Impulse activity and receptor potential of primary and secondary endings of isolated mammalian muscle spindles.

Abstract

SUMMARY1. An isolated muscle spindle preparation from a tail muscle of cat is described. The afferent response to a ramp-and-hold stretch was recorded in individual axons from identified primary and secondary endings.2. Primary endings exhibit a prominent dynamic response, including an initial burst. They also show a well-maintained static discharge. Secondary endings also show a well-sustained static discharge but generally have a much lower dynamic sensitivity. The response of primary and secondary endings of the isolated spindle are similar to the typical responses seen in vivo in group Ia or group II afferent fibres respectively.3. Following impulse blockade by tetrodotoxin, the receptor potential was recorded from primary and from secondary endings in response to ramp-and-hold stretch.4. During the dynamic phase the receptor potential of primary endings consists of a depolarization which has two components. (a) An initial component occurs early during ramp stretch, depends in rate of rise and amplitude on velocity of stretch and is reduced on repetitive stretch; it appears to be responsible for the initial burst. (b) A late dynamic component, which follows, is also dependent on stretch velocity and produces the late dynamic discharge. At the end of ramp stretch the receptor potential falls, and may undershoot, the static level. There is a subsequent adaptive fall during hold stretch, then a maintained static level of receptor potential. On release from stretch the membrane is hyperpolarized.5. Secondary endings usually show a smaller dynamic response, lacking the initial component seen in primary endings. They also generally lack an undershoot following the ramp and have less of a post-release hyperpolarization.C. C. HUNT AND D. OTTOSON 6. Static levels of receptor potential in both primary and secondary endings are related to amplitude of stretch.7. The receptor potentials of primary and secondary endings account for the major features of the impulse responses of these endings to rampand-hold stretch. In primary endings the dynamic frequencies may also depend upon a sensitivity of the impulse initiating site to rate of change of receptor current.

Ionic basis of the receptor potential in primary endings of mammalian muscle spindles.

“…The receptor potential of the primary ending, in response to ramp-and-hold stretch, consists of a characteristic sequence of potential changes which depend upon stretch velocity and amplitude (Hunt and Ottoson, 1975). In the present study, amplitudes of several hundred microns were used with moderate to high ramp velocities.…”

“…Spindles were isolated from thin muscles in the dorsolateral portion of the tail by procedures which have been previously described (Hunt and Ottoson, 1975). The spindle capsule was removed by microdissection in the region of the capsular space, where sensory endings are located.…”

“…The generation of action potentials by primary endings in response to stretch results from nerve-terminal depolarization which spreads to an impulse-initiating site in the primary axon (Hunt and Ottoson, 1975). The depolarization of the sensory terminals presumably results from conductance changes produced by stretch deformation.…”

Interaction of impulse activities originating from individual Golgi tendon organs innervated by branches of a single axon.

“…injection) the tail was skinned. Immediately after being detached at its base from the rest of the body the tail was transferred to a bath containing modified Locke solution (Hunt & Ottoson, 1975;Fukami & Wilkinson, 1977). The thin muscles located in the dorsolateral aspect of the tail were exposed to the solution as quickly as possible by retracting their tendons laterally.…”

Control of dynamic and static nuclear bag fibres and nuclear chain fibres by gamma and beta axons in isolated cat muscle spindels.

“…This mechanical explanation of the action of dynamic axons may account for the second, slow phase of the adaptation process (Crowe & Matthews, 1964) but it is unlikely that it accounts for the whole of the initial rapid adaptation. Hunt & Ottoson (1975) fig. 1 C, F).…”