Something as simplistic as walking to class, or eating a banana is a much more involved process than the superficial and visual aspect, which has been becoming ever more clear throughout this chapter. This subchapter will expand on your knowledge from the previous subchapters to focus on how nerves are able to communicate with muscle fibers and create muscular contractions. Neuro-muscular communication is one of the many highly integrated components involved in the aforementioned examples. This subchapter will also introduce the characteristics and organization of muscle tissues to understand how structural differences in a muscle allow for different contractions to be formed.
In this section you will learn…
The following image provides a visual representation of the three different types of muscle tissue found in humans and an example of each. Types of muscle tissue are not a huge focus of the course but are important background information to keep in mind as you move through the unit. This image is at course level. However, do not focus on differentiating between the muscle types visually.
As can be seen in the above photo, there are three types of muscle tissue: smooth muscle, cardiac muscle, and skeletal muscle. Cardiac and skeletal muscles are both subtypes of striated muscle. Striated muscle is named due to its appearance under a microscope, but is beyond the scope of this course. The appearance and organization of the muscle fibers vary among the different muscle types, and they are found in different locations in the body. Cardiac muscle is found solely in the heart and is responsible for the involuntary contractions that pump blood around your body. Smooth muscle is also under involuntary control and is found in a wide array of organs, including the intestines, urinary bladder, and blood vessels. Skeletal muscle is the only muscle type under voluntary control and is attached to your bones across joints to produce movement during contraction.
Of the three muscle types, skeletal muscle is the primary focus of HK*2810. Many concepts are identical between the three muscle types, but there are some key differences as well. You will learn more about smooth muscle and cardiac muscle during HK*3810.
The following image provides a visual for the structure of a muscle fiber and shows the location of sarcomeres within myofibrils. The different levels of organization within muscles may seem confusing, but this image helps because it focuses on the structure of the cell itself, and the sarcomere, its functional unit. This image is at course level. However, don’t worry about being able to label the muscle fiber. Rather, focus on the organization of myofibrils within muscle fibers and sarcomeres within myofibrils.
Muscle fibers are the cells that make up muscle tissue. Muscle fibers look a little different than the typical cell diagrams that you see during high school biology classes. Muscle fibers are elongated and cylindrical. The cell membrane of a muscle fiber is called the sarcolemma, and the cytoplasm is called the sarcoplasm. Muscle fibers also have a structure called the sarcoplasmic reticulum, which is similar to the endoplasmic reticulum and stores calcium ions (Ca 2+ ). Hundreds to thousands of myofibrils are also found within one muscle fiber. The myofibrils are elongated threads that run parallel to each other and contain the sarcomeres. The sarcomeres are the contractile units of the muscle fibers and are arranged consecutively, separated by Z lines (Z discs).
Did you know the prefixes “myo-” and “sarco-” originate from the Greek words? “Mu”, means “muscle”, and “sark”, means “flesh”. When you hear these prefixes, you can be confident that the word is related to muscles (ex. “myofibrils”, “myofilaments”, “sarcomeres”, “sarcolemma”).
Real-Life Scenario:
Pretend you are working out at the gym and tracking your strength progress. After consistently working out for 4 months, you have noticed your muscles have increased in size and you have gotten stronger. What adaptations are the muscles making to increase in size and strength? Hint: What functional component of the muscle could increase its population to increase force production?
The following image shows the organization of a sarcomere and zooms in on portions of the thick and thin filaments. As you look through this image, try and connect the various labeled components to their role in muscle contraction. As well, pay attention to the relationship between troponin, tropomyosin, and actin.
Sarcomeres are composed of thin (actin) filaments and thick (myosin) filaments. Actin is anchored to z-lines. Myosin is freely mobile and its heads interact with actin for contraction. The myosin heads cannot bind anywhere on actin but rather, must interact with the binding sites on actin. However, while the muscle is relaxed, the binding sites for myosin are covered by the troponin-tropomyosin complex. The steps taken to create a muscle contraction are discussed later in this subchapter.
Are you interested in learning more about the sarcomere? The following image shows an electron micrograph of a sarcomere. One sarcomere runs between the two Z-lines (the white Zs). The lighter areas directly beside the Zs represent the regions where only thin filaments (actin) are found. The region labeled “M” is the area where only thick filaments (myosin) are found. The region between the exclusively actin and myosin regions are where both types of filaments overlap. This electron micrograph helps to visualize where the actin and myosin get their names of “thin” and “thick” filaments, respectively. The individual myosin filaments can be seen because of their thickness, but individual actin filaments cannot be seen because they are so thin.
One way to remember the organization of sarcomeres within a myofibril is to think about the z-lines. The z-lines are the part of the sarcomere that look like the letter “z”. The letter “z” indicates the end of the alphabet and the z-line indicates the ends of the sarcomeres.
The following diagram outlines the process of muscle contraction, beginning with the arrival of an action potential at a neuromuscular junction, and ending with a shortening of the muscle. This image is useful because it provides a big picture explanation of the mechanisms of muscle contraction, without focusing too much time on the smaller details. This diagram is below course level as it is only a summary, and you will be required to know more of the biochemical details.
The process of muscle contraction (excitation-contraction coupling) is as follows:
Thinking Beyond:
Using what you know about the steps to contract a muscle. Write out the steps for relaxation of a muscle. Hint: What needs to stop occurring/ stop being released in order for the contraction to stop?
The video below provides a review of the process of skeletal muscle contraction. This is a helpful video because it allows you to see the actin and myosin interaction, and therefore muscle contraction, in action.
0:00-1:20 Muscle physiology
1:20 Sliding filament theory
3:35 Calcium release
This video is slightly below course level, as it does not include as many details as you will learn during lecture. However, watching all the steps come together in action can provide a strong framework to understand muscle contraction.
Muscle Contraction. Created by Raghavendra Rao. ©Creative Commons Attribution License (reuse allowed).
Remember that all the myosin heads of a sarcomere act independently. In order for movement to occur, all the myosin heads, on all the sarcomeres, within all the myofibrils, within a muscle fiber, must contract at the same time, and the muscle will shorten. Muscles have tendons that terminate at joints. When the muscle contracts, the tendon pulls on the joint, changing the angle of the two bones connected by the joint, and producing movement.
Ever wonder how Rigor Mortis works? After death, the body is no longer producing ATP, therefore, muscle contraction cannot be undone. This causes permanent muscle contraction.
The following image provides an overview of sources of calcium in skeletal muscle cells. This image is at course level. However, don’t worry about knowing the “terminal cisternae”. It simply refers to the enlarged area of the sarcoplasmic reticulum.
Calcium is the second messenger involved in muscle contraction. In order for actin and myosin interaction to occur, calcium must be present inside the muscle fiber. In skeletal muscle, the calcium ions are stored in the sarcoplasmic reticulum. When the sarcolemma is depolarized, the local current travels along the t-tubules and causes the voltage-dependent dihydropyridine receptor to change shape, yank on the ryanodine receptor and open a calcium channel on the surface of the sarcoplasmic reticulum. Intracellular calcium levels increase. Increased intracellular calcium increases the force of contraction and decreased intracellular calcium decreases the force of contraction.
Once intracellular calcium levels increase, the process of muscle contraction is identical in both types of striated muscle (skeletal and cardiac muscle). However, prior to this step, there are small differences between the two types. We focus on skeletal muscle during HK*2810, and you will learn more about cardiac muscle during HK*3810.
The diagram below shows the sources of calcium for cardiac muscle contraction. As well, the diagram provides a strong overview of the mechanisms by which calcium is recycled during cardiac muscle relaxation.
Cardiac muscle fibers receive calcium from two sources: the sarcoplasmic reticulum and intracellularly. The calcium channels on the sarcoplasmic reticulum are ligand-gated. The ligand that must bind to open these channels is calcium. This is referred to as calcium-induced calcium release (CICR) because calcium both binds the channel and passes through the channel. The calcium that binds the channel is called “trigger calcium” and comes from extracellular sources. However, there is no difference between the extracellular calcium and the calcium from the sarcoplasmic reticulum. They are identical. In order to decrease intracellular calcium levels for muscle relaxation to occur, there are three mechanisms that the cell utilizes:
Thinking Beyond:
What might happen to the heart if the calcium ATPase pumps on the sarcoplasmic reticulum in cardiac myocytes are defective? What effect might this have on heart rate? Heart contractility Contractility is the force or strength of contraction. Hint: Will the cardiac myocytes be able to contract and relax normally?
The image below shows the interaction between dense bodies, thin filaments, and thick filaments in smooth muscle tissue. It provides a nice visual for how all these components work together to cause muscle contraction. You learn how one set of dense bodies, thin filaments, and thick filaments cause muscle contraction, but it can be tricky to see the big picture and think about how multiple sets work together, which is why this picture can be so useful.
The structure of smooth muscle is different than that of striated muscle. Smooth muscle fibers do not have sarcomeres. However, they do have thin and thick filaments. Where the thin filaments are anchored to the Z-line in striated muscle, they are instead anchored to dense bodies in smooth muscle. Many thin filaments can be anchored to one dense body. Similar to striated muscle, myosin pulls on actin during contraction. This causes the dense bodies to move closer together and causes shortening of the muscle fiber.
Think about concentric (shortening) muscle contraction like this… Imagine you’re standing between two bookshelves (bookshelves= z-discs), one on either side of you, and your goal is to bring them together. There are ropes tied to the bookshelves that you can pull to slide the shelves along. (your arms= myosin, ropes= actin). You repetitively pull the ropes towards you, regrasp it and then pull again. This is the actin and myosin interacting to shorten the muscle. Once the bookshelves are right beside you, there is no more myosin that your actin/arms can interact with. That is when no more tension can be produced.
The diagram below provides a nice visual of the process of smooth muscle contraction. It is a helpful resource as it shows the process in a different way by using images, rather than just words. This image is slightly below course level as it does not go into as much detail as your notes, however, it is a nice overview to begin with.
The steps of smooth muscle contraction are listed below (as shown in figure 10):
Clinical Application:
Suppose you are designing a drug that relieves hypertension. If hypertension is high blood pressure caused by increased contraction of smooth muscle in the blood vessels, propose a mechanism for this drug. Hint: What receptor would it bind to? What cellular changes would it induce in the cells it binds to?
Consider the following concepts to help guide your studies:
The questions below can be used to assess your knowledge within this chapter. There are five multiple-choice questions that you should attempt without referring to your notes. The questions will provide you with responses to your answers to guide your studying but should not be used as your only resource.
One of the three types of muscle tissue found in the human body. Smooth muscle is under involuntary control and is found in a variety of internal organs, including the intestines, bladder, and blood vessels.
× Close definitionOne of the three types of muscle tissue found in the human body. Cardiac muscle is under involuntary control and is found solely in the heart.
× Close definitionOne of the three types of muscle tissue found in the human body. Skeletal muscle is under voluntary control and is attached to your joints to produce movement during contraction.
× Close definition × Close definitionThe cell membrane of a striated muscle cell.
× Close definitionThe cytoplasm of a striated muscle cell.
× Close definitionA structure, similar to the endoplasmic reticulum, that is found in striated muscle fibers. Its main function is to store calcium ions.
× Close definitionThe contractile unit of a muscle fiber, composed of sliding protein filaments of actin and myosin.
× Close definitionOne of the two protein filaments that make up a sarcomere; thin filament.
× Close definitionOne of the two protein filaments that make up a sarcomere; thick filament.
× Close definitionA protein complex that covers the binding sites for myosin on actin.
× Close definitionThe process of turning an action potential into a muscle contraction.
× Close definitionA change in membrane potential that passes along a neuron or muscle fiber to transmit an impulse.
× Close definitionA chemical synapse formed between a motor neuron and a muscle fiber.
× Close definitionExtensions of the cell membrane that go into the centre of striated muscle fibers at the Z-lines.
× Close definitionThe release of calcium from the sarcoplasmic reticulum, which is triggered by the binding of calcium to the ligand-gated channels on the sarcoplasmic reticulum.
× Close definitionA structure found in smooth muscle fibers that thin filaments anchor to. Dense bodies are analogous to z-lines in striated muscle.
× Close definitionHuman Physiology Copyright © by Human Physiology Students from University of Guelph is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.
