The contractions of muscles that allow us to move would not appear without the work done by motor units. Sir Charles Sherrington made a great contribution to the study of a human body when he stated that people could move due to the action that occurs between a motor (efferent) neuron and muscle fibers and introduced the concept of a motor unit. The researcher assumed that “each muscle fiber receives innervation from only one motor neuron, and that the muscle fiber faithfully responds to every impulse of the motor neuron” (Floeter 1).
As can be seen, their basic structure is rather simple and includes only an efferent neuron and muscle fibers. Motor units can be of different sizes, depending on the type of contraction of the muscles. The smallest ones deal with the smallest neurons and fibers and turn into slow motor units with low myosin ATPase activity. An intermediate size of neuron and diameter of muscle fiber means that the motor unit is fast-oxidative with high myosin ATPase activity.
Finally, the largest fast-glycolytic units with high myosin ATPase activity come (“Motor Units” par. 7). As a rule, the muscles include different types of motor units in various proportions depending on the individual’s genetics; the quadriceps, for example, has a relatively equal proportion of slow and fast units. In this way, the extraocular muscles are fast motor units, while postural muscles are slow ones.
Of course, it is impossible just to memorize all the information about the motor units in the spare of the moment. Still, its constituents are rather easy to remember, as there are only two of them. The connection between size and types is also clear. If the constitutes are large, the motor unit is also large and fast. The contradiction can provide an example: the eyes and extraocular muscles are small, so the units are fast. If motor neurons and muscle fibers are small, the motor unit is small and slow. The back and postural muscles in it are big, so the units are slow. Dealing with slow and fast motor units, it is critical to remember two opposites, slow-oxidative and fast-glycolytic. The intermediate part will have the characteristics of both of them and can be easily recollected when combining the names of these two: fast (glycolytic) + (slow) oxidative = fast-oxidative.
One of the latest researchers connected with this topic focuses on “determining the selectivity of access to large numbers of independent motor units through intrafascicular multielectrode stimulation of the peripheral nerve” (McDonnall, Clark, and Normann 599). In other words, scientists try to investigate the connection between nerves and motor units, influencing them from the outside, which seems to be rather unusual.
Motor units play an important role in our lives, as with the help of them, we can control our movements and cope with day-to-day activities. They make our muscles contract and give us the ability to blink, talk, brush hair, etc. Even the simplest movements would be unobtainable for us without motor units. They are the vital components of any action. This topic is extremely important, as it allows us to understand not only the way in which the human body moves but also what triggers the movements. It proves that the organism is a complicated system where all elements are connected, and even toe and brain can have something that makes them united.
Works Cited
Floeter, Mary. Structure and Function of Muscle Fibers and Motor Units 2010. Web.
McDonnall, Daniel, Gregory Clark, and Richard Normann. “Selective Motor Unit Recruitment via Intrafascicular Multielectrode Stimulation.” Canadian Journal of Physiology and Pharmacology 82.8-9 (2004): 599-609. Print.
Motor Units. n.d. Web.