Communication within a neuron is electrical. When a neuron is sufficiently stimulated, it reaches a threshold of excitation, triggering an action potential—a rapid reversal of the electrical charge across the membrane.
This process follows the All-or-None Law, meaning an action potential either fires at full strength or not at all; the intensity of a stimulus is communicated by the frequency of firing, not the size of the electrical charge.
In myelinated axons, the impulse travels via saltatory conduction, where the electrical charge effectively leaps from one Node of Ranvier to the next, making the process much faster than continuous conduction in unmyelinated fibers.
When the electrical impulse reaches the axon terminal, it must cross a physical gap called the synaptic cleft. This transition requires converting the electrical signal into a chemical one.
Neurotransmitters stored in vesicles are released into the cleft and bind to specific receptor sites on the postsynaptic neuron, much like a key fitting into a lock.
The effect on the receiving neuron depends on the type of neurotransmitter: excitatory chemicals increase the likelihood of the next neuron firing, while inhibitory chemicals decrease that likelihood.
It is vital to distinguish between the three types of neurons based on their structure and location in the signal arc:
| Feature | Sensory Neuron | Relay Neuron | Motor Neuron |
|---|---|---|---|
| Function | PNS to CNS (Input) | Within CNS (Connector) | CNS to PNS (Output) |
| Dendrites | Long | Short | Short |
| Axon | Short | Short | Long |
| Cell Body | Located to the side | Central | Central |
Directional Flow: Always verify the direction of the impulse in diagrams. It moves from dendrites soma axon terminal. Examiners often flip diagrams to test this.
The 'All-or-None' Principle: Remember that a stronger stimulus does NOT create a 'bigger' action potential. It creates MORE action potentials per second (higher frequency).
Structure-Function Mapping: Be prepared to explain how specific structures like the myelin sheath or Nodes of Ranvier directly contribute to the efficiency of the nervous system.
Terminology Precision: Do not confuse 'axons' with 'dendrites'. Axons Away from the cell body; Dendrites Deliver to the cell body.