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notes - NPB12 Lecture 9 ventroposterior thalamus S1 Dorsal...

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Unformatted text preview: NPB12 Lecture 9 ventroposterior thalamus S1 Dorsal root ganglion cell (RA neuron) Dorsal column nucleus Anatomical Divergence: One neuron provides input to many post-synaptic neurons This particular dorsal root ganglion cell will send its axon to the dorsal column nucleus, where it will make synaptic contacts with many (hundred) neurons. The fact that one neuron will make synaptic contacts with many post-synaptic neurons is the concept of divergence. The inputs of one neuron will diverge to many different neurons. Convergence: Many neurons converge and provide input onto a single post-synaptic neuron This is a general principle of the central nervous system, each neuron sends an axon to, and makes a synapse with, many other neurons (divergence), and each neuron receives a synaptic input from many different neurons (convergence). After an amputation there is a change in the receptive fields across many neurons, such that they now respond to a different finger. Merzenich et al. (1984) J. Comp. Neurol.224:591-605 Donald Hebb If two neurons often fire action potentials at the same time, the synapse is strengthened. If two neurons rarely fire action potentials at the same time, the synapse is weakened. Inhibition plays a key role in brain circuits These cortical neurons will provide excitatory input to their neighboring neurons, using glutamate as the neurotransmitter to open Na+ channels in the post-synaptic membrane. T hey also excite inhibitory interneurons, which then release GABA, opening Cl- channels in their post-synaptic neurons. recurrent inhibition chandelier cell Computational models of this circuitry are very complex, yet still fall far short of the capabilities of the brain. T his theory predicts that cortical neurons will have receptive fields on one finger, but not on two fingers, because the skin of one finger gets stimulated together more often than the skin of two different fingers. The balance of excitation and inhibition that results from the local circuitry is a major influence on the receptive field properties of any given neuron in the central nervous system. B A release GABA VeqNa+ C Open Clchannels release glutamate Open Na+ channels Chloride goes in and drives the membrane potential down, the two A's overpower the B even though that will drive it up normally AA membrane potential (mV) B A BB BB B BBB AAA Activation Threshold VeqClVeqK+ tim e Allard et al. (1991) J. Neurophysiol. 66:1048-1058 Neurovascular Island Transplant Cortical Representation Normal (before injury) P R M I Th Right after surgery P R M I Th Long time after surgery PR M I Th How can neurons change their synaptic weights? We have already seen neuromodulators can do that (e.g. dopamine) -Increase binding of glutamate -Increase number of glutamate channels -Increase open time of glutamate channels -Do the opposite for GABA channels Is there another mechanism that the neurons could use? YES! The n-methyl-d-aspartate (NMDA) channel could do it. NMDA receptors are found at the same synapses as the non-NMDA receptors (Na+ channels). Normally this receptor does not let ions pass when the cell is near the resting potential because of a Mg++ block. blocks other ions from going through If the post-synaptic cell is depolarized (in an action potential), the Mg++ goes away and the receptor lets both Na+ and Ca++ into the cell. Ca++ acts as a second messenger to start processes that change the response at the non-NMDA receptor. Long Term A Record stimulate Potentiation LTP glutamate B This has been called long-term potentiation, which means the response is bigger for a long time. "Long time" can mean anything from tens of minutes, to hours, to days, to weeks. There have also been molecules discovered stim stim that result in what is called long term depression, which means that a synapse will go from activating a poststim stim A A A A synaptic neuron to being unable to cause a post-synaptic neuron from firing an action potential membrane potential (mV) activation threshold time minutes, hours, days later What is this plasticity good for? Are phantom limbs and phantom limb pain good? Muscle memory are "late synapses" PRACTICE MAKES PERFECT Presumably practicing something will cause different sets of neurons to fire action potentials at the same time, causes changes in receptive field properties across the cortex. These new receptive field properties will allow for better sensory discriminations, or combinations of thoughts, that will allow you to perform better at some tasks, from digging post-holes to solving fourth order differential equations. If this is in fact the purpose of the brain plasticity, then there is a clear evolutionary advantage of being able to acquire new skills and to learn new things because you will be better able to adapt to changes in the environment. Attention is also necessary for this to work. You don’t get plasticity (or at least as much) unless you attend to the stimuli and are trying to do something with it. Neuromodulators and the NMDA receptor are both important. Attention activates the neuromodulators norepinephrine and acetylcholine, which serve to increase the sensitivity of your cortical neurons to their inputs. Essentially what happens is that many parts of your cortex process new information, whether it be the visual cortex noting that there are many spiders on the ceiling falling down, or the auditory and language parts of the cortex processing the verbal instructions that a deadly mosquito is in the room, that activates the attention system to sensitize the somatosensory cortex. You can get good at very complex things Drop a ball into a cup: need to overcome gravity with the help of friction. you can't tickle yourself because of the sensory feedback, you know it's going to happen, unlike when someone else tickles you. balance a serving tray necessary 1 force overcompensating holding onto the cup, when dropping the ball 1. friction of the surface 2. how heavy it is time Cortical hierarchy: Receptive field complexity increases as you go The projections from SI to area 5 to area 7a to area 7b form what is know as the "cortical hierarchy". This concept is that as information is passed to more and more cortical areas, more and more information is extracted from the convergence of inputs. S1 primary somatosensory cortex rfs < 1 finger anesthetic okay S2 secondary somatosensory cortex rfs 1+ finger anesthetic okay Area 5 rfs hand / arm anesthetic bad sub-cortical and cortical areas involved in pain and affect PV parieto-ventral area rfs 1+ finger anesthetic bad Parietal lobe areas (7a, 7b many others) rfs hand / arm anesthetic very bad ...
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