Potenziale d'azione | NEUROSCIENZE - Lezione 4

Summary

The script discusses the complex process of nerve impulse transmission in neurons, focusing on the resting potential, action potential, and the refractory period. It explains how neurons maintain a charge difference across their membrane, the role of sodium and potassium ions, and the significance of the myelin sheath in saltatory conduction. The refractory period ensures signal transmission in one direction, preventing the signal from reversing or getting stuck. The explanation is enriched with detailed descriptions of the electrical and chemical gradients, ion channels, and the importance of maintaining the resting potential for proper nerve function.

Takeaways

• 🧠 The nervous system's primary function is to receive stimuli, process them, and calibrate a response to adapt to the surrounding environment.
• 💫 Neurons, the main cells of the nervous system, have two main characteristics: excitability and conductivity, allowing them to generate and transmit electrical impulses.
• ⚡ The transmission of the nerve impulse, an electrical signal, is facilitated by the structure of charges along the neuron's plasma membrane.
• 🌐 Ions and molecules with charge cannot simply diffuse across the cell membrane; they must pass through membrane channels, which are protein structures that facilitate their passage.
• 🔋 At rest, the neuron's membrane is slightly polarized, with a difference in charge between the inside and outside, predominantly negative inside and positive outside.
• 🚀 The action potential is triggered when a stimulus is strong enough to change the membrane potential from its resting state of -70mV to a threshold of -50mV, initiating a chain of events leading to depolarization.
• 🔄 The depolarization phase involves the opening of sodium channels, allowing sodium ions to enter the cell, which rapidly changes the membrane potential to positive.
• 🔄 The repolarization phase occurs when potassium channels open, allowing potassium ions to leave the cell, restoring the membrane potential back to its resting state.
• 🔄 The hyperpolarization phase is a brief period where the membrane potential goes below the resting potential due to the slow closing of potassium channels.
• 🛑 The refractory period is a time when the neuron cannot initiate another action potential, ensuring that the signal is transmitted in one direction only, preventing the signal from backtracking or stopping.
• 🦈 In humans, nerve impulses are transmitted in a saltatory (jumping) fashion along myelinated axons, from one node of Ranvier to the next, allowing for faster signal transmission.

Q & A

What is the primary function of neurons in the nervous system?

Neurons in the nervous system serve to receive stimuli, process them, and generate an electrical impulse to adapt to the surrounding environment.

What are the two main characteristics of neurons?

The two main characteristics of neurons are excitability, which is the ability to generate an electrical impulse in response to a stimulus, and conductivity, which is the ability to transmit this impulse to other cells.

What is the resting membrane potential of a neuron?

The resting membrane potential of a neuron is the difference in electrical charge across the cell membrane when the cell is not transmitting any signal, typically around -70 millivolts.

What ions are predominantly inside and outside the neuron at rest?

At rest, there are more positive charges (Na+ ions) outside the cell and more negative charges (K+ ions and phosphate groups) inside the cell.

How does the neuron maintain the difference in charge across its membrane?

Neurons maintain the charge difference through ion channels and pumps, such as the sodium-potassium pump, which actively transport ions across the membrane using ATP energy.

What is the action potential and how is it triggered?

The action potential is a rapid, temporary change in the membrane potential of a neuron, triggered when a stimulus causes a significant change in the membrane potential, reaching a threshold of about -50 millivolts.

What happens during the depolarization phase of an action potential?

During depolarization, voltage-gated sodium channels open, allowing Na+ ions to flow into the cell, causing the inside of the cell to become positively charged and reaching up to +35 millivolts.

What is the role of potassium channels in the repolarization phase?

In the repolarization phase, potassium channels open to allow K+ ions to flow out of the cell, which helps to restore the resting membrane potential by reducing the positive charge inside the cell.

What is hyperpolarization and why does it occur?

Hyperpolarization is a temporary state where the membrane potential becomes more negative than the resting potential, typically reaching around -90 millivolts. It occurs because potassium channels close slowly, allowing more K+ ions to leave the cell than necessary.

What is refractory period and why is it important for neural signal transmission?

The refractory period is a time when a neuron cannot generate another action potential, regardless of the strength of the stimulus. It is important for ensuring that neural signals are transmitted in one direction only, preventing the signal from being blocked or reversing.

How does the presence of myelin sheaths affect the speed of neural signal transmission in humans?

In humans, myelin sheaths around the axons of nerve cells increase the speed of signal transmission by allowing the impulse to 'jump' from one node of Ranvier to the next, rather than being conducted along the entire length of the axon.