Autonomic Nervous System

Overview of the Autonomic Nervous System (ANS)

  • The ANS is crucial for maintaining homeostasis in the body.
  • It modulates the activity of various organs based on changing physiological conditions.
  • Key effectors include:
    • Smooth muscle
    • Cardiac muscle
    • Adipose tissue
    • Glands

Motor Division - Visceral Motor System

Sympathetic Division
  • Function: Prepares the body for physical exertion, associated with states of arousal/stress.
  • Often termed "Fight or Flight" response, characterized by:
    • Increased heart rate (HR)
    • Elevated blood pressure (BP)
    • Enhanced respiration rate
    • Increased blood glucose levels
    • Heightened mental alertness
    • Pupil dilation for improved visual input
    • Decreased activity in pain receptors, digestion, and urinary output
  • Origin: Thoracolumbar region (T1-L2 vertebrae), involving lateral horns of the spinal cord.
Parasympathetic Division
  • Function: Promotes calming and recovery processes, often called "Rest and Digest".
  • Key features include:
    • Decreased HR, BP, and respiration rate
    • Pupil constriction for lesser visual acuity
    • Enhanced digestion, nutrient absorption, and gland secretion
    • Increased urinary output
  • Origin: Craniosacral region (cranial nerves and sacral nerves).

Breakdown of ANS Regions

  • Cranial: Parasympathetic
  • Cervical: Neither
  • Thoracic: Sympathetic (T1-L2)
  • Lumbar: Neither (L3-5)
  • Sacral: Parasympathetic
  • Note: Somatic nerves (C1-L5) originate from diverse gray horns but share the same anterior root.

Reflex Activities in ANS

  • Reflex pathway includes:
    • Afferent (sensory) fibers detecting stimuli
    • Short and long reflex arcs leading to a response via postganglionic neurons and peripheral effectors.

Visceral Motor Neurons

  • The preganglionic neuron originates in the lateral gray horn, traveling via the ventral root.
  • This neuron forms a synapse with a second neuron in a ganglion to create the postganglionic neuron.

Sympathetic Division Specifics

  • Preganglionic Neurons: Myelinated neurons located in lateral gray horns from spinal segments T1-T12 and L1-L2.
  • Chain Ganglia: Located near the spinal cord, responsible for quick reflex processing:
    • Paired ganglia: 3 cervical, 11 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.
  • Collateral Ganglia: Not paired, located anterior to the spinal cord, innervate abdominal cavity organs. Key parts:
    • Celiac
    • Superior mesenteric
    • Inferior mesenteric
  • Adrenal Medullae: Neurons directly innervate the adrenal medulla, triggering the release of neurotransmitters like epinephrine and norepinephrine.

Postganglionic Neurons in the Sympathetic Division

  • Postganglionic neurons feature long, unmyelinated fibers.
    • Preganglionic fibers enter the chain ganglion via white rami; postganglionic fibers exit via gray rami.

Parasympathetic Division Details

  • Preganglionic Neurons: Originating from the nuclei of cranial nerves (III, VII, IX, X) and sacral segments (S2-S4).
  • Key cranial nerves:
    • III: Oculomotor - pupil diameter control
    • VII: Facial - provides secretory functions to salivary glands
    • IX: Glossopharyngeal - controls parotid salivary gland
    • X: Vagus - regulates cardiovascular, respiratory, and digestive functions.
  • Ganglia:
    • Terminal ganglia: Near target organs (e.g., CN III, VII, IX)
    • Intramural ganglia: Embedded within the wall of target organs.
  • Postganglionic Neurons: Generally short connections between the ganglia and the target organ.

Neural Circuit Examples

  • Widespread Response: Sympathetic preganglionic fibers can synapse with many postganglionic fibers, allowing rapid widespread responses.
  • Localized Response: Sympathetic fibers may synapse with just a few postganglionic neurons to target isolated responses without affecting other systems.

Illustration of Reflexes and Responses

  • A pedestrian narrowly avoids an accident, resulting in a delayed return to normal heart and respiratory rates.
    • Likely due to sympathetic activation releasing epinephrine/norepinephrine.

Additional Questions

  • Why does sympathetic function remain intact after certain spinal injuries?