The 'initial burst' of muscle spindle afferents with or without terminals on the bag1 intrafusal muscle fibre

Mechanosensory encoding in ex vivo muscle-nerve preparations

Our objective was to evaluate an ex vivo muscle-nerve preparation used to study mechanosensory signalling by low threshold mechanosensory receptors (LTMRs). Specifically, we aimed to assess how well the ex vivo preparation represents in vivo firing behaviours of the three major LTMR subtypes of muscle primary sensory afferents, namely type Ia and II muscle spindle (MS) afferents and type Ib tendon organ afferents. Using published procedures for ex vivo study of LTMRs in mouse hindlimb muscles, we replicated earlier reports on afferent firing in response to conventional stretch paradigms applied to non-contracting, that is passive, muscle. Relative to in vivo studies, stretch-evoked firing for confirmed MS afferents in the ex vivo preparation was markedly reduced in firing rate and deficient in encoding dynamic features of muscle stretch. These deficiencies precluded conventional means of discriminating type Ia and II afferents. Muscle afferents, including confirmed Ib afferents were often indistinguishable based on their similar firing responses to the same physiologically relevant stretch paradigms. These observations raise uncertainty about conclusions drawn from earlier ex vivo studies that either attribute findings to specific afferent types or suggest an absence of treatment effects on dynamic firing. However, we found that replacing the recording solution with bicarbonate buffer resulted in afferent firing rates and profiles more like those seen in vivo. Improving representation of the distinctive sensory encoding properties in ex vivo muscle-nerve preparations will promote accuracy in assigning molecular markers and mechanisms to heterogeneous types of muscle mechanosensory neurons.

Effects of changes in pH on the afferent impulse activity of isolated cat muscle spindles

Muscle spindle activity has been shown to decrease in the sustained contracting muscle. The effect has been assumed to result from a declining fusimotor drive. Since accumulation of metabolites including H(+), lactate and CO(2) might also affect the receptor in the fatiguing muscle, the impulse activity of muscle spindles isolated from the cat tenuissimus muscle was characterized under varying degrees of extracellular pH, thus excluding any effect on fusimotor activity, blood supply and extrafusal muscle fibers. The isolated receptor was exposed to bathing fluids of pH 6.4, 7.4 and 8.4, and afferent discharge activity was recorded from the spindle nerve. Both primary and secondary endings responded similarly to changes in pH. Resting discharge frequency usually decreased with decreasing pH and increased with increasing pH. A sudden break-off in activity was observed with about 40% of primary endings and about 30% of secondary endings at pH 6.4. Experiments with slow stretch stimulation indicated that this effect was caused by a rising threshold of firing at the encoder site of the endings. With brief ramp-and-hold stretches, we tested the effects of changes in pH on the dynamic and static sensitivity of primary and secondary endings. When pH was reduced from 7.4 to 6.4, the initial burst activity at the beginning of the ramp phase increased in primary and secondary endings and the dynamic response increased in secondary endings, demonstrating that the dynamic properties of muscle spindle endings were usually augmented in the acidic milieu. The static properties rose as well because the static index of both types of ending increased significantly. By contrast, dynamic and static properties of both primary and secondary endings decreased significantly, when pH was increased from 7.4 to 8.4. The amplitude of tension that was measured during the passive stretch stimuli very slightly decreased in the acidic solution and very slightly increased in the alkaline solution. The decrease in the resting discharge activity at low pH supports those previous observations, which demonstrate a reduced peripheral input from muscle spindle afferents to the spinal motor nuclei during fatigue in the isometric contracting muscle. The present finding indicates that an attenuated afferent discharge is not only caused by a decreasing central activation of gamma-motorneurons, but may additionally be supported by a direct effect of protons on the muscle receptor itself. The accompanying augmentation of stretch sensitivity is suggested to correspond to the well-known increase in physiological tremor during exhaustive exercise.

Effects of the calcium antagonist nifedipine on the afferent impulse activity of isolated cat muscle spindles

The impulse activity of muscle spindles isolated from the cat tenuissimus muscle was investigated under varying concentrations of the L-type calcium channel blocker nifedipine. At a concentration of 25 microM nifedipine impulse activity was clearly diminished in both primary and secondary endings. However, low concentrations of the drug (5-10 microM) exerted unexpected excitatory effects. The dynamic properties of primary endings in particular were augmented; those of secondary endings were also increased, although only slightly. A detailed analysis of the afferent discharge patterns obtained under ramp-and-hold stretches yielded the following effects of 10 microM nifedipine. (1) The initial burst at the beginning of the ramp phase of a stretch was increased in primary endings; (2) the peak dynamic discharge frequency at the end of the ramp phase was considerably increased in most primary endings; (3) the sensitivity of the peak dynamic discharge value to varying amplitudes and velocities of stretch was significantly enhanced in primary endings, and also increased, although only slightly, in secondary endings; (4) the rise in the discharge frequency during the ramp phase of a stretch was augmented in both types of ending, the effect being again stronger in primary endings; (5) the fast adaptive decay of the impulse frequency following the ramp phase of a ramp-and-hold stretch was significantly increased in primary endings, but remained unaffected in secondary endings. The enhanced dynamic properties of primary endings were also observed under small sinusoidal stretch stimuli (10 microm, 40 Hz), where nifedipine induced a significant shift in the position of the 1:1 driven action potentials toward smaller phase values. In view of an increase in tension in the isolated muscle spindle and an increased initial burst in primary endings in the presence of nifedipine, it is suggested that the drug facilitates the attachment of cross-bridges in the poles of the intrafusal muscle fibers. The increase in the dynamic properties of primary endings points to the possibility that the drug preferentially affects the nuclear bag(1) fiber. The inhibitory effect on the afferent discharge rate at high doses of the drug is interpreted as the consequence of a calcium channel block in the membranes of the sensory endings. The membrane potential of sensory endings appears to be highly dependent on sustained Ca(2+) conductance.

Functional roles of the proprioceptive system in the control of goal-directed movement

This article explored functional roles of the proprioceptive system during the control of goal-directed movements. Proprioceptive information contributes to the control of movement through both reflex and central connections. Spinal and transcortical reflex loops establish a servomechanism which provides automatic corrections of unexpected changes in muscle length and allows compensation for undesirable irregularities in the mechanical properties of muscles by modulating limb stiffness at the subconscious level. Central connections provide the control system with information about peripheral states which is used in voluntary components of movement control. Before the initiation of movement, proprioceptive information about initial limb orientation becomes a basis for the programming of motor commands. During a movement, proprioceptive input about velocities and angular displacements of a limb is used to regulate movement by triggering planned sequences of muscle activation and modulating motor commands. After movement, feedback produced by responses is compared with previously stored information, verifying the quality of the movement. Considering potential roles of the reflex and central connections, the proprioceptive system seems to constitute an important aspect of motor control mechanisms, providing the control system with efficiency and flexibility in the regulation of goal-directed movements.

Time-dependent fusimotor effects on the discharge of cat primary muscle spindle afferents induced by a long-lasting succinylcholine infusion

The responses of 46 Ia afferents from the tibial anterior muscle of the cat to repetitive ramp-and-hold stretches were investigated under a succinylcholine (SCh) infusion of 120 micrograms.kg-1.min-1 lasting 15 to 25 min. It was possible to distinguish four consecutive phases of the effect of the SCh on the responsiveness of the Ia afferents. The first three of these four phases have already been described. We analysed in more detail the changes from Phase III to the end of Phase IV. Static fusimotor effects were dominant in the discharge patterns obtained during Phase III; dynamic fusimotor effects prevail at the end of Phase IV. Our observations were quantified by comparing the mean values of initial activity, final static value, dynamic response and slow receptor adaptation read from the discharge patterns obtained during Phase III with the mean values of the same parameters obtained from discharge patterns from the end of Phase IV: the two mean values were significantly different for each of the four measurements. This change from Phase III to the end of Phase IV is highly specific for each spindle. To demonstrate this spindle specificity, discharge patterns were selected from among those produced by each Ia afferent in Phase III and at the end of Phase IV. Each of these discharge patterns was assigned to one of six categories. Category I displays purely dynamic fusimotor effects and Category VI purely static fusimotor effects. Categories II, IV and V display combinations of static and dynamic fusimotor effects with an increasing admixture of static fusimotor effects. The spindle-specific change from Phase III to the end of Phase IV is defined in terms of the specific degree of change from a higher-number to a lower-number category in the case of each of the 46 Ia afferents. In the discussion a combination of activity by the two nuclear bag fibres of a spindle is deduced from the specific discharge pattern of each category. The conclusion from these considerations is that results obtained from the administration of SCh have to be interpreted with great caution in making any statement about the existence of a dynamic bag1 fibre in a spindle.

Oscillations in the discharge frequency of cat primary muscle spindle afferents during the dynamic phase of a ramp-and-hold stretch

Under a ramp-and-hold stretch the Ia afferent responds to the beginning of the ramp phase with an initial frequency peak, which is followed during the ongoing dynamic phase of stretching by further frequency peaks. In this investigation the behavior of the initial peak is compared with the behavior of the subsequent peaks under four experimental conditions: (i) increasing prestretch of the muscle, (ii) increasing stretch rate, (iii) change in the waiting time before a ramp stretch, (iv) variations in the degree of prestretch preceding a ramp stretch, which is then started from a medium degree of prestretch. Under three of these four experimental conditions the initial peak behaves qualitatively differently from the subsequent peaks. This result gives rise to the interpretation that the causes of the initial peak and the subsequent peaks are different. The initial peak is caused by a largely synchronous opening of acto-myosin bonds in the polar parts, whereas the subsequent peaks may be caused by intrinsic oscillatory properties of the receptor potential.

Subdivision of primary afferents from passive cat muscle spindles based on a single slow-adaptation parameter

38 primary afferents originating from de-efferented cat tibialis anterior muscle spindles were investigated. Ramp-and-hold stretches of the host muscle were performed with a varying amount of muscle pre-stretch while recording the primary afferent discharges. From the discharge responses an interspike interval function was estimated. This revealed a slow adaptation during the hold phase of stretch which could be approximated quite well by a power function. The slow-adaptation power function exponent (SAE) was found to be rather independent of the amount of pre-stretch applied to the host muscle and grouped around a value characteristic for each afferent. These 'characteristic SAEs' showed a clearly bimodal distribution within the population of primaries studied. Moreover, the distribution around both modes was narrow enough to justify the subdivision of the primary afferents according to their characteristic SAE as either high-SAE (10 afferents; 26%) or low-SAE (28 afferents; 74%) afferents. The most likely explanation for this bimodality of slow-adaptation behavior in primary afferents is given by the assumption that the afferent discharge of the passive spindle is mainly provided from a branch innervating either the bag1 (for high-SAE units) or the bag2 and chain (for low-SAE units) intrafusal fibers.