SIMULTANEOUS RESPONSES OF GROUPS I AND II CAT MUSCLE SPINDLE AFFERENTS TO MUSCLE POSITION AND MOVEMENT

1. Chronically implanted microelectrode wires in the L7 and S1 dorsal root ganglia were used to record unit activity from cat hindlimb primary and secondary muscle spindle afferents. Units could be reliably recorded for several days, permitting comparison of their activity with homonymous muscle EMG and length during a variety of normal, unrestrained movements. 2. The general observation was that among both primary and secondary endings there was a broad range of different patterns of activity depending on the type of muscle involved and the type of movement performed. 3. During walking, the activity of a given spindle primary was usually consistent among similar step cycles. However, the activity was usually poorly correlated with absolute muscle length, apparently unrealted to velocity of muscle stretch, and could change markedly for similar movements performed under different conditions. 4. Spindle activity modulation not apparently related to muscle length changes was assumed to be influenced by fusimotor activity. In certain muscles, this presumption leads to the conclusion that gamma-motoneurons may be activated out of phase with homonymous alpha-motoneurons as well as by more conventional alpha-gamma-motoneuron coactivation. 5. Simultaneous recordings of two spindle primary afferents from extensor digitorum longus indicated that spindles within the same muscle may differ considerably with respect to this presumed gamma-motoneuron drive. 6. Spindle secondary endings appeared to be predominantly passive indicators of muscle length during walking, but could demonstrate apparently strong fusimotor modulation during other motor activities such as postural changes and paw shaking. 7. Both primary and secondary endings were observed to undergo very rapid modulation of firing rates in response to presumed reflexly induced intrafusal contractions. 8. It is suggested that the pattern of fusimotor control of spindles may be tailored to the specific muscle and task being performed, rather than necessarily dominated by rigid alpha-gamma coactivation.

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Activation of GABAB receptors causes presynaptic inhibition at synapses between muscle spindle afferents and motoneurons in the spinal cord of bullfrogs

Journal of Neuroscience ◽

10.1523/jneurosci.09-05-01502.1989 ◽

1989 ◽

Vol 9 (5) ◽

pp. 1502-1515 ◽

Cited By ~ 45

Author(s):

YY Peng ◽

E Frank

Keyword(s):

Spinal Cord ◽

Muscle Spindle ◽

Presynaptic Inhibition ◽

Gabab Receptors ◽

Muscle Spindle Afferents

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Firing properties of muscle spindles supplying the intrinsic foot muscles of humans in unloaded and freestanding conditions

Journal of Neurophysiology ◽

10.1152/jn.00539.2018 ◽

2019 ◽

Vol 121 (1) ◽

pp. 74-84 ◽

Cited By ~ 6

Author(s):

T. P. Knellwolf ◽

A. R. Burton ◽

E. Hammam ◽

V. G. Macefield

Keyword(s):

Muscle Spindle ◽

Postural Sway ◽

Center Of Pressure ◽

Muscle Spindles ◽

Medial Malleolus ◽

Proprioceptive Information ◽

Muscle Spindle Afferents ◽

Tungsten Microelectrode ◽

Firing Properties ◽

Intrinsic Muscles

We recently developed an approach for recording from muscle spindles in the intrinsic muscles of the foot in freestanding humans by inserting a tungsten microelectrode into the posterior tibial nerve behind the medial malleolus of the ankle. Here we characterize the behavior of muscle spindles in the small muscles of the foot in 1) seated subjects with the leg horizontal and the foot naturally plantarflexed and 2) standing subjects. In the first study, recordings were made from 26 muscle spindle afferents located within flexor digiti minimi brevis ( n = 4), abductor digiti minimi ( n = 3), quadratus plantae ( n = 3), plantar interossei ( n = 4), flexor digitorum brevis ( n = 3), dorsal interossei ( n = 2), and lumbricals ( n = 2), with one each supplying abductor hallucis, adductor hallucis, and flexor hallucis brevis. The identity of another two muscle afferents was unknown. The majority of the units were silent at rest, only seven (27%) being spontaneously active. Because of the anatomic constraints of the foot, some spindles supplying muscles acting on the toes responded to movements of one or more digits. In the second study, 12 muscle spindle afferents were examined during standing. The ongoing discharge of eight spindle afferents covaried with changes in the center of pressure during postural sway. We conclude that the majority of spindle endings in the small muscles of the foot are silent at rest, which may allow them to encode changes in conformation of the foot when it is loaded during standing. Moreover, these muscle spindle afferents can provide useful proprioceptive information during standing and postural sway. NEW & NOTEWORTHY We have characterized the firing properties of muscle spindles in the intrinsic muscles of the human foot for the first time. The majority of the spindle endings are silent in seated subjects, and most fire tonically during standing, their discharge covarying with center of pressure during postural sway. We conclude that spindle endings in the intrinsic muscles of the foot provide useful proprioceptive information during free standing.

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Effects of tonic muscle pain on fusimotor control of human muscle spindles during isometric ankle dorsiflexion

Journal of Neurophysiology ◽

10.1152/jn.00862.2018 ◽

2019 ◽

Vol 121 (4) ◽

pp. 1143-1149

Author(s):

Lyndon J. Smith ◽

Vaughan G. Macefield ◽

Ingvars Birznieks ◽

Alexander R. Burton

Keyword(s):

Muscle Spindle ◽

Muscle Pain ◽

Tibialis Anterior Muscle ◽

Muscle Spindles ◽

Muscle Spindle Afferents ◽

Firing Rates ◽

Leg Muscles ◽

Tonic Muscle ◽

Voluntary Contractions ◽

Ankle Dorsiflexors

Studies on anesthetized animals have revealed that nociceptors can excite fusimotor neurons and thereby change the sensitivity of muscle spindles to stretch; such nociceptive reflexes have been suggested to underlie the mechanisms that lead to chronic musculoskeletal pain syndromes. However, the validity of the “vicious cycle” hypothesis in humans has yielded results contrasting with those found in animals. Given that spindle firing rates are much lower in humans than in animals, it is possible that some of the discrepancies between human experimental data and those obtained in animals could be explained by differences in background fusimotor drive when the leg muscles are relaxed. We examined the effects of tonic muscle pain during voluntary contractions of the ankle dorsiflexors. Unitary recordings were obtained from 10 fusimotor-driven muscle spindle afferents (6 primary, 4 secondary) supplying the ankle dorsiflexors via a microelectrode inserted percutaneously into the common peroneal nerve. A series of 1-min weak contractions was performed at rest and during 1 h of muscle pain induced by intramuscular infusion of 5% hypertonic saline into the tibialis anterior muscle. We did not observe any statistically significant increases in muscle spindle firing rates of six afferents followed during tonic muscle pain, although discharge variability increased slightly. Furthermore, a participant’s capacity to maintain a constant level of force, while relying on proprioceptive feedback in the absence of visual feedback, was not compromised during pain. We conclude that nociceptive inputs from contracting muscle do not excite fusimotor neurons during voluntary isometric contractions in humans. NEW & NOTEWORTHY Data obtained in the cat have shown that muscle pain causes a marked increase in the firing of muscle spindles, attributed to a nociceptor-driven fusimotor reflex. However, our studies of muscle spindles in relaxed leg muscles failed to find any effect on spindle discharge. Here we showed that experimental muscle pain failed to increase the firing of muscle spindle afferents during weak voluntary contractions, when fusimotor drive sufficient to increase their firing is present.

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