FUNBio 7. Excitable Membranes 2024 PDF

Fundamentals in Biology FUNBIO.7 Biological Membranes: Excitable membranes Dr Ian Woods Dept. Anatomy and Regenerative Medicine DATE: 11/10/2024  Learning outcomes At the end of this lecture, the learner will be able to Define excitable cells/tissues and explain why are they important....

Fundamentals in Biology FUNBIO.7 Biological Membranes: Excitable membranes Dr Ian Woods Dept. Anatomy and Regenerative Medicine DATE: 11/10/2024  Learning outcomes At the end of this lecture, the learner will be able to Define excitable cells/tissues and explain why are they important. Differentiate between the terms excitation, irritation and stimulus Define the basic properties of excitable tissues Explain the basic concepts of the resting membrane potential and action potential Demonstrate an awareness of how excitable cells are stimulated Discuss the cystic fibrosis transmembrane conductance regulator (CFTCR) in membrane polarisation/ the balance of salt and water 2 FUNBIO.12 Biological membranes: Excitable membranes  Membrane potential The cell membrane forms a barrier between the extracellular fluid and the intracellular fluid. The membrane has different permeabilities to different ions. Differences in the amount of different cations (+ve charge) and anions (-ve charge) on the inside and outside of the membrane means that there is a difference in charge across the membrane. The membrane itself is not charged. Ion Extracellular Intracellular Relative Concentration Concentration Permeability (mM) (mM) Na+ 150 15 1 K+ 5 150 25-30 A- 0 65 0 (Anions) 3 FUNBIO.12 Biological membranes: Excitable membranes  Membrane potential This difference in charge is the membrane potential. The separation of charge provides the potential to do electrical work. Potential difference (voltage) is measured in Volts (V), membrane potential is small in magnitude and is measured in millivolts (mV). 1000th of a volt The inside of the cell membrane is normally negatively charged compared to the outside. The resting membrane potential is typically -70mV. The inward and outward flow of ions changes the membrane potential. 4 FUNBIO.12 Biological membranes: Excitable membranes  Voltmeter Measurement of -70mV Reference 0 Recording - + Membrane potential electrode electrode (outside) (inside) The membrane potential can measured using a voltmeter. The reference electrode is positioned outside the cell. The recording is done using a microelectrode which pierces the cell membrane. At rest, the inside of the cell is more negatively charged than the outside (-70mV) 5 FUNBIO.12 Biological membranes: Excitable membranes Changes in membrane potential Voltmeter -80mV -70mV 0mV 0 Reference 0 Recording 0 - + - + - + electrode electrode (outside) (inside) Cell Membrane - - - - - - - + + + + - - + + - - + - + - + - - - - - - + - - - + + - - + - + - + + + + - + - - - + - - - - + - - + + + + - + + + + + - - - Hyperpolarized Polarized, Depolarized at Resting potential 6 FUNBIO.12 Biological membranes: Excitable membranes  Excitable tissues Definition All cells have a membrane potential- a difference in charge on either side of the cell membrane. The cells of excitable tissues can produce a rapid, transient change in membrane potential when excited. These transient changes in membrane potential serve as electrical signals that stimulate a response. rapid movement across cell membranes for electrical signalling Muscles, nerves and some glands are excitable tissues. Excitable tissues allow the body to coordinate rapid and appropriate responses to specific stimuli. 7 FUNBIO.12 Biological membranes: Excitable membranes  Examples of excitable tissues Nervous tissue Glandular tissue 8 FUNBIO.12 Biological membranes: Excitable membranes  Membrane potential and membrane transporters Membrane potential is maintained by ion transporters in the cell membrane. Different transporters determine the permeability of the membrane for different ions. Transporters scan be activated or inhibited by specific stimuli or can be active all the time. Excitation normally results from the activation of ion channels which allow cations to enter the cell. Text Entry of Na+ or Ca2+ ions results in depolarization. The membrane potential rapidly becomes more positive. 9 FUNBIO.12 Biological membranes: Excitable membranes  Ion channels and ion pumps Ion channels are transmembrane proteins specifically involved with the transport of inorganic ions like Na+, K+, Ca2+, or Cl-. Ion channels are “gated”, i.e. they open in response to a specific stimulus, such as a change in membrane potential (voltage-gated ion channels) or the binding of a neurotransmitter (ligand-gated ion channels) or stretching of the membrane (mechanically-gated). Some channels are always open (leak channels). 10 FUNBIO.12 Biological membranes: Excitable membranes  Channels and Ion Pumps Ion channels are integral membrane proteins that contain a pore which allows the regulated flow of selected ions across the plasma membrane. Ion flux is passive and driven by the electrochemical gradient for the ions Voltage-gated ion channels Voltage-gated ion channels (VGICs) are primarily responsible for the generation and propagation of action potentials in excitable tissue E.g. along the axons of neurons, between muscle cells at gap junctions. lipid based membrane structure VGICs are a class of transmembrane proteins that form ion channels that are activated by changes in the electrical membrane potential near the channel. The change in membrane potential alters the conformation of the channel proteins, regulating their opening and closing. 11 FUNBIO.12 Biological membranes: Excitable membranes  Ion channels and ion pumps opens channels*** Ligand-gated ion channels Ligand-gated ion channels (LGICs) are activated or inhibited by the binding of specific biochemical molecules. At chemical synapses and neuromuscular junctions these regulatory molecules are are neurotransmitters. Together, the combined activity of ligand- and voltage-gated ion channels gives rise to many complex physiological processes from cardiac and skeletal muscle contraction to the more enigmatic behaviours of the CNS such as cognition and memory. https://physoc.onlinelibrary.wiley.com/doi/epdf/10.1113/JP275877 12 FUNBIO.12 Biological membranes: Excitable membranes  Ion channels and ion pumps Ion Pumps, also called ATPases, are transmembrane proteins that actively move ions and/or solutes against a concentration or electrochemical gradient across biological membranes. Pumps generate a membrane potential by creating an electrochemical gradient across the membrane. builds up sodium ions then releases and diffuses across membrane Ion pumps can be distinguished from ion channels on the basis that ion pumps actively transport ions against a concentration gradient, while ion channels allow ions to passively flow down a concentration gradient. 13 FUNBIO.12 Biological membranes: Excitable membranes  Ion channels and ion pumps Transporters can be classified as either primary (pumps) or secondary active transporters based on the method they use to move ions across the gradient. Primary active transporters are usually transmembrane ATPases, that hydrolyse ATP to produce energy in order to transport ions up a concentration gradient. E.g. Na+/K+ ATPase. Secondary active transporters, also known as co-transporters, transport ions or other solutes) against their concentration gradient by using the electrochemical gradient created across the membrane by the transport of a different ion. E.g. sodium/ glucose co-transporter. 14 FUNBIO.12 Biological membranes: Excitable membranes  Ion channels and ion pumps There are two types of secondary active transporters, which are classified based on the direction that they move ions. Antiporters transport two different ions or solutes in opposite directions across the membrane. One moves with the concentration gradient (high to low) which powers the movement of the other against the gradient (low to high). Examples: CLCN3, NHE3. Symporters transport two different ions or solutes in the same direction, moving one with the concentration gradient (high to low), and the other against the concentration gradient (low to high). Examples: KCC2, NCC, NIS, NKCC2 15 FUNBIO.12 Biological membranes: Excitable membranes  Membrane transport Primary active transport – Typically moves molecules against their electrochemical gradient. – Uses energy in the form of ATP – E.g. Na+/K+ ATPase, Proton pumps Secondary active transport (indirect) – Moves molecules together Symport (in the same direction) – E.g. sodium-dependent co-transport carrier system – (glucose and AA transporters) Antiport (in opposite directions) - E.g. Sodium-proton exchangers (NHE) 16 FUNBIO.12 Biological membranes: Excitable membranes  Excitation Excitation is the transition from a normal, resting membrane potential to a changed action potential state in response to a stimulus. The change in membrane potential is due to a change in permeability for a specific ion. It is an electrical response in excitable cells. The action potential state will produce specific responses. The action potential state in muscle cells will cause muscle contraction. The action potential state in neurons will cause the release of chemicals which control other neurons or target tissues. The action potential state in glandular tissues may cause the release of secretions e.g. saliva, or hormones e.g. adrenaline. 17 FUNBIO.12 Biological membranes: Excitable membranes  Other properties of excitable tissue Conductivity The ability to pass the excitation impulses through the tissue. Along axons or Neuron to neuron via synapses. Muscle cell to muscle cell via gap junctions. Lability The ability to vary excitation impulse (action potential) frequency to regulate the magnitude of responses. Contractility Muscle cells specifically- The ability to change size, shape and tension in response to a stimulus. 18 FUNBIO.12 Biological membranes: Excitable membranes  Stimulus A stimulus is a detectable physical or chemical change in the environment of a sensory receptor on a responsive cell. The stimulus will trigger a predictable response in the target cells. External stimuli are usually physical or chemical: Normal temperature fluctuations- thermoreceptors Membrane stretching- touch receptors Pleasant or unpleasant odours- olfactory receptors Internal stimuli can be: Biological (hormones), Physical (blood pressure), Chemical (blood glucose). The minimum stimulation needed to produce a response is the stimulation threshold. Insufficent stimulation to produce a response is sub-threshold. 19 FUNBIO.12 Biological membranes: Excitable membranes  Irritation Changes in the physical or chemical environment that affect cellular responses in non- excitable cells. Non-excitable cells include: - Red blood cells - Fibroblasts - Interstitial cells Irritation may stimulate a response that is electrical, metabolic, affects cell proliferation (hyperplasia) or affects cell growth (hypertrophy). 20 FUNBIO.12 Biological membranes: Excitable membranes  How is the resting membrane potential generated? Maintaining the resting membrane potential depends on 3 factors: Ion distribution on either side of the membrane Permeability of the membrane for different ions Na+/K+ ATPase activity Ion Extracellular Intracellular Relative Concentration Concentration Permeability (mM) (mM) Na+ 150 15 1 K+ 5 150 25-30 A- 0 65 0 21 FUNBIO.12 Biological membranes: Excitable membranes  The role of Na+/K+ ATPase in resting potential generation 3 Na+ The Na+/K+ ATPase uses one ATP molecule to pump 3 Outside Na+ ions out of the cell in exchange for 2 K+ ions. So, a Inside net loss of +ve ions inside the cell membrane. 2 K+ ATP The action of the pump results in a higher ADP + Pi concentration of K+ inside the cell than outside, favouring outward diffusion. K+ Na+ Na+ K+ Na+ The cell has a significant number of K+ leak channels which allow K+ to leave the cell down a concentration K+ K+ K + K+ K+ Na+ gradient. This further contributes to the –ve membrane potential. There are normally very few Na+ leak channels allowing Na+ to enter the cell. 22 FUNBIO.12 Biological membranes: Excitable membranes  How is excitation achieved? depolarization- ‘all or nothing”- a change within a cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell compared to the outside. Excitation requires rapid depolarization of the membrane potential. Positively charged ions, Na+ or Ca2+ follow an electrochemical gradient into the cell. The permeability of the membrane for one of these ions has to increase. Permeability is increased by the activation of a type of ion channel. The process of generating and propagating an electrical impulse (action potential) may need a combination of different ion channels. 23 FUNBIO.12 Biological membranes: Excitable membranes  An action potential in a nerve cell 1. The resting membrane potential is maintained at -70mV by the action of the Na+/K+ ATPase and the outward leak of K+ ions. 1 Failed initiations- didnt reach the threshold 24 FUNBIO.12 Biological membranes: Excitable membranes  An action potential in a nerve cell 2. A stimulus, for example exposure to a neurotransmitter, stretching of the cell membrane or phosphorylation by a kinase causes activation of Na+ channels. Na+ ions enter the cell and depolarize the membrane. Summation of the stimuli may allow the depolarization threshold to be reached for triggering an action potential. 2 25 FUNBIO.12 Biological membranes: Excitable membranes  An action potential in a nerve cell cell potential flips from -70 to +40 3. Once membrane depolarization reaches a threshold (usually around -55mV), this new membrane potential triggers the rapid opening of voltage gated Na+ channels. 3 The large flow of Na+ into the cell causes a large magnitude depolarization of the membrane. Once the membrane reaches the peak of the action potential, the voltage gated Na+ channels will close. 26 FUNBIO.12 Biological membranes: Excitable membranes  An action potential in a nerve cell 4. At the action potential peak, voltage- gated K+ channels open which together With the K+ leak channels allow the out -flow of K+ ions. 4 The membrane repolarizes to wards its resting potential. refactory period-takes time for cells to pump sodium ions out so it can go back to resting state 27 FUNBIO.12 Biological membranes: Excitable membranes  An action potential in a nerve cell 5. The membrane repolarizes beyond its normal resting potential and hyperpolarizes due to the efflux of K+. The voltage-gate K+ channels close and the normal ion distribution is recovered by the Na+/K+ ATPase during the refractory period. 5 The resting membrane potential returns to -70mV and another action potential can be initiated if the stimulus persists. 28 FUNBIO.12 Biological membranes: Excitable membranes  CFTCR and membrane transport The cystic fibrosis transmembrane conductance regulator (CFTCR) is a Cl- channel found in many cell types. In epithelial tissues, CFTCR plays a role in NaCl secretion which is coupled to water secretion. In the lungs CFTRC plays a critical role in hydrating the mucus in the airways. disfunctional sodium ion channel- reduced sodium In Cystic fibrosis (CF), mutation of the CFTCR gene results in reduced NaCl secretion and the accumulation of a thick, sticky mucus in the airways. This makes breathing difficult. FUNBIO.12 Biological membranes: Excitable membranes 29  CFTCR in excitable cells CFTCR is also widely expressed in the peripheral and central nervous system. Membrane Cl- transport affects nerve cell excitability. Cl- outward current is excitatory (depolarizing) in PNS neurons. Cl- inward current is inhibitory (hyperpolarizing) in CNS neurons. The CF mutation reduces CFTCR activity and reduces some excitation responses. Some effects seen in CF are modulated by the nervous system, not just epithelial transport. E.g. gut motility, coughing response. Children with CF have a reduced coughing response to irritants present in the lungs, like thick mucus. builds up in lungs 30 FUNBIO.12 Biological membranes: Excitable membranes  CFTCR and membrane depolarization 7. Coughing reflex stimulated 1. Stimulus at bronchial epithelium 6. Action potential 5. Depolarization of nerve cell membrane Cl- 2. Activation of GPCR AC P Cl- 4. CFTCR opening leading ATP cAMP Cl- to Cl- efflux Cl- Cl- PKA 3. Protein kinase A activation results in phosphorylation of CFTCR 31 FUNBIO.12 Biological membranes: Excitable membranes Spinal Cord Injury: The spinal cord plays a key role in transmitting electrical signals. Spinal Cord Injury can disrupt these signals Spinal Cord Injury can be devastating and can lead to paralysis, pain and loss of key bodily functions How it occurs: Cells called neurons send electrical signals from the brain to the body How Injury Occurs: Impacts to the spinal cord result in fracture of the spine. If the damage spreads to the delicate cord tissue, a Spinal Cord Injury can occur! Injury damages neurons Nerve connections are broken most axons develop as a baby and lose ability to repair/ regrow axons if injured, so very hard to repair spinal cord injurys Nerves below the injury site are unable to send or receive signals! But…what if we DON’T NEED nerves to send signals? What if we don’t need nerves to send signals? Courtine et al. (2023)  Subdural Stimulator Stimulator is implanted against the nerves that control walking Courtine et al. (2023) Therapeutic Electrical Stimulation External electric field Provides stimulus! Stimulator is implanted against the nerves that control walking  D. Chiras, Human Biology 9th Ed, Ch 3 43-50, Ch13 275-278. L. Sherwood, Human Physiology 9th Ed, Ch 2 pp 77-84. Learning L.R. Reznikov (2017). Cystic fibrosis Resources and the nervous system. Chest, 151 (5), 1147-1155. Lorach, H., et al. Nature 618, 126–133 (2023). https://doi.org/10.1038/s41586- 023-06094-5 Youtube: 2-minute Neuroscience 44 FUNBIO.12 Biological membranes: Excitable membranes Thank you F O R M O R E I N F O R M AT I O N P L E A S E C O N TA N T NAME SURNAME EMAIL: [email protected] 45