Autonomic nervous system | Anatomy2Medicine
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Autonomic nervous system

Autonomic Nervous System

    • Organization
      • The autonomic nervous system (ANS) has two divisions: sympathetic and parasympathetic.

 

  • Synapses between neurons are made in autonomic ganglia.
  • Parasympathetic ganglia are located in or near the effector organs.

 

      • Sympathetic ganglia are located in the paravertebral chain
    • Each division has two neurons in the peripheral distribution of the motor innervation
    • A preganglionic neuron has its cell body in the central nervous system

(CNS).

        • Preganglionic sympathetic neurons originate in the thoracolumbar (MCQ)
        • spinal cord segments T1–L3. (MCQ)
        • Preganglionic parasympathetic neurons originate in the nuclei of cranial nerves (CNs) III, VII, IX, and X and in spinal cord segments S1–S4.  (MCQ)
      • A postganglionic neuron has its cell body in a ganglion in the peripheral nervous system (PNS).
      • The adrenal medulla is a specialized ganglion of the sympathetic nervous system.
        • Preganglionic fibers synapse directly on chromaffin cells, which act like postganglionic cell bodies.

 

  • Chromaffin cells contain phenylethanolamine N-methyltransferase, (MCQ)

 

        • which converts norepinephrine to epinephrine.
      • Thus, the adrenal medulla secretes 80% epinephrine and 20% norepi-nephrine. (MCQ)
    • Neurotransmitters

 

  • In all sympathetically innervated organs, except sweat glands, adrenergic neu- rons release norepinephrine.
  • Whether in the sympathetic or parasympathetic nervous system, cholinergic neurons release acetylcholine.

 

      • Peptidergic neurons in the parasympathetic nervous system release neurocrine peptides such as vasoactive inhibitory peptide
    • Adrenergic Receptors
      • alpha1 Receptors

 

  • dominant alpha receptor subtype on the postsynaptic target cell membrane. (MCQ)

 

        • They are located on (MCQ)
          • vascular smooth muscle
          • gastrointestinal (GI) and bladder sphincters
          • radial muscle of the eye.

 

  • They are excitatory and produce contraction through activation of phospholipase C, leading to formation of inositol triphosphate (IP3) and an increase in intracellular Ca2+. (MCQ)

 

      • Alpha 2-Receptors
        • dominant -receptor type on the presynaptic side of adrenergic nerve terminals.
        • They are located in

 

  • presynaptic nerve terminals

 

          • platelets, fat cells, and walls of the gut.
        • They are inhibitory (MCQ)

 

  • produce relaxation by inhibition of adenylate cyclase and by decreasing cyclic AMP (cAMP). (MCQ)

 

      • Beta1-Receptors,
        • are found mostly in cardiac muscle cells
        • are excitatory
        • produce increased heart rate and contractility
        • activate of adenylate cyclase and by increasing cAMP levels. (MCQ)
      • Beta 2-Receptors
        • are found in smooth muscle and in secretory effectors.
        • They are inhibitory

 

  • produce relaxation (eg, dilation of bronchioles) by increasing cAMP levels. (MCQ)

 

        • They are more sensitive to epinephrine than norepinephrine. (MCQ)
      • Beta 3-Receptors have limited distribution.

 

  • Low levels are present in adipose tissue and the GI tract. (MCQ)

 

        • They are excitatory
        • stimulate lipolysis and GI motility by increasing cAMP levels. (MCQ)
    • Cholinergic Receptors
      • Nicotinic receptors

 

  • postsynaptic receptors in ganglia located in the heart, smooth muscle, and exocrine glands.

 

        • They are activated by low concentrations of acetylcholine or nicotine
        • They are inhibited by ganglionic blockers (eg, hexamethonium) and high concentrations of acetylcholine.
        • They produce excitation by acting as nonspecific cation channels, increasing the influx of Na+ and K+ down their electrochemical gradients. (MCQ)
      • Muscarinic receptors
        • responsible for most parasympathetic postsynaptic effects
        • located in the heart, smooth muscle, and glands.
        • They are activated by acetylcholine and muscarine
        • They are inhibited by atropine.
        • They are inhibitory in the heart (eg, decreasing heart rate)
        • They are excitatory in smooth muscle and glands (eg, increasing secretion).
        • They produce inhibition by decreasing cAMP, which leads to opening of K+ channels (MCQ)

 

  • They produce excitement by increasing IP3-mediated release of Ca2+ from intracellular stores. (MCQ)

 

    • Central Coordination
      • Electrical information reaches the medulla-pons area primarily through the nucleus tractus solitarius, (MCQ)
      • chemical information reaches medulla-pons area mostly through the area postrema.

 

  • The medulla-pons area contains the vasomotor, respiratory, pneumotaxic, and swallowing and vomiting centers.
  • Efferent autonomic information leaves central nuclei in sympathetic and parasympathetic tracts.

 

      • Sympathetic efferents descend in the intermediolateral column of the spinal cord and are transferred from there to sympathetic preganglionic neurons.
      • Parasympathetic efferents leave the central nuclei primarily by way of the vagus nerves.

 

  • Fibers of the sacral region of the spinal cord descend in the mediolateral area.
  • The nucleus tractus solitarius and area postrema have extensive communi- cations with nuclei that generate efferent information: the nucleus ambiguus, the dorsal motor nucleus, and the rostral and caudal ventrolateral medulla nuclei. (MCQ)