Smooth muscle is non-striated and involuntary. Smooth muscle myocytes are spindle shaped with a single centrally located nucleus. Types of muscle : The body contains three types of muscle tissue: skeletal muscle, smooth muscle, and cardiac muscle, visualized here using light microscopy. Visible striations in skeletal and cardiac muscle are visible, differentiating them from the more randomised appearance of smooth muscle. Skeletal muscle contains different fibers which allow for both rapid short-term contractions and slower, repeatable long-term contractions.
Skeletal muscle fibers can be further subdivided into slow and fast-twitch subtypes depending on their metabolism and corresponding action. Most muscles are made up of combinations of these fibers, although the relative number substantially varies. Slow-twitch fibers are designed for endurance activities that require long-term, repeated contractions, like maintaining posture or running a long distance.
The ATP required for slow-twitch fiber contraction is generated through aerobic respiration glycolysis and Krebs cycle , whereby 30 molecules of ATP are produced from each glucose molecule in the presence of oxygen. The reaction is slower than anaerobic respiration and thus not suited to rapid movements, but much more efficient, which is why slow-twitch muscles do not tire quickly. However, this reaction requires the delivery of large amounts of oxygen to the muscle, which can rapidly become rate-limiting if the respiratory and circulatory systems cannot keep up.
Due to their large oxygen requirements, slow-twitch fibers are associated with large numbers of blood vessels, mitochondria, and high concentrations of myoglobin, an oxygen-binding protein found in the blood that gives muscles their reddish color. Fast-twitch fibers are good for rapid movements like jumping or sprinting that require fast muscle contractions of short duration.
Unlike slow-twitch fibers, fast twitch-fibers rely on anaerobic respiration glycolysis alone to produce two molecules of ATP per molecule of glucose. While much less efficient than aerobic respiration, it is ideal for rapid bursts of movement since it is not rate limited by need for oxygen. Lactate lactic acid , a byproduct of anaerobic respiration, accumulates in the muscle tissue reducing the pH making it more acidic, and producing the stinging feeling in muscles when exercising.
This inhibits further anaerobic respiration. While this may seem counter-intuitive, it is a feedback cycle in place to protect the muscles from over-exertion and resultant damage.
As fast-twitch fibers generally do not require oxygenation, they contain fewer blood vessels and mitochondria than slow-twitch fibers and less myoglobin, resulting in a paler color. While there is evidence that each person has a unique proportion of fast-twitch versus slow-twitch muscles determined by genetics, more research is required.
In the sliding filament model, the thick and thin filaments pass each other, shortening the sarcomere. Movement often requires the contraction of a skeletal muscle, as can be observed when the bicep muscle in the arm contracts, drawing the forearm up towards the trunk. The sliding filament model describes the process used by muscles to contract.
It is a cycle of repetitive events that causes actin and myosin myofilaments to slide over each other, contracting the sarcomere and generating tension in the muscle. To understand the sliding filament model requires an understanding of sarcomere structure. A sarcomere is defined as the segment between two neighboring, parallel Z-lines. Z lines are composed of a mixture of actin myofilaments and molecules of the highly elastic protein titin crosslinked by alpha-actinin.
Actin myofilaments attach directly to the Z-lines, whereas myosin myofilaments attach via titin molecules. Surrounding the Z-line is the I-band, the region where actin myofilaments are not superimposed by myosin myofilaments. The I-band is spanned by the titin molecule connecting the Z-line with a myosin filament.
The region between two neighboring, parallel I-bands is known as the A-band and contains the entire length of single myosin myofilaments. Within the A-band is a region known as the H-band, which is the region not superimposed by actin myofilaments. Within the H-band is the M-line, which is composed of myosin myofilaments and titin molecules crosslinked by myomesin.
Titin molecules connect the Z-line with the M-line and provide a scaffold for myosin myofilaments. Their elasticity provides the underpinning of muscle contraction.
Titin molecules are thought to play a key role as a molecular ruler maintaining parallel alignment within the sarcomere. Another protein, nebulin, is thought to perform a similar role for actin myofilaments. The molecular mechanism whereby myosin and acting myofilaments slide over each other is termed the cross-bridge cycle. During muscle contraction, the heads of myosin myofilaments quickly bind and release in a ratcheting fashion, pulling themselves along the actin myofilament.
At the level of the sliding filament model, expansion and contraction only occurs within the I and H-bands. The myofilaments themselves do not contract or expand and so the A-band remains constant. The sarcomere and the sliding filament model of contraction : During contraction myosin ratchets along actin myofilaments compressing the I and H bands. During stretching this tension is release and the I and H bands expand. The A-band remains constant throughout as the length of the myosin myofilaments does not change.
The amount of force and movement generated generated by an individual sarcomere is small. However, when multiplied by the number of sarcomeres in a myofibril, myofibrils in a myocyte and myocytes in a muscle, the amount of force and movement generated is significant. ATP is critical for muscle contractions because it breaks the myosin-actin cross-bridge, freeing the myosin for the next contraction. With each contraction cycle, actin moves relative to myosin.
Muscles contract in a repeated pattern of binding and releasing between the two thin and thick strands of the sarcomere. Cardiocytes are branched, allowing them to connect with several other cardiocytes, forming a network that facilitates coordinated contraction. See more from our free eBook library.
Newark: University of Delaware, Biological Sciences. How do muscles grow? Len Kravitz, Ph. Muscle Attachment and Actions. Muscular System Pathologies. When you select "Subscribe" you will start receiving our email newsletter. Use the links at the bottom of any email to manage the type of emails you receive or to unsubscribe. See our privacy policy for additional details. Learn Site. Artery walls include smooth muscle that relaxes and contracts to move blood through the body 3.
They are usually found in regions near the agonist and often connect to the same bones. Because skeletal muscles move the insertion closer to the immobile origin, fixator muscles assist in movement by holding the origin stable. If you lift something heavy with your arms, fixators in the trunk region hold your body upright and immobile so that you maintain your balance while lifting. Skeletal muscle fibers differ dramatically from other tissues of the body due to their highly specialized functions.
Many of the organelles that make up muscle fibers are unique to this type of cell. The sarcolemma is the cell membrane of muscle fibers. The sarcolemma acts as a conductor for electrochemical signals that stimulate muscle cells. Connected to the sarcolemma are transverse tubules T-tubules that help carry these electrochemical signals into the middle of the muscle fiber. Myofibrils are made up of many proteins fibers arranged into repeating subunits called sarcomeres.
The sarcomere is the functional unit of muscle fibers. See Macronutrients for more information about the roles of sugars and proteins. The main function of the muscular system is movement. Muscles are the only tissue in the body that has the ability to contract and therefore move the other parts of the body. Muscles often contract to hold the body still or in a particular position rather than to cause movement.
Another function related to movement is the movement of substances inside the body. The cardiac and visceral muscles are primarily responsible for transporting substances like blood or food from one part of the body to another.
The final function of muscle tissue is the generation of body heat. As a result of the high metabolic rate of contracting muscle, our muscular system produces a great deal of waste heat.
Many small muscle contractions within the body produce our natural body heat. When we exert ourselves more than normal, the extra muscle contractions lead to a rise in body temperature and eventually to sweating. Skeletal muscles work together with bones and joints to form lever systems. The muscle acts as the effort force; the joint acts as the fulcrum; the bone that the muscle moves acts as the lever; and the object being moved acts as the load.
There are three classes of levers, but the vast majority of the levers in the body are third class levers. A third class lever is a system in which the fulcrum is at the end of the lever and the effort is between the fulcrum and the load at the other end of the lever. The third class levers in the body serve to increase the distance moved by the load compared to the distance that the muscle contracts.
The tradeoff for this increase in distance is that the force required to move the load must be greater than the mass of the load. For example, the biceps brachia of the arm pulls on the radius of the forearm, causing flexion at the elbow joint in a third class lever system.
A very slight change in the length of the biceps causes a much larger movement of the forearm and hand, but the force applied by the biceps must be higher than the load moved by the muscle. Nerve cells called motor neurons control the skeletal muscles. Each motor neuron controls several muscle cells in a group known as a motor unit. When a motor neuron receives a signal from the brain, it stimulates all of the muscles cells in its motor unit at the same time.
The size of motor units varies throughout the body, depending on the function of a muscle. Muscles that need a lot of strength to perform their function—like leg or arm muscles—have many muscle cells in each motor unit.
One of the ways that the body can control the strength of each muscle is by determining how many motor units to activate for a given function. This explains why the same muscles that are used to pick up a pencil are also used to pick up a bowling ball. Muscles contract when stimulated by signals from their motor neurons.
Motor neurons release neurotransmitter chemicals at the NMJ that bond to a special part of the sarcolemma known as the motor end plate. The motor end plate contains many ion channels that open in response to neurotransmitters and allow positive ions to enter the muscle fiber. The positive ions form an electrochemical gradient to form inside of the cell, which spreads throughout the sarcolemma and the T-tubules by opening even more ion channels.
Tropomyosin is moved away from myosin binding sites on actin molecules, allowing actin and myosin to bind together. ATP molecules power myosin proteins in the thick filaments to bend and pull on actin molecules in the thin filaments. Myosin proteins act like oars on a boat, pulling the thin filaments closer to the center of a sarcomere.
As the thin filaments are pulled together, the sarcomere shortens and contracts. Myofibrils of muscle fibers are made of many sarcomeres in a row, so that when all of the sarcomeres contract, the muscle cells shortens with a great force relative to its size.
Muscles continue contraction as long as they are stimulated by a neurotransmitter.
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