Lesson 7 — Synaptic and Neuromuscular Transmission


Reabsorption of Neurotransmitters

Read pages 380 - 381

Why don't neurotransmitters continue to start new nerve impulses over and over again? 

To prevent repeated stimulation of dendrites, the neurotransmitter must be broken down by enzymes or transported back into the presynaptic neuron.

For example, if the neurotransmitter is acetylcholine, then the enzyme cholinesterase breaks it down and the fragments are reabsorbed into the synaptic knob. If the neurotransmitter is dopamine or norepinephrine , then it is reabsorbed directly into the presynaptic neuron.


© Getty Images

Neurotoxins – Medical Uses

Deadly neurotoxins produced by some organisms such as the rattlesnake and black widow spider could have medical implications. One neurotoxin found in rattlesnake venom blocks receptors on postsynaptic neurons. If a neurotoxin blocks receptors on postsynaptic neurons (as rattlesnake venom is able to do), it could prevent the binding of the neurotransmitter acetylcholine, thereby blocking the transmission of a nerve impulse to the muscle and causing muscle paralysis. If we can mimic the activity of the rattlesnake venom to block painful stimulus, it could be effective in medical treatment.

Scientists have developed insecticides that inactivate cholinesterase, causing acetylcholine to stay in the synapse longer and continually stimulate the muscles until they become fatigued and no longer contract. The insect dies because it can no longer contract diaphragm muscles to get oxygen into its body.

The venom from a female black widow spider stimulates release of synaptic vesicles into the synapse. This increases the number of neurotransmitters in the synapse, resulting in nerve and muscle fatigue. This knowledge can be useful in preventing transmission of pain impulses. 


© Getty Images

© Getty Images
Did You Know?
Reserpine was isolated from Indian snakeroot plant. It prevents catecholamine neurotransmitters from being released into the synaptic cleft. Historically, it has been used to treat insanity, fever, and snakebites.

Recently, reserpine has been developed into a drug to control involuntary movement in patients suffering from Huntington’s disease.


Self-Check

  1. Summarize the events involved in impulse transmission starting at the presynaptic neuron and ending at postsynaptic neuron.

  2. Identify the function of neurotransmitters in the nervous system.

  3. Place the following events in the correct sequence:
    1. An action potential is initiated in the dendrite.
    2. Neurotransmitters are released into the synaptic cleft.
    3. Sodium gates are opened, and sodium ions rush into the dendrite.
    4. A wave of depolarization arrives at the axon terminal.

  4. Explain what happens to the neurotransmitters after an action potential has been started in the postsynaptic neuron.

  5. To review the anatomy of the synaptic gap and the functions of the structures, complete the following drag-and-drop activity involving structures and functions .

    structures and functions drag-and-drop activity

    Match the term with the number on the diagram, and match the function with the structure.
    (Click the "Home icon" in the lower right-hand corner to toggle between the two activities.)

Self-Check Answers

  1. The following events summarize, in correct sequence, the events involved in impulse transmission from the presynaptic neuron to the postsynaptic neuron. 

    • A wave of depolarization arrives at the axon terminal.

    • Synaptic vesicles fuse with the presynaptic membrane. 

    • Neurotransmitters are released from the vesicles into the synaptic cleft.

    • Neurotransmitters diffuse to the postsynaptic membranes and lock into complementary receptors.

    • Sodium gates are opened and sodium ions rush into the postsynaptic neuron or dendrite of the second neuron.

    • An action potential is initiated in the postsynaptic membrane of the dendrite.

    • Neurotransmitters are decomposed by enzymes and reabsorbed by the presynaptic membrane.

  2. The function of neurotransmitters in synaptic transmission is to carry the neural signal from one neuron to another. Neurotransmitters can carry the neural signal from a neuron to an effector such as a muscle cell.  Because the neurotransmitters can be formed only in the synaptic knob of the axon terminals, one-way impulse transmission is assured. 

  3. The correct sequence of events is d, b, c, and a.

  4. If the neurotransmitter is acetylcholine, the enzyme cholinesterase (found in the synaptic cleft) decomposes the acetylcholine and the smaller fragments are reabsorbed by the presynaptic membrane. If the neurotransmitter is norepinephrine, it is reabsorbed directly.


    1. axon: a wave of depolarization arrives from this structure 
    2. mitochondria: these structures supply ATP for synthesis of neurotransmitters and for active transport 
    3. presynaptic vesicles: the structure that stores the neurotransmitter 
    4. presynaptic membrane: the synaptic vesicles fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft. 
    5. neurotransmitter: a chemical stored in the synaptic vesicle and released from the synaptic vesicle (acetylcholine)
    6. postsynaptic membrane: sodium ions move through gates in this structure 
    7. receptor molecules: structures that have complementary shapes to the transmitter substance and, when combined with the transmitter, function to open sodium gates 
    8. synaptic knob: sodium rich extracellular fluid is found here 
    9. synaptic cleft: the gap between the presynaptic membrane and the postsynaptic membrane. 
    10. postsynaptic neuron: a new action potential is generated in this structure 
    11. presynaptic neuron: this structure contributes to the release of neurotransmitter from the synaptic vesicles

    1. axon
    2. mitochondria
    3. presynaptic vesicles
    4. presynaptic membrane
    5. neurotransmitter
    6. postsynaptic membrane
    7. receptor molecules
    8. synaptic knob
    9. synaptic cleft
    10. postsynaptic neuron
    11. presynaptic neuron

Biology 30 © 2008  Alberta Education & its Collaborative Partners ~ Updated by ADLC 2019