Accommodation in Spinal Motoneurons of the Cat

155

SUMMARY1. The patterns of medial gastrocnemius (MG) motor unit firing in response to MG muscle stretch have been studied in decerebrate cats, using intracellular recording techniques. In most of the units, the motoneurone axonal conduction velocity and cell input resistance were measured, and the maximum amplitude and wave form of the group Ia composite EPSP evoked by MG nerve stimulation were determined. The mechanical properties of the muscle unit portion of each motor unit were also studied.2. Motor unit firing patterns were classified into two groups, 'tonic' and 'phasic'. With few exceptions, tonic motor units showed sustained firing throughout MG stretches of varying duration, while most phasic units did not fire at all to the same stimulus. Tetanization of the afferents did not convert any of the previously phasic units to tonic firing.3. MG motor units in this study were divided into three groups on the basis of the muscle unit twitch properties. Units with short twitch time to peak values (< 35 msec) were subdivided into two groups according to twitch tension output: (a) type F, with twitch tension > 1*5 g, and (b) type F*, with twitch tension < 1U5 g. Units with slow twitch time to peak (> 35 msec) were classified as type S.4. The presence or absence of tonic firing during sustained MG stretch was found to be significantly related to the following factors: (a) the motor unit twitch type, in that tonic firing was observed in 100 % of type S, 70 % of type F*, and only 10 % of type F units; (b) the apparent motoneurone size, in that tonic units tended to have higher cell input resistance values and slower axonal conduction velocities than phasic units; and (c) the density and spatial organization of the group Ia synaptic input, in that the MG monosynaptic EPSPs found in tonic motor units tended to be both larger in amplitude and longer in duration than those found in phasic units.5. The intrinsic properties of MG motor units, the characteristics of the group I a synaptic input to the motoneurones and the unit firing patterns elicited by MG muscle stretch appear to be mutually interrelated, forming a pattern of motor units within the MG pool which is analogous to the pattern thought to characterize the motor units belonging to 'slow' and 'fast' muscles. In most respects this pattern of MG motor units appears to be a continuum without clearly separable subgroups.

Section: Discussionmentioning

confidence: 88%

Firing patterns of gastrocnemius motor units in the decerebrate cat

Burke¹

1968

155

“…Such a qualitative distinction may be found in cell properties not studied here, such as in accommodative mechanisms (Sasaki & Otani, 1961;Ushiyama, Koizumi & Brooks, 1966) or in the organization patterns of synaptic input to different motoneurones (see Sasaki & Tanaka, 1964;Bosemark, 1965).…”

Section: Discussionmentioning

confidence: 97%

Motor unit types of cat triceps surae muscle

Burke¹

1967

334

171

SUMMARY1. Motor units, defined as including a motoneurone (cell body, dendrites and axon) plus the muscle unit innervated, have been examined in the triceps surae motor pool of pentobarbital anaesthetized cats.2. The technique of intracellular stimulation and recording which was used permitted measurement of the axonal conduction velocity, post-spike hyperpolarization duration and input resistance of individual motoneurones, and the correlation of these properties with the characteristics of the twitch and tetanus responses of the muscle unit innervated by the cell elicited by direct intracellular stimulation.3. On the basis of muscle unit speed of contraction, motor units were divided into two groups: (a) fast twitch, or F, type with twitch time to peak (TwTp) less than or equal to 30 msec, and (b) slow twitch, or S, type with TwTp of 40 msec or greater. The twitch tensions (TwTen) produced by type F units were significantly larger (median value = 18 g) than the tensions generated by type S units (TwTen median value = 1*6 g). Type F muscle units had much higher tetanus fusion frequencies (median = 85 pulses/see) than the S type (median 25 pulses/sec), and tended to have smaller tetanus to twitch tension ratios (Tet/Tw) (median = 2.6) than type S units (median = 5.4).4. The gastrocnemius heads contained a mixture of F and S types of muscle units, the proportions found being about 3 to 1 respectively. Units encountered in the soleus muscle were uniformly of type S. The characteristics of gastrocnemius and soleus type S motor units were not identical but appeared to represent quantitative differences in units of the same qualitative type.5. Motoneurones innervating type F muscle units had faster axonal conduction velocities, shorter post-spike hyperpolarizations and lower input resistances than those supplying type S units. However, no combination of motoneurone properties alone was sufficient to separate unambiguously types F and S motor units.

“…post-synaptically. Such post-synaptic factors would include the relation between membrane potential and threshold for impulse initiation and differences in adaptation to membrane potential changes (Eccles, 1961;Sasaki & Otani, 1961). Systematic variation in the amount of depolarization necessary to reach threshold for a conducted action potential would be sufficient to explain differences in the responsiveness of elements of a motoneurone pool provided that reflex excitability was always parallel for the various tests.…”

Section: Discussionmentioning

confidence: 99%

A comparison of monosynaptic and polysynaptic reflex responses from individual flexor motoneurones

Perl¹

1962

Evidence from a number of types of experiments has shown that gradation of muscular contraction is accomplished by changes both in the number of motoneurones active and in the frequency of discharge of individual motoneurones (Creed, Denny-Brown, Eccles, Liddell & Sherrington, 1932). It is also well known that different spinal reflexes have different abilities to excite fractions of the motoneurone pool supplying a particular muscle. On the other hand, there appears to be little information on one important feature of the functional organization of motoneurones particularly relevant to quantitative control of skeletal muscle contraction, namely, the degree of influence various reflex pathways have on a particular motoneurone in comparison to other motoneurones of the same pool.The present study examines the effectiveness of the monosynaptic and several polysynaptic reflex arcs in discharging individual knee flexor motoneurones. These experiments should be useful in indicating whether a given motoneurone responds equally well to different reflex pathways or whether variation in excitation throughout a particular pool is dependent upon the nature of the reflex process. Information on such a point would be valuable in helping to ascertain the manner in which different mechanisms combine to determine the participation of motoneurones in activity. For instance, it has been postulated that a significant control of motoneurone excitability is exerted via the fusimotor (gamma) effect upon the afferent limb on the monosynaptic reflex, the Group I fibres from the muscle spindle (Eldred, Granit & Merton, 1953). Fitting the motor control of the spindle into the framework of mechanisms concerned with the functional organization of motoneurones would require knowledge of how the effectiveness of the monosynaptic drive varies among members of a pool in comparison to other excitatory pathways.The motoneurones studied were taken from the pool monosynaptically excited by Group I volleys from the peripheral nerves supplying the