3.6.2.1 Nerve impulses

• The resting potential is the voltage across the neuron's membrane when it is not transmitting an impulse (typically around -70 mV in mammals).
• The mechanism of the resting potential:

  • Sodium-Potassium Pump: Actively transports 3 Na⁺ ions out of the neuron and 2 K⁺ ions into the neuron, using ATP.
  • Selective permeability: The neuron is more permeable to K⁺ ions than Na⁺ ions, so K⁺ ions can diffuse out more easily than Na⁺ ions can diffuse in.
  • This creates an electrochemical gradient, with a higher concentration of Na⁺ ions outside and a higher concentration of K⁺ ions inside the neuron.
• During the resting potential, the inside of the neuron membrane is more negative as positive ions (Na+ and K+) are pumped out while the outside of the neuron is more positive.
• An action potential is a temporary reversal of the membrane potential that propagates along the axon.
• An unmyleinated neuron causes slower reponse to stimuli because there is no saltatory conduction so the impulse is unable to ‘jump‘ from node to node. This leads to more depolarisation over length of membrane.
• In myelinated neurons, the fatty sheath of myelin around the axon acts as an electrical insulator, preventing ion exchange through the Schwann cells.
• In myelinated neurons, there are breaks in the myelin insulator called nodes of Ranvier. The action potential ‘jumps’ from node to node in a process known as saltatory conduction.
• An action potential travels faster in a myelinated axon than an unmyelinated axon of the same diameter.
• Outline how an action potential is generated and propagated along a neuron.

  • Depolarisation: A stimulus opens Na⁺ channels, and Na⁺ enters, reducing negativity inside.
  • When the threshold is reached, more Na⁺ channels open, causing depolarization.
  • Repolarisation: K⁺ channels open, allowing K⁺ to exit and restore negativity inside.
  • Propagation: Local currents depolarize adjacent regions, propagating the action potential along the axon
• Explain the refractory period in neurons.

  • During the refractory period, Na⁺ channels are inactivated, preventing a second action potential.
  • This ensures one-way impulse transmission and limits frequency.
• The factors that affect the speed of nerve impulse conduction include myelination, axon diameter and temperature.
• Axon diameter: Larger diameters reduce resistance and increase speed.
• Temperature: Higher temperatures speed up ion diffusion as ions have higher kinetic energy.
• What is a generator potential?

  • A small change in membrane potential caused by a stimulus.
  • If the stimulus is strong enough to reach the threshold, an action potential is triggered.
• Nerve impulses are faster, short-lived, and localised.
• The threshold potential is the minimum voltage required to open voltage-gated Na⁺ channels, initiating an action potential.
• Sensory neurons: Transmit impulses from receptors to the CNS.
• Motor neurons: Transmit impulses from the CNS to effectors like muscles or glands.
• Depolarisation occurs when sodium channels open, allowing sodium ions to rush into the cell, making the inside of the neurone more positive.
• Repolarisation is when potassium channels open, allowing potassium ions to leave the cell, restoring the negative charge inside the neurone.
• The 'all or nothing' principle states that an action potential is only generated if the stimulus reaches or exceeds a certain threshold. If the threshold is not reached, no action potential will occur.
• Hyperpolarisation occurs when the membrane potential becomes more negative than the resting potential, usually due to the prolonged opening of potassium ion (K⁺) channels or the opening of chloride ion (Cl⁻) channels.