Mechanosensitive afferents of femoral-saphenous vein.

Reflex contraction of cat hindlimb skeletal muscles can be induced by mechanical stimulation of afferents in the femoral-saphenous vein. Afferent fibers innervating this vein have been previously reported. Hence, it is evident that mechanoreceptors must be present in the femoral-saphenous vein, yet there have never been direct recordings of sensory nerves that respond to mechanical stimulation of the vein. This study recorded the activity of these venous mechanoreceptors and tested their response to increased intravenous pressure. The activity of single venous afferents was recorded from thin filaments of the saphenous nerve. The afferent response was recorded during external mechanical probing of the vein wall and increases in intravenous pressure. Their venous pressure threshold and adaptation to static pressures were determined. All the afferents responded to both types of mechanical stimuli. Static increases in venous pressure resulted in two types of afferent discharge behavior: slowly adapting and rapidly adapting. Venous pressure thresholds varied considerably with a range of 22.2-174.7 mmHg. The estimated conduction velocities are 2.8-7.8 m/s, suggesting type III, A-delta-myelinated mechanoreceptors. These results demonstrate the presence and mechanical sensitivity of venous afferents that transduce intravenous pressure. Their role in the reflex responses to vein distension is unclear and awaits further investigation.