Neck afferents and muscle sympathetic activity in humans: implications for the vestibulosympathetic reflex

Ray, Chester A., and Keith M. Hume. Neck afferents and muscle sympathetic activity in humans: implications for the vestibulosympathetic reflex. J. Appl. Physiol. 84(2): 450–453, 1998.—We have shown previously that head-down neck flexion (HDNF) in humans elicits increases in muscle sympathetic nerve activity (MSNA). The purpose of this study was to determine the effect of neck muscle afferents on MSNA. We studied this question by measuring MSNA before and after head rotation that would activate neck muscle afferents but not the vestibular system (i.e., no stimulation of the otolith organs or semicircular canals). After a 3-min baseline period with the head in the normal erect position, subjects rotated their head to the side (∼90°) and maintained this position for 3 min. Head rotation was performed by the subjects in both the prone ( n = 5) and sitting ( n = 6) positions. Head rotation did not elicit changes in MSNA. Average MSNA, expressed as burst frequency and total activity, was 13 ± 1 and 13 ± 1 bursts/min and 146 ± 34 and 132 ± 27 units/min during baseline and head rotation, respectively. There were no significant changes in calf blood flow (2.6 ± 0.3 to 2.5 ± 0.3 ml ⋅ 100 ml−1 ⋅ min−1; n = 8) and calf vascular resistance (39 ± 4 to 41 ± 4 units; n = 8). Heart rate (64 ± 3 to 66 ± 3 beats/min; P = 0.058) and mean arterial pressure (90 ± 3 to 93 ± 3; P < 0.05) increased slightly during head rotation. Additional neck flexion studies were performed with subjects lying on their side ( n = 5). MSNA, heart rate, and mean arterial pressure were unchanged during this maneuver, which also does not engage the vestibular system. HDNF was tested in 9 of the 13 subjects. MSNA was significantly increased by 79 ± 12% ( P < 0.001) during HDNF. These findings indicate that neck afferents activated by horizontal neck rotation or flexion in the absence of significant force development do not elicit changes in MSNA. These findings support the concept that HDNF increases MSNA by the activation of the vestibular system.

Neck muscle and trigeminal input to the upper cervical cord and lower medulla of the cat

Experiments on chloralose-anaesthetized cats have shown that low-threshold neck muscle afferents project to laminae IV and V in the dorsal horn of the upper cervical cord, to lamina VI including the region which encompasses the central cervical nucleus, as well as to extensive regions of the ventral horn. At posterior medullary levels projections also exist to laminae IV, V, and VI of the spinal nucleus of V (although those to lamina IV are circumscribed), to the deep layers and lateral margin of the cuneate nucleus, and to the inferior olive. These projections are both from low- and high-threshold afferents. Evidence of a functional relationship between the trigeminal and neck muscle afferent system was found both in the upper cervical cord and lower medulla. About 40% of units in both regions receive a convergent input and when convergence could not be demonstrated, prior stimulation of one modality in some instances affected the responsiveness of the unit to the other modality. A motor role was found for some trigeminal afferent projections to the upper cervical cord. Trigeminal afferents consistently activated antidromically identified motoneurons of splenius, biventer cervicis, and complexus.

Projections of extraocular, neck muscle, and retinal afferents to superior colliculus in the cat: their connections to cells of origin of tectospinal tract

Unit recordings were made in the superior colliculus of cats anesthetized with chloralose and with Pentothal. Electrical stimulation of extraocular muscle afferents and neck muscle afferents excited more units in the superior colliculus than did a variety of moving and stationary visual stimuli. Units responding to neck muscle afferent stimulation fell into three populations; one population firing with a short latency and following stimulus presentation up to 1/s, a second population with a long latency and following stimulus presentation at frequencies lower than 15/min, and a third population exhibiting paired firing. The latencies and firing patterns of the third population combined the characteristics of each of the first two patterns. It is suggested that these characteristics of unit discharges stem from the existence of two pathways from neck muscle afferents to the superior colliculus. The projection is predominantly bilateral. Units responding to neck muscle afferent stimulation are distributed throughout the superior colliculus on the basis of their latencies. Long-latency responses predominate in the superficial layers of the superior colliculus and short-latency responses, while more common in the intermediate and deep layers, predominate in the tegmentum. Extraocular muscle afferent projections to the superior colliculus constitute the single richest projection found in these experiments. While the response patterns and latencies are similar to those of the neck muscle afferents, long-latency responses are the most common and dominate in all collicular regions. Few units in the tegmentum could be excited by extraocular muscle afferents. Both extraocular muscle and neck muscle afferents show considerable convergence with one another and with retinal afferents within the superior colliculus. Cells of origin of the tectospinal tract were identified within the superior colliculus and tegmentum by antidromic excitation from the upper cervical cord. These cells were distributed predominantly within the intermediate and deep layers of the superior colliculus, and sparsely in the superficial layers and tegmentum. Almost 50% of the cells of origin of the tectospinal tract receive a convergent input from extraocular muscle and neck muscle afferents and from the retina. About 30% of the cells were inexcitable to the stimuli employed in these experiments. The significance of these projections is discussed with respect to superior collicular function in the cat and i