Biol 3455 Autonomic System

Somatic Nervous System

Controls voluntary movements by consciously directing upper motor neurons in the primary motor cortex down to lower motor neurons, which directly stimulate skeletal muscles. It involves a single-neuron pathway.

Autonomic Nervous System

regulates involuntary visceral effectors automatically (smooth muscle, cardiac muscle, glands, and adipocytes).

What is the two neuron pathway

1. Preganglionic neuron

2. Postganglionic neuron

Preganglionic Neuron

Cell body originates in the CNS (brain or spinal cord) and synapses onto a ganglion.

Postganglionic neuron

Cell body sits in an autonomic ganglion and projects to the target effector organ.

Dual Divisions of ANS

1. Sympathetic Division (Fight of Flight)

2. Parasympathetic Division (Rest-and Digest)

Sympathetic Division location

Thoracolumbar outflow ($T_1$ to $L_2$ spinal segments).

Effects of Sympathetic Division

Prepares the body for stress/emergencies by increasing alertness metabolic rate, heart rate, blood pressure, airway dilation, and energy mobilization while inhibiting digestion/urinary functions.

• activation of sweat glands

Location of Parasympathetic Division

Craniosacral outflow (Brainstem cranial nerves III, VII, IX, X, and sacral segments S_2–S_4).

Effects of Parasympathetic Division

Conserves energy, reduces metabolic blood pressure/heart rates, and promotes digestion, nutrient absorption, urination, and defecation.

Sympathetic Preganglionic Fibers

myelinated and are located between T1 and L2 of the spinal cord.

role of Sympathetic preganglionic fibers

pass through spinal ventral roots and then through white rami communicantes to reach the sympathetic ganglia

Sympathetic ganglia

cluster of cell bodies of postganglionic neurons

What are the 3 main ganglion layouts of Sympathetic preganglionic fibers

1. Sympathetic Chain Ganglia

2. Collateral Ganglia

3. Adrenal Medullae

Sympathetic Chain Ganglia

Found in rows along the vertebral column; innervates body wall, head, neck, and thoracic cavities via unmyelinated postganglionic fibers passing through gray rami communicantes.

How many cervical sympathetic chain ganglia

3

How many thoracic sympathetic chain ganglia

10-12

How many Lumbar sympathetic chain ganglia

4-5

How many sacral sympathetic chain ganglia

4-5

coccygeal chain ganglion

1

One preganglionic fiber

synapses on many postganglionic neurons

Collateral Ganglia

Unpaired clusters anterior to the vertebral column (includes celiac, superior mesenteric, and inferior mesenteric ganglia) that supply abdominopelvic viscera.

Adrenal Medullae

Modified sympathetic ganglia acting as neuroendocrine cells. When stimulated, they release epinephrine and norepinephrine directly into the bloodstream for widespread, long-lasting hormonal effects.

First Step of Motor Command

anterior root of each spinal nerve contains the axons of somatic motor and visceral neurons

Second Step of Motor Command

Spinal nerve forms lateral to invertebral foramen, where posterior and anteiror roots unite

third step of Motor Command

Posterior ramus contains somatic motor and visceral motor fibers that innervate the skin and skeletal muscles in the back

Fourth step of Motor Command

Axons in the relatively large anterior ramus supply the ventrolateral body surface, structures in the body wall, and limbs

Fifth Step of Motor Command

white ramus communicans is the first branch from the spinal nerve and carries visceral motor fibers to a nearby sympathetic ganglion. Because these preganglionic axons are myelinated this branch has a light color and is therefore known as white ramus

sixth step of motor command

Gray ramus communicans contains postganglionic fibers that innervate glands and smooth muscles in the body wall/limbs. These fibers are unmyelinated and have a dark gray color. Gray rami are associated with each spinal nerve

Seventh step of Motor command

Sympathetic nerve contains preganglionic and postganglionic fibers innervating structures in the thoracic cavity

Parasympathetic preganglionic fibers

long, running mostly through cranial nerves (the vagus nerve [X] alone accounts for ~75% of output) or sacral pelvic nerves.

Terminal Ganglia

located close to the target organ

Intramural Ganglia

Embedded directly inside the target’s organ’s walls (urinary bladder and digestive tract)

Collateral Ganglia

▪Anterior to the vertebral column

▪Originate as paired ganglia (left and right), but are typically unpaired in adults

▪Preganglionic fibers from the seven inferior thoracic spinal segments synapse at the celiac ganglion or the superior mesenteric ganglion

types of Collateral Ganglia

Celiac ganglia

Superior mesenteric ganglion

Inferior mesenteric ganglion

Celiac Ganglia

pair of interconnected masses of gray matter located at the base of the celiac trunk

Where does postganglionic fibers innervate in celiac ganglia

the stomach, liver, gallbladder, pancreas, and spleen

Superior Mesenteric Ganglion

near the base of the superior mesenteric artery

Where does postganglionic fibers innervate in superior mesenteric ganglion

–small intestine and proximal two-thirds of the large intestine

Inferior mesentric ganglion

near the base of the inferior mesenteric artery

Where does postganglionic fibers innervate in inferior mesenteric ganglion

fibers innervate the kidneys, urinary bladder, last segment of large intestine and sex organs

Adrenal medulla

–modified sympathetic ganglion at the center of each adrenal gland

▪Postganglionic neurons have very short axons

▪Preganglionic neurons synapse on secretory cells.

–When stimulated, they release epinephrine (adrenaline) and norepinephrine (noradrenaline) into the bloodstream

Role of Neurotransmitters

function as hormones to cause changes in the metabolic activities of different target cells, including cells not innervated by sympathetic postganglionic fibers

What do preganglionic neurons release when stimulated

Acetycholine ACh

Varicosities

bead-like swellings along an axon that release neurotransmitters over a broad area instead of a single synaptic terminal

Where are varicosities releasing ACh located

the body wall, the skin, the brain and skeletal muscle

Adernergic Receptors

membrane G protein-coupled receptors that bind to norepinephrine and epinephrine

Classes of Adernergic receptors

Alpha receptors - more sensitive to norepinephrine

Beta Receptors

Alpha 1 receptors

–More common

–Found primarily in smooth muscle cells

–Stimulation has excitatory effect (release of intracellular calcium ions)

Alpha 2 Receptors

–Found on both pre- and postsynaptic sympathetic and parasympathetic neurons

–Stimulation lowers cAMP levels in cytoplasm and has inhibitory effect

–Coordinates activities of the ANS

Beta Receptors

▪Located in skeletal muscles, lungs, heart, liver, etc.

▪Stimulation increases intracellular c A M P levels and triggers changes in metabolic activity

Beta 1 receptors

cause increase in metabolic activity in muscles and the heart

Beta 2 Receptors

stimulation triggers relaxation of smooth muscles along respiratory tract

Beta 3 Receptors

stimulation leads to lipolysis, the breakdown of triglycerides in adipocytes

Cranial nerves 3,7,9,10

fibers that leave the brain

What do Fibers that leave the sacral segments of the spinal cord form

Pelvic Nerves

Synapse at intramural ganglia in the walls

of the kidneys, urinary bladder, terminal portion of large intestine and sex organs

Major effects of Parasympathetic Division

–Constriction of pupils and focusing the lenses on near objects

–Secretion by digestive glands, such as salivary, gastric, intestinal, duodenal glands, the pancreas and the liver

–Secretion of hormones that promote the absorption and use of nutrients by peripheral cells

–Changes in blood flow and glandular activity are associated with increased energy

–Increased smooth muscle activity along the digestive tract

–Stimulation and coordination of defecation

–Contraction of the urinary bladder during urination

–Constriction of the respiratory passageways

–Reduction in heart rate and the force of contraction