In consideration of the responses of the person with a broken neck, I feel the need to make a distinction between two pathways of response. That is in a normal person we think of the stimulus going into a sensory neuron then to intermediate neurons up the spinal cord to the brain, then back down the spinal cord to a motor neuron giving the visible response. Yet in the patient with the broken neck certain stimuli give visible responses and clearly the pathway for this cannot include the brain. It can include the spinal cord, so one inference is that in a normal person there are similar pathways of response whereby the stimulus goes into the sensory neuron to the spinal cord and intermediate neurons and then to the motor neuron. In addition, there are similar pathways which do not include intermediate neurons such as the monosynaptic reflex arc. I wonder if perhaps the rate at which stimuli elicit a response is related to the amount of intermediate neurons in that particular pathway. For example, a knee jerk response may occur faster than a withdrawal of the foot after stepping on a tack. This would be due to the fact that a knee jerk response goes through only a few intermediate neurons, whereas the foot withdrawal would go through an innumerable amount more intermediate neurons in the brain.

Interesting and appropriate thoughts. Indeed, an implication is that one can get from spinal sensory to spinal motor neurons without the "brain" (LOTS of ways, all polysynaptic, the monosynaptic reflex arc is an anomaly which we'll talk about later in the course, much talked about by neurobiologists because its (relatively) tractable but highly misleading for thinking about how the nervous system works in general). And yes, indeed, different routes take different amounts of time, in part because of the different numbers of axons and synapses needing to be crossed. But there are almost certainly even more significant reasons for varying time delays. If you want to think about those you'll really be at the cutting edge of neurobiology. PG