Actomyosin filaments and ATP - heart of rigor mortis

Transcription

1 World Journal of Pharmaceutical Sciences ISSN (Print): ; ISSN (Online): Published by Atom and Cell Publishers All Rights Reserved Available online at: Review Article Actomyosin filaments and ATP - heart of rigor mortis 1 Sonia Garg, 2 Shilekh Mittal and 1 Naveenta Gupta 1 Associate Professor, Physiology and 2 Associate professor, Forensic Medicine, G.G. S. Medical College, Faridkot, Punjab, India ABSTRACT Received: / Revised: / Accepted: / Published: It is a well-known fact that contractile substance of the muscle fibril is actomyosin, a complex composed of two protein filaments- actin (thin filament) and myosin (Thick filament), which require adenosine triphosphate (ATP), as a source of energy for their sliding movement. So after death due to non-availability of ATP, the thick and thin filaments can't slide away from each other and the muscles stay in a contracted state, known as rigor mortis (stiffness of body). Although this helps explain how the rigor mortis comes and goes, it also help us to explore how the gradual progression of rigor mortis plays a part in solving crimes by estimating the time since death. Keywords: ATP, Muscle contraction, Rigor mortis. INTRODUCTION Soon after the time of death, a body becomes rigid. This stiffening is the result of a biochemical process called Rigor Mortis, Latin word for "stiffness of death". This condition is common to all deceased humans, but is only a temporary state. Within hours of the time of death, every muscle in the body contracts and remains contracted for a period of time. So our objective is to assess the biological cause of the rigor mortis, how it comes and goes and also to explore the role of rigor mortis in solving crimes. To answer all these, we should first know the process of muscle contraction [1]. As we all know that, muscle fibers are composed of tissue units called sarcomeres. The components of an individual sarcomere are a thick and thin filament, myosin and actin, respectively. Strands of actin take on a double helix shape and are surrounded by a long protein strand called tropomyosin spotted with various small protein complexes called troponin. Underneath the tropomyosin are myosin active sites, where myosin is able to bind to the actin. Along a strand of myosin are "heads" that protrude towards the actin. Actin and myosin are the central actors in muscle contraction [2,3,4]. Initiation of Contraction by the Nervous System *Corresponding Author Address: Dr Sonia Garg, MD, Associate Professor, Physiology, G.G. S. Medical College, Faridkot, Punjab. Mob. no , drsoniamittal@rediffmail.com

2 The main steps involved in a single contraction are as follows. 1. Voluntary activity from the brain or reflex activity from the spinal cord computes that a contraction is needed. 2. The impulse is passed down the spinal cord to a motor neuron, and an action potential passes outwards in a spinal nerve, carried by an axon linking the motor neuron to all its muscle fibers. 3. The axon branches to supply all its muscle fibres (motor unit), and the action potential is conveyed to a neuromuscular junction on each muscle fiber. 4. At the neuromuscular junction, the action potential causes the release of packets or quanta of acetylcholine into the small space (synapse) between the axon and the muscle fiber. 5. Acetylcholine causes the electrical resting potential of the muscle fiber membrane to change, and this then initiates a new action potential that passes in both directions along the surface of the muscle fiber. Sonia et al., World J Pharm Sci 2016; 4(9): The action potential spreads deep inside the muscle fiber, carried by transverse tubules. 7. Where transverse tubules touch parts of the sarcoplasmic reticulum, the sarcoplasmic reticulum releases calcium ions. 8. The calcium ions cause the movement of troponin and tropomyosin on their thin filaments, which then enables the myosin molecule heads to "grab and swivel" their way along the thin filament. 9. A power stroke follows, where in the myosin heads drop the ADP and Pi, which holds the heads in a cocked back position and move laterally thereby moving the actin filament at the same time. 10. Finally ATP binds to the myosin heads, thereby detaching from the actin. 11. Upon release from the actin, ATP breaks down into ADP and Pi, giving energy to return the myosin to its cocked position, thereby renewing the cycle [5, 6]. 1. Acetylcholine is released at neuromuscular junction 2. AP is propagated along membrane & down T-tubule 3. Ca released from SR via a voltage gated Ca channel 4. Ca binds to Troponin-C - conformation changes in TnI & TnT favor tropomyosin opening actin myosin binding sites. 5. ATP split favors myosin cross-bridges attach-detach from actins... pulls filament toward M-line. 6. Ca is removed by Ca-pump (uptake by SR) 7. Tropomyosin blocks actin sites and muscle relaxes. Contraction is turned off by the following sequence of events. 12. The acetylcholine at the neuromuscular junction is destroyed by an enzyme (acetyl cholinesterase), and this terminates the stream of action potentials along the muscle fiber surface. 13. The sarcoplasmic reticulum ceases to release calcium ions, and immediately starts to resequester all the calcium ions that were just released. 14. Without calcium ions, a change in the configuration of troponin and tropomyosin blocks the action of the myosin molecule heads so that they cannot reach the thin filaments any more, and contraction ceases [5, 7, 8]. 399

3 Sonia et al., World J Pharm Sci 2016; 4(9): The supply of ATP is central in the continuing process of muscle contraction. ATP originates from three sources; the phosphagen system, glycogenlactic acid system, and aerobic respiration. In the phosphagen system, muscle cells store a compound called creatine phosphate in order to replenish the ATP supply quickly. The enzyme creatine kinase breaks the phosphate from this compound and the phosphate is added to ADP. This source of ATP can only sustain muscle contraction for 8 to 10 seconds. The glycogen-lactic acid uses the muscles' supply of glycogen. Through anaerobic metabolism, the glycogen is broken down and creates ATP and the byproduct lactic acid. This method does not require oxygen and is able to supply more ATP than the phosphagen system, but occurs at a slower rate. Finally, the aerobic respiration allows glucose to be broken down into carbon dioxide and water in the presence of oxygen. This method creates most of the ATP and for extended periods of time [8, 9]. With all of the information on how muscles work, it is now possible to explain the process behind rigor mortis. Death terminates aerobic respiration because the circulatory system has ceased. Therefore, the muscles rely on the phosphagen and anaerobic metabolism methods to acquire ATP. As stated above, these sources only provide a small amount of ATP. This lack of ATP disables the myosin heads from detaching from the actin. Meanwhile, calcium ions leak from extracellular fluid and the sarcoplasmic reticulum, which is unable to recall the ions, into the muscle fiber. The 400 ions perform their task as if the body were alive, disengaging the tropomyosin and troponin from the myosin active sites. The muscle contracts when the myosin shifts, but the lack of ATP prevents it from detaching, and the muscle remains contracted. Such a process occurs in all muscles as the body becomes rigid known as rigor mortis [8, 10] Now what actually happens in rigor mortis? It has been observed that rigor initiated when ATP concentration falls to 85% of normal and maximum rigor is achieved when ATP concentration reaches 15% normal [11]. The rigor develops at the same rate as the ATP disappears. At the maximum of rigor, there is no ATP at all. Thus it can be concluded that there exists a close relation between availability of ATP and development of rigor mortis as was found by Caspersson and Thorrell [12]. ATP content during development of Rigor mortis Two or three hours after a person or animal dies, the muscles start to stiffen. This phenomenon progresses in a downward, head-to-toe direction. In 12 to 18 hours the body becomes stiff as a board. At this stage, you can move the joints only by force, breaking them in the process. It takes about two days for rigor mortis to fade, and once it does, decay sets in [13]. Rigor mortis at the scene of crime has a lot of significance to police, medical examiners and lawyers in the criminal justice system. It's a clue in understanding the circumstances of someone's unexpected and possibly violent death. Although it's an imperfect

4 marker of the time of death, rigor mortis is useful because it's like an alarm clock set to go off and stop ringing within a known time span. [13,14]. CONCLUSION We have seen the role of actomyosin filaments and ATP in the contraction and relaxation of muscle. Thus after death when body runs out of ATP, rigor mortis develops. During rigor mortis, another process called autolysis takes place. This is the self-digestion of the body's own cells. Rigor mortis Sonia et al., World J Pharm Sci 2016; 4(9): ends not because the muscles relax, but because autolysis takes place. Rigor mortis is a piece of the forensic jigsaw puzzle, and combined with other details, it can help detectives and medical examiners figure out what happened. Several variables also affect the progression of rigor mortis, and investigators must take these into account when estimating the time since death. But the disadvantage of rigor mortis is that it leaves a lot of room for doubt so forensic pathologists should rely on other indicators as well that provide greater certainty as to time of death. 401

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