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Class notes

Comparison of somatic and autonomic nervous structure
  • ANS functions to coordinate the activity of the viscera, to maintain normal body function and cope with changes in the external environment
  • ANS is mainly a motor system (though there are sensory fibres that travel in along the viscera and run along the ANS)
  • Note the different arrangement for motor neurons in the somatic NS (cell body in ventral horn of spinal cord, or in the brainstem. Then the neuron runs to the effector, where ACh is released. These fibres are myelinated and are large) to the autonomic NS - which supplies SMC, cardiac muscle, glands (chain of two neurons. They always synapse outside the CNS in an autonomic ganglion, where ACh is the transmitter. Preganglionic neuron from spinal cord to autonomic ganglion, then postganglionic neuron runs from ganglion to target organ. Preganglionic = myelinated, postganglionic = unmyelinated. Preganglionic neurons synapse on several postganglionic neurons).
  • ANS subdivided into parasympathetic and sympathetic, that oppose each other (depending on circumstances, one takes over the other).
    • Parasympathetic: housekeeper. Functions to conserve and restore energy (e.g. after nice evening meal, put feet up, relax; digestion - persistalsis, GIT secretions, respiratory rate is slow, bronchoconstriction, relaxes sphincters in urination, HR slows, erection)
    • Sympathetic: fight or flight. Anxiety, channel energy into parts of body to save you (digestion on hold, GNG increases in liver, bronchodilation, blood redirected from viscera and periphery, towards muscles, adrenal medulla releases NA and A to intensify sympathetic response, sweat increases, goosebumps)
      • System overload (really high stress) - opens your bowels.
  • Erection: Point and Shoot
  • Sympathetic NS is more widely distributed throughout the body. Every spinal nerve contains sympathetic fibres
  • Preganglionic autonomic neurons:
    • Sympathetic arise from T1-T2
    • Parasympathetic: brainstem and sacral region of spinal cord

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Overview of sympathetic system

Overview of sympathetic system
  • Sympathetic trunk runs parallel to the spinal cord on each side, where the sympathetic ganglia sit, and there are fibres passing between the ganglia (making a chain)
  • Some fibres synapse in the sympathetic trunk to supply peripheral effectors
  • In the abdominal viscera, the preganglionic fibres go straight through the sympathetic trunk and synapse with peripheral ganglia
  • Roughly one sympathetic ganglion per spinal nerve
  • Lateral horn = cluster of sympathetic nerves originating
  • Connectors between spinal nerve and sympathetic nerve = rami communicans
    • Ventral = white ramus communicans
    • Dorsal = grey ramus communicans
  • Fibre from lateral horn may or may not synapse in the sympathetic ganglion at the same level
  • Diagram shows a T10 cross section
    • This is the case for T1-L2 (sympathetic)

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Sensation from the heart

  • Brain confuses heart pain as pain in the skin area (referred pain), because the sympathetic neurons for the heart go to T1-4 dorsal horn. Hence heart pain is felt in the T1-4 dermatomes (referred pain), in the arm/shoulder
  • Neck pain = pericardium
  • Fullness in the bladder etc travel along parasympathetic nerves, so there is a different site of referred pain.
  • Pain sensations (resulting from ischemia) travel to the Spinal cord with sympathetic nerves
  • These fibres terminate in the dorsal horn of T1-4 segments
  • Hence heart pain is felt in the T1-4 dermatomes (referred pain)
  • Other sensations (from chemo/baroreceptors) travel with the vagus n. (parasympathetic)

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Sympathetic pathways to the abdominal viscera

  • Pathways supplying abdominal viscera

Autonomic9.png

Thoracic vertebra dissection

Autonomic6.png

  • Note that the white rami are anterior to the grey rami
  • Splanchnic nerves contain the preganglionic nerves from the lateral horn (sympathetic) to supply the GIT. There are greater, lesser, and least splachnic nerves
  • Greater splachnic fibres synapse with the coeliac ganglion surrounding the coeliac artery
    • Fibres form plexuses that run around arteries to supply the gut
    • Aorticorenal ganglion (run from coeliac ganglion to supply kidneys
  • Superior mesenteric ganglion (surrounding SMA) supplies SMA; includes splanchnic nerve
  • Inferior mesenteric ganglion
    • Don't come from splanchnic nerves, but come from lumbar ganglia (L1-L2; "lumbar splanchnics")
    • Then travel out to targets (hindgut, pelvic organs: hypogastric nerves and hypogastric plexus)

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Autonomic10.png

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Sympathetic pathway to the head

Sympathetic pathway to the head
  • Supplies sweat glands
  • T1-T4 origin, travel up to a series of ganglia in the neck, then travel out via blood vessels to target (e.g. eye)
  • Compression of sympathetic trunk (e.g. Pancoast's tumour, causing Horner's syndrome): miosis (pupil constriction), ptosis (droopy eyelid) and anhydrosis (no sweating). Unilateral.
  • Sympathetic: pupil dilation, sweating, and fully opened eye

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Comparison of somatic, parasympathetic and sympathetic systems

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  • Parasympathetic ganglia tend to be located in or near their target organ

Overview of parasympathetic nervous system

Spinal cord sources of parasympathetic nerves

Autonomic23.png

  • Preganglionic neurons arise from:
    • (i) brainstem - reach their targets via cranial nerves III, VII, IX and X (details not required, except for CN X)
    • (ii) spinal segments S2-S4 - reach target via pelvic splanchnic nn.
  • Vagus nerve (CN X) supplies neck, viscera, heart, lungs and GIT
  • CNX = vagus nerve, and wanders (unlike all the others, which stay in the head and neck), to supply thoracic viscera (heart, lungs, gut up to midgut)
  • Pelvic = S2-S4 pelvic splanchnic nerves (parasympathetic, while the thoracic/lumbar are sympathetic)
  • Vagus also sends somatic motor fibres to muscles in larynx (recurrent laryngeal nerve)
  • Vagus nerve travels in carotid sheath (with internal/common carotid a's and internal jugular vein)
    • Nerve sits posterior to the artery
Vagus nerve

Cardiac plexuses

Branches of the vagus n. mix with cardiac branches of the sympathetic trunk to form the cardiac plexuses which supply the SA node of the heart Autonomic16.png

Oesophageal plexus and vagal trunks

Oesophageal plexus and vagal trunks
  • Formed by remaining fibres after supplying heart and lungs
  • Supply to these plexuses comes from both left and right vagus nerves (lung on one side, gut on one side, both supplied by both).
  • Vagal trunks (mix of Rt and Lt), pass through diaphragm to supply the stomach
  • Some fibres join with sympathetic plexuses to supply various organs. They don't synapse in the ganglia - vagal fibres synapse in the walls of the organs in tiny ganglia
Prevertebral plexuses
  • Vagus stops supply at transverse colon
  • Pelvic splanchnic nerves supply the pelvic organs, and some run up to join the IMA plexus
  • Prevertebral plexuses are formed by both sympathetic & parasympathetic fibres

Official notes

Handy images

See also