1. Articulation of the coxobasal joint of the posterior leg of the shore crab, Carcinus maenas, through its full 105° arc of movement translates into a linear displacement of the levator receptor strand of 3-4 mm (crab carapace widths 60-75 mm, respectively) (Fig. 2). 2. Both levator afferents, the distally inserting L1 (diameter approximately 30 μm) and the proximally inserting L2 (13 μm), respond qualitatively similarly to trapezoidal length changes of the levator strand with graded amplitude, decrementing receptor potentials that are passively conducted to the thoracic ganglion, and that have both dynamic and static response components (Figs. 1, 3). 3. Both the 'resting' membrane potential and the depolarized level of the static response component are linearly related to the receptor length. This linearity is fairly well delimited by the in situ receptor length range (Fig. 4). 4. The peak dynamic potential increases linearly with stretch velocity. At more extended receptor lengths this linear relationship is dualphased with two distinct slopes of velocity sensitivity (Figs. 5 C, 6). 5. Graded amplitude depolarizing transients often produce spike-like initial responses to stretches of high velocity, particularly at short receptor lengths. Similar graded 'spikes' may occur in response to adequate depolarizing pulses, indicating a degree of voltage-dependent membrane activation (Figs. 5, 7, 8). 6. Injected inward (depolarizing) current pulses of increasing strength (3-30 nA) yield progressively shorter membrane time constants (τm =T84% = 43-8 ms); equivalent hyperpolarizing currents give significantly larger values of τm. Maximal length constant (λ) values of 12.6 mm (L1) and 4.9 mm (L2) were estimated from responses to stretch stimuli (Figs. 3B, 8). 7. A comparison of the sensory responses of the two levator afferents with those of the single depressor afferent reveals differences in the degree of active membrane responsiveness of these afferents, and in their relationship between stretch velocity and peak dynamic amplitude. Possible reasons for these differences are discussed.
ASJC Scopus subject areas
- Ecology, Evolution, Behavior and Systematics
- Animal Science and Zoology
- Behavioral Neuroscience