Chapter 11: Part 2: Neurophysiology
What causes RMP?
1. Difference in ionic concentrations inside and outside the cell 2.Difference in membrane permeabilities to these ions
What are the two types of graded potential?
-2 types of graded potential: 1. Receptor Potential/generator potential: /produced when a sensory receptor is activated by light/stimulus, this stimulates opening of K+ ion channels causing K+ to diffuse out and Na+ in, causing a brief change in RMP 2. Postsynaptic Potential: /Occurs between 2 neurons: the axon terminal of one neuron and the cell body or dendrites of another, this is one way neurons can communicate to each other /postsynaptic in skeletal muscle is known as end plate potential
What is resting membrane potential? Does every cell have a membrane potential? What mV changes happen in the three stages of changing RMP?
-ALL cells have a resting membrane potential -A resting neuron when its not sending electrical impulse has a resting membrane potential -Only excitable cells can change their RMP -RMP=potential difference (mV) that exist across the membrane -When a neuron is -70 mV it is polarized/at its resting potential because its more negative inside than outside of the cell -If it were to get more negative: from -70 to -90, hyperpolarization would be occuring (any polarization past -90 is hyperpolarization) -If it were to get less negative -70 back to -50, depolarization would be happening -If it went from -50 back to -70, repolarization would be happening -this electrical difference/difference of ions is only at the plasma membrane
How are Na+ and K+ ions distributed across the cell membrane? What maintains this distribution? Does Na+ diffuse in or out through leakage channels? Does K+ diffuse in or out through leakage channels? Is the membrane more permeable to K+ or to Na+?
-K+ and protein anions (A-) mostly inside -Na+ and Cl- mostly outside -Therefore: Gradient for K+ to diffuse out and a gradient for Na+ to diffuse in using Na+ leakage channels and K+ leakage channels -Theoretically, if membrane were only permeable to K+, (only K+ leakage channels), RMP would be -90 mV, in reality the RMP is -70 -The membrane is 25x more permeable to K+ than than Na+ because there are 25x more K+ channels /losing so much positive K+ ions increases negative membrane potential but some Na+ entry back through leakage channels slightly reduces the negative membrane potential from -90 to -70 -The Na+ and K+ pump keeps the ionic gradients in a steady state/maintains the distribution
What type of transport are leakage channels? What would happen to the membrane potential if the Na+/K+ pump stopped working?
-Leakage channels are passive so they don't use ATP -This means ions move down their concentration gradients through this 2 way diffusion until they reach equilibrium/an equal number of Na+ and K+ ions inside and out -If Na+/K+ pump stopped working: Neurons function because they have a -70 RMP but once equilibrium is reached, you've removed this gradient and therefore the RMP disappears and the cell can no longer function-this occurs at death
How are Na+ and K+ distributed across a cell membrane?
-More K+ inside the cell than out -More Na+ outside than in
What does voltage mean? What does potential difference mean? What does current mean? What does resistance mean?
-Voltage: The measure of potential energy generated by separated electrical charges-think about the concentration of of charged molecules on one side of the membrane. You can measure the voltage of the plasma membrane which is the difference in charged ions. -Potential (potential difference): Voltage measured between 2 points: the inside and outside of the cell -Current: flow of electrical charge/when charged ions move from one side to the next there is an electrical current -Resistance: hindrance to charge flow/resisting flow of ions, the higher the resistance, the better the electrical insulator /the greater the voltage the greater the current. The greater the resistance the smaller the current.
What is a graded potential? What is an Action potential?
1. Graded potential -Brief, localized changes in RMP -Graded: size of change/magnitude in RMP is proportional to the stimulus-the stronger the stimulus, the more the voltage changes and the farther the current flows -Decremental: decreases with distance -Usually occurs in dendrites or cell body -Can be either depolarizations or hyperpolarizations -Any graded potential produced on cell body or dendrites moves towards axon hillock (beginning of axon) as it moves farther from the site of stimulus the graded potential gets more negative and goes from -40 to -60 -The purpose of graded potential is to produce action potential-send stimulus to the brain say like hey theres light 2. Action Potential (also called a nerve impulse) -An AP is a brief reversal of membrane potential followed by a return to RMP -Occurs in neurons and muscle cells, both of which are excitable-can only occur in excitable cells -In neurons, AP only in axons -Does not decay with distance -Initially activated by graded potentials that spread towards the axon
What channels create the difference of ions?
Created by two classes of ion channels: leakage and gated channels: allow for ions to go through the membrane from one side of the cell to the next 1. Leakage channels (or non-gated channels) -ions are free to flow through channels from area of high concentration to low, so it moves down its concentration gradient 2. Gated channels -passive transport down concentration gradient -gated only opens in presence of specific chemicals -3 types: a.) Chemically gated ion channels/ligand gated-open in response to specific chemicals-binding of the appropriate neurotransmitter (acetylcholine) b.) Voltage-gated ion channels- open in response to changes in membrane potential/voltage which is typically between -50 and -55 when voltage gated ion channels open c.) mechanically gated-which is a physical deformation of receptors
What is the RMP of a neuron in mv? What is threshold in mv? What happens when a neuron reaches threshold? Can an Action potential normally vary in size? Can action potentials summate?
RMP=-70 -If the graded potential is sufficient enough to move across the membrane to the axon an AP will be generated -Threshold=-50 or -55 is a sufficient electrical stimulus that is going to cause a threshold to open VGC-VG Na+ channels in the first phase of AP and start a positive feedback cycle-positive feedback cycle aplifies depolarization by opening more Na+ channels -If you hit threshold the AP will happen completely-so it cannot summate-action potentials are all or none this means each AP is its own separate event because you have refractory periods -When the intensity of the stimulus is increased, the size of the AP does not become larger. Rather, the frequency or number of AP's increases -Below this: subthreshold-not sufficient stimulus
Differentiate between the Absolute refractory period and the Relative refractory period. Explain the cause of each.
Refractory Periods: 1. Absolute Refractory Period (ARP): -The time following (during) an AP/stimulus during which another AP cannot be generated also enforces one-way transmission of the AP -Begins with opening of Na+ channels and ends when Na+ channels begin to reset to resting state -No chance of another AP because it corresponds to VG Na+ channels already being open and no AP can be generated until they close -Therefore, because of VG Na+ channels being open each AP is a separate all-or-none event 2. Relative refractory period: -Follows the absolute refractory period -During RRP most VG Na+ channels have returned to resting state but some VG K+ channels are still open -RRP corresponds to voltage gated K+ channels being open -Need a greater than normal stimulus to elicit an AP because K+ channels are still open so the cell is getting more negative which is the opposite of whats needed for an AP
Describe what occurs during an action potential. What causes each of these 3 phases?
Stages in AP and what occurs in each stage: At rest Na+ and K+ VGC are closed 1. Depolarization: membrane becomes more positive (+30 mV) -Cause: when a graded potential arives at axon hillock, if its stimulus is sufficient it reaches a threshold and opens VG Na+ channels and positive ions/Na+ enters into the cell through voltage gated Na+ channels-takes about a milisec to close and open 2. Repolarization: membrane heading back to -70/returning towards RMP -Cause: VG Na+ channels close by inactivating (depolarization is self limiting) and voltage gated K+ channels open (opening began at threshold) and because you have more K+ inside than out, positively charged K+ ions leave the cell making the cell more negative (+ charged ions leaving balances + ions that entered) 3. Hyperpolarization: during AP you overshoot your RMP causing it to become more negative, past -70 (to about -90) -happens because those voltage gated K+ channels stay open too long/close slowly, causing RMP to be overshot
Describe the difference between Voltage gated Na+ channels and voltage gated K+ channels. What is happening to Na+ permeability during depolarization? What is happening to K+ permeability during repolarization? Hyperpolarization? Does the Na+/K+ pump play a role in the action potential?
Voltage gated Na+ channels: -Closed at rest. Open in response to depolarization which comes from graded potential that arrives at axon hillock after a delay -within one millisec they open and close because inactivation gate comes in and blocks the channel -short period when this can occur -causes depolarization, during depolarization Na+ floods the cell and makes it less negative (adds +30) -Na+ permeability highest during depolarization Voltage gated K+ channels: -Closed at rest. Open in response to depolarization which comes from graded potential that arrives at axon hillock after a delay -takes a millisecs to open: they open slowly at depolarization and also close slowly which causes hyper polarization -K+ channels are most important for creating RMP -K+ leaving causes repolarization -K+ channels closing slowly causes hyperpolarization -K+ permeability highest during repolarization Na+/K+ pump helps to return to original ion distribution, pump does not cause depolarization, repolarization or hyperpolarization it just helps to return to and maintain that -70 RMP
How do neurons function? What are the two phenomena that result in a membrane potential?
neurons function by changing their membrane potential (the difference in ions on either side of the membrane) because they can change the concentration of charged ions that means they are excitable (can send and receive information) 2 ways neurons can change their membrane potential: 1. Nerve impulses=action potential (AP) 2. Chemical transmission=synapses -where they are transferring electrical impulse from one cell to the next -all cells have a membrane potential that can range from -50 to -100 mv
