Relationship of noradrenergic locus coeruleus neurones to vestibulospinal reflexes.

The electrical activity of presumably noradrenergic locus coeruleus (LC) neurones was recorded in decerebrate cats during roll tilt of the animal at 0.15 Hz, +/- 10 degrees, leading to sinusoidal labyrinth stimulation. Among the tested units, some of which projected to the lumbosacral spinal cord, 56.7% responded to animal tilt. Most of these neurones were activated during side-up and depressed during side-down tilt of the animal, while a smaller proportion of units showed the opposite response pattern. This predominant response pattern of LC neurones and coeruleospinal (CS) neurones to animal tilt was opposite in activation polarity to that of vestibulospinal (VS) neurones projecting to the same segments of the spinal cord. Both the VS and the CS neurones exert a direct excitatory influence on ipsilateral limb extensor motoneurones. However, VS neurones excite corresponding Renshaw (R) cells, though due to activation of limb extensor motoneurones and their recurrent collaterals, the CS neurones may inhibit them. It appears, therefore, that during side-down animal tilt, the motoneurones innervating the ipsilateral limb extensors are excited by the increased discharge of VS neurones, while the corresponding R-cells are disinhibited due to the reduced discharge of CS neurones. The functional coupling between ipsilateral limb extensor motoneurones and the corresponding R-cells would then increase, just at the time in which these motoneurones are driven by the excitatory VS volleys, thus limiting the response gain of limb extensors to labyrinth stimulation. This hypothesis is supported by two facts: (1) R-cells linked with limb extensor motoneurones discharge during side-down tilt, thus firing in phase with the excitatory VS volleys, and (2) functional inactivation of the noradrenergic LC neurones increases the gain of the vestibulospinal reflexes acting on limb extensors.