Transmission mediated with and without spikes at connexions between large second-order neurones of locust ocelli

Abstract

Large second-order neurones of locust ocelli (L-neurones) make both excitatory and inhibitory connexions with each other. Small, graded depolarisations and hyperpolarisations are transmitted at the excitatory connexions but, at the inhibitory connexions, spikes in the presynaptic neurone are required for transmission. 1. L-neurones spike, usually once only, when hyperpolarisations end rapidly. The hyperpolarisations can be caused either by illumination of the ocellus, or by injection of current. The amplitude of a spike depends both upon the amplitude and the duration of the preceding hyperpolarisation (Fig. 1B). 2. At the excitatory connexions, the resting potential of the presynaptic neurone normally lies depolarised from the threshold for transmission, so that both small hyperpolarisations and depolarisations effect changes in postsynaptic potential (Fig. 2A, B). Over periods of several minutes, there is no sign of decrement in transmission at these connexions (Fig. 2D). Spikes in the presynaptic neurone usually ensure that the postsynaptic neurone also spikes. 3. At the inhibitory connexions, the postsynaptic potential decrements within 10–20 ms (Fig. 3A). Because of this, rapidly rising presynaptic potentials, such as spikes, are required for transmission. Also, presynaptic hyperpolarisations do not effect changes in postsynaptic potential. Following an inhibitory postsynaptic potential, transmission at an inhibitory connexion remains depressed for about a second (Figs. 3B, C). 4. All three members of one anatomical class of L-neurone (L1–3; C.S. Goodman 1976) of a lateral ocellus make reciprocal inhibitory connexions with each other (Fig. 7B; Table 1). Some of these neurones are presynaptic at excitatory connexions with another class (L4–5; Fig. 7A). Many L-neurones do not project to the whole area of the retina, and most project to the dorsal or ventral halves (Fig. 6). The excitatory connexions may sharpen responsiveness to decreases in illumination, and the inhibitory connexions may enhance the detection of rapid movements of large objects, such as the visual horizon.