Autonomic Nervous System

ANS is the portion of the nervous system that handles unconscious vegetative functions

• Makes routine homeostatic adjustments in physiological systems
• System of polysynaptic reflexes modified by higher brain centers
• It is essentially a separate motor system; but uses same sensory system as somatic n.s.

Comparison of Organization of ANS to SNS

ANS components

1. Preganglionic neuron - cell body in CNS, 1^st order neuron
   1. Usually myelinated axon
   2. Extends to a ganglion outside of CNS
   3. All release acetylcholine (Ach) and are excitatory
2. Ganglion - group of nerve cell bodies lying outside of CNS
   1. Location where preganglionic n. synapses with postganglionic n.
   2. Types of ganglia
      1. Sympathetic chain ganglia - series of 22 interconnected paravertebral ganglia lying on each side of vertebral column extending from skull to coccyx
      2. Prevertebral ganglia - collateral ganglion lying anterior to the vertebral column   ex.
         celiac ganglion - base of artery
         superior mesenteric ganglion - at base of artery
         inferior mesenteric ganglion - at base of artery
      3. Peripheral or terminal ganglion - lies directly in or adjacent to the walls of the target organ
3. Postganglionic neuron - cell body outside of CNS, 2^nd order neuron
   1. Mostly unmyelinated axons
   2. Extends to target organ

ANS divisions

• Sympathetic - preparation for crisis or stress
• Parasympathetic - visceral activity during repose

Sympathetic division (thoracolumbar)

1. Preganglionic n. - arises in lateral horn of T-1 to L-3 spinal segments and leaves by ventral root
2. Leaves spinal nerve via white ramus and synapses with postganglionic n. in
   1. Sympathetic chain ganglia or
   2. Prevertebral ganglia or in 1 case
   3. Adrenal medulla = modified ganglion
3. Postganglionic n. - cell body located in ganglion
   1. Travels directly to target organ or
   2. First re-enters spinal nerve via gray ramus & then travels to target organ

Flow chart in class

Parasympathetic division

(cranial & sacral outflows)

1. Preganglionic n. - arises in brainstem nuclei or lateral horn of spinal cord segments S-2 to S-4
   1. Travels in cranial nerves III (oculomotor), VII (facial), IX (glossopharyngeal) and X (vagus) or sacral spinal nerves S2-4
2. Synapse with postganglionic n. in peripheral ganglia
3. Postganglionic n. - cell body located in ganglion
   1. Short neuron
   2. Travels directly to target organ

Chemical transmitters

Sympathetic division

1. Preganglionic n.  - cholinergic, release Ach
   1. Effects are local, of short duration & excitatory
2. Postganglionic n. - some cholinergic, most adrenergic, nitroxidergic
   1. Cholinergic innervate sweat glands, vessels of skeletal muscle and brain
   2. Adrenergic release norepinephrine from sympathetic varicosities
      1. Effects are widespread and prolonged, removed slowly
   3. Nitroxidergic - releases nitric oxide, causes vasodilation in skeletal muscle and brain
3. Adrenal medulla - secretes mainly epinephrine (adrenaline, affects both receptor types) and norepinephrine
4. Adrenergic receptors on target organs, both have subtypes
   1. Alpha-receptors on smooth muscle, use 2^nd messenger mechanism
      1. Causes intracellular Ca⁺² release and vasoconstriction or
      2. Reduces cAMP and inhibits cell activity
      3. More sensitive to norepinephrine
   2. Beta-receptors on cardiac muscle and organs, use 2^nd messenger mechanism
      1. cAMP changes cellular metabolic rate

Parasympathetic division

1. Preganglionic n. - cholinergic, release Ach like sympathetic
2. Postganglionic n. - cholinergic, effects are localized and of short duration
3. Cholinergic receptors
   1. Nicotinic receptors on ganglion cells, always excitatory
   2. Muscarinic receptors on target organs, excitatory or inhibitory

Most vital organs have a dual innervation, often with opposing effects.

Autonomic plexus - network of nerves containing both sympathetic and parasympathetic fibers

• Produces autonomic tone or resting level of activity and coordinates activity, ex:
  Cardiac plexus - regulates heart action, sympathetic increases and parasympathetic decreases activity
  Celiac plexus (solar plexus) - abdominal organs, sympathetic decreases and parasympathetic increases
  Hypogastric plexus - affects organs in the pelvic area, digestive organs, urinary organs (sympathetic decreases and parasympathetic increases) and reproductive glands (sympathetic increases and parasympathetic variable)