Final answer:
The straplike muscle whose spasms can result in wryneck or torticollis is the sternocleidomastoid. This muscle is responsible for head movements and is innervated by the spinal accessory nerve. Involuntary contractions can lead to a condition called cervical dystonia.
Step-by-step explanation:
Spasms of the sternocleidomastoid muscle often result in wryneck or torticollis. This major muscle of the neck is responsible for the lateral flexion and rotation of the head. When both left and right sternocleidomastoids contract, the head flexes forward; when they contract individually, they cause rotation to the opposite side. The spinal accessory nerve innervates the sternocleidomastoid. Spasmodic contractions in this muscle can cause the neck to twist and turn involuntarily, leading to the condition known as cervical dystonia or spasmodic torticollis, which is depicted in medical illustrations such as Figure 15.1.2, showcasing abnormal head positioning.