Question

After death, metabolism shuts down and ATP runs out. As a result, calcium pumps at the sarcoplasmic reticulum stop, so Ca⁺² ions diffuse out into the sarcoplasm.   Produce a diagram describing the contraction cycle and use it to explain what will be the consequence of a sudden calcium increase in the absence of ATP for the muscles of this person’s body?

Answer 1

• This is a diagram for the contraction of muscles. The process occurs in the following steps:

1. Electrical signal arrives at the cell causing the release of Ca⁺² from the sarcoplasmic reticulum (which is like an endoplasmic reticulum (ER) in the muscle cell.)

2. Ca+2 binds to troponin so that troponin will change shape and unbind, or let go, of actin. Troponin is attached to the protein tropomyosin which lies within the groove between actin filaments in muscle tissue.

3. A phosphate attached to the myosin heads is released as the myosin head binds to the actin forming a cross bridge

4. : The remaining ADP molecule on the myosin head is released in a power stroke when the myosin pulls the actin

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

Figure 10-7 Principles of Antomy and Physiology, 11/e, 2006 John Wiley and Sons

• Rigor mortis, is commonly known as rigor, which is the postmortem muscle stiffening of the body.
• Immediately after death, the muscles of the body contract in the same way 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, when the body is 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 which causes muscle contraction. The muscles remain in the contracted state till 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.

• After the death of a person, breathing and circulation stop and hence muscle cells lack oxygen and 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.