Neural Control of Muscle

Neural Control of Muscle

Parveen Bawa (Simon Fraser University, Canada) and Kelvin E. Jones (University of Alberta, Canada)
DOI: 10.4018/978-1-4666-6090-8.ch001
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Abstract

The purpose of this chapter is to introduce the reader to the development of ideas and concepts about the manner in which the central nervous system controls muscle contraction. The motor unit, the quantum of muscle contraction, is fundamental to concepts of the neural control of muscle and will be the focus of discussion. The population of motor units comprising a skeletal muscle have a diverse range of physiological and anatomical properties. The Size Principle of motor unit recruitment is a concept that proposes a simple strategy for exploiting the diversity of the motor unit population to produce graded force output. The Size Principle has a great deal of empirical support, but also faces criticism about the extent of generalization to all types and forms of movement. As the key principles of motor units are discussed, methods of measuring and methodology for analysing motor unit activity and whole muscle activities are introduced.
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Background

Muscle and Its Motoneuron Pool

Skeletal muscle is composed of thousands of muscle fibres; each fibre a multi-nucleated cell. In this chapter we will concern ourselves with muscles of the limbs, which generally have a simpler structure; motoneurons innervating them lie in the spinal cord of vertebrates. The geometry of muscle fiber orientation relative to the long axis of the muscle and tendon varies in limb muscles. Muscle fibres can lie in parallel to the long axis of the muscle; all active fibres contract together with the force vector along the principal axis. A typical example of a parallel geometry is the biceps brachii (Loeb & Gans, 1986). In pennate (or pinnate) muscles, fibres attach to the tendon at an angle, called the angle of pennation. The force exerted along the line of action is less than if the fibres contracted along the main axis of the muscle. The directions of contraction of muscle fibres in different regions of large muscles are important for interpreting motor unit recruitment data (Staudenmann et al., 2009). Most of the muscles are multifunctional and their muscle fibres contribute different amounts of force in various directions (Jones et al., 1993; Ter Haar Romeny et al., 1984).

Motoneurons

The contraction of a muscle is controlled by a pool of alpha motoneurons1 in the spinal cord. The morphology and electrophysiology of motoneurons within a pool vary over a wide range. The physiology of each motoneuron is well matched to the properties of the muscle fibres it innervates (Henneman, 1985; Henneman & Mendell, 1981; Kernell et al., 1999). The cell body, also referred to as the soma or perikaryon, and dendrites of a limb motoneuron lie in the ventral horn of the spinal cord. A long myelinated axon exits the spinal cord through the ventral spinal root and travels to the muscle where it enters the muscle, divides into branches and terminals that make synaptic connections with muscle fibres. In a healthy adult, each muscle fibre receives input from a single motoneuron. A single motoneuron with all its muscle fibres is what Sherrington defined as the motor unit in 1925. All the muscle fibres innervated by the same motoneuron, the muscle unit (Burke, 1981), are of a uniform fibre type; and the number of muscle fibres connected to one motoneuron is the innervation ratio. Innervation ratio has been shown to be as high as 2000 in the cat gastrocnemius muscle which means that an action potential in the motoneuron results in 2000 synchronous muscle fiber action potentials in a large motor unit. This is an example of a system with high amplification.

Motoneurons that innervate limb muscles are typically in a resting inactive state until synaptic inputs excite them to an active state; this process is called recruitment. The number of motoneurons active at any time is associated with the force requirements, more motoneurons are recruited when more force is required. When a motoneuron is recruited to an active state for a long period, action potentials are generated at rates ranging roughly from 5- 30 impulses/s depending on the muscle and the force level. The time between action potentials, called the interspike interval (ISI), is not constant. The distribution of ISIs for an active motoneuron is unimodal with a variance resulting from various sources of probabilistic noise and electrophysiological properties (Jones & Bawa, 1997; Matthews, 1996). Motoneurons that are active simultaneously generate action potentials that are mostly independent of action potentials of other motoneurons. There is some level of synchronization of motorneuron action potentials in limb muscles that likely result from common input sources (Keen et al., 2012; Mochizuki et al., 2005).

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