Temporomandibular and Body Joints: Anatomy
Temporomandibular Joint (TMJ)
The temporomandibular joint (TMJ) is the single mobile articulation of the head. It is a synovial joint with an articular disc made of fibrous tissue. It allows for the following movements:
- Descent and ascent (lowering and raising the jaw)
- Occlusal contact of the two dental arches
- Protrusion (jaw moves forward)
- Retraction (jaw moves backward)
- Lateralization (jaw moves left or right)
- Circumduction (a combination of all the previous movements)
Muscles of Mastication
The muscles responsible for chewing are:
- Masseter muscle: Originates from the zygomatic arch and inserts into the body of the mandible. Its action is to lift the jaw.
- Internal Pterygoid: Originates from the pterygoid apophysis and inserts into the inner angle of the jaw. Its action is to lift the jaw.
- Temporalis: Originates from the temporal fossa and inserts into the coronoid apophysis.
These three muscles contribute to dental arch occlusion (the union of the two dental arches) by lifting the jaw.
- Digastric: It has two bellies. The anterior belly originates from the lower jaw, and the posterior belly originates from the mastoid apophysis. It inserts into the hyoid bone. Its action is hyoid elevation or depression of the mandible.
- Geniohyoid: Originates from the internal part of the jaw and inserts into the hyoid bone. Its action is to lower the jaw.
Muscles of Facial Expression
These muscles are innervated by the facial nerve (VII cranial nerve):
- Occipitofrontalis: Covers the entire cranial vault and has a fibrous band.
- Orbicularis Oculi: Surrounds the outside of the orbits.
- Muscles of the nose: Open and close the nostrils.
Muscles of the Neck
- Platysma: A superficial muscle layer that covers the anteromedial and lateral regions of the neck. Its action is to depress the jaw and lower lip.
- Scalenus Medius: Originates from the transverse apophyses of the cervical vertebrae and inserts into the first rib. Its function is the elevation of the 1st rib and rib cage during inspiration.
- Sternocleidomastoid: Originates from the mastoid apophysis and inserts into the clavicle and sternum. Its function is the lateralization of the head.
Joints of the Body
- Atlanto-occipital Joint: A diarthrosis condyloid joint between the occipital bone and the atlas (C1 vertebra), allowing for flexion and extension of the head.
- Atlantoaxial Joint: Consists of two points of articulation between the atlas (C1) and the axis (C2): between the articular apophyses (a diarthrosis arthrodia joint) and between the arch of the atlas and the odontoid process of the axis (a diarthrosis trochoid joint). This allows for rotational movements of the head.
- Intervertebral Joints (between articular processes): Diarthrosis arthrodia joints, allowing for turning, flexion, and extension of the spine.
- Costovertebral Joints: Both the articulation between the vertebral body and the head of the rib (costovertebral joint) and between the transverse process and the rib tubercle (costotransverse joint) are diarthrosis arthrodia joints.
- Intervertebral Joints (between vertebral bodies): Semi-mobile joints (amphiarthrosis) that are larger in the lumbar region. These joints are important for the support function of the spine.
- Sacroiliac Joints: Synovial joints between the sacrum and the ilium. They have minimal mobility due to their irregular surfaces and strong ligaments. The articular surfaces are covered with hyaline cartilage.
- Pubic Symphysis: A cartilaginous joint between the two superior pubic rami.
Movements of the Spine
The movements of the spine include:
- Antero-posterior bending (flexion and extension)
- Lateral bending
- Rotation
Joints of the Ribs
Each rib articulates with the superior and inferior facets of the bodies of two adjacent vertebrae, coinciding with the intervertebral disc.