Proprioception and myoclonus.

This review focuses on sensory information originating from muscle spindles and its role in proprioception and motor control. The first part reminds of the structural and functional properties of these muscle mechanoreceptors, with arguments for an independent fusimotor command, i.e. the gamma-motoneurons, that would regulate spindle mechanical sensitivity in keeping with the requirements of ongoing motor action. The possibility that dysfunction of the fusimotor system might be responsible for clinical signs is discussed with respect to the hyperexcitability of the sensorimotor cortex that is observed in myoclonus of cortical origin. What is known about the spindle afferents projections into the spinal cord and about the dysfunction of the spinal sensorimotor networks in patients with neurological disorders, is put together in the second part. It is stressed on the significant complexity of the monosynaptic reflex in spite of its "simple" organization. The monosynaptic reflex constitutes the only possible way for testing the excitability of motoneurons and spinal networks. This method is extensively used clinically to examine changes in the nervous system with diseases. When studying changes from the norm, it is important to understand how the reflex functions in neurologically normal conditions. Different mechanisms such as pre-synaptic inhibition, post-activation depression and motoneuronal intrinsic properties are reviewed as they may induce changes in reflex amplitude and have therefore consequences for interpretation of spinal excitability.

Increased muscle spindle sensitivity to movement during reinforcement manoeuvres in relaxed human subjects.

1. The effects of reinforcement manoeuvres, such as mental computation and the Jendrassik manoeuvre, on muscle spindle sensitivity to passively imposed sinusoidal stretching (1.5 deg, 2 Hz) in relaxed subjects were analysed. 2. The unitary activity of 26 muscle spindle afferents (23 Ia, 3 II) originating from ankle muscles was recorded using the microneurographic method. Particular care was paid to the subjects' state of physical and mental relaxation. 3. The results showed that the activity of 54 % of the Ia afferents was modified during mental computation. The modifications took the form of either an increase in the number of spikes (mean, 26 % among 11 Ia fibres) or a shortening in the latency of the response to sinusoidal stretching (mean, 13 ms among 3 Ia fibres), or both. They were sometimes accompanied by an enhanced variability in the instantaneous discharge frequency. The three secondary endings tested exhibited no change in their sensitivity to stretch during mental computation. 4. The increased sensitivity to passive movements sometimes began as soon as the instructions were given to the subjects and sometimes increased during mental computation. In addition, the increased sensitivity either stopped after the subjects gave the right answer or continued for several minutes. 5. During the performance of a Jendrassik manoeuvre, the Ia units underwent changes similar to those described above for mental computation. 6. It was concluded that muscle spindle sensitivity to movement can be modified in relaxed human subjects. The results reinforce the idea that the fusimotor system plays a role in arousal and expectancy, and contribute to narrowing the gap between human and behaving animal data.

Comparison of the depression of H-reflexes following previous activation in upper and lower limb muscles in human subjects.

When conditioning-testing (C-T) stimuli are applied to Ia afferents to elicit H-reflexes, the test reflex is abolished immediately following the conditioning reflex. As the C-T interval is increased, the test response slowly begins to recover, taking several hundred milliseconds to attain control values. The time course of this recovery is known as the H-reflex recovery curve. H-reflex recovery curves were compared using surface EMG and single motor unit activities in lower limb soleus and upper limb flexor carpi radialis (FCR) muscles in seven healthy human subjects. Under rest conditions, the recovery of H-reflexes and single motor unit activity was slow for soleus; the recovery was not complete even in 1 s. In comparison, the recovery was very fast for FCR motor units, occurring in 200-300 ms. The effects of rate of stimulation (0.1-10.0 imp/s) were also examined on the magnitude of H-reflex responses. The reflex response declined with increasing rate of stimulation, the decline being slightly greater in soleus than in FCR. When these phenomena were examined with voluntary facilitation of the spinal cord, the time of recovery shortened and the effect of stimulus rate also diminished. Changes with background facilitation were greater in FCR than in soleus. The differences between the two muscles are attributed mainly to differences in presynaptic inhibition in the two spinal segments, and/or to the differences in dynamics of the transmitter release in terminals of Ia afferents synapsing with slow soleus motoneurons and those synapsing with the fast FCR motoneurons.

Muscle spindle activity following muscle tendon vibration in man.

Muscle spindle primary endings originating from the Tibialis anterior, Extensor Digitorum Longus and Lateral Peroneal muscles were recorded by the microneurographic technique. Their resting activity and stretch sensitivity after muscle tendon vibration (80 Hz, 30 s) were compared with those in the previbratory period. Most of the units (73%) exhibited a decreased spontaneous firing rate whereas a few others either conserved (13.5%) or increased (13.5%) their resting discharge after vibration. A complete recovery necessitated 40 s. The static stretch sensitivity of the units was decreased during the 3 s following vibration exposure and returned to the control level (about 14 s). The results are discussed in the light of previous psychophysiological studies reporting an altered position sense and a development of involuntary muscle contractions in postvibratory periods.

Task-dependence of muscle afferent monosynaptic inputs to human extensor carpi radialis motoneurones.

The task-dependence of homonymous muscle afferent inputs was investigated in motor units of the extensor carpi radialis muscles during voluntary isometric contraction involving either the activation of agonist extensor muscles (wrist extension) or the co-activation of antagonist extensor and flexor muscles (hand clenching). The effectiveness of the muscle afferent monosynaptic inputs was tested by delivering either tendon taps or electrical stimulation to the radial nerve. In both cases, the motor unit responses, which took the form of narrow peaks in the peri-stimulus time histograms, were found to be significantly greater during hand clenching. The parallel enhancement of the responses to both mechanical and electrical stimulations observed during hand clenching could not be explained in terms of changes in the muscle spindle responsiveness. The enhancement of the motor units' responsiveness was apparent during the first 0.5 ms of the peaks in the peri-stimulus time histograms, taken to be uncontaminated by any polysynaptic components. It may therefore have reflected an increase in the amplitude of the excitatory monosynaptic potentials generated by the muscle spindle primary afferents. This is interpreted in terms of changes in the presynaptic inhibition, which might be depressed as the result of the large-scale activation of palm and finger cutaneous afferents liable to occur during hand clenching.

Peripheral proprioceptive modulation in crayfish walking leg by serotonin.

In vitro serotoninergic modulation of intracellularly recorded sensory responses was examined in primary afferent terminals of a crayfish leg proprioceptor, the coxo-basal chordotonal organ (CB CO). The effects of different concentrations of serotonin (5-HT) on static and dynamic sensory responses were analysed following bath or pressure applications of the monoamine directly on the strand of the mechanoreceptor. Consequently, the reported effects result from the direct peripheral action of 5-HT on the sensory organ itself. Serotonin modulates the sensory activity by modifying the sensory discharge frequency. The firing discharge of the primary afferents is increased in a dose-dependent manner. The maximal effect is obtained with a concentration of 10(-6) M. Higher concentrations are less effective, and for 20% of the recorded cells, 10(-4) M 5-HT induces a decrease of the sensory discharge, i.e. has an inhibitory effect. Alteration in the pattern of sensory firing, resulting in bursting discharge, was observed in some units. All the recorded sensory units were responsive to the neuromodulator whatever their functional properties. The effects of 5-HT lasted as long as the amine was applied and were reversible after wash. The results suggest that 5-HT could exert a modulatory action on the proprioceptive feedback, by peripheral action on the sensory organ. The natural modalities of 5-HT action are discussed on the basis of immunohistochemistry data suggesting: (i) connections between CB CO and central serotoninergic cells, (ii) 5-HT content in sensory cells of the CB CO. Since the CB CO is involved in the control of leg movement and position, the modulation of its primary afferents might influence the organization of the locomotor pattern. The functional significance of this peripheral sensory neuromodulation was approached by the analysis of the motor reflex activity.

Serotonin and proctolin modulate the response of a stretch receptor in crayfish.

The modulatory effects of proctolin and a biogenic amine (5-hydroxytryptamine; 5-HT) have been investigated on a leg mechanoreceptor in the crayfish. Single afferent sensory units were recorded extra- and intracellularly during imposed sinusoidal movement to the receptor strand; all responses were facilitated by a bath application of proctolin and 5-HT at various concentrations (10(-9)-10(-6) M).

Lack of correspondence between histochemical and structural fiber typing in antennal muscles of the rock lobster Palinurus vulgaris.

1. Sarcomere lengths and fine structure were examined in three histochemical fiber types of antennal muscles of the rock lobster. 2. Sarcomere lengths are distributed over a continuum of values from 6.5 to 19 microns. 3. Although a correlation between ATPase activity and sarcomere length is demonstrated, fibers with high ATPase activity do not have the sarcomere length typical of fast contracting fibers. 4. These fibers deviated from the typical fast structure in having long sarcomeres (greater than 6.5 microns) and in having some unusual ultrastructural characteristics (absence of the H-band, presence of Z-tubules, high thin to thick ratio, 5:1) associated with other more classical features. 5. This finding demonstrates that sarcomere length measurements do not always accurately predict the physiological performance of a single muscle fiber. 6. The fiber type composition of two antagonistic antennal muscles is compared and the functional significance of the results is discussed with respect to their role in behavior.

Use of the fluorescent retrograde tracer, Granular blue, in crustacean motor system studies.

Retrograde axonal transport and fluorescent retrograde labelling of neurons were investigated in a crustacean motor system. A fluorescent tracer (Granular blue) was injected into an antennal muscle of rock lobster in order to identify the motor neurons innervating it. After an appropriate post-injection survival time, the fluorescent tracer was found to have labelled motor neurons in the central ganglion. This result indicates that after uptake by the axon terminals, the fluorescent tracer was retrogradely transported through axons of motor neurons to the parent cell bodies.