Nerve and Muscle Connection Patterns; Bone Shapes and Muscle-Tendon Attachments

The structures that you learn about in anatomy textbooks all have causes, which include genes, proteins & mechanical forces.

Sensory nerves connect the spinal cord to the skin in stripes called "Dermatomes". Diagrams of human dermatomes are shown in a Wikipedia article on this subject.

How nerves are guided to these connections is not understood; Selective adhesion has been a popular theory, but isn't very specific or tested.

This same word "dermatome" also has a specific meaning in Embryology, where it refers certain parts of temporarily-existing blocks of cells along the right and left sides of the notochord and the neural tube. The inner layer of the skin (the dermis) is made (entirely?) of cells from the dermatome parts of somites, and we have the same number of dermatomes, in the sense of stripes of sensory innervation.

It is possible, maybe even very likely, but hasn't really been proven, that there is some direct causal relation between somite formation and the formation of dermatomes in the sense of stripes of skin innervation.

Please suggest what experiments or other observations that would be able to prove or disprove theories about nerve guidance to skin.

Motor nerves also connect to different parts of the body in 30-plus stripe-like (slice-like) bands called myotomes. Their geometric pattern is very similar to dermatomes. Another similarity is that somites each have a subdivision called a myotome. Using the same word to refer to parts of somites and also to regions of innervation of muscle creates the impression of (at least) a direct causal relation More evidence is needed.

Try to imagine what guidance mechanism(s) could cause the myotome cells of each somite to attract or pull motor nerves to the slices of muscular tissue that are also called "myotomes". What guides the muscle cells, and what guides the nerves?

Each somite also stimulates development of its own branch from the aorta, which also must have some mechanism. Some mechanism (or combination) causes ("controls?") the biceps muscle (and its tendons) to attach at certain locations ("Origins" = whichever skeletal attachment is closer to the midline of the body; and "Insertions" = the skeletal attachment sites farther from the midline.

For example, one theory is (was?) that each tendon attachment site on bone surfaces might secrete a chemotactic attractant substance, toward which tendons would either grow or crawl or be pulled, and to which the tendons would selectively attach.
Can we invent testable predictions of this or alternative theories?

Chemotaxis in slime molds was first revealed by observations that cell aggregations formed exactly opposite each other when cultured on both surfaces of dialysis membranes (through which only comparatively small molecules can diffuse). A few years later, John Bonner proved chemotaxis even more conclusively by pumping water slowly across glass surfaces on which slime mold amoebae were aggregating. Those amoebae on the upstream side of the water flow did not aggregate, but aggregation did occur on the downstream side. Can you visualize why chemotaxis predicts that effect of flow?

Figure out how to discover sizes or other properties of chemotactic attractant substances using differences in water flow rates, and membranes that differ in the maximum sizes of molecules that can pass through them.

Visualize how to test theories about muscle and nerve guidance by implanting small thin plastic barriers into embryos, for example at locations through which sensory and motor nerves normally extent. For example, what could you learn from whether or not nerves turned to go around permeable and impermeable barriers.


back to index page