propagation of action potentials along a neuron

• Resting State: 

  • a neuron is like a tiny electrical wire and, has a resting membrane potential of about -70 mV.

  • the inside of the neuron is more negative than the outside.

  • difference in charge is due to the uneven distribution of ions:

  outside = more positively charged Na+

  inside = more negatively charged K+
• Depolarisation:

  • excitation occurs when a stimulus reaches the neuron

  • some sodium channels on the neuron's membrane open

  • positively charged Na+ rush into the neuron

  • inside of the neuron is less negative.

  •depolarisation reaches a threshold level of around -55 mV to -50 mV, it triggers an action potential.
• Threshold and Action Potential

  • Once the threshold is reached: voltage-gated sodium channels open and a massive influx of Na+ occurs.

  • This causes a rapid change in the membrane potential from -70 mV to about +30 mV.

  • inside of the neuron becomes positively charged.

  • sudden spike in voltage is the action potential.

  (The action potential is an "all-or-nothing" event, meaning it either happens fully or not.)
• Repolarisation:

  • After the action potential, the neuron quickly repolarises.

  • Voltage-gated sodium channels close and, voltage-gated potassium channels open.

  • K+ rush out of the neuron

  • membrane potential returns to its resting state of -70 mV.
• Refractory Period:

  • neuron enters a short refractory period after an action potential.

  • During this time, it cannot generate another action potential due to the sodium channels being temporarily inactive and needing to reset before they can open again.
• Unmyelinated Neurons:

  • Unmyelinated neurons are like regular electrical wires without any special covering.
  • When an action potential happens in an unmyelinated neuron, it travels along the entire length of the neuron's membrane, one step at a time. This is called continuous conduction.
  • The speed of conduction in unmyelinated neurons is relatively slow.
• Myelinated Neurons:

  • Myelinated neurons have a special myelin coating around their axons, which acts like an insulating layer.
  • The myelin covers the axon in segments with small gaps in between, called nodes of Ranvier.
  • When an action potential occurs in a myelinated neuron, it "jumps" from one node of Ranvier to another. This is called saltatory conduction.
  • The conduction speed in myelinated neurons is much faster, depending on the axon's diameter and myelination thickness.