The Nature and Etiology of Normal and Alcohol Withdrawal Tremor

Alcohol withdrawal tremor is principally a postural-type tremor. That is, the tremor manifests itself most when the limbs are voluntarily maintained in a stationary position. The tremor of alcohol withdrawal is typically 5 to 20 times as large and approximately the same frequency as normal postural tremor. The fact that withdrawal tremor is uncorrelated in both outstretched hands argues against the involvement of a central pacemaker in the genesis of the tremor. Both withdrawal and normal physiologic type hand tremors are associated with synchronous firing of extensor motor units in phase with tremor velocity at the frequency of limb resonance. Both kinds of tremor are reduced by the p-adrenergic receptor blocking drug, propranolol. The reduction in tremor is caused by a decrease in the synchrony of motor units and not a decrease in the total number of units involved in maintaining limb posture. It is proposed that increased tremor during withdrawal is due to the entrainment by 1 A muscle spindle afferents of asynchronously firing motoneurons. Tremor is also increased by entrainment of motoneurons firing synchronously at 9 to 10 Hz due to an oscillation in the stretch reflex control system. Tremor reduction following propranolol is due to a decrease in entrainment because of decreased 1 A afferent inflow to the spinal cord or because of reduced sensitivity of alpha motoneurons to 1 A spindle afferent discharges.