Size and inspiratory input timing as factors determining the recruitment order of the phrenic motoneurones

Hilaire, G.; Monteau, R.

Journal de Physiologie 75(7): 765-781


ISSN/ISBN: 0021-7948
PMID: 232726
Accession: 006424321

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Experiments were done on anesthetized cats. The activity of 68 phrenic motoneurons and of 40 inspiratory medullary neurons located in the dorsal respiratory nucleus, whose axons lie in the spinal cord and connect with the phrenic motoneurons, were recorded with extracellular microelectrodes. The activity of 110 phrenic motoneurons was studied on thin filaments of dilacered phrenic nerves. The recruitment delay of the phrenic motoneurons after the onset of inspiration (estimated by the gross phrenic nerve discharge) shows a bimodal distribution: some motoneurons are recruited early, others late with a delay of recruitment > 10% of the inspiratory duration. The conduction velocities of the early recruited motoneurons are often slower than those of the late recruited motoneurons, but the 2 groups overlap: the conduction velocity has a unimodal distribution. As conduction velocity is claimed to be closely related to the size of the axon, it can be said that recruitment order is not only dependent on the size of the motoneurons. Antidromic activation latency of the phrenic motoneurons is not constant throughout the respiratory cycle. It shows slight fluctuations that depend on the excitability of the soma: variations of the antidromic latency can reveal variation of cell excitability. Three motoneuron types: P, T and PT were distinguished. A tendency to synchronism in the discharge of 2 phrenic motoneurons is frequently present when the 2 studied motoneurons are recruited either early or late: motoneurons which have the same type of recruitment most likely receive a common excitatory drive. A tendency to synchronism in the discharge of an early and a late recruited phrenic motoneuron is uncommon, indicating that early and late recruited motoneurons do not receive the same excitatory inputs. When a tendency to synchronism occasionally exists, the late recruited motoneuron that shows synchronism with an early recruited one is probably a type P/T motoneuron. The study of the synaptic connections between the medullary inspiratory neurons of the dorsal nucleus that send their axons to the phrenic motoneurons and these motoneurons reveals that the early recruited inspiratory medullary neurons excite the early recruited motoneurons, while the late recruited medullary neurons excite the late recruited motoneuron. Type P/T motoneurons may receive excitatory inputs from both early and late recruited medullary neurons. The recruitment order of phrenic motoneurons depends mainly on the timing of the excitatory drive that they receive. The phrenic motoneurons of type P (early recruited) are often small in size and are excited by the early recruited medullary neurons; those of type T and P/T (late recruited), the size of which is frequently greater, are controlled by the late recruited neurons. Type P/T motoneurons may receive an early drive too weak to trigger an early recruitment.