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Human Physiology - Neurons and synapses

Grade 12IBBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Neurons are specialized cells that transmit electrical impulses. The basic structure includes dendrites, the cell body (soma), and an axon ending in synaptic terminals.

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The resting membrane potential is typically around −70 mV-70 \text{ mV}. This is maintained by the Na+/K+Na^{+}/K^{+}-ATPase pump, which actively transports 3Na+3 Na^{+} ions out of the cell for every 2K+2 K^{+} ions pumped in, creating an electrochemical gradient.

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An action potential is an 'all-or-nothing' event. It is triggered only if the graded potential reaches the threshold potential of approximately −55 mV-55 \text{ mV}.

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Depolarization occurs when voltage-gated Na+Na^{+} channels open, allowing Na+Na^{+} to rush into the neuron, changing the membrane potential from negative to positive (reaching about +30 mV+30 \text{ mV}).

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Repolarization occurs as voltage-gated Na+Na^{+} channels close and voltage-gated K+K^{+} channels open, allowing K+K^{+} to exit the cell, restoring the negative internal charge.

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Hyperpolarization (the refractory period) occurs when the membrane potential briefly becomes more negative than −70 mV-70 \text{ mV} because K+K^{+} channels are slow to close. This prevents the impulse from traveling backwards.

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Myelination, provided by Schwann cells, allows for saltatory conduction. The action potential 'jumps' between the Nodes of Ranvier, significantly increasing the speed of impulse transmission.

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Synaptic transmission involves the arrival of an action potential at the presynaptic terminal, triggering the opening of voltage-gated Ca2+Ca^{2+} channels. The influx of Ca2+Ca^{2+} causes synaptic vesicles to fuse with the membrane and release neurotransmitters via exocytosis.

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Acetylcholine is a common neurotransmitter. It is broken down in the synaptic cleft by the enzyme acetylcholinesterase to prevent continuous stimulation of the postsynaptic neuron.

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Neonicotinoids are synthetic compounds that bind to acetylcholine receptors in insect central nervous systems. They cannot be broken down by acetylcholinesterase, leading to overstimulation, paralysis, and death of the insect.

📐Formulae

Vrest≈−70 mVV_{rest} \approx -70 \text{ mV}

Vthreshold≈−55 mVV_{threshold} \approx -55 \text{ mV}

3Naout+:2Kin+3 Na^{+}_{out} : 2 K^{+}_{in}

Vpeak≈+30 mVV_{peak} \approx +30 \text{ mV}

💡Examples

Problem 1:

Explain why a resting neuron has a negative internal charge of approximately −70 mV-70 \text{ mV} compared to the outside.

Solution:

The Na+/K+Na^{+}/K^{+} pump moves 3Na+3 Na^{+} out for every 2K+2 K^{+} in. Additionally, the membrane is more permeable to K+K^{+} than Na+Na^{+} due to 'leak' channels.

Explanation:

Because 33 positive ions leave for every 22 that enter, a net positive charge accumulates outside. Furthermore, the leakage of K+K^{+} out of the cell along its concentration gradient leaves behind large, negatively charged proteins and organic anions inside the cytoplasm.

Problem 2:

Calculate the net change in ion concentration ratio for 33 cycles of the Na+/K+Na^{+}/K^{+}-ATPase pump.

Solution:

9Naout+:6Kin+9 Na^{+}_{out} : 6 K^{+}_{in}

Explanation:

Each cycle of the pump consumes 11 molecule of ATPATP to export 3Na+3 Na^{+} and import 2K+2 K^{+}. Therefore, 33 cycles result in 3×3=9Na+3 \times 3 = 9 Na^{+} ions exiting and 3×2=6K+3 \times 2 = 6 K^{+} ions entering.

Problem 3:

A toxin blocks voltage-gated K+K^{+} channels. Predict the effect on the action potential graph.

Solution:

The membrane would fail to undergo rapid repolarization.

Explanation:

Depolarization would occur normally as Na+Na^{+} enters, but because K+K^{+} cannot exit through the blocked voltage-gated channels, the cell remains at a positive potential (+30 mV+30 \text{ mV}) for an extended period, preventing the neuron from resetting to its resting state.

Neurons and synapses - Revision Notes & Key Diagrams | IB Grade 12 Biology