Download 19 Transmission of the action potential to smooth muscle and

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NERVE ENDINGS AT SMOOTH MUSCLE
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Postganglionic neurons branch extensively over the surface of smooth muscle fibres
There are NO END PLATES
Some of the endings contain acetylcholine vesicles, other endings contain noradrenaline
There are vesicle-containing VARICOSITIES along the axons of these nerves; the neurotransmitter seems to be
released from these along the whole axon
Thus, one neuron innervates many effector cells
This is called synapse en passant- “synapse in passing”
NERVE ENDINGS AT CARDIAC MUSCLE
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Cholinergic and adrenergic fibres innervate the sinoatrial node, atrioventricular node and the bundle of His.
Noradrenergic fibers also pass into the ventricular muscle
In the ventricle, the contacts between the noradrenergic fibers and the muscle are synapses en passant
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The smooth muscles receive noradrenergic and cholinergic nerve endings; in some muscle the cholinergic is
excitatory and the noardrenegic are inhibitory; in other tissues its vice versa.
The excitatory neurotransmitter, when released, produces a small discrete partial depolarization- looks like a small
end plate potential. These are called excitatory junction potentials.
Similarly, the inhibitory neurotransmitter produces inhibitory junction potentials
These potentials are summative and can bring the cell to threshold
The potentials spread electrotonically.
JUNCTIONAL POTENTIALS
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DENERVATION HYPERSENSITIVITY
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IT IS WELL KNOWN: If you severe a nerve supply to a muscle, it becomes abnormally sensitive to acetylcholine.
The denervated skeletal muscle will also atrophy.
Smooth muscle does NOT atrophy, but does become abnormally sensitive
This is due to increased synthesis of neurotransmitter receptors.
The upregulation of receptor synthesis is due to the decreased neurotransmitter release
Hypersensitivity is limited to the structures immediately innervated by the severed neuron
References: Ganong Review of Medical physiology, 23rd ed, chapter 6