Understanding Action Potentials: All or Nothing Responses and Stimulus Encoding - Prof. Ni, Assignments of Biology

Download Understanding Action Potentials: All or Nothing Responses and Stimulus Encoding - Prof. Ni and more Assignments Biology in PDF only on Docsity! Cellular Neurobiology BIPN 140 Fall 2004 Problem Set #1 1. Explain the following statement, “Action potentials are all or nothing responses.” How is stimulus strength encoded assuming this is true? An action potential can only occur if the stimulus is strong enough to raise the membrane potential to the threshold. Any stimulus below threshold won’t result in an action potential. Stimuli above threshold generate action potentials that are the same amplitude as those generated at threshold. Given this property, the stimulus strength governs the rate at which the membrane potential returns to threshold, because it determines how quickly the membrane capacitance becomes charged. Thus stimulus strength governs the frequency with which action potentials are produced and stimulus strength is often coded by the frequency of action potentials. 2. What is axoplasmic transport? What is the fastest form? The slowest? Which type of axoplasmic transport would be affected if kinesin were “knocked out”? Dynein? Axoplasmic transport is the movement of cargo to or away from the soma. The fastest form is anterograde at a speed of 400 mm/day. The slowest is anterograde at a speed of 1 mm/day. If kinesin were knocked out, there would be no fast anterograde transport. If dynein were knocked out, there would be no retrograde transport. 3. In the paper discussed at the end of the second lecture, what were the methods used to find the structure of the potassium channel? How did they verify that the structure was the same in humans? KcsA and MthK bacteria were used as a source of the potassium channels, because bacteria could be grown in large numbers and they had small potassium channels, which made the channels easy to crystallize. The crystals were examined by x-ray crystallography to determine the 3-D structure of the channel. The amino acid sequences of the bacterial potassium channels were compared to those of humans, and the sequences for the selectivity filter and the glycine residue for the gating hinge were found both in humans and bacteria. This suggests that the human channel is highly conserved from the bacterial ones. 4. In action potential propagation, both the active and passive properties are important. Which properties are rate limiting? Why? Active properties are rate limiting. Passive spread occurs almost at the speed of light - the speed of electrical current flowing in a conductor (saline). Active spread is dependent on the speed at which ion channels undergo conformational changes, which is relatively slow. 5. Jim was conducting a survey of glial cell types when he dropped all of his slides on the floor! Being a reckless as well as clumsy scientist, he has failed to label any of them, but he does have a microscope with which he can observe the slides. Help Jim 1 figure out what cell type is on each slide, and provide locations (PNS or CNS) as well as known functions for each cell type. a. Jim observes a cell that has wrapped itself around the axons of several neurons. These are oligodendrocytes, which are located in the CNS. They promote the fast transmission of action potentials through their production of myelin. b. Jim observes a cell that has wrapped itself around the axon of a single neuron. He remembers reading a paper that suggests that some axons can regrow in the presence of this glial cell. These are Schwann cells, which are located in the PNS. By myelinating axons, they promote the conduction of action potentials. Recent studies have suggested that trophic factors secreted by Schwann cells can direct axonal regrowth. (In humans, Schwann cells may also secrete factors that impair axonal regrowth into the spinal cord. This is currently a hot research area). c. Jim observes a cell that is close to a synaptic cleft. He observes the same cell type at a blood-brain barrier. These are probably astrocytes, which are located in the CNS. Near the synaptic cleft they perform potassium buffering, neurotransmitter uptake, and neurotransmitter release to modulate synaptic activity. Astrocytes also form the blood-brain barrier, which prevents foreign particles and many molecules including drugs from direct access to the brain via the circulation. d. Jim observes a cell that is engulfing debris. These are microglia, which are located in the CNS. They remove debris and foreign material from the brain. They are also involved in the immune response of the brain. e. Jim observes a cell to which several undifferentiated neurons appear to be attached. These are radial glial cells. They are located in the CNS. They guide neurons to their final locations in the brain during development, and also secrete trophic factors. f. Jim observes many of these cells surrounding the central canal of the spinal cord. These are ependymal cells, which secrete cerebral spinal fluid (CSF). They are considered to be part of the CNS. 6. Why is PET used infrequently relative to fMRI to image neural activation? PET is not in everyday use because the emission of positrons into living tissue is enough of a safety concern that it is not possible to conduct many trials in a single subject. fMRI also has temporal resolution, allowing researchers to correlate activity with performance on several different tasks rather than with all tasks the subject has performed since receiving the injection. This makes it far easier to set up control and 2