The Muscle Twitch

The Muscle Twitch

A muscle Twitch is one contraction of a muscle. In reality any motion that your body creates is the result of many muscle twitches working at once. However, when we work in laboratory settings, we test different models of muscle mechanics. We use this concept called the muscle Twitch. Looking at this graph time is along the X axisSecond cervical vertebra; has the odontoid process (dens) for pivoting head (“no” motion). and tension is along the Y axis. This graph shows that after a muscle is stimulated, it builds tension to a maximum point. It then loses tension as it relaxes. Notice that there is a little bit of time between the stimulus and when the muscle starts to create tension. This is correct this is called the latent. This accounts for those events at the neuromuscular junctionThe connection between a motor neuron and a muscle fiber. using acetylcholinealso know as ACh A neurotransmitter that stimulates muscle contraction. to create depolarizationThe loss of electrical charge across a membrane, triggering an action potential.. The upslope of this graph corresponds with the phase when the muscle contracts. All of those little cross bridges form and break, and the sarcomeres shorten. This relaxation phase is when the muscle is returning to an optimal length using gravity and elastic forces.  Microscopically, during this relaxation phase, calcium leaves the sarcoplasm of the muscle fiber. It is reloaded into the sarcoplasmic reticulum.

Muscle Size and Tension

The graph in the previous slide is what we would expect optimally from an average sized muscle in your body. In reality larger muscles take a longer time to get to their peak tension after the stimulus is applied. For example, tiny eye muscles need to react quickly. They create movements very soon after being stimulated and reach maximum tension rapidly. These muscles are also those fast twitch muscles that we learned are more abundant in people who do sprinting. Another example are the muscles of your calf like the soleusPlantarflexor Under the gastrocnemius; helps stand or walk by pointing toes downward. and gastrocnemiusPlantarflexor / Flexor Large calf muscle; pushes you forward when you walk or jump which are actually quite large muscles. They both take a long time longer time to get two peak tension after the application of the stimulus. We also saw that these are called slow twitch muscles. They are more abundant in people who are fine-tuned for marathon running rather than sprinting. The one consistent thing you see in this graph is the latent periodThe short delay between muscle stimulation and contraction. for all of these muscles. It doesn’t matter what kind of muscle it is. Once the stimulus is applied, there is the same amount of time between the stimulus and the start of tension. This is true for all of the muscles in your body. The process involves releasing acetylcholine at the neuromuscular junction. This excites a muscle. The time for this process is the same for every muscle.

Tetanus

These graphs show a muscle’s response when it is bombarded with action potentials. The muscle has very little time to fully relax between them. So these muscles reflected in these graphs are experiencing a stimulus going through the latent. Going through the contraction. But not being allowed to go through the relaxation period.   On the graph on the left this is what is called unfused or incomplete tetanusA muscle contraction with brief periods of relaxation between stimuli.. This is where a muscle is given a chance to relax. However, it’s not given enough time to relax fully. As the muscle continues to be bombarded by action potentials, it starts from a higher level of tension every time. Each muscle twitch increases the tension. Eventually, the muscle reaches a point where it says, “Hey, I can’t create more tension. Can you please stop asking me to?”

In the graph on the right this is what is called complete or fused tetanusA sustained, maximal muscle contraction without relaxation..   This is when there is absolutely no relaxation offered to the muscle. The action potentials are coming so quickly. They bombard the muscle with such frequency that the muscle has no chance to relax. The same effect occurs with fused tetanusIn this context, sustained muscle contractions due to calcium or electrolyte imbalances. as with unfused tetanus. At some point in time the muscle just comes to its maximum tension. Here in complete or fused tetanus the muscle will reach the absolute maximum tension. As you can see over in the graph about unfused tetanus, the muscle gets really close to maximum tension. However, it can’t actually reach maximum tension.