We are wired to maintain our visual axes parallel to the horizon. This involves a series of joint and muscle mechanoreceptors in the neck (for a review of joint mechanoreceptors, click here, muscle mechanoreceptors, click here). These muscle and joint mecanoreceptors receptors, through connections in the midbrain (or mesencephalon as we neuro geeks like to call it) and pons, interact with the vestibular system to keep our head (and our bodies) upright, by firing our extensor muscles.
To explain this reflex, think of a dog sitting to get a treat. As he looks up while sitting down he has to extend his head, extend his front legs and fires all the axial extensor muscles associated with performing this action. The opposite would also happen, but with the flexors, if he were to bend forward to take a drink; fire front flexors and rear extensors to bend down. There are many more reflexes (tonic neck, cervcio ocular, etc) that could be the subject of another post.
As we have learned from the principle of facilitation (see recent post here), when we fire pur extensors, we fire into the extensor pool, and as a result, ALL extensors get to benefit. The advantage of the receptors in the cervial spine is that the upper four fire DIRECTLY into the flocculo nodular lobe of the cerebellum, and thus have a PROFOUND EFFECT on extensor tone in general.
The Gait Guys. Stretching your neurology on a daily basis.