Test 1 Questions and Answers

Test 1 Questions and Answers - Test 1 questions and...

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Test 1 questions and answers **Note that the answers below do not contain diagrams because they are hard for me to post the answers this way and it is not meant to imply that diagrams are not acceptable. Diagrams explaining the same material were just a good as written answers.** 1. The crossed extensor reflex causes contraction of hip flexors by propagating an action potential down an alpha motor neuron. Describe the processes involved in getting the alpha motor neuron to threshold. How would the processes differ if the alpha motor neuron had a very large cell body versus a very small cell body? The presynaptic interneurons would release neurotransmitter (probably glutamate) to open ligand gated ion channels (probably Na+) on the alpha motor neuron to produce depolarization, EPSPs, in the post synaptic alpha motor neuron. This depolarization sets up a local current across the dendrites and the cell body of the alpha motor neuron. Local current is a longitudinal separation of charge, whereby negative ions are attracted to the depolarized region leaving positive regions across the cell body such that the axon hillock is then affected (depolarized). There would have to be summation of the EPSPs in order to get enough depolarization to get the axon hillock to threshhold. Multiple EPSPs would have to summate, either via temporal summation, where a single presynaptic cell producs another EPSP before the effects of the first were over, or spatial summation, whereby multiple presynaptic interneurons produce simultaneous EPSPs, whereby the effects of each of the EPSPs add. Summation of depolarization and transmission of this larger depolarization to the axon hillock via local current would get the axon hillock to threshold and open voltage dependent Na+ channels to produce an action potential. Local current decays across a cell body because not all ions move and due to transmembrane leak. If the alpha motor neuron had a large cell body the decay of local current would be larger and you would need more EPSPs to summate to get the alpha motor neuron to threshold. If the cell body were smaller you would have less decay and would require fewer EPSPs and less summation to get the alpha motor neuron to threshold.
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