Some Factors in the Activation of Crab Movement Receptors

The elastic organ spanning the propodite-dactylopodite (PD) joint in the leg of the crab Carcinus maenas was first described by Burke (1954) and shown by him to have mechanoreceptive function. Text-fig. I is a diagram of the organ in situ showing its relation to the surrounding structures. The nerve cells of the organ are bipolar neurons, with ovoid somata, proximal running axons, and long distal processes which Burke was able to trace clearly for distances of 200 fL from the cells. The microanatomy of the PD neurons has been studied with the electron microscope by Whitear (1960). She describes scolopidial structures, each containing the distal processes of two cells, and states that this is the only type of end-organ structure visible in the preparations. She has also examined the carpopodite-propodite (CP2) organ of Carcinus and finds that the scolopidia there generally contain only one ending of normal appearance (M. Whitear, personal communication). Text-fig. 2 is a composite schematic diagram of the structure of two such cells. By the use of single-unit recording techniques Wiersma & Boettiger (1959) have shown that two distinct classes of mechanoreceptors are present in the PD organ: one class signals the position of the joint and the other responds to the movement of the joint. Single units of the latter class respond only to movement in one direction. Wiersma (1959) has shown that in the CP2 organ of Carcinus most of the movement units respond to motion in the direction that causes shortening of the elastic strand with which the sensory cells are associated; but he did not propose a mechanism for stimulation. The movement receptors are unique not only in their unidirectional sensitivity to movement but also in their discharge pattern. A very sensitive movement receptor shows a definite threshold with respect to the speed of the stimulating movement. At threshold it discharges spikes slowly and irregularly, and as the speed is increased the discharge becomes more rapid and regular. When the speed has been increased to the point where the cell is firing at about 40/sec. the discharge becomes quite regular, but a further increase in movement speed does not elicit a smooth additional increase in firing frequency. Indeed an increase of as much as tenfold in speed does not elicit a rise of the basic 40/sec. frequency. Instead, such vigorous driving causes occasional double or triple firings; but the doublets or triplets still recur at the 40/sec. rate.