Sunday, 12 May 2013

Neuroanatomy - Autonomic Nervous System


ANS
ANS innervates everything except skeletal muscle
Sympathetic nervous system only needed to respond to stress
Parasympathetic nervous system maintains normal physiological function

Autonomic nerves:
  • ganglia are outside cerebrospinal axis,
  • produce extensive nerve plexuses,
  • ACh and Noradrenaline,
  • post-ganglionic fibres are non-myelinated,
  • on interruption -> incomplete paralysis (because organs have inherent activity and autonomic nerves only modify their activity)

Somatic nerves:
  • ganglia are inside cerebrospinal axis (no peripheral ganglia),
  • do not produce extensive nerve plexuses,
  • ACh,
  • post-ganglionic fibres are myelinated,
  • on interruption -> complete paralysis

Hypothalamus and Subthalamic nucleus are the main centres of autonomic regulation

Generally both sympathetic and parasympathetic innervate together having opposite effects. exceptions include:

Sympathetic only:
  • blood vessels,
  • spleen,
  • sweat glands,
  • hair follicles

Parasympathetic only:
  • gastric glands,
  • ciliary muscles,
  • pancreatic glands

Sympathetic:
  • thoracolumbar output,
  • wide distribution,
  • ganglia are away from organs,
  • long postganglionic fibres,
  • Noradrenaline and ACh,
  • necessary in stress [1st neuron is short (ACh),
  • 2nd is long (Noradrenaline),
  • synapsing occurs in chain ganglia or collateral ganglia,
  • excitatory to many organs,
  • inhibitory to others (digestive),
  • effects widespread and persistent].
  • can cause glycogenolysis, glycolysis, vasoconstriction at site of bleed, release of RBCs from spleen, bronchodilation, vasodilation in blood vessels of skeletal muscle and vasoconstriction in blood vessels of skin.

Parasympathetic:
  • craniosacral output,
  • limited distribution to head,
  • neck and trunk,
  • ganglia are near organs,
  • short postganglionic fibres,
  • ACh. [1st neurons are long, emerge from brain stem or sacral spinal cord, run with spinal or pelvic nerves, ACh. 2nd neurons are short, ACh, excitatory/ inhibitory].
  • decreases HR and Respiration,
  • moves cilia,
  • stimulates GI and causes defecation, urination, stimulates digestion, miosis (protection from excess light??)

In the entire PNS adrenaline is released only in adrenal medulla as NT.
Dopamine is also released on renal vasculature
*ganglia - nicotinic receptor

Enteric Nervous System:
Auerbach’s plexus: ACh, CGRP, NPY
Meissner’s plexus: substance P, 5-HT, ACh, Noradrenaline

Disorders of ANS:
  • erectile dysfunction,
  • orthostatic hypotension,
  • gastroparesis,
  • urinary incontinence,
  • decreased or absent sweating