Intercostal muscle spindle activity and its γ‐motor control

139

117

SUMMARY1. Synchronization of intercostal motoneurones was studied by the construction of cross-correlation histograms which related the firing times of paired groups of efferent inspiratory or expiratory discharges recorded from filaments of the external or internal nerves of anaesthetized or decerebrate cats.2. The principal feature of the histograms was always a central peak but the time course of the central peak showed considerable variation. Three forms of synchronization were defined on the basis of the time course of the central peak: (i) short-term synchronization (Sears & Stagg, 1976), where the peak was narrow, extending over about + 3 ms but sometimes with weak shoulders to about + 5 ms; (ii) broad-peak synchronization where the peak was wider than this (often + 20 ms or more) but where there were no strong periodicities; (iii) high-frequency oscillation (h.f.o.) synchronization, which was named from the related phenomena in medullary and phrenic recordings (Cohen, 1979), where there were periodic peaks on either side of the central peak with a frequency in the range 60-120 Hz. Combinations of these forms of synchronization were seen in some histograms.3. When different animals were compared, broad peak synchronization was seen in association with light anaesthesia and with polysynaptic excitation of the motoneurones from muscle spindle afferents. 4. In individual animals, additional anaesthesia depressed both broad peak and h.f.o. synchronization.5. Raising PA, co2, which increased the respiratory drive to the motoneurones, favoured short-term or h.f.o. synchronization at the expense of broad-peak synchronization.6. In three decerebrate animals only short-term or h.f.o. synchronization was seen. 7. Spinal cord lesions above or below the segments ofinterest promoted broad-peak synchronization, even with high PA, co, or deep anaesthesia.8. We conclude: (i) that short-term synchronization, due mainly to the branching of presynaptic axons, is generated mainly by those axons which transmit the respiratory drive, that drive providing most of the excitation of the motoneurones

Section: Discussionmentioning

confidence: 99%

Variations in the time course of the synchronization of intercostal motoneurones in the cat

Kirkwood

Sears

Tuck

et al. 1982

139

117

“…The two measurements made on individual afferents were in good agreement. A small number of the afferents showed a particularly irregular firing pattern, due to partial synchrony of their discharges with the cardiac cycle (Critchlow & Euler, 1963;Ellaway & Furness, 1977). An example is shown in Fig.…”

Section: Discussionmentioning

confidence: 99%

Excitatory post‐synaptic potentials from single muscle spindle afferents in external intercostal motoneurones of the cat.

Kirkwood¹,

Sears²

1982

SUMMARY1. The discharges of muscle spindle afferents from the external intercostal muscles of anaesthetized, paralysed cats were recorded from dorsal roots in continuity. The dynamic responses, regularities of firing and conduction velocities of the afferents were measured and used to characterize the afferents as primary-like or secondary-like.2. The synchronization of afferent discharges was investigated by the construction of cross-correlation histograms from the simultaneously recorded discharges of pairs of afferents. The discharges of primary-like afferents with high dynamic responses were found to be synchronized within a few msec. The cardiac pulse was a strong contributory factor in this synchronization.

“…Godwin-Austin, 1969) and the elevation ofthe rib due to the action of more rostral external intercostal and levator costae muscles. No attempt was made in the present study to analyse such complex interactions, nor the extent to which a-y co-activation 534 LEVATOR COSTAE MOTONEURONES was involved (Sears, 1963(Sears, , 1964bCritchlow & von Euler, 1963), but this spindle response illustrates the complexity of the proprioceptive signals to the C.N.S. as the configuration of the rib cage changes during breathing (cf.…”

Section: Mu8cle Proprioceptors and Afferentsmentioning

confidence: 99%

“…The topographic distribution of activity in the levator costae muscles throughout the rib cage was surprisingly different from that of the external intercostal muscles, which are also inspiratory in function (Bronk & Ferguson, 1934;Sears, 1963;1964b;Critcholow & von Euler, 1963;Duron, 1973). Whereas the activity of the external intercostal muscles in eupnoea was strongest in the rostral thoracic segments ( Fig.…”

Section: T4mentioning

confidence: 99%

Central and proprioceptive influences on the activity of levator costae motoneurones in the cat.

Hilaire¹,

Nicholls²,

Sears³

1983

SUMMARY1. The role of central respiratory drive, muscle spindles and tendon organs in producing respiratory movements has been studied in the cat by recording from motoneurones supplying a set of small axial muscles inserted between each rib and the vertebra immediately rostral, the levatores costae.2. The levator costae muscles are active during normal inspiration but activity is progressively stronger in the muscles located in more caudal thoracic segments. Intracellular recordings from levator costae motoneurones show a characteristic central respiratory drive potential (c.r.d.p.), comprising phases of depolarization during inspiration alternating with expiratory phased hyperpolarization due to post-synaptic inhibition.3. Loading or unloading the levator costae muscles increases and decreases, respectively, their normal inspiratory activity. Electrophysiological and histological analysis of levator costae afferents reveals that each muscle contains three to five spindles and two to three tendon organs.4. By dissecting the levator costae nerve, afferent discharges from muscle spindle primary and secondary endings and those from tendon organs were recorded 'in continuity' and used to trigger an averager for analysing the synaptic potentials they evoke in levator costae motoneurones. Monosynaptic excitation and oligosynaptic inhibition originate from spindle (primary and secondary endings) and from tendon organ afferents respectively. Peculiarly a monosynaptic excitation has been evoked from a tendon organ.5. The action of the levator costae muscle on the respiratory movements of the ribs in the different thoracic spaces was analysed in relation to their anatomical properties and to the activity of the other respiratory muscles.6. The levator costae muscles, because of their relatively small size and well-defined population of proprioceptors, appear to provide a favourable preparation for studying the integrative action of the motoneurone.