Fall in the firing rate of muscle spindles during stretching.

Investigation of the dynamic responses of 500 muscle receptors to stretching of the triceps surae muscle of the de-efferented hind limb of anesthetized cats revealed in 23 cases a transient fall in the firing rate (FFR) of muscle spindle receptors while muscle stretching was in progress. FFR arises in the same receptor during stretching of the muscle in different forms and at different speeds, approximately in the same region of stretching (the "FFR zone"). With an increase in the velocity of muscle stretching the FFR effect is intensified. It is suggested that the FFR effect is due to temporary removal of the load from the area of the spindle where the receptor endings are located. The conditions for this may be created by the nonhomogeneous structure of the extrafusal muscle, and also by the different mechanical properties of the intrafusal muscle fibers.

[Rate of occurrence and reproducibility of the initial burst of frequency of muscle spindle receptors]. / Vstrechaemost'i vosproizvodimost' nachal'nogo vspleska chastoty u retseptorov myshechnykh vereten.

The initial burst (IB) of activity of the passive muscle spindle receptors occurs sometimes during muscle stretching with minor speed starting from the completely relaxed state. Its rare occurrence, unstability and variability reject the suggestion of acceleration-sensitivity in the muscle spindle receptors. The receptors respond sometimes to initial enhancement of rigidity in the extra- and intrafusal muscle fibers. The IB occurs as the result of interaction between both types of fibers during stretching.

Characteristics of adaptation of muscle stretch receptors of dynamic type.

Experiments on cats with a de-efferented triceps surae muscle showed that some stretch receptors of dynamic type adapt quickly within the first few seconds after stretching of the muscle and its keeping at the new length, whereas other receptors reveal two components of adaptation-fast and slow. Units with different types of adaptation are found in the same muscle; their afferents belong mainly to group I, although there is a zone of overlapping with group II afferents. These differences are manifested at high values of initial stretching of the muscle. It is suggested that the slow component of adaptation is based on a mechanical factor depending on the visco-elastic properties of the tissues in the region of the sensory nerve endings.

Identification of muscle spindles by direct and indirect electrical stimulation of intrafusal muscle fibers.

In two series of acute experiments on 25 anesthetized cats methods of rapid identification of muscle spindles based on electrical stimulation either of fusimotor nerve fibers or directly of the intrafusal muscle fibers were tested. The methods used are more reliable tests for the identification of muscle spindles than the generally accepted method of investigating the velocity of conduction of excitation along a single afferent fiber, for receptors both of the joints and of the deep tissues may also be innervated by fibers of the same diameter.

[Decrease in the frequency of muscle spindle discharges during muscle stretching]. / Umen'shenie chastoty razriadov myshechnykh vereten pri rastiazhenii myshtsy.

When studying responses of 500 muscle receptors to stretching of the m. triceps of the cat deafferented hindlimb, 23 cases revealed a transient decrease in the firing rate (DFR) of muscle spindles' receptors. The DFR occurs in the same receptor at different forms and rates of stretching approximately at the same area of stretching ("the DFR area"). When increasing the rate of stretching, the DFR effect is enhanced. The DFR effect seems to be due to a short-lasting unloading of the spindle area which contains the receptor terminals. The unhomogenous structure of the extrafusal muscle as well as various mechanical properties of the intrafusal muscle fibers can provide necessary conditions for the above effect.

[Muscle relaxation and muscle spindle adaptation]. / Relaksatsiia myshtsy i adaptatsiia myshechnykh vereten.

In experiments on deefferented m. triceps surae of the cats, the unequal correlation between speed of muscle relaxation during interval from 2.5 to 40 sec after the beginning of static stretch and the rate of adaptation of three functionally different groups of muscle spindles, was revealed. The dependence of the adaptation of muscle spindle endings on relaxation of both extra-and intrafusal muscle fibers, is discussed.

[Spontaneous activity of the passive muscle spindles of the cat triceps surae]. / Spontannaia aktivnost' passivnykh myshechnykh vereten trekhglavnoi myshtsy goleni koshki.

In a slack deeferented m. triceps surae about 45% of 400 stretch receptors under study have a spontaneous discharge. The following facts indicated that some intrafusal fibres in the slack muscles are presumably in the prestretch state: 1) the isotonic contraction of the muscle diminished or abolished the spontaneous discharge; 2) at the beginning of the stretch of the slack muscle the initial slowing in the discharge frequency of the unit sometimes appeared; this slowing did not depend on the type of unit and occured in a unit among many others in the same preparation and under the same conditions of experiment; 3) the dynamic thresholds of spontaneously discharging units were lower than that of units without spontaneous discharge. It is suggested that a static stretch occurs in some muscle spindles, if the distance between two points of big fibres insertion on extrafusal fibres or tendon fascicules is longer than the length of intrafusal fibres. The initial slowing at the beginning of stretch is presumably due to viscous damping forces which must be higher in more viscous extrafusal fibres in comparison to that of intrafusal fibres.

[Adaptational characteristics of muscle stretch receptors of the dynamic type]. / Kharakteristiki adaptatsii myshechnykh retseptorov rastiazheniia dinamicheskogo tipa.

In cats with deefferented m. triceps, some stretch receptors of the dynamic type were shown to adapt rapidly after stretching of the muscle and keeping it at the new length, whereas other receptors revealed rapid and slow components of the adaptation. The units with different kinds of adaptation coexist in the same muscle, their afferents being related mainly to the group I, although overlapping with the group II afferents does exist. The above differences are revealed at high values of the muscle initial stretch. It seems that the adaptation slow component is based on a mechanical factor depending on the viscous-elastic properties of the tissue in the area of sensitive nerve terminals.

[Identification of muscle spindles by direct and indirect electric stimulation of intrafusal muscle fibers]. / Identifikatsiia myshechnykh vereten putem priamoi i nepriamoi élektricheskoi stimuliatsii intrafuzal'nykh myshechnykh volokon.

In two series of experiments on 25 anesthetized deefferented cats methods of quick identification of muscle spindles were used. The first method was based on the postcontractory sensory discharge in a muscle spindle after brief strong electrical stimulation of the muscle nerve. In another approach the concentric electrodes inserted into the muscle were used. If the intrafusal muscle fibers were selectively contracted by direct electrical stimulation the sensory discharge appeared without any increase in the total muscle tension. It seems that these methods enable to identify the unit under study more precisely in comparison with conduction velocity measurements, as the group I and II afferents are sometimes present in muscle nerve from joint and deep tissues.

[Effect of prolonged preliminary static stretching of de-efferented muscle on the dynamic response of muscle receptors]. / Vliianie dlitel'nosti predshestvuiushchego staticheskogo rastiazheniia deéfferentirovannoi myshtsy na dinamicheskii otvet myshechnykh retseptorov

Speed of adaptation of muscle receptor discharges during static stretching of the muscle was shown to depend on the grade and duration of muscle tension reducing which, in its turn, is the function of muscle length. The increase in the receptor firing rate during the dynamic phase of muscle stretching (the dynamic response) depends both on the grade and speed of stretching and on whether the muscle is "balanced" after previous stretching. The data obtained suggest insignificant increase in the relaxed muscle length on changing of angle in the talocrural joint within the physiological range.

[Functional specialization of stretch receptors in the triceps surae muscle of cats]. / Funktsional'naia spetsializatsiia retseptorov rastiazheniia v trekhglavoi myshtse goleni koshki

The behavior of 300 receptors in response to stepwise changes in muscle length was studied in passive m. triceps surae of cats anesthetized with nembutal. Distinct differences in receptor behavior were demonstrated, and receptors were divided into three functional groups according to their sensitivity to slow stretch (0.6 mm/sec in the limit range of 10 mm). The 1st group (elementary dynamic receptors) showed obvious sensitivity (over 70/sec) to dynamic components of stimulus and did not respond to static stretch in the physiological range. It was impossible to study the silent period during muscle contraction and stretch. The units of the 2nd group (elementary static receptors) reached maximal discharge frequency of 20--25/sec in the same range of muscle stationary stretch. The dynamic response of these receptors slightly exceeds the static response. The 3rd group, comprising 80% of units, served as universal detector of both dynamic and static phases of a stimulus. Some units showed significant dynamic response (over 70/sec) and moderate static response (about 30--40/sec), others exhibited moderate dynamic and static responses. The behavior of the receptors of universal type is similar to that of the well--known primary and secondary endings of the muscle spindles. Asto the endings of two elementary types they should be further indentified.