Understanding Brain Dissection and Histology Part 2

Neurobiology Exam 1 Lecture Summary: • Neural circuits tend to be replete/stocked with diverging, converging, and reciprocal connections. • The brain is comprised of the forebrain, midbrain and hindbrain, which consist of even smaller subdivisions which serve different functions • Forebrain: • Diencephalon (Preoptic Area, Retina, Hypothalamus, Thalamus) • Telencephalon (Cerebral Cortex, Olfactory Bulb, Telencephalic Nuceli) • Cerebral Cortex (Piriform Cortex, Neocortex, Hippocampus) • Telencephali Nuclei (Pallidum, Striatum, Septum, Amygdala) • Midbrain: • Colliculi (Superior and Inferior) • Tegmentum • Hindbrain: • Cerebellum, Pons, Medulla • Major players in our understanding of neurons: Descartes, Schwann & Scheldon, Ramon y Cajal, Camillo Golgi, Charles Sherrington • Studying the brain requires some sort of framework: informed by neuropsychology and neuroethology. Neurobiology Exam 1 Resting and Action Potential What is a resting potential vs. an action potential? Resting Potential (Membrane Potential): The voltage difference (electrical potential) across the cell membrane that a cell exhibits when it is not receiving external inputs (resting) -The voltage difference between the inside and outside of the neuron -Resting Potential = -70mV Action Potential: An all-or-none electrochemical signal generated by neurons in response to above-threshold stimulation. -A brief, transient reversal of polarity of the membrane potential that sweeps along the membrane of a neuron. -Action Potential = +40mV Ion: An atom or molecule with a net positive or negative electrical charge (K+ or Na+) What mechanisms generate resting potentials? Resting Potentials Ionic Basis of the Resting Potential: (Key discovery made on squid giant axons) The axon’s intracellular fluid differs from the extracellular fluid in ion composition. Means that the neurons are actively pumping sodium ions out across their cell membranes and pumping potassium in. Concentration of K+ much higher INSIDE axon ! Concentration of Na+ much higher OUTSIDE axon Changing this relative concentration of an ion across the membrane can affect the resting potential. Resting Potential Membrane Diagram: Factors Which Induce an Ion to Cross a Membrane: 1.) Action of an ion pump 2.) A difference in the concentration of ions on two sides of the membrane (due to the selectively permeable membrane) 3.) An electrical potential difference across the membrane Neurobiology Exam 1 Ion Pumps: Specialized transporter molecules that pump ions into and out of neurons which requires energy in the form of ATP due to the fact they are moving ions against their concentration gradients. *ATP is the brain’s main currency of metabolic energy Na+/K+-ATPases: Enzyme that moves K+ in and Na+ out of a cell. 2 Potassium IN — 3 Sodium OUT — 1 ATP USED Na+/K+-ATPase Diagram: Selective Permeability: The ability of a membrane to allow some ions or small molecules to pass into or out of the cell regularly, while restricting the passage of others. **The resting neural membrane is slightly permeable to K+ ions but almost completely impermeable to other ions such as Na+. Means that there are little K+ channels letting it inside the membrane. **The reason why neurons are negatively charged at rest is that the resting neuronal membrane is slightly permeable to potassium ions but almost completely impermeable to other ions, including sodium. this means that some of the potassium ions pumped into the neuron by the Na+/K+ATPase flow right back out, down their concentration gradient. As this occurs, the neuron’s extracellular space becomes positively charged relative to the neuron’s interior, setting up a voltage difference across the neuronal membrane. More specifically, the net movement of potassium ions out of the neuron causes the neuron to develop their negative membrane potential. (Page 37) **The movement of the K+ ions are fighting against two opposing forces: Concentration gradient and electrical potential gradient Neurobiology Exam 1 Concentration Gradient: A difference in the concentration of ions across two sides of a cell membrane. -In regard to K+, it is trying to push K+ out Electrical Potential Gradient: A difference in electrical potential between the inside of a cell and its surrounding extracellular space. -In regard to K+, it is trying to pull K+ in Gradient Diagrams: Electrochemical Equilibrium Diagram: 1.) The initial condition is created by the Na+/K+-ATPases 2.) K+ ions will start to diffuse down along their concentration gradient/outside membrane 3.) The buildup of charges on either side of membrane causes the electrical potential gradient to move the K+ back into the cell 4.) The equilibrium state is created when outward flow K+ is balanced by inward flow K+ Neurobiology Exam 1 *As the neuron’s inside becomes more negative, the positive potassium ions become ever more reluctant to exit the neuron because they are attracted to the net negative charge inside the cell and repelled by the net positive charge outside of the neuron. Hence, the two governing gradients. (Page 37) The net movement of potassium through all of these mechanisms is what sets a neuron’s membrane potential -Flow of K+ due to Na+/K+-ATPase pump -Flow of K+ due to electrical gradient -Flow of K+ due to concentration gradient Equilibrium Potential: The membrane potential at which these two opposing forces are balanced in regards to potassium. The formula used to calculate this as well as other observations, suggests that push and pull of potassium across the neuronal membrane is indeed the major cause for the neuronal resting potential. **SIDE NOTE: Because the membranes are so thin and are generally a good electrical insulator (meaning ions cannot cross pure lipid bilayers), electrical charges on one side of the membrane can be electrostatically attracted to opposite charges on the other side. When the inside of the neuron is more negative, little negative charges develop to line this side of the membrane and little positive charge develop to line the outside of the membrane. This line up — opposite charges lined up on either side of the membrane which is acting like an electrical insulator — creates a capacitor. It takes time for the capacitor to charge (ions to accumulate on either side) and once charged, they take time to discharge. The fact that capacitors take time to change their charge affects how rapidly a neuron’s membrane potential can change and consequently, how rapidly neuronal signals can propagate along the neuronal membrane. (Page 39) Ion Channels: A transmembrane protein with a central pore through which ions can flow. Leak Channel: A type of ion channel that is permeable to potassium and opens near a neuron’s resting potential, allowing potassium ions to “leak” out of a neuron at rest. What mechanisms generate action potentials? Action Potentials Hodgkin-Huxley Model: Discovered the role of K+ in setting the membrane potential. They systematically varied the K+ concentrations outside of a frog muscle fiber and found that the fiber’s membrane potential became less negative as extracellular K+ increased. The data they plotted matched the data predicted based on the equilibrium potential Nernst equation for K+ except at very low concentrations of K+ which is when the membrane’s permeability to Na+ comes into play. -At the beginning of an action potential, the axonal membrane suddenly becomes highly permeable to Na+ ions. Neurobiology Exam 1 Rising Phase: Voltage-gated ion channels allow Na+ ions to flow. -These channels open and Na+ rushes in causing depolarization. Depolarization: Negative internal charge of the cell briefly becomes positive and increases the probability that the neuron will fire an action potential. -Making a cell’s membrane potential more positive or less negative (e.g. changing the potential from -70 mV to +40 mV.) -The Na+ ions flow down their concentration gradient into the cell Voltage-Gated Ion Channel: Class of transmembrane proteins that form ion channels that are activated by changes in the electrical membrane potential near the channel. Either depolarization or repolarization. Graded Potential: The strength of the current pulses can vary leading to different degrees of depolarization with different outcomes. -Changes in membrane potential that vary in size, as opposed to being all-or-none. They arise from the summation of the individual actions of ligand-gated ion channel proteins, and decrease over time and space. They do not typically involve voltage-gated sodium and potassium channels. Action Potential Threshold: The membrane potential at which voltage-gated Na+ channels open and, consequently, the action potential is triggered. When the K+ channels open, the repolarization of the membrane begins. -The membrane potential at which the positive feedback between the membrane depolarization and sodium influx kicks in is call the action potential threshold. **Action potentials are ALL-OR-NONE **There can be varying degrees of depolarization but NOT action potentials Feedback Loop for the Action Potential: The more the membrane is depolarized beyond the threshold, greater increase in Na+ permeability, more Na+ enters the membrane, the increased Na+ causes additional/further depolarization. Repolarization: The process that brings the membrane potential back towards its resting value after a strong depolarization. -Includes slow opening of voltage-gated K+ channels and rapid closure of Na+ channels. -Takes the membrane potential to -75 mV Hyperpolarization: Change in the cell’s membrane potential to become more negative than its resting potential. This inhibits action potentials and is considered part of the refractory period. -The K+ channels close and the membrane potential is restored/extra negative. Refractory Period: A period following an AP when the neuron cannot generate another AP. Neurobiology Exam 1 Graph of the Phases of the Action Potential Diagram: Diagram of Membrane Stages During Action Potential: Resting Membrane Depolarization Repolarization/Hyperpolarization Return to Resting Membrane Neurobiology Exam 1 Resting Membrane: Impermeability of the membrane to Na+, while some K+ flows out. Depolarization: Voltage-Gated Na+ ion channels open, Na+ influx; Voltage-gated K+ ion channels slowly begin opening. Repolarization: Voltage-Gated Na+ ion channels snap shut; Voltage-gated K+ ion channels open, K+ efflux. Hyperpolarization: Inhibition of action potentials causing a refractory period to “recharge” and get back to the normal resting membrane potential. **More detailed explanation** -What keeps the neuronal membrane at rest -70mV = impermeable to Na+, some K+ flows out -During the rising phase of the AP, the membrane becomes highly permeable to Na+ making it rush down its concentration gradient into the axon causing depolarization to +40mV. This only lasts for 1ms due to the fact the Na+ voltage-gated channels snap shut 1ms after they open (Sodium Channel Inactivation). -During the falling phase the membrane becomes less permeable to Na+ and much more permeable to K+ which causes K+ to rush out of the cell, causing the overall hyperpolarization to -75mV. The K+ voltage-gated channels open at the same time as the Na+ ones but open more slowly. The difference in speed/length of channels being open is what causes the difference in polarization. If they were both open at the same time for the same amount of time, Na+ and K+ would be moving simultaneously and they would end up canceling each other out. -Soon after, K+ returns to its resting level at -70mV ! *There is overlap seen in squid axons between the sodium influx and potassium efflux which is energetically inefficient. This overlap in mammals is minimal and indicates evolution has made the action potential generation more efficient in mammals than squid. How do action potentials travel along axons? Axon Hillock: This is where action potentials are generated. They have a high density of voltage gated Na+ channels making them the first place that reaches the threshold for the action potential initiation. ! -Action potential propagation is directional — moves ! away from the axon hillock.! -The massive influx of Na+ ions at one location of the ! axonal membrane tends to cause depolarization in ! nearby areas and will trigger Na+ influx at adjacent ! locations on the axon (as long as those locations are ! not in the refractory period) which in turn causes the ! action potential to travel along the axon following ! these influxes and this continues on until to AP has ! reached the end. Neurobiology Exam 1 **Can think of it like dominos with the axon hillock being the initial domino being pushed and the wave of the rest going down is the propagation of the AP down the axon. Propagation Speed of Action Potentials: Travel relatively slowly and depend on several factors: -Time it takes voltage gated Na+ channels to open -Time it takes for Na+ ions to flow into the axon -Time for successive patches of axonal membrane to reach the threshold for AP triggering *If you increase the diameter of an axon, the propagation speed increases BUT this is costly metabolically and takes up lots of space. Myelin Sheath: A multilayered wrapper of myelin-rich glial cell membranes around an axon. This increases the speed of action potential propagation BUT myelinated segments of the axon cannot generate action potentials. Node of Ranvier: A small gap in the myelin sheath, usually occurring at regular intervals along the axon where Na+ rushes in during an action potential. Saltatory Conduction: Action potentials can “jump” from one node of ranvier to another further down the axon. This increases the speed of the action potential propagation. Since the most time consuming part of an action potential propagation is the depolarization of a membrane patch to the threshold for the action potential triggering. In myelinated axons, there are significantly fewer patches of membrane which must be depolarized than in unmyelinated axons. Pros of Myelination: -Decreases signal transmission time/faster conducting time -Saves metabolic energy needed to propagate the potential -Good, evolutionarily evolved design Cons of Myelination: -Myelin is bulky ! -Not entirely metabolically cost free; must be built during development and then maintained *Mostly long axons are myelinated due to the fact if closely placed neurons were all myelinated they would be bulkier and have to be further apart from one another which means the connections to one another would have to be longer. The rule of thumb in the brain is to save wiring. So to reduce the amount of wiring neurons that are close together don’t usually have myelin. This is why there is no myelin in the gray matter of the brain. *Squids do not have myelin so they compensate for this with large axons. ! Processes: Any projection from the cell body of a neuron. Neurobiology Exam 1 How do neurons differ from one another? Neuronal Differentiation: I. Function ! II. Anatomy ! III. Neurotransmitters ! IV. Receptors ! V. Ion Channels ! ! I. Neuronal Functional Differences: Sensory (Afferent) Neurons: Respond to sensory stimuli. Usually have long dendrites, short axon. (Take in signals) Motor (Efferent) Neurons: Deliver output to muscles or glands. Usually have short dendrites, long axon. (Send out signals) Interneurons/Projection Neurons: Send information between sensory neurons and motor neurons. Interneurons mainly connect to nearby neurons which gives them short dendrites and short axons. Projection neurons connect to distant neurons in the brain which gives them short dendrites and long axon. (Process signals) ! II. Neuron Anatomical Differences: Bipolar Neurons: Have two processes — an axon and dendrite projecting from each opposing side of the neuron cell body. Usually specialized sensory neurons like in the retina look like this. Monopolar Neurons: Considered either unipolar or pseudo unipolar. They have a single short process leaving the cell body which leads to longer axons and dendrites. Usually most of the body’s sensory neurons look like this. Multipolar Neurons: Have three or more processes protruding from the cell body usually one axon and multiple dendrites. This is the most common neuron type found in spinal motor nerves, pyramidal neurons, Purkinje cells.