Neural control of the hearts in the leech,Hirudo medicinalis
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The electrical activity of the heart inHirudo medicinalis is correlated with the rhythmic discharge of segmental heart motor neurons (HE cells). Excitatory junctional potentials from the HE motor neurons summate in the heart muscle cells and give rise to large plateau-like potentials with associated spikes called bursts.
Individual heart muscle cells isolated by enzymatic dissociation of the heart are capable of producing a myogenic polarization rhythm.
The peripheral branches of the HE motor neurons are capable of producing antidromic burst activity (peripheral neurogenic rhythm) independently of the heart's myogenic rhythm when central activity in the HE cells is experimentally suppressed. HE motor neurons synaptically interact with one another in the periphery: their peripheral bursts can be coordinated and orthodromic activity in an HE cell can elicit antidromic activity in other ipsilateral HE cells whose central activity is suppressed experimentally. Antidromic bursting in HE cells is not normally observed when they are expressing their normal central activity rhythm. These observations indicate that there is a peripheral nerve plexus comprising the HE cells' peripheral branches that is capable of spreading the HE cells' activity along the heart tube.
The heart produces a myogenic contractile rhythm as well as its polarization rhythm, and these two activities appear to be associated.
The HE motor neurons reset and entrain the heart's electrical and contractile myogenic rhythms. Since the HE cells' activity is the output of the heartbeat central pattern generator, the pattern generator must be the governing oscillator determining the animal's heartbeat.
KeywordsMotor Neuron Central Pattern Generator Nerve Plexus Heart Tube Peripheral Branch
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