Video: Motor neurons

After a long day of studying, it feels really good to just go outside and take a walk. It seems like such a simple thing, moving our arms and legs, but it's far from simple. Every time we move a limb, a muscle needs to contract. It's able to do so because of information it receives from neurons coming from the central nervous system.

And in this tutorial, we're going to flex our brain muscles as we learn about these motor neurons.

Motor neurons are neurons that come from the brain or spinal cord. They can supply muscles or glands. Thus, we have somatic motor neurons and autonomic motor neurons.

Autonomic or visceral motor neurons supply glands and smooth muscles of viscera. When we talk about motor neurons, we're usually referring to somatic motor neurons, which are those that supply skeletal muscle fibers. These somatic motor neurons can be divided into two groups: upper motor neurons and lower motor neurons.

Upper motor neurons are those which have cell bodies in the motor cortex or in the brainstem. Their axons form descending pathways of motor control and there are quite a few, such as the corticospinal tracts, reticulospinal, vestibulospinal, and tectospinal tracts. They form synapses with lower motor neurons, either via interneurons or directly.

The lower motor neurons are those that directly supply the muscle fibers. They include cranial nerves which have their cell bodies in nuclei of the brainstem, supplying muscles of the head and neck; and spinal nerves which have their cell bodies in the spinal cord supplying muscles of the trunk and limbs.

These motor neurons are also known as efferent neurons as they carry motor information away from the central nervous system towards effector organs such as the muscles. This is in contrast to afferent neurons which bring information from sensory receptors towards the central nervous system.

Based on their external diameters, motor neurons can be classified into alpha, beta, and gamma motor neurons. In this tutorial, we'll be looking at the alpha and gamma motor neurons. Let's start with the big alpha motor neurons.

These are large multipolar neurons. Their axons are heavily myelinated, belonging to the A alpha group of nerve fibers. Thus, they also have a high conduction velocity of nearly 70 to 120 meters per second. These neurons have their cell bodies in the anterior horn of the spinal cord gray matter. The cell bodies are clustered, forming motor pools in motor nuclei that extend as a column through the cord. The clusters include a mix of alpha and gamma motor neurons.

The arrangement of motor neurons in the spinal cord nuclei varies depending on the type of muscle being supplied. This is known as topographic organization of the spinal cord.

The spinal cord has cervical, thoracic, lumbar, and sacral segments. Throughout the cord, the neurons supplying axial muscles, such as those of the trunk, form a continuous column of medial motor nuclei. The lateral motor nuclei contain neurons supplying the limb muscles. Hence, these are found in the cervical and lumbosacral enlargements of the cord.

In these nuclei, the neurons that supply the flexor muscles are more posterior, while those that supply the extensors are located more anteriorly. For each limb, the neurons that supply the proximal muscles, such as the biceps brachii in the upper limb, are more medial, while those that supply the distal muscles, like the lumbrical muscles of the hand, are more lateral. Thus, the spinal cord keeps its motor neurons well organized.

The long axons of these neurons exit the spinal cord through the anterior root of the spinal nerve and extend through the body to supply the extrafusal fibers of skeletal muscles. These are the fibers that are responsible for generating the force of muscular contraction. It's through these neurons that the motor cortex and other regions of the brain and spinal cord can create voluntary and involuntary movements.

Each alpha motor neuron supplies a single skeletal muscle. However, the motor neuron divides into branches, supplying multiple muscle fibers. The motor neuron, together with all the muscle fibers it supplies, forms what's known as a motor unit. An action potential generated by the motor neuron will cause contraction of all the muscle fibers in that motor unit.

The total number of motor units within a single muscle can range from a few hundred to a few thousand, depending on the muscle's size and precision of movement.

The number of muscle fibers supplied by each motor neuron is known as the innervation ratio. This ratio varies, depending on the type of activity the muscle has to perform. For instance, fine and delicate movements, such as eye movements that use extraocular muscles, will use motor units with fewer muscle fibers being supplied by a single neuron, thus ensuring precise control.

On the other hand, coarse movements, such as jumping, will involve muscles such as the gastrocnemius muscle, which have a large number of muscle fibers supplied by a single motor neuron.

Though each skeletal muscle can have a mix of slow and fast-twitch fibers, the muscle fibers belonging to a single unit have the same type of fiber.

Small motor neurons form motor units with slow-twitch or Type I fibers, which are fatigue-resistant; intermediate size neurons form motor units with fast-twitch fibers, also known as Type IIa fibers, which have a moderate resistance to fatigue; and the largest motor neurons form motor units with fast-twitch fibers that fatigue easily, the Type IIx fibers.

You can learn more about the differences between these fibers in our video on skeletal muscle fibers.

Motor units follow a size principle, commonly known as Henneman’s size principle, where they are recruited in the ascending order of their size.

The smallest motor neurons are activated for movements requiring lesser force of contraction. The muscle fibers in these motor units don't fatigue easily. When more force is required, like for lifting heavier objects, motor neurons of progressively larger size get recruited. Muscle fibers innervated by large motor neurons, which produce large amounts of force, can fatigue quickly.

It's important to remember the neurons we're talking about here are alpha motor neurons supplying the extrafusal fibers of skeletal muscles.

Gamma motor neurons have a smaller diameter than alpha motor neurons and are less heavily myelinated. They have a slower conduction velocity of nearly 15 to 30 meters per second. Unlike the alpha motor neurons, they do not supply the extrafusal fibers of muscles. Instead, they innervate the intrafusal fibers of the muscle spindle.

So, what is a muscle spindle?

A muscle spindle is a collection of specialized muscle fibers located in skeletal muscles throughout the body. These specialized fibers are known as intrafusal fibers, since they are within the capsule of the spindle. The fibers are small, and their central portions have very few actin and myosin filaments making them non-contractile.

The ends of the intrafusal fibers can contract. However, this contraction is small and does not significantly contribute to the overall force of muscle contraction.

The muscle spindle is surrounded by extrafusal fibers, which are our regular muscle fibers. The extrafusal fibers are supplied by alpha motor neurons and can contract.

The intrafusal fibers are arranged in parallel to the extrafusal fibers, enabling them to sense stretch of the muscle. Structurally, there are two types of intrafusal fibers: nuclear chain and nuclear bag fibers. The nuclear chain fibers have nuclei arranged linearly in a row, while the nuclear bag fibers have nuclei in a bunch at the equatorial region.

Functionally, nuclear bag fibers are of two kinds: nuclear bag 1 and bag 2 fibers, also known as dynamic and static nuclear bag fibers. The nuclear chain and nuclear bag 2 fibers detect static stretch of the muscle, while nuclear bag 1 fibers primarily detect dynamic stretch.

Unlike the extrafusal fibers, the intrafusal fibers have both sensory and motor innervation. The central non-contractile region of the muscle spindle serves as a sensory receptor, a proprioceptor, responding to the stretch of a muscle and providing the brain with information regarding the static and dynamic state of the muscle. It is innervated by two groups of sensory neurons: group Ia and II fibers.

Group Ia fibers are also known as anulospiral or primary endings as they spiral around both the nuclear bag and chain fibers. Group II fibers are also known as flower spray or secondary endings, which end on either side of the anulospiral endings and innervate the static nuclear bag and nuclear chain fibers.

Both group Ia and II fibers sense information regarding static stretch; that is, the length of the muscle at any given time, increasing their firing as the length of the muscle increases. But the group Ia fibers can also detect the rate of change of length, picking up dynamic information using those nuclear bag 1 or dynamic nuclear bag fibers.

When the muscle stretches, the muscle spindle stretches in parallel, and since its length increases, the firing rate of neurons increases. On the other hand, when the muscle contracts, the length of the muscle spindle reduces and the firing rate of the neurons decreases. As the muscle continues contracting, the ends of the muscle spindle come closer together. The muscle spindle becomes lax and unloaded, and thus becomes insensitive to further decrease in muscle length.

This is where gamma motor neurons come in. They innervate the ends of the muscle spindle and stimulate the ends of the fibers to contract, keeping the spindle taut, and retaining its sensitivity to stretch even as the muscle contracts.

This is how gamma motor neurons regulate the sensitivity of the muscle spindle and thus the central nervous system uses alpha-gamma coactivation to ensure that the spindle stays sensitive as the muscle shortens during a contraction. This muscle spindle not only provides the brain with conscious awareness of stretch of the muscle, angles of joints, and positions of the limbs, it also participates in an important reflex known as the stretch reflex, which helps to stabilize joints, maintain posture, and protect muscles from overstretching.

That concludes this tutorial on the motor neurons.

Learn more about the sensory and motor systems with our study units and quizzes at Kenhub.