PROPERTIES OF AFFERENT NERVE-FIBERS SUPPLYING THE SAPHENOUS-VEIN IN THE CAT

1. We examined the responses of primary afferent neurones supplying a vascularly isolated segment of the saphenous vein to mechanical and ch emical stimuli in anaesthetized cats. Activity was recorded from centr ally cut axons of afferent nerve fibres which were isolated from the s aphenous nerve near its junction with the femoral nerve. 2. A total of thirty units responded to one of these stimuli and twenty-three of th em were activated by local mechanical stimulation of the venous wall. Most receptive fields were circular spots. The response of the isolate d venous segment to distension was tested in fifteen out of thirty uni ts and eight out of fifteen were activated. Intravasal threshold press ures inducing discharges were in the range of 35-250 mmHg with a mean of 120 mmHg. 3. Twenty-seven out of the thirty units were tested for b oth mechano- and chemosensitivity. Thirteen were classified as A fibre s and fourteen as C fibres with conduction velocities of 5-30 m s(-1) and less than 2.5 m s(-1) respectively. Twenty fibres (12/13 A, 8/14 C ) were mechanosensitive. Two-thirds of the mechanosensitive A (8/12) a nd all of the mechanosensitive C fibres (8/8) responded to at least on e of the chemical stimuli used: hypertonic saline, bradykinin (BK) or capsaicin. 4. The remaining seven units (6 C, 1 A) were activated by i njection of BK into the isolated venous segment but failed to respond to mechanical stimuli. Six were found during five experiments in which BK was used as a search stimulus. Injection of bradykinin into the is olated venous segment repeatedly induced an increase in systemic blood pressure. 5. The proportion of unmyelinated fibres responding to mech anical stimulation of the venous segment was systematically examined i n three experiments and amounted Do about 1% of the unmyelinated affer ents in the saphenous nerve. 6. In conclusion, a small proportion of a fferent nerve fibres in the saphenous nerve responds to presumably nox ious mechanical and/or chemical stimuli applied to the saphenous vein. These fibres, together with some chemospecific venous afferents, may be capable of encoding nociceptive information from the vein especiall y under pathological conditions.