5. The diagram shows the structure of a motor neuron.
[VISUALIZATION: Diagram of a typical vertebrate motor neuron. Label the following parts: Dendrites, Cell body (soma) with nucleus, Axon hillock, Axon, Myelin sheath, Nodes of Ranvier, Axon terminal (synaptic terminal).]
(a) Identify the structure labelled A, which insulates the axon. [1]
(b) Outline how the resting potential is established and maintained in a neuron. [3]
(c) Describe the process of synaptic transmission at a cholinergic synapse. [5]
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Identify the structure labelled A, which insulates the axon. [1]
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Look for the fatty layer that wraps around the axon in segments.
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Myelin sheath
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Outline how the resting potential is established and maintained in a neuron. [3]
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Think about the balance of ions inside and outside the neuron and the proteins responsible for moving them across the membrane.
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Resting potential is established and maintained primarily by the sodium-potassium pump, which actively transports 3 Na+ ions out for every 2 K+ ions in, using ATP. Additionally, the membrane is significantly more permeable to K+ ions than Na+ ions at rest due to the presence of K+ leak channels, allowing K+ to diffuse out down its concentration gradient. This net outward movement of positive charge makes the inside of the neuron negative relative to the outside (typically around -70mV).
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Describe the process of synaptic transmission at a cholinergic synapse. [5]
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Trace the signal: Action potential arrival -> Calcium influx -> Neurotransmitter release -> Receptor binding -> Postsynaptic effect -> Neurotransmitter removal.
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1. An action potential propagates down the axon and arrives at the presynaptic terminal. 2. Depolarization of the terminal membrane opens voltage-gated calcium (Ca2+) channels. 3. Ca2+ ions flow into the presynaptic terminal down their electrochemical gradient. 4. The influx of Ca2+ triggers the fusion of synaptic vesicles (containing the neurotransmitter acetylcholine, ACh) with the presynaptic membrane. 5. ACh is released into the synaptic cleft by exocytosis. 6. ACh molecules diffuse across the synaptic cleft and bind to specific cholinergic receptors (ligand-gated ion channels) on the postsynaptic membrane. 7. Binding of ACh opens these channels, allowing sodium (Na+) ions to flow into the postsynaptic neuron. 8. This influx of positive charge causes depolarization of the postsynaptic membrane, known as an excitatory postsynaptic potential (EPSP). 9. To terminate the signal, ACh is rapidly broken down in the synaptic cleft by the enzyme acetylcholinesterase (AChE) into acetate and choline. Choline may be reabsorbed by the presynaptic terminal.
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