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OVERVIEW OF EXCITABLE TISSUEDR Sahar SasiPhysiology Part 1 My Notes

This Lecture is dedicated for all medical student and for whom preparing for basic science exams .Hope you will find it helpful.

What are the Excitable Tissue??cells and tissues in which excitation is accompanied by action potential, distributed along the cellular membrane. This is a property of the bodies of nerve cells and their processesnerve fibers, muscle fibers . What is excitability? An ability of specialized cells to respond to certain stimuli by producing electrical signals known as action potential at its membrane . Give examples of excitable tissues Nerve cells, muscle (cardiac/skeletal) What are 2 basic properties of excitable cell membranes? 1. The membranes have an electrical excitability across the membrane. (In response to depolarization of the membrane above a certain threshold voltage) and may transmit an impulse along the membrane2. The membranes contain a variety of ion channels (pores) that may be opened or closed, allowing specific ions to flow across. What is Resting Membrane Potential (RMP)? Membrane potential difference (trans membrane voltage) that exists when cell membranes of excitable tissues are not producing an action potential (at rest)

Why does RMP occur?Because of build-up of negative charges inside the cells and an equal build-up of positive charges outside the cells. The greater the difference in charges across the membrane, the larger the membrane potential (voltage) What is the RMP of nerve cells? -90mV (the potential inside the nerve fiber cells is 90 mV more negative than the potential of the outside) What is the RMP of skeletal muscle fibers? -80 to -90mV What is the RMP of non-excitable tissue? +20mV out, -20mV in A cell that exhibits a membrane potential is said to be polarized 2 main factors contribute to RMP. They are? 1. Distribution of ions across the membrane2. Permeability of the membrane to Na and K

What is the distribution of ions in extra and intracellular fluid?Extracellular: rich in Na and Cl ionsIntracellular: mainly K & organic phosphates + proteins Explain about the permeability of the membrane to Na and K Plasma membrane permeability to K is 50-100 times greater than its permeability to Na The ion concentrations do not normally change very quickly (with the exception of calcium). However, the membrane permeabilities can change in a fraction of a milisecond, as a result of? Activation of ligand-gated or voltage-gated ion channels The change in membrane potential can be large or small, depending on how many ion channels are activated and what type they are. This type of changes of the membrane potential are referred to as graded potentials Explain a bit on graded potentials - affect locally- Non-propagated potential- In contrast to action potentials, AP have a constant amplitude and time course, and propagated along the neighboring cell membranes. 2 types of voltage-gated ion channels are involved in these 2 phases. What are they? 1. Na channels2. K channels

Voltage-gated Na channels have 2 separate gates. What are they?1. Activation gates: close in resting membrane, open at threshold2. Inactivation gates: open in resting membrane, also open at threshold Following stimulation of excitable cells, a graded potential causes membrane to depolarize to a critical level that is called threshold . Explain depolarization phase Rapid opening of Na+voltage-gated channels leads to Na+ inflow. this leads to loss of membrane polarization. (MP = +30mV). An action potential rises to a constant and maximum strength each time Explain threshold depolarization Also stimulate slower opening of voltage gated K+ channels. That its opening coincides with voltage-gated Na+ channels closing. What is the effect of K outflow? K+ outflow causes the resting membrane potential to be restored = repolarization phase (MP = -70mV)

While the voltage-gated K channels are open, a large enough outflow of K may lead to ???? After-hyperpolarization What is hyperpolarization? Polarization more negative than the resting level (about -90mV) As the voltage-gated K channels close, what happens? RMP returns to the resting level (-70mV) What is the refractory period? The period when excitable cells cannot generate another action potential There are two types of refractory periods. What are they? 1. Absolute refractory period 2. Relative refractory period What is the absolute refractory period? Refers to the time period during which a 2nd action potential cannot be initiated, even with a very strong stimulus. It coincides with Na channels activation and inactivation. Inactivated Na channels cannot reopen, they must first return to resting state. What is relative refractory period? Refers to the time period during which a 2nd action potential can be initiated, but only with a larger than threshold stimulus (supra threshold). It coincides with the period when the voltage gated K channels are still open after inactivated Na channels have returned to their resting state Explain propagation (conduction) of action potentials As Na flows in, depolarization increases to threshold depolarization. This open voltage-gated Na channels in adjacent patches of cell membrane

What is relative refractory period? Refers to the time period during which a 2nd action potential can be initiated, but only with a larger than threshold stimulus (supra threshold). It coincides with the period when the voltage gated K channels are still open after inactivated Na channels have returned to their resting state Explain propagation (conduction) of action potentials As Na flows in, depolarization increases to threshold depolarization. This open voltage-gated Na channels in adjacent patches of cell membrane What is the All-or-None principle? A principle of action potential generation. The depolarization process travels over the entire membrane but if it does not generate sufficient voltage to stimulate the next area of the membrane, the spread of depolarization stops.

Compare between different excitable tissues in terms of resting membrane potential The initiation and conduction of nerve and muscle action potentials are similar, but they have different RMP's. Neuron = -70mV Skeletal and cardiac muscle fibers = -90mV Compare between different excitable tissues in terms of velocity of conduction Nerve action potentials are 18x faster than muscle Compare between different excitable tissues in terms of duration of AP Neuron = 0.5 - 2.0 msecSkeletal muscle fibers = 1.0 - 5.0 msecCardiac & smooth muscle fibers = 10 - 300 msec What helps spread the action potential deep with the skeletal muscle fiber? Transverse tubules. The T tubule action potentials cause release of calcium ions inside the muscle fiber

Cardiac muscle tissues contract without neural stimulation. This property is called AutomaticityThis uploaded video will explain the RMP. Click play

Resting Membrane Potential When a neuron is not sending a signal, it is "at rest." When a neuron is at rest, the inside of the neuron is negative relative to the outside. Although the concentrations of the different ions attempt to balance out on both sides of the membrane, they cannot because the cell membrane allows only some ions to pass through channels (ion channels). At rest, potassium ions (K+) can cross through the membrane easily. Also at rest, chloride ions (Cl-)and sodium ions (Na+) have a more difficult time crossing. The negatively charged protein molecules (A-) inside the neuron cannot cross the membrane. In addition to these selective ion channels, there is a pump that uses energy to move three sodium ions out of the neuron for every two potassium ions it puts in. Finally, when all these forces balance out, and the difference in the voltage between the inside and outside of the neuron is measured, you have the resting potential. The resting membrane potential of a neuron is about -70 mV (mV=millivolt) - this means that the inside of the neuron is 70 mV less than the outside. At rest, there are relatively more sodium ions outside the neuron and more potassium ions inside that neuron.

Action PotentialThe resting potential tells about what happens when a neuron is at rest. An action potential occurs when a neuron sends information down an axon, away from the cell body. Neuroscientists use other words, such as a "spike" or an "impulse" for the action potential. The action potential is an explosion of electrical activity that is created by a depolarizing current. This means that some event (a stimulus) causes the resting potential to move toward 0 mV. When the depolarization reaches about -55 mV a neuron will fire an action potential. This is the threshold. If the neuron does not reach this critical threshold level, then no action potential will fire. Also, when the threshold level is reached, an action potential of a fixed sized will always fire...for any given neuron, the size of the action potential is always the same. There are no big or small action potentials in one nerve cell - all action potentials are the same size. Therefore, the neuron either does not reach the threshold or a full action potential is fired - this is the "ALL OR NONE" principle.

Action potentials are caused when different ions cross the neuron membrane. A stimulus first causes sodium channels to open. Because there are many more sodium ions on the outside, and the inside of the neuron is negative relative to the outside, sodium ions rush into the neuron. Remember, sodium has a positive charge, so the neuron becomes more positive and becomes depolarized. It takes longer for potassium channels to open. When they do open, potassium rushes out of the cell, reversing the depolarization. Also at about this time, sodium channels start to close. This causes the action potential to go back toward -70 mV (a repolarization). The action potential actually goes past -70 mV (a hyperpolarization) because the potassium channels stay open a bit too long. Gradually, the ion concentrations go back to resting levels and the cell returns to -70 mV.

And there you have it...the Action Potential

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