Cell Physiology of Neurons Lecture 2022-2023 PDF

Physiology of Neurons Electrochemical properties and communication Dr. Harry Witchel Discipline Leader in Physiology Room 204 Trafford Centre for Medical Research 01273 873 549 [email protected] Synapses Action Pot. Learning Outcomes Synaptic Transmission: Chemical or Electrical Graded Pot....

Physiology of Neurons Electrochemical properties and communication Dr. Harry Witchel Discipline Leader in Physiology Room 204 Trafford Centre for Medical Research 01273 873 549 [email protected] Synapses Action Pot. Learning Outcomes Synaptic Transmission: Chemical or Electrical Graded Pot. Equilibrium Excitability Action potentials Physiology of Neuronal Excitability Equilibrium Potentials Graded potentials 2 Synapses Action Pot. Excitability Equilibrium Graded Pot. Recommended Reading Tortora (Global ed.)  Chapter 12 (Nervous Tissue) • Electrical Signals & Synapses Guyton & Hall (More detailed text book, 14th ed)  Chapter 46: Synapses (you can skip the first 3 pages)  Review Chapter 4: transport  Review Chapter 5: membrane potentials & action potentials Synapse Lecture (HJW) from Module 102 3 Synapses Action Pot. Excitability Equilibrium Graded Pot. Electrical Synapses Faster coupled via gap junctions Bidirectional Much smaller gap = 3.5 nm No plasticity (thus no learning)*E No amplification 4 Synapses Action Pot. Excitability Equilibrium Graded Pot. *No* amplification in electrical synapse Signal is always weakened as it is transmitted from pre-synaptic to post-synaptic cell Pre Post Signal will not transmit if post-synaptic cell is much bigger than pre-synaptic cells Excitatory pre-synaptic signal cannot inhibit the post-synaptic cell 5 Synapses Action Pot. Excitability Equilibrium Graded Pot. Summation of Signals Spatial summation = A neuron determines whether to fire based on the “add together” of all the tiny signals it is receiving from several other neurons synapsing on it (from both excitatory and inhibitory inputs). In this way small depolarisations (if there are many) can reach threshold  See diagram of convergent neuronal signalling Temporal summation = When the input neuron is firing fast enough so that the receiving neuron can “add together” the many tiny signals, ultimately reaching threshold.  This happens when the receiving neuron’s ability to recover from the tiny input (depolarisation) is slow enough that the next signal arrives while the receiving neuron has not yet recovered from the previous signal (i.e. it is still slightly depolarized) 6 Synapses Action Pot. action potential DEP OLA RISA T Equilibrium THRESHOLD ISAT ION Excitability IO N OVERSHOOT R REPOLA Graded Pot. If you understand this slide, then you understand the next 5 slides RESTING STATE After-hyperpolarization 7 For clear animation explaining how AP works (Harvard) see https://youtu.be/oa6rvUJlg7o Synapses Action Pot. Excitability Equilibrium Graded Pot. The Action Potential in 4 steps At rest K+ that is going out of cell clamps the membrane potential negative (e.g. -70 mV) An external factor (e.g. synaptic activity) causes the membrane to depolarise slightly. If the voltage reaches threshold, then Na+ conductance shoots up, Na+ current goes into cell, membrane potential depolarises (voltage  +) With a time-delay, Na+ conduction diminishes (inactivation), K+ conductance increases, so K+ leaves cell, voltage returns to resting potential (i.e. the membrane repolarises) 8 How to answer an exam question on drawing an AP (BSMS): http://youtu.be/ETm_ZCHJ0Nc Synapses Action Pot. Excitability Equilibrium Graded Pot. Action Potential: Initial Depolarisation The cell starts at rest (-70 mV)  Inward rectifier K+ channels are open, K+ flowing out is the dominant current  Resting membrane potential is near EK Something causes the cell to become less negative  Depolarisation: inside the cell the voltage becomes less negative (or more positive)  Could be a nearby cell depolarising  Could be synaptic transmission where a neurotransmitter opens a ligand-gated channel 9 Synapses Action Pot. Excitability Equilibrium Graded Pot. Action Potential: Positive Feedback of Depolarisation The initial depolarisation causes a few of the Na+ channels to open  Na+ permeability increases, Na+ current flows through channels into cell The additional current of Na+ going into the cell  more depolarisation (ie the membrane potential moves closer to 0 mV) This acts as a positive feedback loop When the voltage goes above the threshold voltage (approx. -50 mV), the cell is committed to an AP  APs are “all-or-none”. The positive feedback of ↑ Na+ channel conductance and ↑ voltage continues until the membrane becomes quite positive (> +30 mV)  when Vm > 0, call this period the “overshoot” 10 Synapses Action Pot. Excitability Equilibrium Graded Pot. Action Potential: Repolarisation Repolarisation = the voltage becomes less positive (or more negative) inside the cell Due to the passage of time, 2 delayed-action events occur  Na+ channel inactivation  ↓ Na+ current going in  Delayed rectifier K+ channels open  ↑ K+ going out These cause the membrane to be less positive and more negative inside 11 Synapses Action Pot. Excitability the amount of time it takes for neuron’s membrane to be ready for a second stimulus once it returns to its resting state following an excitation Refractory period occurs mostly during afterhyperpolarisation (AHP) Graded Pot. period of time during which neuron is incapable of reinitiating an AP Equilibrium Refractory Period 12 Synapses Graded Pot. Equilibrium Excitability Action Pot. Action Potential: After-hyperpolarisation After-hyperpolarisation (AHP) = at the end of an AP the voltage inside temporarily goes slightly more negative than at rest, followed by a return to the resting membrane potential When the voltage goes below -60 mV, the inward rectifier K+ channels open again; they stay open until next depolarisation  These normally clamp the voltage toward EK, and are responsible for maintaining the resting membrane potential During AHP:the ↑ K+ permeability and ↓ Na+ permeability  the membrane potential moves closer to E(K) 13 Synapses Action Pot. Excitability Equilibrium Graded Pot. Coding of intensity by neurons From Carpenter Core concept Action potentials are all or none  they carry no information about the size of the stimulus that elicited them So how do neurons code the intensity of their synaptic input? θ (theta) = threshold voltage for AP Firing frequency represents the intensity of activity  Increasing threshold lowers firing frequency (see figure)  Increasing excitatory synaptic activity increases firing frequency  When lengthy (>10 msec) synaptic currents are small, they create a higher threshold potential for action potential generation than larger currents do,  This is due to accommodation of Na+ current (which inactivates during the slower subthreshold depolarization) Different neurons for different strength stimuli  Light touch receptors vs. pain receptors (see sensory receptor lectures) 14 Synapses Action Pot. Excitability Equilibrium Graded Pot. Excitability of Neurons: Threshold Excitability = how easy to start nervous signalling  “sensitivity” in sensory cells  “irritability” in muscle or effector cells  Risk of seizure or spasms Increased threshold lowers excitability  Threshold = voltage above which action potential fires Excitability changes are the basis of psychotropic pharmacology  Changes in threshold have profound health & behavioural effects 15 Synapses Action Pot. Excitability Equilibrium Channels: Open, Closed, Inactivated Channels are proteins  Sometimes they conduct ions • Sometimes they do not conduct  These are different “conformational states” Voltage-gated channels change states based on transmembrane voltage  These channels open when membrane becomes positive inside • Channels in the open state can conduct = increased permeability • Inward rectifiers are the opposite of other channels  Channels close when membrane repolarises (when negative inside) Inactivated ≠ Closed Graded Pot.  although they both are non-conducting  Inactivated = when a channel stops conducting (after a delay) when the membrane is positive inside 16 Synapses Action Pot. Excitability Equilibrium Graded Pot. Voltage depends on ion permeabilities 1 When Na+ channels open, the membrane tends to become positive inside  [Na+] is higher outside the cell than inside Membrane voltage is described in terms of what happens to the intracellular face of the membrane  When the inside of the membrane is positive with respect to the extracellular facing part of the membrane, we say, “The membrane is positive”  The extracellular space of all cells is electrically joined and thus has the same voltage everywhere  The extracellular fluid is considered the electrical ground 17 Synapses Action Pot. Excitability Equilibrium Graded Pot. Voltage depends on ion permeabilities 2 When K+ channels open  the membrane tends to become negative inside  K+ ions travel from inside to outside • Due to the chemical gradient  Because [K+] is higher inside than outside the cell  This exit of K+ causes the membrane to become negative inside When Ca2+ channels open  the membrane tends to become positive inside  [Ca2+] is higher outside than inside  Calcium passively goes inward 18 Synapses Increased permeability to K+ makes membrane more negative The voltage of the cell membrane is determined by inter-related feedback loops Formally: Graded Pot. Excitability Increased permeability to Na+ makes membrane more positive Equilibrium Action Pot. Ionic permeabilities control Voltage At rest, Vm = ~ EK • because conductance of K+ is >> conductance of Na+ or Ca2+ Vm = gKEK + gNaENa + gCaECa + gClECl gK + gNa + gCa + gCl if gK >> than gNa, gCa and gCl Then Vm = ~ EK g = conductance 19 Synapses Action Pot. Excitability Equilibrium Graded Pot. Lidocaine = Lignocaine Local anaesthetic Apply topically Raises the threshold And thus lowers excitability Which stops action potentials locally 20 Synapses Action Pot. Excitability Equilibrium Graded Pot. Carbamazepine = anticonvulsant Carbamazepine inactivates sodium channels  Among other actions Raises AP threshold and Lowers excitability Other Na+ channel blockers: Antiarrhythmic drugs (Class 1) e.g. quinidine Works by lowering conduction velocity  Which extends the refractory period Fugu fish (pufferfish) poison (tetrodotoxin (TTX)) 21 Synapses Action Pot. Excitability Equilibrium Graded Pot. In Class Question Glibenclamide is a sulfonylurea administered to manage type II diabetes. It works (in part) by increasing the excitability of pancreatic beta cells, which leads to an increase in insulin secretion. What is its most likely mechanism?  Inhibiting a Na+ channel  Inhibiting a K+ channel  Inhibiting a Ca2+ channel  Activating a Cl- channel 22 Synapses Action Pot. The Two Forces on Each Ion The chemical force  Also called diffusional force Graded Pot. Equilibrium Excitability  Is based upon the difference in concentration across the membrane  E.g. If there is 10X as much Na+ outside than inside, the chemical force on Na+ channels is 60 mV directed into the cell The electrical force  This is based on Vm (the membrane potential, which varies over time) This is the basis of the equilibrium potential 23 Synapses Excitability + + + - + + - - - - + - - K+ K+ - K+ K+ Membrane at rest Vm = -80 mV + + + - - + K+ 90 mV K+ Electrical Force - - Na+ - - - Na+ + + - - - + + - + - Na+ Na+ Na+ Na+ K+ Na+ Na+ Chemical Force (diffusional) [K+]extracell = 5 mM [K+]intracell = 140 mM + + + + Na+ Na+ Compare K+ to Na+ More Na+ extracellularly http://youtu.be/f3JEyg7WgOo mV -60 Equilibrium + K+ K+ Graded Pot. -80 mV Electrical Force -80 mV Action Pot. [K+] is ALWAYS high inside the cell Na+ Na+ Chemical Force (diffusional) 24 Synapses Action Pot. Excitability Equilibrium Electrical Force EK is also called the reversal potential of K+  EK is the voltage where K+ flowing out = K+ flowing in because electrochemical forces on K+ are + in equilibrium  This occurs when the diffusion (chemical) forces pushing K+ out of the cell equal the voltage (electrical) forces pushing K+ into the cell The Nernst Equation is used to calculate the equilibrium potential:  You do not need to memorise the equation + + - + + - - - - + -90 mV Graded Pot. EK – The Equilibrium Potential - - K+ K+ + K+ K+ - K+ K+ K+ + + - - + K+ 90 mV K+ Chemical Force (diffusional) 25 Synapses Action Pot. Excitability Equilibrium Graded Pot. Notes for Previous Slide regarding +ve vs -ve This is information for experts and the truly curious  You definitely do not need to know this information On the previous slide, I drew the electrical force on K+ going inward as negative.  This is correct.  By definition, when a force drives a positive charge outward, it is a positive force. • So if a force drives a positive charge inward, the force is negative • For negative ions, the forces are reversed • So a force that drives a negative charge inward is a positive force. In the previous diagram I drew a chemical force driving positive K+ outward as positive  Yet, when we talk about the equilibrium potential of K+, the voltage of EK is said to be negative.  This is an oddity based on the discovery of these forces in the 1950s  By definition, the equilibrium potential (which is a way of describing the chemical force) is described as “the electrical voltage that exactly counter-balances (i.e. is equal and opposite to) the chemical force.” This is sometimes called “the reversal potential”.  Thus, the equilibrium potential = −1 x the chemical force On exams, it will be less confusing for you to simply state the magnitude (ie positive) of the force and draw an arrow showing the direction of the force 26 Synapses Action Pot. Excitability Equilibrium Graded Pot. Equilibrium Potentials The more permeable the cell membrane is to K+, the more the membrane potential approaches the value of EK If the membrane was 1000X more permeable to K+ than to any other ion, the membrane potential would = EK EK = -90 mV 27 Synapses Graded Pot. Equilibrium Excitability Action Pot. Equilibrium Potentials ENa = +60 mV EK = -90 mV ECa = +123 mV mV) ECl = -40 mV (in neurons –65 Different tissues have slightly different numbers, and different books quote slightly different figures Core memorization 28 Synapses Action Pot. Excitability Equilibrium Graded Pot. Open ion channels control voltage: examples Na+ K+  If many Na+ channels are conducting but there are no other currents, the membrane potential (Vm) will go to +60 mV  If many K+ channels are conducting, but there are no other currents, the membrane potential (Vm) will go to -90 mV Both Na+ & K+  If both K+ channels and Na+ channels were open, and if the cell was exactly equally permeable to Na+ and K+ (which is unlikely), the membrane potential would go to the average between their equilibrium potentials (i.e. -15 mV) 29 Synapses Action Pot. Excitability Equilibrium Graded Pot. Mnemonic for equilibrium potentials A CAt is as easy as 1-2-3 A NAg retires early at 60. A CLub for fighting Ali Baba’s 40 thieves A King will die at 90. Remember that for the first two positive events the voltages are positive, while the next two negative events have negative voltages. Easy things are positive; so are cats. A nag retiring early is good news, both for the nag and for you. Fighting Ali Baba’s 40 thieves all at once is quite scary (negative). When the king dies, it is negative. 30 Synapses Action Pot. Excitability Equilibrium Graded Pot. Nernst Equation For Deriving Equilibrium Potentials  [ K ]o 5 EK 61.5log  61.5log  89mV [ K ]i 140  [ion]o RT E 2.3 log zF  [ion]i    Where 2.3 X (RT/zF) = ~ 60 mV, 31 if z = +1 Synapses Action Potentials are: A Stereotyped electrical signal Short-duration Voltage Action Pot. Excitability Equilibrium Graded potentials vs action potentials in most neurons, skeletal and cardiomyocytes a spike Time Graded Pot. Always the same — “All or none” Require time to start because of conformational changes 32 Synapses Action Pot. Graded potentials *Decrease as they move along* Graded Pot. Equilibrium Excitability Electrically localised Last a long time much Flatter in shape Are conducted almost instantly  in receptor cells (eg rods & cones) http://youtu.be/N4Z_Bl337BQ Variable In duration and voltage 33 Passive conduction of voltage Graded Pot. Equilibrium Brief channel opening allows ions to flow depolarised here Flow Decreased by Resistance Myelin: High resistance Shields electric field + +   + + Now membrane is   Excitability  + +  Synapses Action Pot. Electrotonic Conduction in Axon Cytoplasm: Low resistance Instantly transmits electric field Basis of: saltatory conduction, all summation, graded potentials Larger diameter  Lower resistance  faster conduction 34 Synapses Action Pot. Excitability Equilibrium Graded Pot. Graded potentials transmit signals Changes in membrane potential do not propagate very far via passive electrical forces Voltage signals diminish as you go farther from the source  This happens because the axon has a finite resistance Depol smaller smaller to +40 smaller RMP For mV changes of cells and APs, the change would be all but extinguished in 1 cm 35 Electrotonic conduction (graded potentials) transmits signal along the length of the axon. The AP is a way for re-amplifying that signal Synapses Graded Pot. Equilibrium Excitability Action Pot. Graded potentials transmit action potentials In essence, APs can only occur at the red triangles, as if the Na+ channels only existed at the red triangles From Carpenter 36 Synapses Action Pot. Excitability Equilibrium Graded Pot. Saltatory Conduction: Graded Pot —> AP When the action potential “jumps” from Node to Node Node of Ranvier Net effect  faster conduction velocity The electrotonic jumps between nodes are very fast. Initiating an Action Potential at each node is slower Long section  Conformational change of ion channels 37 Synapses Action Pot. Excitability Conduction Velocity along the length of the axon Faster when: Myelinated Larger diameter Graded Pot. Equilibrium (lower resistance) Typical conduction velocities  100 m/s for alpha motor fibres (myelinated, 15 um diam.)  1 m/s for C nociceptive fibres (unmyelinated, 0.2 - 1.5 um) 38 Synapses Action Pot. Excitability Equilibrium Graded Pot. Clinical Uses of Conduction Velocity Nerve conduction studies are used for evaluation of paraesthesias  numbness, tingling, burning Evaluation of weakness of the arms and legs Can help diagnose  Peripheral neuropathy  Carpal tunnel syndrome  Ulnar neuropathy  Guillain-Barré syndrome  Facioscapulohumeral muscular dystrophy  Spinal disc herniation 39 Synapses Action Pot. Excitability Equilibrium Graded Pot. Summary Synaptic activity transmits signals between neurons Action potentials transmit signals along the length of neurons  Graded potentials happen at receptors Ion channel activity makes the AP occur in a small region of membrane Interfering with action potentials, synapses or myelin (via trauma or drugs) affects amount and velocity of nervous transmission  Which can affect brain activity (e.g. anaesthetic/psychiatric drugs)  Changes in excitability are critical in pharmacotherapy 40