Excitable Cell and Membrane Potential Lecture Notes PDF

Addis Ababa University CHS, SoM, Dpt of Medical Physiology Lecture Notes On Excitable Cells and Membrane Potential For Pre-clinical I students Abebaye A (BSC, MSc (Physiology and Medical Education), PhD: Physiology),dpt of Medical...

Addis Ababa University CHS, SoM, Dpt of Medical Physiology Lecture Notes On Excitable Cells and Membrane Potential For Pre-clinical I students Abebaye A (BSC, MSc (Physiology and Medical Education), PhD: Physiology),dpt of Medical Physiology, SoM, CHS, AAU 1  Objectives At the end of this lecture, we are expected to :- Discus about neuroglia Explain the excitable cells: Neurons List down the types of membrane potential Discuses how action potential is formed and propagate Discuses synaptic transmission, synapse and types Correlates by Abebaye A (AAU-pd) 2  Excitable Cells and Neuroglia Excitable Cells : Forming excitable tissues (nerve and muscles) Non- excitable cells: Neuroglia Excitability Capability of responding to stimulus by changing their membrane potential by Abebaye A (AAU-pd) 3 Neuroglia Supporting cells and out numbered Proliferate throughout life (Glioma: tumor), no synaptic potential, silent cells Central 1. Astrocytes  Maintain BBB, metabolic activities, regulation of electrolytes and NTS, inflammation, neuroplasticity, release lactate  Supply nutrients, guide neuronal development,  Act as K+ and NT buffers  Predominantly permeable to K+  Can synthesize NT: GABA, glutamate  Neuroglia … 2. Microglial CNS innate immune cells Together with astrocytes Produce cytokines, clear A, phagocytosis, Response to injury and stress Rapidly activated by brain injury This causes them to proliferate, change shape & become phagocytic Gliosis: Over-activation of microglia and astrocytes and other neuroglia Proliferation, hypertrophy, change in shape, secret inflammatory cytokines  Neuroglia … 3. Ependymal cells Form plexus (epithelium on the wall of ventricles) and blood-CSF barrier Produce CSF 4. Oligodendrocytes Myelin sheath of axon of neuron centrally One cell covers more than one axon Neuroglia … Peripheral 1. Schwan cells Cover axon of a neuron at periphery 2. Satelites Encapsulate dorsal root & cranial nerve ganglia. Regulate autonomic ganglia  Neurons Fundamentals cells in the nervous tissues/nervous system Generate and propagate electrical impulse (action potential) Longitivity Consume more energy Independent to insulin Not replicate themselves  Neurons… New neurons are not formed  But possible in: hippocampus, involved in memory and navigation, and the olfactory bulb About 86 billion /brain and form 100 trillion connections Important for Sensation, integration, commanding (movement, reflexes) Motor function: Reaction  Organization of nervous system 1. Central Nervous System The brain + spinal cord The center of integration & control Sensory: somatic and autonomic Motor : somatic and autonomic 2. Peripheral Nervous System The nervous system outside CNS Consists of: 31 pairs of Spinal nerves Carry info to & from the spinal cord 12 pairs of Cranial nerves Carry info to & from the brain Figure 1: Organization of nervous systems Sensory and motor Communicate the CNS with the effectors 10  Structures and functions of a neuron Dendrites Receive information from other cells , input region of a neuron Spines increase surface area Form graded membrane potential and convey to the cell body Cell body/soma Contain organelles (nucles , mitochondria, ER/Nissl body), golgi apparatus Materials are synthesized Called nuclei in the CNS and ganglia in the PNS Contains many bundles of protein filaments (neurofibrils) Help maintain shape, structure, and integrity of the cell.  Structure and functions of a neuron … Axon Single long (up to 2m) extension from cell body and generate AP and conduct it Graded potential converted into AP (axon hillock) Axon terminal: secretory region Contain mitochondria in its terminal Transport Antrograded and supported by Kinesin, requires energy Fast : 400 mm/day such as vesicles containing peptides, NTs, some degenerative enzymes and slow (1-5 mm/day) : soluble proteins, cytoskeletal proteins and supported by Kinesin Retrograded : virus/bacteria for lysosomal degradation and supported by Dynein by Abebaye A (AAU-PhD) 12  Structure and functions of a neuron … Myelin sheath Fatty lipo-protein substance that covers the axon; Protects cells, increases impulse conduction speed Schwan cells/oligodendrocytes Node of Ranvier Important for impulse conduction  Demyelination Multiple Sclerosis (MS) The most common demyelinating disease of CNS and is purely CNS disease Degeneration of myelin sheath myelin removed proliferation of astrocytes formation of gliotic scar conduction of impulses in the axons is impeded. This CNS disorder Guillain-Barre Syndrome (GBS) Infection with a virus or bacteria induced Aab destruction of peripheral nerve Muscle weakness and/or tingling sensations (paresthesia) Structure and functions of a neuron … Fig 2: Structure of a neuron by Abebaye A (AAU-pd) 15  Classifications of neurons Chemical they release Cholinergic: Sympathetic preganglionic fiber, PNS: pre and post ganglionic fiber Adrenergic: Chromaffin cells Dopaminergic: VTA, Substantia nigra pars compacta, infundibulum Function: Sensory, integrating and motor neurons  Classifications of neurons … Number of process from the cell body (neurites) Unipolar : one process Bipolar : retina (bipolar cells), olfactory epithelium, Mechanoreceptors: touch, pressure, pain Multipolar : Predominant in the NS of vertebrates Single axon and many dendrites Large area for receiving synaptic input eg. Spinal motor neurons Anatomical classification of neurons Fig 2.1 : Structural classification of neurons  Classifications of Neurons … Electrical activity Silent neurons: unchanging RMP in the absence of external stimulation Pacing/beating neurons: Repetitive firing at constant stimuli External stimulation can change the firing rate of the cell or inhibit it altogether. Bursting Neurons: Fires spontaneously without external stimuli Do not fire at fixed regular intervals Have patterns of neuronal activity Fires AP rapidly followed by decrease in the rate  Neuronal Reactions to Injury Neuronal Degeneration Gradual loss of neuronal structure (axon) Results in:- Loss of synapse Degeneration of myelin sheath Organelle rearrangement: swelling of soma after axonal injury: ER degeneration + chromatolysis (reactive changes in the cell body of damaged neurons, dispersal and redistribution of Nissl substance) Basophilic structure composed of ribonucleic acid (RNA) and proteins in aggregate with rough endoplasmic reticulum Debris of degeneration products taken by microglia Process during degeneration Focal swelling followed by axon fragmentation  Structure and functions of a neuron … Site of neuronal degeneration Parkinson’s disease, Alzheimer's, disease Huntington disease Traumatic brain injuries, infection, tumor, stroke, ROS, inflammation, protein and metal accumulation, mitochondrial dysfunction, amyotrophic lateral sclerosis( problem of motor neurons involved for voluntary muscle movement and breathing) , disruption of cellular/axonal transport (4 and 5 may be regarded as secondary effects), aging, fragmentation of neuronal Golgi apparatus, dysfunction of neurotrophins (NTFs) Fig 3: Neuronal Degeneration and risk factors by Abebaye A (AAU-pd) 21 Factors involved for neurodegeneration: Summary Abnormal protein involved in Parkinson’s disease These proteins are neurotoxin UPS: Ubiquitin-Proteasome system: complex proteins with or normal shape They are protein involved in the degeneration of neurons Fig 3.1: Neuronal Degeneration  Neuronal Reactions to Injury… Neuronal Regeneration Neurons raise in the third month of intrauterine life, not after birth CNS cell regeneration is limited However, neurogenesis is made in Olfactory bulb neurons Hippocampal cells And peripheral axon Axon regrow, reconnect to the muscles & sensory receptors of the hand But neuroglia can be replaced Process of Neuron Regeneration at Periphery Fig 4: Processes of peripheral axon Regeneration  CNS neurons Regeneration is limited? Glia scar formation Regeneration process inhibitory factors Lack of myelin clearance Inflammatory reactions Myelin-associated inhibitors (MAIS) and the chondroitin sulfate proteoglycans (CSPGS) Not produced by Schwan cells  Electrophysiology and Membrane potential(MP) - Electrophysiology: studies about the electrical activity of cells - MP: Charge difference across cell membrane - Net inside is negative and outside is positive (Vm = (Φi) - (Φo). - The difference not static - Neurons are polarized at rest Asymmetrical distribution of ions Ion Inside (mM) Outside e.g. plasma (mM) Na+ 12 145 K+ 140 4 Cl- 4 115 HCO3 - 12 30 prosein - 140 10 Ca++ 0. 0001 2 by Abebaye A (AAU-pd) 26  Membrane potential… all cells have membrane potentials or charge difference The Range of Em: -90 + 30mv (depending on the cell type). Any change in membrane permeability of ions causes a change in Em.  Electrochemical gradients Na+ electrochemical gradient facilities its influx Chemical (diffusion) gradient favours Na+ influx K+ efflux favoured by diffusion gradient & opposed by electrical gradient Until equilibrium potential developed For is K+ -90mv and for Na+ is +66mv Movement of ions stop by Abebaye A (AAU-pd) 28  Nernst potential Membrane Potential prevents diffusion of ions It is equilibrium potential No net movement of the ion For a single ion: assumes K+ to be the main ionic species involved in the resting state, that is, Pk >> PNa Atomic no of k+=19, 2 electrons in the first, Valence electron: electron in 8 in the 2nd and 3rd ,one in the outer most the outer most shell of the shell , the valence of k+= one element by Abebaye A (AAU-pd) 29 The Nernst potential of ions by Abebaye A (AAU-pd) 30  Membrane potential measurement Using microelectrodes  Electrolyte containing pipette inserted into the inside (recording electrode)  Differential electrode with zero reading put in ECF Voltmeter measure the potential difference b/n the inside & outside The voltage change area= electrical dipole Fig 5: Voltmeter by Abebaye A (AAU-pd) 31  Types of membrane potentials A. Resting membrane potential (Er), RMP o Membrane potential when cells are at rest, no net movement of ions o Magnitude vary according to cell type o Excitable cells (- 70 mV to - 90 mV) have larger RMP than non-excitable cells o - 53 mV epithelial cells, -8.4 mV RBC, -20 to -30 mV fibroblasts, and -58 mV adipocytes o Is nearer to equilibrium membrane potential for K+ o b/se of the leaked ion channels, more for K+ o The concentrations of K+ and Na+ never reach equilibrium because the Na+/K+ ATPase pump is always working. by Abebaye A (AAU-pd) 32 Factors for resting membrane potential Differential membrane permeability to K+ & Na+ The electrogenic nature of the Na+/K+ pump The presence of intracellular impermeable anions The presence of non-gated ion channels Cardiac -60 mv: unstable pacemaker cells Neurons -70 mv Skeletal muscles -80 mv Smooth muscle -40 -60 mv: cells fluctuate  Resting membrane potential … o Determinants  Passive determinants Leaked ion channels, diffusion of both K+ & Na+ More for Na+ than K+ Differences in plasma membrane permeability and at rest, a membrane is  Impermeable to large anionic cytoplasmic proteins  Very slightly permeable to Na+  25 times more permeable to K+ and fairly permeable to Cl- Unequal distribution of ion Diffusion gradient by Abebaye A (AAU-pd) 34  Resting membrane potential …  Active determinants Na+- K+ pumps Negative charge inside (active determinants) Cause differences in ionic composition ICF has high [K+] (150 mmol/L), low [Na+] (15 mmol/L) ECF has low [K+] (5 mmol/L), high [Na+] (145 mmol/L) Fig 6: Na+- K+ pump by Abebaye A (AAU-pd) 35 K+-Na+ pump Fig 7: Processes of K+-Na+ Exchange by Abebaye A (AAU-pd) 36  Ion channels Proteins in the cell membrane Ions influx and effluxes Non-gated Open and leaked and found in cell bodies and along axon Gated Ligand, voltage (can be inactive and found at axon hillock), light, mechanical gated Chemical (at synapse) and mechanical gated (at sensory receptors) ion channels can be desensitized by Abebaye A (AAU-pd) 37 Types of ion channels... Figure 8: Types of ion channels 38  Ion channels… Voltage gated Na+ and K+ channel Na+ channel has activation (m-gate) and inactivation (h) gate m-gate: n-gate h-gate: Fig 9: K+ and Na+ channel by Abebaye A (AAU-pd) 39 Effects of opening and closing of these voltage gated ion channels Figure 10: effects of opening and closing of K+ and Na+ channel  Type of membrane potential… B) Graded membrane potential (GMP)  Amount of transient local changes in membrane potential  Depolarization/ hyperpolarization  The size depends on the strength of the stimulus (physical, chemical)  Propagate passively with decrement Magnitude  Decay (currents leaked out)  Can be summed  Has no threshold  Has no refractory period strength of the stimulus Fig 4: Relation between stimulus strength and GMP by Abebaye A (AAU-pd) 41 Features of Graded Membrane potential Figure 11: Characteristics of grade membrane potential  Mechanism of change in EP and response Physical, mechanical, chemical….)  Sensory receptors  Transform stimulus energy/Transduction  Ion channels opened  Inward flow of current (eg. Na+)Receptor potential/Generator potential Depolarization Response Action Potential, if threshold is reached  Types of graded membrane potential Synaptic potentials (EPP, EPSP, IPSP) Receptor/generator potential Pacemaker potential by Abebaye A (AAU-pd) 44  D) Action Potential (AP) Def: Large, rapid, transit change in membrane potential Self-propagating for large distance without attenuation In excitable membrane (axolemma, sarcolemma) Used to transmit information +ve charge (Na+) inter when it started and +ve charge (k+) leave the cell when it ended Becomes frequent when Ca+2 in the blood less Hypocalcemia tetanus by Abebaye A (AAU-pd) 45  Features of AP  All/none phenomena  Needs threshold  Has refractory period  Can not be summed  The same magnitude  Propagate actively without decrement  Always towards the depolarization  Its magnitude not affected by the intensity of stimuli  Stimuli affect its frequency by Abebaye A (AAU-pd) 46 AP generation mechanism and triggering zone Beginning No AP why?? AP trigger Zone: Cell Integration and initiation of AP. When the graded potential reaches into the triggering zone, its size reduced and below the threshold so no AP formed Fig 12: AP formation mechanism by Abebaye A (AAU-pd) 47 Phases and Processes of AP Fig 13: Phases and Processes of AP  Phases of AP and responsible factors Na+ channels refractory Na+ and K+ channels open for initial depolarization g Na+ > g K+ at the beginning of AP The rising phase of the AP is the result of an  in Na+ conductance. Repolarization phase is a result of a  in Na+ conductance and delayed  in K+ conductance. Fig 14: Phase of AP and responsible factors by Abebaye A (AAU-pd) 49  Refractory period of AP Time where 2nd AP not formed flowing the 1st Ion channels closed and can be absolute and relative Is because of closure of inactivation gate Inactivation gate opened and activation gate is going to open Refractory period More for cardiac muscles cells than skeletal muscle From-55 (threshold to +30mv) Fig 15: Refractory period by Abebaye A (AAU-pd) 50 Frequency of AP Fig 16: Frequency of AP by Abebaye A (AAU-pd) 51 Graded potentials Action potentials Depending (the size) on the stimulus strength Towards depolarization and amplitude is all- & can be depolarizing or hyperpolarizing. or none, the stimulus intensity is coded by frequency Amplitude small Large (about 100mv) Very short duration Relatively high, 3-5ms Channels open by sensing ligand from in/out, By sensing voltage change movement, T0,cytoplasm signals The ions involved are usually Na+, K+, or Cl-. The ions involved are Na+ and K+ and there is And no refractory period refractory period Summed Not summed Travel passively with decreasment Travel by generating new AP and no decreasment , actively Created by external stimuli Created by membrane depolarization to threshold. Graded potential is the responsible In neurons, post synaptic dendrites or soma Occur where voltage-gated Na+ and K+ or membrane region where sensory stimuli is channels are highly concentrated received - axon hillock In non neural cell can occur in any region of - node of ranviers the membrane by Abebaye A (AAU-pd) 52 Propagation of action potential -Movement of AP to axon terminal - salutatory or continuous ways of conduction Origin of depolarization Local current flow and adjacent membrane depolarized Fig 17:AP conduction by Abebaye A (AAU-pd) 53  Propagation of AP cont’…. Continuous conduction AP formed at every segment of axon membrane Occurs in muscle fibers and unmyelinated axons Saltatory conduction Occurs in myelinated axons The impulse leaps from one node of Ranvier to another Results in much faster conduction of an impulse No AP’S formed are few by Abebaye A (AAU-pd) 54 Continuous propagation Fig 18: Cabling way of Propagation of AP by Abebaye A (AAU-pd) 55 AP propagation in Myelinated neuron Fig 19: Saltatory way of AP propagation by Abebaye A (AAU-pd) 56 Factors affecting impulse conduction speed Myelination Membrane resistance Myelin sheath thickness Luminal resistance Internode distance in myelinated neurons Capacitance Temperature Metabolic activity Fiber size by Abebaye A (AAU-pd) 57  Conduction velocity cont’… Types of nerve fibers I. Type A myelinated 4-20µm diameter with 140m/sec conduction speed carry somatic motor and somatic sensory info II. Type B 2-4µm diameter with 18m/sec, myelinated Carry autonomic motor and visceral sensory info III. Type C < 2µm diameter and unmyelinated 1m/sec and carry autonomic motor and visceral sensory info by Abebaye A (AAU-pd) 58 AP conduction velocity and fiber types A-alpha fibre A-beta fibre A-delta fibre C fibre Fig 20: Different types of nerve fibers by Abebaye A (AAU-pd) 59 Propagation of AP is one way 2 1 Only forward movement: membrane behind always in absolute refractory period 3 4 Fig 21: One way propagation of AP by Abebaye A (AAU-pd) 60 Synaptic transmission by Abebaye A (AAU-pd) 61  Chemical synapse, its functional components: pre, post synaptic ells and cleft Calcium influx across conc gradient and sensed by synaptotagmin proteins and Ca++ taken into mitochondria or released through an active calcium pump Fig 15 chemical synapse Multiple EPSPs needed to trigger AP in post cell axon And added spatially or temporally postsynaptic cell:  Neuron → AP EPSP,IPSP  Muscle → contraction  Glands → secretion by Abebaye A (AAU-pd) 62  Synapse con’t… Axon terminals of one neuron form junction with parts of another : 1. Dendrites (axo-dendritic synapse)  Very common 2. Soma (axo-somatic synapse) 3. Axon (axo-axonic synapse) 4. Sipine of dendrites (axo-spinic) Rarely dendrodendritic synapse may be found by Abebaye A (AAU-pd) 63  Chemical synapse and its Characteristics Use chemicals as mediators Signals not directly transmitted between cells, but indirectly via chemicals The transmission is one way and delayed (steps) Conduction speed is slow Synaptic fatigue occur: Calms neuronal excitability The synapse modulated The synapse is inhibitory or excitatory by Abebaye A (AAU-pd) 64  Chemical synapse…. NTs action ceased Re-up taking Taken by glial cells, Diffuse into blood vessels Breaking down by Abebaye A (AAU-pd) 65  Synaptic fatigue Unable to neurons responding to stimulus Caused by: Depletion of NTs in the presynaptic terminals Desensitization of postsynaptic membrane receptors Down regulation of postsynaptic membrane receptors Accumulation of wastes around neuronal environment Subsequent closing of ion channels by Abebaye A (AAU-pd) 66 Excitatory and inhibitory chemical synapse Fig 22: Types of chemical synapse by Abebaye A (AAU-pd) 67 Neurotransmitters by Abebaye A (AAU-pd) 68  Criteria defining neurotransmitters Synthesized in the neurons Small chemicals: fast action  Monoamines ( NE, EP, ser, DA, histamine), ach  Aminoamides (glutamate, GABA, aspartate, glycine)  Purines (ATP) Large peptides: slow and potent action  Endorphin and Opiates, enkephalin, NP-Y, substance P Action of mechanisms  Direct and Indirect by Abebaye A (AAU-pd) 69 Neurotransmitter direct action Inotropic receptors Fig 23: Direct Action of neurotransmitters by Abebaye A (AAU-pd) 70 Neurotransmitter indirect action: using 2nd messenger Neurotransmitter Metabotropic recepotrs Fig 24: Indirect Action of neurotransmitters by Abebaye A (AAU-pd) 71 Life cycle of a neurotransmitter 1-2 = accumulation of precursors, 3 = inter into vesicle, 4-5 = NTs bind to Postsynaptic and autoreceptor for regulating NT synthesis & release 6-9 = removing of NTs. Re-uptaking process need protein Fig 25: Cycle of neurotransmitters by Abebaye A (AAU-pd) 72  Postsynaptic cell response Varies with the NT Excitatory NT: causes a excitatory postsynaptic potential (EPSP) Increases permeability to Na+ and/or Ca+2 Inhibitory NT: causes an inhibitory postsynaptic potential (IPSP) Increases permeability to Cl- or K+ The response is the algebraic sum of the EPSP and IPSP Stimulation causing an AP, if  EPSP >  IPSP and  threshold membrane potential Stimulation leading to facilitation  EPSP >  IPSP, but < threshold membrane potential Inhibition  EPSP <  IPSP by Abebaye A (AAU-pd) 73 Synaptic transmission and drugs by Abebaye A (AAU-pd) 74  Synaptic transmission and drugs Drugs always affect Receptors Ion channels Enzymes and transporters like NTs re-uptake proteins Drugs with similar structure to transmitter substances can affect protein receptors in pre/postsynaptic membranes. Agonists or antagonists They can act Pre, post-synaptically or in the cleft by Abebaye A (AAU-pd) 75  Drug actions… Re-uptake inhibitor (anti-depressants) Control amount of NTS released (amphetamine, cocaine) Control enzyme action (MA Oxidase inhibitors, GABA transaminase inhibitors) Modulate receptor and vesicular proteins Tetanus toxin (from spider) Neuroleptics (antipsychotics): antagonist at dopamine receptors Barbiturates and benzodiazapines:  GABA receptor function by Abebaye A (AAU-pd) 76  Drug actions… Curare (from frogs) : antagonist at nicotinic receptors Atropine (from mushroom) : antagonist at muscarnic receptors Nicotine (from tobacco) : agonist at nicotinic receptors Muscarine (from fungus) : agonist at muscarnic receptors by Abebaye A (AAU-pd) 77 Synaptic Plasticity Experience based change in the connectivity of neurons Increase : Potentiation Short term Change in the activities of existing structure Long term New connectivity New structure formed Decrease: Depression Bases for learning and memory Learning increases synaptic connectivities, is the basis for long term memory Learning and memory require the formation of new neural networks in the brain Changes in the pre-and post synaptic cells  Factors affecting Neuronal Excitability 1.Ionchannel density 2. Metabolic eg. pH : Acidosis ↓ Neuronal excitability (depression) More H+ Na+-K+ ATPase. Alkalosis ↑ Neuronal excitability (GABA transmission) 3. Hypoxia: ↓ PO2 ↓ Neuronal excitability 4. Ischaemia: ↓ Excitability Conduction block 5. Drugs and Chemicals i. Caffeine (Coffee) and Theophylline (Tea): ↓ Threshold ↑ Neuronal excitability 6. Temperature and basal metabolic activities 79 Synaptic abnormalities: Synaptopathy by Abebaye A (AAU-pd) 80  Synaptopathy NTS imbalance: glutamate excitotoxicity Ion channel problems Receptor abnormalities Enzymes Docking proteins Depressive disorder, schizophrenia, Alzheimer's disease, Huntington disease, autism, epilepsy, hearing impairment Infection, toxic substances, injury, stroke, noise  Toxins or antibodies affect transmission Venom toxins Bungarotoxin (from cobras) : antagonize n receptors in the NMJ: paralyzes & respiratory failure)  Botulinium toxin (clostridium bacillus): damage nicotinic receptor Myasthenia gravis :autoimmune disease  Aab destroyed nicotinic receptor in NMJ  Muscle becomes weak Lambert-Eaton Syndrome Aab against Ca2+ channels in presynaptic nerve endings at NMJ. by Abebaye A (AAU-pd) 82 Electrical Synapses by Abebaye A (AAU-pd) 83  Electrical Synapses By gap junction (involves channels comprised of connexins that link cells) Can be found in non-neural cells (epithelial cells, astrocytes, muscle cells) Narrow gap ( about 2-4 nm) Permeability of junction mediated by conformation of the connexins by Abebaye A (AAU-pd) 84 Functional anatomy of Electrical synapse Fig 26: Electrical synapse by Abebaye A (AAU-pd) 85  Characteristics of Electrical synapse Short last changes MP change in one  transmit to another cell No synaptic delay Conduct in both direction Significance: Intracellular signaling during embryonic development Synchronization of impulses (electrically couple groups of cells) Signal is rapid ( escape stimuli) The disadvantage is not modulated by Abebaye A (AAU-pd) 86