Excitatory and inhibitory actions on phrenic motoneurones

SUMMARY1. Isometric contractions of the medial gastrocnemius (fast twitch) and soleus (slow twitch) muscles were recorded in kittens ranging in age from 3 to 112 days, as well as in adult cats.2. It was confirmed that the speed of contraction of the gastrocnemius muscle becomes progressively faster during the first few weeks after birth, whereas contraction times of the soleus muscle show little changes or a slight prolongation during the period of post-natal development.3. The properties of gastrocnemius (fast alpha) and soleus (slow alpha) motoneurones were examined with intracellular electrodes in kittens at three different stages; 16-20, 61-71 and 100-112 days in age.4. The axonal conduction velocities of both gastrocnemius and soleus motoneurones increased monotonically throughout the period of development and showed no correlation with post-natal changes in contraction times of the innervated muscles.5. The duration of after-hyperpolarization in soleus motoneurones became progressively longer with age, while that in gastrocnemius motoneurones remained virtually unchanged during development.6. The relation between the duration of after-hyperpolarization and the axonal conduction velocity in kitten motoneurones was similar to that observed in axotomized motoneurones of adult cats.7. It is suggested that fast and slow alpha motoneurones show postnatal differentiation in terms of the duration of after-hyperpolarization and that axotomy leads to dedifferentiationn' of the motoneurone properties.8. Post-natal changes in the contractile properties of skeletal muscles were independent of the changes in the duration of after-hyperpolarization of the innervating motoneurones. However, it remains uncertain whether muscle differentiation is independent of the discharge pattern of the innervating motoneurones.

Section: Discussionmentioning

confidence: 99%

Differentiation of motoneurones and skeletal muscles in kittens.

Huizar¹,

Kuno²,

Miyata³

1975

“…These authors have suggested that such rapid accommodation is an important factor in limiting the frequency and duration of discharge in respiratory neurones. In contrast, phrenic motoneurones have shown no evidence of rapid accommodation, and even when subjected to continuous synaptic bombardment during the active phase the occasional increase in the firing threshold did not limit the duration of discharge (Gill & Kuno, 1963). The measurement of electrical constants of the phrenic motoneurone membrane has revealed that the resistance and time constant are both greater than those of lumbosacral motoneurones measured by the same method.…”

Section: Effects Of C02mentioning

confidence: 99%

Properties of phrenic motoneurones

Gill¹,

Kuno²

1963

Self Cite

Descending impulses from the medullary respiratory centre produce a rhythmic inspiratory discharge in phrenic motoneurones. In quiet breathing of an anaesthetized cat single phrenic units discharge at a frequency of 5-30/sec (Pitts, 1946). Pitts (1943) has postulated that the discharge frequency of phrenic motoneurones is determined by a balance between the degree of synaptic excitation of these neurones and their recovery subnormality. In contrast, Calma (1952) has suggested that the site of mechanisms responsible for the long depression appears to be in the medullary respiratory centre. In view of these discrepancies the present study was undertaken to evaluate the factors that may be responsible for determining the frequency at which phrenic motoneurones can discharge.With lumbosacral motoneurones it is generally agreed that the afterhyperpolarization following each spike and recurrent inhibition through Renshaw cells limit their ability to discharge at high frequencies. The slowly discharging 'tonic' motoneurones, for example, have a longer afterhyperpolarization and a more prominent recurrent inhibition than the rapidly discharging 'phasic'

“…Gill & Kuno, (1963b) recorded hyperpolarization only when stimulating the contralateral phrenic nerve. The most pronounced changes were seen in the experiment illustrated in Fig.…”

Section: Introductionmentioning

confidence: 96%

An analysis of the inhibition of phrenic motoneurones which occurs on stimulation of some cranial nerve afferents

Biscoe¹,

Sampson²

1970

SUMMARY1. Inhibition and excitation of spontaneous phrenic nerve discharges in response to stimulation of the sinus, glossopharyngeal, aortic and superior laryngeal (SLN) nerves has been investigated in cats.2. The inhibition, in response to a single shock, had a latency of 5-10 msec and lasted for 20-40 msec; the response to SLN stimulation was the most prolonged.3. Excitation of phrenic motoneurones also occurred and was seen either before or after the end of the inhibition of the inspiratory burst and sometimes also during expiration.4. Intravenous strychnine blocked the inhibition. 5. Intracellular recording from phrenic motoneurones showed that hyperpolarization was evoked by each nerve during the central phrenic depolarizing potential (CPDP) but only rarely in the interval between these potentials.6. When the CPDP was suppressed, hyperpolarization could sometimes be evoked.7. There were no changes in amplitude or time course of hyperpolarization during the passage of current through the cell membrane. No change in membrane conductance could be shown by passing current pulses.8. Raising the pressure in the carotid sinus also depressed phrenic activity and evoked a hyperpolarization whilst the CPDP was suppressed.9. Inspiratory interneurones in the brain stem were suppressed by nerve stimulation and by a rise in carotid sinus pressure.