Biology | Neural signalling

Hello Biologists! Welcome to Neural Signalling

This chapter, "Neural signalling," sits right in the middle of our unit on Interaction and Interdependance. Why? Because the nervous system is the ultimate communication network—it dictates how your body interacts with the environment and how its internal systems (like muscles and glands) depend on rapid instruction.

Think about the last time you pulled your hand away from something hot. That reaction wasn't magic; it was an incredibly fast electrical-chemical message. In these notes, we're going to break down how those messages—called nerve impulses—are generated, propagated, and passed from one cell to the next.

Don't worry if terms like depolarization sound intimidating at first. We will use simple analogies to make the electrical language of the brain understandable!

The Basic Unit: The Neuron

The entire nervous system, from your brain to the tip of your toes, relies on specialized cells called neurons. These cells are designed for rapid transmission of signals.

Key Components of a Neuron

• Dendrites: Receive signals from other neurons or sensory receptors. They are the 'input cables.'
• Cell Body (Soma): Contains the nucleus and cellular machinery; processes the incoming signals.
• Axon: A long, thin projection that transmits the nerve impulse away from the cell body towards other neurons, muscles, or glands. This is the 'transmission cable.'
• Myelin Sheath: A fatty insulating layer (formed by Schwann cells in the peripheral nervous system). Its job is to speed up the impulse transmission.
• Axon Terminal: The end point where the neuron communicates with the next cell via a synapse.

Analogy: Think of a neuron like an ethernet cable. The dendrites are the port where you plug in the signal; the axon is the cable itself, and the myelin sheath is the protective rubber coating that keeps the signal fast and clean.

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1. Establishing the Resting Potential

Before a neuron can fire a message, it must establish a stable state, known as the resting potential. This is the electrical difference (or voltage) across the neuron's membrane when it is not transmitting a signal.

For most neurons, the resting potential is approximately -70 mV (millivolts). The inside of the cell is negative relative to the outside.

How the Resting Potential is Maintained

This negative charge is primarily established and maintained by the action of the Sodium-Potassium (\(Na^+/\text{K}^+\)) Pump. This is an example of active transport, meaning it requires ATP energy.

Step-by-Step Action of the \(Na^+/\text{K}^+\) Pump:

1. Three Sodium ions (\(Na^+\)) from the inside of the cell bind to the pump.
2. The pump hydrolyzes ATP, changing its shape.
3. The three \(Na^+\) ions are released outside the cell.
4. Two Potassium ions (\(\text{K}^+\)) from the outside bind to the pump.
5. The pump changes shape again and releases the two \(\text{K}^+\) ions inside the cell.

Key Takeaway: Because 3 positive charges are pumped OUT for every 2 positive charges pumped IN, there is a net loss of positive charge inside the cell. This, combined with large, negatively charged protein molecules trapped inside the axon, ensures the resting potential is maintained at -70 mV.

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2. Generating and Propagating the Action Potential

A nerve impulse is an electrical signal that travels along the axon, and it is technically referred to as an action potential. This signal is a rapid, temporary change in the membrane potential.

The All-or-Nothing Principle

Action potentials only fire if the stimulus reaches a critical level known as the threshold potential (usually around -55 mV). If the stimulus is too weak, nothing happens. If it reaches the threshold, a full, maximum-strength action potential is fired. You cannot have a "small" action potential.

The Three Phases of the Action Potential

Phase 1: Depolarization (Becoming Positive)

If the threshold is reached:

1. Voltage-gated \(Na^+\) channels snap open rapidly.
2. Since the concentration of \(Na^+\) is much higher outside the cell, \(Na^+\) floods into the axon down its concentration and electrical gradient.
3. This influx of positive charge makes the membrane potential rapidly increase (becoming less negative, then positive), peaking at about +30 mV.

Mnemonic Aid: Depolarization is the Drive in of \(Na^+\).

Phase 2: Repolarization (Returning to Negative)

Immediately after the peak:

1. The voltage-gated \(Na^+\) channels close and become inactive.
2. The slower voltage-gated \(\text{K}^+\) channels finally open.
3. \(\text{K}^+\) rushes out of the cell, down its concentration gradient. This outflow of positive charge rapidly brings the membrane potential back down towards negative values.

Phase 3: Refractory Period and Restoration

The \(\text{K}^+\) channels are slow to close, causing the membrane potential to briefly overshoot the resting potential (it dips below -70 mV). This period is called hyperpolarization.

During the subsequent refractory period, the neuron cannot fire another action potential. This ensures that the impulse travels in only one direction and limits the frequency of firing. The \(Na^+/\text{K}^+\) pump then works continuously to restore the original resting ion distribution.

Propagation of the Impulse

Once an action potential is generated, it must travel down the length of the axon.

The influx of \(Na^+\) during depolarization at one point generates local currents that diffuse rapidly to the adjacent section of the axon, causing the next section's voltage-gated \(Na^+\) channels to reach the threshold and open. This process repeats all along the axon, moving the signal forward.

The Importance of Myelination (Saltatory Conduction)

In unmyelinated axons, the impulse must regenerate continuously, slowing the process. However, in myelinated axons, the impulse transmission is much faster.

• The myelin sheath acts as an electrical insulator.
• Voltage-gated channels only exist in the small gaps between the myelin segments, called the Nodes of Ranvier.
• The action potential jumps rapidly from one Node of Ranvier to the next, a process called saltatory conduction (from the Latin saltare, meaning 'to leap'). This dramatically increases the speed of neural communication—vital for fast interactions!

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3. Synaptic Transmission: Chemical Communication

When the electrical impulse reaches the end of the axon, it usually cannot jump directly to the next neuron or effector cell. Instead, the signal must cross a tiny gap called the synaptic cleft using chemical messengers called neurotransmitters. The entire junction is called the synapse.

• The neuron sending the signal is the presynaptic neuron.
• The cell receiving the signal is the postsynaptic cell.

The Process of Synaptic Transmission (Step-by-Step)

Step 1: Arrival of the Action Potential

The electrical impulse reaches the axon terminal of the presynaptic neuron.

Step 2: Calcium Influx

The depolarization caused by the action potential opens voltage-gated \(\text{Ca}^{2+}\) channels in the presynaptic membrane. \(\text{Ca}^{2+}\) (Calcium ions) rush into the terminal, down their concentration gradient.

Step 3: Neurotransmitter Release

The sudden increase in intracellular \(\text{Ca}^{2+}\) concentration triggers the synaptic vesicles (sacs containing neurotransmitters) to move towards and fuse with the presynaptic membrane. The neurotransmitters are then released into the synaptic cleft via exocytosis.

Analogy: Think of \(\text{Ca}^{2+}\) as the key that unlocks the door (the vesicle fusion) to release the message (the neurotransmitter).

Step 4: Binding and Postsynaptic Response

The neurotransmitter molecules rapidly diffuse across the synaptic cleft and bind to specific receptor proteins located on the postsynaptic membrane.

Binding causes ligand-gated ion channels (chemically controlled) to open. The resulting flow of ions (like \(Na^+\) or \(\text{Cl}^-\)) changes the postsynaptic membrane potential.

• If the postsynaptic potential becomes more positive (e.g., \(Na^+\) influx), it is an excitatory synapse, making the postsynaptic neuron more likely to fire an action potential.
• If the postsynaptic potential becomes more negative (e.g., \(\text{Cl}^-\) influx), it is an inhibitory synapse, making the postsynaptic neuron less likely to fire.

Step 5: Removal of Neurotransmitter

The signal must be brief and precise. To stop the effect, neurotransmitters are rapidly removed from the synaptic cleft by one of two ways:

• Enzymatic breakdown: Specific enzymes (like acetylcholinesterase, which breaks down acetylcholine) rapidly deactivate the neurotransmitter.
• Reuptake: The presynaptic neuron takes the neurotransmitter back up to be recycled and repackaged into vesicles.

Common Mistakes to Avoid

Do not confuse the electrical signal within the neuron (action potential) with the chemical signal between neurons (neurotransmitters). They are two distinct, sequential steps in neural communication.

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Summary of Neural Signalling

This complex process is the foundation for all nervous system interactions, allowing us to respond quickly to stimuli and coordinate complex movements.

Key Takeaways for Interaction and Interdependance

• Resting Potential: Maintained by the \(Na^+/\text{K}^+\) pump (3 \(Na^+\) out, 2 \(\text{K}^+\) in), creating a negative internal charge (~-70 mV).
• Action Potential: An all-or-nothing electrical event involving rapid depolarization (\(Na^+\) in) followed by repolarization (\(\text{K}^+\) out).
• Speed: Myelination allows for rapid saltatory conduction, ensuring critical speed for body coordination.
• Synapse: The action potential triggers \(\text{Ca}^{2+}\) influx, which causes the release of chemical neurotransmitters into the cleft to influence the next cell.