Onset Of Voluntary Contraction Research Articles

Inadequate Modulation of Excitability With Voluntary Dorsiflexion in Parkinson's Disease

Freezing phenomenon at onset of movement causes gait disturbance in Parkinson's disease (PD), but the pathophysiology is unclear. We studied motor property at onset of dorsiflexion in PD. In 9 patients with PD and 8 normal subjects, motor evoked potential was recorded from the tibialis anterior muscle under 3 conditions: at rest, during tonic contraction, and at onset of contraction. Motor threshold, size of motor evoked potential and the relationship between the intensity of transcranial magnetic stimulation, and the size of motor evoked potentials (recruitment gain) were examined. Motor threshold decreased with voluntary contraction in both PD and normal subjects, but the threshold at rest and during tonic contraction was lower in Parkinson's disease. The size of motor evoked potential with maximal stimulus intensity increased with voluntary contraction in both groups; this tendency was more pronounced in normal subjects. The recruitment gain during contraction was steeper than at rest in normal subjects. However, there was no such increase in PD. There was no increase in recruitment gain with voluntary contraction in PD, which was obvious in normal subjects, especially at onset of voluntary contraction. Modulation of motor excitability at onset of voluntary contraction was impaired in PD.

WS2-4 Inappropriate modulation of antagonistic inhibition at onset of voluntary contraction in Parkinson's disease

WS2-4 Inappropriate modulation of antagonistic inhibition at onset of voluntary contraction in Parkinson's disease

EMG for assessing the recovery of voluntary movement after acute spinal cord injury in man

Objective Multi-channel electromyogram (EMG) was used to examine the pattern and time-course of voluntary contraction recovery in subjects with acute traumatic spinal cord injury (SCI), concentrating on the latest time after injury at which a given muscle would begin to show voluntary recruitment. Methods We conducted repeated measures of voluntary contractions of 12 lower limb muscles (for all subjects) and 12 upper-limb muscles (for subjects with cervical injury), beginning within days of the injury and extending for 1 or more years post-injury. The EMG interference pattern was scored in a blinded fashion from tape records. Results We recruited 229 subjects, including 152 from whom repeated measures were made. Several different patterns of recovery were identified. For persons with motor-incomplete injury to the cervical or thoracic spine, EMG recruitment had not yet occurred by 5 weeks post-injury in roughly 1/2 of all lower limb muscles, and prolonged delays between injury and recruitment onset were sometimes seen. Injury to the thoracolumbar spine was frequently associated with very long delays (i.e. >1 year) between injury and resumption of volitional contraction of distal lower limb muscles. Discussion The incidence of neurologically incomplete SCI is rising. In such subjects, delays of 1 or more months between injury and the onset of voluntary contraction are common for muscles of the distal upper limbs (for cervical injury) and lower limbs. Given the abbreviated period of in-patient rehabilitation now routine in the United States, these subjects in particular will benefit from frequent follow-up evaluations to assess spontaneous recovery and design appropriate rehabilitation strategies to maximize functional independence. Moreover, the potential for delayed recovery must be considered when designing and implementing novel clinical interventions for treating SCI, to better differentiate between spontaneous and treatment-related improvements in neurologic function.

Chapter 31 Central control of information transmission through the intraspinal arborizations of sensory fibers examined 100 years after Ramón y Cajal

About 100 years ago, Santiago Ramón y Cajal reported that sensory fibers entering the spinal cord have ascending and descending branches, and that each of them sends collaterals to the gray matter where they have profuse ramifications. To him this was a fundamental discovery and proposed that the intraspinal branches of the sensory fibers were "centripetal conductors by which sensory excitation is propagated to the various neurons in the gray matter". In addition, he assumed that "conduction of excitation within the intraspinal arborizations of the afferent fibers would be proportional to the diameters of the conductors", and that excitation would preferentially flow through the coarsest branches. The invariability of some elementary reflexes such as the knee jerk would be the result of a long history of plastic adaptations and natural selection of the safest neuronal organizations. There is now evidence suggesting that in the adult cat, the intraspinal branches of sensory fibers are not hard wired routes that diverge excitation to spinal neurons in an invariable manner, but rather dynamic pathways where excitation flow can be centrally addressed to reach specific neuronal targets. This central control of information flow is achieved by means of specific sets of GABAergic interneurons that produce primary afferent depolarization (PAD) via axo-axonic synapses and reduce transmitter release (presynaptic inhibition). The PAD produced by single, or by small groups of GABAergic interneurons in group I muscle afferents, can remain confined to some sets of intraspinal arborizations of the afferent fibers and not spread to nearby collaterals. In muscle spindle afferents this local character of PAD allows cutaneous and descending inputs to differentially inhibit the PAD in segmental and ascending collaterals of individual fibers, which may be an effective way to decouple the information flow arising from common sensory inputs. This feature appears to play an important role in the selection of information flow in muscle spindles that occurs at the onset of voluntary contractions in humans.

Selectivity of the central control of sensory information in the mammalian spinal cord.

Afferent feedback from muscle proprioceptors, as well as movement-induced activation of skin receptors plays an important role in the patterning of motor activity for stepping and postural control. An important component in this control is the presynaptic GABAergic modulation of the synaptic effectiveness of muscle and cutaneous afferents, known to change in phase with the locomotor cycle, during the execution of voluntary movements, or after a peripheral nerve injury. Recent electrophysiological studies, together with ultrastructural observations, indicate that the distribution of GABAa synapses in the intraspinal arborizations of muscle spindle and tendon organ afferents is not homogeneous. Namely, that some collaterals are the targets of one, or more, GABAergic interneurones, while other collaterals of the same fibre receive no GABAergic connections. In addition, both PAD and inhibition of PAD have a local character. This allows, at least in principle, decoupling the information arising from common sensory inputs. A spatially restricted modulation of PAD could play a significant role in the adjustment of the synaptic effectiveness of Ia afferents at the onset of voluntary contractions in humans, during movement-induced stimulation of the skin, or during the compensation of motor activity following partial denervation of muscles. Changes in the synchronization of the PAD-mediating interneurones can also have a profound effect on the information transmitted by a given set of afferent fibres. Data are presented that in the anesthetized cat, variation in the spontaneous activity of a population of dorsal horn neurones in laminae III-VI, that respond to stimulation of low-threshold cutaneous afferents, produce correlated fluctuations of monosynaptic reflexes by means of pre- and postsynaptic mechanisms. It is suggested that correlated changes in the level of PAD can also play a significant role in the presynaptic adjustment of the synaptic effectiveness of the afferent fibres during specific motor tasks.

Presynaptic selection of afferent inflow in the spinal cord

The synaptic effectiveness of sensory fibers ending in the spinal cord of vertebrates can be centrally controlled by means of specific sets of GABAergic interneurons that make axo-axonic synapses with the terminal arborizations of the afferent fibers. In the steady state, the intracellular concentration of chloride ions in these terminals is higher than that predicted from a passive distribution, because of an active transport mechanism. Following the release of GABA by spinal interneurons and activation of GABA A receptors in the afferent terminals, there is an outwardly directed efflux of chloride ions that produces primary afferent depolarization (PAD) and reduces transmitter release (presynaptic inhibition). Studies made by intrafiber recording of PAD, or by measuring changes in the intraspinal threshold of single afferent terminals (which is reduced during PAD), have further indicated that muscle and cutaneous afferents have distinctive, but modifiable PAD patterns in response to segmental and descending stimuli. This has suggested that PAD and presynaptic inhibition in the various types of afferents is mediated by separate sets of last-order GABAergic interneurons. Direct activation, by means of intraspinal microstimulation, of single or small groups of last-order PAD-mediating interneurons shows that the monosynaptic PAD elicited in Ia and Ib afferents can remain confined to some sets of the intraspinal collaterals and not spread to nearby collaterals. The local character of PAD allows cutaneous and descending inputs to selectively inhibit the PAD of segmental and ascending intraspinal collaterals of individual muscle spindle afferents. It thus seems that the intraspinal branches of the sensory fibers are not hard wired routes that diverge excitation to spinal neurons, but are instead dynamic pathways that can be centrally controlled to address information to selected neuronal targets. This feature appears to play an important role in the selection of information flow in muscle spindles that occurs at the onset of voluntary contractions in humans.

Changes in presynaptic inhibition of Ia fibres at the onset of voluntary contraction in man.

1. Two independent methods were used, in man, to assess changes in presynaptic inhibition of I a terminals at the onset of selective voluntary contractions: (1) measurement of the amount of heteronymous monosynaptic I a facilitation (from the quadriceps muscle to soleus motoneurones) to provide an assessment of the amount of ongoing presynaptic inhibition exerted on the I a fibres responsible for the facilitation; (2) measurement of the inhibition of H reflexes 40-60 ms after a short vibration to the tibialis anterior tendon to estimate the excitability of the interneurones mediating presynaptic inhibition from tibialis anterior I a afferents to the I a afferents of the test H reflex (soleus or quadriceps). 2. At the onset of an isolated voluntary plantar flexion of the foot (gastrocnemius-soleus contraction) the heteronymous facilitation from quadriceps to soleus was increased, reflecting a decreased presynaptic inhibition of the quadriceps I a terminals on soleus motoneurones. Vibratory inhibition of the soleus H reflex was decreased, reflecting an inhibition of transmission of presynaptic inhibition to homonymous soleus I a afferent terminals. 3. At the onset of the same gastrocnemius-soleus contraction there was, on the contrary, an increased vibratory inhibition of the quadriceps H reflex indicating a facilitation of transmission of presynaptic inhibition to homonymous quadriceps I a afferent terminals. 4. At the onset of an isolated voluntary knee extension (quadriceps contraction) the opposite pattern was seen: the heteronymous facilitation from quadriceps to soleus was decreased and the vibratory inhibition of a soleus H reflex was increased, whereas the vibratory inhibition of the quadriceps H reflex was decreased. 5. These results indicate that presynaptic inhibition of I a afferent terminals on motoneurones of contracting muscles is decreased, permitting I a activity to contribute to excitation of voluntarily activated motoneurones. On the contrary, presynaptic inhibition of I a fibres to motoneurones of muscles not involved in the contraction is increased. It is argued that the former must be supraspinal in origin. 6. It is concluded that the control of presynaptic inhibition of I a fibres at the onset of movement may be organized so as to aid in achieving selectivity of muscle activation, i.e. so as to increase motor contrast.

Cardiovascular responses at the onset of exercise with partial neuromuscular blockade in cat and man.

1. In decerebrated cats the cardiovascular, heart rate and blood pressure responses to static muscle contractions were followed from the onset of stimulation of the cut L7-S1 ventral roots. Heart rate and blood pressure were also followed during maximal voluntary and electrically induced static muscle contractions in man using one leg. In both cat and man contractions were performed under control conditions and tubocurarine-induced neuromuscular blockade. 2. In the cat, heart rate and blood pressure increased 1.7 s after the onset of the contraction. No cardiovascular responses were seen when the muscle contraction was blocked by tubocurarine. 3. In man, both heart rate and blood pressure increased at the onset of voluntary contractions. Partial curarization reduced strength to 39% of control. The heart rate response was unaffected by tubocurarine while the blood pressure response was reduced from 61 to 32 mmHg. 4. Electrical stimulation of the muscles resulted in 75% of voluntary strength in man. The heart rate response was delayed one R-R interval in the electrocardiogram but was as large as during voluntary contractions. During partial curarization the heart rate response was significantly smaller and the blood pressure response was reduced from 11 to 8 mmHg. 5. In conclusion, processes in active muscles elicit an increase in heart rate and blood pressure which depends on the intensity of the muscle contraction developed. However, the immediate cardiovascular responses at the onset of voluntary muscle contractions cannot be accounted for by reflexes generated in the working muscles alone.