2 Solns prob set 3

2 Solns prob set 3 - BILD 2, Multicellular Life ­...

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Unformatted text preview: BILD 2, Multicellular Life ­ ­Problem set #3 solutions Page 1 1. Because it is rare for synaptic input from a single presynaptic neuron to elicit an action potential in a postsynaptic cell (the exception is the neuromuscular junction), no one presynaptic input can dominate the activity in any given postsynaptic neuron. I (a) Instead, at every point in time, the presynaptic inputs sum, and their integrated effect on Vm at the spike ­initiating zone determines whether the postsynaptic cell will produce an action potential. (b) In addition, the amount of transmitter released from the presynaptic axon terminal (as the result of an action potential arriving along the axon) varies; in many cases the presynaptic neuron fails to release transmitter, and when transmitter is released, the amount varies. (C) Furthermore, PSPs can be either excitatory or inhibitory, and of course all of the PSPs ­ ­ not just the excitatory ones ­ ­sum. As a result, whether or not a postsynaptic neuron produces an action potential depends on the activity level and neurotransmitter release in ALL of its presynaptic neurons, and that can vary a lot. 2. a. A ganglion is a cluster of neuron cell bodies that are located outside of the central nervous system. b. A nerve is a collection of axons that are located outside of the central nervous syste. c. A neuromuscular junction (like all synapses) consists of a presynaptic axon terminal, a synaptic cleft, and a postsynaptic cell. At the neuromuscular junction, the axon terminal is the ending of a spinal motor neuron, and the postsynaptic cell is a skeletal muscle fiber. 3. a. An afferent neuron carries information from the periphery into or through the central nervous system to the "highest center" where that information will go ­ ­typically to the cerebral cortex. Afferent neurons carry information toward the cortex. b. An efferent neuron carries information from the "highest center" where the information originated ­ ­often the cerebral cortex, but not always ­ ­ through the central nervous system or out to the periphery. Efferent neurons carry information away from the cortex. c. An interneuron is one that is entirely contained within the central nervous system. (An interneuron can be either excitatory or inhibitory, and it can be afferent, efferent, or neither.) d. An excitatory neuron is one that increases the probability of action potentials in the cells that are postsynaptic to it. BILD 2, Multicellular Life ­ ­Problem set #3 solutions Page 2 4. a. Visual information is sent along pathways through the brain (from the retina, through a the lateral geniculate nucleus, and then) to the bilaterally symmetric primary visual projection cortex in the occipital lobe of the cortex. b. Auditory information is sent along pathways through the brain to the bilaterally symmetric auditory projection cortex in the temporal lobe of the cortex. c. Olfactory information is sent to the bilaterally paired olfactory cortex, which is located in a different part of the temporal lobe of the cortex. d. Motor output is initiated and coordinated first in the primary motor cortex, but it also depends on inputs to the motor cortex from neurons in the cerebellum and the basal ganglia. 5. I'm typing these answers, so I am using: i. primary visual cortex to perceive black shapes on a white background, ii. visual association cortex and other parts of the cortex to recognize that the shapes are letters and the letters form words, iii. yet other regions of association cortex to remember what I've learned and to formulate answers, iv. and still other regions of association cortex to translate what I've remembered into words, that will be black shapes on a white background, v. motor pathways including the primary motor cortex, the cerebellum, and the basal ganglia to produce the high ­dexterity movements of my fingers as they type, vi. back to my visual cortex to perceive the black shapes on a white background that my fingers have produced, vii. back to regions of association cortex to interpret the black shapes and decide whether my answer is correct. 6. a. Motor neurons in the autonomic nervous system synapse onto internal structures OTHER THAN skeletal muscle fibers: e.g., smooth muscle fibers in the walls of internal organs and the blood vessels, muscle fibers in the walls of blood vessels, muscle fibers in the heart, and the cells of some glands. In contrast, motor neurons in the voluntary nervous system synapse onto skeletal muscle fibers. Period. b. i. Neurons in the two branches of the autonomic nervous system typically synapse onto the very same target organs. As a result, all of these organs receive input from both the sympathetic AND the parasympathetic nervous system. BILD 2, Multicellular Life ­ ­Problem set #3 solutions Page 3 ii. Synaptic input from sympathetic neurons speeds up the heart rate, whereas synaptic input from parasympathetic neurons slows down the heart rate. iii. The path that forms the connection between the CNS and target organs consists of two neurons in both the sympathetic and the parasympathetic branches of the autonomic nervous system. In both branches, the synapse between the first neuron in the chain synapses onto the second neuron in the chain in a ganglion. iv. For sympathetic pathways, the cell body of the first neuron in the two ­neuron chain (that is, the one that has its cell body inside the CNS) is located in the thoracic and upper lumbar spinal. For parasympathetic pathways, the cell body of the first neuron in the two ­neuron chain is located either in the brainstem or in the sacral region of the spinal cord. 7. a. Primary somatosensory cortex is the first part of the cerebral cortex that receives information from sensory neurons that are specialized to receive information such as touch, pressure, muscle stretch, pain, temperature, etc. b. Information from the face and mouth is sent to the most lateral region of the somatosensory cortex on both sides of the brain; information from the legs and feet is sent to the most medial region of the somatosensory cortex on both sides of the brain. c. Primary motor cortex is the part of the cortex where movements are initiated and patterned; it is the last region of the cortex where motor information is processed before it is sent through the brain, down the spinal cord, and out to the skeletal muscle fibers in the periphery. d. The map in the primary motor cortex is very similar to the map in the primary somatosensory cortex. That is, information that will control the face and mouth is sent from the most lateral region on each side of the primary motor cortex, and information that will control movements of the legs and feet is sent from the most medial region of the primary motor cortex. PLEASE NOTE: In this context, the word "information" refers to signals carried along axons as action potentials and across synapses by chemical synaptic transmission. 8. a. The components that produce long ­term potentiation include i. A glutamatergic presynaptic neuron (that is, a presynaptic neuron that releases the neurotransmitter glutamate from its presynaptic terminals) ii. A postsynaptic neuron that produces two different kind of glutamate receptors; one kind is called NMDA receptors after a drug that activates these receptors, and the other is called AMPA receptors after a drug that activates these receptors. NMDA does not activate AMPA receptors; AMPA does not activate NMDA receptors. iii. Glutamate (the natural ligand for NMDA and AMPA receptors) and extracellular magnesium (Mg2+). BILD 2, Multicellular Life ­ ­Problem set #3 solutions Page 4 The steps in LTP: i. Before LTP, there are NMDA receptors in the membrane of the postsynaptic neuron, but few or no AMPA recepetors. At the normal value of the resting potential in the postsynaptic neuron, although NMDA receptors are present in the membrane, their ion channel is blocked by Mg2+ ions that are bound to the outside of the receptor, blocking the ion channels. Thus, even if the presynaptic receptor releases glutamate, there will be little or no responses in the postsynaptic neuron because the only postsynaptic receptor ion channels that are opened by the glutamate are the AMPA receptors, and there aren't many of them in the membrane. (There are a lot of them inside the cell, but they produce a postsynaptic potential only if they are inserted in the membrane. ii. However, if the postsynaptic cell is depolarized ­ ­say by synaptic input onto that cell from other presynaptic neurons ­ ­at the same time that the presynaptic glutamatergic cell releases glutamate, the situation is different. Depolarization of the postsynaptic membrane causes the Mg2+ to leave the ion channel in each NMDA receptor complex. iii. Now when glutamate binds to the NMDA receptors, an ion channel opens in each receptor complex opens, and this ion channel allows Ca2+ to enter the postsynaptic cell. iv. As you learned in BILD 1, Ca2+ is a very powerful intracellular signaling molecule, and in this case, it activates intracellular events that cause AMPA receptors to be inserted into the postsynaptic membrane, increasing the number of glutamate receptors that are available to bind glutamate. v. Because mores glutamate receptors are now in the postsynaptic membrane, more ions can enter the postsynaptic cell when glutamate is present in the synaptic cleft, so the PSPs produced in the postsynaptic cell in response to each synaptic event is bigger. vi. This change in the number of AMPA receptors in the postsynaptic membrane is maintained for a long time, so it's called "long ­term" potentiation. b. Addiction is a highly complex process, and we've discussed only the simplest description. However, our description is an important component of all addictions. The steps are: i. Dopaminergic neurons in the brain (that is, neurons that release the neurotransmitter dopamine) are activated by a chemical substance. ii. The axons of these dopaminergic neurons synapse onto neurons in another region of the brain that is strongly associated with feelings of intense pleasure. (One such location is the bilaterally paired nucleus accumbens.) iii. With repeated stimulation of the dopaminergic neurons, cells of the nucleus accombens accommodate to the input (that is, they respond less strongly to it), so the dopaminergic neurons must be stimulated more strongly to produce the same feelings of pleasure, and withdrawal of the addictive substance that stimulated the dopaminergic neurons produces feeling of wretchedness. BILD 2, Multicellular Life ­ ­Problem set #3 solutions Page 5 iv. Most drugs of addiction affect activity of the dopaminergic neurons, either directly (e.g. nicotine) or indirectly (e.g., opium and heroin). 9. a. The intensity of a sensory stimulation is encoded two ways: i. In the specialized sensory receptive membrane, it is encoded in the amplitude of the receptor potential, and ii. Along the axon, it is encoded in the number and frequency of the action potentials. b. A neuron that is postsynaptic to the sensory neuron would receive this information as a change in the amount and timing of neurotransmitter released into the synaptic cleft. If the action potentials coming to the synapse were close together in time (i.e., high frequency action potentials), temporal summation could increase the size of EPSPs and hence increase the probability of action potentials in the postsynaptic neuron. c. All sensory pathways between the periphery and the brain consist of many neurons linked together by chemical synapses. Information is transmitted along axons as action potentials, and it travels across the synapses connecting the neuron in the chain by way of chemical synaptic transmission. 10. a. Hair cells in the cochlea change their transmembrane potential ­ ­and hence their release of synaptic transmitter ­ ­when the cilia ("hairs") are bent relative to the rest of the cell. The cilia are bent when the tectorial membrane, in which the cilia are embedded, moves with respect to the basilar membrane, in which the base of each hair cell is anchored. b. The change in the release of neurotransmitter by hair cells onto the first neuron in the auditory pathway (it's called the sensory neuron) changes the transmembrane potential in the sensory neuron, which modifies the number and frequency of action potentials transmitted along the axon of the sensory neuron. c. THIS INTRODUCES SOME NEW INFORMATION. GOOD FOR YOU FOR WORKING THE PROBLEM SET! The intensity of a sound (and its pitch as well) is encoded in the number of hair cells excited, their position along the cochlear canals, and the number and frequency of action potentials transmitted along the axon of the sensory neurons. 11. a. The steps in the response of photoreceptors to light: i. In the dark, retinal photoreceptors are somewhat depolarized because their plasma membrane contains Na+ channels that remain open and allow a small but steady inward Na+ current. The membrane also contains the usual "leak" K+ channels, but the current through the open Na+ channels make the membrane somewhat more depolarized than in most cells. at rest ii. Because the photoreceptor molecules are slightly depolarized in the dark, they continuously release neurotransmitter molecules onto postsynaptic neurons (called bipolar cells and horizontal cells). BILD 2, Multicellular Life ­ ­Problem set #3 solutions Page 6 iii. When rhodopsin molecules absorb a photon of light (one photon per rhodopsin molecule), the retinal part of the rhodopsin undergoes photoisomerization. That is, the cis ­retinal in the molecule changes to trans ­retinal, which triggers an intracellular second ­messenger cascade causing the voltage ­gated Na+ channels to close. iv. When the Na+ channels close, the current through the K+ channels remains, and it causes the photoreceptor to hyperpolarize, reducing or halting its release of neurotransmitter. b. The response of a photoreceptor to light is conveyed to the next cells along the pathway as a drop in the amount of neurotransmitter that binds to postsynaptic recepetors. c. Primary visual cortex is the first location in the cerebral cortex where information arrives. It is in the occipital lobe. d. A visual signal travels through the retina from photoreceptors to bipolar cells and from bipolar cells to retinal ganglion cells. The axons of retinal ganglion cells transmit the signal as action potentials that travel along the axon to the axon terminals, which are located in the lateral geniculate nucleus. (Some of the visual information is carried across the midline when some of the axons cross the midline in a structure called the optic chiasm.) e. The retinal ganglion cells synapse onto neurons that have their cell bodies in the lateral geniculate nucleus and that have axons that extend to primary visual cortex. f. Thus there are a minimum of two chemical synapses within the retina, one in the lateral geniculate, and then the synapses onto cortical neurons. ...
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This note was uploaded on 02/18/2012 for the course BILD 2 taught by Professor Schroeder during the Spring '08 term at UCSD.

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