Double innervation of the anterior belly of the digastric muscle

Abstract

We came across a very rare case in which the anterior belly of the digastric muscle was innervated by the twigs of the facial nerve in addition to those of the mylohyoid nerve. The anomaly was discovered in the cadaver of an 84-year-old Japanese male bequeathed for a training seminar in gross anatomy at Kumamoto University in 2003. One twig issued from the marginal mandibular branch of the facial nerve and entered the central region of the anterior belly of the digastric muscle on the lower surface. The other twig issued from the stylohyoid branch of the facial nerve, descended along the lateral margin of the stylohyoid muscle and entered the anterior belly of the digastric muscle on the lower surface near the intermediate tendon. The twig from the marginal mandibular branch was distributed to the shallow (lower) and central region near the medial margin of the anterior belly. The twig from the stylohyoid branch was distributed to the shallow and lateral region of the anterior belly. These two twigs communicated with the mylohyoid nerve at several peripheral parts. Textbooks on general anatomy make mention of only one nerve, the mylohyoid, supplying the anterior belly of the digastric muscle. However, the present case manifests that the anterior belly receiving twigs from the mylohyoid and facial nerves is formed with the second brachial component as well as the first.

Anatomy and variations of digastric muscle

“…The mylohyoid nerve of the inferior alveolar nerve of the mandibular nerve of the trigeminal nerve innervates the anterior belly of the digastric muscle, and the facial nerve innervates the posterior belly of the digastric muscle [10]. However, there are some reported cases that the anterior belly of the digastric muscle was innervated by the branches of the facial nerve in addition to the mylohyoid nerve [1415]. Furthermore, Kawai et al [15] observed that when the branches of the facial nerve innervate the anterior belly of the digastric muscle, they seemed to be specifically the branches of the stylohyoid nerve.…”