A myotome is described as 'a group of muscles innervated by a single spinal nerve root'. They are clinically useful due to their ability to determine the presence and level of damage to the spinal cord. In this article, we shall explore the embryonic origins of myotomes, their distribution in the adult body, and their practical applications.
The development of skeletal muscle begins with the appearance of somites around twenty days after conception. By this time, the trilaminar disc has formed and the mesoderm has divided into different areas. The area closest to the neural tube is known as the paraxial mesoderm.
As the embryo continues to progress, the paraxial mesoderm will further differentiate into segments called somites. Originally 44 pairs of somites form, although some of them will regress in time until 31 remain, each corresponding to one of the spinal nerves present in the adult human body.
Each somite consists of a dorsal and ventral region. The ventral part is known as the sclerotome, which is responsible for the formation of vertebrae and ribs. The dorsal section is referred to as the dermomyotome, which will eventually develop into muscle as the embryo continues to develop.
Most muscles in both the upper and lower limbs receive innervation from more than one spinal nerve root, resulting in their formation from multiple myotomes. For example, the biceps brachii muscle is responsible for elbow flexion. It is innervated by the musculocutaneous nerve, which is composed of the C5-7 spinal nerve roots. These three nerve roots can be said to be related to elbow flexion.
The following table outlines which movement is most commonly associated with each of the myotomes:
When evaluating a potential spinal cord injury, myotome function may be tested to determine the presence and location of any damage. Myotomes can be graded on a scale of 0-5 to assess power. A grade of 0 indicates total paralysis, 1 a visible or palpable contraction, 2 active movement with full range of motion without gravity, 3 with gravity, 4 with gravity and moderate resistance in a muscle specific position, and 5 full ROM and full resistance in a muscle specific position as expected of an otherwise healthy individual.
To conclude, myotomes are a useful and clinically relevant tool for determining the extent and nature of spinal cord damage. It is important to understand their development, distribution, and functioning in order to accurately diagnose and treat any damage.
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