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Impulse conduction at the branch point of afferent axons in dorsal root ganglion (DRG) has been studied using intracellular recording from frog DRG neurons in vitro. The least conduction interval (LCI, the minimum inter-response interval) was determined for pairs of impulses to successfully propagate through the branch point into the dorsal root. At 21 degrees-23 degrees C, average branch point LCI was significantly longer than for afferent fibers in the peripheral nerve. This result suggested that the branch point would limit the maximum frequency of action potentials that could conduct into the dorsal root. This was found to be the case. The maximum frequency of impulses in short trains (less than or equal to 40 ms) which could conduct into the dorsal root without failure (363 Hz) was accurately predicted by branch point LCI and was far less than the maximum frequency predicted from the LCI of axons in the peripheral nerve (610 Hz). Branch point LCI was correlated (r = -0.78) with the natural log of peripheral axon conduction velocity (CV). However, the relationship of LCI and CV was different for different types of neurons and the shape of the somatic action potential was found to be a reliable predictor of branch point LCI. Neurons with long-duration somatic action potentials with a shoulder on the falling phase tended to have low CV and invariably had long LCI's. Neurons with brief, smooth action potentials had short LCI's regardless of CV. These cells, which appear to be the most differentiated type, have found a way to minimize branch point LCI which is virtually independent of their axonal CV. For the latter neurons, branch point LCI was correlated (r = 0.42) with the reciprocal of the hyperpolarization level, at the cell body, required to block conduction through the branch point, suggesting that the proximity of the cell body to the branch point might play a role in determining the LCI of some neurons. Over a range of 12 degrees C to around 35 degrees C, branch point LCI was inversely related and maximum firing frequency directly related to temperature. At high temperatures (30 degrees-40 degrees C) conduction failure occurred at sites having particularly long LCI's. It is concluded that a) these axon branch points act as low-pass filters and set the maximum frequency of conducted impulses that can access the central nervous system; b) certain varieties of DRG neurons exhibit more branch point filtering action than others; and c) warming, within limits, reduces branch point filtering action.

Instructional multimedia is creating a revolution in universities. Academic staff are being urged to transfer instruction to a format suitable for either CD/ROM and/or the Internet, principally the World Wide Web. Many, it seems, simply take their existing course materials, add image and sound without proper consideration of the nature of the medium in which they are seeking to instruct and present the product to their students. In essence, it appears they ignore the need to motivate their students to work with their instructional multimedia materials. It is a contention of this paper that the user‐interface to instructional multimedia is strategically important: if it is poorly designed students will not be intrinsically motivated to make use of the product or to learn with it. Interfaces that motivate learners are realistic, easy to use, challenging and engaging. Superior interfaces have some of the elements of a game: they provide the user with a functional model of task, content and processes; they encourage exploration and engagement; and they demonstrate cognisance of design considerations such as interactivity, functionality, learner control and cognition.

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