Fetal responses to acute hypoxemia include bradycardia, increase in blood pressure, and peripheral vasoconstriction. Peripheral vasoconstriction contributes to the redistribution of the cardiac output away from ancillary vascular beds toward myocardial, cerebral, and adrenal circulations. We investigated the effect of alpha-adrenergic receptor blockade on this fetal response. Fluorescent microspheres were used to measure cardiac output distribution during basal and hypoxemic conditions with and without phentolamine treatment. Phentolamine altered basal cardiac output distribution, indicating a basal alpha-adrenergic tone, but this was mainly noted at the earlier stages of incubation. During hypoxemia, phentolamine prevented vasoconstriction in the carcass. At day 19 of incubation, the percent cardiac output distributed to the carcass increased by 20% compared with a decrease in the control group by 17%. Phentolamine markedly attenuated the subsequent redistribution of the cardiac output toward the brain (from +102% in the control group to -25% in the phentolamine-treated group) and the heart (from +196% in the control group to +69% in the phentolamine-treated group). In the chick embryo, alpha-adrenergic mechanisms contribute to the maintenance of basal vascular tone and to the redistribution of the cardiac output away from the peripheral circulations toward the brain and heart during hypoxemic conditions.
SYNOPSIS In 35 normal subjects electromyographic silent periods were constantly evoked bilaterally in the masseter muscles during maximal contraction after unilateral electrical stimulation over the infraorbital or mental nerve. Findings in this study and data obtained in 30 patients suffering from trigeminal (26) and facial (four) nerve lesions suggest that the silent period evoked according to our methods is cutaneous in origin. The trigeminal sensory root forms the afferent limb of the silent period reflex. Its central pathway is thought to pass both crossed and uncrossed through the pons. Determination of the cutaneous silent period might be of value for the demonstration of trigeminal nerve lesions and to supplement results concerning other brain-stem reflexes.The electromyographic silent period (SP) refers to a transitory, relative or absolute decrease of EMG activity evoked in the midst of an otherwise sustained contraction (Shahani and Young, 1973). The term SP should be reserved for reflex pauses following a stimulus of some kind -for example, an electrical shock-to peripheral nerves and the skin overlying them; phasic reflex contractions-for instance, a tendon tap; or sudden decreases in the load against which a muscle is contracting (unloading reflex). Several factors are involved in the SP, including recurrent inhibition of motoneurones (Renshaw inhibition), Golgi tendon organ inhibitory activity, a pause in muscle spindle input, cutaneous factors, and descending impulses from subcortical or cortical areas, as well as other mechanisms yet to be described (Shahani and Young, 1973;Struppler, 1975).In the masseter muscles SPs have been observed after acoustic stimuli, a tap on the chin, and stimulation applied to the surface of the tongue and to the masseter muscle belly (Hoffman and Tonnies, 1948;Struppler et al., 1960;
Eighteen patients with intracranial trigeminal nerve lesions were investigated electrodiagnostically. The trigeminal motor root function was studied on the basis of the jaw reflex and masseter myograms and was disturbed in all cases. The jaw reflexes were abnormal in 16 and the masseter myograms in eight cases. An impressive trigeminal sensory root function was obtained from blink reflex, which was abnormal in 12 cases. The results show the diagnostic value of bilateral recording of the jaw reflex in such patients.
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