اقوى شرح action potential بالتفصيل 💫🔥💯. скачать в хорошем качестве

اقوى شرح action potential بالتفصيل 💫🔥💯.

1An *action potential* is a rapid, temporary change in the electrical membrane potential of a neuron that allows it to transmit signals. This process is fundamental for communication between neurons and from neurons to muscles or glands. Here's a detailed breakdown: **1. Resting Potential**: Before an action potential occurs, the neuron is in a resting state, where the inside of the neuron is negatively charged compared to the outside. This is due to an unequal distribution of ions, mainly: *Sodium ions (Na⁺)* are more concentrated outside the neuron. *Potassium ions (K⁺)* are more concentrated inside the neuron. This state of charge difference is called the **resting membrane potential**, and it is typically around **-70 millivolts (mV)**. The neuron maintains this resting potential using the **sodium-potassium pump**, which actively transports 3 Na⁺ ions out of the neuron and 2 K⁺ ions into the neuron, consuming ATP. **2. Threshold and Depolarization**: When a neuron receives a stimulus (such as from a neurotransmitter or a physical stimulus like touch), it can cause a slight change in the membrane potential. If this change is strong enough to reach a critical level called the *threshold potential* (about **-55 mV**), an action potential is triggered. *Depolarization**: Once the threshold is reached, **voltage-gated sodium channels* in the neuron's membrane open, allowing Na⁺ ions to rush into the cell due to the concentration gradient. As Na⁺ floods into the neuron, the inside of the neuron becomes more positively charged, reversing the polarity from around *-70 mV* to about **+30 mV**. **3. Repolarization**: After the depolarization peak, the neuron needs to return to its resting state. This process is called **repolarization**. *Sodium channels close**, and **voltage-gated potassium channels* open. K⁺ ions now flow out of the neuron, restoring the negative charge inside the neuron. This outward movement of K⁺ brings the membrane potential back toward its resting level. **4. Hyperpolarization**: In some cases, too many potassium ions leave the neuron, causing the membrane potential to drop slightly below the resting potential, a phase called **hyperpolarization**. The membrane potential might fall below **-70 mV**, making the inside of the neuron even more negative than during the resting state. This is corrected over time by the sodium-potassium pump, which restores the normal ionic concentrations and resting membrane potential. **5. Refractory Period**: After an action potential, the neuron goes through a *refractory period* where it temporarily cannot generate another action potential. There are two phases of this period: **Absolute Refractory Period**: During depolarization and the early phase of repolarization, another action potential cannot be initiated, no matter how strong the stimulus is. **Relative Refractory Period**: During hyperpolarization, a new action potential can only be triggered by a much stronger stimulus than usual. **6. Propagation of Action Potential**: Once an action potential is initiated at one part of the neuron (usually the **axon hillock**), it travels along the axon toward the axon terminal. The action potential moves in a **wave-like manner**, with each segment of the axon undergoing depolarization and repolarization. In *myelinated axons**, the action potential "jumps" between the gaps in the myelin sheath called **Nodes of Ranvier* in a process known as **saltatory conduction**. This speeds up signal transmission. In **unmyelinated axons**, the action potential moves continuously along the axon, which is slower. **7. Importance of Action Potentials**: **Neuronal Communication**: Action potentials are the fundamental way neurons communicate with each other, relaying information from one neuron to the next, or from neurons to muscles and glands. **Muscle Contraction**: Motor neurons use action potentials to signal muscles, causing them to contract. **Sensory Processing**: Sensory neurons generate action potentials in response to stimuli (such as light, touch, sound), which are then interpreted by the brain. **Summary**: An action potential is a rapid electrical event in which a neuron's membrane potential temporarily becomes positive due to the influx of sodium ions. This process allows neurons to communicate by transmitting signals along axons, playing a critical role in nervous system function. طب بشري طب الأسنان كلية التمريض العلاج الطبيعي الصيدلة