Nerve Impulses

Dendrites: Highly branched structures that receive signals from other neurons or sensory receptors, increasing the neuron’s ability to communicate with other cells.
Nucleus: The central organelle of the neuron that contains the cell’s genetic material (DNA) and is responsible for regulating cellular activities and maintaining the health of the neuron.
Axon Hillock: A specialized region at the junction of the cell body and axon that integrates incoming signals; it is the site where action potentials are initiated when the summed signals exceed a specific threshold.
Axon: A long, slender projection that transmits impulses away from the neuron’s cell body to target cells, which can be other neurons, muscles, or glands. The axon is often covered by myelin, a fatty substance that helps speed up signal transmission through insulation.
Astrocyte: A type of glial cell in the brain and spinal cord that provides structural support, transports nutrients to neurons, maintains extracellular ion balance, and participates in the repair of the nervous system after injury.

Information is carried along a neuron as an electrical impulse, which is crucial for communication within the nervous system.

The neural signal is not a continuous electrical current but rather:
- A fleeting change in the potential difference across the neuron’s cell surface membrane – known as the action potential. This change sweeps along the neuron from one end to the other, enabling rapid communication.

A neuron at rest is termed a resting neuron and is metabolically active:
- Sodium-Potassium Pumps: Utilize ATP to maintain ionic concentrations, vital for the generation of action potentials. For every three sodium ions (Na+) moved out of the cell, two potassium ions (K+) are moved in, establishing a concentration gradient essential for neuronal excitability.

Potassium Ions: Diffuse out of the neuron through specific channels, following their concentration gradient, while sodium ions diffuse more slowly due to lower membrane permeability. This unequal distribution of ions results in a negatively charged inside (-70mV) compared to the outside of the neuron, maintaining the resting membrane potential and storing potential energy.

Depolarization occurs when the charge difference across the membrane is reduced, leading to excitability. A neuron is termed depolarized when the potential difference decreases, moving towards zero.

The cell surface membrane has voltage-gated sodium and potassium channels:
- At resting potential (-70mV), sodium channels are closed.
- During depolarization, if the potential difference reaches a threshold (around -40mV), sodium channels open, allowing Na+ ions to rush into the neuron, creating a rapid change in potential that peaks at +40mV.
- As sodium channels close after this peak, potassium channels open, allowing K+ ions to flow out of the cell, returning the potential difference back down to around -75mV. This rapid process of membrane potential change is critical for transmitting nerve impulses and lasts about 4 milliseconds.

After an action potential, the neuron enters a refractory period during which it cannot fire another action potential immediately. This refractory period is essential for the unidirectional propagation of the action potential and ensures that neurons do not become overstimulated:
- Sodium channels must reset to their closed state before they can open again, maintaining a proper signal transmission.

The influx of Na+ ions in a depolarized area creates local circuits that attract ions from adjacent sections, initiating depolarization in the next regions. This local current flow allows the action potential to propagate along the entire length of the neuron.

Initial Stimulation: The axon is stimulated, and an action potential is initiated.
Sodium Influx: Na+ ions rush into the neuron, significantly depolarizing the membrane.
Potassium Efflux: K+ ions exit the neuron, repolarizing the membrane back to its resting state.
Re-establishment: The sodium-potassium pump restores the membrane potential to its resting state, ensuring readiness for the next signal.

The phases of action potential can be depicted on a graph showing the transitions from resting potential to depolarization and back:
- Resting potential → Depolarization → Repolarization → Overshoot → Resting potential restored.

Action potentials are consistent in size and follow a binary pattern, behaving like electrical pulses that are either ON (producing a full action potential) or OFF (no action potential).
The strength of stimuli is encoded in the frequency of action potentials rather than their size, while the brain interprets the type of stimulus based on which neurons are activated and their frequency of firing, allowing for complex signals to be conveyed.