Parasympathetic nervous system

From Ganfyd

Along with the sympathetic nervous system is responsible for the autonomic control of the body. Parasympathetic neurons generalise arise from sections of the spinal cord above and below those involved in the sympathetic nervous system and its actions often, but not always, oppose those of the sympathetic system.

Innervates

• Innervates heart, eye and internal organ systems
  • constricts pupil of eye
  • slows heart rate
  • increased salivation
  • cooperates in ejaculation
  • selectively regulates and promotes functioning of internal organ systems
• More restricted and focal than the sympathetic system
• ganglia all lie close to or enclosed within structures they innervate
• Pre-ganglionic neurones situated either within brainstem or in middle sacral segments of spinal cord
  • enteric ganglia innervated by parasympathetic pre-ganglionic neurones

Cranial outflow:

• occulomotor (III) nerve to iris (sphincter pupillae) and ciliary muscles of eye, via small cilliary ganglion
• facial and glossopharyngeal nerves (VII and IX) innervate lacrimal gland of orbit and salivary and mucus glands of mouth, nose and pharynx via three ganglia (pterygopalatine, submandibular, otic)
• vagus (X) caries extensive preganglionic outflow to pacemaker and nodal regions of heart, ganglia in lung (pulmonary plexuses) and enteric ganglia in alimentary tract (submucosal and myenteric) from pharynx to end of midgut (oesophagus, stomach, duodenum and pancreas, small intestine and part of colon)

Sacral outflow

• Middle sacral segments of spinal cord (S2-4).
• Cell bodies of preganglionic neurons in intermediate zone of grey matter
• axons distributed to enteric ganglia of intrinsic plexuses (submucosal and myenteric) plexuses of hindgut (caudal part of colon and rectum) and to ganglia in wall of urinary bladder and associated with accessory reproductive organs (pelvic ganglia
• B fibres like those of sympathetic
• cholinergic

Postganglionic axons

• unmyelinated and usually very short.
• many are cholinergic (e.g. to sphincter pupillae)
• wide variety of other transmitters,
  • VIP, substance-P, cholecystokinin, somatostatin
  • Nitric oxide : responsible in some sites for non-adrenergic non-cholinergic inhibitory responses
• Preganglionic ratio 1:1 of 1:few

Enteric ganglia

in 2 nerve plexuses

• • Myenteric
    • between longitudinal and circular smooth muscle layers of gut wall
  • submucosal plexus
    • lying in the submucosal layer of the gut wall
• can allow peristalsis independent of external innervation
• innervated by preganglionic parasympathetic nerve fibres
  • increase peristalsis and blood flow to gut wall
• innervated by postganglionic sympathetic nerve fibres
  • decrease peristalsis and blood flow
  • increases tone of pyloric and anal sphincters.