Chapters
In this article, we will discuss and explain changes to the membrane potential of neurons which includes how the resting potential is maintained, the events that take place during an action potential, and how the resting potential is restored during the refractory period. Moreover, we will also describe and explain the quick transmission of an impulse in a myelinated neuron regarding saltatory conduction. So, let us get started.
Transmission of Nerve Impulses
- Neurons transfer electrical impulses which are transmitted quite rapidly along the cell surface membrane of the neuron from one end to another
- These electrical impulses do not involve the flow of electrons, unlike the normal electric current
- These impulses, referred to as action potentials, take place through brief changes in the electrical charge distribution across the cell surface membrane
- The quick movement of sodium and potassium ions across the membrane of the axon causes the action potential
Resting Potential
- In a resting axon, which refers to an axon that does not transmit impulses, the interior of the axon always has a slightly negative electrical potential as compared to the exterior of the axon
- This potential difference is approximately -70 mV which implies that the interior of the axon has an electrical charge about 70mV lower than its exterior. This is referred to as resting potential.
- Many factors play their role in maintaining this resting potential
Maintenance of Resting Potential
Factor # 1: Sodium-potassium pumps in the membrane of the axon
Due to these pumps, sodium ions move out of the axon and the potassium ions move into the axon. The pump proteins utilize energy from ATP hydrolysis so that these ions can move continuously against their concentration gradient
Factor # 2: many large, negatively charged molecules known as anions inside the axon
It attracts potassium ions, minimizing the chance of them diffusing out of the axon
Factor # 3: Axon membranes are impermeable to the ions
The diffusion of sodium ions through the axon membrane cannot occur when the neuron is at rest
Factor # 4: Voltage-gated channel proteins in the axon membrane close
It prevents sodium and potassium ions to diffuse through the axon membrane
Action Potentials
The channel proteins present in the axon membrane enable sodium and potassium ions to pass through. Based on the electrical potential or voltage, they open and close across the axon membrane and are referred to as voltage-gated channel proteins. With the stimulation of an action potential, for instance, by a receptor cell in the neuron, the following things happen:
- The opening of sodium channel proteins in the axon membrane occurs
- Passing of sodium ions into the axon down the electrochemical gradient
- It results in the reduction of potential difference across the axon membrane because the inside of the axon becomes less negative. The process is referred to as depolarization
- It stimulates the opening of voltage-gated sodium channels. Thus, enabling more sodium ions to enter and resulting in more depolarization.
- It is an example of positive feedback which refers to a small starting depolarization that results in greater and greater levels of depolarization
- When the potential difference reaches approximately -50mV, called the threshold value, several more channels open and several more sodium ions enter, resulting in the inside of the axon reaching a potential of approximately +30mV. It results in the generation of the action potential.
- The current flows to the next section of the axon as a result of the depolarization of the membrane at the site of the first action potential. It leads to depolarization and causes the opening of sodium ion voltage-gated channel proteins.
- The current flow occurs due to the diffusion of sodium ions along the axon from the region of high concentration to the region of low concentration
- It stimulates the production of another action potential in this section of the axon membrane and the process goes on
- In the body, it enables the beginning of action potential at one end of the axon and then passes along the whole length of the axon membrane
Repolarization and the Refractory Period
- In a short period of approximately 1ms, after an action potential in a section of axon membrane is generated, the closure of all the sodium ion voltage-gated channel proteins occurs, preventing the further diffusion of the sodium ions into the axon
- In this section of the axon membrane, the potassium ion voltage-gated channel proteins open to enable the diffusion of potassium ions out of the axon, down the concentration gradient
- It results in the return of the potential difference to normal (approximately -70Mv). This process is referred to as repolarization.
- In fact, hyperpolarization occurs for a short period. It occurs when the potential difference across this section of the axon membrane becomes more negative than the normal resting potential for a short period.
- The closure of the potassium ion voltage-gated channel proteins occurs and the sodium ion channel proteins in this section of the membrane start responding to depolarization again
- This section of the axon is in the recovery period and is unresponsive. It is referred to as the refractory period
Speed of Conduction of Impulses
The conduction speed of an impulse means how quickly the impulse is transferred along the neuron. The following two factors determine the speed of conduction of impulses:
- Whether the myelin is present or absent
- The axon’s diameter
Myelination
- The conduction speed is extremely slow in the unmyelinated neurons
- By insulating the axon membrane, the presence of myelin boosts the speed at which the action potentials can be transmitted along the neuron
- Depolarization cannot take place in the sections of the axon that are surrounded by a myelin sheath because a myelin sheath prevents the diffusion of sodium and potassium ions
- Action potentials can only take place at nodes of Ranvier
- The local circuits of current that stimulate depolarization in the next section of the axon membrane exist between the nodes of Ranvier
- It implies that the action potential jumps from one node to another. This is referred to as saltatory conduction
- It enables the rapid transmission of an impulse as compared to an unmyelinated axon of the same diameter
Diameter
- The conduction speed of an impulse along neurons with thicker axons is greater than along with the thinner ones
- Thicker axons contain an axon membrane which has a greater surface area over which diffusion of ions can take place
- It boosts up the diffusion rate of sodium and potassium ions, which in turn leads to an increase in the rate at which action potentials and despoliation occur
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