Neurons transmit signals through dendrites and axons. Electrical impulses, or action potentials, begin at the axon hillock when membrane voltage reaches threshold, triggering ion flow. This depolarizes the membrane, generating a one-way signal down the axon toward the nerve terminal.

Structure and Signal Flow

• Neurons communicate through dendrites (input) and axons (output).

• Dendrites receive incoming signals.

• Axon carries outgoing signals from the cell body to the nerve terminal.

Nerve Impulses (Action Potentials)

• Action potential: rapid, brief reversal of electrical polarity across the neuron’s membrane.

• Enables fast communication along the axon.

Resting Membrane Potential

• Neurons are polarized: inside is more negative than outside.

• Typical resting membrane potential: ~ -70 mV.

• Caused by ion concentration gradients:

  • More Na⁺ (sodium) outside the cell.

  • More K⁺ (potassium) inside the cell.

• Maintained by the sodium-potassium pump:

  • Pumps 3 Na⁺ out and 2 K⁺ in.

Signal Reception and Depolarization

• Stimulation typically occurs at dendrites.

• Excitatory signals open ligand-gated sodium channels, allowing Na⁺ to enter.

• This depolarizes the membrane (makes it less negative).

• Depolarization spreads to the axon hillock (trigger zone).

Action Potential Generation

• Action potential occurs if membrane voltage reaches threshold (~ -55 mV).

• Voltage-gated Na⁺ channels open rapidly → Na⁺ influx causes further depolarization.

• Positive feedback: more depolarization opens more Na⁺ channels.

• Voltage-gated K⁺ channels open more slowly.

Phases of Action Potential

• Rising phase: rapid depolarization due to Na⁺ influx.

• Peak: Na⁺ channels begin closing; K⁺ channels fully open.

• Falling phase: K⁺ efflux repolarizes the membrane.

• Hyperpolarization: excess K⁺ leaves the cell → membrane becomes more negative than resting.

• Membrane potential is restored by diffusion and Na⁺/K⁺ pump.

Refractory Period

• Absolute refractory period:

  • Na⁺ channels inactivated.

  • No new action potential possible.

• Relative refractory period:

  • Some K⁺ channels still open.

  • Stronger-than-normal stimulus needed for new action potential.

Action Potential Propagation

• Na⁺ influx at one point causes local depolarization → triggers action potential in adjacent membrane.

• Action potential travels one direction only due to refractory period.

• Usually propagates from axon hillock to nerve terminal, not backwards to cell body.

• Directionality ensured by ion channel distribution and refractory propertie