Autonomics and Somatic Motor Systems Quiz

Autonomics - Visceral Motor System

• The autonomic nervous system (ANS) controls visceral functions, including smooth muscle, cardiac muscle, and glands
• The ANS is a two-neuron system:
  • Preganglionic neurons (preganglionic axons) originate in the CNS, extend to autonomic ganglia, and release acetylcholine
  • Postganglionic neurons (postganglionic axons) extend from the autonomic ganglia to the target, and release neurotransmitters.
• The ANS has two primary divisions:
  • Sympathetic division
  • Parasympathetic division

Somatic vs. Autonomic Systems

• The somatic motor system uses a single neuron to extend from the Central Nervous System (CNS) to the skeletal muscle
• The autonomic system uses a chain of two neurons to extend from the CNS to the target (smooth muscle or gland)
  • a presynaptic (preganglionic) neuron
  • a postsynaptic (postganglionic) neuron

Somatic Motor

• The brachial plexus is a network of nerves.
• Major nerves in the upper limb include the axillary, radial, musculocutaneous, ulnar, and median nerves.

Visceral Motor

• Cranial nerves (III, VII, IX, X) are part of the parasympathetic system.
• Sacral outflow (S2-S4) involves the parasympathetic system.

Visceral Motor Pathways

• Parasympathetic pathways often use cranial nerve pathways.
• The preganglionic fibers, which originate in the cranial nucleus, synapse with postganglionic neurons in ganglia located near the target organ.
• The preganglionic fibers of the parasympathetic nervous system are long, myelinated axons.
• The postganglionic fibers are short, unmyelinated axons.
• The neurotransmitter in both divisions is acetylcholine

Introduction to the Autonomic Nervous System (ANS)

• The ANS is a component of the peripheral nervous system (PNS).
• It regulates visceral functions like heart rate, blood pressure, digestion, and urination.
• The ANS consists of sensory and motor components.
  • Sensory (afferent) : carries information to the CNS from visceral organs.
  • Motor (efferent) : controls visceral organs.
    • Somatic: controls skeletal muscles.
    • Visceral: controls smooth muscle, cardiac muscle, and glands.

Two Divisions of the ANS

• Sympathetic Division: "Fight, flight, or fright" response

  • Originates in the thoracic and lumbar regions of the spinal cord.
  • Has a chain of ganglia along the spinal cord
  • Long postganglionic fibers.
  • Typically has norepinephrine as the neurotransmitter, with some acetylcholine release at some synapses.

• Parasympathetic: "Rest and digest."

  • Originates in the brain stem and sacral spinal cord
  • Short postganglionic fibers.
  • Typically uses acetylcholine as the neurotransmitter

Sympathetic Division

• Increases heart rate, breathing rate and blood flow to skeletal muscles.
• Dilates pupils and airways
• Inhibits motility of the digestive and urinary tracts.

Parasympathetic Division

• Heart rate and breathing are at low-normal levels.
• Gastrointestinal tract digests food.
• Pupils are constricted.

Parasympathetic Pathways

• Craniosacral outflow, meaning nerves originate in the brain stem and the sacral area of the spinal cord.
• The postganglionic neurons are located in or near the target organ, and the neurotransmitter is acetylcholine.
• The parasympathetic system does not contain nerves that innervate the body wall or limbs.

Cranial Outflow

• Originates in the brain, and innervates organs of the head, neck, thorax, and abdomen.
• Pre-ganglionic fibers run via cranial nerves III, VII, IX, and X.
• Synapse with second motor neurons in head ganglia or in ganglia in the walls of target organs.
• Pre-ganglionic fibers travel with cranial nerve X and pass through several autonomic plexuses.

Sacral Outflow

• Originates in the sacral spinal cord and innervates the lower abdominal and pelvic organs.
• Preganglionic fibers exit the spinal cord (S2-S4) and form pelvic splanchnic nerves.

Distribution of Parasympathetic Nerve Fibers

• Most of the innervation occurs through cranial nerves
• Acetylcholine is the main neurotransmitter
• Innervates many organs in the body

Autonomic Plexuses

• Networks of parasympathetic and sympathetic fibers
• All autonomic plexuses contain both sympathetic and parasympathetic fibers
• Ganglia in these networks are almost exclusively sympathetic

Sympathetic Pathways

• Three main pathways:
  • Pathway 1: Synapses on the same level in that sympathetic trunk
  • Pathway 2: Synapse at a higher or lower level in the sympathetic trunk
  • Pathway 3: Passes through the sympathetic trunk and synapses in a collateral ganglion.
• The pathways involve white rami communicantes, spinal nerves (ventral rami), and gray rami communicantes.

Collateral (Prevertebral) Ganglia

• Unpaired ganglia arranged anterior to the vertebral column.
• Located mainly in the abdomen and pelvis (on or near the surface of the aorta)

White and Gray Rami Communicantes

• White rami are preganglionic fibers
  • Connect spinal nerves to the sympathetic trunk
• Gray rami are postganglionic fibers
  • Return to spinal nerves to innervate peripheral tissues.

Visceral Reflexes

• Are involuntary reactions to changes in internal organs.
• Involve the CNS and sensory impulses carried on visceral fibers.
  • Baroreceptors are sensory receptors that detect pressure changes
• The glossopharyngeal nerve (CN IX) carries information about changes in blood pressure.

Comparison of Sympathetic and Parasympathetic Divisions Effects on Organs

• Various organs have distinct responses based on sympathetic and parasympathetic inputs.

Important Considerations

• The sympathetic branch of the ANS innervates superficial and deep structures of the body.
• Sympathetic innervation leads to nervous system responses such as sweating, pupil dilation, goosebumps, etc.