Nerve Impulse

• A nerve impulse is caused by the movement of ions (charged particles) across the membrane of the neuron.
• When a neuron is not carrying an impulse, ions are pumped in and out of the axon. The inside of the axon becomes negatively charged, and the outside becomes positively charged.

Key Concepts

1. Threshold:

• The minimum stimulus required to start a nerve impulse.
• Any stimulus below the threshold has no effect.
• A stimulus at or above the threshold causes an electrical impulse to travel along the neuron.

2. All or Nothing Law:

• The size of the impulse is not affected by the size of the stimulus (as long as it is above the threshold level).
• A mild or a strong stimulus will cause the same impulse to travel along the axon.
• The amount of pain felt depends on:
• The number of neurons stimulated.
• The frequency with which they send their impulses.

3. Ionic Movement During an Impulse:

• When the threshold is reached, the axon or dendrite changes its permeability to ions.
• This causes the inside of the axon to become positive and the outside to become negative.
• The change in charge moves along the axon as a chain reaction.
• ATP is needed for these changes.

4. Refractory Period:

• This is the time needed by a nerve cell to recover before it can send another impulse.

Speed of the Impulse

The speed at which an impulse travels along a neuron depends on;

5. Myelin presence:

• If there is no myelin, the speed of the impulse decreases.
• If myelin is present, the impulse can jump between the nodes of Ranvier, increasing its speed.

6. Neuron diameter:

• The greater the diameter of the dendrite or axon, the faster the impulse travels

Synapse

• A synapse is where two neurons come into close contact. It consists of:
  • The pre-synaptic neuron (axon terminal).
  • The post-synaptic neuron (dendrite).
  • The tiny gap between them is called the synaptic cleft.
• Impulses cannot cross the synapse.

How a Synapse Works

7. An impulse reaches the axon terminal of the pre-synaptic neuron.
8. This stimulates neurotransmitter swellings in the pre-synaptic neuron to release neurotransmitters.

• Neurotransmitters are chemical substances such as noradrenaline and dopamine.
• They are produced in the cell bodies or neurotransmitter swellings.

9. The neurotransmitter diffuses across the synaptic cleft.
10. On reaching the post-synaptic neuron, the neurotransmitter binds to protein receptors on its cell membrane.
11. This causes a new impulse to be generated in the post-synaptic neuron.
12. After transmission, the neurotransmitter is either:

• Broken down in the synaptic cleft by special enzymes.
• Reabsorbed by the pre-synaptic neuron.

Functions of Synapses

1. Transmit impulses: Allow the transfer of signals from one neuron to another or to an effector.
2. Control impulse direction: Ensure impulses travel in one direction only.
3. Prevent overstimulation of effectors: Regulate the response by stopping neurotransmitter production when stimulation is constant, allowing us to adapt to stimuli such as noise or pain.