Welcome to Nerve Impulses and Synapses!

Ever wondered how you can pull your hand away from a hot stove before you even realize it’s hot? Or how you’re reading these words right now? It’s all thanks to nerve impulses. In this chapter, we’ll explore how your body sends "electrical" messages along cells called neurones and how those messages jump across the gaps between them. Don't worry if it seems like a lot of physics and chemistry mixed into biology—we will break it down step-by-step!

1. The Structure of a Neurone

Before we look at the electricity, we need to know what the "wires" look like. A neurone is just a specialized cell. You need to know these parts:

  • Cell Body: Contains the nucleus and lots of ribosomes (to make neurotransmitters).
  • Dendrites: Small extensions that receive signals from other neurones.
  • Axon: A long fiber that carries the impulse away from the cell body.
  • Myelin Sheath: A fatty "insulation" layer made of Schwann cells. It speeds up the impulse.
  • Nodes of Ranvier: Tiny gaps in the myelin sheath.

Analogy: Think of the axon as a long copper wire and the myelin sheath as the plastic coating around it. The signal travels much faster when the wire is insulated!

2. The Resting Potential: The "Ready" State

When a neurone isn't sending a signal, it is at rest. However, it isn't "off." It is actually working hard to stay "charged" and ready to fire, like a stretched rubber band.

The inside of the neurone is negatively charged compared to the outside. This difference in charge is called the resting potential, and it is usually about -70mV (millivolts).

How is this charge maintained?

It’s all about moving ions (charged particles):

  1. The Sodium-Potassium Pump: This protein uses ATP (energy) to actively pump 3 Sodium ions (\(Na^+\)) out of the cell and 2 Potassium ions (\(K^+\)) in.
  2. Potassium Leak: There are more "open" potassium channels than sodium channels, so \(K^+\) ions diffuse back out faster than \(Na^+\) can leak back in.
  3. Result: More positive ions end up outside the cell than inside, making the inside negative.

Quick Review: The resting potential is maintained by the active transport of ions. Remember: 3 Na out, 2 K in.

Key Takeaway:

The resting potential is a state of "readiness" where the inside of the neurone is negative (-70mV) compared to the outside.

3. The Action Potential: Sending the Signal

When a stimulus (like a touch or sound) is strong enough, the neurone "fires." This is called an action potential. It happens in four main stages:

Step 1: Depolarisation

The stimulus causes Voltage-gated Sodium Channels to open. \(Na^+\) ions rush into the cell because they are attracted to the negative charge and are moving down a concentration gradient. The inside becomes positive (about +40mV).

Step 2: Repolarisation

Once the charge hits +40mV, the sodium channels close and Voltage-gated Potassium Channels open. \(K^+\) ions rush out of the cell, carrying the positive charge away. The inside starts becoming negative again.

Step 3: Hyperpolarisation

The potassium channels are a bit slow to close, so too many \(K^+\) ions leave. The charge drops even lower than the resting potential (e.g., -80mV). This is sometimes called the Refractory Period.

Step 4: Return to Resting Potential

The channels close, and the Sodium-Potassium pump works to return everything to the original -70mV state.

Mnemonic: Depolarisation = Door opens for Sodium. Repolarisation = Returning to negative.

Did you know? Action potentials are "All-or-Nothing." If the stimulus doesn't reach a certain threshold, nothing happens. If it does reach it, the signal is always the same size! A stronger stimulus just means more frequent signals, not bigger ones.

Key Takeaway:

An action potential is a rapid reversal of charge across the membrane. It follows the all-or-nothing principle.

4. Factors Affecting Speed

Not all nerve impulses travel at the same speed. Three things make them faster:

  1. Myelination: In myelinated neurones, the impulse "jumps" from one Node of Ranvier to the next. This is called Saltatory Conduction. It is much faster than traveling the whole length of the axon.
  2. Axon Diameter: The wider the axon, the less resistance there is, so the signal moves faster.
  3. Temperature: Higher temperatures increase the rate of diffusion of ions and the speed of enzyme activity (for ATP production).

Common Mistake: Students often think the myelin sheath "conducts" the electricity. Actually, it acts as an insulator that forces the electricity to jump!

5. Synaptic Transmission

A synapse is the tiny gap between two neurones. Because the electrical impulse cannot jump across air/fluid, it must be turned into a chemical signal called a neurotransmitter.

The Process (Cholinergic Synapse):

  1. The action potential arrives at the presynaptic knob.
  2. This causes Calcium (\(Ca^{2+}\)) channels to open, and calcium ions enter the neurone.
  3. The calcium makes tiny bubbles called vesicles (containing the neurotransmitter Acetylcholine) fuse with the membrane.
  4. The Acetylcholine is released into the gap (the synaptic cleft) and diffuses across.
  5. The Acetylcholine binds to receptor proteins on the next neurone (the postsynaptic membrane).
  6. This opens Sodium channels in the next neurone, starting a new action potential!

Cleaning up:

To stop the signal from firing forever, an enzyme called Acetylcholinesterase breaks down the neurotransmitter so it can be recycled.

Don't worry if this seems tricky! Just remember: Calcium in -> Vesicles move -> Transmitter out -> Sodium in.

6. Summation

Sometimes, one single signal from one neurone isn't enough to trigger an action potential in the next one. The neurone needs to "add up" the signals. This is called Summation.

  • Temporal Summation: One single presynaptic neurone fires many times in a short period.
  • Spatial Summation: Many different presynaptic neurones all fire at the same time to one postsynaptic neurone.
Key Takeaway:

Synapses use neurotransmitters to pass signals. Summation ensures that only important or strong enough signals are passed on.

Quick Review Box

Resting Potential: -70mV, maintained by Na/K pump.
Action Potential: Depolarisation (\(Na^+\) in) then Repolarisation (\(K^+\) out).
Saltatory Conduction: Impulse jumping between Nodes of Ranvier.
Synapse: Gap where chemicals (neurotransmitters) pass the message.
Refractory Period: Time when a neurone cannot fire again; ensures signals only travel in one direction.