What is a Motor Unit? 

A motor unit is a functional unit of muscle contraction and each muscle in the body includes many motor units. It consists of alpha motor neurons, axons, and muscle fibers. Each motor neuron comes from the anterior horn of the spinal cord and supplies nearly 4-100 muscle fibers. Examples include gastrocnemius muscle that contains 2000 muscle fibers/motor neurons. A collection of motor neurons that supply a muscle is called a motor pool. 

What are the Types of Motor Units?

When the motor units are stimulated by an action potential, the fibers present in the motor unit develop muscle tension. Although the tension developed is common in all motor units, the response of each motor unit is not the same. The difference in the motor units is based on the muscle tension that is stimulated and the resistance of the motor unit to fatigue. The two types of motor units that are commonly used are: 

1. Type I or type S (slow): It is fatigue-resistant because of the extensive blood supply to maintain aerobic metabolism. It is also known as slow-twitch oxidative which is based on slow tension development time and the use of oxygen. It develops a low peak force with a long period, about 60-120 milliseconds. As they are fatigue resistant, these motor units are recruited by the central nervous system on muscle activation and will be recruited continuously until it remains active. The type I motor units are mainly involved in low-intensity work like maintaining posture. They are also referred to as tonic motor units as they provide muscle tone. 

2. Type II: It develops a high peak force which is for a short period between 10-50 milliseconds. It is also called fast-twitch motor units as the response is very quick. It is recruited after type I motor units to provide shorter bursts and higher muscle tension as required. It is divided into two categories that differ in resistance to fatigue: 

• Type IIb or type FR (fast-resistant): It is the last motor unit that is recruited when the muscle gets activated and also the first to get stopped when the force from the muscle is no longer needed. It contains oxidative and glycolytic enzymes. It is also called type IIx in the human muscle which is prone to fatigue and produces great force when stimulated. 

• Type IIa or type FF (fast-fatigable): The peak force and the fatigue resistance lie between the classification of type I and type IIb. Type IIa contains glycolytic enzymes and is also known as intermediate motor units. It also gets recruited after type I but before type IIb. Similarly, it gets to stop or being activated after type IIb but before type I.  

Every muscle contains all three types of motor units. These motor units can increase force output from a low level to a high level and also provide each muscle with a certain degree of muscular endurance.  

What is Meant by Motor Unit Recruitment?

It is defined as the activation of the additional similar motor units that have increased strength of voluntary muscle contraction. The strength of the muscle contraction of the central nervous system can be increased by increasing the number of motor units (spatial recruitment) and also increasing the firing frequency at which individual motor units optimize the summated tension generated (temporal recruitment). Both the processes occur at the same time. When all the motor units are recruited, the increased firing frequency becomes a common procedure to increase the motor strength. The recruitment of muscle fibers during exercise:  

• Light intensity exercise: Type I and it is slow. 
• Medium intensity exercise: It includes type I, type IIa (FR). 
• High-intensity exercise: It includes type I, type IIa, and type IIx (FF). 

It has been found that type I is always recruited during exercise.  

How are Motor Units Recruited? 

The recruitment of motor units happens according to the order of their size. When the activation of muscle takes place, the first motor units that are fired appear small in size, and the tension that is generated is also weak. From small to bigger motor units, the larger ones are recruited with increasing strength of muscle contraction. As a result, there is an addition of sequentially bigger and stronger motor units that lead to a smooth increase in the strength of the muscles. This recruitment of muscles in an orderly fashion is known as the size principle or the Henneman size principle. 

But there are few exceptions in the activation of motor units based on their size order. The recruitment pattern varies for a different movement that depends on various factors like central control and sensory feedback. For example, during an injury, the relationship between the size of the motor neuron and the size of the motor neuron that the muscle fibers innervate is lost. But with time, a size-dependent branching of axons occurs for rematching of motor neuron and muscle unit size which results in the size-ordered organization of motor units and gets restored. 

The Sequence of Motor Unit Recruitment 

The sequence starts with type I motor units that are similar to the type of S units and progress to type II units which include type IIa and ends with units similar to type IIb that are very active at high force output. In the case of walking the recruitment is type S and during jumping, it is type II motor units. 

What is Electromyography (EMG)? 

Electromyography is used to study motor units. It records the electrical activity generated by skeletal muscles. The signals are recorded as graphs or in numbers which consist of muscle activity and electrical noise. The signals increase with increased contraction force. An increasing number of motor units is stimulated to increase contraction force. There are two components of EMG: nerve conduction and needle EMG. The nerve conduction study helps in assessing the ability of motor neurons to send electrical signals and needle type help in evaluating electrical signals. In the case of myopathy, the size of the motor unit potential is small and the shape of the motor units is abnormal in case of nerve injury.  

Content and Applications   

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for 

• Bachelor of Science in Zoology 
• Bachelor of Science in General Physiology 
• Master of Science in Human Physiology    
• Masters in Anatomy and Physiology