The sagittal vestibulocollic reflex and its interaction with neck proprioceptive afferents in the decerebrate cat.

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Abstract
The sagittal vestibulocollic reflex (vcr) evoked by nose-up, nose-down movements of the head, was studied in the neck extensor muscle biventer cervicis in the decerebrate cat. Nose-down movements of the head increased and nose-up movements decreased electromyographic (EMG) activity in the biventer cervicis muscles of the left and right sides. At low frequencies of sinusoidal head movement (0.1-0.5 Hz), the gain of the sagittal vcr was approximately constant, and EMG modulation showed a phase lead 40.degree. with respect to head position. At higher frequencies (2-5 Hz), vcr gain increased at a rate close to 40 dB/decades and phase lead increased to approach 150.degree.. The relation between head movement and vcr activity may be described by a transfer function containing 2 lead terms, with time constants of 0.07 and 0.23 s, and 2 lag terms, with time constants of 5.3 and 9.9 s. When movements of the head were accompanied by stretching of the biventer cervicis muscles, the gain of the sagittal vcr was increased 3-fold, at all frequencies between 0.1 and 5 Hz, with no substantial change in phase. Sinusoidal stretching of the biventer cervicis muscles, with the head stationary, evoked a stretch reflex (cervicocollic reflex, ccr) the behavior of which was similar to that of a 2nd-order system described by a transfer function containing 2 lead terms with time constants of 0.07 and 0.16 s. This difference in dynamics between the vcr and the ccr indicates that the lag terms in the vcr transfer function reflect the frequency-response properties of the vestibular pathway to the biventer cervicis muscles, as they do not appear when the same muscles participate in the ccr. The vectorial differences between the frequency-response of the sagittal vcr, with and without concomitant stretching of the biventer cervicis muscles is quantitatively similar to the frequency-response of the ccr evoked by sinousoidal stretching. The inputs from the vestibular and stretch receptors thus appear to sum linearly to produce the increase in vcr gain, at least over the frequency range 0.1-1 Hz. Since most head movements, and all voluntary head movements, involve a rotation of the head in relation to the neck, the potentiation of the gain of the vcr by afferents from stretch receptors (presumably muscle spindles) in the neck muscles is an important factor in the normal reflex stabilization of head position.