Skeletal muscle, also known as striated or voluntary muscle, is muscle tissue that individuals can consciously control. It attaches to the skeleton and enables voluntary movements like walking, running, jumping, and lifting. Skeletal muscles display a striped appearance under a microscope due to the orderly arrangement of muscle fibers. They connect to bones through tendons and collaborate in pairs or groups to generate coordinated movements around joints. Skeletal muscles possess a high capacity for adaptation, increasing in size and strength with consistent exercise and training.
1. Whole Muscle:
At the highest level of organization, we consider a skeletal muscle to be a complete muscle. Each muscle forms a separate organ made up of muscle tissue, blood vessels, nerves, and connective tissue.
The connective tissue that envelops the entire muscle is termed the epimysium.
2. Muscle Fascicles:
The muscle fibers are grouped into bundles called fascicles within each skeletal muscle.
A layer of connective tissue surrounds the bundles, called the perimysium.
3. Muscle fiber:
A muscle fiber, also known as a muscle cell or myofiber, is the individual contractile unit of skeletal muscle.
Each muscle fiber is a long, cylindrical, multinucleated cell with a high content of myofibrils.
Myofibrils are contractile structures within muscle fibers composed of repeating units called sarcomeres.
4. Sarcomere:
The sarcomere, responsible for the striated appearance of skeletal muscle, serves as the functional unit of muscle contraction.
Sarcomeres consist of thick and thin myofilaments. Thick filaments primarily consist of myosin, while thin filaments primarily consist of actin.
The interaction between myosin and actin during contraction is the basis of muscle contraction.
5. Myofibrils:
The Myofibrils are thread-like structures that run the muscle fiber’s length and contain numerous end-to-end sarcomeres.
Myofibrils are composed of repeating sarcomeres, giving them a striated appearance.
6. Myofilaments:
Myofilaments, the protein filaments within a sarcomere, play a role in generating muscle contraction.
The sarcomere organizes thick filaments (myosin) and thin filaments (actin) in a way that enables them to slide past each other during muscle contraction.
7. Muscle Proteins:
In addition to myosin and actin, skeletal muscles contain various proteins involved in muscle contraction, including troponin and tropomyosin, which regulate the interaction between myosin and actin.
Other structural proteins, like titin, provide elasticity and maintain the alignment of sarcomeres.
8. Muscle Fiber Membrane:
The plasma membrane of a muscle fiber is called the sarcolemma.
The sarcolemma contains numerous invaginations called T-tubules that help transmit action potentials deep into the muscle fiber.
9. Sarcoplasm:
The cytoplasm of a muscle fiber goes by the name sarcoplasm and holds typical cell components like mitochondria, nuclei, and the endoplasmic reticulum.
In muscle fibers, the endoplasmic reticulum is referred to as the sarcoplasmic reticulum (SR). It stores and releases the essential calcium ions required for muscle contraction.
10. Motor Units:
A motor unit consists of a single motor neuron and all its innervated muscle fibers.
Motor units vary in size, with small motor units controlling fine motor movements and larger motor units used for more powerful movements.
11. Neuromuscular Junction:
The neuromuscular junction is the point of contact between a motor neuron and a muscle fiber.
When an action potential reaches the neuromuscular junction, it triggers the release of acetylcholine, which initiates muscle contraction.
The organization of skeletal muscle from the whole muscle down to the individual sarcomeres allows for precise control of muscle contraction and movement. This hierarchical structure enables the body to produce a wide range of movements with varying degrees of force and precision, depending on the functional requirements of different muscles and activities.
