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Motor neurons are responsible for controlling the force exerted by muscle fibres. They are divided into two groups: alpha motor neurons and gamma motor neurons. Alpha motor neurons innervate extrafusal fibres, which are highly contracting fibres that supply muscles with power. On the other hand, gamma motor neurons innervate intrafusal fibres, which contract only slightly and are necessary to keep the muscle spindle taut and sensitive to stretch. The rate at which motor neurons fire action potentials signals the amount of force to be exerted by a muscle. Additionally, the size principle governs the force generated by a muscle, with stronger inputs leading to the recruitment of more motor neurons and increased force.
| Characteristics | Values |
|---|---|
| Type of neuron | Multipolar neuron |
| Function | Controls the amount of force exerted by muscle fibres |
| Groups | Alpha motor neurons and gamma motor neurons |
| Alpha motor neurons | Innervate extrafusal fibres, the highly contracting fibres that supply the muscle with its power |
| Gamma motor neurons | Innervate intrafusal fibres, which contract only slightly |
| Rate code | Signals the amount of force to be exerted by a muscle |
| Size principle | Governs the amount of force generated by a muscle |
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What You'll Learn
Motor neurons control muscle force
Motor neurons control the amount of force exerted by muscle fibres. There are two principles that govern the relationship between motor neuron activity and muscle force: the rate code and the size principle.
The rate code is a signal sent by motor neurons to indicate the amount of force to be exerted by a muscle. An increase in the rate of action potentials fired by the motor neuron causes an increase in the amount of force that the motor unit generates. When the motor neuron fires a single action potential, the muscle twitches slightly, and then relaxes back to its resting state. If the rate of firing of the motor neuron increases, such that a second action potential occurs before the muscle has relaxed back to baseline, then the second action potential produces a greater amount of force.
The size principle states that 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. With stronger inputs, more motor neurons will be recruited, resulting in more force applied to the muscle.
Motor neurons are divided into two groups. Alpha motor neurons innervate extrafusal fibres, the highly contracting fibres that supply the muscle with its power. Gamma motor neurons innervate intrafusal fibres, which contract only slightly. The function of intrafusal fibre contraction is not to provide force to the muscle; rather, gamma activation of the intrafusal fibre is necessary to keep the muscle spindle taut, and therefore sensitive to stretch, over a wide range of muscle lengths.
Different types of muscle fibres are innervated by small, intermediate, and large motor neurons. Small motor neurons innervate slow-twitch fibres, intermediate-sized motor neurons innervate fast-twitch fatigue-resistant fibres, and large motor neurons innervate fast-twitch fatigable muscle fibres. Most muscles contain both fast- and slow-twitch fibres, but in different proportions.
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Alpha motor neurons
Motor neurons are divided into two groups: alpha motor neurons and gamma motor neurons. Alpha motor neurons are large, multipolar lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibres of skeletal muscle and are directly responsible for initiating their contraction.
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Gamma motor neurons
Motor neurons are divided into two groups: alpha motor neurons and gamma motor neurons. Alpha motor neurons are larger and more abundant than gamma motor neurons. They innervate extrafusal fibers, the highly contracting fibers that supply the muscle with its power.
The presence of myelination in gamma motor neurons allows a conduction velocity of 4 to 24 meters per second, significantly faster than non-myelinated axons but slower than alpha motor neurons.
When the central nervous system sends out signals to alpha neurons to fire, signals are also sent to gamma motor neurons to do the same. This process, called alpha-gamma co-activation, maintains the tautness of muscle spindles. As the extrafusal fibers contract, the intrafusal fibers also contract to maintain a consistent length ratio in the muscle. This increases sensitivity to stretch and provides fine compensations of muscle length and velocity.
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Fast-twitch fibres
Fast-twitch muscle fibres, also known as type II muscle fibres, are responsible for enabling rapid, explosive movements. They are essential for activities requiring short, powerful bursts of energy, such as sprinting, jumping, powerlifting, and sports like football. These fibres are recruited when there is a large input onto motor neurons, causing the activation of intermediate-sized and large motor neurons.
There are two types of fast-twitch muscle fibres: type IIa and type IIx (formerly known as IIb). Type IIa fibres are considered intermediate muscle fibres, as they can be used for longer periods before fatigue sets in. They are commonly engaged during endurance exercises such as sprints and powerlifting. On the other hand, type IIx fibres excel at providing rapid force, making them crucial for quick, unexpected movements. However, their performance is short-lived due to rapid fatigue.
To effectively train fast-twitch muscle fibres, individuals should focus on exercises that push their muscles to the point of fatigue or failure. This can be achieved through strength training, high-intensity workouts, and exercises like lifting weights. The phosphagen system (ATP-PC system) is the first to be activated during the use of fast-twitch muscle fibres, providing up to 30 seconds of maximum effort. Subsequently, the glycolytic system takes over, utilising glucose to generate energy for up to three minutes.
While everyone is born with a mix of fast-twitch and slow-twitch muscle fibres, the ratio can vary depending on training habits and physical activities. Elite strength or power athletes may exhibit a higher proportion of type II muscle fibres, allowing them to excel in their respective sports. However, it's important to note that a balance of both types of fibres is beneficial, and individuals are advised to engage in activities that build both fast-twitch and slow-twitch muscle fibres for overall health and fitness.
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Slow-twitch fibres
Motor neurons control the force exerted by muscle fibres. The two principles that govern the relationship between motor neuron activity and muscle force are the rate code and the size principle. Small motor neurons innervate slow-twitch fibres, which generate less force than fast-twitch fibres. However, they are able to maintain these lower force levels for longer periods.
Slow-twitch muscle fibres are essential for everyday tasks, such as walking, sitting, and standing. They are also used for low-intensity activities like sweeping the floor or pushing a grocery cart. These fibres are also necessary for endurance exercises and longer workout routines. They are used for maintaining posture and making other low-force movements.
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Frequently asked questions
A multipolar neuron activates a muscle.
Motor neurons are divided into alpha motor neurons and gamma motor neurons.
Alpha motor neurons innervate extrafusal fibres, which are the highly contracting fibres that supply the muscle with power.
Gamma motor neurons innervate intrafusal fibres, which contract only slightly. They keep the muscle spindle taut and sensitive to stretch.
An increase in the rate of action potentials fired by the motor neuron causes an increase in the amount of force that the motor unit generates.
