Skeletal Muscle

Almost half of an adult’s body weight comes from muscle, one of the four main types of tissues found in the body. Although there are three different types of muscle tissue, they all share some basic characteristics. Muscle tissue is often referred to as excitable, meaning the electrical state of the plasma membrane can change. This change, known as depolarization, occurs in response to signals from the nervous system, endocrine system or as a reaction to various intercellular signaling molecules.. Regardless of muscle tissue type, the end result of depolarization is a shortening of fibers resulting in muscle contraction.

Muscle cells, also known as muscle fibers, share many of the same basic structures and functions as other cells. They have nuclei, cell membranes, mitochondria, cytoskeleton, as well as the other organelles found in body cells. They perform metabolism, communicate with other cells, and are prone to cell damage and death. The most significant difference between muscle cells and other cells of the body is the density of proteins found within and surrounding the muscle cell. These proteins perform essential functions needed to allow the cell to expand and contract, which is the basis for movement in the body.

The most significant and most abundant constituents of muscle fibers are the contractile proteins actin and myosin. Simply put, myosin proteins pull on actin proteins, resulting in a shortening of muscle fibers, which leads to muscle contraction. The arrangement and specific mechanisms of action between actin and myosin varies between the different types of muscles. Regulatory proteins, troponin and tropomyosin, play a role in preventing some types of muscle contraction in the absence of nervous system stimulation. Structural proteins such as titin, actinin, and nebulin help to connect proteins within muscle fibers and maintain the shape and structure of those fibers, even during contraction. Another protein, dystrophin, serves to anchor the various proteins to the cell membrane and surrounding extracellular matrix. The protein elastin comprises the elastic fibers that are responsible for allowing muscles to stretch up to 1.5 times their resting length and recoil back to their original shape after contraction and relaxation. Many other proteins are found throughout muscles, serving to form channels across cell membranes.

In addition to the proteins found in muscle cells, a variety of ions and other molecules assist in contraction. Sodium and potassium ions allow for the transmission of signals, known as action potentials, across the membrane of the muscle cell. Calcium ions play a critical role in initiating contraction within muscle cells. Molecules like myoglobin, creatine, and adenosine triphosphate (ATP) are all essential for normal muscle maintenance and action.

There are four functions of muscle tissue.

First, and most obviously, muscle tissue is responsible for producing body movements. For example, in the last unit, we discussed flexion and extension, adduction and abduction. These are body movements. Not as obvious, but still important, are the pumping action of the heart carried out by cardiac muscle, and the peristaltic (milking) movements within the digestive tract.

Second, the muscular system stabilizes the body position. Without muscle activity, not just movement would cease; also, the constant battle of the body against gravity would be lost.

Third, muscles store and move substances within the body. For example, digestive enzymes are kept in the pancreas by a specialized circular muscle called a sphincter. When a meal arrives and is detected by the sphincter, it relaxes and releases its cargo of digestive juices. Lymph fluid is pushed around by muscular action, having no pump of its own.

Finally, muscles play an important role in thermoregulation by generating heat. We are only intermittently aware of the heat generated by muscles. When you are cold and start to shiver, you are subconsciously causing contraction of many muscles to generate heat. About 70-80% of the energy used by muscles is lost as heat, so in some ways they are better heat generators than they are movement generators. Normal body movements are responsible for a significant portion of the heat that contributes to the normal body temperature (core temperature 38°C; surface temperature 37°C).

A common theme in anatomy and physiology is that form and function are closely related. This is especially true for the types of muscles. What are the similarities between these classes of muscles? What are the major differences? How do these similarities and differences allow these muscles to perform their unique functions?

Skeletal muscle is named such because of its connection to the skeleton. This relationship allows contractions of the muscles to move the body or maintain posture. Skeletal muscle is under voluntary control, meaning it requires conscious thought and intentional stimuli from the nervous system in order to contract. You have more than likely seen what happens when an individual loses consciousness; their control over skeletal muscle immediately ceases.

The movement of our skeletal muscles is the job of many different areas of the brain, but we will wait until a discussion of the nervous system to tackle that. Generally speaking, these kinds of movements are referred to as motor movement. The neurons of the nervous system that cause these movements, coincidentally, are called motor neurons. Upper motor neurons are those that originate in the brain as well as those that send signals within the brain and down the spinal cord. Lower motor neurons extend from the spinal cord to the muscle or group of muscles they innervate.

A key feature of skeletal muscle cells is the arrangement of the contractile proteins actin and myosin. The highly organized structure is called a sarcomere. Sarcomeres are arranged end to end in a repeating fashion throughout the muscle. This gives the appearance of striations (stripes). The sarcomere is the basic unit of muscle contraction. One sarcomere is about 2.5 μm long. When the sarcomeres collapse on themselves and become shorter, the entire muscle becomes shorter. The large rectus femoris muscle of the leg contains perhaps 150,000 sarcomeres end to end; as each of these shortens by a tiny amount, the entire muscle is contracted.

In skeletal muscle, thousands of sarcomeres, arranged end to end, form long tubes called myofibrils. These tubes are wrapped together in connective tissue and make up the bulk of the skeletal muscle cell (muscle fiber). Due to the length of each muscle fiber, many nuclei are scattered along the length of the cell. The average skeletal muscle cell is about 3 cm in length, but they can vary from about 1 mm (stapedius muscle of the middle ear) to over 50 cm (sartorius muscle of the leg).

Several dozen myofibrils are found inside of each muscle cell, or muscle fiber. This is called a syncytium and is the result of embryonic muscle cells fusing to form a long tube.

The cell membrane of the skeletal muscle cell has the rather grand and strange name sarcolemma. It’s just like the cell membrane of any other cell; no one knows why this strange name persists.

Two Latin names appear over and over again in anatomical descriptions of muscle.

Sark or sarx (Greek σάρξ) means “flesh” in Greek, and so all the “sarco–” words come from that root: sarcoplasm, sarcolemma, sarcomere.

μῦς orMus, which became myo–, is the Greek word for mouse! The action of muscles like the biceps was thought to look like a mouse, probably by the same Greeks who named the constellations. They had a very active imagination.

Similarly, instead of the cytoplasm of muscle cells, we refer to the sarcoplasm.

muscle fiber = muscle cell

sarcolemma = cell membrane = plasmalemma

sarcoplasm = cytoplasm

sarcoplasmic reticulum = smooth endoplasmic reticulum

Lower motor neurons run throughout the muscle, but do not extend deep into each individual muscle cell. An elaborate system of tubules functions to relay the signal from the neuron resting just above the cell membrane to all of the sarcomeres inside the cell. These tubules, called T tubules, are like little folds of plasma membrane that dip into the cell. Flanking the T tubules on each side are tiny bags of calcium known as sarcoplasmic reticula. The sarcoplasmic reticulum is a specialized form of smooth endoplasmic reticulum which stores calcium ions (Ca₂⁺) and releases them when needed. They are necessary in the contraction process.

Because the electrical and calcium management systems of the muscle cell must work together, as we’ll see later, they are in close proximity to each other. One cylindrical T tubule is flanked by two flattened sacs of smooth endoplasmic reticulum in a standardized arrangement called a triad.

Sarcomere Structure

Light micrographs of skeletal muscle reveal striations that occur from a regular array of actin and myosin proteins. These striations that give striated muscle its name. Each band is called a sarcomeres, the contractile unit of muscle.

The actin filaments are held together at the Z disc (line) by a protein called actinin. The Z disc is the dark line that splits the light areas in the micrographs seen here. The Z disc defines the borders of the sarcomere. Using electron microscopy, we can see the relationship between the thick filaments of myosin and the thin filaments of actin. There are regions where only actin filaments are found near the Z discs, regions where only myosin filaments are found near the middle of the sarcomere or the M line, and regions where they overlap. These regions changes as the sarcomeres shorten or spring back to their original shape after contraction.

We will see in more detail later how the centrally located myosin proteins reach across the gap and bind to the actin proteins and pull them towards the center of the sarcomere. As more myosin heads bind and pull in on actin, the sarcomere continues to decrease in length. After contraction, the actin binding sites on actin are covered and structural proteins like titin and elastin help return the sarcomere to its original shape. The process is repeated every time a muscle contracts. This is referred to as the sliding filament model.