The motor fields of individual segmental nerves have been mapped electro-physiologically in the longitudinal muscle of the earthworm, Lumbricus terrestris. The anterior pair of segmental nerves (SN I) innervates approximately the anterior two-thirds of its segment and a small portion of the segment just anterior to it. The posterior pair of segmental nerves (SNII-III) innervates approximately the posterior two-thirds of its segment and a small portion of the segment just posterior to it. Adjacent nerves (both intrasegmental and intersegmental) have partially overlapping motor fields; that is, adjacent segmental nerves innervate some of the same longitudinal muscle fibres.
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Drewes et al. (1974) studied this question.
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