Thumb Flexor Muscles Research Articles

The senses of force and heaviness at the human elbow joint

The present-day view of the neural basis for the senses of muscle force and heaviness is that they are generated centrally, within the brain, from copies of motor commands. A corollary of the motor discharge generates a sense of effort which underlies these sensations. In recent experiments on force and heaviness sensations using thumb flexor muscles, a rather different explanation has been invoked: Subjects were proposed to rely predominantly on inputs of a peripheral origin, in particular, the signals of muscle spindles. The present experiments have been carried out at the elbow joint to determine whether these new ideas apply more widely. The effects of fatigue of elbow flexor muscles have been studied in force and heaviness matching tasks using three exercise regimes, a sustained maximum voluntary contraction (MVC), a maintained contraction of 35 % MVC, and a maintained contraction of 35 % MVC combined with muscle vibration at 80 Hz. In force-matching experiments, subjects were required to contract both arms and while the reference arm generated the target force under visual control, it was matched by the indicator arm without visual feedback. During the 100 % MVC exercise, force in the exercising reference arm fell rapidly to almost a half of its original value over 90 s while force in the indicator did not fall, leading to a significant overestimation of the reference force. During the 35 % MVC exercise, subjects also overestimated the reference force and this persisted at 5 and 10 min after the exercise. When 35 % MVC was combined with vibration, the amount by which the indicator arm overestimated the reference force was significantly reduced. In heaviness matching experiments, subjects could move their arms through a small range. The reference arm was loaded with a weight, and weights were added or removed from the indicator until heaviness felt the same in the two arms. There was a small, but significant fall in the matching weight used after 100 % MVC exercise, that is, the weight held by the fatigued arm felt lighter. The 35 % exercise did not alter heaviness sensation while 35 % MVC exercise with vibration led to a significant reduction in perceived heaviness. To conclude, while the results of these experiments on elbow flexors are not as clear cut as for thumb flexors, the central effort hypothesis falls short, in a number of respects in explaining the data which are able to be interpreted in terms of a peripheral afferent contribution to the senses of force and heaviness.

Ciclosporine

Anterior interosseous neuropathy is a rare cause of weakness of flexion of the thumb interphalangeal and index and middle finger distal interphalangeal joints, most commonly caused by trauma, compression, or inflammation. Bilateral anterior interosseous nerve palsies are rare, and other neuromuscular disorders may present with a similar pattern of weakness. We describe 2 patients who initially presented for orthopedic evaluation for suspected anterior interosseous neuropathy and were subsequently diagnosed with inclusion body myositis, an uncommon inflammatory myopathy affecting adults, with a predilection for finger and thumb flexor muscles. Electromyography, serologic and radiographic studies, and muscle biopsy can aid in diagnosis and help to distinguish inclusion body myositis from an anterior interosseous neuropathy.

The fusimotor and reafferent origin of the sense of force and weight

Signals associated with the command the brain sends to muscles are thought to create the sensation of heaviness when we lift an object. Thus, as a muscle is weakened by fatigue or partial paralysis (neuromuscular blockade), the increase in the motor command needed to lift a weight is thought to explain the increasing subjective heaviness of the lifted object.With different fatiguing contractions we approximately halved the force output of the thumb flexor muscles, which were then used to lift an object. For two deafferented subjects the perceived heaviness of the lifted object approximately doubled, in keeping with the central-signal theory. However, for normal subjects this resulted in objects feeling the same or lighter, inconsistent with the central-signal theory but consistent with the expected effects of the conditioning contractions on the sensitivity of peripheral receptors. In separate experiments we subjected the forearm muscles to complete paralysis with a non-depolarising neuromuscular blocking agent and then allowed them to recover to approximately half-force output. This also resulted in objects feeling lighter when lifted by the semi-paralysed thumb, even though the motor command to the motoneurons must have been greater. This is readily explained by reduced lift-related reafference caused by the prolonged paralysis of muscle spindle intrafusal fibres.We conclude that peripheral signals, including a major contribution from muscle spindles, normally give rise to the sense of exerted force. In concept, however, reafference from peripheral receptors may also be considered a centrally generated signal that traverses efferent and then afferent pathways to feed perceptual centres rather than one confined entirely to the central nervous system. These results therefore challenge the distinction between central- and peripheral-based perception, and the concept that muscle spindles provide only information about limb position and movement.

Influence of tactile afferents on the coordination of muscles during a simulated precision grip

The mechanisms by which the nervous system coordinates multiple muscles for the control of finger movements are not well understood. One possibility is that groups of muscles may be enlisted into synergies by last-order inputs that project across multiple motor nuclei. In this study we investigated the role that tactile input might play in coupling together the activities of motor units in two muscles involved in generating the precision grip. Cross-correlation analysis was used to assess the degree of synchrony in the discharge times of pairs of motor units recorded from index-finger and thumb flexor muscles while human subjects performed an isometric task that mimicked a precision grip. The magnitude of synchrony is thought to reflect the extent to which divergent last order inputs provide common synaptic input across motor neurons. Synchrony was evaluated under two simulated-gripping conditions: gripping with normal tactile input and gripping when tactile input from the digit pads was eliminated by applying flexion forces to fittings glued to the finger nails. Synchrony between motor units of index finger flexor and thumb flexor muscles, while substantial, was not significantly different across the two tactile-input conditions. These findings suggest that tactile input is not required to activate the divergent last-order inputs that couple together the activities of the index finger and thumb flexor muscles during the precision grip.

Estudo anatômico de músculos profundos do antebraço de <em>Cebus apella</em> (Linnaeus, 1766)

Eight adult specimens of Cebus paella were used for anatomical muscles characterization. The animals were donated by the Brazilian Institute of Environment and Recyclable Natural Resources (Ibama – Instituto Brasileiro de Meio – Ambiente), from Sete Lagoas, state of Minas Gerais, Brazil, and sacrificed according to the recommendations of Brazilian College of Animal Experimentation (Cobea – Colégio Brasileiro de Experimentação Animal). This work was approved by ethics committee from UFU (Federal University of Uberlândia). The muscles (1) squared pronator, (2) deep fingers flexor and (3) long thumb flexor were studied. In human beings, the deep muscles of Cebus paella have some general common characteristics, but differ in form, vascularization and innervation. These differences should reflect the functional specialization of those muscles between the two species. The long thumb flexor muscle is important for human beings’ fingers movement since the force of hand in Cebus paella is a necessary characteristic to this species, according to its arboreous habit

Common input to motor units of digit flexors during multi-digit grasping.

The control of whole hand grasping relies on complex coordination of multiple forces. While many studies have characterized the coordination of finger forces and torques, the control of hand muscle activity underlying multi-digit grasping has not been studied to the same extent. Motor-unit synchrony across finger muscles or muscle compartments might be one of the factors underlying the limited individuation of finger forces. Such "unwanted" coupling among finger forces, however, might be desirable when a high level of force coupling is required to prevent object slip during grasping. The goal of this study was to quantify the strength of synchrony between single motor units from extrinsic hand muscles as subjects held a device with a five-digit grasp. During the hold phase, we recorded the normal force exerted by each digit and the electrical activity of single motor units from each of the four divisions of the muscle flexor digitorum profundus (FDP) and one thumb flexor muscle, m. flexor pollicis longus (FPL). The strength of motor-unit synchrony was quantified by the common input strength index (CIS). We found moderate to strong motor-unit synchrony between FPL and the index FDP compartment [CIS: 0.49 +/- 0.03 (SE)] and across most FDP compartments (0.34 +/- 0.02). Weak synchrony, however, was found between FPL and the middle, ring, and little finger FDP compartments (0.25 +/- 0.01). This difference might reflect the larger force contribution of the thumb-index finger pair relative to other thumb-finger combinations in five-digit grasping.

Non-invasive evaluation of central motor tract excitability changes following peripheral nerve stimulation in healthy humans

The interval between muscle stretch and the onset of the long latency electromyographic responses (LLRs) has been theoretically fragmented into an afferent time (AT), taken at the peak of wave N20 of somatosensory evoked potentials and an efferent time (ET), calculated by means of magnetic transcranial stimulation (TCS), the two being separated by a cortical interval (CI). If this were the case, the afferent input should progressively ‘energize’ the sensorimotor cortex during the CI and change the excitability of cortico-spinal tracts. To investigate this, motor evoked potentials (MEPs) from thumb flexor muscles were recorded, whilst a conditioning stimulation of median or ulnar nerve randomly preceded (10–48 msec intervals) magnetic brain TCS. Nerve stimulation was adjusted to motor threshold and amplitudes of conditioned and test MEPs at different nerve-TCS interstimulus intervals were evaluated. Conditioned MEPs were significantly attenuated with nerve-TCS intervals between 16 and 20 msec for elbow and 20 and 22 msec for wrist stimulation. This was followed by MEP potentiation with nerve-TCS intervals corresponding to the sum of AT + CI (mean 23.2 msec, range 21.7–24.8). The onset latency of facilitated conditioned MEPs was about 1 msec briefer than than that of test MEPs, but invariably longer than the latency of MEPs facilitated by a voluntary contraction. This protocol did not demonstrate amplitude facilitation of the segmental H reflex, corroborating the idea that the facilitated part of the conditioning nerve-TCS curve receives a transcortical loop contribution.

Kinematic and electromyographic responses to perturbation of a rapid grasp.

Kinematic and electromyographic responses of the thumb and index finger to load-induced extension of the thumb during a rapid precision grasp of the thumb and finger were studied in four human subjects. Angular position of the index finger metacarpophalangeal (MP) and proximal interphalangeal (PIP) joints was recorded along with the linear position of the thumb tip (TH). Myoelectric activity was recorded from flexor pollicis longus, flexor digitorum superficialis, first dorsal interosseous, and extensor digitorum communis. Loads that extended the thumb were applied ranging from 125 ms prior to movement onset to movement onset. For loads applied earlier than 50 ms before movement onset, the amount of flexion of TH was reduced compared with control trials. Contact between the finger and thumb was attained nonetheless due to an altered trajectory of the fingertip that was generated by reduced flexion of PIP and increased flexion of MP. These finger responses appear to be functional in that contact of the finger pulp on the more distal pulp of the thumb was preserved. With loads delivered near onset of the grasp, there was increased PIP flexion, rather than reduced PIP flexion. These responses to later loads occurred despite greatly reduced magnitudes of TH flexion compared with loads delivered well before onset of the gasp movements. Thus reduced PIP flexion observed with early loads was not simply the result of finger biomechanics. The thumb flexor muscle increased activity 45-55 ms after onset of the load, whereas responses of the finger flexors began 65 ms after load onset. Response magnitudes decreased as loads were introduced nearer to movement onset. Measured reaction times of the finger muscles to thumb extension stimuli averaged 154 ms, which indicated that the responses of the finger muscles were not voluntary responses to the thumb extension. Afferent information generated by perturbation of the thumb during a grasp movement can influence the activity of intrinsic and extrinsic muscles to yield apparently functional compensations in the closing movements. However, temporal limitations exist that appear to offer greater constraints on the use of afferent signals for controlling rapid movements than for sustained grasp.

The corticomotoneurone connection is normal in Parkinson's disease.

Voluntary movements in Parkinson's disease are initiated and executed slowly. It is assumed that the motor cortex and its output pathway are intact and that bradykinesia is due to abnormal motor commands delivered to a normal corticospinal system. We have tested this assumption using electrical stimulation of the motor cortex through the scalp in three patients with severe Parkinson's disease, studied during fluctuations from relatively normal mobility when receiving drugs (ON) to severe bradykinesia when not receiving drugs (OFF). Thresholds and latencies for motor cortex stimulation to excite thumb flexor muscles and the resulting fast mechanical responses were the same in both ON and OFF conditions, even though the patients were unable to execute fast thumb flexion movements voluntarily when OFF. We conclude that the excitability and conduction velocity of the corticospinal motor pathways are normal in Parkinson's disease.