Describe the unilateral action of the highlighted muscle. Depression of scapula Rotation of head to same side Lateral flexion of neck Retraction of scapula Rotation of head so face turns to opposite side Rotation of head to opposite side Extension of head and neck
The Correct Answer and Explanation is:
Correct Answer: Rotation of head so face turns to opposite side
The muscle in question is the sternocleidomastoid, commonly highlighted in anatomical illustrations focusing on unilateral actions of the neck. When this muscle contracts on only one side, it causes the head to rotate such that the face turns toward the opposite side. Simultaneously, it also assists in lateral flexion of the neck toward the same side.
The sternocleidomastoid originates from two locations: the manubrium of the sternum and the medial portion of the clavicle. It inserts on the mastoid process of the temporal bone, located just behind the ear. Its orientation enables it to pull the mastoid process forward and downward when activated, resulting in specific head and neck movements depending on whether one or both muscles contract.
When only one sternocleidomastoid muscle contracts, the resulting motion involves contralateral rotation—the chin moves toward the side opposite the contracting muscle. For example, contraction of the right sternocleidomastoid rotates the head so that the face turns left. This action is often observed during scanning movements or during postural adjustments.
This movement plays a crucial role in both voluntary actions, such as turning the head to look over the shoulder, and involuntary postural responses, like orienting the head in response to sensory stimuli. Additionally, the muscle contributes to lateral flexion, bending the neck toward the same side as the contracting muscle.
The sternocleidomastoid also has bilateral actions, including flexion of the neck and extension of the head at the atlanto-occipital joint, but those occur when both sides contract together. The focus here, however, remains on its unilateral action, which distinctly rotates the head so the face turns to the opposite side.
