Lecture 2 Cells of the Nervous System PDF

Lecture 2 Cells of the Nervous System Neurons Supporting Cells (read at home) The Blood–Brain Barrier (read at home) X.Liu PSYC5130 Warning!! Today is vocabulary-heavy Learn them now! It will pay 10x later Please ask questions along the way Think of this journey as a game X.Liu PSYC5130 Three types...

Lecture 2 Cells of the Nervous System Neurons Supporting Cells (read at home) The Blood–Brain Barrier (read at home) X.Liu PSYC5130 Warning!! Today is vocabulary-heavy Learn them now! It will pay 10x later Please ask questions along the way Think of this journey as a game X.Liu PSYC5130 Three types of neurons Three types of neurons Sensory neurons Motor neurons Interneurons These neurons perform functions essential to tasks such as perceiving, learning, remembering, deciding, and controlling complex behaviors. X.Liu PSYC5130 Sensory, Motor, and Interneurons These three types of neurons relay information in the nervous system. In this example, the person sees the glass of water and sensory nerves relay the sensory information toward the central nervous system. The motor output from the central nervous system allows the person to lift the glass to take a drink. X.Liu PSYC5130 Function of neurons Take in information Make a decision (Maybe) pass that information along X.Liu PSYC5130 Parts of a Neuron X.Liu PSYC5130 X.Liu PSYC5130 Synaptic Connections between Neurons The arrows represent the direction information is traveling. X.Liu PSYC5130 Communication within a neuron Information is transferred via electrical signals X.Liu PSYC5130 Neurons are charged When everything is at rest Voltage, the difference in electric potentials between the two poles X.Liu PSYC5130 Membrane Potential X.Liu PSYC5130 Why? It’s all about the ions … –Ions = small charged particles ▪Cations (positively charged) ▪Anions (negatively charged) The distribution of ions between the intracellular and extracellular are different. The difference causes negative Membrane Potential. X.Liu PSYC5130 Membrane Potential Force of Diffusion Diffusion is the movement of ions from regions of high concentration to regions of low concentration. X.Liu PSYC5130 Membrane Potential Force of Electrostatic Pressure X.Liu PSYC5130 Membrane Potential Diffusion Force = Electrostatic Pressure Force Equilibrium X.Liu PSYC5130 Establishing the Membrane Potential Major “Players”: Ions that matter Organic anions: A Potassium ions: K+ Chloride ions: Cl Sodium ions: Na+ “Lanes”: Mechanisms of Transport Leaking channel (much more K+ channels) Gated channel (e.g., voltage-gated channel, closed at rest) The Sodium/ Potassium Pump X.Liu PSYC5130 Establishing the Membrane Potential X.Liu PSYC5130 Establishing the Membrane Potential Extracellular- Positive Leaking Channel Gated Channel Na/K Pump Intracellular - Negative X.Liu PSYC5130 Establishing the Membrane Potential Extracellular- Positive Leaking Channel Na+ Cl- K+ Na+ Na+ A- K+ Gated Channel ClCl- K+ Na+ Cl- Na/K Pump K+ Intracellular - Negative X.Liu PSYC5130 The Sodium–Potassium Pump Uses up to 40 percent of neuron’s metabolic resources ($3.6 CC Can meal) X.Liu PSYC5130 Ion Channels When ion channels are open, ions can pass through them, entering or leaving the cell. X.Liu PSYC5130 Establishing the Membrane Potential “Players”: Ions that matter Organic anions (A-) : trapped inside Manufactured there Too large to go through channels X.Liu PSYC5130 Establishing the Membrane Potential Extracellular- Positive Leaking Channel A- Gated Channel Na/K Pump Intracellular - Negative X.Liu PSYC5130 Establishing the Membrane Potential “Players”: Ions that matter Organic anions (A-) : trapped inside Manufactured there Too large to go through channels Potassium ions (K+): More inside than outside free to move active pump in (Na/K pump) X.Liu PSYC5130 Establishing the Membrane Potential Extracellular- Positive Leaking Channel A- K+ K+ Gated Channel K+ K+ Na/K Pump K+ Intracellular - Negative X.Liu PSYC5130 the Nernst equation E=RT/zF ln [Ion]out/[Ion]in E= membrane potential z=valence of ion (charge) F=Faraday constant for electrical forces T=absolute temperature (in Kelvin) R=universal gas constant [Ion]=concentration of ion, inside or outside the cell X.Liu PSYC5130 the Nernst equation E=61 mV/z log [Ion]out/[Ion]in E= membrane potential z=valence of ion (charge, can be positive or negative) F=Faraday constant for electrical forces T=absolute temperature (in Kelvin) R=universal gas constant [Ion]=concentration of ion, inside or outside the cell X.Liu PSYC5130 the Nernst equation EK=61 mV/z log [5 mM]out/[100 mM]in EK= -80 mV (at 37 degrees C) X.Liu PSYC5130 EK= -80 mV Resting Potential: –70 mV Extracellular- Positive Leaking Channel A- K+ K+ Gated Channel K+ K+ Na/K Pump K+ Intracellular - Negative X.Liu PSYC5130 Establishing the Membrane Potential “Players”: Ions that matter Organic anions (A-) : trapped inside Manufactured there Too large to go through channels Potassium ions (K+): More inside than outside free to move active pump in (Na/K pump) voltage gradient with A- Sodium ions (Na+): More outside than inside gates closed, locked out, little leakage Active pump out (Na/K pump) X.Liu PSYC5130 Establishing the Membrane Potential Extracellular- Positive Leaking Channel Na+ Na+ Na+ Na+ Gated Channel Na+ Na+ Na/K Pump Intracellular - Negative X.Liu PSYC5130 Equilibrium potentials and the Nernst equation ENa=61 mV/z log [150 mM]out/[15 mM]in ENa= 62 mV (at 37 degrees C) X.Liu PSYC5130 ENa= 62 mV Resting Potential: –70 mV Extracellular- Positive Leaking Channel Na+ Na+ Na+ Na+ Gated Channel Na+ Na+ Na/K Pump Intracellular - Negative X.Liu PSYC5130 Establishing the Membrane Potential “Players”: Ions that matter Organic anions (A-) : trapped inside Manufactured there Too large to go through channels Potassium ions (K+): More inside than outside free to move active pump in (Na/K pump) voltage gradient with A- Sodium ions (Na+): More outside than inside gates closed, locked out, little leakage Active pump out (Na/K pump) Chloride ions (Cl-): More outside than inside X.Liu PSYC5130 Membrane Potential: –70 mV Extracellular- Positive Leaking Channel Na+ Cl- K+ Na+ Na+ A- K+ Gated Channel ClCl- K+ Na+ Cl- Na/K Pump K+ Intracellular - Negative X.Liu PSYC5130 Membrane Potential: -70 mV X.Liu PSYC5130 The next step in our experiment Stimulus X.Liu PSYC5130 We are going to play with a simulator X.Liu PSYC5130 The next step in our experiment Stimulus X.Liu PSYC5130 Hyperpolarization Results from negative current Increase the membrane potential (more negative) Greater negativity inside the neuron Response: graded potentials X.Liu PSYC5130 Graded Potentials A slight change in the Resting Potential Restricted to the vicinity of where it was elicited due to decay with travel (like a wave) Passive – Response essentially mirrors current X.Liu PSYC5130 How about positive current Stimulus X.Liu PSYC5130 Depolarization Results from positive current Decrease the membrane potential Less negativity inside the neuron Response: Action Potential may happen X.Liu PSYC5130 How about positive current Stimulus X.Liu PSYC5130 Comparison Membrane Potential Inside Neuron Current Response Hyperpolarization Increase More negative Negative Graded Potential Depolarization Decrease Less negative Positive Graded Potential or Action Potential X.Liu PSYC5130 The Action Potential Caused by the brief opening of voltage-gated Na+ Channels and then the brief opening of voltage-gated K+ Channels. X.Liu PSYC5130 The Action Potential X.Liu PSYC5130 The Action Potential X.Liu PSYC5130 The Action Potential Na+ channels open rapidly, Na+ enter cell --- depolarizing the neuron After the opening of Na+ channels, K+ channels slowly open, K+ begins to leave cell --- repolarizing, but not strong enough to stop depolarization At the peak, Na+ channels close, no more Na+ enters neuron --depolarization stops X.Liu PSYC5130 Ion permeability X.Liu PSYC5130 The Action Potential K+ continues to leave cell – causing membrane potential to return to the resting level. K+ channels close slowly, so more than “necessary” K+ leaves the cell – causing hyperpolarization (Undershoot) Refractory period – hard to “fire” again X.Liu PSYC5130 Ion permeability X.Liu PSYC5130 The Action Potential K+ channels closed, and Na+ channels reset – causing membrane potential to return to resting level. Re-establishes equilibrium Concentration Gradient = Voltage Gradient X.Liu PSYC5130 Another simulator https://phet.colorado.edu/en/simulations/neuron X.Liu PSYC5130 Tetrodotoxin (TTX) X.Liu PSYC5130 X.Liu PSYC5130 X.Liu PSYC5130 Let’s Put Action Potential into Action! X.Liu PSYC5130 Continue our experiment X.Liu PSYC5130 Recall that there was a threshold of current necessary to produce an AP If we increase the level of the pulse currents beyond the threshold, what happens? – A. The amplitude of the AP increases w/ increases in current. – B. The amplitude of the AP remains constant. Action Potential is All or None X.Liu PSYC5130 The Rate Law The strength of a stimulus is represented by the rate of firing of an axon. The magnitude (size) of each action potential is always constant. Stronger stimuli = more APs X.Liu PSYC5130 Conduction of the Action Potential When an action potential is triggered, its size remains the same as it travels down the axon. The graphs at the top of the figure represent the size (amplitude) of the action potential as it moves along the axon from left to right. X.Liu PSYC5130 X.Liu PSYC5130 Let’s do the beat again! X.Liu PSYC5130 Saltatory Conduction Myelin acts as an insulator that aids conduction. Restricts points at which extracellular Na+ may enter X.Liu PSYC5130 Saltatory Conduction X.Liu PSYC5130 Saltatory Conduction AP Leap Frog X.Liu PSYC5130 Supporting Cells (1 of 3) Supporting Cells of the Central Nervous System Neurons constitute only about half the volume of CNS Neurons have very high rate of metabolism but no means of storing nutrients Cells that support and protect neurons are important to our existence Glia or glial cells are the most important supporting cells X.Liu PSYC5130 Supporting Cells (2 of 3) Supporting Cells of the Central Nervous System (continued) Types of glial cells Astrocytes Nutritive functions Engulf and digest debris in process of phagocytosis Oligodendrocytes Provide support to axons; produce myelin sheath Node of Ranvier Microglia Act as phagocytes; and protect brain X.Liu PSYC5130 Figure 2.10 Structure and Location of Astrocytes The processes of astrocytes surround capillaries and neurons of the central nervous system. Astrocytes regulate chemicals in the synapses (upper panel) and the chemical composition of the fluid surrounding neurons (lower panel). X.Liu PSYC5130 Oligodendrocyte An oligodendrocyte forms the myelin that surrounds many axons in the central nervous system. Each cell forms segments of myelin for several adjacent axons. X.Liu PSYC5130 The Blood–Brain Barrier Blood–Brain Barrier Paul Ehrlich’s experiment over 100 years ago demonstrated existence of blood–brain barrier Barrier is selectively permeable; produced by the cells in walls of brain’s capillaries Regulates composition of extracellular fluid Area postrema is region where barrier is weak Example: can detect toxic substances entering blood and induce vomiting X.Liu PSYC5130 Figure 2.13 The Blood–Brain Barrier The blood–brain barrier is selectively permeable. Some substances, such as water molecules, can pass through the cells of the capillaries passively. Other molecules require expending energy to move between the tightly packed cells of the capillaries. Astrocytes further strengthen the barrier by closely regulating any substances entering the brain from the capillaries. In the rest of the body, larger gaps between the cells in the capillaries allow greater movement of substances in and out of tissues. X.Liu PSYC5130