metabolic effects.

Localized Versus Diffuse Effects

Parasympathetic division has short-lived responses and highly localized control; sympathetic division has longer-lasting responses and systemic effects.

Control of ANS Function

Hypothalamus is the main integrative center of ANS activity.

Homeostatic Imbalances of the ANS

Hypertension is an overactive sympathetic vasoconstrictor response to stress.

Raynaud's disease

Exaggerated vasoconstriction in fingers and toes; results in pale, then cyanotic and painful extremities.

Autonomic dysreflexia

Uncontrolled activation of autonomic neurons in quadriplegics and those with spinal cord injuries above T6; blood pressure skyrockets and is life-threatening.