2008_05_30_16_14_58

2008_05_30_16_14_58 - BILD2 Name KEY Winter Quarter, 2006...

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Unformatted text preview: BILD2 Name KEY Winter Quarter, 2006 1. (5 points) How is an organ diflerent from an organ system? (Definitions are sufficient here.) Organ: grouping of tissues producing ajoint function (e.g. the heart). Organ system: groups of organs with shared responsibility (e.g. cardiovascular system). (10 points) What is the function of each of the following (very briefly; one sentence each): Generator potential - local potential in a sensory neuron that drives action potentials in proportion to the intensity of the physical stimulus. Hippocampus - region of the brain (part of the limbic system) responsible for encoding new (declarative) memories. Myelin - wrapping of axons by glia that produces insulation and speeds up conduction of the action potential. Neuromuscular junction — synapse formed by the motor neuron axon terminal onto the skeletal muscle fiber. Nernst equation - Em = V", = 5810g([ion]m/[ion]in) OR value of Vm at which the ion shows no net flux across the membrane because the force driving it down its concentration gradient is equal and opposite to the electrical force imposed by the charge distribution. (10 points) What is the ionic basis of the resting potential? List the principles involved and indicate how these are fulfilled in a neuron, i.e. what the specific conditions are that produce the RP. Ion concentration gradient: High concentration of 1C inside; low concentration outside. Selective permeability: selectively permeable to K" at rest. ' K" diffuses out across the membrane, down its concentration gradient until it gets so negative inside (near 139 due to the negative counterions remaining behind (mostly negatively charged proteins) that further loss of K” is prevented. BILD 2 Name KEY Winter Quarter, 2006 4. (10 points) What produces the falling phase of the action potential? Why does it actually fall below the resting potential for a little while? The voltage-gated (VG) Na+ channels inactivate when the membrane is depolarized (e.g. at the top of the AP) stopping further Na’ influx, and the VG K‘ channels open allowing K’ to flow out, returning the VIn to RP. VIn falls below RP temporarily because the abundance of VG K” channels still open swamp any residual leak Na” current; hence Vm approaches Ex. Once the VG channels close, the resting leak currents (balance in favor outward K” but still containing some inward Na‘) return the V“, to RP. 5. (10 points) How is an inhibitory postsynaptic potential (IPSP) different from an excitatory PSP (EPSP)? Consider the neurotransmitters involved, ions that flow, and consequences for induction of action potentials in the postsynaptic neuron. An IPSP is a local postsynaptic potential that tends to prevent the Vm from reaching threshold for firing an AP. This is commonly done by either GABA or glycine activating their partner ionotropic receptors which are permeable to Cl'. Since l5.CI is usually below threshold for an AP, activating sufficient Cl' currents will prevent an AP from being fired. (Note: the book points out that transmitters can also activate metabotropic receptors which in some cases link to K’—selective channels; this would be an alternative mechanism for generating an IPSP.) 6. (10 points) From the book: Show where the primary motor cortex and the primary somatosensory cortex are on a rough sketch of the brain. What do they do, i.e. how do they differ functionally from each other? The primary motor cortex lies at the rear of the frontal lobe and sends information (APs) out to the motor neurons of the brain and spinal cord (issuing commands). The primary somatosensory cortex lies immediately behind (and adjacent to) the primary motor cortex and receives sensory information from the body (i.e. from the sensory neurons that transduce the physical stimuli and relay it to the somatosensory cortex). [Consult book figure for sketch: Fig 48.27] BILD 2 Name KEY Winter Quarter, 2006 7. (15 points) How is ATP used to generate muscle contraction? Use diagrams if you like, but clearly indicate the roles of myosin and actin through one complete cycle of ATP usage and replacement. ATP binds to the myosin head group and is cleaved to ADP + Pi but still bound. This cleavage of ATP “coc ” or “activates” the myosin, bending it forward. When the signal comes for muscle contraction, the adjacent actin filament becomes exposed, allowing the activated myosin head group to form a cross-bridge. It does so, and discharges its energy by springing backward, dragging the actin filament with it (along with the attached Z line which contracts the sarcomere) and releases the ADP + Pi. The myosin head group then binds another ATP and repeats the cycle until the message comes to stop contracting (in which case the actin is again covered up so that the myosin can’t bind). 8. (15 points) List the events occurring in a photoreceptor cell that result when a photon strikes the photo pigment retinal What is the Dark Current? Does light produce an excitatory or inhibitory response in the downstream (postsynaptic) bipolar cell (explain)? Light (a photon) strikes a retina] and converts it from cis to trans configuration. This conversion (causes a conformational shift in the protein which) releases a G protein which activates an enzyme (phosphodiesterase) which cleaves cGMP (cyclic GMP) to GMP; As cGMP is destroyed, it can no longer keep open the Na“ channels that it “gates”. The channels close, stopping the Dark Current, i.e. that current carried into the cells by Na‘ in the dark. As a result, the Vm becomes more negative and prevents the release of transmitter which had been occurring in the dark due to the Dark Current. The loss of transmitter (glutamate) release onto the downstream bipolar cells will depend on the kind of (glutamate) receptors the cell has. (If ionotropic, they would be excitatory; stopping transmitter release would have an inhibitory effect. If metabotropic, they could be inhibitory; stopping transmitter release would have an excitatory effect.) BILD 2 Name KEY Winter Quarter, 2006 9. (15 points) What’s the difference between an AMPA receptor and an NMDA receptor? If a glutamate synapse had only NMDA receptors, what would happen when you stimulated the presynaptic input? How would you convert such a synapse (hopefully) to normal? Explain. AMPA receptors open up in response to glutamate (gated by glutamate) and permit (monovalent) cation flux, principally Na+ (& K“), which depolarize the membrane. NMDA receptors are also gated by glutamate but can’t open unless the membrane is already depolarized; when they do open, they mostly let Ca“ in. Presynaptic stimulation of synapse that had only NMDA receptors postsynaptically would do nothing. The NMDA receptors could not by themselves depolarize the membrane. To convert such a synapse to competence (i.e. to get PSPs), one would have to simultaneously depolarize the membrane while stimulating the presynaptic input to release glutamate onto the receptors. Under this condition the receptors could open, let in Ca“, and reth their own AMPA receptors to the synapse and now would be functional as a synapse in the future (a form of LTP). WM); Roger Nicol] first found these “NMDA-only” synapses using only electrodes to stimulate and record. Keeping in mind that you can pass current through an electrode even while recording, how do you think he demonstrated the existence of such synapses? ‘ He depolarized the postsynaptic cell to +50 mV (holding it long enough to inactivate the VG Na+ channels) while stimulating the presynaptic input. This depolarization released block on the NMDA receptors and now he could see current through them! He called them “silent synapses" because they would not fire under normal conditions. These “silent” synapses are presumably available for training, i.e. to get LTP. -.... END .... (see scores histogram on next page) ...
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This note was uploaded on 08/30/2008 for the course BILD 2 taught by Professor Schroeder during the Fall '08 term at UCSD.

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2008_05_30_16_14_58 - BILD2 Name KEY Winter Quarter, 2006...

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