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[A morphological study on the coracobrachialis muscle].

M Koizumi

    Kaibogaku Zasshi. Journal of Anatomy
    |February 1, 1989
    PubMed
    Summary

    The musculocutaneous nerve

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    Area of Science:

    • Anatomy
    • Neuroscience
    • Morphology

    Background:

    • The musculocutaneous nerve (MC) typically pierces the coracobrachialis muscle (cbr).
    • Anatomical variations exist where the MC does not pierce the cbr, running instead alongside the median nerve.
    • The embryological and developmental origins of these variations remain incompletely understood.

    Purpose of the Study:

    • To investigate the anatomical basis for variations in the course of the musculocutaneous nerve relative to the coracobrachialis muscle.
    • To explore the hypothesis that the coracobrachialis is a composite muscle with dual innervation influencing the MC's path.
    • To correlate nerve branching patterns with the observed anatomical variations in humans and prosimians.

    Main Methods:

    • Dissection and observation of the musculocutaneous nerve and coracobrachialis muscle in 240 human arms.
    • Nerve fiber analysis of the MC and its branches to the cbr in 27 selected human arms.
    • Comparative analysis of nerve supply and muscle composition in 4 prosimian arms across 3 species.

    Main Results:

    • The human coracobrachialis muscle receives innervation from three distinct nerve groups: branches from the MC (Rmc), a deep branch from the brachial plexus (Rp), and a superficial branch (Rs) present in a minority of cases.
    • Variations in the MC's course are directly correlated with the relative contribution of Rmc and Rp innervation to the cbr. A higher Rmc to Rp ratio is associated with the MC piercing deeper into the cbr, while a lower ratio leads to a more superficial path or non-piercing course.
    • Prosimian coracobrachialis muscles exhibit a composite structure, supporting the dual innervation hypothesis.

    Conclusions:

    • The variability in the musculocutaneous nerve's path through the coracobrachialis muscle is explained by the muscle's composite nature and the differential innervation ratios between the MC branches (Rmc) and the deep brachial plexus branch (Rp).
    • This study provides a developmental and anatomical explanation for previously unexplained variations in human upper limb anatomy.
    • Comparative anatomy in prosimians supports the hypothesis of a composite coracobrachialis muscle with dual innervation influencing the musculocutaneous nerve's course.

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