Overview nervous system (ANS) manages our physiology.
By regulating organs and organ systems, and their smooth muscles and glands.
ANS Control of Smooth Muscle
Smooth muscle maintains resting tone in absence of nerve stimulation.
Smooth becomes more sensitive when ANS input is cut (=denervation hypersensitivity)
Many types of smooth are spontaneously active and contract rhythmically without ANS input.
ANS input simply increases or decreases intrinsic activity.
Autonomic Neurons
ANS has 2 neurons in its efferent pathway.
1st neuron (=preganglionic neuron) has cell body in brain or spinal cord.
Synapses with 2nd neuron (=postganglionic neuron) in an autonomic ganglion.
Postganglionic axon extends from autonomic ganglion to target tissue.
Divisions of the ANS
ANS has sympathetic and parasympathetic divisions.
Usually have antagonistic effects.
These coordinate physiology with what’s going on in person’s life.
Sympathetic mediates “fight, flight, and stress” reactions.
Parasympathetic mediates “rest and digest” reactions.
Sympathetic Dvisions
Is also called thoracolumbar division because its pregpreganglionics anglionics exit spinal cord from T1 to L2.
Most then synapse on postganglionic in the paravertebral ganglia.
Which form chain of interconnected ganglia paralleling spinal cord.
Divergence: preganglionic branch to synapse with number of postganglionic neurons.
Convergence: Postganglionic receive synaptic input from large number of preganglionic.
Some postganglionics do not synapse in paravertebral ganglion but go to outlying collateral ganglion.
Sympathoadrenal System
The adrenal medulla, located in adrenal gland on top of kidney, appears to be a modified collateral ganglion.
Its secretory cells appear to be modified postganglionic.
That release 85% epinephrine (Epi) and 15% norepinephrine (Norepi) into blood in response to preganglionic stimulation.
Stimulated during mass activation.
Epi is made by methylating Norepi.
Parasympathetic Division
Is also called craniosacral division because long preganglionic originate in midbrain, medulla, pons, and S2-S4.
Synapse on postganglionic in terminal ganglia located next to or within target organ.
Postganglionic has short axon that innervates target.
The long vagus nerve carries most Parasymp fibers.
Innervates heart, lungs, esophagus, stomach, pancreas, liver, small intestine, and upper half of the large intestine.
Preganglionic fibers from S2-4 innervate lower half of large intestine, rectum, urinary and reproductive systems.
ANS Overview
ANS Neurotransmitters
Both Symp and Parasymp preganglionics release ACh.
Parasymp postganglionics also release ACh.
Called cholinergic synapses.
Most Symp postganglionic release Norepi (Noradrenaline).
Called adrenergic synapses.
A small number release ACh
Postganglionic have unusual synapses called varicosities.
Which release NTs along a length of axon.
= synapses en passant
Adrenergic Stimulation
Many useful drugs have been developed to affect ANS receptors.
Drugs that promote actions of a NT are agonists
Drugs that inhibit actions of a NT are antagonists
Cholinergic Stimulation
ACh is used at all motor neuron synapses on skeletal muscle, all preganglionic, and Parasymp postganglionics.
Cholinergic receptors have 2 subtypes:
Nicotinic which is stimulated by nicotine; blocked by curare.
and muscarinic which is stimulated by muscarine (from poisonous mushrooms); blocked by atropine.
Other ANS NTs
Some postganglionic do not use Norepi or ACh.
Called nonadrenergic, noncholinergic fibers.
Appear to use ATP, VIP, or NO as NTs.
NO produces smooth muscle relaxation in many tissues.
Organs With Dual Innervation
Most visceral organs receive dual innervation (supplied by both Symp and Parasymp)
While 2 branches are usually antagonistic, such as their effects on heart rate.
Can be complementary (causes similar effects) such as with salivation.
Or copperative (produce different effects that work together to cause desired effect) such as with micturition, and sexual function.