Problem 16 - Campbell Biology Concepts & Connections

Question

Drugs are often used to relax muscles during surgery. Which of the following chemicals do you think would make a better muscle relaxant, and why? Chemical A: Blocks acetylcholine receptors on muscle cells. Chemical B: Floods the cytoplasm of muscle cells with calcium ions

Step-by-Step Solution

Answer

Chemical A is better because it blocks acetylcholine receptors and prevents muscle contraction.

Step 1: Understand the Role of Acetylcholine

Acetylcholine is a neurotransmitter that binds to receptors on muscle cells, causing them to contract. Blocking these receptors would prevent the muscle from receiving signals to contract.

Step 2: Analyze the Effect of Blocking Acetylcholine Receptors

If acetylcholine receptors are blocked by Chemical A, the muscle cells will not receive the signal to contract, leading to muscle relaxation.

Step 3: Understand the Role of Calcium Ions in Muscle Contraction

Calcium ions are critical for muscle contraction. They allow actin and myosin fibers within muscle cells to interact, causing contraction. Flooding muscle cells with calcium ions would enhance contraction.

Step 4: Analyze the Effect of Increasing Calcium Ions

Chemical B, by increasing calcium ion concentrations in muscle cells, would increase muscle contractions rather than relax the muscles.

Step 5: Compare the Effects

Chemical A causes muscle relaxation by blocking acetylcholine receptors and preventing contraction signals. Chemical B causes muscle contraction by increasing calcium levels.

Step 6: Conclusion

Therefore, Chemical A, which blocks acetylcholine receptors, would make a better muscle relaxant compared to Chemical B, which would enhance muscle contraction.

Key Concepts

Acetylcholine Receptors

To understand how muscle relaxants work, we need to look at acetylcholine receptors.
Acetylcholine is a neurotransmitter, a chemical that nerve cells release to send signals.
It binds to receptors on muscle cells known as acetylcholine receptors.
When acetylcholine attaches to these receptors, it triggers a series of events that lead to muscle contraction. If a chemical was to block these acetylcholine receptors, it would prevent the acetylcholine from binding.
This interruption stops the muscle cells from receiving signals to contract.

• Hence, the muscle remains relaxed.
• Chemical A, as mentioned in the exercise, blocks these receptors.
• This makes Chemical A an effective muscle relaxant.

Calcium Ions

Calcium ions play a crucial role in muscle contraction.
Inside muscle cells, there are tiny structures that store calcium ions.
When acetylcholine binds to its receptor, it causes these structures to release calcium ions. The release of calcium is vital because it directly leads to muscle contraction. When these ions are released into the muscle cell, they interact with proteins to cause the muscle fibers to shorten, resulting in a contraction.
If chemical B were to flood the muscle cell's cytoplasm with more calcium ions:

• The muscle would contract even more efficiently and powerfully.
• Rather than relaxing the muscle, Chemical B would do the opposite and cause it to contract further.
• This makes Chemical B unsuitable as a muscle relaxant.

Muscle Contraction

Muscle contraction is a complex process that involves multiple steps.
First, a nerve impulse reaches the muscle cell, causing the release of acetylcholine.
Acetylcholine binds to its receptors on the muscle cell membrane, which then triggers the release of calcium ions from storage areas within the cell.
These calcium ions interact with muscle proteins, actin and myosin, to enable contraction.

• During this interaction, the muscle fibers slide past one another, shortening the muscle.
• For muscles to relax, the signal for contraction must stop, and calcium ions must be pumped back into their storage areas within the cell.

Therefore, blocking acetylcholine receptors (as with Chemical A) stops this entire chain reaction, resulting in a relaxed muscle.
Flooding cells with calcium ions (as with Chemical B) would keep the contraction ongoing, which is why it's not suitable for muscle relaxation.