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Electrical Properties of Neuron Dr. Atif Mahmood
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Learning Objectives At the end of the lecture, student should be able to: know the basic structure and function of neuron understand its basic physical and electrical properties of neuron to know the factors on which the resting membrane potential depends to appreciate the mechanics of action potential initiation and measurements of action potentials. Know the basics of patch-clamp technique
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Two classes of cells • Nerve cells (neurons) is the basic structural and functional unit of nervous tissue − For electrical signaling − The human brain has 1011 neurons • Glial cells − Not for electrical signaling − 10 to 50 times more glial cells than neurons − Play an essential role in brain metabolism − Support neurons − Cover neurons with myelin − Clean up debris
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Neuron structure • cell body – contains nucleus & organelles • dendrite – conducts signals to cell body • axon – long fibers – specialized for electrical conduction
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Structure of a typical neuron Dendrite Axon Terminal Myelin Sheath Node of Ranvier Axon Cell Body Nucleus Schwann Cell
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Differences between neurons and other cells 1. Neurons have specialized extensions called dendrites and axons. Dendrites bring information to the cell body and axons take information away from the cell body 2. Neurons communicate with each other through an electrochemical process 3. Neurons contain some specialized structures (for example, synapses) and chemicals (for example, neurotransmitters)
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Morphology of Nerve cells (The Neuron)
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Two processes extending from the cell body. (retinal cells, olfactory epithelium cells)
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Galvani frog’s legs experiment
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Ions Cannot Diffuse Across the Hydrophobic Barrier of the Lipid Bilayer
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Ions and Ion Channels [K+] [K+] [Na+] [Ca2+] [Cl-] [Cl-] [Ca2+] [Na+] Cations: + Anions: - Channels are selective to particular ions. Passive vs. Active channels. Permeability is very high to K and Na, medium to Cl and very low to big anions. [K+]in=20 [K+]out [Na+]out=10 [Na+]in Main question: How is the membrane potential is related to [charges] in and out? Outside Inside
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Equilibrium potential for one ion [K+] [K+] Two competing forces: Diffusion by concentration gradient. Motion by voltage gradient. Diffusion Voltage difference Outside Inside
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1.3 Equilibrium potential for one ion [K+] [K+] Diffusion Flux: Jdif(x)= -D d[K+](x) / dx D, diffusion coefficient-> D=µkT/q µ, mobility k, Boltzmann constant q, ion charge x Voltage difference Flux: Jelec(x)= -µz [K+](x) dV(x) / dx µ, mobility z, ion valence, +/-1, +/-2, etc. Equilibrium happens when Jdif(x) + Jelec(x) = 0, which leads to the Nernst equation: E K+ = kT / zq ln [K+]out / [K+]in = -75,-90 mV E K+ is the potential necessary to maintain the concentration gradient [K+]out / [K+]in
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1.3 Equilibrium potential for one ion [K+] [K+] x E K+ = kT / zq ln [K+]out / [K+]in = -75,-90 mV E Na+ = +55 mV E Ca2+ = +150 mV E Cl- = -60,-65mV K+ Na+ Vm = -70 mV E K+ < Vm < E Na+ Compensated by Na-K pump. K+ Na+
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The Bulk Solution Remains Electroneutral
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Ion Channel Selectivity and Ionic Concentration Gradient Result in an Electromotive Force
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Cell membrane: 2-3 nm thick and is impermeable to most charged molecules and so acts as a capacitor by separating the charges lying on either side of the membrane. NB Capacitors, store charge across an insulating medium. Don’t allow current to flow across, but charge can be redistributed on each side leading to current flow. The Lipid Bilayer Acts Like a Capacitor
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+ + + + - - - - Vm = Q/C ∆Vm = ∆Q/C The Lipid Bilayer Acts Like a Capacitor ∆Q must change before ∆Vm can change The voltage (Vm)across a capacitor is proportional to the charge (Q) stored on the capacitor:
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Capacitance is Proportional to Membrane Area - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + - - - - + + + + + + + + Vm = Q/C
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Equilibrium and reversal potentials Equilibrium potentials (The Nernst equation applies when the channels allow only one type of ion to pass through them) Some channels are not so selective, and in this case the potential E is estimated by the Goldman equation Reversal potentials takes a value intermediate between the equilibrium potentials of the individual ion types that it conducts
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Resting membrane potential RMP All cells (not just excitable cells) have a resting potential (membrane potential) : an electrical charge across the plasma membrane The interior of the cell negative Typically -70mV ( varies -40 to -90 mV depends upon the type of neuron)
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Resting membrane potential RMP Neurons are enclosed by a membrane separating interior from extra cellular space The concentration of ions inside is different (more –ve) to that in the surrounding liquid
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-ve ions therefore build up on the inside surface of the membrane and an equal amount of +ve ions build up on the outside The difference in concentration generates an electrical potential (membrane potential) which plays an important role in neuronal dynamics. The ion channels are proteins in the membrane, which lower the effective membrane resistance by a factor of 10,000 (depending on density, type etc)
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Resting membrane potential RMP In order for a potential difference to be present across a membrane lipid bilayer, two conditions must be met. 1.unequal distribution of ions of one or more species across the membrane (ie, a concentration gradient). 2.Two, the membrane must be permeable to one or more of these ion species. The permeability is provided by the existence of channels or pores in the bilayer; these channels are usually permeable to a single species of ions. represents an equilibrium situation at which the driving force for the membrane-permeant ions down their concentration gradients across the membrane is equal and opposite to the driving force for these ions down their electrical gradients.
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Resting Potential In a resting neuron (one that is not conducting an impulse), there is a difference in electrical charges on the outside and inside of the plasma membrane. The outside has a positive charge and the inside has a negative charge.
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Contribution of Active Transport – Factor 1 There are different numbers of potassium ions (K+) and sodium ions (Na+) on either side of the membrane. Even when a nerve cell is not conducting an impulse, for each ATP molecule that’s hydrolysed, it is actively transporting 3 molecules Na+ out of the cell and 2 molecules of K+ into the cell, at the same time by means of the sodium-potassium pump.
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Contribution of facilitated diffusion The sodium-potassium pump creates a concentration and electrical gradient for Na+ and K+, which means that K+ tends to diffuse (‘leak’) out of the cell and Na+ tends to diffuse in. BUT, the membrane is much more permeable to K+, so K+ diffuses out along its concentration gradient much more slowly.
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RESULTS IN: a net positive charge outside & a net negative charge inside. Such a membrane is POLARISED
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Excitation and Conduction Most biological neurons communicate by short electrical pulses called action potentials or spikes or nerve impulses These action potentials are generated by means of influx and out flux of ions through the ion channels embedded in membrane Suitable electrical probe (electrode) and measurement instrumentation (amplifier and read-out) can measure these tiny potentials on the order of few milli-volts
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Excitation and Conduction Nerve cells have a low threshold for excitation – may easily be excited by electrical, chemical or mechanical stimuli Two types of physiochemical disturbances are produced as a result Local or Non-Propagated Potentials such as Synaptic, Generator or Electrotonic Potentials Propagated Disturbances such as Action Potentials or Nerve Impulses
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The Morphology of an Action Potential Action potential is a rapid, reversible, and conductive change of the membrane potential after the cell is stimulated. Nerve signals are transmitted by action potentials. Reduction in membrane potential (depolarization) to "threshold" level leads to opening of Na+ channels, allowing Na+ to enter the cell Interior becomes positive The Na+ channels then close automatically followed by a period of inactivation. K+ channels open, K+ leaves the cell and the interior again becomes negative. Process lasts about 1/1000th of a second.
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Properties of the Action Potential “All or none” phenomenon A threshold or suprathreshold stimulus applied to a single nerve fiber always initiate the same action potential with constant amplitude, time course and propagation velocity. Propagation Transmitted in both direction in a nerve fiber
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The Patch-Clamp Technique This is a novel technique in which physiological currents flowing through the cells can be detected without disrupting the cell or its contents A micropipette (diameter in microns) filled with a buffer solution and carrying a metal electrode is gently touched to the cell membrane and the membrane contents are sucked in. This is called a patch-clamp.
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THE END