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In anatomy and physiology, there is a muscle in the muscular system termed the depressor anguli oris. This muscle sits near the mandible, or jaw, muscle, and is inserted into the angle of the mouth. It can also be located by looking at the side of the jaw, as the muscle extends from the jaw and continues to connect to the lips and sides of the mouth. The depressor anguli oris aids in producing a frowning expression on the face.
The nerve which surrounds the depressor anguli oris is called the mandibular branch of the facial nerve, which also passes through the platysma and the triangularis. This nerve includes the muscles of the chin and lip which communicate with each other for proper functioning and movement. The facial nerve contains thousands of neurons which fire and send messages to allow for facial movement of the chin and mandible.
Surrounding the depressor anguli oris are blood vessels which bring nutrients to the lip muscles for proper functioning. Oxygen and various other nutrients, such as protein, are carried through the blood vessels and into the depressor region. Without an adequate supply of nutrients passing through these vessels on a daily basis, the lip muscles and mouth area would become completely unmovable or would only function in certain ways without their full extent or range of motion. When a muscle isn't used due to the lack of nutrient absorption and assimilation, muscle atrophy may occur and the muscle may waste. As in the case of the depressor anguli oris, however, nutrients seem to flow through the vessels into the muscle easily.
In order for the depressor anguli oris to work and contract the lip muscles, it must first be activated by a neuron. A neuron is a tiny, electrically charged cell which contains messages and commands. In the case of muscle contraction, a group of neurons are affected by a message sent by the brain to move a certain area of the body. When it is desired to frown, the brain sends an electrical current through the neurons and into the depressor anguli oris to promote the angles of the mouth to turn downward. Without neurons and their functions, the depressor anguli oris would not perform its own function properly.