Special Sub-Topic: Eat Up! - Muscles of Mastication
|The muscles of mastication are derived from the first branchial arch in the embryo; which cranial nerve supplies them?|
V (trigeminal). While most of the other facial muscles are supplied by the facial nerve (VII), the four muscles of mastication are supplied by the trigeminal nerve (V), in particular by the mandibular branch of the trigeminal (V3). This branch is the final branch of the trigeminal, and is the largest branch. As well as the muscles of mastication, this branch supplies other structures in the lower face, including the teeth and the inside wall of the cheek.
|All four muscles of mastication attach the mandible (lower jaw).|
t. This makes sense as the mandible is essential in chewing food - in fact, it is the only bone that moves during mastication! The different attachments by each muscle provide different movements of the jaw - up and down movement, and side-to-side movement.
|Which of the four muscles of mastication arises from the zygomatic arch and inserts on the ramus of the mandible?|
Masseter. Masseter is a quadrilateral muscle, consisting of both superficial and deep fibers - despite this, it is considered a superficial muscle of mastication, the superficial part being much larger. Its insertion point, the ramus of the mandible, can be palpated in the living; follow your jaw line with your fingers from your chin (called the mental protuberance) to the back of the jaw. That square corner you feel is called the angle, and the ramus is the flat part of the mandible above and around the angle.
|The lateral and medial pterygoid muscles arise from the lateral and medial pterygoid plates respectively. Which unpaired skull bone are these plates part of?|
Sphenoid. The sphenoid bone sits at the base of the skull, and is shaped rather like a butterfly (it has greater and lesser wings). There is a pterygoid process on each side, each containing a lateral and medial pterygoid plate (four plates in total). This is where the pterygoid muscles arise. As well as being the origin of these muscles, the sphenoid bone also forms part of the orbit (eye socket).
|This muscle consists of both vertical and horizontal fibers. Its vertical fibers close the mandible, while the horizontal fibers retract the jaw back. Which of these is it?|
Temporalis. Temporalis (also called the temporal muscle) is very large and spans the side of the head. It can be felt in the living by placing the fingers over the temple, and clenching and unclenching the jaw. It is covered by a sheet of connective tissue, called the temporal fascia (or temporal aponeurosis). The muscle arises from the temporal fossa, which is a large but shallow depression on the side of the skull. It inserts onto the coronoid process of the mandible - from a side view, the top of the mandible is shaped rather like a capital "M", with two processes pointing upward. The anterior process (left in the "M") is the coronoid process, while the posterior (right in the "M") is called the condyloid process.
|Although not strictly a muscle of mastication, which two-bellied muscle helps open the jaw due to its attachment from below?|
Digastric. The phrase "two-bellied" may have given you a hint, as this is what "digastric" means. There is a digastric muscle on each side underneath the jaw, each with an anterior belly and a posterior belly linked in the middle by tendon. This tendon attaches the hyoid bone in the throat - upon contraction, digastric raises the hyoid bone. If the hyoid bone is held still (by other surrounding muscles), digastric's contraction causes a lowering of the mandible, thus opening the jaw. Other muscles underneath the jaw, such as geniohyoid and mylohyoid have a similar action on the jaw.
From the incorrect answers: "hyoid" is of course a bone, and not a muscle at all, and I just made up "lower pterygoid".
|The movements of the mandible are up and down, and side-to-side. Animals with different feeding strategies are specialized for different movements. For which group (out of omnivores, herbivores, and carnivores) is the up and down movement particularly specialized?|
Carnivores & Carnivore & C. Carnivores have a diet of primarily meat. They have very pronounced canine teeth for tearing up their food, but don't actually need to chew on it very much, so the up and down movement serves them best, as they need to have strong muscles to chomp down on their prey. They don't have much of a need for side-to-side movements, as they frequently swallow quite large chunks of meat without chewing much.
Herbivores on the other hand eat primarily plant material, and need to chew the tough matter before swallowing. Their jaws move side-to-side to allow them to do this sufficiently. They don't have a need to grasp prey in their mouths, thus they have no need for an up and down movement.
Omnivores eat both plant and animal matter, so their jaws allow both up and down, and side-to-side movements. However, these movements are not as specialized in the omnivore as they are in the carnivore and herbivore. Humans are omnivores, thus you can quite easily move your jaw around in these movements.
|Which muscle(s) of mastication is/are not involved in closing the mandible?|
Lateral pterygoid. Only the lateral pterygoid is not involved in closing the mandible. It actually has the opposite effect of opening the jaw - it attaches to the neck of the condyloid process. As it contracts, the lateral pterygoid pulls the condyloid process forward where it comes against the articular eminence (part of the temporal bone of the skull). As the condyloid process cannot move forward due to the articular eminence, it actually moves downward, depressing the mandible thus opening the jaw.
|The medial pterygoid follows the same angle as masseter - arising from the medial pterygoid plate, where does it attach on the mandible?|
Ramus and angle. As it follows the same angle as masseter, the medial pterygoid has the same action as masseter, i.e. closing the mandible. It is also involved in side-to-side movement of the jaw - by contracting both the lateral and medial pterygoids on one side, the jaw is pulled to that side. You can "wiggle" your jaw by contracting the pterygoids on one side alternatively with those on the other side.
|Small and large movements in chewing are made along a different horizontal axis through the mandible - this is because otherwise vital structures such as the inferior alveolar nerve and artery could be torn on the bone. Which part of the mandible do these structures pass through?|
Mandibular foramen. The axis of small movements is between the two heads of the condyloid processes on each side of the mandible. During large movements, this axis moves further down in the ramus of the mandible - this is where the mandibular foramen is situated. The inferior alveolar artery and nerve pass into the mandible through this foramen, and exit via the mental foramen, which is situated on the anterior of the mandible, at the side of the chin.
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