Neuron Structure and Resting Potential

Neurons are specialised cells that transmit electrical impulses. They have three main parts: the cell body (soma), dendrites (receive signals), and an axon (transmits signals). At rest, a neuron maintains a resting membrane potential of approximately −70 mV.

Neuron Structure

Dendrites

Receive signals from other neurons

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Cell Body

Contains nucleus; integrates signals

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Axon

Carries impulse away from cell body

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Axon Terminals

Release neurotransmitters at synapse

Resting potential (−70 mV): Maintained by the Na⁺/K⁺ ATPase pump, which actively transports 3 Na⁺ out and 2 K⁺ in per cycle. This creates a negative charge inside the neuron relative to outside.

The Action Potential

An action potential is a rapid reversal of the membrane potential that propagates along the axon. It follows an all-or-nothing principle -- if the threshold is reached (−55 mV), a full action potential fires; below threshold, nothing happens.

Stages of an Action Potential

1. Resting State

−70 mV. Na⁺ channels closed. K⁺ leak channels open.

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2. Depolarisation

Stimulus reaches threshold (−55 mV). Voltage-gated Na⁺ channels open. Na⁺ rushes in. Membrane reaches +30 mV.

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3. Repolarisation

Na⁺ channels close. Voltage-gated K⁺ channels open. K⁺ flows out. Membrane returns toward −70 mV.

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4. Hyperpolarisation

K⁺ channels slow to close. Membrane briefly overshoots to about −80 mV (refractory period).

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5. Return to Resting

Na⁺/K⁺ pump restores resting potential (−70 mV).

Saltatory conduction: In myelinated neurons, the action potential "jumps" between gaps in the myelin sheath called nodes of Ranvier. This dramatically increases transmission speed (up to 120 m/s compared to 1 m/s in unmyelinated fibres).

Synaptic Transmission and Neurotransmitters

A synapse is the junction between two neurons. Signals cross the synaptic cleft via chemical messengers called neurotransmitters. This process converts the electrical signal to a chemical signal and back to electrical.

Steps of Synaptic Transmission

1

Action potential arrives at the axon terminal (presynaptic neuron).

2

Voltage-gated Ca²⁺ channels open, and calcium ions flow into the terminal.

3

Ca²⁺ causes synaptic vesicles to fuse with the membrane and release neurotransmitters into the synaptic cleft (exocytosis).

4

Neurotransmitters bind to receptors on the postsynaptic membrane, opening ion channels.

5

This generates an excitatory (EPSP) or inhibitory (IPSP) postsynaptic potential. The neurotransmitter is then removed by enzymatic breakdown, reuptake, or diffusion.

Excitatory Neurotransmitters

Acetylcholine -- neuromuscular junctions, memory

Glutamate -- main excitatory NT in the brain

Noradrenaline -- alertness, fight-or-flight

Inhibitory Neurotransmitters

GABA -- main inhibitory NT in the brain

Serotonin -- mood regulation, sleep

Dopamine -- reward, motivation (can be both)

Key Vocabulary

Action Potential

A rapid, transient reversal of the membrane potential that propagates along the axon as an electrical impulse.

Synapse

The junction between two neurons where chemical signals (neurotransmitters) are used to transmit information across the synaptic cleft.

Neurotransmitter

A chemical messenger released from the presynaptic neuron that binds to receptors on the postsynaptic neuron to transmit a signal.

Myelin Sheath

An insulating fatty layer around the axon (produced by Schwann cells in PNS or oligodendrocytes in CNS) that speeds up impulse transmission via saltatory conduction.

Worked Examples

1

Explain why the action potential is described as "all-or-nothing."

Step 1: A stimulus must depolarise the membrane to the threshold level (−55 mV) for an action potential to fire.

Step 2: If the threshold is reached, a full action potential is generated -- it always reaches the same peak voltage (+30 mV) regardless of stimulus strength.

Step 3: If the stimulus is below threshold, no action potential occurs. There is no partial or graded action potential. Stronger stimuli are encoded by higher frequency of action potentials, not greater amplitude.

2

Describe the role of calcium ions in synaptic transmission.

Step 1: When an action potential arrives at the axon terminal, voltage-gated Ca²⁺ channels open.

Step 2: Ca²⁺ ions flow into the presynaptic terminal down their concentration gradient.

Step 3: The influx of Ca²⁺ triggers synaptic vesicles to move to and fuse with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft by exocytosis.

3

Explain how myelination increases the speed of nerve impulse transmission.

Step 1: The myelin sheath acts as an electrical insulator, preventing ion leakage across the membrane in myelinated regions.

Step 2: Ion exchange can only occur at the nodes of Ranvier -- gaps between the myelin segments.

Step 3: The action potential therefore "jumps" from node to node (saltatory conduction), which is much faster than continuous conduction along an unmyelinated axon.

Knowledge Check

Select the correct answer for each question. Click "Check Answer" to see if you are right.

Question 1

The resting membrane potential of a typical neuron is approximately:

+30 mV −55 mV −70 mV 0 mV

Question 2

During depolarisation, which ion rushes into the neuron?

K⁺ (potassium) Cl⁻ (chloride) Ca²⁺ (calcium) Na⁺ (sodium)

Question 3

Saltatory conduction occurs because:

Action potentials jump between nodes of Ranvier in myelinated axons The axon has no myelin sheath Neurotransmitters travel faster in thicker axons The Na⁺/K⁺ pump works faster in myelinated neurons

Question 4

What triggers the release of neurotransmitters from synaptic vesicles?

Influx of Na⁺ ions into the terminal Influx of Ca²⁺ ions into the terminal Efflux of K⁺ ions from the terminal Binding of GABA to postsynaptic receptors

Question 5

Which neurotransmitter is the main inhibitory neurotransmitter in the brain?

Glutamate Acetylcholine GABA Noradrenaline

Neural Communication