The refractory period allows the voltage-sensitive ion channels to return to their resting configurations. Very quickly, the membrane repolarizes, so that it can again be depolarized. Neural control initiates the formation of actin—myosin cross-bridges, leading to the sarcomere shortening involved in muscle contraction. These contractions extend from the muscle fiber through connective tissue to pull on bones, causing skeletal movement.
The pull exerted by a muscle is called tension, and the amount of force created by this tension can vary. This enables the same muscles to move very light objects and very heavy objects.
In individual muscle fibers, the amount of tension produced depends on the cross-sectional area of the muscle fiber and the frequency of neural stimulation.
The number of cross-bridges formed between actin and myosin determine the amount of tension that a muscle fiber can produce. Cross-bridges can only form where thick and thin filaments overlap, allowing myosin to bind to actin. If more cross-bridges are formed, more myosin will pull on actin, and more tension will be produced. The ideal length of a sarcomere during production of maximal tension occurs when thick and thin filaments overlap to the greatest degree.
If a sarcomere at rest is stretched past an ideal resting length, thick and thin filaments do not overlap to the greatest degree, and fewer cross-bridges can form. This results in fewer myosin heads pulling on actin, and less tension is produced. As a sarcomere is shortened, the zone of overlap is reduced as the thin filaments reach the H zone, which is composed of myosin tails. Because it is myosin heads that form cross-bridges, actin will not bind to myosin in this zone, reducing the tension produced by this myofiber.
If the sarcomere is shortened even more, thin filaments begin to overlap with each other—reducing cross-bridge formation even further, and producing even less tension. Conversely, if the sarcomere is stretched to the point at which thick and thin filaments do not overlap at all, no cross-bridges are formed and no tension is produced. This amount of stretching does not usually occur because accessory proteins, internal sensory nerves, and connective tissue oppose extreme stretching.
The primary variable determining force production is the number of myofibers within the muscle that receive an action potential from the neuron that controls that fiber. When using the biceps to pick up a pencil, the motor cortex of the brain only signals a few neurons of the biceps, and only a few myofibers respond.
In vertebrates, each myofiber responds fully if stimulated. When picking up a piano, the motor cortex signals all of the neurons in the biceps and every myofiber participates. This is close to the maximum force the muscle can produce. As mentioned above, increasing the frequency of action potentials the number of signals per second can increase the force a bit more, because the tropomyosin is flooded with calcium. The body contains three types of muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle.
Skeleton muscle tissue is composed of sarcomeres, the functional units of muscle tissue. Muscle contraction occurs when sarcomeres shorten, as thick and thin filaments slide past each other, which is called the sliding filament model of muscle contraction.
ATP provides the energy for cross-bridge formation and filament sliding. Regulatory proteins, such as troponin and tropomyosin, control cross-bridge formation.
Excitation—contraction coupling transduces the electrical signal of the neuron, via acetylcholine, to an electrical signal on the muscle membrane, which initiates force production. The number of muscle fibers contracting determines how much force the whole muscle produces. Skip to content Chapter The Musculoskeletal System.
Learning Objectives By the end of this section, you will be able to: Classify the different types of muscle tissue Explain the role of muscles in locomotion. Skeletal Muscle Fiber Structure.
Concept in Action. Sliding Filament Model of Contraction. ATP and Muscle Contraction. Figure With each contraction cycle, actin moves relative to myosin. The power stroke occurs when ADP and phosphate dissociate from the myosin head. The power stroke occurs when ADP and phosphate dissociate from the actin active site. Regulatory Proteins. Excitation—Contraction Coupling. This diagram shows excitation-contraction coupling in a skeletal muscle contraction. The sarcoplasmic reticulum is a specialized endoplasmic reticulum found in muscle cells.
Control of Muscle Tension. Exercises Which of the following statements about muscle contraction is true? However the neurotransmitter from the previous stimulation is still present in the synapse.
What factors contribute to the amount of tension produced in an individual muscle fiber? What effect will low blood calcium have on neurons? What effect will low blood calcium have on skeletal muscles? Answers B In the presence of Sarin, acetycholine is not removed from the synapse, resulting in continuous stimulation of the muscle plasma membrane. At first, muscle activity is intense and uncontrolled, but the ion gradients dissipate, so electrical signals in the T-tubules are no longer possible.
The result is paralysis, leading to death by asphyxiation. This is why dead vertebrates undergo rigor mortis. The cross-sectional area, the length of the muscle fiber at rest, and the frequency of neural stimulation. Neurons will not be able to release neurotransmitter without calcium.
Previous: Next: Chapter When the muscle is stretched beyond this length the number of active cross bridges decreases because the overlap between the actin and myosin fibers decrease. As the muscle becomes shorter than the optimum length the thin filaments at opposite ends of the sarcomere first begin to overlap one another and interfere with each other's movements.
This results in a slow decrease in tension as the sarcomeres get shorter. Then as the sarcomeres get shorter the thick filaments come into contact with the Z lines and the decrease in tension with decreasing length becomes even steeper. Regulation of Force Generated by Whole Muscles The force of muscle contraction can be increased by increasing the frequency of action potentials to an individual fiber.
This force ranges from the force generated by a single twitch to the force generated by maximum tetanic tension. This range in tension, or force, generated by a fiber only accounts for a fraction of the whole range of force that a whole muscle can generate.
The whole muscle can generate greater force by increasing the number of individual fibers that contract in a process called recruitment. Recruitment The nervous system exerts most of its control over muscle force by varying the number of active motor units. Recruitment is the term used to describe an increase in the number of active motor units.
Motor units themselves vary in the number of fibers they stimulate and in the size of the fibers within each unit. Size Principle According to the size principle when a muscle is called upon to generate small forces only smaller motor units are stimulated. When larger forces are needed larger motor units are recruited. This enables fine movements to be controlled by the smaller increments of force generated by the smaller motor units.
When greater force is required, the larger increments come from the larger motor units. The gradual recruitment of ever larger motor units is also a reflection of the fact that larger motor units have larger motor neurons which require more stimulation to fire. This is illustrated in the figure above. As the action potential frequency in the controlling upper motor neuron increases the lower motor neurons become active in order of increasing size.
Also, the force of contraction increases as the larger motor units with increasing numbers of fibers are recruited. Velocity of Shortening The speed with which a muscle contracts is also important in movement.
When a muscle contracts isotonically under increasing loads the contractions display the following effects:. The latent period time lag between stimulation and shortening increases. When the velocity of shortening is plotted as a function of load, as the load increases the velocity of shortening gradually decreases. The velocity of shortening is an important concept because different types of fibers differ in their velocity of shortening. In other words, certain types of muscle fibers can shorten faster than others.
Types of Fibers Speed of Contraction Under isometric contraction muscles vary in the speed they reach maximum tension. This is because there are fast-twitch and slow-twitch fibers. Certain muscles e. Some contain predominantly fast-twitch fibers e. Fast-twitch fibers also have higher maximum shortening velocities compared to slow-twitch. The difference between fast-twitch and slow-twitch depends on the type of myosin. Fast myosin hydrolyzes ATP at a faster rate and this leads to more cross bridge cycles per second compared to slow myosin.
Primary Mode of ATP Production Glycolytic fibers have a high cytosolic concentration of glycolytic enzymes and few mitochondria. These fibers are bigger and have fewer capillaries.
Oxidative fibers are rich in mitochondria and have a high capacity to produce ATP by oxidative phosphorylation. These fibers are smaller and have more capillaries. These fibers also have an oxygen binding protein called myoglobin. This molecule reversibly binds with oxygen like hemoglobin and serves as an oxygen buffer. It supplies oxygen to oxidative fibers when oxygen is temporarily cut off. Myoglobin gives the muscle fibers a reddish-brown color.
These fibers are often referred to as red muscle while glycolytic fibers are called white muscle. Glycolytic fibers produce ATP less efficiently by glycolysis but can function with little oxygen. Pyruvate builds up in these fibers and is converted to lactic acid. Oxidative fibers have a greater need for oxygen but are more resistant to fatigue than glycolytic fibers. Three Types of Skeletal Muscle Fibers.
Slow oxidative fibers contain slow myosin and produce most of their ATP by oxidative phosphorylation. These fibers also tend to be small in diameter and generate less force. Fast oxidative fibers also have a high oxidative capacity but have fast myosin.
In size and force generation these fibers are intermediate. Fast glycolytic fibers contain fast myosin and have a high glycolytic capacity. These fibers tend to be the largest and to generate the most force. All muscles have all three types but in different proportions. Size of Motor Unit and Order of Recruitment The three fiber types are segregated into separate motor units. The slow oxidative fibers have smaller fibers and are associated with the smaller motor units that tend to be recruited first for movements requiring a small force.
The fast glycolytic fibers have larger fibers and are associated with the larger motor units that tend to be recruited last for movement requiring greater force. The fast oxidative fibers are intermediate between the two. Resistance to Fatigue Muscles differ in their ability to resist fatigue. Fatigue occurs when muscles are stimulated at higher frequencies and when larger forces are generated. High Intensity Exercise e.
Strong contractions constrict blood vessels decreasing oxygen delivery and increase dependence on glycolysis. Lactic acids build up and lowers the pH. Low Intensity Exercise e. Fatigue develops more slowly and is probably due to depletion of energy reserves. Very High Intensity Exercise May induce neuromuscular fatigue due to a depletion of acetylcholine at synaptic terminals. Complex psychological factors are also involved with fatigue.
Long Term Responses of Muscle to Exercise Aerobic exercise low intensity; long duration converts some fast glycolytic fibers to fast oxidative fibers. This is associated with an increase in the number of mitochondria, capillaries and a decrease in fiber diameter. High intensity exercise e. There is a decrease in mitochondria, an increase in glycolytic enzymes and an increase in fiber diameter. Muscle growth is due to an increase in fiber diameter due to an increase in the myofibrils in the muscle fiber.
It was covered in your anatomy course. Muscle Receptors for Coordinated Activity Two types of receptors detect the movement of muscles and communicates this information to the CNS in order to coordinate muscle activity:. Muscle Spindles Muscle spindles, or stretch receptors, consist of 2 - 12 modified muscle fibers called intrafusal fibers enclosed in a sheath of connective tissue surrounded by regular skeletal muscle fibers extrafusal fibers. The expanded central region of these fibers is where the nuclei of these fibers are located and is called the central bag region.
The contractile component of these fibers is located on either side of this central region. There are two types of sensory receptors associated with the intrafusal fibers that detect the degree of stretch of these fibers which in turn monitors the degree of stretch of the entire muscle:. Annulospiral endings This is the end of a type Ia afferent fiber that wraps around the central bag region.
Troponin and tropomyosin run along the actin filaments and control when the actin binding sites will be exposed for binding to myosin. Thick myofilaments are composed of myosin protein complexes, which are composed of six proteins: two myosin heavy chains and four light chain molecules.
The heavy chains consist of a tail region, flexible hinge region, and globular head which contains an Actin-binding site and a binding site for the high energy molecule ATP. The light chains play a regulatory role at the hinge region, but the heavy chain head region interacts with actin and is the most important factor for generating force.
Hundreds of myosin proteins are arranged into each thick filament with tails toward the M-line and heads extending toward the Z-discs. Other structural proteins are associated with the sarcomere but do not play a direct role in active force production.
Titin, which is the largest known protein, helps align the thick filament and adds an elastic element to the sarcomere. Titin is anchored at the M-Line, runs the length of myosin, and extends to the Z disc.
The thin filaments also have a stabilizing protein, called nebulin, which spans the length of the thick filaments. Watch this video to learn more about macro- and microstructures of skeletal muscles.
The arrangement and interactions between thin and thick filaments allows for the shortening of the sarcomeres which generates force. It is important to note that while the sarcomere shortens, the individual proteins and filaments do not change length but simply slide next to each other.
This process is known as the sliding filament model of muscle contraction Figure Tropomyosin winds around the chains of the actin filament and covers the myosin-binding sites to prevent actin from binding to myosin. The troponin-tropomyosin complex uses calcium ion binding to TnC to regulate when the myosin heads form cross-bridges to the actin filaments.
Cross-bridge formation and filament sliding will occur when calcium is present, and the signaling process leading to calcium release and muscle contraction is known as Excitation-Contraction Coupling. Skeletal muscles contain connective tissue, blood vessels, and nerves. There are three layers of connective tissue: epimysium, perimysium, and endomysium.
Skeletal muscle fibers are organized into groups called fascicles. Blood vessels and nerves enter the connective tissue and branch in the cell. Muscles attach to bones directly or through tendons or aponeuroses.
Skeletal muscles maintain posture, stabilize bones and joints, control internal movement, and generate heat. Skeletal muscle fibers are long, multinucleated cells.
The membrane of the cell is the sarcolemma; the cytoplasm of the cell is the sarcoplasm. The sarcoplasmic reticulum SR is a form of endoplasmic reticulum. Muscle fibers are composed of myofibrils which are composed of sarcomeres linked in series. The striations of skeletal muscle are created by the organization of actin and myosin filaments resulting in the banding pattern of myofibrils.
These actin and myosin filaments slide over each other to cause shortening of sarcomeres and the cells to produce force.
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