Depolarisation

Depolarisation is a process that occurs in neurons and muscle cells when the membrane potential of the cell becomes less negative, or even positive. This process is crucial for the transmission of nerve impulses and the contraction of muscle cells.

Overview

Depolarisation is the first phase of an action potential, a brief electrical event that allows nerve impulses to be transmitted along the axon of a neuron. During depolarisation, the membrane potential of the neuron changes from its resting state of about -70 millivolts (mV) to a positive value of about +40 mV. This change in membrane potential is caused by the rapid influx of sodium ions (Na+) into the cell through voltage-gated sodium channels.

Depolarisation also occurs in muscle cells, where it triggers the release of calcium ions (Ca2+) from the sarcoplasmic reticulum, leading to muscle contraction.

Mechanism

Depolarisation begins when a stimulus, such as a nerve impulse or a neurotransmitter, causes the voltage-gated sodium channels in the neuron's membrane to open. This allows sodium ions, which are in higher concentration outside the cell, to rush into the cell down their concentration gradient. The influx of positive sodium ions makes the inside of the cell less negative, or even positive, compared to the outside. This change in membrane potential is what we call depolarisation.

Once the membrane potential reaches about +40 mV, the sodium channels close and voltage-gated potassium channels open. Potassium ions (K+), which are in higher concentration inside the cell, then rush out of the cell, making the inside of the cell more negative again. This process is known as repolarisation.

Role in Nerve Impulses

Depolarisation is crucial for the transmission of nerve impulses. When a neuron is at rest, its membrane potential is about -70 mV. When a nerve impulse arrives at the neuron, it triggers depolarisation, causing the membrane potential to rise to about +40 mV. This change in membrane potential propagates along the axon of the neuron, allowing the nerve impulse to be transmitted from the cell body to the axon terminals.

Role in Muscle Contraction

In muscle cells, depolarisation triggers the release of calcium ions from the sarcoplasmic reticulum. The calcium ions then bind to troponin, a protein in the muscle fibers, causing the muscle to contract.

See Also

• Hyperpolarisation
• Resting potential
• Action potential
• Nerve impulse
• Muscle contraction

Depolarisation Resources
Latest articles Most recent articles Ongoing trials	Wikipedia