Due Tues 12 April 2005 at 8 AM. Print out the figure in #1 and draw your traces on the sheet. Type out answers to 2-5.
(1) 3 points Given the picture below, draw three traces that show (a) Tritonia's body position, (b) activity in a dorsal flexion neuron, and (c) activity in a ventral flexion neuron.
Answer drawn below
(2) 4 points Regarding Tritonia swim behavior, describe (a) the role of sensory information in initiation of the behavior, (b) the initial movement(s), (c) the progressive changes in cycle period over time, and (d) the termination of swim movements.
Swimming initiates (a) when epithelial cells make contact with tube-feet of predatory starfish (Getting 1983, p.90). (b) The first stage is a reflex withdrawal in which brachial tufts (gills), rhinophores, and the oral veil are retracted, as well as local regions around the site of contact are withdrawn. Next the body is elongated in the rostral-caudal axis and the oral veil and posterior foot margins form two paddle-like areas (Getting 1983, p.90). Swimming begins with a rapid bilateral ventral flexion lasting 1-2 s, propelling Tritonia upwards off the floor, and continues with alternating dorsal and ventral flexions for 2-20 cycles. (c) the reflex withdrawal is to remove body parts in the immediate vicinity from dangerous contact with a predator, the elongation and formation of paddles is to promote the postural changes that will aid in swimming, the first ventral flexion removes the animal from the sea floor, and the alternating series of dorsal-ventral flexions propels Tritonia into the tidal currents that can carry it away from predators. the swim cycle is not itself a purposeful for of directed locomotion (swim behavior), but rather maintains buoyancy. (d) Swim cycles do not remain constant, but typically decrease to a minimum (fastest period) in the second cycle and then continue to increase from 3 to 9 s before the swim terminates (Getting 1983, p. 94). (e) Swimming terminates with a dorsal flexion, which is less intense than preceding ones. Termination occurs when DSI neurons no longer succeed in raising C2 neurons above threshold via DSI-C2 synaptic excitation (Getting 1983, pp. 114, also see 110,112).
(3) 2 points How does the resetting experiment of Getting 1983 (Fig. 6) support the hypothesis that C2 is part of the CPG, and not a motor efferent
A hyperpolarizing stimulus that silences C2 permanently phase-shifts the swim cycle for the duration of the hyperpolarizing stimulus.
(4) 3 points Propose a functional role for why DSI neurons transiently enhance the C2-DFN synapses during Tritonia swim behavior. Argue your case based on information contained in Katz et al. 1994 and Getting 1983.
The C2-DFN enhancement is dynamic and brief, and decays moments after DSI stimulation terminates. This ensures that the dorsal flexion triggered by the phase switch from ventral to dorsal flexions is most effective and strong as temporally close to the phase transition as possible, and does not last long so that it does not affect the next ventral flexion phase (Katz 1994, p.730).
(5) 3 points Propose a functional role for why DSI neurons cause the inhibitory component of the C2-DSI mixed E-I synapse to be enhanced during Tritonia swim behavior. Argue your case based on information contained in Katz et al. 1994 and Getting 1983.
DSI enhancement of the inhibitory component of the C2-DSI mixed synapse is important because the ehnancement of inhibition is long-lasting and in fact outlasts each swim cycle. This means that each ensuing swim cycle in DSI is superimposed on the long-lasting C2-DSI inhibition from the previous cycle, which contributes to the termination of the swimming behavior.