Which of the following statements about a chemical synapse is true?

A. Calcium ions relay the signal generated by an action potential in one neuron across the gap to a second neuron.

B. It allows signal transmission from one neuron to another through a wave of electrochemical change.

C. Signal transmission occurs by diffusion of a compound called a neurotransmitter through the intercellular space between neurons.

D. Its presynaptic component undergoes chemical breakdown in response to a signal, and the breakdown product carries the signal to the postsynaptic component.

Diagram of a chemical synapse
Diagram of a chemical synapse

The correct answer is C. Signal transmission occurs by diffusion of a compound called a neurotransmitter through the intercellular space between neurons.

The nerve impulse has to be transmitted from one nerve cell to another in order for animals to stay alive and for all the organs of the body to function correctly. A nerve action potential (impulse) is transmitted from the dendrites through the cell body and along the axon of the nerve cell.

The nerve impulse of a cell reaches the ends of the neuron at the presynaptic terminals of the axon. The synapse is where the axon of one nerve meets the dendrites of another nerve cell.

There is a space known as the synaptic cleft (space) present at the synapse. Neurotransmitter chemicals are packed into vesicles at the presynaptic terminals of the nerve cell axon.

The vesicles fuse with the plasma membrane and release the neurotransmitters into this cleft, where they diffuse across to the postsynaptic terminal of the dendrite of the next nerve cell.

The chemicals bind to receptors on the postsynaptic terminal which then triggers a new impulse in this new neuron.

Enzymes act to break down any extra neurotransmitters that are present in the synaptic cleft. The levels of different neurotransmitters are very important, and diseases can result if too many or too few of these substances are produced or remain in the synaptic cleft.

The neuron

In order to understand what happens at a chemical synapse, one has to know the structure and function of a typical nerve cell or neuron.

Nerve impulses are also known as action potentials and involve a movement of ions into and out of the nerve cell. A typical neuron consists of a series of extensions known as dendrites, which then connect to the part known as the cell body.

From the cell body there extends another elongated section known as the axon. In many neurons, this axon is also surrounded by a glycolipid layer of fat known as myelin.

The cell body is the section of the nerve cell which contains organelles that keep the cell alive and functioning. These organelles include the mitochondria, endoplasmic reticulum, nucleus, and Golgi apparatus.

The nerve impulse

The nerve impulse travels from the dendrites, through the cell body, and along the axon. The end of the axon of one nerve cell meets up with the dendrites of the next neuron at the region known as a synapse.

The nerve impulse is generated by the movement of specific charged atoms (ions) across the neuron plasma membrane. The transport of these ions results in changes in the charge from inside compared with outside of the nerve cell.

This impulse is a change in voltage that occurs across the plasma membrane of the cell. When a neuron is stimulated it causes many sodium ions to move into the neuron through gated channels in the membrane.

This causes a change in the voltage or charge of the membrane since when at rest there are more sodium ions outside than inside. There are also potassium ions that move across the membrane and impact the voltage.

Once enough sodium has moved into the cell the impulse is triggered. After a while, the cell returns to a resting state as more potassium ions move into the cell through potassium gated channels.

Eventually, the cell returns to rest with more sodium outside and potassium inside. This process is repeated along the neuron and is how the impulse is propagated.

The chemical synapse

The synapse is the region where two nerve cells meet. The nerve impulse of one nerve cell has to be transmitted to the next nerve cell across the synaptic cleft between the two cells.

The nerve cell of the cell carrying the impulse to the cleft has terminals at the end of the cell which at the cleft are known as presynaptic terminals. The neuron that is to receive the signal on the other side of the cleft has postsynaptic terminals present.

The presynaptic terminals are on the axon of the neuron while the postsynaptic terminals are on the dendrites of a nerve cell.

The process

When a nerve action potential arrives at the presynaptic terminals on the axon of the nerve cell, vesicles form which then fuse at the membrane. These vesicles contain chemicals known as neurotransmitters.

The vesicles fuse with the plasma membrane on the presynaptic terminal and release the neurotransmitters into the synaptic cleft.

The trigger for this process of vesicle movement and chemical release into the cleft is the increased concentration of calcium ions.

The neurotransmitter in the synaptic cleft then binds to receptors present on the postsynaptic terminal on the opposite nerve cell.

This binding activity then stimulates an action potential in this new nerve cell. There are enzymes that degrade any extra neurotransmitters that may be present in the cleft.

It is important to realize that there is also a synapse where a nerve cell meets a muscle or other effector organ. In the case of a muscle the synapse is known as a neuromuscular junction, and here too a neurotransmitter, namely, acetylcholine, traverse the synaptic cleft.

Problems involving neurotransmitters

It is very important that the amount of neurotransmitters present is carefully controlled since too much or too little can cause human disorders such as depression and Parkinson’s disease, to name two examples.

Low levels of the neurotransmitter dopamine, has been linked to the development of Parkinson’s disease in people. Researchers also believe that problems with the neurotransmitter serotonin may be related to the development of clinical depression.

References

  1. Editors of Encyclopedia Britannica (2019). Synapse. Retrieved from Encyclopedia Britannica.
  2. Editors of Encyclopedia Britannica (2019). Action potential. Retrieved from Britannica.com.
  3. RL Dorit, WF Walker, RD Barnes (1991).  Zoology. Philadelphia: USA, Saunders College Publishing.
  4. Editors of Encyclopedia Britannica (2019). Neurotransmitter. Retrieved from Britannica.com.
  5. Editors of Encyclopedia Britannica (2019). Neuron. Retrieved from Britannica.com.

LEAVE A REPLY

Please enter your comment!
Please enter your name here