1. 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 Ia EPSP was small.6. The results are discussed in relation to problems concerning the pyramidal control of hand and finger movement.

Journal Article↗

PubMed1968

Conduction velocity in proximal and distal portions of forelimb axons in the baboon.

1. Peripheral nerves of the baboon's forelimb were stimulated at different sites, and the latencies of antidromic action potentials were measured in intracellular records from forelimb motoneurones.2. The conduction velocity of single motor axons was slower in the brachial plexus than in the nerves of the arm and forearm. This proximal slowing of conduction velocity was more marked for rapidly conducting axons than for the more slowly conducting ones.3. Gross recordings from dorsal and ventral roots showed that the conduction velocity was slower in the brachial plexus than in the arm for fast afferent as well as efferent nerve fibres.4. The proximal slowing of conduction velocity was shown to be due neither to errors of measurement nor to proximo-distal differences of temperature.

Journal Article↗

PubMed1967

Responses of the pyramidal tract to stimulation of the baboon's motor cortex.

1. The arm area of the baboon's precentral motor cortex was stimulated by brief surface-anodal pulses, and the discharge of the corticospinal tract (the ;pyramidal tract waves') was recorded by an electrode resting on the dorsolateral surface of the cervical spinal cord.2. Some properties of the pyramidal tract waves were described, and they were also studied in relation to the firing of single cortico spinal fibres.3. The results led to the conclusion that the later pyramidal tract waves (the ;I waves') were almost exclusively due to a semi-synchronous repetitive discharge of the same fast cortico spinal fibres as those responsible for the initial wave (the ;D wave').4. Some problems concerning the origin and significance of the I waves were discussed.

Action Potentials↗

PubMed1967

Post-synaptic effects of cortical stimulation on forelimb motoneurones in the baboon.

1. The arm area of the baboon's precentral motor cortex was stimulated by brief surface-anodal pulses, and the post-synaptic potentials elicited in contralateral forelimb motoneurones were studied by intracellular recording.2. Strong cortical stimuli elicited a rapid series of excitatory and, in some cells, inhibitory post-synaptic potentials (EPSPs and IPSPs respectively). Comparisons with the simultaneously recorded response of the pyramidal tract indicated that these post-synaptic potentials were due to a repetitive discharge of fast pyramidal fibres. Thus, the later synaptic events were mostly due to a repetition of the early monosynaptic EPSP and early IPSP respectively.3. Inhibition was seen more often in cells whose monosynaptic EPSP had a small maximal size than in those whose monosynaptic EPSP was larger. The net depolarization produced by a strong cortical stimulus was related to the maximal size of the early monosynaptic EPSP.4. In the Discussion, an interpretation is suggested for previous findings concerning the spinal distribution of late synaptic effects elicited by cortical stimulation.

Action Potentials↗

PubMed1966

Input resistance, electrical excitability, and size of ventral horn cells in cat spinal cord.

Experiments on cat lumbosacral alpha motoneurones showed that, in comparison with cells possessing rapidly conducting axons, the cells with slowly conducting axons have the higher input resistance, that they need weaker stimulating currents to reach the threshold for repetitive firing, and that they need a relatively larger increment in current strength for a given increase in firing rate. Measurements of the number and diameters of dendritic trunks gave larger values for the larger cell bodies. The discussion deals with the interrelation between cell geometry, electrical properties, and the reflex action of alpha motoneurones.

Animals↗

PubMed1966

Algebraical summation in synaptic activation of motoneurones firing within the 'primary range' to injected currents.

1. In intracellular studies of cat lumbar motoneurones constant synaptic stimuli such as stretch, contraction or a high-frequency stimulation of a cut afferent nerve have been superimposed on firing in response to injected currents.2. As long as the slope relating spike frequency to injected current remained constant, which by definition is the ;primary range' of firing, algebraical summation of superimposed synaptic stimulation prevailed. Added quantities in these experiments were then between 2.0 and 56.2 impulses/sec for excitation, between -2.8 and -21.8 impulses/sec for inhibition.3. Data were obtained correlating firing rates with amount of synaptic potential and current respectively.4. Theoretical implications are dealt with in the Discussion.

Animals↗

PubMed1966

Synaptic stimulation superimposed on motoneurones firing in the 'secondary range' to injected current.

1. This paper extends the work on the ;primary range' of firing (Granit, Kernell & Lamarre, 1966), in lumbar motoneurones to the ;secondary range'. By definition the latter begins when, with stronger currents, the linear curve relating firing rate to injected current in the primary range undergoes a fairly sudden increase of slope.2. It was shown that motoneurones firing at the higher frequencies of the secondary range were partially inactivated. Yet such firing rates were within the physiological range.3. Algebraical summation of firing rates, when present in the secondary range, implied at the same time that the synaptic amount added was diminished by comparison with what it had been within the primary range.4. Superimposed synaptic excitatory stimuli did not (as in the ;primary range') regularly add their effect algebraically on to the rate of firing achieved by injected currents alone. More commonly the synaptic effect of the constant input underwent a progressive increase throughout the secondary range.5. Superimposed inhibitory stimuli regularly reduced the slope constant as determined by trans-membrane current alone and, by counter-acting inactivation, made the motoneurone approach the mode of firing characteristic of the ;primary range'.6. The latter finding emphasizes the significance of analysing firing motoneurones with the aid of ;slope constants' and provides inhibition with a new role in the integrative behaviour of motoneurones, as considered in the Discussion.

Animals↗

PubMed1965

Reduction of the monoamine stores in the terminals of bulbospinal neurones following stimulation in the medulla oblongata.

Explore the source record for details and available documents.

Animals↗

PubMedDate not supplied

Discharge properties of motoneurones: how are they matched to the properties and use of their muscle units?

A general survey is given of old as well as more recent findings concerning matches between electrophysiological properties of motoneurones and contractile properties of their muscle fibres. Mechanisms for creating and maintaining such matches are discussed. It is pointed out that it is not sufficient to describe the variation of functional motoneurone characteristics simply in terms of 'fast' or 'slow': all properties seem continuously graded and there is cytochemical evidence for several, seemingly independent parameters of functional specialisation.

Adaptation, Physiological↗