Nerve Impulse Conduction
Signal Propagation in Nerve Cells
Nerve cells, or neurons, transmit signals through both passive and active propagation.
Passive Propagation (Cable Properties)
- Responses are graded: A larger stimulus results in a larger response.
- Responses summate: Multiple stimuli at synapses result in a summed response.
Active Propagation
Unlike simple wires, neurons possess active electrical properties triggered by changes in membrane potential (Vm). These properties enable the conduction of electrical signals over long distances without signal degradation.
Action Potential
The action potential is a rapid, transient change in membrane potential that travels down the neuron’s axon.
Effect of Refractory Period on Impulse Propagation
The refractory period is a period of time after an action potential during which it is difficult or impossible to generate another action potential. This period has significant effects on impulse propagation.
- Absolute Refractory Period: A second action potential cannot be generated, regardless of the stimulus strength.
- Relative Refractory Period: Action potentials can be generated, but with:
- Increased threshold: A stronger stimulus is required to overcome hyperpolarization.
- Reduced amplitude: Fewer sodium (Na+) channels are available to open, limiting the influx of Na+ ions.
Nerve Impulses – Propagation of Action Potentials
During depolarization:
- The outside of the membrane becomes more negative as positive charges move away.
- The inside of the membrane becomes more positive as positive charges move toward it.
The absolute refractory period prevents the immediate generation of another action potential, ensuring unidirectional signal propagation.
Action potentials can spread in both directions from the point of stimulation. However, the refractory period prevents backward propagation, ensuring the signal travels down the axon.
Conduction of Action Potentials
Two main types of action potential conduction exist:
- Saltatory Conduction: Occurs in myelinated axons.
- Continuous Conduction: Occurs in non-myelinated axons.
Saltatory Conduction
Saltatory conduction is the propagation of action potentials along myelinated axons. Myelin acts as an insulator, forcing the action potential to "jump" between the gaps in the myelin sheath called Nodes of Ranvier.
Functional Consequences of Saltatory Conduction in Myelinated Axons
- Increased Conduction Velocity: Saltatory conduction significantly increases the speed of action potential propagation.
- In small, non-myelinated axons, conduction velocity is approximately 0.25 m/sec.
- In large, myelinated axons, conduction velocity can reach up to 120 m/sec.
Continuous Conduction
Continuous conduction occurs in non-myelinated axons. The action potential propagates along the entire length of the axon, with each adjacent segment of the membrane depolarizing and repolarizing in sequence. This process is slower than saltatory conduction due to the lack of myelin insulation.
