Whether in whispering a syllable or lifting a heavy weight, we require the coordinated use of muscles. The human body has numerous muscles for moving our various joints. We have various types of muscles in our body, some for involuntary contractions, such as cardiac muscles and some for voluntary contractions, such as skeletal muscles. Skeletal muscles can vary in sizes and shapes. Some of the most common and larger weight bearing muscles are those in our limbs- such as the biceps muscles in our arms or the quadriceps in our thighs – more commonly referred to as the quads.
Muscle are composed of muscle fibers that extend from tendon to tendon that connect to our bone. Fibers can vary in sizes depending on the size and type of muscle. The muscle fibers are grouped together into bundles that lie alongside in parallel or in series depending on the size of the muscle. The smallest unit of a muscle fiber is known as a sarcomere which is responsible for producing the muscular contraction – or the smallest unit of force from the muscle fibers.
Muscle fibers are innervated by the alpha motor neuron (MN) that resides within our spinal cord. Motor neurons will innervate several muscle fibers scattered throughout the muscle and not necessarily the fibers lying next to each other. This type of distributed innervation ensures the MN’s electrical activity can stimulate the whole muscle globally. The alpha motor neuron and all the muscle fibers it innervates forms one motor unit (MU). When the alpha motor neuron receives neuronal excitation, it is activated and it will electrically stimulate each of the various muscle fibers it is connected to. Muscle fibers are physiologically compatible with their motor neurons and larger fibers are connected to larger neurons.
The total number of fibers that a neuron is connected to determines the innervation ratio. One important thing is that MN has a one to many mapping to its muscle fibers. This means that one MN is connected to many fibers throughout the muscle. This leads to distributed and smooth muscle control. On the other hand, each muscle fiber is typically innervated by just one motor neuron in a one on one mapping. However, one exception exists in the muscles of the tongue.
A motor unit will be activated when the alpha motor unit is electrically stimulated and reaches its action potential. The signal passing down to the fibers causes the muscle fibers to contract and cumulatively we have a motor unit twitch– it is the building block of the force produced by a muscle. When the MU is repeatedly activated, we have continuous bursts of MU twitches. When the MU is activated faster, the twitches will summate or tetanize to our muscle force.
This is the idea behind force. Each muscle is composed of hundreds of MUs depending on the size of the muscle. The brain sends the command or neural excitation to each motor unit. With increasing excitation, more and more and larger units are activated. Hence the force output increases and also smooth out as the twitches summate.