| Myotome | |
|---|---|
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| Scheme showing the manner in which each vertebral centrum is developed from portions of two adjacent segments. (Myotome labeled in upper right.) | |
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| Transverse section of a human embryo of the third week to show the differentiation of the primitive segment. ao. Aorta. m.p. Muscle-plate. n.c. Neural canal. sc. Sclerotome. s.p. cutis-plate. ("Muscle-plate" is an older term for "myotome.") | |
| Latin | myotomus |
| Gray's | subject #17 80 |
| Carnegie stage | 13/14 |
| Precursor | somite |
| Gives rise to | muscle |
| Code | TE E5.2.0.0.0.0.6 |
In vertebrate embryonic development, a myotome is a group of tissues formed from somites. These somites develop into the body wall muscle.
Each myotome divides into a dorsal epaxial part and a ventral hypaxial part.
The myoblasts from the hypaxial division form the muscles of the thoracic and anterior abdominal walls.
The term "myotome" is also used to describe the muscles served by a single nerve root.[1] It is the motor equivalent of a dermatome.
Each muscle in the body is supplied by a particular level or segment of the spinal cord and by its corresponding spinal nerve. The muscle and its nerve make up a myotome.[2]
The epaxial muscle mass loses its segmental character to form the extensor muscles of the neck and trunk of mammals. In fishes, salamanders, caecilians, and reptiles, the body musculature remains segmented as in the embryo, though it often becomes folded and overlapping, with epaxial and hypaxial masses divided into several distinct muscle groups.
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Clinical Significance [link]
In humans myotome testing can be an integral part of neurological examination as each nerve root coming from the spinal cord supplies a specific group of muscles. Testing of myotomes, in the form of isometric resisted muscle testing, provides the clinician with information about the level in the spine where a lesion may be present.[3] During myotome testing, the clinician is looking for muscle weakness of a particular group of muscles. Results may indicate lesion to the spinal cord nerve root, or intervertebral disc herniation pressing on the spinal nerve roots.
Myotome distributions of the upper and lower extremity are as follows;[4][5]
- C1/C2-neck flexion/extension
- C3-neck lateral flexion
- C4-shoulder elevation
- C5-shoulder abduction
- C6-elbow flexion/wrist extension
- C7-elbow extension/wrist flexion
- C8-thumb extension
- T1-finger abduction
- L2-hip flexion
- L3-knee extension
- L4-ankle dorsi-flexion
- L5-great toe extension
- S1-ankle plantar-flexion
- S2-knee flexion
See also [link]
References [link]
- ^ myotome at eMedicine Dictionary
- ^ Apparelyzed: Myotomes & Dermatomes
- ^ Magee, David. J (2006). "1". Orthopaedic Physical Assessment (4th ed.). St. Louis: Elsevier. pp. 1–63. ISBN 978-1-4160-3109-3.
- ^ Magee, David. J (2006). "3". Orthopaedic Physocal Assessment (4th ed.). St. Louis: Elsevier. pp. 121–181. ISBN 978-1-4160-3109-3.
- ^ Magee, David. J (2009). "9". Orthopaedic Physocal Assessment (4th ed.). St. Louis: Elsevier. pp. 467–566. ISBN 978-1-4160-3109-3.
External links [link]
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Somite
A somite is a division of the body of an animal or embryo.
Somites are bilaterally paired blocks of paraxial mesoderm that form along the head to tail axis of the developing embryo in segmented animals. In vertebrates, somites subdivide into the sclerotomes, myotomes and dermatomes that give rise to the vertebrae of the vertebral column, rib cage, and part of the occipital bone; skeletal muscle, cartilage, tendons, and skin (of the back).
The word "somite" is also used in place of the word "metamere." In this definition, the somite is a homologously paired structure in an animal body plan, such as is visible in annelids and arthropods.
Development
The mesoderm forms at the same time as the other two germ layers, the ectoderm and endoderm. The mesoderm at either side of the neural tube is called paraxial mesoderm. It is distinct from the mesoderm underneath the neural tube which is called the chordamesoderm that becomes the notochord. The paraxial mesoderm is initially called the “segmental plate” in the chick embryo or the “unsegmented mesoderm” in other vertebrates. As the primitive streak regresses and neural folds gather (to eventually become the neural tube), the paraxial mesoderm separates into blocks called somites.
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