Digastric muscle
n., plural: digastric muscles
[daɪˈɡæstrɪk ˈmʌ.sl̩]
Definition: a muscle with two fleshy bellies separated by a fibrous insertion
Table of Contents
Digastric Definition
The digastric muscle is a paired muscle located under the jaw, consisting of the anterior and posterior “bellies”. It aids in jaw movement and swallowing. Dysfunction of the digastric muscle can lead to issues such as difficulty swallowing or speaking.
The digastric muscle (also digastricus) is a tiny muscle situated beneath the mouth. It is so named because it has two “bellies”. Thus, this specific muscle is referred to as the “digastric muscle”.
Other muscles with two distinct muscle bellies include the suspensory muscle of the duodenum, the occipitofrontal, and the omohyoid.
One further piece of information regarding the digastric muscle may be discovered in its very name. This muscle’s name comes from the Greek word “dis”, which means double or twofold, and the Latin word “gaster,” which means belly. Together, these words provide a perfect description of the structure of this muscle, which consists of two muscle bellies.
The suprahyoid aponeurosis is a wide layer of connective tissue that comes from the tendon of the digastric muscle on each side and connects to the body and the larger cornu of the hyoid bone.
The structure of a person’s digastric muscle could be different from the norm. This is called an atypical or abnormal digastric muscle. These differences can be present from birth.
Digastric Muscle Anatomy |
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| Origin | The anterior belly arises from the digastric fossa of the mandible, close to the midline
In temporal bone, the notch of the mastoid process forms the posterior belly |
| Insertion | It inserts on the hyoid bone via the intermediate tendon |
| Nerve Supply | The mandibular division of the trigeminal nerve is responsible for providing nerve supply to the anterior belly.
Sometimes both the facial and mylohyoid nerve supply the anterior belly as well. The facial nerve supplied the posterior belly |
| Blood Supply | The blood supply to the anterior belly is from the facial artery which is the branch of the external carotid artery.
The posterior auricular artery, which ends by anastomosing with the occipital artery, is responsible for the blood supply to the posterior belly i.e.: |
| Functions |
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Data source: Shoaib Zaheer of Biology Online
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A digastric muscle is one of the suprahyoid group of muscles consisting of two bellies united by a central tendon which is connected to the body of the hyoid bone.
- Origin: by posterior belly from the digastric groove medial to the mastoid process.
- Insertion: by anterior belly into the lower border of the mandible near the midline.
- Action: elevates the hyoid when the mandible is fixed; depresses the mandible when the hyoid is fixed.
- Nerve supply: posterior belly from the facial, anterior belly by the nerve to the mylohyoid from the mandibular division of trigeminal
Etymology: from the Greek word “dis”, which means double or twofold, and the Latin word “gaster,” which means belly.
Synonym: musculus digastricus; biventer mandibulae; musculus biventer mandibulae; two-bellied muscle.
Structure
The digastric muscle divides into two muscular bellies joined by a circular tendon in the middle.
Separate cranial nerves supply both anterior as well as posterior belly of the digastric muscle and they also have distinct embryological origins.
Despite every individual possessing both left and right digastric muscles, the singular form of the term is frequently utilized in anatomical discourse to refer to the muscle. This is similar to how the term “deltoid” is used even though each shoulder only has one deltoid muscle. Consequently, the term “digastric” is also used to refer to both the right and left digastric muscles.
The internal jugular vein does not pass through the digastric muscle, but it does run near the neck. The spinal accessory nerve (CN XI) does not pass through the digastric muscle, but it does innervate it.
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Posterior belly
The posterior belly of the digastric muscle comes from the mastoid process of the temporal bone. which is a prominent bony projection located behind the ear. The muscle then descends inferiorly and anteriorly, passing through the carotid triangle and attaching to the intermediate tendon.
In the middle of its path from the mastoid process of the skull to the mandible at the chin, the digastric muscle transforms into a tendon and travels through a tendinous pulley that is linked to the hyoid bone.
The digastric muscle is attached to the hyoid bone by a tendon that is formed by the fusion of the two muscular bellies of the muscle.
The development of the digastric muscle can be traced back to the second pharyngeal arch, which is one of the six embryonic structures that give rise to the various components of the head and neck region.
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Anterior belly
The digastric fossa of the mandible is a depression that is present on the inner side of the lower border of the jaw. This depression is near the symphysis and gives rise to the anterior bellies. The anterior belly then proceeds downward and backward.
The anterior body receives its supply of nerve fibers from the trigeminal nerve through the mylohyoid nerve. The mylohyoid nerve is a branch of the inferior alveolar nerve. Its origin may be traced back to the first arch of the pharynx.
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Intermediate tendon
The intermediate tendon of the digastric muscle is a strong, fibrous band that is crucial to the proper function of the muscle. It acts as a fulcrum and allows both bellies of the digastric muscle to work together to elevate the hyoid bone and depress the mandible during various activities such as swallowing and speaking.
The intermediate tendon of the digastric muscle is a strong and flexible structure that runs through the hyoid bone’s hyoid fascia, a fibrous connective tissue that forms a loop in the neck.
The intermediate tendon plays a role in maintaining the position of the hyoid bone and stabilizing it during movements of the head and neck.
Damage to the intermediate tendon of the digastric muscle can result in a range of symptoms, including trouble swallowing, hoarseness, and neck or jaw pain. These injuries can be the result of muscular damage or stress, as well as underlying medical problems such as malignancies or infections.
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Variations
There are a lot of different variations.
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- The anterior belly can sometimes be duplicated, or it may give rise to extra slips that extend toward the jaw or mylohyoid muscle. It can also cross over and connect with a corresponding slip on the other side.
- The anterior belly might be absent, while the posterior belly attaches to either the hyoid bone or the center of the jaw.
- Sometimes the tendon will go in front of the stylohyoid muscle, and only very rarely will it go behind it. From the body of the hyoid bone to the chin is where the mentohyoideus muscle may be found.
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Triangles
The digastric muscle forms different triangles, which are explained below:
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- Submandibular or Digastric triangle: It is a small triangle present in the lower part of the neck, just below the mandible (jawbone). This triangle is formed by the anterior belly and the posterior belly of the digastric muscle, and the lower border of the mandible. Each submandibular triangle contains the following: the submandibular gland, submandibular lymph nodes, hypoglossal nerve, facial artery, and facial vein. (Kim & Loukas, 2019)
- Superior carotid or Carotid triangle: It is bounded by the sternocleidomastoid, the posterior bellies of the digastric, and the anterior border of the trapezius muscle.
- Submental triangle: It is present in the middle of the neck, just below the chin. On the lateral side, it has the anterior belly of the digastric muscle, medially it is limited by the middle line of the neck, and the hyoid bone inferiorly.
- Inferior carotid or muscular triangle: This triangle is present in the anterior part of the neck. It is bordered by the sternocleidomastoid muscle, the omohyoid muscle, and the anterior border of the trapezius muscle.
Referral Pattern for Trigger Points
A myofascial trigger point (MTrP) can develop in any of the anterior or posterior bellies of the digastric muscle, which can cause pain. The referred pain from an MTrP in the posterior belly may radiate to the area above the mastoid process, as well as to the throat and under the chin. (Physiopedia, 2023)
Palpation
Due to the muscle’s location, conducting an external examination of the posterior digastric muscle can be challenging. To examine the anterior digastric muscle, begin by identifying the hyoid’s lateral borders. Next, use your thumbs to palpate the jaw’s inferior surface on either side of the midline, which will enable you to assess the anterior digastric muscle.
During the examination, the patient is requested to swallow to determine the anterior digastric’s location. While the hyoid is lifted upwards, a protrusion of the anterior belly can be felt beneath the fingertips to pinpoint the exact position of the anterior digastric muscle. (Physiopedia, 2023)
Functions
The digastric muscle functions in the body are as follows:
- It is principally accountable for mouth opening by lowering the jaw, which occurs through the contraction of its posterior belly. The digastric muscle anterior belly assists in the elevation of s the hyoid bone during swallowing and speech production, and it also assists in pulling the tongue backward.
- In addition to its role in mandibular depression and hyoid bone elevation, the digastric muscle also plays a crucial role in stabilizing the hyoid bone and the larynx during swallowing and speech. It is also involved in the process of chewing and biting by keeping the lower jaw stabilized and fixed in position.
- The suprahyoid muscles and digastric muscles also work together to aid in speech production.
Other Animals
Although the digastric muscles are present in a wide range of other animals as well, including mammals, reptiles, and birds. The precise attachment locations might differ from one species to another.
- In mammals, the digastric muscle is generally similar to that of humans, consisting of two bellies that originate from different areas and join together to attach to the hyoid bone.
- In reptiles, the digastric muscle is similarly involved in jaw movement and swallowing. However, it is typically less well-developed than in mammals, as reptiles generally have simpler jaw structures and do not require as much muscular power to open their mouths.
- In birds, the digastric muscle is present but is modified to accommodate their unique anatomy. The muscle in birds is indeed divided into three parts: the anterior, posterior, and intermediate bellies. The anterior and posterior bellies function similarly to those in mammals, while the intermediate belly extends from the posterior belly to attach to the sternum. The digastric muscle is important for opening the beak and for swallowing. However, it is much more complex than in mammals, with multiple bellies that are arranged in a complex pattern around the skull. This modification also allows birds to elevate their sternum during breathing and flying.
Note it!
Question: What can cause dysfunction of the digastric muscle?
Answer: Dysfunction of the digastric muscle can be caused by injury, infection, or neurological disorders that affect the trigeminal nerve. Additionally, overuse of the muscle from activities such as excessive chewing or singing can also lead to dysfunction.
Take the Digastric muscle – Biology Quiz!
References
- Kenhub. (2023). Digastric muscle Retrieved 27 March 2023, from https://www.kenhub.com/en/library/anatomy/digastric-muscle
- Physiopedia. (2023). Digastric Muscle. Retrieved 27 March 2023 from https://www.physio-pedia.com/Digastric_Muscle
- Kim, S. D., & Loukas, M. (2019). Anatomy and variations of digastric muscle. Anatomy & Cell Biology, 52(1), 1. https://doi.org/10.5115/acb.2019.52.1.1
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