Autonomic Nervous System

Sympathetic Innervation of Adrenal Gland

  • The adrenal gland consists of the adrenal cortex and adrenal medulla.
  • Sympathetic Preganglionic Neuron:
    • Releases neurotransmitters to stimulate chromaffin cells in the adrenal medulla.
    • Primary neurotransmitter: Epinephrine.

Effects of the Sympathetic Nervous System

  • Target Organs and Main Effects:
    • Eye:
    • Target: Smooth muscle cells around pupil
    • Neurotransmitter: Norepinephrine (NE)
    • Effect: Dilation of pupil.
    • Salivary Glands:
    • Target: Cells of salivary glands
    • Neurotransmitter: NE
    • Effect: Increased secretion in certain cells (via B₁ and B₂ receptors).
    • Cardiac Muscle:
    • Target: Cardiac muscle cells
    • Neurotransmitter: NE
    • Effect: Increased heart rate and contraction force (via B₁ receptors).
    • Lungs:
    • Target: Smooth muscle cells of bronchus
    • Effect: Dilation of bronchioles (bronchodilation).
    • Sweat Glands:
    • Target: Cells of sweat glands
    • Neurotransmitter: ACh (Acetylcholine)
    • Effect: Increased secretion (muscarinic receptors).
    • Blood Vessels:
    • Target: Blood vessels to skin
    • Neurotransmitter: NE
    • Effect: Vasoconstriction (via α₁ receptors).
    • Liver:
    • Target: Cells of liver
    • Neurotransmitter: NE
    • Effect: Increased release of glucose (via B₂ receptors).

Sympathetic Pathways

  • Preganglionic Nerves:

    • Arise from T1-L2 spinal cord region (thoracolumbar division).
    • Fibers pass through the white ramus communicantes into sympathetic trunk ganglia.
    • Can synapse:
    • At the same level.
    • At higher/lower levels.
    • In distal collateral ganglia (abdomen/pelvis).

  • Postganglionic Pathways:

    • To the Head (T1-T4):
    • Synapse in the superior cervical ganglion.
    • Stimulate blood vessels and dilator muscles of eyes.
    • To the Thorax (T1-T6):
    • Synapse in cervical trunk ganglia (middle and inferior cervical).
    • Innervate heart and thyroid (mostly skin).
    • To the Abdomen:
    • Travel via splanchnic nerves; synapse at celiac and superior mesenteric ganglia.
    • Innervate stomach, intestines, liver, spleen, kidneys.
    • To the Pelvis (T10-L2):
    • Innervate pelvic organs and distal large intestine.

Adrenal Medulla Function

  • Role in Sympathetic Response:
    • Preganglionic fibers travel from thoracic splanchnic nerves to the celiac ganglion.
    • Terminate directly in hormone-producing cells of the adrenal gland.
    • Secrete norepinephrine and epinephrine (noradrenaline/adrenaline).

Effects of the Parasympathetic Nervous System

  • Main Targets:
    • Eye: Constriction of pupil; adjustment for near vision (via oculomotor nerve).
    • Salivary Glands: Increase in secretion (facial and glossopharyngeal nerves).
    • Heart: Decrease in heart rate (vagus nerve).
    • Digestive Tract: Contraction and stimulation of secretions.
    • Urinary Bladder: Relaxes sphincters and promotes urination.

ANS Overview

  • Neurotransmitter Types:
    • Cholinergic: Releases ACh (for all postganglionic neurons, both systems).
    • Adrenergic: Releases Norepinephrine (responses can be either excitatory or inhibitory).
  • Dual Innervation:
    • Antagonistic interactions:
    • Sympathetic: Stimulates heart rate, inhibits digestion.
    • Parasympathetic: Stimulates digestion, reduces heart rate.
  • Homeostatic Control:
    • Regulated by CNS (spinal cord, brain stem, hypothalamus).
    • Mediates various behaviors and organ functions related to stress and relaxation.