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Vocabulary: skeletal muscle, muscle fiber, myofibril, sarcomere, thin filament, thick filament, motor unit
Hey there. This is Eric Simon, and welcome to another MP3 tutor session for tough terms. In today’s tutorial, I’ll talk about the structural organization of muscle and include a general overview of how it contracts.
Are you ready to get started? Remember: no pain, no gain!
Most body movements occur as a result of contraction of the specialized tissue called muscle. There are actually multiple types of muscle, but we will only discuss one kind in this tutorial: skeletal muscle.
Skeletal muscle is used to move the bones of the body. Skeletal muscle is also under voluntary control, which means that it is consciously controlled by impulses from the nervous system.
A skeletal muscle, such as the bicep muscle of the upper arm, consists of one large bundle containing many muscle fibers. Each muscle fiber is a single, elongated muscle cell.
Remember one of biology’s central principles: form fits function. Can you apply this principle to think of a good reason why muscle fibers are long, as opposed to round like many other cells?
You’re correct if you guessed that it has to do with the fact that muscle fibers contract. Imagine a round cell contracting: it wouldn’t be able to change shape much. However, a long cell, such as a muscle fiber, can substantially shorten when it contracts.
Each muscle contains many muscle fibers, and each fiber contains many myofibrils, which are contracting threads. Within each myofibril are repeated structural units called sarcomeres.
Sarcomeres are the functional unit of muscles, meaning that sarcomeres are the smallest unit that undergoes contraction. Sarcomeres are lined up end-to-end in a myofibril. Thus, when a series of sarcomeres all contract, the entire myofibril shortens. How do sarcomeres contract?
To understand this, we need to introduce a couple of other players. Within a sarcomere, there are thin filaments and thick filaments.
Thin filaments consist primarily of the protein actin and are attached to the ends of the sarcomere. Thick filaments are made up of the protein myosin and sit in the middle of a sarcomere.
When a muscle fiber is stimulated to contract, a thick filament binds to the surrounding thin filaments and pulls them toward the center of the sarcomere. This causes the thin filaments to slide past the thick filament. Because the thin filaments are attached to the end of the sarcomere, this causes the sarcomere to shorten, or contract. This process is referred to as the sliding filament model of muscle contraction.
Note that the length of the thick and thin filaments never changes; they just slide past each other. This sliding together of the thick and think filaments causes the sarcomere to contract.
Did you follow that? Let’s summarize by talking our way through a muscle contraction.
Individual muscle cells, called muscle fibers, receive a signal from the nervous system to contract. This causes thick filaments within the muscle fiber to grab thin filaments and pull them, sliding the filaments past each other. The thin filaments are attached to the end of the sarcomere, which are the functional unit of muscles. Therefore, when the thin filaments slide past the thick filaments, they cause the sarcomeres to shorten. Sarcomeres are lined up end-to-end in a myofibril, and shortening of a series of sarcomeres will cause contraction of an entire myofibril. Myofibrils are bundled together into a muscle fiber, and groups of muscle fibers combine to form a muscle.
Muscle fibers contract in an all-or-none manner. In other words, a muscle fiber either contracts or it doesn’t, and if it does contract, it will always contract with the same force and for the same length of time. However, you can obviously flex your bicep with varying amounts of force, such as when you lift a coffee cup versus when you lug a biology book. How does this happen?
To answer this, we need to understand one more term, the motor unit. A motor unit is a neuron and all of the muscle fibers that it connects to. Every muscle fiber is connected to only a single neuron. However, a single neuron can join any number of muscle fibers into a single motor unit. Some neurons connect to only a few muscle fibers, while others connect to thousands of muscle fibers. Therefore, a muscle can contract with varying intensity based on the number of motor units that are recruited into action.
I hope this tutorial helped you learn about skeletal muscle. If you need more help, you can listen to it again or consult your biology textbook and lecture notes. Good luck with your studying, and remember: no pain, no gain!