Neck reflexes

Turning the head relative to the body excites spindles in neck muscles and afferents from the cervical vertebral joints. This evokes reflex contractions of neck muscles (i.e., cervicocollic reflexes) and limb muscles (i.e., cervicospinal reflexes) with both tonic and phasic components. The Cervicocollic reflexes contract neck muscles which are stretched and therefore act to reorientate the head on the body. Vestibulocollic and Cervicocollic vestibulocollic reflexes are synergistic. The Cervicospinal reflexes cause contraction of limb muscles in reaction to fast head movement. In standing humans a force that throws the head backwards on the trunk activates all the limb extensors, while a force throwing the head forwards activates all the limb flexors. The Vestibulospinal and Cervicospinal reflexes are antagonistic. When the head and trunk are tilted as one to the left, and the vestibulospinal reflex causes left arm extension. Although when the trunk alone is passively tilted to the left (with the head remain fixed in relation to space) the cervicospinal reflex will flex the left arm. In the more common case that the head is tilted whereas the trunk remains stationary, these contrasting reflexes cancel out.