transmission of nerve impulse

Definition

Target type: biologicalprocess

The neurological system process in which a signal is transmitted through the nervous system by a combination of action potential propagation and synaptic transmission. [GOC:curators, ISBN:0815316194]

The transmission of a nerve impulse, also known as an action potential, is a complex electrochemical process that allows the nervous system to communicate and transmit information throughout the body. Here's a detailed breakdown of the process:

**1. Resting Potential:**

- The neuron is in a polarized state, with a negative charge inside the cell membrane and a positive charge outside.
- This difference in charge is maintained by the sodium-potassium pump, which actively pumps three sodium ions out of the cell for every two potassium ions pumped in.
- The concentration of potassium ions is higher inside the cell, while the concentration of sodium ions is higher outside.
- This creates a resting potential of about -70 millivolts (mV).

**2. Depolarization:**

- When a neuron receives a stimulus, such as a neurotransmitter binding to its receptors, it causes the cell membrane to become more permeable to sodium ions.
- Sodium ions rush into the cell, driven by both the concentration gradient and the electrical gradient.
- This influx of positive charge causes the membrane potential to become less negative, a process known as depolarization.

**3. Threshold Potential:**

- If the depolarization reaches a critical threshold of about -55 mV, an action potential is triggered.
- This threshold is essential to ensure that only strong enough stimuli can initiate an action potential.

**4. Action Potential:**

- Once the threshold is reached, voltage-gated sodium channels open, allowing a large influx of sodium ions into the cell.
- This rapid influx of sodium ions causes a rapid and significant depolarization, reversing the membrane potential to a positive value, typically around +30 mV.
- This change in membrane potential is known as the action potential.

**5. Repolarization:**

- As the sodium channels close, voltage-gated potassium channels open, allowing potassium ions to flow out of the cell.
- This outflow of positive charge brings the membrane potential back towards its negative resting value, a process known as repolarization.

**6. Hyperpolarization:**

- The potassium channels remain open slightly longer, leading to a brief period of hyperpolarization, where the membrane potential becomes even more negative than the resting potential.

**7. Refractory Period:**

- During the repolarization and hyperpolarization phases, the neuron enters a refractory period, where it is less likely to fire another action potential.
- This period ensures that action potentials travel in one direction and prevents the signal from going backward.

**8. Propagation:**

- The action potential travels down the axon, the long, thin fiber that extends from the neuron's cell body.
- The action potential does not travel down the axon as a wave, but rather as a series of new action potentials generated at each point along the axon.
- This "leapfrogging" of action potentials is facilitated by the myelination of the axon, which acts as an insulator, increasing the speed of transmission.

**9. Synaptic Transmission:**

- At the end of the axon, the action potential reaches the synapse, a junction between the neuron and another cell, such as a muscle cell, a gland, or another neuron.
- The action potential triggers the release of neurotransmitters, chemical messengers that travel across the synaptic cleft to bind to receptors on the postsynaptic cell.
- This binding initiates a new signal in the postsynaptic cell, either an excitatory or inhibitory signal.

**10. Integration:**

- Neurons receive input from multiple other neurons, and the overall effect on the postsynaptic neuron is determined by the summation of all the excitatory and inhibitory signals.
- If the net effect is excitatory, the postsynaptic neuron may fire an action potential of its own, and the cycle of nerve impulse transmission continues.
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