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The muscles responsible for opening the jaw are known as the muscles of mastication. These muscles are associated with movements of the jaw (temporomandibular joint) and develop from the first pharyngeal arch. They are innervated by a branch of the trigeminal nerve (CN V), the mandibular nerve. The four primary muscles of mastication are the masseter, temporalis, medial pterygoid, and lateral pterygoid. The masseter and temporalis muscles are responsible for closing the jaw, while the medial pterygoid is responsible for closing and lateral movements. The lateral pterygoid is responsible for opening the jaw, as well as deviation to either side and anterior movement. Jaw opening is also assisted by accessory muscles, including the digastric, geniohyoid, mylohyoid, and others. These muscles coordinate the full integration of mandibular movement during opening and closing.
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What You'll Learn
Jaw opening is a ballistic movement
The muscles that open the jaw include the anterior digastric, mylohyoid, and inferior head of the lateral pterygoid. Jaw-closing muscles, on the other hand, include the masseter, temporalis, medial pterygoid, and superior head of the lateral pterygoid. The jaw-closing muscles are larger and have more motor units because they do most of the work during chewing.
The jaw-opening reflex serves a protective function. Unlike jaw-closing motor neurons, which are inhibited during the jaw-opening phase, jaw-opening motor neurons are not inhibited during jaw closing. This allows a protective jaw-opening reflex to be initiated by high-threshold nociceptors if, for example, the teeth slide off food and make contact with the soft tissues of the mouth.
Ballistic movements are observed in nature. For example, chameleons exhibit ballistic tongue projection, and trap-jaw ants have extremely rapid mandible strikes. In humans, ballistic movements include tongue protrusion and mastication or chewing, which is a rhythmical movement controlled by a central pattern generator in the brainstem.
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Muscles that open the jaw
The muscles of mastication are associated with movements of the jaw. There are four primary muscles of mastication: the masseter, temporalis, medial pterygoid, and lateral pterygoid. While the first three muscles are responsible for closing the jaw, the lateral pterygoid is responsible for opening the jaw, as well as deviation to either side and anterior movement of the jaw. The lateral pterygoid muscle has two heads: the superior and inferior. The inferior head of the lateral pterygoid, along with the anterior digastric and mylohyoid, are the muscles that depress the mandible and thus open the jaw.
The masseter muscle is the most powerful muscle of mastication. It is responsible for jaw-closing and can exert very high forces. Conversely, it can also exert very low forces with incredible precision, such as during speech. The temporalis muscle is also responsible for jaw-closing and is crucial for chewing as it raises the mandible and helps with the jaw's grinding and crushing actions.
The medial pterygoid muscle is responsible for closing and lateral movements of the mandible. It has two heads: the deep and superficial. The lateral pterygoid muscle, on the other hand, is responsible for opening the jaw, as well as side-to-side movement and anterior movement of the jaw. This muscle is the major protractor of the mandible due to its horizontally oriented muscle fibres.
In addition to the primary muscles of mastication, there are also accessory muscles of mastication that are directly or indirectly associated with mandibular function. The accessory muscles that are directly associated with mandibular function assist in jaw opening and closing through their attachments to the hyoid bone, the mandible, and other bones. These include the digastric, geniohyoid, mylohyoid, omohyoid, sternohyoid, sternothyroid, stylohyoid, and thyrohyoid muscles.
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Masticatory myalgia and myospasm
Masticatory muscle pain (MMP) is the primary cause of chronic non-odontogenic orofacial pain in humans, affecting about 12–14% of the adult population. MMP is typically felt in the face, jaws, and preauricular area, but can radiate to the ear, teeth, head, and neck. Masticatory myalgia is characterised by a dull persistent ache overlying the jaw and temple muscles, with occasional reference to other structures such as the head, neck, ear, and teeth. Symptoms can also include a restricted opening, fatigue, and stiffness. Signs include tenderness of the muscles with limited ROM. Localized myalgia presents as sore or aching muscles, usually affecting bilaterally at the region of masseter and temporalis muscles.
Masticatory myospasm, also known as muscle cramp, is an acute condition resulting from a sudden, involuntary, and continuous tonic contraction of the muscle. It is characterised by acute shortening of a single muscle and may lead to trismus (lockjaw), pain on movement, and a severely limited range of motion of the mandible. These characteristics, coupled with their sudden onset at rest, allow clinicians to differentiate myospasm from other masticatory muscle disorders. Myospasm is an acute local disorder, while myofascial pain and chronic centrally mediated myalgia are more chronic regional disorders.
The masseter, temporal muscle, sternocleidomastoid muscle, and trapezius muscle are all examples of muscles that tense in response to alterations in mental status, which may induce muscle pain related to mental disorders or chronic stress. Increased muscle tension can be a local result of trauma, physiological function or dysfunction, and a defensive response to psychological burden. The central mechanism of muscle pain development due to long-term overactivity mostly relates to those muscles that tense during psychological discomfort, anxiety, anger, bad mood, or stress.
The predominant complaint of patients with masticatory muscle disorders is myalgia, which is often reported as having a sudden onset and being recurrent. The pain originates in the muscles; therefore, any restriction of mandibular movement is due to extracapsular muscular pain. All masticatory muscle disorders are not the same clinically, and proper diagnosis is important as the treatment of each disorder differs. Some muscle disorders occur and resolve in a relatively short period of time, but when these conditions are not resolved, more chronic pain disorders may result. With time, the central nervous system (CNS) can play an important role in maintaining the muscle disorder.
The mandible rotates forward during opening, and the temporomandibular joint allows lateral movements. Muscles that depress the mandible and thus open the jaw include the anterior digastric, mylohyoid, and inferior head of the lateral pterygoid. Jaw-closing muscles consist of the masseter, temporalis, medial pterygoid, and superior head of the lateral pterygoid. Jaw opening is a relatively ballistic movement that is not against a load and, consequently, relies on myosin heavy-chain isoforms with fast conduction and fatigue-resistant qualities. The pterygoid muscles and the cranial belly of the digastricus muscle are located on the medial side of the mandible and are not readily palpable.
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Jaw-opening reflex
The jaw-opening reflex (JO reflex) is a disynaptic reflex triggered by specific orofacial stimuli, such as tapping on the lips, teeth, or palate, or noxious stimulation of the face, intraoral tissues, muscles, and joints. This reflex is the V equivalent of the flexion-withdrawal reflex of the limbs. In humans, the mandible rotates forward during opening, and the temporomandibular joint allows for lateral movements as well. Muscles that depress the mandible and open the jaw include the anterior digastric, mylohyoid, and inferior head of the lateral pterygoid. The pterygoid muscles, including the lateral pterygoid, are located on the medial side of the mandible and are devoted to opening the jaw.
The jaw-opening reflex serves a different function than the jaw-closing reflex. Unlike jaw-closing motor neurons, which are centrally inhibited during the jaw-opening phase of mastication, jaw-opening motor neurons are not subject to inhibition during jaw closing. This lack of inhibition allows for a protective jaw-opening reflex to be initiated by high-threshold nociceptors. For example, if the sharp teeth slide off a piece of food and make contact with the soft tissues of the mouth, the jaw-opening reflex can be activated to prevent injury.
The jaw-closing reflex (JC reflex) is the myotatic (stretch) reflex of the JC muscles, where the stretch of muscle spindle afferents activates JC motor neurons through monosynaptic and polysynaptic pathways. The JC reflex provides rapid compensation for changes in food resistance during chewing and modulates JC muscle tension in response to changes in gravitational load on the mandible during locomotion. The jaw-jerk reflex, a type of JC reflex, is used to judge the integrity of the upper motor neurons projecting to the trigeminal motor nucleus. It is elicited by a mechanical tap over the mandible with a tendon reflex hammer and can be used to differentiate pathologies such as cervical spondylotic myelopathy from multiple sclerosis (MS) or amyotrophic lateral sclerosis (ALS).
The jaw-opening reflex is typically not part of a standard neurological examination and is performed when there are other signs of damage to the trigeminal nerve. Studies have shown that the jaw jerk reflex is influenced by age and sex, with a general increase in latency observed with increasing age and higher amplitude impulses observed in females compared to males.
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Lateral pterygoid muscle
The lateral pterygoid muscle is a craniomandibular muscle that plays a crucial role in the inferior temporal region. It is one of the four muscles of mastication, along with the medial pterygoid, temporalis, and masseter muscles. All these muscles act upon the temporomandibular joint (TMJ) to enable chewing (mastication) and biting. The lateral pterygoid muscle is located deep to the temporalis and masseter muscles, spanning between the sphenoid bone and temporomandibular joint. Its muscle belly is separated by a small horizontal fissure into two heads: the superior (upper) and inferior (lower). The superior head is formed by the most superomedial fibres of the muscle, while the inferior head is much wider and originates from the lateral surface of the lateral pterygoid plate of the sphenoid bone.
The lateral pterygoid muscle is supplied by the lateral pterygoid nerve, a branch of the mandibular nerve (CN V3), itself a branch of the trigeminal nerve (CN V). The primary function of the lateral pterygoid muscle is to pull the head of the condyle out of the mandibular fossa along the articular eminence to protrude the mandible. A unilateral action of the lateral pterygoid muscle causes contralateral excursion, a form of mastication usually performed in concert with the medial pterygoids. When they work independently, they can move the mandible side to side. Unlike the other three muscles of mastication, the lateral pterygoid alone can assist in depressing the mandible (opening the jaw). When both bellies of the lateral pterygoid muscle contract bilaterally, the mandible travels anteriorly (protrusion).
The lateral pterygoid muscle is also involved in the generation of horizontal forces during mastication and parafunctional activities. It is active during mandibular movements, including protrusion (forward movement of the mandible), abduction (depression of the mandible), and mediotrusion (mandibular condyle movement towards the midline). The muscle works particularly during speaking, singing, and clenching. The lateral pterygoid muscle is the only one among the masticatory muscles with horizontally arranged fibres. It exhibits a range of mediolateral and superoinferior movements.
The lateral pterygoid muscle is located on the medial side of the mandible, along with the pterygoid muscles and the cranial belly of the digastricus muscle. These muscles are not easily palpable, but atrophy of the pterygoids on one side may be evident as relative deepness of the supraorbital fossa on the same side. The lateral pterygoid muscle comprises the medial wall of the infratemporal fossa and is contained within the masticator space. This space is bounded by the superficial layer of the deep cervical fascia.
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Frequently asked questions
The lateral pterygoid muscle is responsible for opening the jaw, as well as deviation to either side and anterior movement.
Jaw opening is assisted by sets of paired muscles, including the digastric, geniohyoid, mylohyoid, omohyoid, sternohyoid, sternothyroid, stylohyoid, and thyrohyoid muscles.
The lateral pterygoid muscle has a triangular shape with two heads: the superior and inferior. It has horizontally oriented muscle fibres, making it the major protractor of the mandible.
The trigeminal nerve transmits signals from the brain to the lateral pterygoid muscle, regulating its contraction and relaxation.
Yes, certain conditions can impact jaw opening. For example, a lateral pterygoid muscle spasm can cause pain and difficulty opening the mouth. Similarly, temporalis muscle atrophy can weaken the jaw-closing ability, leading to challenges in chewing and potentially opening the jaw.
