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Neurophysiology: Neural Signaling 1, NOTE: When the Potassium (K+) and…

- - - - Equilibrium Potential and the Nernst Equation
        Provide an example of the Equilibrium Potential
        
        When a cell divided by a membrane without any Ion Channels
        
        Inside the cell there is a Large concentration of K+ and a Large concentration of A-
        
        Outside the cell there is a Small concentration of K+ and a Small concentration of A-
        
        Vin - Vout = 0
        
        Once the K+ Ion Channel is installed in the membrane the K+ ions will move along the K+ Concentration Gradient from Inside of the Cell (High Concentration) to Outside of the Cell (Low Concentration)
        
        As a result, there will be more K+ OUTSIDE the cell than INSIDE the cells
        
        Therefore, the Inside of the cells will be More Negative due to the High A- concentration and Low K+ concentration
        
        Vin - Vout = - 30 mV
        
        Outside of the cell will be More Positive due to the High K+ concentration and Low A- concentration
        
        As a result, the K+ outside of the cell will be repelled by the Like Charges outside of the cell and attracted by the Opposite Charges inside of the cell
        
        Vin - Vout = -80 mV
        
        Therefore, due to the presence of an electrical potential difference across the membrane, the tendency of the K+ ions to move down the Concentration Gradient via Diffusion will be exactly matched by the amount of K+ ions being pushed back into the Cell due to Electrostatic Forces
        
        And there is NO net flux of K+ ions
        
        Jdiff = Jdrift
        
        This is known as the Equilibrium Potential
        
        Equilibrium Potential and Nernst Equation
        Define the Equilibrium Potential (Reverse Potential)
        
        Equilibrium Potential for a particular ion is the electrical potential difference/voltage across the membrane which exactly counters the tendency of the ion to move down its concentration gradient
        
        A given ion will move in order to drive the membrane potential towards its equilibrium potential
        
        Explain Nernst Equation Formula
        
        Concentration Gradient = ([K+]out)/ ([K+]in)
        
        Nernst equation is used to determine Equilibrium Potential for an ion
        
        Nernst Equation = EK
        
        EK = Vin - Vout
        
        EK = (RT)/(zF) ln ([K+]out)/([K+in])
        
        Ohms Law
        Explain the Ohms Law Equation in relation to Equilibrium Potential
        
        Ions are driven across the membrane at a rate proportional to the difference between membrane potential (V) and the equilibrium potential of the ion (Ei)
        
        Ii = gi(V-Ei)
        
        Ii is the Current of an Ion
        
        V is the Membrane Potential
        
        Ei is the Equilibrium Potential of an Ion
        
        gi = 1/Ri
        
        Ei = (RT/zF) ln [C]out/[C]in
        
        When the Equilibrium Potential of an Ion is reached the:
        
        Ii = 0
        
        gi GREATER than 0
        
        Ions
        Outline the arrangement of K+, Na+, Ca2+ and Cl- Ions outside and inside of the cell
        
        K+
        
        Inside the cell there is MORE K+
        
        Outside the cell there is LESS K+
        
        Ei = -80 mV
        
        Na+
        
        Inside of the cell there is LESS Na+
        
        Outside of the cell there is MORE Na+
        
        Ei = 62 mV
        
        Ca2+
        
        Inside of the cell there is MORE Ca2+
        
        Outside of the cell there is LESS Ca2+
        
        Ei = 123 mV
        
        Cl-
        
        Inside of the cell there is LESS Cl-
        
        Outside of the cell there is MORE Cl-
        
        Ei = - 65 mV
- - - - Voltage-Gated Ion Channels
        Outline the features of the Voltage-Gated Ion Cannels
        
        Voltage-Gated Ion Channels have the following features:
        
        Voltage Sensor
        
        Voltage Sensor is located on the membrane spanning domain
        
        Voltage-Sensor senses a stimulus and signals for he gate to Open/Close
        
        Vestibule
        
        Vestibule is forms the entrance to the aqueous pore
        
        Vestibule is charged and attracts ions around the pore mouth
        
        Gate
        
        Gate regulates the ion flow
        
        Gate is modified by mechanical, electrical or chemical actions
        
        Selectivity Filter
        
        Narrow Selectivity Filter is a region of the channel at which the Aqueous Pore is very narrow
        
        Selectivity Filter distinguishes which ions enter the pore
        
        Voltage-Gated Sodium (Na+) Channels
        Explain the mechanism of action of the Voltage-Gated Sodium (Na+) Channels
        
        Voltage-Gated Na+ Channels have an inward current due to the influx of Na+ ions into the Cell
        
        Voltage-Gated Na+ Channels have 2 gates:
        
        Activation gate
        
        Inactivation Gate
        
        Activation Gate is more likely to open at increased voltage, this happens fast
        
        Inactivation Gate is more likely to close at increased voltages, this happens slowly
        
        In order for Na+ Ions to flow into the cell, BOTH the Inactivation gate and the Activation gate need to be OPEN
        
        Resting State:
        
        Inactivation gate is open and thee Activation Gate is closed
        
        Therefore, no Sodium (Na+) ions enter the cell
        
        Activated State
        
        During Depolarization, there is an increase in the Voltage (Membrane Potential)
        
        An Increase in the Voltage, stimulates the opening of the Activation Gate
        
        Both the Activation Gate and the Inactivation Gate are open
        
        This allows for an influx of Na+ Ions into the cell
        
        Inactivated State
        
        During the sustained increase in Membrane Potential (Voltage) the Inactivation Gate closes
        
        BUT the Activation Gate remains open
        
        Channel no longer allows the Na+ Ions to move through
        
        Once the action potential is over, the channel returns to Resting State
        
        Voltage-Gated K+ Channels
        Describe the mechanism of action of the Voltage-Gated K+ Channels
        
        Voltage-Gated K+ Channels have an outward current due to the increased efflux of K+ Ions
        
        Voltage-Gated K+ Channels have an Activation Gate
        
        Activation Gate is more likely to be open during increased voltages
        
        Activation gate of the Voltage-Gated K+ Channels opens more slower than the Voltage-Gated Na+ Channels
        
        Resting State
        
        During the Resting Membrane Potential the Activation Gate is closed
        
        There will be no efflux of K+ ions out of the cell
        
        Activation State
        
        During Depolarization, there is an increase in the Membrane Potential (Voltage) and the Activation gate will open
        
        There will be an efflux of K+ Ions out of the cell
- - - - How Does the Length Constant affect the Action Potential conduction velocity ?
        
        The Longer the Length Constant, the faster the Action Potential conduction velocity
        
        The Shorter the Length Constant, the Slower the Action Potential conduction velocity
        
        Length Constant is a measure of how far a voltage travels along the axon before it decays to zero
        
        How Does the Time Constant affect the Action Potential conduction velocity
        
        The Faster the Time Constant, the Faster the Action Potential conduction velocity
        
        The Slower the Time Constant, the Slower the Action Potential conduction velocity
        
        Time Constant measures the time it takes for a neuron to 'fully' charge to a stable voltage if a voltage change was applied anywhere in the neuron
        
        Myelin Sheath
        
        Myelin Sheath of the Axons increases the speed of propagation of the action potential
        
        Myelin Sheath contains Lipids which provide for Insulation and give the White Colour of the White Matter
        
        Auto-Immune destruction of the myelin sheath is known as Multiple Sclerosis
        
        Relationship between Myelin and Membrane Capacitance
        Explain the relationship between Myelin and Membrane Capacitance
        
        Myelin Sheath Reduces Membrane Capacitance
        
        Membrane Capacitance is the ability of the membrane to separate charge
        
        Cations are Positive Ions and Anions are Negative Ions
        
        The Lower the Membrane Capacitance, the Higher the speed of propagation of the Action Potential along the Axon
        
        The Higher the Membrane Capacitance, the Slower the speed of propagation of the Action Potential along the Axon
        
        Myelin and Membrane Capacitance
        Describe the Membrane Capacitance in an Unmyelinated Axon
        
        Unmyelinated Axon has an increased Membrane Capacitance
        
        When the Axon is unmyelinated, the Cations will enter the Axon
        
        These Cations will then travel along the Axon very quickly to cause a Voltage Changes in neighboring ares of the Axon
        
        Therefore, due to the High Membrane Capacitance, more Anions (-) remain along the Inner Surface of the Axon and and attract more Cations (+) to the Outer Surface of the Axon
        
        Therefore, Less cations will be depolarize other parts of the Axon Membrane
        
        Describe the Membrane Capacitance in a Myelinated Axon
        
        Myelinated Axon has a decreased Membrane Capacitance
        
        In a Myelinated Axon, the Cations will enter the Axon
        
        These Cations will travel along the Axon very quickly to cause Voltage Changes in the neighbouring areas of the Axon
        
        Due to a decreased Membrane Capacitance Less Anions will remain along the Inner Surface of the Axon
        
        As a result, Less Cations will be attracted to these Anions and remain along the Outer Surface of the Axon
        
        Therefore, more Cations will be available to depolarize other areas of the Axon Membrane