NERVE PHYSIOLOGY
Potentials in Nerve Fibers: The structure of the neuron has a cell body and two types of fibers, the dendrites and the axon. It has a membrane and a sheath around its fibers called the myelin sheath. The nerve fibers are excited by stimuli and conduct excitation throughout their lengths. The mechanism of excitation is inextricable tied up with various electrical potentials and currents in the fiber. The potentials found in the nerve, and it deals with how they lead one another, and how they take part in excitation and conduction of nerve fibers.
The Resting Potential of a Nerve fiber: Diagram of a polarized membrane and measurement of resting potential. +’ve ions are on the out side and –‘ve ions on the inside of the membrane of the nerve cell. These makes a resting potential across the membrane, which can measured by hooking up a voltameter with one electrode at the cut end of a nerve fiber and the other on the outside of the fiber.
Electrical Stimulation: Based on polarization, it in turn, the chain of events that gives nerves their enhanced properties of irritability and conduction. These events can be studied by applying electrical stimuli to the membrane of the nerve fiber and then recording the electrical changes that occur. IT can do with the help of one pair of electrodes put on a nerve – one electrode inside and the other outside of fiber. When electrical stimuli are applied through 2 electrodes on the outside of the fiber, each electrode has different sign. One is +’ve and the other is –‘ve, just as are the two electrodes of a battery. The electrode that carries the –‘ve sign is called the cathode, the one with +’ve sign is called anode. The voltage or potential difference between the cathode and anode is the strength of the stimulus.
Neurons send messages electrochemically. This means that chemicals cause an electrical signal. Chemicals in the body are "electrically-charged" -- when they have an electrical charge, they are called ions. The important ions in the nervous system are sodium and potassium (both have 1 positive charge, +), calcium (has 2 positive charges, ++) and chloride (has a negative charge, -). There are also some negatively charged protein molecules. It is also important to remember that nerve cells are surrounded by a membrane that allows some ions to pass through and blocks the passage of other ions. This type of membrane is called semi-permeable.
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 Potential
The current flows arising from the electrotonic potential brings to the action of the nerve itself. The electrotonic currents disturb the equilibrium itself. The electrotonic currents disturb the equilibrium of the membrane and set off a set of events in the nerve fiber that is called the action potential.
The spike Potential: The most explosive and couspicuous event set off in the membrane is the spike potential, it is also called as nerve impulse. What happens is this: the out flowing electrotonic currents at the cathode, when they get sufficiently strong, bring about a collapse of the polarization of the membrane. And also it has something to do with changing resistance or chemical forces maintaining polarization. As a result, the small electrotonic potential leaps very suddenly into a relatively large –‘ve potential, the spike potential.
Properties of the Spike Potential: Threshold stimulus and potential
Action potentials occur only when the membrane in stimulated (depolarized) enough so that sodium channels open completely. The minimum stimulus needed to active an action potential is called the threshold stimulus. The threshold stimulus causes the membrane potential to become less negative. If the membrane potential reaches the threshold potential, the voltage regulated sodium channels all open. Sodium ions rapidly diffuse inward, and depolarization occurs.
All-or-None Law: Action potentials occur maximally or not all, in other words, there in no such things as partial or weak action potential. Either the threshold potential is reached an action potential occurs, or it isn’t reached and no action potential occurs.
The 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 by an exchange of ions across 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.
Potentials in Nerve Fibers: The structure of the neuron has a cell body and two types of fibers, the dendrites and the axon. It has a membrane and a sheath around its fibers called the myelin sheath. The nerve fibers are excited by stimuli and conduct excitation throughout their lengths. The mechanism of excitation is inextricable tied up with various electrical potentials and currents in the fiber. The potentials found in the nerve, and it deals with how they lead one another, and how they take part in excitation and conduction of nerve fibers.
The Resting Potential of a Nerve fiber: Diagram of a polarized membrane and measurement of resting potential. +’ve ions are on the out side and –‘ve ions on the inside of the membrane of the nerve cell. These makes a resting potential across the membrane, which can measured by hooking up a voltameter with one electrode at the cut end of a nerve fiber and the other on the outside of the fiber.
Electrical Stimulation: Based on polarization, it in turn, the chain of events that gives nerves their enhanced properties of irritability and conduction. These events can be studied by applying electrical stimuli to the membrane of the nerve fiber and then recording the electrical changes that occur. IT can do with the help of one pair of electrodes put on a nerve – one electrode inside and the other outside of fiber. When electrical stimuli are applied through 2 electrodes on the outside of the fiber, each electrode has different sign. One is +’ve and the other is –‘ve, just as are the two electrodes of a battery. The electrode that carries the –‘ve sign is called the cathode, the one with +’ve sign is called anode. The voltage or potential difference between the cathode and anode is the strength of the stimulus.
Neurons send messages electrochemically. This means that chemicals cause an electrical signal. Chemicals in the body are "electrically-charged" -- when they have an electrical charge, they are called ions. The important ions in the nervous system are sodium and potassium (both have 1 positive charge, +), calcium (has 2 positive charges, ++) and chloride (has a negative charge, -). There are also some negatively charged protein molecules. It is also important to remember that nerve cells are surrounded by a membrane that allows some ions to pass through and blocks the passage of other ions. This type of membrane is called semi-permeable.
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 Potential
The current flows arising from the electrotonic potential brings to the action of the nerve itself. The electrotonic currents disturb the equilibrium itself. The electrotonic currents disturb the equilibrium of the membrane and set off a set of events in the nerve fiber that is called the action potential.
The spike Potential: The most explosive and couspicuous event set off in the membrane is the spike potential, it is also called as nerve impulse. What happens is this: the out flowing electrotonic currents at the cathode, when they get sufficiently strong, bring about a collapse of the polarization of the membrane. And also it has something to do with changing resistance or chemical forces maintaining polarization. As a result, the small electrotonic potential leaps very suddenly into a relatively large –‘ve potential, the spike potential.
Properties of the Spike Potential: Threshold stimulus and potential
Action potentials occur only when the membrane in stimulated (depolarized) enough so that sodium channels open completely. The minimum stimulus needed to active an action potential is called the threshold stimulus. The threshold stimulus causes the membrane potential to become less negative. If the membrane potential reaches the threshold potential, the voltage regulated sodium channels all open. Sodium ions rapidly diffuse inward, and depolarization occurs.
All-or-None Law: Action potentials occur maximally or not all, in other words, there in no such things as partial or weak action potential. Either the threshold potential is reached an action potential occurs, or it isn’t reached and no action potential occurs.
The 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 by an exchange of ions across 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.
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