Question
A neuron from a spinal cord was examined. The propagation of an action potential in the neuron was slower than that of a typical sensory neuron. This was because Select one: a. axon length is much longer in sensory neurons. b. the neurotransmitters were blocked in this spinal neuron. c. myelination was absent in the spinal cord neuron. d. the nodes of Ranvier were absent in sensory neurons.
Solution
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Answer
The correct answer is **c. myelination was absent in the spinal cord neuron.**<br /><br />Here's why:<br /><br />* **Myelination** is the key factor affecting the speed of action potential propagation. Myelin acts as an insulator, allowing the action potential to "jump" between the Nodes of Ranvier (saltatory conduction). This significantly increases the speed of transmission. The absence of myelination would drastically slow down the propagation.<br /><br />Let's look at why the other options are incorrect:<br /><br />* **a. axon length is much longer in sensory neurons:** While axon length *can* influence the overall time it takes for a signal to travel, it doesn't directly affect the *speed* of propagation along the axon itself. A longer axon simply means the signal has a longer distance to cover.<br /><br />* **b. the neurotransmitters were blocked in this spinal neuron:** Neurotransmitters are involved in the synaptic transmission *between* neurons, not the propagation of the action potential *along* a single neuron's axon. Blocking neurotransmitters would affect communication between neurons, not the speed of the signal within a neuron.<br /><br />* **d. the nodes of Ranvier were absent in sensory neurons:** Nodes of Ranvier are *essential* for rapid saltatory conduction in myelinated neurons. Their absence would actually *slow down* conduction, not speed it up. Sensory neurons typically *are* myelinated and *do* have Nodes of Ranvier, facilitating rapid signal transmission.<br />
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