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JHU BME 580.422 Biological Systems II Neurons Reza Shadmehr Retinal ganglion cell (neuron) and the surrounding blood vessels David Becker, Univ. College London This is a ferret retinal ganglion cell injected with Lucifer Yellow and Neurobiotin. The image also includes blood vessels with hemoglobin. The shadow from the neuron has made some of the hemoglobin appear dark. The image was captured by confocal microscopy. Neurons have four functional regions: • Input component (dendrite) Apical dendrites • Trigger area (soma) • Conductive component (axon) Inhibitory synapse • Output component (synapse) Cell body Excitatory synapse Nucleus basal dendrites axon Axon hillock Presynaptic myelin Node of Ranvier cell axon Presynaptic terminal Synaptic cleft Postsynaptic dendrite Postsynaptic cell Kandel et al. (2000) Principles of Neural Science Glia: support cells for neurons • Produce myelin to insulate the nerve cell axon. • Take up chemical transmitters released by neurons at the synapse. • Form a lining around blood vessels: blood-brain barrier. neuron axon mylein Oligodendrocyte synapse dendrite neuron neuron astrocyte blood vessel R. D. Fields, Sci Am April 2004 Injury in a peripheral nerve When a peripheral nerve is cut, the portion of the axon that was separated from the cell body dies. Cell body The glia cells that produce the myelin sheath around the dying axon shrink, but stay mostly in place. Node of As the cell body re-grows the axon, it Ranvier uses the path that is marked by the glia cells. injury In this way, the glia cells act as a road Myelin Myelin map for the injured neuron to find its sheath sheath previous destination. Neurons Neurons in different parts of the CNS are very similar in their properties. Yet the brain has specialized function at each place. The specialized function comes from the way that neurons are connected with sensory receptors, with muscles, and with each other. Kandel et al. (2000) Principles of Neural Science The conductive component (axon) propagates an action potential • An action potential is a spike that lasts about 0.5 ms. • Signal travels down the axon no faster than ~100 m/sec. • Action potentials do not vary in size or shape. All that can vary is the frequency. Voltage (mv) An action potential recorded by putting an electrode inside the giant axon of a squid Timing marker 2 ms Kandel et al. (2000) Principles of Neural Science Recording the electrical activity of neurons 100 mV 0 volts Intra-cellular recording in the soma Extra-cellular recording 0 volts Intra-cellular recording in the axon Recording electrode Low-pass filtered signal Timing Electrode signal (uV) pulses Filtered signal (V) Extra-cellular recordings Extra-cellular recordings: spike sorting Recordings from the human thalamus (Haiyin Chen, Fred Lenz, and Reza Shadmehr) Kandel et al. (2000) Principles of Neural Science Neurotransmitters A neuron can produce only one kind of neurotransmitter at its synapse. The post-synaptic neuron will have receptors for this neurotransmitter that will either cause an increase or decrease in membrane potential. Acetylcholine (ACh) Released by neurons that control muscles (motor neurons), neurons that control the heart beat, and some neurons in the brain. Antibodies that block the receptor for ACh in the muscle cell cause myasthenia gravis, a disease characterized by fatigue and muscle weakness. In Alzheimer’s disease, ACh releasing neurons die in the brain. Glutamate and GABA These are two different amino acids that serve as neurotransmitters in the brain. Glutamate excites the post-synaptic cell. In contrast, GABA inhibits the firing of the post-synaptic neuron. In HD, GABA producing neurons in the basal ganglia die, causing uncontrollable movements. Cell injury causes excessive release of glutamate. Neurotransmitters Dopamine Patients with Parkinson’s disease exhibit a deficiency of this neurotransmitter in their brain. Depending on the receptor, Dopamine can either excite or inhibit the post-synaptic cell. May signal reward prediction errors. Serotonin Serotonin has been implicated in sleep, mood, depression, and anxiety. Depending on the receptor, Serotonin can either excite or inhibit the post- synaptic cell. Prozac is a common drug that alters the action of Serotonin (it inhibits the re-uptake of Serotonin, resulting in increased concentration of this neurotransmitter in the synaptic junction). Second messengers Second messengers are chemicals within the post-synaptic cell that are trigged by the action of the neurotransmitter. (Neurotransmitter is the first messenger.) Second messengers affect the biochemical communication within post-synaptic cell. Whereas a neurotransmitter has an effect that lasts only a few milliseconds, the second messenger’s effect may last as long as many minutes. When a neurotransmitter binds to its receptors on the surface of the neuron’s synapse, the activated receptor binds G proteins on the inside of the membrane. The activated G protein causes an enzyme to convert ATP to cAMP. The second messenger cAMP exerts a variety of influences on the cell, ranging from changes in the function of ion channels in the membrane to changes in the expression of genes in the nucleus. Modifiability of connections results in learning and adaptation With repeated activation of pre- and post-synaptic neuron, their connection via the synapse gets stronger. This is called Long-term Potentiation (LTP) for an excitatory synapse and Long-term depression (LTD) for an inhibitory synapse. Over the long-term, a neuron can grow and make more synapses or shrink and prune its synapses. Setup for inducing Long-term Potentiation (LTP) Rat hippocampus slice Intracellular stimulating Electrode Intracellular recording Electrode CA1 Hippocampal Schaffer Slice CA1 Collaterals CA3 Perforant Path Mossy Fibers DG Modified from Blitzer et al., Biol Psychiat. 57:113 (2005) Stimulating Electrode Recording Electrode An action potential in the CA3 axon results in the release of glutamate at the synapse. Glutamate crosses the synaptic junction and opens sodium and calcium channels, resulting in an EPSP in the CA1 synapse. Stimulating Electrode (CA3 neuron) glutamate (Excitatory Post-Synaptic Potential) (High Frequency Stimulation) Recording Electrode (CA1 neuron) Following a brief period of high frequency stimulation of the CA3 axon, the EPSP in the CA1 synapse in response to the CA3 action potential is increased Excitatory post-synaptic potential (EPSP) recorded in CA1 synapse in response to a single action potential in the CA3 axon Control After 100Hz stimulation 1 of CA3 for 1 minute 2 1 mV 10 ms 250 EPSP slope (% baseline) CA1 neuron 200 2 High frequency 150 stimulation CA3 neuron 1 100 0 -20 0 20 40 60 80 TIME (min relative to high frequency stimulation) What are the mechanisms of LTP? The induction of LTP in the CA3-CA1 synapse involves mostly changes in how the CA1 synapse responds to the glutamate released from the CA3 synapse. The CA1 synapse becomes highly sensitive to a small amount of glutamate. How long after the high frequency stimulation does LTP last? In the brain slice preparation, LTP can last many hours. In the behaving rat, LTP in the hippocampus can last for more than a year. Invention of functional imaging of the brain. When neurons are active, they consume more energy. The vascular system responds to the change in their activity by increasing the blood in the vessels that are near these neurons. By imaging the blood flow, one can make a rough estimate of where in the brain neurons are more active than before. Optical imaging: image the visible light that reflects off the surface of the brain. The brighter the light, the more oxygenated blood it carries. PET: Positron Emission Tomography. A radioactive substance is injected into the blood stream. Detectors estimate amount of blood flow at a given location in the brain by the amount of radiation detected from there. FMRI: functional magnetic resonance imaging. Strong magnetic fields are used to detect amount of oxy-hemoglobin in a particular region of the brain. Intrinsic optical signal response to neural activity Increased activity of neurons results in a small decrease in the oxy-hemoglobin. This decrease is visible in the light that reflects off the surface of the cortex (the image becomes darker) and can be optically measured using a camera. 4.9 mm 300ms Frostig RD et al. (1990) Proc Natl. Acad. Sci. 87:6082. 800ms 1.3s 1.8s 0.1% 5 sec 2.3s Composite image of the blood vessel pattern overlying somatosensory cortex of a rat with the optical signal superimposed. The image shows the blood vessel pattern as imaged through the dura when the camera was focused on the surface of the brain. Signals are average of 24 trials, where the trial begins with stimulation of one whisker of the rat and continues in the 5 sec period. Each signal is an average of 2.8s 24 trials. X-axis is time and Y-axis is fractional change in signal. By convention, the signals are shown as up-going although cortical activation actually causes a decrease in light reflectance. FMRI response to neural activity Increased activity of neurons results in a small decrease in the oxy-hemoglobin. This decrease often cannot be detected by fMRI. About 3 seconds after the increased activity in the neuron, the capillary dilates and dramatically increases the amount of oxy-hemoglobin. This produces a very large increase in the fMRI signal. Percent signal change in the visual cortex 6 1500ms 5 Stimulus Onset Asynchrony 4 500ms (15-17 seconds) 100ms 3 2 500ms 100ms 1 0 1500ms 0 1500ms 0 1 2 3 4 5 6 7 8 9 10 11 Time (sec) How old are the cells in a person’s brain? Carbon dating of DNA The levels of 14C in the atmosphere have been stable over long time periods, with the exception of a large addition of 14C in 1955–1963 as a result of above ground nuclear bomb tests. 14C levels from modern samples are by convention given in relation to a universal standard and corrected for radioactive decay, giving the Δ14C value. 14C half-life is 5730 years. After the test ban treaty in 1963, there has been no above-ground nuclear detonation leading to significant 14C production. 14C levels spanning the last decades were measured in cellulose taken from annual growth rings of local pine trees. The levels have dropped after 1963, not primarily because of radioactive decay, but due to diffusion and equilibration with the oceans and the biosphere (that is, taken up in water and in plants and animal). Spalding, Bhardwaj, Buchholz, Druid, and Frisen (2005) Cell 122:133-143. Cerebellar gray matter is on average about 2 years younger than the person. Cortical gray matter is on average about 5 years younger. 14C in the atmosphere reacts with oxygen and forms CO2, which enters the biotope through photosynthesis. Our consumption of plants, and of animals that live off plants, results in 14C levels in the human body paralleling those in the atmosphere. Most molecules in a cell are in constant flux, with the unique exception of genomic DNA, which is not exchanged after a cell has gone through its last division. The level of 14C integrated into genomic DNA should thus reflect the level in the atmosphere at any given time point. The determination of 14C levels in genomic DNA was used to retrospectively establish the birth date of cells in the human body. Birth of person Amount of 14C in DNA (occipital lobe) Average age of the tissue Spalding, Bhardwaj, Buchholz, Druid, and Frisen (2005) Cell 122:133-143. Summary Axons propagate action potentials, resulting in the release of neurotransmitter at the synapse. Second messengers are chemicals within the post-synaptic cell that are trigged by the action of the neurotransmitter. Modifiability of synaptic strength results in learning and adaptation. The vascular system responds to the change in the neuron’s activity by increasing the blood in the vessels that are near these neurons. The neurons that we have in adulthood are mostly the neurons that we had in very early childhood. However, there is some turnover, as the average age of cells in the cortex is 5 years younger than the person.
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