• Introduction
  • The anatomy of a motor unit
  • How are motor units recruited?
  • Different types of motor units
  • Slow motor units
  • Fast motor units
  • Type IIa motor units
  • Type IIb/IIx motor units
  • Final thoughts
  • Sources
  • Action potential: a brief reversal of electric polarization of the membrane of a nerve cell or muscle cell.
  • Atrophy: loss of muscle mass and force production capability.
  • Axon: a long projection of a nerve cell that sends signals away from the cell body.
  • Capillaries: small blood vessels that form networks throughout the bodily tissues.
  • Cell body: located in the spinal cord, the cell body maintains the neuron’s structure, carries genetic information, and provides energy to drive activities. The cell body also contains a nucleus.
  • Central nervous system: the nervous system consisting of the brain and spinal cord.
  • Dendrite: branch-like extensions at the ends of the neuron responsible for receiving, processing, and transferring signals from other neurons to the cell body. 
  • Mitochondria: the powerhouse of the cell.
  • Motor unit: a motor neuron and all muscle fibers innervated by it.
  • Motor pool: a collection of motor units.
  • Myoglobin: an oxygen-binding protein located primarily in muscles.
  • Soma: cell body of a neuron.

Introduction

A motor unit refers to a single motor neuron and the group of muscle fibers it innervates. This makes it the smallest functional unit of the nervous system as well as the final output of motor commands. A collection of motor units a known as a motor pool. 

The number of muscle fibers controlled by a single motor neuron varies greatly. For example, extraocular muscles (found in the eye) only have ten muscle fibers controlled by one motor neuron. On the other hand, a calf muscle has approximately 1000-2000 muscle fibers activated by a single motor neuron. Fewer motor units tend to be more precise whereas a higher number produces more power in gross movements. The more motor units are active, the more muscle fibers are recruited, leading to a stronger muscle contraction.

Muscles are a combination of multiple motor units. When needed, the maximal number of motor units in a muscle can be recruited simultaneously to increase the force of contraction. However, this cannot last for very long due to the energy needed to sustain the contraction. To prevent complete muscle fatigue, motor units are usually not all simultaneously active. Instead, some motor units rest while others are active, allowing for longer muscle contractions. The central nervous system uses this mechanism to utilize a skeletal muscle efficiently.

This post explains what motor units are, and what makes them so important for human movement.

The anatomy of a motor unit

A motor unit consists of a motor neuron and all the muscle fibers activated by it. The motor neuron consists of three components; a cell body (soma), dendrites, and an axon. 

  • The cell body, or soma, is located in the spinal cord. It maintains the neuron’s structure, carries genetic information, and provides energy to drive the cell’s activities. The soma also contains a nucleus as well as specialized organelles.
  • Dendrites are branch-like extensions found at the ends of the neuron. They are responsible for receiving, processing, and transferring signals from other neurons to the cell body. 
  • An axon, also known as a nerve fiber, is a long extension of the neuron responsible for carrying impulses away from the cell body and produce movement, etc. The axons themselves are enclosed in a myelin sheath, which increases the speed of the impulse. The thicker this sheath is, the faster the impulse is.

The human body has approximately 500,000 motor neurons that carry information from the central nervous system (CNS) to the muscles, and other peripheral systems (organs, glands). Because motor neurons carry information away from the CNS, they are also known as efferent neurons. The efferent fibers projecting out of the spinal cord are some of the longest in the body – the longest ones extending from the spinal cord to the toes. 

Motor neurons can also be divided into two categories; upper motor neurons and lower motor neurons. The upper motor neurons are confined to the central nervous system and responsible for initiating voluntary movement. The lower motor neurons are the efferent neurons of the peripheral nervous system. They connect the central nervous system with the innervated muscle fibers. Together, they form a complex system that controls both voluntary and involuntary movements in the human body. 

How are motor units recruited?

Skeletal muscles convert energy into a mechanical output, which is required for creating movement. Instead of activating all motor units simultaneously, the muscles must be energy efficient and produce force more gradually. This is done by altering the firing rate of the motor unit, as well as recruiting motor units with different mechanic and metabolic properties. 

When stimulated by an action potential from the motor neuron, all the muscle fibers in the motor unit are activated simultaneously at full force. This phenomenon is known as the all-or-none law. To put it simply, a single nerve fibre will always give a maximum response if the stimulus exceeds an activation threshold, or none at all.

Motor units are also recruited in an order from smallest to largest. This is known as the size principle. Thus, slow motor units are fired first due to their lower activation threshold, whereas naturally larger fast-twitch fibers are only recruited when smaller fibers are unable to produce enough force. 

The overall force exerted my a muscle during voluntary contraction is dependent on the number of motor units recruited simultaneously (motor unit synchronization) and the rates at which they discharge action potentials (rate coding).

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Motor Units

A single motor neuron and the muscle fibers activated by itCan be divided into Type I & Type II motor unitsAll muscles have both Type I & Type II motor unitsA motor unit activates all muscle fibers or none at allMotor units are activated from smallest to largest

Different types of motor units

Both motor units and the motor neurons vary in size. Smaller motor neurons tend to innervate fewer muscle fibers, creating motor units that produce smaller forces. Large motor neurons usually innervate larger and more powerful motor units. Motor units also differ in the types of muscle fibers they innervate. In most skeletal muscles, smaller motor units innervate type I muscle fibers, and vice versa.

Although all motor units respond to stimuli, they have distinct properties. The two main differences are; twitch time (how quickly the motor unit produces tension in the muscle) and fatigue resistance. Based on these characteristics, motor units can be divided into two categories: Type I (slow motor units) and Type II (fast motor units).

Humans have roughly the same amount of both fast and slow-twitch muscle fibers, and all muscles are comprised of both muscle fiber types (although their proportions differ between individuals and muscle groups). This means that each muscle has the ability to produce high levels of force yet still maintain some degree of muscular endurance.

Slow motor units

Type I motor units, also known as slow (S) motor units, are highly resistant to fatigue and focus on postural control as well as sustained physical performance. Hence, why they are sometimes called tonic motor units. Because of their smaller size, type I motor units have a lower activation threshold. Thus, they are activated first during a muscle contraction. They also produce significantly less force than type II motor units, which are only activated when a stronger contraction is needed.

Slow motor units contain a high number of mitochondria (the powerhouse of the cell) that are used in aerobic (with oxygen) energy production. They also have a large amount of capillaries which provides oxygen and nutrients to the muscle while removing unwanted byproducts. Additionally, slow motor units also contain myoglobin – a protein that binds iron and oxygen. This gives them their signature red color. As a result, slow motor units have a high oxygen capacity, which makes them excel in sustained muscle contractions.

Fast motor units

Fast motor units, also referred to as type II motor units, produce a high peak force in a short amount of time. This makes them essential in sports that require strength, power and speed. Although fast motor units produce a significant amount of force relatively quickly, their downside is that they tire faster than slow motor units.

This lower oxidative capacity is due to having less mitochondria, fewer capillary beds, and lower myoglobin content. To offset this, fast motor units tend to have greater glycogen storage that can be used for anaerobic (without oxygen) energy production. This generates lots of energy in a short amount of time, but creates lactic acid as a side product.

Fast motor units are larger in size, resulting in a higher activation threshold. Because of this, they are only activated when a higher muscle tension is required. This is also why type II motor units are sometimes called phasic motor units. 

Fast motor units can be further divided into two subcategories; type IIa and type IIb/IIx. 

Type IIa motor units

Type IIa motor units are often referred to as fast fatigue-resistant (FR) motor units. Similar to slow motor units, type IIa motor units also have a high amount of mitochondria, capillary beds, and myoglobin content. As a result, they have a high oxidative capacity while still generating around twice the force of slow motor units.

However, when needed, type IIa motor units can also switch to anaerobic respiration to rapidly produce more energy. Thus, producing more tension than slow motor units but also fatiguing quicker. In a way, their force production and fatigue-resistance fall somewhere between type I and type IIb motor units. 

Type IIb/IIx motor units

Type IIb/IIx motor units, or fast fatiguable (FF) motor units, rely mostly on anaerobic respiration for energy production. This is due to the fact that type IIb/IIx motor units have the least amount mitochondria, capillaries, and myoglobin. This also gives them a more pale color.

Being the largest motor units in size they also produce the highest amount force out of any motor unit, albeit at the expense of endurance capability. With this in mind, type IIb/IIx motor units are the last to be recruited when a muscle is activated, and the first ones to relax when the muscle is no longer needed.

Here is a chart of the characteristics of different muscle fibers.

Motor Unit Type

I

IIa

IIb/IIx

Contraction Speed

Slow
(90-140ms)

Fast
(50-100ms)

Very Fast
(40-90ms)

Fatigue Resistance

High

Medium

Low

Force Production

Low

High

Very High

Mitochondria Content
(powerhouse of the cell)

High

High

Low

Myoglobin Content
(a protein that binds iron & oxygen and gives blood its red colour)

High

High

Low

Capillary Content
(capillaries provide muscles with oxygen and nutrients while removing unwanted byproducts)

High

High

Low

Oxidative Capacity
(ability to use oxygen for energy production)

High

Medium

Low

Glycolytic Capacity
(ability to store and break down glycogen for intense exercises)

Low

High

High

Muscle Fiber Diameter

Small

Medium

Large

Muscle Fiber Color

Dark Red

Dark Red

Pale Red

Motor Neuron Size
(larger neurons provide faster activation)

Small

Large

Very Large

ATPase Level
(enzyme that controls glycogen breakdown and ATP synthesis)

Low

Medium

High

The more motor units are recruited, the more force is produced by the muscle.

Final thoughts 

Motor units are considered the basic functional units of a skeletal muscle. Their activity represents the final output of the central nervous system. Thus, controlling physical movements of the human body.

All motor units innervate the same type of muscle fiber (either slow-twitch or fast-twitch) to ensure that muscles are activated gradually. The more motor units are activated during a muscle contraction, the more force is produced. 

Throughout the years, sports scientists have been able to pinpoint the mechanic and metabolic properties of different motor units. This also resulted in a better understanding of how they can be trained most effectively. Regardless of the muscle fiber type, there is an overwhelming evidence that muscle fibers (and therefore motor units) change in size as a response to the physical demands. There are even signs that motor units can convert from one type to another. Changes between type IIa and IIb being the most common, whereas conversion from type II to type I can occur after a significant deconditioning or spinal cord injury.

Did you learn anything new about motor units? Let us know in the comments.

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