The distribution of monosynaptic excitation from the pyramidal tract and from primary spindle afferents to motoneurones of the baboon's hand and forearm

Abstract

SUMMARY1. Intracellular records were obtained from motoneurones innervating muscles of the baboon's forearm and hand. Monosynaptic excitatory postsynaptic potentials (EPSPs) were elicited by stimulation of motor cortex (CM EPSPs) and peripheral nerves (Ia EPSPs).2. CM EPSPs were larger on average in motoneurones innervating intrinsic hand muscles and extensor digitorum communis (EDC) than in neurones of other forearm muscles.3. Among motoneurones of the median nerve, the CM EPSP tended to be larger for cells with more rapidly conducting axons than for those with more slowly conducting axons. Among motoneurones of EDC the opposite tendency was found.4. The afferent fibres responsible for the Ia EPSP nearly always had a lower stimulus threshold than that of motor axons in the same nerve. Some observations were made concerning the distribution of heteronymous Ia EPSPs.5. Among motoneurones of a given nerve, those with large Ia EPSPs tended to receive larger CM EPSPs than did cells in which the I a EPSP was small.6. The results are discussed in relation to problems concerning the pyramidal control of hand and finger movement.

Motor Unit Synchronization Is Increased in Biceps Brachii After Exercise-Induced Damage to Elbow Flexor Muscles

“…For example, in biceps brachii motor units, we found a large-amplitude, low-frequency oscillation that peaks at 1-2 Hz, with minimal high-frequency coherence, which is in agreement with previous studies (Farmer et al 1993a;Kim et al 2001). These findings suggest that the relative contribution of the oscillatory inputs at low and high frequencies is different in arm and hand muscles, and supports the observations of a reduced corticospinal projection to proximal as opposed to distal muscles (Clough et al 1968;Phillips and Porter 1964). Nonetheless, we did observe a weak but significant association between motor unit synchronization and high-frequency (10 -30 Hz) coherence that was stronger after eccentric exercise, suggesting that the weak inputs generating synchronization in biceps brachii also contribute to high-frequency coherence.…”

“…These findings are interpreted as a change in the number and/or excitability of branched corticospinal inputs used to perform these tasks (Farmer et al 1990;Schmied et al 1999). However, most measurements of the strength of motor unit synchronization in humans are performed in hand or wrist muscles, which have strong corticospinal projections to motor neurons (Clough et al 1968;Phillips and Porter 1964). In contrast, corticospinal inputs to biceps brachii motor neurons are generally weaker than in hand muscles (Palmer and Ashby 1992), and this is reflected in reduced synchrony in proximal muscles (Kim et al 2001).…”

Recovery of Thumb and Finger Extension and Its Relation to Grasp Performance After Stroke

“…The degree of motor deficits post stroke is determined by the amount of damage to the corticospinal system (Pineiro et al 2000). Our finding that the majority of deficits in grasp performance post stroke may be due to an execution problem is consistent with the understanding that corticospinal system structures constitute the neural substrate for skilled muscle action (Clough et al 1968;Strick 1996, 2002;Fetz and Cheney 1980;Palmer and Ashby 1992;Porter and Lemon 1993).…”

Functional properties of monkey motor cortex neurones receiving afferent input from the hand and fingers

“…It is known that there are direct monosynaptic connexions from the motor cortex to spinal motoneurones in monkeys, apes and man, animals which possess the ability to produce fractionated finger movements, while they are lacking in animals such as cat and dog which do not possess the capacity for relatively independent finger movements (Kuypers, 1973). These direct corticomotoneuronal connexions are preferentially distributed to motoneurones supplying distal motoneurones (Kuypers, 1960) where they exert powerful e.p.s.p.s (Phillips & Porter, 1964;Clough, Kernell & Phillips, 1968;Porter, 1970).…”