Peripheral Nervous System: Sensory and Motor Divisions

• The functional organization of the peripheral nervous system (PNS) involves sensory and motor divisions.

Sensory Nervous System

• The sensory nervous system is also known as the Afferent nervous system.
• It conveys information in the form of impulses to the central nervous system (CNS).
• Somatic sensory input is consciously perceived from receptors, including special senses and skin.
• Visceral sensory input is not consciously perceived and comes from the viscera and blood vessels.

Motor Nervous System

• The motor nervous system is also known as the Efferent nervous system.
• It conveys information in the form of impulses from the CNS to effector organs.
• Somatic motor output is controlled consciously or voluntarily, with the effector being skeletal muscle.
• Autonomic motor output is not consciously or involuntarily controlled and functionally maintains homeostasis.
• Effectors include glands, smooth muscle, and cardiac muscle.
• The autonomic motor system is further divided into the sympathetic and parasympathetic systems.

Somatic Nervous System

• Processes are consciously perceived or controlled
• Somatic sensory neurons detect and transmit information from the special senses, skin, and proprioceptors to the CNS.
• Somatic motor neurons initiate and transmit nerve signals from the CNS to skeletal muscles.

Autonomic Nervous System

• The autonomic nervous system is also called the visceral nervous system.
• Processes are regulated below the conscious level.
• It functionally maintains homeostasis.
• Visceral sensory neurons detect stimuli from the viscera and blood vessels to the CNS.
• Autonomic motor neurons transmit signals from the CNS to cardiac and smooth muscles and glands.

Sympathetic Division

• The Sympathetic Division is known as "fight or flight".
• Mobilized during periods of exertion (activity), excitement (stress), or emergency increases heart and respiration rate, blood flow to skeletal and cardiac muscle, and sweating

Parasympathetic division

• The Parasympathetic Division is known as "rest-and-digest” or “feed and breed".
• It Primarily promotes maintenance functions and is for energy conservation.
• SLUD is the acronym for salivation, lacrimation, urination, and defecation.
• Parasympathetic decreases heart rate and blood pressure
• It increases motility and secretion in the digestive system, the excretion of waste products, and sexual arousal.
• The body spends most of the time under parasympathetic dominance.

Autonomic Nerve Pathways

• The autonomic nerve pathway consists of a two-neuron chain.
• The cell body of the first neuron is located within the CNS.
• Synapses occur with the cell body of the second neuron located in a ganglion.
• The axon of the second neuron (post-ganglionic fiber) innervates the effector organ.

Differences Between Autonomic and Somatic Nervous Systems

• Target tissues for the somatic nervous system are skeletal muscles.
• Target tissues for the autonomic nervous system include smooth and cardiac muscle, kidneys, lungs, blood vessels, GI tract, liver, bladder, eyes, salivary, and sweat glands.
• The somatic nervous system response is excitation of skeletal muscle contraction whereas the autonomic nervous systems response includes inhibition and excitation of smooth and cardiac muscle and glandular secretion.
• The somatic nervous systems regulation is voluntary/conscious whereas the autonomic nervous system regulation is involuntary/unconscious.
• Origins for the somatic nervous system include the motor cortex, medulla, and dorsal horns of the cord.
• Origins for the autonomic nervous system include brain stem nuclei and varying regions of the cord.
• Myelination is present in the somatic nervous system however, in the autonomic nervous system, preganglionic fibers are myelinated, and postganglionic fibers are not.
• The neurotransmitter for the somatic nervous system is Acetylcholine (ACh).
• The neurotransmitters for the autonomic nervous system are Acetylcholine, norepinephrine, and epinephrine.
• Receptor subtype for the somatic nervous system is Nicotinic.
• Receptor subtypes for the autonomic nervous system are Nicotinic, muscarinic, a, and β adrenergic.

Anatomic Differences Between Parasympathetic and Sympathetic

• Preganglionic cell bodies are located in the cranial and sacral origins for the parasympathetic system, and in the thoracic and lumbar origins for the sympathetic system.
• Preganglionic fibers are long in the parasympathetic system and short in the sympathetic system.
• Ganglia in the parasympathetic system are located near the target organ or within the wall of the organ, whereas in the sympathetic system, they are located near the spinal cord.
• Postganglionic fibers are short in the parasympathetic system and long in the sympathetic system.
• The parasympathetic system has few preganglionic axon branches whereas the sympathetic system has extensive branching.
• The degree of response in the parasympathetic system is local, but in the sympathetic system, it is either mass activation (many systems simultaneously) or local.

Parasympathetic Anatomy

• Oculomotor nerve (CN III) affects the ciliary muscle of the lens for accommodation and the sphincter pupillae muscle.
• Facial nerve (CN VII) affects lacrimal glands, nasal cavity glands, and glands of the palate and oral cavity; also, submandibular and sublingual salivary glands.
• Glossopharyngeal nerve (CN IX) affects the parotid salivary gland.
• Vagus nerve (CN X) affects thoracic and abdominal viscera.
• Splachnic nerves (S2-S4) affect lower abdominal viscera and pelvic viscera.

Sympathetic Anatomy

• The sympathetic system is more complex than the parasympathetic system.
• Cell bodies for the sympathetic system are housed in the lateral horn of the spinal cord (between T1 and L2), and fibers exit through anterior roots.
• Sympathetic fibers leave the spinal nerve to enter either the left or right sympathetic trunk (immediately lateral to the spinal cord).
• The trunks contain the sympathetic ganglia, and the sympathetic ganglia are interconnected by bundles of axons to form the trunk.
• There is roughly one sympathetic ganglia per spinal nerve.
• Fibers can synapse with neurons in the same trunk ganglion.
• Sympathetic fibers can ascend or descend to synapse with another trunk ganglion or pass through without synapsing (collateral ganglia).

Cervical Ganglia

• Superior: postganglionic fibers to head and neck, some thoracic
• Middle: postganglionic fibers to thoracic viscera
• Inferior: postganglionic fibers to thoracic viscera

Rami Communicantes

• Connects the spinal nerves to each sympathetic trunk
• White ramus communicans carries pre-ganglionic axons from T1-L2 to the sympathetic trunk.
• Gray ramus communicans carries post-ganglionic axons from the sympathetic trunk to the spinal nerve.
• Post-ganglionic fibers lack myelin, hence color and name

Sympathetic Splanchnic Nerves

• Preganglionic fibers that do not synapse in a sympathetic trunk ganglion

• Travel to most of the abdominal and pelvic viscera

• Terminate in collateral (prevertebral) ganglia

• Greater thoracic splanchnic nerves preganglion fibers originate from T5 - T9 and synapse in the Celiac ganglion.

• Lesser thoracic splanchnic nerves preganglion fibers originate from T10 - T11 and synapse in the superior mesenteric ganglion.

• Least thoracic splanchnic nerves preganglion fibers originate from T12 and synapse in the superior mesenteric ganglion.

• Lumbar splanchnic nerves preganglion fibers originate from L1 - L2 and synapse in inferior mesenteric ganglion.

• Sacral splanchnic nerves, postganglion fibers originate from sympathetic sacral ganglion.

Prevertebral (Collateral) Ganglia

• Located anterior to the vertebral column on the anterolateral wall of the aorta, and are only located in the abdominopelvic cavity
• The Celiac ganglion's postganglionic fibers innervate the stomach, spleen, liver, gall bladder and proximal portion of the duodenum, and a part of the pancreas.
• The superior mesenteric ganglion's postganglionic fibers innervate the distal half of the duodenum, the rest of the small intestine, the proximal portion of the large intestine, a part of the pancreas, the kidneys, and the proximal part of the ureters.
• The inferior mesenteric ganglion's postganglionic fibers innervate the distal part of the large intestine, rectum, urinary bladder, distal ureters, and most of the reproductive organs.

Sympathetic Pathway

• This is the path by which a sympathetic fiber exits the sympathetic ganglion
• Four types of pathways include the spinal nerve pathway, the postganglionic sympathetic nerve pathway, the splanchnic nerve pathway, and the adrenal medulla pathway.

Spinal Nerve Pathway

• The preganglionic fiber synapses in the sympathetic trunk ganglion with the postganglionic fiber.
• The postganglionic fiber travels out of the ganglion via the gray ramus at the same "level".
• The fiber joins the spinal nerve and extends to its target organ.
• Targets in the skin of the torso, neck, and limbs including sweat glands, blood vessels of the skin, and arrector pili muscles.

Postganglionic Sympathetic Nerve Pathway

• The preganglionic fiber synapses in the sympathetic trunk ganglion with the postganglionic fiber.
• The postganglionic fiber does not use the gray ramus, the fiber exits the ganglion and directly extends to its target organ. Targets include the esophagus, heart, lungs, and thoracic blood vessels, innervated structures of the head, including sweat glands and blood vessels and some eye muscles

Splanchnic Nerve Pathway

• The preganglionic fiber passes through the sympathetic trunk ganglion without synapsing and extends to the collateral (prevertebral) ganglia to synapse with the postganglionic fiber.
• The postganglionic fiber travels out of the collateral ganglion and extends to its target organ.
• Targets include the abdominal and pelvic organs.

Adrenal Medulla Pathway

• The preganglionic fiber directly innervates the adrenal medulla
• It extends through the sympathetic trunk and collateral ganglion and terminates on neurosecretory cells.
• There are no postganglionic fibers in this pathway.
• The adrenal medulla is a modified sympathetic ganglion and does not give rise to postganglionic fibers.
• The Adrenal medulla secretes hormones into the bloodstream: 20% NorE, and 80% Epinephrine (Epi). This reinforces the activity of the sympathetic nervous system.

Adrenal Medulla

• It is located superior to the kidney.
• It consists of two parts, the cortex (outer portion), and the medulla (inner portion).
• The Modified sympathetic ganglion does not give rise to postganglionic fibers but secretes hormones into the bloodstream.
• Hormones include 20% NorE, and 80% Epinephrine (Epi) which reinforces the activity of the sympathetic nervous system.

Neurotransmitters of the ANS

• The two main neurotransmitters of the ANS include Norepinephrine (NorE) and Acetylcholine (ACh).
• Neurons are named for which neurotransmitter they synthesize and release.
• Cholinergic fibers release ACh.
• All sympathetic and parasympathetic preganglionic fibers
• All parasympathetic postganglionic fibers
• Sympathetic postganglionic fibers that innervate sweat glands in skin and blood vessels in muscle
• Adrenergic fibers release NorE - Almost all sympathetic postganglionic fibers
• Receptors are named for what they bind
• Cholinergic receptors bind Ach and are found on cell bodies of cells responding to ACh.
• Adrenergic receptors bind NorE and are found on cell bodies of cells responding to NorE.

Varicosity

• Terminal end of a sympathetic fiber and is analogous to a synaptic knob.
• The network of swellings along the terminal branch overlays the innervated tissue.
• The neurotransmitter is released along the terminal length of the axon as opposed to the terminal end which yields a greater response.

Cholinergic Receptors: Nicotinic

• Named because they are sensitive to nicotine
• Location includes post-ganglionic cell bodies, the adrenal medulla, and skeletal muscles.
• They are activated by ACh
• Binding of ACh to the nicotinic receptor is always excitatory.
• Produces EPSPs

Cholinergic Receptors: Muscarinic

• Named because they are sensitive to muscarine, a mushroom toxin.
• The location is on Effector cell membranes in the parasympathetic system and selected cells in the sympathetic system.
• They are activated by ACh from parasympathetic postganglionic fibers from some sympathetic fibers innervating sweat glands and blood vessels (of skeletal muscle)
• Binding of ACh to the muscarinic receptor can be either excitatory or inhibitory, depending on the subclass of receptor.

Adrenergic Receptors

• There are two major types which include Alpha (a) receptors: a₁ & a₂ and Beta (β) receptors: B1, B2, & β3.
• They are bound by either Epi or NorE, depending on the subtype.
• They can be excitatory or inhibitory, depending on the subtype.
• All are coupled with G-protein.

Interactions Within the ANS

• Tone is the basal level of activity of both systems.
• Both systems are always active, but the dominant level of activity is dependent on the homeostatic need at a given time.
• When one system is increased above its tonic level with a simultaneous decrease in the other system, it is called dominance.
• Different organ systems have different types of dominance that exhibit the tone of the dominant system.
• Antagonistic effects occur when sympathetic and parasympathetic systems frequently innervate the same organ therefore generally exert opposite effects on a particular organ, which is then called dual reciprocal innervation.

(a) Parasympathetic Division The "Rest-and-Digest" Division

• Originates in the cranial and sacral regions of the CNS by way of CN III CN VII Pons Medulla CN IX CN X S2 S3 S4 Pelvic - splanchnic nerves
• Tends to have long preganglionic axons and short postganglionic axons.
• Neurotransmitters and receptors include: ACh is released from preganglionic axons and binds cholinergic (specifically, nicotinic) receptors. ACh is released from postganglionic axons, which binds cholinergic (muscarinic) receptors. • Main effects include: Pupil constriction; Increases motility and activity of digestive system; Storage of fuel molecules in liver and adipose connective tissue; Stimulates secretion of lacrimal (tear), nasal, and digestive system glands; Returns body to homeostasis Decreases heart rate and causes bronchoconstriction

(b) Sympathetic Division The "Fight-or-Flight" Division

• Originates in the T1-L2 segments of the spinal cord. Right T1-L2
• 2
• Tends to have shorter preganglionic axons and longer postganglionic axons. • Preganglionic axon has many branches and extensive divergence. • Neurotransmitters and receptors include: ACh is released from preganglionic axons and binds cholinergic (specifically, nicotinic) receptors. Postganglionic axons release either ACh or NE such as: ACh binds (muscarinic) receptors for sweat glands NE binds adrenergic receptors (all other structures). Nicotinic • Main effects: Pupil dilation; Increases heart rate and causes bronchodilation; Vasoconstricts surface blood vessels and vessels to most visceral organs but vasodilates coronary arteries and arteries to skeletal muscle Stimulates secretion of sweat glands,; contractions of arrector pili vessels; Decreases motility and activity of digestive system,; Release of fuel molecules from liver and adipose connective tissue into the blood; Releases epinephrine and NE to prolong sympathetic effects. vessels Ganglia (sympathetic trunk or prevertebral) -
• Ganglia lie close to organ or within effector wall (e.g., terminal or intramural). Main effects ganglia close to the spinal cord main effects intramural