Intro | Golgi Tendon Organ | Motor Neuron | Muscle | Muscle Spindle | Sensory Neurons | Tendon
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The Motor Neuron forms synaptic junctions with either extrafusal muscle fibers (skeletal muscle) or intrafusal muscle fibers (thread-like muscle that adjusts tension). Stimulation of these motor neurons induces contraction or shortening of the muscle fibers. Alpha motor neurons induce the contraction of extrafusal muscle fibers upon stimulation, whereas gamma motor neurons induce the contraction of intrafusal muscle fibers upon stimulation. Alpha motor neurons control muscle contraction involved in voluntary movement, whereas gamma motor neurons control muscle contraction in response to external forces acting on the muscle. In response to these external forces, the gamma motor neurons induce the involuntary, reflexive movement called the stretch reflex. Intrafusal motor neurons adjust the length of intrafusal muscle fibers to maintain an appropriate level of tension on the muscle spindle receptor. The control of intrafusal muscle fibers occurs independently of the length of skeletal muscle fibers. This independent function allows the spindle to maintain a high degree of sensitivity over a wide range of muscle lengths, and in effect acts as a means of encoding muscle length.
The smallest functional component of the motor system is the motor unit, which is composed of the motor neuron and all muscle fibers that it innervates (Parent, 1996). The innervation ratio defines the number of muscle fibers innervated by a single motor neuron. Low innervation ratios (approximately 100) occur in the hand, where fine gradations in muscle force are needed for fine motor control. Conversely, high innervation ratios (approximately 2000) occur in larger muscles such as the biceps of the upper arm where accuracy of movement is relatively insignificant.
When a motor neuron fires, the neurotransmitter acetylcholine is released in the neuromuscular junction. Depolarization is propagated in both directions along the muscle fibers innervated until the ends are reached. Upon completion of this signal propagation, the muscle undergoes a brief contraction. The activation of motor neurons is modulated by local circuits in the spinal cord and by pathways descending from motor centers of the brain. These motor centers will be discussed in Tutorial 33.
There are two types of gamma motor neurons that work in association with the two types of muscle spindle sensory fibers, the dynamic gamma motor neuron (involved in responses to dynamic stretch) and the static gamma motor neuron (involved in the response to steady-state length).
Parent, A. (1996). Carpenter's human neuroanatomy (9th ed.). London: Williams & Wilkins.