The Autonomic Nervous System

Autonomic nervous system (ANS)

 is involuntary arm of peripheral nervous system (PNS); also known as visceral motor division

sympathetic and parasympathetic

Divided into two separate divisions, … nervous systems; constantly work together to maintain homeostasis

Main differences between motor divisions of PNS:

Recall that somatic motor division neurons innervate skeletal muscle; leads to voluntary muscle contractions, initiated consciously

Autonomic motor division neurons innervate smooth muscle cells, cardiac muscle cells, and glands; produce involuntary actions 

ANS motor neurons do not directly innervate their target like somatic motors neurons; require a two-neuron circuit:

Preganglionic neuron – initial efferent neuron; cell body resides within CNS; all axons release acetylcholine

Postganglionic neuron – cell body resides in autonomic ganglion in PNS; axons travel to target cells; trigger specific changes (inhibitory or excitatory responses) by releasing either acetylcholine or norepinephrine

Sympathetic nervous system

 preganglionic axons are usually short and postganglionic axons are usually long

Parasympathetic nervous system 

preganglionic parasympathetic axons are long while postganglionic axons are short

Sympathetic nervous system exhibits following characteristics:

Preganglionic cell bodies originate in thoracic and upper lumbar spinal cord giving rise to name, thoracolumbar division

Sympathetic ganglia are generally located near spinal cord, where preganglionic axons synapse with postganglionic neuron cell bodies; postganglionic axons proceed to target

 “Fight or flight” division of ANS; prepares body for emergency situations

Vital role in maintenance of homeostasis when body is engaged in physical work 

Mediates body’s responses to emotion

Parasympathetic nervous system exhibits the following characteristics:

Preganglionic cell bodies are located within nuclei of several cranial nerves in brainstem and sacral region of spinal cord giving rise to name, craniosacral division

Cranial nerves innervate structures of head and neck, thoracic viscera, and most abdominal viscera

Sacral nerves innervate structures within pelvic cavity

Cell bodies of postganglionic neurons are usually located near target organ; requires only a short axon to make connection

“Rest and digest” division; role in digestion and in maintaining body’s homeostasis at rest

Balance between parasympathetic and sympathetic nervous systems:

actions of parasympathetic division directly antagonize those of sympathetic division; together, maintain a delicate balance to ensure that homeostasis is preserved

Sympathetic chain ganglia

where most of postganglionic cell bodies are found; run down both sides parallel with vertebral column; “chainlike” appearance (hence name)

Neurotransmitters

 bind to specific protein-based receptors embedded in plasma membranes of target cells; following slides summarize sympathetic nervous system neurotransmitters and target cell receptors with which they bind

Acetylcholine (ACh)

 neurotransmitter used in excitatory synapses between sympathetic preganglionic axons and postganglionic neurons; postganglionic axons then transmit action potentials to target cell

At synapse with their target cells, postganglionic axons release one of three neurotransmitters: 

ACh, epinephrine (adrenalin), or norepinephrine (noradrenalin; most frequently utilized neurotransmitter released into synapses between postganglionic axons and target cells)

Classes of sympathetic receptors: 

Adrenergic receptors bind to epinephrine and norepinephrine; two major types of adrenergic receptors, alpha and beta, are further classified into subtypes:

Alpha-1 receptors 

 in plasma membranes of smooth muscle cells of many different organs, including blood vessels in skin, GI tract, and kidneys, arrector pili muscles in dermis, and certain organs of genitourinary tract

Alpha-2 receptors

 in plasma membranes of preganglionic sympathetic neurons instead of in peripheral target cells

Beta-1 receptors 

in plasma membranes of cardiac muscle cells, certain kidney cells, and adipose cells

Beta-2 receptors

 in plasma membranes of smooth muscle cells lining airways of respiratory tract (bronchioles), and in wall of urinary bladder, skeletal muscle fibers, and cells found in liver, pancreas, and salivary glands

Beta-3 receptors

 primarily in adipose cells and smooth muscle cells in walls of digestive tract

Muscarinic receptors

 on sweat glands in skin

Nicotinic receptors 

in membranes of all postganglionic neurons within sympathetic ganglia and adrenal medullae

Alpha-2 receptors 

 in plasma membranes of preganglionic sympathetic neurons instead of in peripheral target cells

Effects on cardiac muscle cells:

when norepinephrine binds to beta-1 receptors it causes following changes:

Ion channels open on cardiac muscle cells; raises both rate and force of contraction

Amount of blood delivered to tissues and blood pressure both increase; maintains homeostasis during increased physical activity

Effects on smooth muscle cells:

 when norepinephrine binds to specific receptors it mediates following changes

Constriction of blood vessels serving digestive, urinary, and integumentary system

 occurs when norepinephrine binds to alpha-1 receptors; decreases blood flow to these organs

Dilation of bronchioles

 occurs when norepinephrine binds to beta-2 receptors; increases amount of air that can be inhaled with each breath

Dilation of blood vessels serving skeletal and cardiac muscle

 occurs when norepinephrine binds to beta-2 receptors; increases blood flow; allows for an increase in physical activity

Contraction of urinary and digestive sphincters 

 occurs when norepinephrine binds to receptors  on smooth muscle (e.g., norepinephrine binding to alpha-1 receptors cause contraction of the internal urinary sphincter); ; makes emptying bladder and bowel more difficult during increased physical activity

Relaxation of smooth muscle of digestive tract 

 occurs when norepinephrine binds to beta-3 receptors; slows digestion during increased physical activity

Dilation of pupils 

occurs when norepinephrine binds to alpha-1 receptors; causes dilator pupillae muscles to contract; causes pupil to allow more light into eye

Constriction of blood vessels serving most exocrine

glands

 occurs when norepinephrine binds to beta receptors on blood vessels serving various glands (like salivary glands); decreases secretion, except in sweat glands

Effects on cellular metabolism:

during times of sympathetic activation, nearly all cells, especially skeletal muscle, require higher amounts of ATP; to meet this higher energy demand norepinephrine has three effects when it binds to:

Beta-3 receptors on adipocytes; triggers breakdown of lipids; releases free fatty acids into bloodstream

Beta-2 receptors on liver cells; triggers release of glucose from glycogen and synthesis of glucose from other resources

Binds to beta-2 receptors on cells of pancreas; triggers release of hormone glucagon; increases blood glucose levels

Effects on secretion from sweat glands

 sympathetic nervous system attempts to maintain body temperature homeostasis during periods of increased physical activity

Postganglionic sympathetic neurons release ACh onto sweat gland cells in skin

ACh binds to muscarinic receptors that increase sweat gland secretion

This is a component of a negative feedback loop that corrects elevated body temperature

Effects on cells of adrenal medulla:

 adrenal medulla sits on top of each kidney; in direct contact with preganglionic sympathetic neurons; medulla is composed of modified sympathetic postganglionic neurons with following functions

ACh is released from preganglionic neurons; binds to nicotinic receptors on adrenal medulla cells

ACh stimulates medullary cells to release norepinephrine and epinephrine into bloodstream; considered hormones rather than neurotransmitters

Act as long-distance chemical messengers; interface between endocrine and sympathetic nervous systems

Effects on other cells

 sympathetic nervous system influences many other target cells, all with mission of maintaining homeostasis during increased physical or emotional stress 

Enhances mental alertness by increasing neuron activity in association areas of cerebral cortex 

Temporarily increases tension generated by skeletal muscle cells during a muscle contraction; why people have been known to perform unusual feats of strength under influence of an “adrenaline (epinephrine) rush”

Increases blood’s tendency to clot, which can be useful if a person is injured during a “fight” or a “flight” situation

Postganglionic sympathetic neurons trigger contraction of arrector pili muscles, which produces “goose bumps” 

Cause ejaculation of semen via effects on smooth muscle cells of male reproductive ducts

Gross and Microscopic Anatomy of Parasympathetic Nervous System 

“Rest and digest” division of ANS 

Role in body’s maintenance functions, such as digestion and urine formation

Known as craniosacral division based on association with cranial nerves and pelvic nerves from sacral plexus

Parasympathetic cranial nerves – associated with oculomotor (CN III), facial (CN VII), glossopharyngeal (CN IX), and vagus (CN X) nerves 

Vagus nerves 

main parasympathetic nerves that innervate most thoracic and abdominal viscera

Branches of vagus nerves contribute to cardiac, pulmonary, and esophageal plexuses

Parasympathetic sacral nerves 

make up pelvic nerve component of this division; innervates last segment of large intestine, urinary bladder, and reproductive organs

Sacral nerve branches form pelvic splanchnic nerves; form plexuses in pelvic floor

Some preganglionic neurons synapse with terminal ganglia in associated plexuses; most synapse in terminal ganglia within walls of target organs

Nicotinic receptors

 located in membranes of all postganglionic neurons

Muscarinic receptors

located in membranes of all parasympathetic target cells

Effects on cardiac muscle cells 

Parasympathetic activity decreases heart rate and blood pressure

Preganglionic parasympathetic neurons travel to heart with vagus nerve (CN X)

Constriction of pupil 

 involves CN III, ciliary ganglion, and sphincter pupillae muscle; reduces amount of light allowed into eye

Accommodation of lens for near vision 

 involves CN III and contraction of ciliary muscle; changes lens to a more rounded shaped

Constriction of bronchioles (bronchoconstriction) 

involves CN X; reduces air flow through bronchioles

Contraction of smooth muscle lining digestive tract 

 involves CN X; produces rhythmic contractions called peristalsis; propels food through digestive tract

Relaxation of digestive and urinary sphincters 

involves CN X and sacral nerves; promotes urination and defecation

Engorgement of penis or clitoris

occurs when stimulated by sacral nerves in male or female respectively

Although parasympathetic division only innervates specific blood vessels, many blood vessels dilate when system is activated, due to a reduction in sympathetic activity 

Effects on glandular epithelial cells:

 parasympathetic division has little effect on sweat glands but does increase secretion from other glands:

CN VII stimulation stimulates tear production from lacrimal glands and mucus production from glands in nasal mucosa

CN VII and IX stimulation leads to increased production of saliva from salivary glands

CN X stimulates secretion of enzymes and other products from digestive tract cells 

Effects on other cells:

parasympathetic division has no direct effect on cells that mediate metabolic rate, mental alertness, force generated by skeletal muscle contractions, blood clotting, adipocytes, or most endocrine secretions

Each of above bodily functions returns to a “resting” state during periods of parasympathetic activity; allows for replenishment of glucose storage and other fuels

Fuel replenishment is critical for allowing sympathetic nervous system to function properly when needed

dual innervation

Both divisions innervate many of same organs where their actions antagonize one another, a condition called …

Dual innervation allows sympathetic division to become dominant and trigger effects that maintain homeostasis during physically demanding periods

Parasympathetic division regulates same organs, preserving homeostasis between periods of increased physical activity

Autonomic tone

refers to fact that neither division is ever completely shut down; constant amount of activity from each division 

Sympathetic tone

dominates in blood vessels; keeps them partially constricted

Parasympathetic tone

dominates in heart; keeps heart rate at an average of 72 beats per minute