Throughout the nervous system, the predominant mode of neuron to neuron and neur

Question

Throughout the nervous system, the predominant mode of neuron to neuron and neuron to effector (muscle or gland) transmission involves the conversion of an electrical event in the presynaptic neuron (A.P.) to a chemical event (secretion of neurotransmitter) and then back to an electrical event (A.P.) in the postsynaptic cell. Interference with any step in the sequence has significant effects on nerve and/or effector function.

Botulinum toxin causes paralysis. It binds to presynaptic sites at the motor end plate and prevents the release of neurotransmitter from vesicles. Predict the consequences (closer to threshold, further from threshold, no change) of blocking the release of Ach in the neuromuscular junction on the following.

A. Presynaptic resting potential: ?

B. Presynaptic action potential amplitude: ?

C. Presynaptic Ca++ influx: ?

D. Presynaptic transmitter release: ?

E. Postsynaptic resting potential: ?

F. Postsynaptic action potential frequency: ?

G. Muscle contractile strength: ?

Explanation / Answer

Botulin toxin prevents the release of the neurotransmitter from the presynaptic neuron.
A. Presynaptic resting potential: No change. Botulin toxin does not affect the resting potential of the presynaptic neuron. -------------------------- B. Presynaptic action potential amplitude: No change. Botulin toxin does not affect the amplitude of the action potential of the presynaptic neuron. ------------------------- C. Presynaptic Ca++ influx: No change. Botulin toxin does not affect the presynaptic Ca2+ influx. ----------------------------- D. Presynaptic transmitter release: Blocked. Botulin toxin specifically blocks the release of the neurotransmitter from the presynaptic neuron. The action potential in the presynaptic neuron occurs when the potential is close to the threshold. But the release of neurotransmitter based on this action potential is blocked. -------------------------- E. Postsynaptic resting potential: No change. Botulin toxin does not affect the resting potential of the postsynaptic neuron. -------------------------- F. Postsynaptic action potential frequency: Blocked. As the signal from the presynaptic neuron is blocked, the action potential of the postsynaptic neuron also is blocked. As the neuron is at the resting potential, it is further from the threshold. ----------------------- G. Muscle contractile strength:  No change. When action potentials of the postsynaptic neuron are blocked, the signal does not reach the targeted muscle. Hence the muscle does not contract. But the strength of the muscle remains the same. -------------------------- B. Presynaptic action potential amplitude: No change. Botulin toxin does not affect the amplitude of the action potential of the presynaptic neuron. ------------------------- C. Presynaptic Ca++ influx: No change. Botulin toxin does not affect the presynaptic Ca2+ influx. ----------------------------- D. Presynaptic transmitter release: Blocked. Botulin toxin specifically blocks the release of the neurotransmitter from the presynaptic neuron. The action potential in the presynaptic neuron occurs when the potential is close to the threshold. But the release of neurotransmitter based on this action potential is blocked. -------------------------- E. Postsynaptic resting potential: No change. Botulin toxin does not affect the resting potential of the postsynaptic neuron. -------------------------- F. Postsynaptic action potential frequency: Blocked. As the signal from the presynaptic neuron is blocked, the action potential of the postsynaptic neuron also is blocked. As the neuron is at the resting potential, it is further from the threshold. ----------------------- G. Muscle contractile strength:  No change. When action potentials of the postsynaptic neuron are blocked, the signal does not reach the targeted muscle. Hence the muscle does not contract. But the strength of the muscle remains the same. -------------------------- F. Postsynaptic action potential frequency: Blocked. As the signal from the presynaptic neuron is blocked, the action potential of the postsynaptic neuron also is blocked. As the neuron is at the resting potential, it is further from the threshold. ----------------------- G. Muscle contractile strength:  No change. When action potentials of the postsynaptic neuron are blocked, the signal does not reach the targeted muscle. Hence the muscle does not contract. But the strength of the muscle remains the same.

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