Autonomics - Visceral Motor System

Autonomics - Visceral Motor System

• The autonomic nervous system regulates visceral functions like heart rate, blood pressure, digestion, and urination.
• It is part of the peripheral nervous system (PNS).
• It contains two main divisions: sympathetic and parasympathetic.
• The somatic motor system uses one motor neuron to connect the central nervous system (CNS) to skeletal muscle.
• The autonomic system uses two motor neurons (preganglionic and postganglionic) with a synapse in ganglia to reach the target structure (smooth muscle or gland).
• Material in lectures refers to the required textbook.
• Purchasing online materials is not required, but may be a helpful supplement.
• External resources from online compendiums like Wikipedia may not be entirely accurate.

Somatic and Autonomic Nervous Systems

• The somatic motor system has a direct pathway from the CNS to the skeletal muscle, with one neuron.
• The autonomic system has a two-neuron pathway, preganglionic and postganglionic neurons, with a synapse in a ganglion.

Somatic Motor

• The brachial plexus includes nerves like the axillary, radial, musculocutaneous, ulnar, and median nerves.
• These nerves supply the upper limb.

Visceral Motor

• The autonomic nervous system innervates many organs in the body.
• Specific cranial nerves are involved in the parasympathetic innervation of various organs.
• Structures like lungs, heart, stomach, pancreas, liver and gallbladder, intestines, and bladder are innervated by specific autonomic pathways.
• Preganglionic fibers synapse in various ganglia.

Somatic and Autonomic Nervous System: Comparison

• The Autonomic nervous system consists of two neuron pathways with preganglionic and postganglionic neurons.
• The somatic nervous system has a direct pathway from the CNS to the skeletal muscle.
• Effector organs in the somatic nervous system include skeletal muscles.
• Effector organs in the autonomic nervous system include smooth muscle, cardiac muscle, and glands.

Introduction to the Autonomic Nervous System (ANS)

• The ANS is the general visceral motor component of the PNS.
• It controls smooth muscle, cardiac muscle, and glands.
• This system regulates heart rate, blood pressure, digestion, and urination.

Two Divisions of the ANS

• Sympathetic division: "fight or flight," originates in the thoracic and lumbar spinal cord.
• Parasympathetic division: "rest and digest," originates in the brain and sacral spinal cord.

Sympathetic Division

• Increases heart rate and breathing.
• Increases blood flow to skeletal muscles.
• Dilates pupils and airways.
• Inhibits motility of digestive and urinary tracts. It responds to dangerous situations.

Parasympathetic Division

• Active when the body is at rest.
• Conserves energy. Regulates "housekeeping" activities.
• Lowers heart rate and breathing rate.
• Digests food.
• Constricts pupils.

Parasympathetic Division

• Contains craniosacral outflow of neurons in the cranial (brain) and sacral regions of central nervous system (CNS).
• The ganglia are located near or in the target organs.
• Does not innervate body wall or limbs.

Cranial Outflow

• This division arises in the brain (CN III, VII, IX, X) and innervates many structures of the head, neck, thorax, and abdomen.

Sacral Outflow

• Originates in the sacral part of the spinal cord (S2-S4).
• Innervates organs in the lower abdomen and pelvis.

Distribution of Parasympathetic Nerve Fibers

• The parasympathetic nervous system controls certain areas of the body.

Parasympathetic Pathways

• CN III, VII, and IX synapse in one of four head ganglia, innervating structures of the eye, lacrimal gland, nasal mucosa, and salivary glands.
• CN X travels through autonomic plexuses, synapsing in intramural ganglia near the target organs (heart, lungs, and digestive tract).
• Sacral outflow (S2-S4) uses the inferior hypogastric plexus and intramural ganglia in the target organ.

Autonomic Plexuses

• Networks of parasympathetic and sympathetic nerve fibers.
• Include cardiac, pulmonary, esophageal, celiac, superior and inferior mesenteric plexuses.

Sympathetic Division

• Issues from thoracic and lumbar regions of the CNS (T1-L2).
• Innervates visceral organs and structures in superficial body regions.
• Contains more ganglia.

Sympathetic Division (page 20)

• Thoraco-lumbar outflow: pre-ganglionic cell bodies in spinal cord, lateral gray horns.
• Bodies of 2nd neurons located in two places; sympathetic trunk or prevertebral/collateral ganglia.

Sympathetic Trunk Ganglia

• Paired ganglia that run along the sides of the vertebrae.
• Linked together by short nerves to form long sympathetic trunks or chains.
• Connected to ventral rami of spinal nerves through white and gray rami communicantes.

Sympathetic vs. Dorsal Root Ganglia

• Dorsal root ganglia: contain sensory neuron cell bodies.
• Sympathetic chain ganglia: contain motor neuron cell bodies.

Collateral (Prevertebral, Preaortic) Ganglia

• Unpaired and not segmentally arranged.
• Located in abdomen and pelvis, usually on the surface of the aorta.
• Include celiac, superior mesenteric, inferior mesenteric, and inferior hypogastric ganglia.

White and Gray Rami Communicantes

• White rami: contain preganglionic fibers to sympathetic trunk ganglia.
• Gray rami: contain postganglionic fibers to peripheral structures.
• Rami connect spinal nerves to sympathetic trunk.

Sympathetic Pathways

• Preganglionic fibers leave the lateral gray horn of the thoracolumbar spinal cord; they pass to the next neuron via a spinal nerve then a white ramus communicans.
• Reaches the sympathetic trunk or collateral ganglia.
• Synapses with the postganglionic fibers leading to the target organ.

Visceral Reflexes

• Generalized similarities between somatic and visceral reflexes exist.
• Baroreceptors in the carotid sinus monitor blood pressure.
• Changes stimulate the glossopharyngeal nerve and the cardiac center in the medulla oblongata.
• Results in parasympathetic nervous system activity, causing decreased heart rate to lower blood pressure.

Comparison: Sympathetic vs. Parasympathetic

• Sympathetic and parasympathetic divisions have opposite or opposing effects on many organs.
• Various physiological responses such as eye dilation, gland secretion, blood vessel dilation, and heart rate are regulated.

Important Considerations

• Sympathetic division innervates superficial structures and those within the body's inner tube.
• Sympathetic stimulation includes sweating, piloerection, and blood vessel constriction.