Spatiotemporal mapping of the vestibulocollic and cervicocollic reflexes
Left splenius muscle shows activation in response to translations between 30 and 50 ms after onset of sled acceleration. Subject was seated up right in chair and translated to his left and his right twice each. During each translation activity was shown on the left splenius muscle within 50 ms.
In the US, over 6,000 new cases of spinal cord injuries arising from severe cervical injuries are reported annually, of which more than half are in people under 30 1. Two of the most common activities associated with cervical injury are automobile accidents and contact sports. To avoid cervical injury from hyperextension of the neck, the body must quickly sense the direction of a collision and react by activating collic reflexes to stabilize the head. The vestibulo-collic reflex (VCR) senses motion of the head in space and contracts neck muscles to maintain head stability in space. Meanwhile, the cervico-collic stretch reflex (CCR) senses head on trunk position and acts on neck muscles to stabilize the head with respect to the trunk.
Although external perturbations may occur from any direction, existing cervical posture research has largely focused on disturbances applied in the fore and aft directions. As a result, little is known about head stabilization in the other directions. Postural reflexes may be directionally tuned such that there is greater reflexive stabilization, and therefore less movement of the head relative to the trunk and less susceptibility to injury, in a particular direction of perturbation. By understanding the conditions associated with increased postural instability, modifications to existing safety equipment, or the creation of new complementary safety equipment, can be made to better protect individuals during perturbations where reflexes provide inadequate protection.
Muscles are activated more effectively by side to side translations than by front to back translation. Subjects were translated upright in 3 directions; side to side, front to back and 45Â° in between. Left splenius muscles were well activated during side to side translations and there was no significant activation during front to back translation.
To examine human postural reflexes a wide variety of directional stimuli and contexts simulating natural perturbations encountered in minor car accidents or by athletes on the field is necessary. Natural perturbations can come from any direction, but prior studies have largely focused on anterior-posterior or lateral translations. Our unique sled/rotator allows independent translational and rotational stimuli to provide perturbing linear and/or rotational accelerations in any direction in the horizontal plane.
In addition to the physical stimulus itself, a variety of contextual variables may play a significant role when an individual is perturbed in everyday life the individual may be voluntarily turning their head or have their head turned to a non-centered position at the time of collision. During high contact sports, such as football, or where there is high risk of head injury, such as automobile racing or riding a bicycle or motorcycle, protective helmets are worn to protect the head. However, the additional mass of the protective helmet may have a deleterious effect on reflexive head stabilization, thereby increasing susceptibility to neck injury. Through our work we hope to characterize the directional tuning of reflexive head movements and examine the changes that occur when the head is not centered on the trunk, actively moving, and loaded with an external mass.
Center, T.N.S.C.I.S., Spinal Cord Injury: Facts and Figures at a Glance. 2008, The University of Alabama at Birmingham: Birmingham. p. 2.