How many motor neurons are in a motor unit

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Chapter Plant Responses to the Environment. Full Table of Contents. This is a sample clip. Motor neurons are not merely the conduits of motor commands generated from higher levels of the hierarchy. They are themselves components of complex circuits that perform sophisticated information processing.

As shown in Figure 1. Two terms are used to describe the anatomical relationship between motor neurons and muscles: the motor neuron pool and the motor unit. If a muscle is required for fine control or for delicate movements e. That is, each motor neuron will innervate a small number of muscle fibers , enabling many nuances of movement of the entire muscle. If a muscle is required only for coarse movements e. A motor neuron controls the amount of force that is exerted by muscle fibers.

There are two principles that govern the relationship between motor neuron activity and muscle force: the rate code and the size principle. The upper trace on the oscilloscope shows the action potentials generated by the alpha motor neuron. The lower trace shows the force generated by the isometrically contracting muscle.

PLAY 1: Single spikes by the motor neuron produce small twitches of the muscle. PLAY 2: Multiple spikes in succession summate to produce larger contractions.

PLAY 3: Very high rates of spikes produce maximal contraction called tetanus. Because motor units are recruited in an orderly fashion, weak inputs onto motor neurons will cause only a few motor units to be active, resulting in a small force exerted by the muscle Play 1. With stronger inputs, more motor neurons will be recruited, resulting in more force applied to the muscle Play 2 and Play 3.

Moreover, different types of muscle fibers are innervated by small and larger motor neurons. Small motor neurons innervate slow-twitch fibers ; intermediate-sized motor neurons innervate fast-twitch, fatigue-resistant fibers ; and large motor neurons innervate fast-twitch, fatigable muscle fibers. The slow-twitch fibers generate less force than the fast-twitch fibers, but they are able to maintain these levels of force for long periods.

These fibers are used for maintaining posture and making other low-force movements. Fast-twitch, fatigue-resistant fibers are recruited when the input onto motor neurons is large enough to recruit intermediate-sized motor neurons. These fibers generate more force than slow-twitch fibers, but they are not able to maintain the force as long as the slow-twitch fibers. Finally, fast-twitch, fatigable fibers are recruited when the largest motor neurons are activated.

These fibers produce large amounts of force, but they fatigue very quickly. They are used when the organism must generate a burst of large amounts of force, such as in an escape mechanism. Most muscles contain both fast- and slow-twitch fibers, but in different proportions. Thus, the white meat of a chicken, used to control the wings, is composed primarily of fast-twitch fibers, whereas the dark meat, used to maintain balance and posture, is composed primarily of slow-twitch fibers.

Upper trace of oscilloscope represents the action potentials of a descending pathway axon. With low rates of activity of the descending pathway, only small alpha motor neurons are activated, producing small amounts of muscle force lower trace of oscilloscope. With increasing rates of descending pathway activity, intermediate-size alpha motor neurons are activated in addition to the small neurons.

Because more motor units are activated, the muscle produces more force. Finally, with the highest rates of descending activity, the largest alpha motor neurons are recruited, producing maximal muscle force. The motor system requires sensory input in order to function properly. In addition to sensory information about the external environment, the motor system also requires sensory information about the current state of the muscles and limbs themselves.

The muscle spindle signals the length of a muscle and changes in the length of a muscle. The Golgi tendon organ signals the amount of force being applied to a muscle. Muscle contraction and actin—myosin interactions : Skeletal muscle contracts following activation by an action potential.

The binding of acetylcholine at the motor end plate leads to intracellular calcium release and interactions between myofibrils to elicit contraction. A motor unit is comprised of a single alpha-motor neuron and all the muscle fibers it innervates. Rectus femoris : The rectus femoris muscle is one of the four quadriceps muscles of the human body.

These muscles may have as many as a thousand fibers in each motor unit. A motor unit consists of a single alpha motor neuron and all of the corresponding muscle fibers it innervates; all of these fibers will be of the same type either fast twitch or slow twitch. When a motor unit is activated, all of its fibers contract.

Groups of motor units often work together to coordinate the contractions of a single muscle. All of the motor units that subserve a single muscle are considered a motor unit pool.

The number of muscle fibers within each unit can vary. Thigh muscles, for example, can have a thousand fibers in each unit, eye muscles might have ten.

The smaller the motor unit, the more precise the action of the muscle. Muscles requiring more refined motion are innervated by motor units that synapse with fewer muscle fibers. Motor unit recruitment is the progressive activation of a muscle by the successive recruitment of motor units to accomplish increasing gradations of contractile strength. The activation of more motor neurons will result in more muscle fibers being activated, and therefore a stronger muscle contraction.

Motor unit recruitment is a measure of how many motor neurons are activated in a particular muscle. The tips of the terminal branches expand into small synaptic bulbs, which fit into grooves along the surface of the muscle fiber.

Together, the expanded axon tips and the nearby muscle fiber membrane make up a neuromuscular junction. A motor unit is the term applied to a single motor neuron and all of the muscle fibers that it stimulates. When a motor neuron fires, all the muscle fibers in the motor unit contract at once.