Dominic Stone
The muscles that close the jaw are known as the muscles of mastication. These include the masseter, temporalis, and internal and external pterygoids. The supra and infra-hyoid muscles also play a role in jaw movement. The trigeminal nerve supplies these muscles, and they are responsible for most of the work associated with mastication or chewing. The power stroke of chewing is a forward and medial movement, similar to a file being moved across a metal surface.
| Characteristics | Values |
|---|---|
| Main muscles that close the jaw | Masseter, Temporalis, Internal and External Pterygoids |
| Other muscles that close the jaw | Supra and Infra-hyoid muscles |
| Muscle pain disorders | Masticatory Myofascial pain, Masticatory Myalgia, Myospasm |
| Inflammatory condition of the muscles | Myositis |
| Involuntary, aimless, repetitive oral activity | Bruxism |
| Imbalance of forces within the muscles | Temporomandibular Joint (TMJ) dysfunction |
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What You'll Learn
The masseter, temporalis, and pterygoid muscles close the jaw
The masseter, temporalis, and pterygoid muscles are responsible for closing the jaw. These muscles work together to produce the powerful forces necessary for chewing and other jaw movements.
The masseter muscle is a major muscle of mastication, originating from the zygomatic arch and inserting at the mandible. It is a strong muscle that raises the mandible and aids in the lateral movement of the jaw. The masseter can be easily assessed by having the patient clench their jaw and evaluating the volume and firmness of the muscle.
The temporalis muscle is another important muscle involved in jaw closure. It originates from the temporal bone and inserts at the coronoid process of the mandible. The temporalis muscle is also active during chewing and plays a role in the forward and lateral movements of the jaw.
The pterygoid muscles, including the internal and external pterygoids, are smaller muscles located on the medial side of the mandible. They are not easily palpable but are crucial for jaw function. The internal pterygoid opposes the action of the deep masseter, turning the lower edges of the mandible inwards.
These muscles work in coordination with other muscles and nerves to produce the complex movements of the jaw. For example, during mastication, the jaw-closer and jaw-opener motor neurons receive alternating waves of excitatory input, allowing for the smooth and rapid opening and closing of the jaw.
Disorders of these muscles, such as myofascial pain, myalgia, and myospasm, can result in symptoms like jaw pain, restricted opening, fatigue, and stiffness. Bruxism, or teeth grinding, is another condition associated with the dysfunction of these jaw-closing muscles and can lead to tooth wear, temporomandibular joint issues, and headaches.
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The trigeminal nerve controls the muscles of mastication
The trigeminal nerve, also known as the fifth cranial nerve, is responsible for sensation in the face and motor functions such as biting and chewing. It is the most complex of the cranial nerves and carries sensory information from the face and body through parallel pathways in the central nervous system. The trigeminal nerve has three major branches: the ophthalmic nerve (V1), the maxillary nerve (V2), and the mandibular nerve (V3). The ophthalmic and maxillary nerves are purely sensory, while the mandibular nerve supplies motor as well as sensory functions.
The mandibular division of the trigeminal nerve controls the movement of eight muscles, including the four muscles of mastication: the masseter, the temporal muscle, and the medial and lateral pterygoids. The other four muscles are the tensor veli palatini, the mylohyoid, the anterior belly of the digastric, and the tensor tympani. With the exception of the tensor tympani, all these muscles are involved in biting, chewing, and swallowing.
The masseter muscle is a rectangular muscle that covers most of the lateral aspect of the ramus. It consists of three layers: the superficial, intermediate, and deep layers. The fibres of the muscle originate from the inferior zygomatic arch and the anterior two-thirds of the zygomatic arch, connecting to the posterior aspect of the zygomatic bone. The masseter functions to elevate and approximate the teeth, with the intermediate and deep muscle fibres retracting the mandible and the superficial fibres protruding it.
The pterygoids can be examined by having the patient move the jaw from side to side against resistance and protrude the jaw. Contraction of each muscle causes deviation of the jaw to the opposite side, so weakness in the pterygoid muscles will result in deviation of the open jaw to the ipsilateral side. The lateral pterygoid muscle is the sole muscle of mastication that causes depression of the mandible, although this is largely due to gravity. It also assists with protrusion and side-to-side movement of the mandible.
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Bruxism is the involuntary clenching or grinding of teeth
Bruxism is a condition characterised by the involuntary and repetitive clenching, grinding, or gnashing of teeth. It can occur during sleep or while awake, with sleep bruxism being more widely studied and wakeful bruxism being more common. The forces exerted on the teeth and jaw during bruxism are strong enough to cause tooth wear and destruction, as well as temporomandibular joint (TMJ) dysfunction and associated pain.
Bruxism can be caused by various factors, including stress, anxiety, sleep disorders, and certain medications. It is often associated with psychological, neurological, and orodental conditions. The condition can lead to several symptoms, such as tooth erosion, headaches, jaw pain, and TMJ disorders.
As bruxism can have negative consequences for oral health, it is important to recognise and address the condition. Treatment options include the use of custom mouth guards and botulinum toxin injections to relax the jaw muscles and reduce pain. Lifestyle changes, such as reducing caffeine and alcohol consumption and quitting smoking, can also help manage the condition. Additionally, stress management techniques like meditation, exercise, and cognitive behavioural therapy may reduce bruxism symptoms.
While occasional teeth grinding may not require intervention, frequent and prolonged bruxism can lead to health issues such as cracked teeth, headaches, and TMJ disorders. Therefore, it is advisable to consult a healthcare provider for a diagnosis and appropriate treatment plan if symptoms of bruxism persist or occur regularly. Regular dental exams are also recommended to monitor for any signs of bruxism-related damage.
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Masticatory myofascial pain is characterised by a dull, aching pain
The masseter and temporalis are the two muscles that close the jaw. Masticatory myofascial pain is characterised by a dull, aching pain in the jaw and temple muscles, which can also affect other structures such as the head, neck, ear, and teeth. This pain is usually persistent and can be accompanied by a restricted opening, fatigue, and stiffness. It is a type of muscle pain disorder that is similar to other skeletal muscle disorders found in other parts of the body.
Masticatory myofascial pain is often associated with trigger points, which are small bumps, nodules, or knots in the muscle that cause pain when touched and sometimes even when they are not. These trigger points can be identified by applying pressure to the muscle, which is known as palpation. Palpation is performed using a 2-second sustained pressure of 2 kg of force. The presence of these trigger points can lead to referred pain, which increases during muscle function.
The diagnosis of myofascial pain (MFP) is given when there are one or two taut bands of muscles identified within the affected muscle, and painful trigger points are found within these bands. This condition can affect multiple muscles in the body, including the masticatory and cervical muscles. It is important to note that myofascial pain syndrome is different from fibromyalgia, although the two conditions are often confused.
Masticatory myofascial pain can be assessed by evaluating the volume and firmness of the masseter muscle when the patient clenches their jaw. Additionally, the pterygoid muscles, which are also involved in jaw movement, can be examined by having the patient move their jaw from side to side against resistance and protrude their jaw.
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Myospasm is an acute, involuntary muscle contraction
The masseter and pterygoid muscles are responsible for closing the jaw. Myospasm, or muscle cramp, is an acute, involuntary, and continuous tonic contraction of the muscle. It is a type of muscle spasm that can occur in any muscle of the body, including the jaw.
Myospasm is characterised by a sudden, involuntary, and continuous muscle contraction that does not relax. This can result in a painful tightness in the affected muscle, which can last for just a few seconds or up to several minutes. In some cases, myospasm may be a sign of an underlying health condition, such as neurological or thyroid-related problems. However, it is important to note that muscle spasms are very common and usually not a cause for concern.
The jaw, or mandible, is controlled by a group of muscles called the muscles of mastication. These muscles include the masseter and the pterygoids. When these muscles contract, they cause the jaw to close. Myospasm in the masseter and pterygoid muscles can lead to acute jaw closure, resulting in a sudden, involuntary closing of the jaw.
Myospasm in the jaw can be treated through various methods such as muscle relaxants, stretching, and massage. Applying heat or ice to the affected area may also help relieve the symptoms. In some cases, a healthcare provider may prescribe muscle relaxers to ease severe or frequent myospasm symptoms. However, these medications can have side effects such as drowsiness, dizziness, and nausea.
To prevent myospasm in the jaw, individuals can perform flexibility exercises, maintain proper jaw alignment, and avoid teeth clenching or grinding (bruxism). Bruxism is a common condition characterised by involuntary, aimless, and repetitive oral activity that can lead to tooth wear, temporomandibular joint dysfunction, and muscle pain. Maintaining good oral hygiene and seeking professional advice for jaw or tooth pain can also help prevent myospasm in the jaw.
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Frequently asked questions
The muscles that close the jaw are the masseter, temporalis, and the internal and external pterygoids.
The masseter can be tested by having the patient clench their jaw and evaluating the volume and firmness of the muscles.
Masticatory myofascial pain is a dull, persistent ache overlying the jaw and temple muscles. It is similar to other skeletal muscle disorders in other parts of the body.
The deep masseter raises the mandible and everts (turns outwards) the lower edges of the mandible.
