muscle tissue
hapter 10
Overview of Muscular Tissue
Explain the structural differences among the three types of muscular tissue.
Smooth Muscle tissue: Fusiform (tapered at ends), Short, Nonstriated, Contain only one centrally located nucleus
Cardiac Muscle Tissue: Short, bifurcated, and striated, One or two centrally located nuclei, Intercalated discs between cells
Skeletal Muscle Tissue: Long, cylindrical, parallel, and unbranched
Multinucleated with nuclei along periphery
Compare the functions and special properties of the three types of muscular tissue.
Smooth Muscle Tissue: Involuntary muscle movements and motions moves materials through organs
Cardiac Muscle Tissue: Involuntary contraction and relaxation pump blood into the heart
Skeletal Muscle Tissue: Moves along skeleton Responsible for body movements
locomotion (moving the whole body in external space), Heat production
Skeletal Muscle Tissue
Explain the importance of connective tissue components, blood vessels, and nerves to skeletal
Fascia – the most superficial (outermost) connective tissue. It surrounds and supports muscles and helps anchor them to other structures in the body.
Then there are three deeper layers that organize and bind muscle cells:
Epimysium surrounds the entire muscle. The epimysium is the muscle's personal "packaging" that provides strength and protection while enabling independent, smooth movement. (epi=entire)
Perimysium is surrounded by the epimysium and another wrapper for muscle fibers called fascicles. Fascicular organization allows the nervous system to produce specific movements by activating only certain muscle fibers or fascicles. (peri=part)
Endomysium are think connective tissue layer that surround each fascicle muscle fiber Inside (endo=inside, individual cell)
Blood vessels bring oxygen and nutrients to the muscle and carry away waste
Nerve fibers send signals from your brain telling the muscle when to contract
Describe the microscopic anatomy of a skeletal muscle fiber.
Skeletal muscle fibers can range from 100 micrometers up to 11.8 in. They have multiple nuclei. The nuclei help make these proteins: contractile structural, enzymatic (provide energy and support reaction)
Distinguish thick filaments from thin filaments.
Thin Filaments: Made out of actin. Have “active sites” in order for binding. Have tropomyosin around the actin in order to prevent myosin binding when not intended. Troponin is on tropomyosin and it allows for shape change for the tropomyosin to then allow for the binding. This shape change in the troponin is allowed by the release of calcium in the muscle cell.
Thick Filaments: Made out of multiple myosin molecules. Contains heads with cross-bridges that the actin binds to during muscle contraction.
Describe the functions of skeletal muscle proteins
The contractile proteins generate force during muscle contraction and include myosin and actin (create movement)
The regulatory proteins determine when a muscle contracts and relaxes and include troponin and tropomyosin. (controls when the movement happens)
Contraction and Relaxation of Skeletal Muscle Fibers
Outline the steps involved in the sliding filament mechanism of muscle contraction. Includes contraction cycle steps, excitation-contraction coupling, length-tension relationship
Muscle contraction happens when thin filaments (actin) slide past thick filaments (myosin) inside the muscle fiber, making the sarcomere shorter.
Muscle fiber is stimulated by a nerve impulse
Calcium ions are released and bind to the troponin
Calcium+troponin releases tropomyosin along the thin filament in order for myosin to bind
ADP and phosphate molecule attach to myosin head, it is now “cocked”
Myosin head release the phosphate and attach itself to the actin
As myosin heads move, ADP is released
Myosin heads halt to add ATP, which detaches the myosin head from the actin binding site
The ATP is decomposed and turned into ATP and phosphate
Process continues when there is muscle contraction
Describe how muscle action potentials arise at the neuromuscular junction.
Muscles have motor neuron axons, and attached to the muscle fibers are neuromuscular junctions
Neuromuscular junctions have nerve terminals
Once action potential reaches nerve terminal, it releases neurotransmitter acetylocholine (ACh)
Acetylocholine binds to nicotinic receptors on the end plate
The nicotinic receptors open for sodium to enter the cell (end plate potential)
Outside the end plate, there are voltage channels which allow for more sodium to go to the cell membrane (threshold)
Voltage potassium channels open for the potassium to move out (resting)
This action potential spreads to the entire muscle unit
Muscle Metabolism
Describe the reactions by which muscle fibers produce ATP
Creatine Phosphate: When the muscle is resting, extra ATP gives its energy to creatine to form creatine phosphate. When the muscle starts to contract, creatine phosphate quickly gives its energy back to ADP to make ATP again. This process happens very fast with the help of an enzyme called creatine kinase. It provides energy for the first few seconds of muscle activity, but only lasts about 15 seconds before other energy sources are needed.
ATP → ADP (energy used)
ADP + creatine phosphate → ATP (energy restored)
Anaerobic glycolysis: Muscles make ATP without using oxygen by breaking down glucose (sugar). Slower than creatine phosphate, so when creatine phosphate runs out, muscles switch to glycolysis for energy. The glucose can come from the blood or from stored glycogen in the muscle. (come from carbs you eat) One glucose molecule produces only a small amount of ATP (2 ATP) the broken down glucose forms pyruvic acid. If there is not enough oxygen, pyruvic acid is turned into lactic acid. This system only lasts about 1 minute but helps provide energy for short, intense activities.
Aerobic respiration: Muscles make ATP using oxygen. It breaks down glucose, pyruvic acid, or fats to produce carbon dioxide, water, and a large amount of ATP. Most of the energy used during rest or moderate activity comes from this process, which happens in the mitochondria. It produces a lot of ATP (about 36 per glucose), but it works more slowly and requires a steady supply of oxygen. Muscles store some oxygen in myoglobin and can become more efficient with training.
Distinguish between anaerobic glycolysis and aerobic respiration
Anaerobic Glycosis: Makes ATP w/ glucose. Produces lactic acid to help glycolysis last longer. Works quickly but short term. Occurs in cytoplasm
Aerobic respiration: Makes ATP w/oxygen from breaking down pyruvic acid/glucose/fats. Works slower but long term Occurs in mitochondria.
Describe the factors that contribute to muscle fatigue.
Muscle fatigue is when muscles can no longer contract properly even with nerve signals.
Possible causes of muscle fatigue are:
Low levels of ATP (especially in intense exercise)
Lactic acid build up with the increase of hydrogen ions making muscle more acidic, slows down enzymes and muscle function
Ion imbalance between the release of potassium and sodium
Sarcoplasmic reticulum and sarcolemma damaged during long exercise making it harder for calcium control
Control of Muscle Tension
Describe the structure and function of a motor unit and define motor unit recruitment.
Each muscle fiber is innervated by only one motor neuron (nerve single from brain to muscle). A motor unit is made of one motor neuron and all the muscle fibers it controls. Size of a motor unit depends on the muscle and its function in your body. Small motor units control few muscle fibers and produce small precise movements (eye, fingers). Large motor units control many muscle fibers and produce stronger movements (thigh, back). Motor Unit Recruitment (Simple Explanation). Motor Recruitment is when your body activates more motor units to make a muscle contract stronger. The more motor units that are turned on, the stronger the contraction becomes. Some muscles have very large motor units that can produce up to 50 times more force than small ones. If all motor units are activated at once, the muscle produces its maximum strength But this can’t last long because it uses too much energy
Light object (like a feather) → only small motor units are used
Heavy object (like a textbook) → many + large motor units are recruited
Maximum force
If all motor units are activated at once, the muscle produces its maximum strength
But this can’t last long because it uses too much energy
Explain the phases of a twitch contraction
Each single isolated contraction in a muscle fiber is called a twitch. Each twitch goes through three phases
Latent Period: Action potential spreads along the sarcolemma and calcium ions are released from the sarcoplasmic reticulum. This is when excitation is being linked to contraction, but the muscle has not started to shorten yet.
Contraction phase: calcium ions bind on troponin and tropomyosin shifts away from binding sites for the myosin cross bridge to form. Sarcomeres are actively shortening, peak tension.
Relaxation phase: tension decreases as Ca2+ ions are pumped out of the sarcoplasm back into sarcoplasm reticulum. Crossbridge cycling stops.
Describe how frequency of stimulation affects muscle tension, and how muscle tone is produced.
A single action potential causes one twitch with rising tension during contraction and falling tension during relaxation. If stimuli happen before the muscle fully relaxes, twitches overlap and increase tension. Very frequent stimulation causes a sustained contraction with high tension. Muscle tone is produced when some motor units stay slightly active at rest, creating small overlapping twitches that keep a low level of continuous tension.
Distinguish between isotonic and isometric contractions
Isotonic contractions: Muscle tension stays CONSTANT. Two types are concentric, eccentric. Concentric muscle shortens by upward movement. Eccentric muscle tension diminishes, and muscle lengthens by downward movement.
Isometric contractions: Muscle tension stays constant, no movement. Ex holding our posture.
Types of Skeletal Muscle Fibers
Compare the structure and function of the three types of skeletal muscle fibers.
Slow oxidative fibers (Type 1): Muscles contract slowly. Have many mitochondria for aerobic respiration. Muscle fibers are small and thin and red with the color coming from high myoglobin amount, but low glycogen. They make very slow contractions over long periods of time but are resistant to fatigue. Used for low intensity activities.
Fast oxidative (Type IIa): Muscles have fast contractions and use both aerobic and anaerobic to produce ATP.Produces more force than slow fibers, but isn’t as resistant to fatigue. Used for movement such as walking or jogging. Fibers are pale red and thicker than slow fibers with moderate amounts of myoglobin and glycogen.
Fast Glycolic (Type IIx) Muscles have fast contractions and use anaerobic glycolysis to produce ATP. Thickest muscle fiber and pale pink, high amounts of glycogen and low amounts of myoglobin. Produce the most amount of force and fatigues most easily. Used for higher resistance exercise.
Exercise and Skeletal Muscle Tissue
Describe the effects of exercise on different types of skeletal muscle fibers
Endurance exercise focuses on slow-oxidative (SO) fibers. These fibers stay energized by using oxygen to produce massive amounts of ATP, and training makes them even more efficient by increasing the number of mitochondria and myoglobin (oxygen storage) within the cells. The body undergoes angiogenesis, growing a denser network of tiny blood vessels called capillaries to fast-track oxygen delivery and waste removal. Muscles don't grow much in size; keeping them lean allows oxygen to reach the deepest parts of the muscle quickly.
Resistance exercise focuses on Fast-Glycolytic (FG) fibers. These fibers generate massive force by breaking down ATP rapidly to create frequent "cross-bridges" between muscle proteins, leading to the high-intensity contractions needed for lifting heavy weights. When you perform resistance training, your muscles don't necessarily grow more cells; instead, they experience hypertrophy, where individual muscle fibers become thicker by adding more myofibrils (the structural units of contraction). Because this process relies on building up new structural proteins within the fiber, athletes often consume high-protein diets to provide the "bricks" needed for this expansion. This results in the significantly larger, more defined muscle mass typically seen in bodybuilders and power athletes.
Cardiac Muscle Tissue
Describe the main structural and functional characteristics of cardiac muscle tissue
Cardiac muscle tissue is found only in the heart. Its cells, called myocytes, make up most of the heart wall and are responsible for the heartbeat. Cardiac muscle is involuntary, meaning it networks itself automatically. It is striated, but its cells are branched and usually have two nuclei. A key feature of cardiac muscle is the intercalated discs, which are structures that hold the cells tightly together so they can work as one unit during contractions.
Smooth Muscle Tissue
Describe the main structural and functional characteristics of smooth muscle tissue
Smooth muscle fibers are long, spindle-shaped cells with a single nucleus. They are found in the walls of hollow organs such as blood vessels, airways, glands, skin, and the eyes. Smooth muscle is involuntary, meaning it networks itself automatically. It is nonstriated because its muscle proteins are not arranged in a repeating pattern. Smooth muscle helps control movements of internal organs and is part of the visceral (organ) motor system.
Aging and Muscular Tissue
Explain the effects of aging on skeletal muscle
Muscle loss that happens with aging is called sarcopenia and is not reversible. In sarcopenia, muscle fibers gradually die and are replaced with connective tissue and fat, which cannot contract. As a result, muscles produce less force, leading to reduced strength and difficulty with posture and movement. Aging also leads to fewer fast glycolytic fibers, smaller motor units, and relatively more slow oxidative fibers, which further reduces powerful movements.
Describe the disorders that affect muscular tissue
Without enough physical activity, muscle loss becomes more severe, leading to problems with movement, balance, and posture. This can reduce quality of life and cause joint weakness because muscles are less able to support bones. It also increases the risk of falls and injuries in older adults due to reduced strength and stability