Summary
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1.
Bilateral pairs of heart accessory neurons (HA cells) have been identified on the ventral surface of the fifth and sixth segmental ganglia ofHirudo medicinalis. These neurons are rhythmically active and fire in a metastable phase relation to the heartbeat central pattern generator and heart activity.
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2.
The HA neurons project into the contralateral roots of their own ganglion and innervate the contralateral heart. The HA neurons of the fifth ganglion (HA (5) cells) send axons into the contralateral anterior connectives and branch in several more anterior ganglia to form axons that project into the contralateral roots. The HA(6) neurons are similar but their intersegmental projection is directed posteriorly.
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3.
The HA neurons receive rhythmic inhibitory synaptic input directly from the neurons of the heartbeat central pattern generator (HN interneurons). The HA cells also receive excitatory synaptic input, which can be spontaneous or driven by discharge in N mechanosensory cells.
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4.
The HA neurons can induce myogenic activity in quiescent hearts and accelerate the rhythm of hearts that are already myogenic. The HA neurons can also increase the beat tension in hearts that are entrained by phasic excitatory drive from heart motor neurons (i.e. the activity of the heartbeat central pattern generator). These influences, are mediated by tonic activity in the HA neurons.
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5.
The HA neurons appear to modulate the myogenic properties and contractile strength of the hearts.
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Calabrese, R.L., Maranto, A.R. Neural control of the hearts in the leech,Hirudo medicinalis . J. Comp. Physiol. 154, 393–406 (1984). https://doi.org/10.1007/BF00605238
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DOI: https://doi.org/10.1007/BF00605238