Neurophys 2 edited PDF

Equilibrium Potentials Ion Intracellular Concentration (mM) Extracellular Concentration (mM) Nernst Equilibrium Potential (mV) Na+ 10 145 +72 K+ 120 4.5 -88 Cl- 35 116 -32 Ca+2 0.0001 1.0 +123 The membrane potential of any cell depends on: • The relative permeability of the membr...

Equilibrium Potentials Ion Intracellular Concentration (mM) Extracellular Concentration (mM) Nernst Equilibrium Potential (mV) Na+ 10 145 +72 K+ 120 4.5 -88 Cl- 35 116 -32 Ca+2 0.0001 1.0 +123 The membrane potential of any cell depends on: • The relative permeability of the membrane to each ion • The concentration of the ion on either side of the membrane If the membrane potential is close to the Nernst potential of a particular ion, it usually means that the membrane is more permeable to that ion Question: • The membrane potential is about -75 mV in many neurons ▪ However, the Nernst potential for potassium is close to -90 mV ▪ Why is the membrane potential of a neuron close to, but not the same, as the equilibrium (Nernst) potential for K+? • The Goldman Field equation is below – modified form of the Nernst equation • p = the permeability of the membrane to a particular ion V𝑚 = −61 × 𝑙𝑜𝑔10 𝑝𝐾 𝐾+ 𝑖 +𝑝𝑁𝑎+ 𝑁𝑎+ 𝑖+𝑝𝐶𝑙 𝐶𝑙− 𝑜 𝑝𝐾 𝐾+ 𝑜+𝑝𝑁𝑎+ 𝑁𝑎+ 𝑜+𝑝𝐶𝑙 𝐶𝑙− 𝑖 Membrane Potentials and Channels • The potential across the membrane depends on concentration gradients and the permeability (or its inverse, the resistance) of the membrane to each ion ▪ When the membrane is permeable to more than one ion, then the Goldman Field equation is necessary to predict the membrane potential • Membranes are poorly permeable to charged particles – movement of an ion across the membrane is dependent on the presence of channels ▪ Pores in the membrane that allow movement of an ion ▪ Most channels are selective to relatively few ions – those ions typically have the same charge Membrane Potentials and Channels • Channels are often dynamic ▪ They can open or close in response to a variety of stimuli… ▪ … which means membrane permeability and the membrane potential can change, often very quickly • Channels will change their open/closed states depending on what they’re “built” to detect ▪ Voltage – voltage-gated channels ▪ Stretch or mechanical deformation – mechanoreceptors or osmoreceptors ▪ Intracellular messengers ▪ Extracellular messengers – ionotropic receptors • A ligand binds to a receptor which is also a channel – binding opens the channel, and allows an ion across the membrane The Action Potential • The axon, axon hillock, and the synaptic terminal have a unique population of channels that allows for the production of action potentials • An action potential: ▪ Requires the presence of sodium voltage-gated channels (or sometimes calcium voltage-gated channels) ▪ Relies on positive feedback ▪ Always results in a membrane voltage change that is the same size ▪ Occurs very quickly – the membrane becomes more positive (depolarized) in a matter of milliseconds Where do action potentials occur? • The axon hillock, the axon (or in myelinated axons the nodes of Ranvier) and the synaptic terminals possess a large population of sodium voltage-gated channels (Na+ VGC) in the membrane ▪ K+ VGC are also present in these areas – they help to quickly terminate the action potential • The next 8 slides show an animated model of how the events of the action potential take place Axon hillock Step 1: The Resting Membrane Potential • The Na+/K+ ATPase uses ATP to pump Na+ out of the axon, and K+ in ▪ K+ concentrations are high inside the axon, and low outside (vice-versa for Na+) • K+ is high inside the axon, so it diffuses out ▪ diffusional, or chemical, force acting on K+ • Membrane becomes negative inside the axon ▪ Negatively-charged proteins, ions cannot leave the cell with K+ • The attractive force of the negatively-charged membrane balances out the diffusional force driving K+ out ▪ This balance establishes the resting membrane potential at about -70 mV (inside membrane negative) Step 2: Depolarization • The inside of the axonal membrane becomes more positive, and a Na+ VGC opens ▪ channels are opened by more positive charges inside membrane ▪ threshold = membrane potential at which all Na+ VGC will end up opening (~ -55 mV) • Na+ VGC opening leads to other Na+ VGC opening, eventually all open ▪ positive feedback, Na+ diffuses into the cell, making membrane more positive, allowing more Na+ in • Inside of the axon becomes completely depolarized ▪ diffusion gradient (high Na+ outside, low inside) as well as electrical force (inside negative) drives Na+ into the cell • K+ VGC open, Na+ VGC close after ~ 1 msec Step 3: Repolarization • Na+ VGC are closed, no further Na+ entering the axon ▪ close after about 1 msec ▪ Are unable to open for 1-2 msec – they are “locked” ▪ After 1-2 msec, Na+ VGC will “unlock” – but only if the membrane is replolarized (becomes inside-negative again) • K+ rapidly leaves the axon ▪ high K+ inside axon and positive charge inside the membrane strongly drive K+ out ▪ K+ VGC and regular K+ channels are both open, allowing rapid K+ exit • Na+ VGC are ready to re-open: ▪ when membrane potential is -70 mV (repolarization) ▪ after they’re “unlocked” (1 – 2 msec after closing) Voltage-gated channels • The sodium voltage-gated channel has 2 gates: ▪ The activation gate – this gate opens as soon as threshold is reached (i.e. the membrane depolarizes to -55 mV) ▪ The inactivation gate – this gate closes very soon after the activation gate opens, after Na+ has rushed into the cell • The inactivation gate will not open again unless: ▪ 1-2 msec has passed since it has closed (it’s “locked”) ▪ The cell membrane becomes inside-negative again (repolarized) • The potassium voltage-gated channel does not have an inactivation gate – it opens when the cell depolarizes, and closes once the cell is inside-negative again ▪ It is slower to open than the sodium voltage-gated channel Refractory periods of the neuronal action potential Absolute refractory period: • Inactivation gate of the Na+ VGC is closed • Another action potential is impossible until this gate opens 0 1 2 3 4 5 6 msec Refractory periods of the neuronal action potential Relative refractory period: • Inactivation gate is open, activation gate is closed for the Na+ VGC • The cell is hyperpolarized – the membrane potential is lower than resting membrane potential • A larger stimulus is necessary to reach threshold Threshold 0 1 2 3 4 5 6 msec Properties of Action Potentials • All-or-none events ▪ Begin when a threshold voltage (usually 15 mV positive to resting potential) is reached ▪ There are no “small” or “large” APs – each one involves maximal depolarization → all Na+ channels open once threshold is reached • Initiated by depolarization • Have constant amplitude ▪ Action potentials don’t summate – information is coded by frequency, not amplitude ▪ the size of the depolarization stays the same size no matter how far it travels along axon • Have constant conduction velocity along a fiber ▪ Fibers with a large diameter conduct faster than small fibers. • Myelinated fiber velocity in m/s = diameter (um) x 4.5 • Unmyelinated fiber velocity in m/s = square root of diameter (um) Velocity of action potentials along myelinated and unmyelinated axons Note the increase in conduction velocity with increased size of the axon diameter • True of myelinated and unmyelinated axons Why does myelin speed up conduction? • Below is a non-myelinated axon model ▪ Blue lines = membrane, charges represent the overall intracellular and extracellular voltages ▪ When one part of the membrane depolarizes, it reaches threshold and an action potential occurs ▪ The neighbouring part of the axon needs to depolarize → reach threshold → AP before the action potential progresses further down the axon ▪ The action potential is reproduced all the way along the length of the axon – continuously ▪ This is known as continuous conduction - - ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ++ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ++ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Saltatory conduction • Saltatory = “jumping” ▪ In a myelinated axon, the nodes of Ranvier are the only parts of the axon expressing voltage-gated channels ▪ The parts in-between are myelinated, and therefore insulated • The myelin insulation allows the electrical field from the depolarization to “jump” to the next node of Ranvier • This is very fast compared to depolarizing each piece of membrane along the axon ▪ The portions covered by myelin do not experience action potentials – they can’t, there’s no ion channels and myelin keeps ions from crossing the cell membrane • Therefore it’s the positive “electric field” from one node of Ranvier that brings the next node of Ranvier up to threshold Saltatory conduction • The “ “ indicates the positive electric field that jumps from one node of Ranvier to the next Speed of Impulse – Fiber Size • A Fibers ▪ Largest fibers, 5-20 μm, myelinated ▪ Conduct impulses at 12-130 m/sec or 280 miles/hr ▪ Large sensory nerves for touch, pressure, position, heat, cold ▪ Final common pathway for motor system • B Fibers ▪ Medium fibers, 2-3 μm, non-myelinated ▪ Conduct impulses at 15 m/sec or 32 miles/hr ▪ From viscera to brain and spinal cord, autonomic efferents to autonomic ganglia • C Fibers ▪ Smallest fibers, non-myelinated ▪ Conduct impulses at 0.5-2 m/sec or 1-4 miles/hr ▪ Impulses for pain, touch, pressure, heat, cold from skin and pain impulses from viscera ▪ Visceral efferents to heart, smooth muscle and glands The Chemical Synapse • Chemical synapses are associated with excitable cells • The presynaptic neuron releases a neurotransmitter (NT) that binds to receptors embedded in the post-synaptic cell membrane ▪ The “chemical” part of the chemical synapse ▪ The presynaptic terminal of the axon is the site of NT release • The NT crosses the synaptic cleft ▪ The tiny distances (20 nm) from pre-synaptic to post-synaptic membrane are small enough that diffusion is an efficient transport mechanism • Binding of the neurotransmitter to a receptor can affect the postsynaptic cell in a wide variety of ways ▪ The synapse is usually between a dendritic spine or an axon terminal – the dendritic spine expresses the receptor for the NT Neurotransmitter vesicles • Vesicles are synthesized and packaged in the rER and Golgi and transported down the axon via microtubules ▪ Known as fast axonal transport – the “molecular motor” kinesin transports the vesicles towards the synaptic terminal, like a train along a track of microtubules • Neurotransmitters (non-peptide) are synthesized in the cytosol of the presynaptic terminal and transported into vesicles ▪ Transported into the vesicle using a proton gradient generated by a proton pump • Vesicles then bind to the actin within the presynaptic terminal cytoskeleton and are transported to release sites (active zone) close to the synapse Basic steps of NT release 1. AP arrives at the presynaptic terminal 2. Depolarization leads to opening of voltage-gated calcium channels 3. Calcium enters the presynaptic terminal (as per its Nernst potential) 4. Calcium binds to a protein associated with neurotransmitter-filled vesicles 5. Neurotransmitter is released into the cleft as the vesicles fuse with the presynaptic membrane 6. Neurotransmitter binds to a receptor Basics of synaptic transmission • Calcium entry is mediated by opening of Ca+2 VGC ▪ Not from intracellular store release ▪ The whole point of the action potential is to open Ca+2 VGC in the presynaptic terminal → Ca+2-induced exocytosis of NT into the synaptic cleft Essential details of vesicle release • Vesicle release needs to be highly regulated, but quick ▪ Vesicle fusion and NT release takes 1 – 5 msec post-AP • Key players: ▪ v-SNAREs –a protein complex of proteins attached to vesicles • They “force” the vesicle to fuse with the presynaptic membrane and dock with t-SNARES • synaptobrevin is a v-SNARE ▪ t-SNARES – a protein complex attached to the pre-synaptic membrane → “grabs” the v-SNAREs • Syntaxin and SNAP-25 are t-SNAREs ▪ Complexin – a molecule that prevents premature release after v-SNAREs and t-SNARES engage with each other ▪ Synaptotagmin – a calcium-binding protein • When calcium binds, it “knocks” complexin off the v-SNARE-tSNARE complex Steps of vesicle release 1. v-SNARES and t-SNARES “zipper” together ▪ Synaptotagmin and complexin prevent premature fusion and release after zippering 2. AP → depolarization → Ca+2 VGC opening → calcium influx into the presynaptic terminal 3. Calcium binds to synaptotagmin → disengagement of complexin 4. The synaptic vesicle fuses when complexin disengages → release of NT into the synapse 5. The v-SNAREs and t-SNARES disengage, and the vesicle is re-used ▪ This occurs after intracellular calcium levels decrease Clinical Relevance - Botox • The toxins produced by Clostridium botulinum are some of the deadliest known ▪ They impair the assembly and function of v-SNAREs and t-SNARES • This impairs fusion of vesicles with the presynaptic membrane ▪ Used therapeutically (in tiny doses) to reduce muscle spasticity, treat migraines… and decrease wrinkles • Prevents release of acetylcholine from motor neuron pre-synaptic terminals, which is necessary to excite contraction in skeletal muscle ▪ 7 main types of botox – medical applications use Botox A • Botox A binds to SNAP-25, a v-SNARE Neurotransmitter removal • Neurotransmitters don’t stay bound to receptors forever: ▪ Degraded by enzymes in the synapse • Example – acetylcholinesterase degrades acetylcholine to acetate and choline ▪ Reabsorbed by nearby astrocytes ▪ Reabsorbed by the presynaptic terminal ▪ Diffuse out of the cleft and carried away by blood Effects of Neurotransmitters • The effects of NT can vary: ▪ Different neurons will release different NTs from the presynaptic terminal ▪ Different postsynaptic cells may contain different receptors ▪ Some NTs cause cation channels to open, which results in: • Depolarization for sodium and (to a lesser extent) calcium • Hyperpolarization for potassium ▪ Some NTs cause anion channels to open, which results in a graded hyperpolarization ▪ Many NTs cause a G-protein or other intracellular cascade of second messengers… • These can open or close channels for longer periods, change kinase activity, even change gene expression Effects of Neurotransmitters • Ionotropic receptors open an ion channel when they bind to their ligand ▪ NMDA receptor – binds the NT glutamate → sodium and calcium channel opening ▪ Nicotinic acetylcholine receptor – binds to acetylcholine → sodium channel opens ▪ GABA(a) and glycine receptors – bind to GABA and glycine respectively → Cl- channel opens • Many (most?) metabotropic receptors are linked to G-protein signaling (see table next slide) ▪ Only the bold, underlined neurotransmitters and receptors will be asked in exams NT behaviour Receptor Signal Ach - excite Nicotinic M1, M3, M5 → Ionotropic, sodium channel → increases in calcium (metabotropic) Ach – inhibit M2, M4 → Decrease in calcium or cAMP or opens a Gprotein-gated K+ channel (metabotropic) GABA – inhibit GABAa GABAb → Ionotropic, Chloride channel → Decreased cAMP, G-protein-gated K+ channel (metabotropic) Glutamate - excite NMDA, AMPA mGlu1 and mGlu5 → Ionotropic sodium + calcium channels → IP3, calcium increases (metabotropic) Glycine – inhibit Strychnine-sensitive → Ionotropic, Chloride channel Dopamine – excite D1, D5 → Increases cAMP (metabotropic) Dopamine – inhibit D2, D3, D4 → Decreases cAMP, opens G-protein-gated K+ channel (metabotropic) Norepi. - excite Alpha-1 Beta-1 → Increased IP3 and calcium (metabotropic) → Increased cAMP (metabotropic) Norepi. - inhibit Alpha-2 → Decreased cAMP and calcium, opens Gprotein-gated K+ channel (metabotropic) Serotonin – inhibit 5-HT1 → Decreased cAMP and/or calcium (metab) Serotonin – excite 5-HT2, 3, 4 → Many – mostly increased cAMP, calcium, IP3 (metabotropic) General notes on neurotransmitter actions • Acetylcholine ▪ Nicotinic – the NT of the neuromuscular junction, also widely expressed throughout the brain ▪ Excitatory muscarinic – important for cognitive function, memory ▪ Excitatory and inhibitory muscarinic are key for the activity of the autonomic nervous system • GABA – most important inhibitory NT of the “intracranial” CNS • Glycine – most important inhibitory NT of the spinal cord • Glutamate – most common excitatory NT of the CNS - NMDA receptors are very important for learning and memory • Norepinephrine – autonomic nervous system functions, also cortical and limbic system roles – focus on excitatory functions for this lecture • Dopamine – you’ll learn lots about these receptors later → don’t worry about them for this lecture • Serotonin – you’ll also learn lots about these, but 5-HT1 is classically associated with mood disorders → don’t worry about them for this lecture So a neurotransmitter binds to an ionotropic receptor – what’s next? • If it binds to an inhibitory receptor, that results in dendrite hyperpolarization (membrane becomes more negative) ▪ Examples of inhibitory receptors? • If it binds to an excitatory receptor, that results in dendrite depolarization (membrane becomes more positive) ▪ Examples of excitatory receptors? • Activation of ionotropic receptors bring about graded potentials in the dendrites and cell body ▪ If the depolarization or hyperpolarization is large enough, this may be change the membrane potential at the axon hillock Graded Potentials • A graded potential is any change in membrane potential that doesn’t result in an action potential ▪ Include changes in membrane potential that are below the threshold for an action potential or occur in areas of the cell that do not have Na+ VGCs • Properties of graded potentials: ▪ They get smaller (decremental) over time and the further they travel along the cell membrane ▪ They can vary in magnitude ▪ They can “add together”, or summate ▪ They can be excitatory (depolarization) or inhibitory (hyperpolarization) • Excitatory = excitatory post-synaptic potential (EPSP) • Inhibitory = inhibitory post-synaptic potential (IPSP) Graded Potentials Note: • Even if an EPSP is higher than threshold, no AP will occur unless Na+ VGC are present • Graded potentials can vary in size Graded Potentials - Summation • Depolarizing EPSPs are green, hyperpolarizing IPSPs are pink • If multiple EPSPs from different sites (say points 1 and 2) meet at the same time, same place on the membrane → spatial summation 1 2 • Graded potentials last longer than action potentials ▪ If multiple graded potentials add up in a “staircase” fashion over time → temporal summation • This can be seen at point A in the diagram Graded potentials – the whole point of chemical synapses • If we depended only on action potentials for communication between neurons, it would be a very simple form of communication ▪ Digital → all-or-nothing signals • Many different axons synapsing on one neuron can result in a wide array of EPSPs and IPSPs • These EPSPs and IPSPs can be long- or short-lasting, depending on the receptor and how many action potentials are being sent per second • The net result – all of these EPSPs and IPSPs can be integrated at the axon hillock ▪ If the graded potentials bring the hillock to threshold → an action potential (or strings of action potentials, if the graded potential lasts many milliseconds) Graded potentials – the whole point of chemical synapses • Every neuron is therefore a very complicated “computer”, integrating the inputs from all of the neurons that it synapses with, and making a “decision” as to whether those inputs are enough to bring the axon hillock to threshold • We have 1011 neurons with thousands of dendritic spines (and therefore synapses) per neuron • Chemical synapses and graded potentials add an extra level of complexity • Metabotropic receptors can have very long-lasting effects that include protein synthesis and long-lasting intracellular signals Graded Potentials Vs. Action Potentials Characteristic Graded Potentials Action Potentials Origin Arise mainly in dendrites and cell body Arise at trigger zones and propagate along the axon Types of channels Ligand-gated or mechanically gated ion channels Voltage-gated channels for Na+ and K+ Conduction Decremental; permit communication over short distances, degrade over long distances Propagate and thus permit communication over longer distances Amplitude (size) Depending on strength of stimulus, varies from <1 mV to more than 50 mV All-or-none; about 100 mV Duration Longer, ranging from several msec to several min Shorter, ranging from 0.5 to 2 msec Polarity May be hyperpolarizing (inhibitory to generation of an action potential) or depolarizing (excitatory to generation of an action potential) Always consists of depolarizing phase followed by repolarizing phase and return to resting membrane potential Refractory Period Not present, thus summation can occur Present, thus summation cannot occur