Autonomic Nervous System (ANS)

Autonomic Nervous System (ANS) and Higher-Order Functions

• The autonomic nervous system (ANS) is a visceral motor system that operates without conscious instruction and controls visceral effectors, coordinating cardiovascular, respiratory, digestive, urinary, and reproductive functions.
• Higher-order functions include consciousness, learning, and intelligence.

Autonomic vs. Somatic Nervous Systems

• The somatic nervous system (SNS) controls skeletal muscles voluntarily.
• The autonomic nervous system (ANS) controls visceral effectors involuntarily, including smooth muscle, glands, cardiac muscle, and adipocytes.
• The hypothalamus contains integrative centers for the ANS.
• Motor neurons of the CNS synapse on visceral motor neurons in autonomic ganglia, like the SNS uses on upper motor neurons.

Visceral Motor Neurons

• Preganglionic neurons are located in the brainstem and spinal cord.
• A preganglionic fiber is the axon of a preganglionic neuron.
• After leaving the CNS, preganglionic fibers synapse on ganglionic neurons (postganglionic neurons) in autonomic ganglia.
• Postganglionic fibers are axons of the ganglionic neurons.

Divisions of the ANS

• The two divisions of the ANS are the sympathetic and parasympathetic divisions.
  • The sympathetic division is responsible for "fight or flight" responses, preparing the body to deal with emergencies by increasing alertness, metabolic rate, and muscular abilities.
  • The parasympathetic division is responsible for "rest and digest" functions, conserving energy and maintaining resting metabolic rate.

Sympathetic and Parasympathetic Divisions

• The sympathetic and parasympathetic divisions usually have opposing effects.
  • Excitation is caused by the sympathetic division, while inhibition is caused by the parasympathetic division.
  • Both divisions may work independently, with only one division innervating some structures.
  • The two divisions may work together, with each controlling one stage of a complex process.

Sympathetic Activity

• Increased sympathetic activity results in heightened mental alertness, increased metabolic rate, reduced digestive and urinary functions, activation of energy reserves, increased respiratory rate, increased heart rate and blood pressure, and activation of sweat glands.

Parasympathetic Activity

• Increased parasympathetic activity results in decreased metabolic rate, decreased heart rate and blood pressure, increased secretion by salivary and digestive glands, increased motility and blood flow in the digestive tract, and stimulation of urination and defecation.

Sympathetic Division

• The sympathetic division is also known as the thoracolumbar division.
• It has short preganglionic fibers in the thoracic and lumbar segments of the spinal cord, specifically between segments T1 and L2.
• Ganglionic neurons are located in ganglia near the spinal cord.
• It has long postganglionic fibers to target organs.

Sympathetic Chain Ganglia

• Sympathetic chain ganglia are located on either side of the vertebral column.
• A single preganglionic fiber synapses on many ganglionic neurons.
• The fibers interconnect the sympathetic chain ganglia, which makes them appear as a string of pearls.
• Each ganglion innervates a particular body organ or group of organs.

Ganglionic Neuron Locations

• Ganglionic neurons synapse in three locations: sympathetic chain ganglia, collateral ganglia, and adrenal medullae.

Sympathetic Chain Ganglia and Control

• Sympathetic chain ganglia are on both sides of the vertebral column.
• They control effectors in the body wall, thoracic cavity, head, neck, and limbs.

Collateral Ganglia

• Collateral ganglia are anterior to the vertebral bodies, containing ganglionic neurons that innervate abdominopelvic tissues and viscera.

Adrenal Medulla

• The adrenal medulla is the center of each adrenal gland.
• The medulla contains modified sympathetic ganglion cells with very short axons
• When stimulated, it releases neurotransmitters (not at synapses) into the bloodstream, which functions as hormones to affect target cells throughout the body.

Sympathetic Division Fibers

• Preganglionic fibers in the sympathetic division are relatively short, with ganglia located near the spinal cord.
• Postganglionic fibers are relatively long, except at the adrenal medulla.

Sympathetic Chain Ganglia Numbers

• The sympathetic chain ganglia consist of 3 cervical, 10-12 thoracic, 4-5 lumbar, 4-5 sacral, and 1 coccygeal ganglion.

Spinal Nerve Features

• Preganglionic neurons are limited to spinal cord segments T1-L2.
• These spinal nerves have white rami (myelinated preganglionic fibers) and gray rami (unmyelinated postganglionic fibers).
• Preganglionic fibers innervate cervical, inferior lumbar, and sacral sympathetic chain ganglia.
• Chain ganglia provide postganglionic fibers through gray rami to cervical, lumbar, and sacral spinal nerves.

Paths of Postganglionic Fibers

• The paths of unmyelinated postganglionic fibers depend on their targets.
  • Those controlling visceral effectors in the body wall, head, neck, or limbs enter gray ramus and return to spinal nerve for distribution, innervating sweat glands and arrector pili muscles.
  • Those innervating visceral organs in the thoracic cavity (e.g. heart and lungs) form bundles called sympathetic nerves.

Sympathetic Ganglia

• In the head and neck, sympathetic postganglionic fibers leave the superior cervical sympathetic ganglia and supply the regions and structures innervated by cranial nerves III, VII, IX, and X.
• Inferior lumbar and sacral receive preganglionic fibers from T1-L2
• Only thoracic and superior lumbar ganglia (T1-L2) receive preganglionic fibers from white rami
• Every spinal nerve receives a gray ramus from a ganglion of the sympathetic chain.

Collateral Ganglia

• Paired ganglia during development, but typically unpaired in adults due to fusion.
• Preganglionic fibers pass through chain without synapsing, forming splanchnic nerves.

Collateral Ganglia Location

• Found in the posterior wall of the abdominal cavity

Collateral Postganglionic Fibers

• Postganglionic fibers from collateral ganglia leave the ganglia, extend throughout the abdominopelvic cavity, innervate visceral tissues and organs.
• They reduce blood flow and energy use by organs not vital to short-term survival and release stored energy.

Preganglionic Fibers

• Preganglionic fibers from seven inferior thoracic spinal segments end at celiac ganglion or superior mesenteric ganglion.
• Preganglionic fibers from lumbar segments form splanchnic nerves and end at inferior mesenteric ganglion.
• All three ganglia are named after nearby arteries.

Celiac Ganglion & Innervation

• Celiac ganglion is a pair of interconnected masses of gray matter located at the base of the celiac trunk.
  • May form single mass or many interwoven masses
• Its postganglionic fibers innervate the stomach, liver, gallbladder, pancreas, and spleen.

Superior Mesenteric Ganglion

• Near the base of the superior mesenteric artery.
• The postganglionic fibers innervate the small intestine and proximal two-thirds of the large intestine.

Inferior Mesenteric Ganglion

• Near the base of the inferior mesenteric artery.
• Its postganglionic fibers provide sympathetic innervation to the kidneys, urinary bladder, terminal segments of the large intestine, and sex organs.

Adrenal Medulla Function

• The adrenal medulla is a modified sympathetic ganglion at the center of each adrenal gland, innervated by preganglionic fibers that synapse on cells that secrete:
  • Epinephrine (adrenaline)
  • Norepinephrine (noradrenaline) with epinephrine making up 75–80 percent of secretory output.

Neurotransmitters Released by Adrenal Medulla

• Bloodstream carries neurotransmitters throughout the body and causes changes in metabolic activities of different cells, including cells not innervated by sympathetic postganglionic fibers.
• The effects last much longer than those produced by direct sympathetic innervation.
• Hormones continue to diffuse out of bloodstream.

Sympathetic Division and Effector Control

• Sympathetic activities are carried though sympathetic activation
  • It occurs during a crisis and affects peripheral tissues and the CNS activity.
• The entire division responds and is controlled by sympathetic centers in the hypothalamus.

Sympathetic Activation

• Changes caused by sympathetic activation include:
  • increased alertness, feelings of energy and euphoria, increased blood pressure, heart rate, and depth of respiration, elevated muscle tone, and mobilization of energy reserves.

Sympathetic Effects and Stimulation

• Stimulation of sympathetic preganglionic neurons releases acetylcholine (ACh) at synapses with ganglionic neurons, and the effect is always excitatory.
• Ganglionic neurons release neurotransmitters at target organs, where telodendria form branching networks (varicosities).
• Each swollen segment (varicosity) of the target organ is packed with neurotransmitter vesicles and the membrane receptors scattered across target cells.

Neurotransmitters of Sympathetic Ganglionic Neurons

• Most sympathetic ganglionic neurons release norepinephrine (NE) at varicosities, and are called adrenergic neurons.
• Some ganglionic neurons release acetylcholine (ACh), and are called cholinergic neurons, located in the body wall, skin, brain, and skeletal muscles.

Sympathetic Stimulation Effects

• Results primarily from interactions of NE and E with adrenergic membrane receptors (alpha and beta receptors).
• NE stimulates alpha receptors to a greater degree than beta receptors.
• E stimulates both classes of receptors.
• Both are G-protein-coupled receptors.

Alpha Receptors

• Alpha-1 (α1) receptors are more common, found primarily in smooth muscle cells, and stimulation has an excitatory effect.
• Alpha-2 (α2) receptors are found on preganglionic sympathetic neurons, stimulation lowers cAMP levels in cytoplasm and has an inhibitory effect, coordinating activities of ANS.

Beta Receptors

• Beta (β) receptors are located on membranes of cells in skeletal muscles, lungs, heart, liver, etc., and stimulation increases intracellular cAMP levels and triggers metabolic changes.

Beta Receptor Subtypes

• Beta-1 (β1): Stimulation increases metabolic activity.
• Beta-2 (β2): Stimulation triggers relaxation of smooth muscles along respiratory tract.
• Beta-3 (β3): Stimulation leads to lipolysis, the breakdown of triglycerides in adipocytes.

Functions of Postganglionic Fibers

• Majority of sympathetic postganglionic fibers release NE (adrenergic), while a few release ACh (cholinergic), which stimulate sweat glands and dilate blood vessels of skeletal muscles and brain.
• Others release nitric oxide (NO) at nitroxidergic synapses where neurons innervate smooth muscles in blood vessel walls (e.g., in skeletal muscles and brain) to produce vasodilation and increased blood flow.

Parasympathetic Division

• Also known as the craniosacral division
• Has long preganglionic fibres in the brainstem and sacral segments of the spinal cord

Parasympathetic Division Autonomic Nuclei

• Autonomic nuclei are in all parts of brainstem and lateral horns of S2–S4
• Ganglionic neurons are in peripheral ganglia.
• They are within or adjacent to target organs and have short postganglionic fibers in or near target organs.

Ganglionic Neurons

• Ganglionic neurons in peripheral ganglia are made of terminal and intramural ganglion

Parasympathetic Fibers - Terminal Ganglion

• Terminal ganglion are near target organ and usually paired

Parasympathetic Fibers - Intramural Ganglion

• Intramural ganglion are embedded in tissues of target organ and consist of interconnected masses and clusters of ganglion cells.

Parasympathetic Preganglionic Fibers

• Organization of the parasympathetic division has has preganglionic fibers leaving the brain in Cranial Nerve III, VII, IX, and X and control visceral structures in head, and synapse in the ciliary, pterygopalatine, submandibular, and otic ganglia.

Vagus Nerve

• The Vagus nerve provides 75 percent of all parasympathetic outflow.
• It innervates structures in the neck, and thoracic and abdominopelvic cavities, including the distal portion of the large intestine and branches intermingle with fibers of sympathetic division.

Sacral Segments

• Preganglionic fibers in sacral segments of spinal cord carry sacral parasympathetic output but do not join anterior roots of spinal nerves

Pelvic Nerves

• They innervate intramural ganglia in kidneys, urinary bladder, portions of large intestine, and sex organs

Parasympathetic Effects

• Major effects of the parasympathetic division include:
  • Constriction of pupils and focusing on near objects
  • Secretion by digestive glands
  • Absorption and use of nutrients by peripheral cells
  • Changes associated with sexual arousal
  • Increased smooth muscle activity in the digestive tract
  • Stimulation and coordination of defecation, contraction of urinary bladder, constriction of respiratory passageways
  • Reduction in heart rate and force of contraction

Cholinergic Effects

• All parasympathetic neurons release ACh and the effects on the postsynaptic cell vary widely due to different types of receptors, the nature of the second messenger, and effects are localized and short lived.

Acetylcholinesterase

• Most ACh is inactivated at synapse by acetylcholinesterase (AChE) and ACh that diffuses into surrounding tissues is inactivated by tissue cholinesterase.

Cholinergic Receptors

• These receptors for the parasympathetic system are split into Nicotinic and Muscarinic receptors.

Nicotinic receptors

• These receptors are on ganglion cells of the sympathetic and parasympathetic divisions and also occur at neuromuscular junctions of the SNS
• Exposure to ACh causes excitation of ganglionic neuron or muscle fiber

Muscarinic Receptors

• These receptors are found at cholinergic neuromuscular or neuroglandular junctions in parasympathetic division and at cholinergic junctions in sympathetic division.
• They are G protein-coupled receptors and the response is excitatory or inhibitory depending on activation or inactivation of specific enzymes.

Dangerous Environmental Toxins

• Nicotine binds to nicotinic receptors and target autonomic ganglia and skeletal neuromuscular junctions. Nicotine poisoning may result in coma or death.
• Muscarine is produced by some poisonous mushrooms, binds to muscarinic receptors, and targets the parasympathetic division.

ANS Divisions

• Both sympathetic and parasympathetic divisions have distinct characteristics with associated responses

Summary of ANS Characteristics - Sympathetic Division

• widespread effects
• two sets of sympathetic chain ganglia, three collateral ganglia, and two adrenal medullae
• short preganglionic fibers, long postganglionic fibers
• extensive divergence
• preganglionic neurons release ACh; most postganglionic fibers release NE
• effector response depends on second messengers

Summary of ANS Characteristics - Parasympathetic Division

• specific effects
• visceral motor nuclei are associated with cranial nerves III, VII, IX, and X, and with S2-S4
• ganglionic neurons are located in ganglia within or next to target organs
• innervates regions serviced by cranial nerves and organs in thoracic and abdominopelvic cavities
• one-fifth the divergence of sympathetic division
• all neurons are cholinergic
• effects are generally brief and restricted

Dual Innervation

• Most vital organs have innervation by both divisions of ANS
• Two divisions commonly have opposing effects
• Parasympathetic postganglionic fibers travel by cranial nerves to peripheral destinations
• Sympathetic innervation reaches same structures from superior cervical ganglia of sympathetic chain.

Autonomic Plexuses Location

• Autonomic plexuses are nerve networks in the thoracic and abdominopelvic cavities, formed by mingled sympathetic postganglionic fibers and parasympathetic preganglionic fibers.
• They travel with blood and lymphatic vessels that supply visceral organs.

Cardiac and Pulmonary Plexus - Dual Innervation

• The Cardiac and pulmonary plexus have intersecting autonomic fibers in thoracic cavity.
• They contain sympathetic and parasympathetic fibers and parasympathetic ganglia that affect those organs for heart and lungs, respectively.

Esophageal Plexus - Dual Innervation

• The Esophageal plexus contains descending branches of vagus nerves and splanchnic nerves leaving sympathetic chain.
• Parasympathetic preganglionic fibers of vagus nerve enter abdominopelvic cavity with esophagus and fibers enter celiac plexus.

Celiac Plexus- Dual Innervation

• The Celiac plexus, also known as the solar plexus, is associated with smaller plexuses, such as the inferior mesenteric plexus and innervates viscera within abdominal cavity.

Hypogastric Plexus - Dual Innervation

• The Hypogastric plexus innervates digestive, urinary, and reproductive organs of pelvic cavity.
• It contains parasympathetic outflow of pelvic nerves, sympathetic postganglionic fibers from inferior mesenteric ganglion, and splanchnic nerves from sacral sympathetic chain.

Autonomic Tone Importance

• Autonomic motor neurons have a resting level of activity, even without stimulation, and is an important aspect of ANS function.
• Nerves maintain background level of activity and can increase or decrease activity which provides greater range of control
• Significant where dual innervation occurs and more important where it does not occur

Heart Dual Innervation

• Acetylcholine released by parasympathetic postganglionic fibers slows heart rate.
• NE released by varicosities of sympathetic division accelerates heart rate.
• Small amounts of both released continuously, producing autonomic tone.
• Parasympathetic division dominates at rest.

Speeding Up Heart Rate

• When a crisis occurs this speeds heart rate by stimulating sympathetic and inhibiting parasympathetic nerves.

One Division

• Some organs are innervated by only one division. For example: sympathetic control of blood vessel diameter is maintained when NE is released from sympathetic fibers at smooth muscle cells in blood vessel walls.
• Sympathetic tone keeps muscles partially contracted.

Bloodflow

• When more blood flow is needed, the rate of NE release decreases where sympathetic cholinergic fibers are stimulated so that smooth muscle cells relax and blood vessel dilates

Somatic Motor Control - Autonomic

• Centers involved in somatic motor control in the autonomic system are found in all portions of CNS. Lower motor neurons of cranial and spinal reflex arcs and Pyramidal motor neurons of primary motor cortex.
• A Simple reflexes based in spinal cord respond rapidly and automatically to stimuli
• Centers in brainstem control activity of sympathetic and parasympathetic divisions.

Visceral Reflex

• Autonomic, polysynaptic reflexes initiated in viscera.
• Provide automatic motor responses and can be modified, facilitated, or inhibited by higher centers, especially the hypothalamus
• Visceral reflex arc consists of a receptor, sensory neuron, processing center (one or more interneurons), and two visceral motor neurons.

Viscera Long Reflexes

• Autonomic are equivalents of polysynaptic reflexes
• Visceral sensory neurons deliver information to CNS along posterior roots of spinal nerves:
  • Within sensory branches of cranial nerves, and autonomic nerves that innervate visceral effectors
• ANS carries motor commands to visceral effectors:
  • Coordinate activities of entire organ

Viscera Short Reflexes

• Bypasses CNS entirely
• Sensory neurons and interneurons whose cell bodies lie in autonomic ganglia:
  • Interneurons synapse on postganglionic neurons
• Postganglionic fibers distribute motor commands:
  • Control simple motor responses with localized effects:
  • Control activity in one small part of target organ

Reflexes - Higher Order Function

• Long reflexes most important in regulating visceral activities in most organs
• Short reflexes most control and coordination in digestive tract and associated glands
  • Neurons involved form enteric nervous system

Enteric Nervous System

• Ganglia in walls of digestive tract contain cell bodies of visceral sensory neurons, interneurons, and visceral motor neurons.
• Axons form extensive nerve nets and capable of controlling digestive functions independent of CNS.

Higher Levels of Autonomic Control

• Processing centers in the medulla oblongata coordinate complex reflexes.
• Contains centers and nuclei involved in salivation, swallowing, digestive secretions, peristalsis, and urinary function.
• It is regulated by the hypothalamus.

Integration of ANS and SNS Activities

• Many parallels in organization and function.
• Integration occurs at the brainstem and both systems are under control of higher centers.

Higher-Order Functions Characteristics

• These functions are not innate or fixed, they share three characteristics including requiring the cerebral cortex, involving conscious and unconscious information processing, and being subject to adjustment over time.

Memories - Types

• Long-term memories include:

  • Secondary memories: fade with time and require effort to recall
  • Tertiary memories: Do not fade

• Stored information gathered through experience either fact or skill memories

• Fact memories are specific bits of information that are stored separately that skill memories are learned and incorporated at unconscious level with repetition and stored in brainstem related to innate behaviors

Brain Regions and Memory

• The amygdaloid body and hippocampus (components of limbic system) involve memory consolidation, and damage prevents conversion of short-term memories to new long-term memories, existing long-term memories remain intact. The cerebral cortex stores most long-term memories and associates motor and sensory memories.
• Brainstem stores those related to innate behaviors
• Complex skill memories involve integration of motor patterns in basal nuclei, cerebral cortex, cerebellum