Motoneurone Properties and Motor Control
Firing rate modulation of motor unit contraction is discussed in relation to recent experiments concerning the properties and activation patterns of motor units in the first deep lumbrical muscle of the cat’s foot (Ducati, Kernell and Sjöholm; Kernell and Sjöholm; in preparation). Results from these studies showed that firing rate modulation of motor unit tension was of great importance for gradation of the strength of contractions elicited in the lumbrical muscle by cortical stimulation or by pinching the foot pad. It is pointed out that the extent of firing rate modulation that is produced in a motoneurone by maintained synaptic activity will partly depend on the sensitivity of the firing motoneurone to changes in synaptic current intensity. Studies of neurone models suggest that this sensitivity (i. e. the slope for the relation between impulse frequency and the intensity of activating current) would depend markedly on: (i) the size, time course, and ‘summing ability’ of the permeability changes underlying the after-hyperpolarization of the neurone, and (ii) the difference between the threshold potential for spike initiation and the equilibrium potential for potassium (Kernell, 1968, 1971; Kernell and Sjöholm, 1973).
KeywordsFiring Rate Motor Unit Spinal Motoneurones Repetitive Firing Motor Unit Firing
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