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Interaction and interdependence - Neural signalling

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. They consist of a cell body (soma), dendrites that receive signals, and an axon that carries impulses away from the cell body.

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The resting membrane potential is maintained at approximately −70 mV-70\text{ mV}. This is achieved by the sodium-potassium pump which actively transports 3 Na+3\text{ Na}^{+} ions out of the neuron and 2 K+2\text{ K}^{+} ions into the neuron, creating an electrochemical gradient.

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An action potential is a rapid change in membrane potential. It begins with depolarization, where voltage-gated Na+\text{Na}^{+} channels open and Na+\text{Na}^{+} flows into the cell until the potential reaches approximately +30 mV+30\text{ mV}.

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Repolarization follows depolarization as voltage-gated K+\text{K}^{+} channels open and K+\text{K}^{+} ions flow out of the neuron, returning the membrane potential to a negative value.

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Hyperpolarization occurs when too many K+\text{K}^{+} ions exit, causing the potential to drop below −70 mV-70\text{ mV} before the resting potential is restored by the sodium-potassium pump.

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Saltatory conduction occurs in myelinated axons, where the nerve impulse 'jumps' between the Nodes of Ranvier. This significantly increases the speed of impulse transmission compared to unmyelinated fibers.

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Synaptic transmission involves the release of neurotransmitters. When an action potential reaches the presynaptic knob, voltage-gated Ca2+\text{Ca}^{2+} channels open, causing vesicles to release neurotransmitters into the synaptic cleft via exocytosis.

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

📐Formulae

Vresting≈−70 mVV_{\text{resting}} \approx -70\text{ mV}

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

Ratio of Na+:K+=3 out:2 in\text{Ratio of Na}^{+}:\text{K}^{+} = 3\text{ out} : 2\text{ in}

Vpeak action potential≈+30 to +40 mVV_{\text{peak action potential}} \approx +30\text{ to } +40\text{ mV}

💡Examples

Problem 1:

Calculate the net change in intracellular positive charge if the sodium-potassium pump undergoes 150150 cycles.

Solution:

150×(3 Na+ out)=450 positive charges out150×(2 K+ in)=300 positive charges inNet change=300−450=−150\begin{array}{r} 150 \times (3\text{ Na}^{+} \text{ out}) = 450\text{ positive charges out} \\ 150 \times (2\text{ K}^{+} \text{ in}) = 300\text{ positive charges in} \\ \hline \text{Net change} = 300 - 450 = -150 \end{array}

Explanation:

Each cycle of the sodium-potassium pump results in a net loss of one positive charge from the inside of the cell (3−2=13 - 2 = 1). Therefore, 150150 cycles result in a net loss of 150150 positive charges from the cytoplasm.

Problem 2:

Explain why a stimulus that changes the membrane potential from −70 mV-70\text{ mV} to −60 mV-60\text{ mV} does not result in an action potential.

Solution:

The threshold potential is typically −55 mV-55\text{ mV}. Since −60 mV<−55 mV-60\text{ mV} < -55\text{ mV}, the depolarization is insufficient.

Explanation:

Neural signaling follows the 'all-or-nothing' principle. If the stimulus does not reach the threshold potential of approximately −55 mV-55\text{ mV}, voltage-gated Na+\text{Na}^{+} channels will not open in sufficient numbers to trigger a full action potential.