Name ______
Muscle Contraction & Rigor Mortis – 50 Informal Points
Introduction
Think back to the last time you watched a crime scene investigation show on TV or a thriller at the movies. The coroner or medical examiner comes to examine the scene and states that a dead body is in “full rigor” or that “rigor mortis” has set in. You probably figured out that this means that the body is now stiff and the limbs are hard to move, but have you figured out what causes this change? The answer, of course, lies in the science. This stiffening is directly related to the chemical state of your muscles upon death.
ATP is required for successful muscle contraction. You will zoom in on the sarcomere, the smallest unit of a muscle, and see exactly how ATP is used to make a muscle contract and relax. The sarcomere is made up of two important proteins, actin and myosin. You will research the movement of these two fibers in contraction as well as the role other ions, electric stimuli and body systems play in this process. You will study the sequence of events that occur during normal shortening of the sarcomere as well as learn why the muscles can lock up after death.
Procedure
1. Watch the AP crash course at https://www.youtube.com/watch?v=Ktv-CaOt6UQ and a Sliding Filament video found at https://www.youtube.com/watch?v=0kFmbrRJq4w . You may need to watch these a few times to complete the following activities.
2. What is a sarcomere? What is a z-line? Draw and label a diagram below.
A sarcomere is the smallest unit of a muscle that contracts. Z-lines are zig-zagged lines that for the borders of sarcomeres and separate sarcomeres.
3. Describe in words (bullet pointed list or paragraph) as well as with a simple diagram the steps to muscle contraction. Be sure to include sarcomere, ATP, ADP, actin, myosin, Ca+2, phosphate, troponin.
Step 1: Electrical signal arrives at the cell (via a nerve; links nervous system to muscular system), causing the release of Ca+2 from the sarcoplasmic reticulum (which is like an endoplasmic reticulum (ER) in the muscle cell.)
Step 2: Ca+2 binds to troponin so that troponin will change shape and unbind, or let go, of actin
Step 3: A phosphate attached to the myosin heads is released as the myosin head binds to the actin forming a cross bridge
Step 4: The remaining ADP molecule on the myosin head is released in a power stroke when the myosin pulls the actin
Step 5: The ADP molecule combines with the phosphate to create ATP, which triggers the release of myosin from actin; the ATP is then broken down into ADP and a phosphate again
4. Muscle contraction is often called the “sliding filament theory.” Based on what you learned in the last question, why is this a fitting name?
Actin and myosin are both protein filaments and they slide past one another during a muscle contraction.
5. Research the physiology behind the stiffening of muscles at death, known as rigor mortis. Write a paragraph (include a diagram if you wish) explaining what is happening at a biological level.
Immediately after death, the muscles of the body contract in the same manner as they do when the person is alive. Muscle is formed of bundles of long and narrow cells that can span the entire muscle’s length.
Normally, in the resting state, these cells build up the electric potential across their membrane by actively pumping out calcium ions. Upon receiving a signal from a neuron, the muscle cells open the calcium channels in their cell membrane, and the calcium ions rush in due to the voltage difference between the inside and outside of the cell. These ions then interact with actin and myosin filaments to cause muscle contraction. The muscles remain in the contracted state until adenosine triphosphate (ATP) binds to myosin, releasing the myosin and actin filaments from one another. Additionally, muscle cell membrane proteins use ATP to actively pump the calcium ions back out of the cell, restoring the membrane potential and preventing the calcium ions from re-stimulating contraction.
However, when a person dies and breathing and circulation stop, muscle cells lack oxygen and therefore cannot use aerobic respiration to efficiently produce ATP. Respiration continues anaerobically at first, but the muscle cells eventually become so short on ATP that the myosin and actin filaments cannot release from the contracted state and the calcium ions cannot be pumped back out of the muscle cell. Unable to release contraction, all the muscles of the body remain tense, causing rigor mortis.
Overall, the actin and myosin become stuck together, so the muscle cannot relax.
Conclusion Questions
1. How do multiple human body systems work together to cause muscle contraction and movement of the body?
Nervous system sends the impulse from the brain to the skeletal muscle to initiate movement.
Circulatory system delivers oxygen to the muscle cells for aerobic respiration.
Skeletal system is moved as muscles contract.
2. Your friend tried to convince you that the only reason to drink milk and to make sure you get enough calcium is so you can build strong bones. Can you offer him/her another reason?
Calcium is also needed for muscle contractions. Ca+2 binds to troponin so that troponin will change shape
and unbind, or let go, of actin.
3. Explain how it is that actin and myosin in the sarcomere never actually shorten and yet the muscle as a whole does.
Actin and myosin slide past one another causing the muscle to shorten.
4. How do ions and electrical charges play a role in communication with the muscle?
An electrical charge is received from a neuron, stimulating an action potential along the muscle fiber. Ca+2 is an ion that is required for muscle contraction.
5. Summarize the science behind rigor mortis. Why is this state a temporary condition?
Rigor mortis only lasts approximately 72 hours. As the body starts to decay, chemicals in the body eventually begin to breakdown the rigor mortis and allow the muscle to relax. However, there is a buildup of lactic acid at this point, which still makes the muscles stiff.
6. Using what you know about rigor mortis and about energy, what do you think happens inside your muscle when you get a muscle cramp? Why is this not a permanent condition?
Rigor means the actin and myosin filaments are stuck together. As soon as the Ca+2 is released from the
sarcomere, the filaments separate. Muscle cramps tend to happen when Ca+2 is too low.