Properties of afferent nerve fibres supplying the saphenous vein in the cat.

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Abstract
1. We examined the responses of primary afferent neurones supplying a vascularly isolated segment of the saphenous vein to mechanical and chemical stimuli in anaesthetized cats. Activity was recorded from centrally cut axons of afferent nerve fibres which were isolated from the saphenous nerve near its junction with the femoral nerve. 2. A total of thirty units responded to one of these stimuli and twenty‐three of them were activated by local mechanical stimulation of the venous wall. Most receptive fields were circular spots. The response of the isolated venous segment to distension was tested in fifteen out of thirty units and eight out of fifteen were activated. Intravasal threshold pressures 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 both mechano‐ and chemosensitivity. Thirteen were classified as A fibres 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) and all of the mechanosensitive C fibres (8/8) responded to at least one of the chemical stimuli used: hypertonic saline, bradykinin (BK) or capsaicin. 4. The remaining seven units (6 C, 1 A) were activated by injection 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 isolated venous segment repeatedly induced an increase in systemic blood pressure. 5. The proportion of unmyelinated fibres responding to mechanical stimulation of the venous segment was systematically examined in three experiments and amounted to about 1% of the unmyelinated afferents in the saphenous nerve. 6. In conclusion, a small proportion of afferent nerve fibres in the saphenous nerve responds to presumably noxious 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 especially under pathological conditions.