Tuning of vibration sensitive neurons in the central nervous system of a wandering spider,Cupiennius salei Keys

Abstract

1. The first steps of the central nervous integration of sensory input from vibration receptors in the spiderCupiennius salei Keys, take place in the subesophageal ganglionic mass. We describe ventro-medially located neurons which respond to vibrations reaching the spider through the substrate (SVSN, SubstrateVibration Sensitive Neurons). 2. These neurons differ with respect to the convergence of their sensory input. Some neurons (SVSN type 1) only respond to the vibration of the leg corresponding to the ganglion recorded from. Other neurons can be effectively stimulated by vibrating any of the four ipsilateral legs (SVSN type 2) or either one of the two legs of the same segment (SVSN type 4). Still another type receives input from all eight legs. 3. According to their tuning curves (10 Hz–1 kHz) all vibration sensitive neurons in the subesophageal ganglionic mass so far studied behave like bandpass filters. Their best frequencies are in the frequency range of courtship and prey signals encountered by the spider in its natural habitat. Whereas the best frequencies of SVSN type 1 are found at low frequencies (ca. 80 Hz–100 Hz), medium (ca. 400 Hz), and high frequencies (ca. 800 Hz), respectively (Fig. 1), SVSN type 2, 3, and 4 are more broadly tuned to 80 Hz–200 Hz (Figs. 2, 3). 4. SVSN type 2 are more sensitive by about 12 dB to front leg stimulation than to hind leg stimulation (Fig. 2), and SVSN type 4 are more sensitive (by ca. 7 dB) to simultaneous vibration of both segmental legs than to the stimulation of either one of them alone (Fig. 3). 5. Threshold values (displacement of tarsus) at the best frequencies are in the range of 0.1 to 1 μm. They decrease by up to ca. 20 dB if band limited ‘noisy’ stimuli (bandwidth ca. 1/3 octave) are applied instead of sinusoidal vibration (Fig. 4). 6. The relation between stimulus amplitude and spike response is sigmoid. The dynamic ranges of the responses differ in the various neuron types. The low and high frequency SVSN type 1 have a dynamic range of more than 46 dB; in SVSN type 4 it increases if the two segmental legs are stimulated simultaneously instead of alone (Fig. 5).