MUSCULAR TISSUE PT. 1 Flashcards
contributes to homeostasis by producing body movements, moving substances through the body, and producing heat to maintain normal body temperature.
Muscular Tissue
What do bones provide and form in the body?
Leverage and the framework of the body.
Can bones move body parts by themselves?
No, they cannot move body parts by themselves.
What results from the alternating contraction and relaxation of muscles?
Motion results from the alternating contraction and relaxation of muscles.
What percentage of total adult body weight do muscles make up?
40–50% of total adult body weight.
What factors determine the percentage of total adult body weight made up by muscles?
The percentage of body fat, gender, and exercise regimen determine the percentage.
What does your muscular strength reflect?
It reflects the primary function of muscle.
What is the primary function of muscle?
The transformation of chemical energy into mechanical energy to generate force, perform work, and produce movement.
What do muscle tissues stabilize?
They stabilize body position.
What do muscle tissues regulate?
They regulate organ volume.
What do muscle tissues generate?
They generate heat.
What do muscle tissues propel through various body systems?
They propel fluids and food matter through various body systems.
What are the three types of muscular tissue?
Skeletal, cardiac, and smooth.
What is the scientific study of muscles known as?
Myology.
What does “myology” mean?
Myo = muscle; logy = study of.
In what ways do different types of muscular tissue differ?
They differ in microscopic anatomy, location, and how they are controlled by the nervous and endocrine systems.
Why is skeletal muscle tissue named as such?
Because most skeletal muscles move the bones of the skeleton.
What else do some skeletal muscles attach to and move?
The skin or other skeletal muscles.
What characteristic does skeletal muscle tissue have when examined under a microscope?
It is striated.
What causes the striated appearance of skeletal muscle tissue?
Alternating light and dark protein bands (striations).
In what manner does skeletal muscle tissue mainly work?
In a voluntary manner.
How can skeletal muscle activity be controlled?
By neurons (nerve cells) that are part of the somatic (voluntary) division of the nervous system.
To what extent are most skeletal muscles controlled subconsciously?
To some extent.
What skeletal muscle contracts and relaxes without conscious control to allow breathing?
The diaphragm.
What skeletal muscles contract without conscious thought to maintain posture or stabilize body positions?
The skeletal muscles that maintain your posture or stabilize body positions.
Which organ contains cardiac muscle tissue?
Only the heart.
What does cardiac muscle tissue form?
Most of the heart wall.
What characteristic does cardiac muscle tissue share with skeletal muscle tissue?
It is striated.
In what manner does cardiac muscle tissue work?
Involuntary.
What is not consciously controlled in cardiac muscle tissue?
The alternating contraction and relaxation of the heart.
Why does the heart beat?
Because it has a natural pacemaker that initiates each contraction.
What is the built-in rhythm of the heart called?
Autorhythmicity.
What can adjust the heart rate?
Several hormones and neurotransmitters.
How do hormones and neurotransmitters adjust heart rate?
By speeding or slowing the pacemaker.
Where is smooth muscle tissue located?
In the walls of hollow internal structures, such as blood vessels, airways, and most organs in the abdominopelvic cavity.
Where else is smooth muscle tissue found?
In the skin, attached to hair follicles.
What characteristic does smooth muscle tissue lack under a microscope?
Striations.
Why is smooth muscle tissue referred to as smooth?
Because it looks nonstriated.
In what manner does smooth muscle tissue usually work?
Involuntary.
What type of smooth muscle tissue has autorhythmicity?
The muscles that propel food through your digestive canal.
What regulates both cardiac muscle and smooth muscle?
Neurons that are part of the autonomic (involuntary) division of the nervous system and hormones released by endocrine glands.
What are the four key functions of muscular tissue?
Producing body movements, stabilizing body positions, storing and moving substances within the body, and generating heat.
What do movements of the whole body, such as walking and running, and localized movements, such as grasping a pencil, keyboarding, or nodding the head, rely on?
The integrated functioning of skeletal muscles, bones, and joints.
What do skeletal muscle contractions stabilize?
Joints.
What do skeletal muscle contractions help maintain?
Body positions, such as standing or sitting.
What type of muscles contract continuously when you are awake?
Postural muscles.
What is an example of sustained contractions of neck muscles?
Holding your head upright when you are listening intently to your anatomy and physiology lecture.
How is storage accomplished within the body?
By sustained contractions of ringlike bands of smooth muscle called sphincters.
What do sphincters prevent?
Outflow of the contents of a hollow organ.
What makes temporary storage of food in the stomach or urine in the urinary bladder possible?
Smooth muscle sphincters closing off the outlets of these organs.
What pumps blood through the blood vessels of the body?
Cardiac muscle contractions in the wall of the heart.
What helps adjust blood vessel diameter and regulate the rate of blood flow?
Contraction and relaxation of smooth muscle in the walls of blood vessels.
What moves food and substances such as bile and enzymes through the digestive canal?
Smooth muscle contractions.
What pushes gametes (sperm and oocytes) through the passageways of the genital systems?
Smooth muscle contractions.
What propels urine through the urinary system?
Smooth muscle contractions.
What promotes the flow of lymph plasma and aids the return of blood in veins to the heart?
Skeletal muscle contractions.
What is produced when muscular tissue contracts?
Heat.
What is the process of heat production by muscle contractions called?
Thermogenesis.
What is much of the heat generated by muscle used for?
To maintain normal body temperature.
What are involuntary contractions of skeletal muscles known as?
Shivering.
What can increase the rate of heat production?
Shivering.
What enables muscular tissue to function and contribute to homeostasis?
Four special properties.
What are the four special properties of muscular tissue?
Electrical excitability, contractility, extensibility, and elasticity.
What is electrical excitability?
The ability to respond to certain stimuli by producing electrical signals called action potentials (impulses).
What are action potentials in muscles referred to as?
Muscle action potentials.
What are action potentials in nerve cells called?
Nerve action potentials.
What are the two main types of stimuli that trigger action potentials in muscle cells?
Autorhythmic electrical signals and chemical stimuli.
What is an example of an autorhythmic electrical signal?
The heart’s natural pacemaker.
What are examples of chemical stimuli that trigger action potentials?
Neurotransmitters released by neurons, hormones distributed by the blood, or local changes in pH.
What is contractility?
The ability of muscular tissue to contract forcefully when stimulated by a nerve impulse.
What does a skeletal muscle generate when it contracts?
Tension (force of contraction).
What happens if the tension generated by a skeletal muscle is great enough to overcome the resistance of the object being moved?
The muscle shortens and movement occurs.
What is an example of movement caused by muscle contraction?
Lifting a book off a desk.
What happens in some muscle contractions where the muscle develops tension but does not shorten?
The muscle holds the position without movement.
What is an example of a contraction where the muscle does not shorten?
Holding a book in your outstretched upper limb
What is extensibility?
The ability of muscular tissue to stretch, within limits, without being damaged.
What limits the range of extensibility and keeps it within the contractile range of the muscle cells?
The connective tissue within the muscle.
Which type of muscle is subject to the greatest amount of stretching?
Smooth muscle.
What is an example of smooth muscle stretching?
The smooth muscle in the wall of the stomach stretching when it fills with food.
When does cardiac muscle stretch?
Each time the heart fills with blood.
What is elasticity?
The ability of muscular tissue to return to its original length and shape after contraction or extension.
What type of muscle is the focus of much of this chapter?
Skeletal muscle.
What is each of your skeletal muscles?
A separate organ composed of hundreds to thousands of cells.
What are muscle fibers (myocytes)?
Cells of skeletal muscles.
Why are muscle fibers called myocytes?
Because of their elongated shapes.
What are two terms for the same structure in skeletal muscle?
Muscle cell and muscle fiber.
What does skeletal muscle also contain?
Connective tissues surrounding muscle fibers, and blood vessels and nerves.
What must you first understand to learn how contraction of skeletal muscle can generate tension?
Its gross and microscopic anatomy.
What surrounds and protects muscular tissue?
Connective tissue.
What is another name for subcutaneous tissue?
Hypodermis.
What does the subcutaneous tissue or hypodermis separate?
Muscle from skin.
What is the subcutaneous tissue composed of?
Areolar connective tissue and adipose tissue.
What does the subcutaneous tissue provide a pathway for?
Nerves, blood vessels, and lymphatic vessels to enter and exit muscles.
What does the adipose tissue of the subcutaneous tissue store?
Most of the body’s triglycerides.
What does the adipose tissue serve as?
An insulating layer that reduces heat loss.
What does the adipose tissue protect muscles from?
Physical trauma.
What is fascia?
A dense sheet or broad band of irregular connective tissue.
What does fascia line?
The body wall and limbs.
What does fascia support and surround?
Muscles and other organs of the body.
What does fascia hold together?
Muscles with similar functions.
What does fascia allow?
Free movement of muscles.
What does fascia carry?
Nerves, blood vessels, and lymphatic vessels.
What does fascia fill?
Spaces between muscles.
How many layers of connective tissue extend from the fascia to protect and strengthen skeletal muscle?
Three.
What is the epimysium?
The outer layer, encircling the entire muscle.
What type of connective tissue does the epimysium consist of?
Dense irregular connective tissue.
What is the perimysium?
A layer of dense irregular connective tissue that surrounds groups of 10 to 100 or more muscle fibers.
What does the perimysium separate muscle fibers into?
Bundles called muscle fascicles.
What is another name for muscle fascicles?
Muscle fasciculi.
What gives a cut of meat its characteristic “grain”?
Muscle fascicles.
What happens when a piece of meat is torn?
It rips apart along the muscle fascicles.
What is the endomysium?
A layer that penetrates the interior of each muscle fascicle and separates individual muscle fibers from one another.
What is the endomysium mostly composed of?
Reticular fibers.
What are the epimysium, perimysium, and endomysium continuous with?
The connective tissue that attaches skeletal muscle to other structures, such as bone or another muscle.
What do all three connective tissue layers extend beyond the muscle fibers to form?
A ropelike tendon.
What does a ropelike tendon attach?
A muscle to the periosteum of a bone.
What is an example of a ropelike tendon?
The calcaneal (Achilles) tendon of the gastrocnemius (calf) muscle.
What does the calcaneal (Achilles) tendon attach?
The gastrocnemius (calf) muscle to the calcaneus (heel bone).
What is an aponeurosis?
A broad, flat sheet of connective tissue.
What is an example of an aponeurosis?
The epicranial aponeurosis on top of the skull.
Where is the epicranial aponeurosis located?
Between the frontal and occipital bellies of the occipitofrontalis muscle.
It consists of individual muscle fibers bundled into muscle fascicles and surrounded by three connective tissue layers that are extensions of the fascia
Skeletal muscle
What are the 4 functions of Muscular Tissues
- Producing motions. 2. Stabilizing body positions. 3. Storing and moving substances within the body. 4. Generating heat (thermogenesis).
A chronic, painful, nonarticular rheumatic disorder that affects the fibrous connective tissue components of muscles, tendons, and ligaments.
Fibromyalgia
What does the term “algia” mean in fibromyalgia?
Painful condition.
What is a striking sign of fibromyalgia?
Pain that results from gentle pressure at specific “tender points”.
What symptoms are present in fibromyalgia even without pressure?
Pain, tenderness, and stiffness of muscles, tendons, and surrounding soft tissues.
Besides muscle pain, what other symptoms do those with fibromyalgia report?
Severe fatigue, poor sleep, headaches, depression, irritable bowel syndrome, and inability to carry out their daily activities.
Is there a specific identifiable cause of fibromyalgia?
No.
What does the treatment for fibromyalgia consist of?
Stress reduction, regular exercise, application of heat, gentle massage, physical therapy, medication for pain, and a low dose antidepressant to help improve sleep.
What are skeletal muscles well supplied with?
Nerves and blood vessels.
What generally accompanies each nerve that penetrates a skeletal muscle?
An artery and one or two veins.
What are the neurons that stimulate skeletal muscle to contract?
Somatic motor neurons.
What does each somatic motor neuron have?
A threadlike axon that extends from the brain or spinal cord to a group of skeletal muscle fibers.
What does the axon of a somatic motor neuron typically do?
It branches many times, each branch extending to a different skeletal muscle fiber.
What are the microscopic blood vessels that are plentiful in muscular tissue?
Blood capillaries.
What is each muscle fiber in close contact with?
One or more blood capillaries.
What do blood capillaries bring in?
Oxygen and nutrients.
What do blood capillaries remove?
Heat and the waste products of muscle metabolism.
What does a muscle fiber synthesize and use especially during contraction?
Considerable ATP (adenosine triphosphate).
What substances are delivered to the muscle fiber in the blood for ATP production?
Oxygen, glucose, fatty acids, and other substances.
What are the most important components of a skeletal muscle?
Muscle fibers.
What is the diameter of a mature skeletal muscle fiber?
10 to 100 µm.
What is the typical length of a mature skeletal muscle fiber?
About 10 cm (4 in.).
How long can some mature skeletal muscle fibers be?
As long as 30 cm (12 in.).
What do skeletal muscle fibers arise from during embryonic development?
The fusion of a hundred or more small mesodermal cells called myoblasts.
How many nuclei does each mature skeletal muscle fiber have?
A hundred or more.
What happens once fusion of myoblasts has occurred?
The muscle fiber loses its ability to undergo cell division.
When is the number of skeletal muscle fibers set?
Before you are born.
How long do most skeletal muscle fibers last?
A lifetime.
Where are the multiple nuclei of a skeletal muscle fiber located?
Just beneath the sarcolemma.
What is the sarcolemma?
The plasma membrane of a muscle fiber.
What are the tiny tube-shaped invaginations of the sarcolemma called?
T tubules (transverse tubules).
Where do T tubules tunnel from and to?
From the surface toward the center of each muscle fiber.
Why are T tubules filled with interstitial fluid?
Because they are open to the outside of the fiber.
How do muscle action potentials travel?
Along the sarcolemma and through the T tubules.
What does the arrangement of the T tubules ensure?
That an action potential excites all parts of the muscle fiber at essentially the same instant.
What is found within the sarcolemma?
The sarcoplasm.
What is the sarcoplasm?
The cytoplasm of a muscle fiber.
What does sarcoplasm include a substantial amount of?
Glycogen.
What is glycogen composed of?
Many glucose molecules.
What can glycogen be used for?
Synthesis of ATP.
What red-colored protein does the sarcoplasm contain?
Myoglobin.
Where is myoglobin found?
Only in muscle.
What does myoglobin bind to?
Oxygen molecules that diffuse into muscle fibers from interstitial fluid.
When does myoglobin release oxygen?
When it is needed by the mitochondria for ATP production.
Where do mitochondria lie in a muscle fiber?
In rows throughout the muscle fiber.
Why are mitochondria strategically close to contractile muscle proteins?
So that ATP can be produced quickly as needed during contraction.
What are the contractile elements of muscle fibers?
Myofibrils.
What do myofibrils contain?
Overlapping thick and thin filaments.
How does muscle growth occur after birth?
By enlargement of existing muscle fibers, called muscular hypertrophy.
What is muscular hypertrophy due to?
Increased production of myofibrils, mitochondria, sarcoplasmic reticulum, and other organelles.
What causes muscular hypertrophy?
Very forceful, repetitive muscular activity, such as strength training.
Why are hypertrophied muscles capable of more forceful contractions?
Because they contain more myofibrils.
What stimulates an increase in the size of skeletal muscle fibers during childhood?
Growth hormone and other hormones.
What hormone promotes further enlargement of muscle fibers?
Testosterone.
What are the few myoblasts that persist in mature skeletal muscle called?
Satellite cells.
What is the function of satellite cells?
They retain the capacity to fuse with one another or with damaged muscle fibers to regenerate functional muscle fibers.
What happens if the number of new skeletal muscle fibers formed by satellite cells is not enough?
The muscular tissue undergoes fibrosis, replacing muscle fibers with fibrous scar tissue.
What is muscular atrophy?
A decrease in the size of individual muscle fibers due to progressive loss of myofibrils.
What is disuse atrophy?
Atrophy that occurs because muscles are not used.
What causes disuse atrophy?
Reduced nerve impulses to inactive skeletal muscles.
Can disuse atrophy be reversed?
Yes, it is reversible.
What is denervation atrophy?
Atrophy that occurs when the nerve supply to a muscle is disrupted or cut.
What happens to the muscle in denervation atrophy?
Over 6 months to 2 years, the muscle shrinks to about one fourth its original size, and its fibers are irreversibly replaced by fibrous connective tissue
What does the sarcoplasm appear to be stuffed with at high magnification?
Little threads.
What are the small structures in the sarcoplasm called?
Myofibrils.
What are myofibrils?
The contractile organelles of skeletal muscle.
How large are myofibrils?
About 2 µm in diameter and extend the entire length of a muscle fiber.
What makes skeletal muscle fibers appear striped (striated)?
The prominent striations of myofibrils.
What is the sarcoplasmic reticulum (SR)?
A fluid-filled system of membranous sacs that encircles each myofibril.
What is the sarcoplasmic reticulum similar to in nonmuscular cells?
The smooth endoplasmic reticulum.
What are the dilated end sacs of the sarcoplasmic reticulum called?
Terminal cisterns.
What do terminal cisterns do?
They butt against the T tubule from both sides.
What is a triad?
A T tubule and the two terminal cisterns on either side of it.
What does the sarcoplasmic reticulum store in a relaxed muscle fiber?
Calcium ions (Ca²⁺).
What triggers muscle contraction?
The release of Ca²⁺ from the terminal cisterns of the sarcoplasmic reticulum.
What are the smaller protein structures within myofibrils called?
Filaments or myofilaments.
How large are thin filaments?
8 nm in diameter and 1–2 µm long.
What protein are thin filaments composed of?
Actin.
How large are thick filaments?
16 nm in diameter and 1–2 µm long.
What protein are thick filaments composed of?
Myosin.
What filaments are directly involved in the contractile process?
Both thin and thick filaments.
What is the ratio of thin to thick filaments in the regions of filament overlap?
Two thin filaments for every thick filament.
Do the filaments inside a myofibril extend the entire length of a muscle fiber?
No.
What are the compartments that filaments are arranged in called?
Sarcomeres.
What are sarcomeres?
The basic functional units of a myofibril.
What separates one sarcomere from the next?
Z discs.
How far does a sarcomere extend?
From one Z disc to the next Z disc.
What is the A band?
The darker middle part of the sarcomere that extends the entire length of the thick filaments.
What is the zone of overlap in the A band?
The area where thick and thin filaments lie side by side.
What is the I band?
A lighter, less dense area that contains thin filaments but no thick filaments.
What passes through the center of each I band?
A Z disc.
What creates the striations seen in myofibrils and muscle fibers?
The alternating dark A bands and light I bands.
What is the H band?
A narrow band in the center of each A band that contains thick filaments but no thin filaments.
What does the letter I in the I band represent?
Thin filaments.
What does the letter H in the H band represent?
Thick filaments.
What holds the thick filaments together at the center of the H band?
Supporting proteins.
What is the M line?
The middle of the sarcomere, where the supporting proteins hold the thick filaments together.
contain two types of filaments: thick filaments and thin filaments.
Myofibrils
Narrow, plate shaped regions of dense material that separate one sarcomere from the next.
Z discs
Dark, middle part of sarcomere that extends entire length of thick filaments and includes those parts of thin filaments that overlap thick filaments.
A band
Lighter, less dense area of sarcomere that contains remainder of thin filaments but no thick filaments. A Z disc passes through center of each I band.
I band
Narrow region in center of each A band that contains thick filaments but no thin filaments
H band
Region in center of H zone that contains proteins that hold thick filaments together at center of sarcomere.
M line
What are the three kinds of proteins that myofibrils are built from?
Contractile proteins, regulatory proteins, and structural proteins.
What is the function of contractile proteins?
Generate force during contraction.
What is the function of regulatory proteins?
Help switch the contraction process on and off.
What is the function of structural proteins?
Keep the thick and thin filaments in the proper alignment, give the myofibril elasticity and extensibility, and link the myofibrils to the sarcolemma and extracellular matrix.
What are the two contractile proteins in muscle?
Myosin and actin.
What is the main component of thick filaments?
Myosin.
What is the function of myosin?
Functions as a motor protein in all three types of muscle tissue.
What do motor proteins do?
Pull cellular structures to achieve movement by converting the chemical energy in ATP to mechanical energy.
How many myosin molecules form a single thick filament?
About 300.
What is the shape of a myosin molecule?
Two golf clubs twisted together.
What does the myosin tail point toward?
The M line in the center of the sarcomere.
What are the myosin heads?
The two projections of each myosin molecule.
What are the two binding sites on each myosin head?
An actin-binding site and an ATP-binding site.
What is the function of the ATP-binding site on myosin?
Functions as an ATPase, an enzyme that hydrolyzes ATP to generate energy for muscle contraction.
How do the myosin heads project outward?
In a spiraling fashion, each extending toward one of the six thin filaments surrounding each thick filament.
What is the main component of thin filaments?
Actin.
How is an actin filament formed?
By individual actin molecules twisted into a helix.
What is located on each actin molecule?
A myosin-binding site.
What are the two regulatory proteins in the thin filament?
Tropomyosin and troponin.
What prevents myosin from binding to actin in relaxed muscle?
Tropomyosin covers the myosin-binding sites on actin.
What holds tropomyosin in place?
Troponin molecules.
What happens when calcium ions (Ca2+) bind to troponin?
Troponin undergoes a conformational change, moving tropomyosin away from myosin-binding sites on actin, allowing muscle contraction to begin.
What do structural proteins contribute to?
Alignment, stability, elasticity, and extensibility of myofibrils.
What are some key structural proteins?
Titin, α-actinin, myomesin, nebulin, and dystrophin.
What is the third most plentiful protein in skeletal muscle?
Titin.
What is the molecular mass of titin?
About 3 million daltons.
How far does a titin molecule span?
Half a sarcomere, from a Z disc to an M line.
What does titin help with?
Stabilizing the position of the thick filament and providing elasticity and extensibility.
What does titin help prevent?
Overextension of sarcomeres.
What does α-actinin do?
Binds to actin molecules of the thin filament and to titin.
What is the function of myomesin?
Forms the M line and holds the thick filaments in alignment at the M line.
What is the function of nebulin?
Anchors the thin filaments to the Z discs and regulates their length during development.
What is the function of dystrophin?
Links thin filaments to integral membrane proteins of the sarcolemma.
What does dystrophin help with?
Reinforcing the sarcolemma and transmitting tension from sarcomeres to tendons.
What are the two contractile proteins?
Myosin and actin.
What is the function of contractile proteins?
Generate force during contraction.
What are the two regulatory proteins?
Troponin and tropomyosin.
What is the function of regulatory proteins?
Help switch contraction on and off.
Proteins that generate force during muscle contractions
Contractile Proteins
Contractile protein that makes up thick filament; molecule consists of a tail and two myosin heads, which bind to myosin binding sites on actin molecules of thin filament during muscle contraction.
Myosin
Contractile protein that is the main component of thin filament; each actin molecule has a myosin binding site where myosin head of thick filament binds during muscle contraction.
Actin
Proteins that help switch muscle contraction process on and off
Regulatory proteins
Regulatory protein that is a component of thin filament; when skeletal muscle fiber is relaxed, tropomyosin covers myosin binding sites on actin molecules, thereby preventing myosin from binding to actin.
Tropomyosin
Regulatory protein that is a component of thin filament; when calcium ions (Ca2+) bind to troponin, it changes shape; this conformational change moves tropomyosin away from myosin binding sites on actin molecules, and muscle contraction subsequently begins as myosin binds to actin.
Troponin
Proteins that keep thick and thin filaments of myofibrils in proper alignment, give myofibrils elasticity and extensibility, and link myofibrils to sarcolemma and extracellular matrix.
Structural proteins
Structural protein that connects Z disc to M line of sarcomere, thereby helping to stabilize thick filament position; can stretch and then spring back unharmed, and thus accounts for much of the elasticity and extensibility of myofibrils.
Titin
Structural protein of Z discs that attaches to actin molecules of thin filaments and to titin molecule
a-actinin
Structural protein that forms M line of sarcomere; binds to titin molecules and connects adjacent thick filaments to one another
Myomesin
Structural protein that wraps around entire length of each thin filament; helps anchor thin filaments to Z discs and regulates length of thin filaments during development
Nebulin
Structural protein that links thin filaments of sarcomere to integral membrane proteins in sarcolemma, which are attached in turn to proteins in connective tissue matrix that surrounds muscle fibers; thought to help reinforce sarcolemma and help transmit tension generated by sarcomeres to tendons
Dystropin
Organ made up of muscle fascicles that contain muscle fibers, blood vessels, and nerves; wrapped in epimysium.
Skeletal muscle
Bundle of muscle fibers wrapped in perimysium.
Muscle fascicle
Long cylindrical cell covered by endomysium and sarcolemma; contains sarcoplasm, myofibrils, many peripherally located nuclei, mitochondria, T tubules, sarcoplasmic reticulum, and terminal cisterns. The muscle fiber has a striated appearance.
Muscle fiber
Threadlike contractile elements within sarcoplasm of muscle fiber that extend entire length of fiber; composed of filaments.
Myofibril
Contractile proteins within myofibrils that are of two types: thick filaments composed of myosin and thin filaments composed of actin, tropomyosin, and troponin; sliding of thin filaments past thick filaments produces muscle shortening.
Filaments (myofilaments)
What did scientists discover when examining the first electron micrographs of skeletal muscle?
The lengths of the thick and thin filaments were the same in both relaxed and contracted muscle.
What was originally thought about muscle contraction?
It was thought to be a folding process, like closing an accordion.
Why does skeletal muscle shorten during contraction?
Because the thick and thin filaments slide past one another.
What is the model describing this process called?
The sliding filament mechanism.
Why does muscle contraction occur?
Because myosin heads attach to and “walk” along the thin filaments at both ends of a sarcomere, progressively pulling the thin filaments toward the M line.
What happens as a result of the thin filaments sliding inward?
The thin filaments meet at the center of a sarcomere and may even overlap.
What bands or zones narrow and eventually disappear when the muscle is maximally contracted?
The I band and H zone.
Which band remains unchanged during contraction?
The width of the A band.
Do the individual lengths of the thick and thin filaments change during contraction?
No, the individual lengths of the thick and thin filaments remain unchanged.
What happens to the Z discs when the thin filaments slide inward?
The Z discs come closer together.
What causes the shortening of the whole muscle fiber?
The shortening of the sarcomeres.
What does the shortening of the whole muscle fiber lead to?
The shortening of the entire muscle
During __, thin filaments move toward the M line of each sarcomere.
muscle contractions
At the onset of contraction, what does the sarcoplasmic reticulum release into the sarcoplasm?
Calcium ions (Ca2+)
What do the calcium ions (Ca2+) bind to?
Troponin
What does troponin move away from the myosin binding sites on actin?
Tropomyosin
When the binding sites are “free,” what begins?
The contraction cycle
What is the contraction cycle?
The repeating sequence of events that causes the filaments to slide
How many steps does the contraction cycle consist of?
Four
What is the first step of the contraction cycle?
ATP hydrolysis
What does a myosin head include that functions as an ATPase?
An ATP binding site
What does ATPase do?
Hydrolyzes ATP into ADP (adenosine diphosphate) and a phosphate group
What happens to the energy generated from ATP hydrolysis?
It is stored in the myosin head for later use during the contraction cycle
When is the myosin head said to be energized?
When it contains stored energy
What position does the energized myosin head assume?
A “cocked” position, like a stretched spring
At what angle is the myosin head positioned relative to the thick and thin filaments?
90°
What remains attached to the myosin head after ATP hydrolysis?
ADP and a phosphate group
What is the second step of the contraction cycle?
Attachment of myosin to actin
What does the energized myosin head attach to?
The myosin binding site on actin
What is released when the myosin head attaches to actin?
The previously hydrolyzed phosphate group
What is the myosin head referred to as when it attaches to actin?
A cross-bridge
How many heads of a single myosin molecule bind to actin at a time?
Only one head
What is the third step of the contraction cycle?
Power stroke
After a cross-bridge forms, what happens to the myosin head?
It pivots, changing its position from a 90° angle to a 45° angle
What does the myosin head pull during the power stroke?
The thin filament past the thick filament toward the center of the sarcomere
What is generated during the power stroke?
Tension (force)
Where does the energy for the power stroke come from?
The energy stored in the myosin head from the hydrolysis of ATP
What is released from the myosin head after the power stroke?
ADP
What is the fourth step of the contraction cycle?
Detachment of myosin from actin
When does the cross-bridge remain firmly attached to actin?
Until it binds another molecule of ATP
What happens when ATP binds to the ATP binding site on the myosin head?
The myosin head detaches from actin
What enzyme hydrolyzes the newly bound ATP molecule?
Myosin ATPase
How long does the contraction cycle continue?
As long as ATP is available and the Ca2+ level near the thin filament is sufficiently high
What do the cross-bridges keep doing with each power stroke?
Rotating back and forth, pulling the thin filaments toward the M line
How many times per second does each of the 600 cross-bridges in one thick filament attach and detach?
About five times per second
What happens at any one instant during contraction?
Some myosin heads are attached to actin, forming cross-bridges and generating force, while others are detached, getting ready to bind again
What does the movement of cross-bridges apply as the contraction cycle continues?
The force that draws the Z discs toward each other
What happens to the sarcomere as the Z discs move closer together?
The sarcomere shortens
During a maximal muscle contraction, by how much can the distance between two Z discs decrease?
To half the resting length
What do the Z discs pull on as they shorten?
Neighboring sarcomeres
What happens when neighboring sarcomeres shorten?
The whole muscle fiber shortens
What are some of the elastic components in a muscle?
Titin molecules, connective tissue around the muscle fibers (endomysium, perimysium, and epimysium), and tendons that attach muscle to bone
What do skeletal muscle fibers first pull on as they shorten?
Their connective tissue coverings and tendons
What happens to the connective tissue coverings and tendons during muscle contraction?
They stretch and then become taut
What does the tension passed through the tendons do?
Pulls on the bones to which they are attached
What is the result of the tension pulling on the bones?
Movement of a part of the body
Does the contraction cycle always result in shortening of the muscle fibers and the whole muscle?
No
What happens in some contractions when the cross-bridges rotate and generate tension?
The thin filaments cannot slide inward because the tension generated is not large enough to move the load on the muscle
What is an example of a contraction where the thin filaments cannot slide inward?
Trying to lift a whole box of books with one hand
During the power stroke of contraction, what do cross-bridges do?
Rotate and move the thin filaments past the thick filaments toward the center of the sarcomere
What starts muscle contraction?
An increase in Ca2+ concentration in the sarcoplasm
What stops muscle contraction?
A decrease in Ca2+ concentration in the sarcoplasm
What is the concentration of Ca2+ in the sarcoplasm when a muscle fiber is relaxed?
0.1 micromole per liter (0.1 µmol/L)
Where is a huge amount of Ca2+ stored?
Inside the sarcoplasmic reticulum
What happens when a muscle action potential propagates along the sarcolemma and into the T tubules?
It causes the release of Ca2+ from the SR into the sarcoplasm and this triggers muscle contraction.
What is the sequence of events that links excitation to contraction called?
Excitation–contraction coupling
Where does excitation–contraction coupling occur?
At the triads of the skeletal muscle fiber
What does a triad consist of?
A T tubule and two opposing terminal cisterns of the sarcoplasmic reticulum (SR)
What are the two groups of integral membrane proteins that link the T tubule and terminal cisterns?
Voltage-gated Ca2+ channels and Ca2+ release channels
Where are voltage-gated Ca2+ channels located?
In the T tubule membrane
How are voltage-gated Ca2+ channels arranged?
In clusters of four known as tetrads
What is the main role of voltage-gated Ca2+ channels in excitation–contraction coupling?
To serve as voltage sensors that trigger the opening of the Ca2+ release channels
Where are Ca2+ release channels located?
In the terminal cisternal membrane of the SR
What prevents Ca2+ from leaving the SR when a skeletal muscle fiber is at rest?
A given cluster of voltage-gated Ca2+ channels blocking the Ca2+ release channels
What happens when an action potential travels along the T tubule?
The voltage-gated Ca2+ channels detect the change in voltage and undergo a conformational change that ultimately causes the Ca2+ release channels to open.
What happens when Ca2+ release channels open?
Large amounts of Ca2+ flow out of the SR into the sarcoplasm around the thick and thin filaments.
By how much does the Ca2+ concentration in the sarcoplasm increase when Ca2+ release channels open?
Tenfold or more
What does released Ca2+ combine with?
Troponin
What happens when troponin undergoes a conformational change?
Tropomyosin moves away from the myosin binding sites on actin.
What happens when myosin binding sites on actin are free?
Myosin heads bind to them to form cross bridges, and the muscle fiber contracts.
What do Ca2+-ATPase pumps do?
Use ATP to constantly transport Ca2+ from the sarcoplasm into the SR
When do Ca2+ release channels remain open?
As long as muscle action potentials continue to propagate along the T tubules
What happens after the last action potential has propagated throughout the T tubules?
The Ca2+ release channels close.
What happens to Ca2+ as Ca2+-ATPase pumps move it back into the SR?
The Ca2+ level in the sarcoplasm rapidly decreases.
What protein binds to Ca2+ inside the SR?
Calsequestrin
What does calsequestrin allow the SR to do?
Sequester (store) even more Ca2+
How much higher is the concentration of Ca2+ in the SR compared to the sarcoplasm in a relaxed muscle fiber?
10,000 times higher
What happens when the Ca2+ level in the sarcoplasm decreases?
Ca2+ is released from troponin, tropomyosin covers the myosin binding sites on actin, and the muscle fiber relaxes.
What is electrodiagnostic medicine concerned with?
The diagnosis of neuromuscular disorders
What are the two tests that are components of electrodiagnostic medicine?
Nerve conduction velocity studies and muscle response studies
What do nerve conduction velocity (NCV) tests measure?
The speed of nerve impulses conducted through nerves outside the brain and spinal cord
What is an example of nerves that NCV tests examine?
Nerves of your limbs
How are NCV studies performed?
By stimulating a nerve with an electrical impulse applied to the skin and recording the response from a muscle or another portion of a nerve through patches placed on the skin
What conditions are diagnosed using NCV tests?
Carpal tunnel syndrome, herniated discs, and sciatica
What test is performed after a nerve conduction velocity test?
Electromyography (EMG)
What serves as a recording device in an EMG test?
A very thin needle
Where is the recording needle placed in an EMG test?
Through the skin into a muscle
What is the needle in an EMG connected to?
The screen of a device (an oscilloscope)
What does resting muscle produce in an EMG?
No electrical activity
What happens to electrical activity as muscle contraction becomes more forceful?
The level of electrical activity increases
What does the client do once the needle is in place during an EMG?
Contract a muscle
How is muscle activity recorded in an EMG?
On the screen and may also be detected audibly with a speaker
What disorders are diagnosed using EMG?
Muscular dystrophy, myasthenia gravis, and amyotrophic lateral sclerosis
An increase in the Ca2+ level in the sarcoplasm starts the ___. When the level of Ca2+ in the sarcoplasm declines, sliding stops.
sliding of thin filaments
What does the length–tension relationship for skeletal muscle indicate?
How the forcefulness of muscle contraction depends on the length of the sarcomeres within a muscle before contraction begins
At what sarcomere length is the zone of overlap optimal for maximum tension?
2.0–2.4 µm
What is the resting length of sarcomeres in most muscles?
Very close to 2.0–2.4 µm
When does maximum tension (100%) occur?
When the zone of overlap between a thick and thin filament extends from the edge of the H zone to one end of a thick filament
What happens when sarcomeres are stretched to a longer length?
The zone of overlap shortens, and fewer myosin heads can make contact with thin filaments
What happens to tension as sarcomere length increases?
The tension the fiber can produce decreases
What happens when a skeletal muscle fiber is stretched to 170% of its optimal length?
There is no overlap between the thick and thin filaments
Why is tension zero at 170% of the optimal sarcomere length?
Because none of the myosin heads can bind to thin filaments, the muscle fiber cannot contract
What happens when sarcomere lengths become shorter than the optimum?
The tension that can develop decreases
Why does tension decrease when sarcomeres are shorter than the optimum?
Because thick filaments crumple as they are compressed by the Z discs, resulting in fewer myosin heads making contact with thin filaments
How is resting muscle fiber length maintained near the optimum?
By firm attachments of skeletal muscle to bones (via their tendons) and to other inelastic tissues
What happens to cellular membranes after death?
They become leaky
What leaks out of the sarcoplasmic reticulum into the sarcoplasm after death?
Calcium ions
What do calcium ions allow myosin heads to do after death?
Bind to actin
Why can’t the cross bridges detach from actin after death?
Because ATP synthesis ceases shortly after breathing stops
What is the condition in which muscles remain rigid after death?
Rigor mortis
When does rigor mortis begin?
3–4 hours after death
How long does rigor mortis last?
About 24 hours
Why does rigor mortis disappear?
Because proteolytic enzymes from lysosomes digest the cross bridges
develops its greatest tension when there is an optimal zone of overlap between thick and thin filaments.
Muscle fiber
What are the neurons that stimulate skeletal muscle fibers to contract called?
Somatic motor neurons
What does each somatic motor neuron have that extends from the brain or spinal cord to a group of skeletal muscle fibers?
A threadlike axon
In response to what does a muscle fiber contract?
One or more action potentials propagating along its sarcolemma and through its system of T tubules
Where do muscle action potentials arise?
At the neuromuscular junction (NMJ) or neuromuscular synapse (NMS)
What is a synapse?
A region where communication occurs between two neurons, or between a neuron and a target cell
What is the small gap that separates two cells at most synapses called?
Synaptic cleft
Why can’t an action potential “jump the gap” from one cell to another?
Because the cells do not physically touch
How does the first cell communicate with the second cell at a synapse?
By releasing a chemical messenger called a neurotransmitter
What is the end of the motor neuron called?
Axon terminal
What do the axon terminal divide into at the NMJ?
A cluster of synaptic end bulbs
What are the membrane-enclosed sacs inside the synaptic end bulbs called?
Synaptic vesicles
What neurotransmitter is found inside each synaptic vesicle?
Acetylcholine (ACh)
What is the region of the sarcolemma opposite the synaptic end bulbs called?
Motor end plate
How many acetylcholine receptors are found in each motor end plate?
30 million to 40 million
What type of ion channels are ACh receptors?
Ligand-gated ion channels
What are the components of an NMJ?
All of the synaptic end bulbs, the synaptic cleft, and the motor end plate
What is the first step in eliciting a muscle action potential?
Release of acetylcholine
What stimulates voltage-gated channels to open at the synaptic end bulbs?
Arrival of the nerve impulse
Why does Ca2+ flow inward through the open voltage-gated channels?
Because calcium ions are more concentrated in the extracellular fluid
What process do synaptic vesicles undergo to release ACh into the synaptic cleft?
Exocytosis
What happens after ACh diffuses across the synaptic cleft?
Activation of ACh receptors
How many ACh molecules must bind to the receptor on the motor end plate to open an ion channel?
Two
What happens when the ion channel in the ACh receptor opens?
Small cations, most importantly Na+, flow across the membrane
What does the inflow of Na+ into the muscle fiber cause?
Production of muscle action potential
What does the production of a muscle action potential cause?
The sarcoplasmic reticulum to release stored Ca2+ into the sarcoplasm
What enzyme rapidly breaks down ACh?
Acetylcholinesterase (AChE)
Where is AChE located?
On the extracellular side of the motor end plate membrane
What are the breakdown products of ACh by AChE?
Acetyl and choline
When do action potentials in the motor neuron cease?
When ACh is no longer released and AChE breaks down the ACh already present
What happens after the last muscle action potential is produced?
Ca2+ moves from the sarcoplasm back into the sarcoplasmic reticulum
How many NMJs does a skeletal muscle fiber have?
Only one
Where is the NMJ usually located on a skeletal muscle fiber?
Near the midpoint of the fiber
What does this location of the NMJ allow?
Nearly simultaneous activation of all parts of the muscle fiber
What bacterial toxin blocks exocytosis of synaptic vesicles at the NMJ?
Botulinum toxin
What bacterium produces botulinum toxin?
Clostridium botulinum
What happens when botulinum toxin blocks ACh release?
Muscle contraction does not occur
Why can botulinum toxin be lethal?
It paralyzes skeletal muscles, including respiratory muscles, stopping breathing
What is Botox® used for?
To treat strabismus, blepharospasm, vocal cord spasms, chronic back pain, and facial wrinkles
What plant derivative causes muscle paralysis by blocking ACh receptors?
Curare
What is curare used for in medicine?
To relax skeletal muscles during surgery
What family of chemicals slows the enzymatic activity of AChE?
Anticholinesterase agents
What effect do anticholinesterase agents have?
They slow the removal of ACh from the synaptic cleft, strengthening weak muscle contractions
What is an example of an anticholinesterase agent?
Neostigmine
What condition is neostigmine used to treat?
Myasthenia gravis
What else is neostigmine used for?
As an antidote for curare poisoning and to terminate the effects of curare-like drugs after surgery
Located at the tips of axon terminals that contain synaptic vesicles filled with acetylcholine (ACh)
Synaptic end bulbs
released at the neuromuscular junction triggers a muscle action potential, which leads to muscle contraction
Acetylcholine
Unlike most cells of the body, what do skeletal muscle fibers often switch between?
A low level of activity when they are relaxed and using only a modest amount of ATP, and a high level of activity when they are contracting and using ATP at a rapid pace.
What is needed in a huge amount to power the contraction cycle, pump Ca2+ into the sarcoplasmic reticulum, and for other metabolic reactions involved in muscle contraction?
ATP
How long is the ATP present inside muscle fibers enough to power contraction?
Only a few seconds
If muscle contractions continue past a few seconds, what must the muscle fibers do?
Make more ATP
What are the three ways muscle fibers produce ATP?
(1) From creatine phosphate, (2) by anaerobic glycolysis, and (3) by aerobic respiration
What is unique to muscle fibers for ATP production?
The use of creatine phosphate
What ATP production methods can all body cells use?
Anaerobic glycolysis and aerobic respiration
What do muscle fibers do while they are relaxed?
They produce more ATP than they need for resting metabolism.
What is most of the excess ATP used for?
It is used to synthesize creatine phosphate, an energy-rich molecule found in muscle fibers.
What enzyme catalyzes the transfer of a high-energy phosphate group from ATP to creatine?
Creatine kinase (CK) catalyzes the transfer.
What is formed when creatine kinase catalyzes the transfer of a phosphate group from ATP to creatine?
Creatine phosphate and ADP are formed.
What is creatine?
Creatine is a small, amino acid–like molecule synthesized in the liver, kidneys, and pancreas and then transported to muscle fibers.
How much more plentiful is creatine phosphate than ATP in the sarcoplasm of a relaxed muscle fiber?
Creatine phosphate is three to six times more plentiful than ATP.
What happens when muscle contraction begins and the ADP level starts to rise?
Creatine kinase (CK) catalyzes the transfer of a high-energy phosphate group from creatine phosphate back to ADP.
What does this direct phosphorylation reaction quickly generate?
It quickly generates new ATP molecules.
Why is creatine phosphate the first source of energy when muscle contraction begins?
The formation of ATP from creatine phosphate occurs very rapidly.
How do anaerobic glycolysis and aerobic respiration compare to creatine phosphate in ATP production?
These mechanisms take a longer period of time to produce ATP compared to creatine phosphate.
How long can stores of creatine phosphate and ATP provide energy for maximal muscle contraction?
They provide enough energy for muscles to contract maximally for about 15 seconds.
What are the sources of creatine?
Creatine is both synthesized in the body and derived from foods such as milk, red meat, and some fish.
How much creatine do adults need to synthesize and ingest daily to make up for urinary loss?
Adults need to synthesize and ingest a total of about 2 grams of creatine daily.
What is creatinine?
Creatinine is the breakdown product of creatine.
What type of movements have some studies shown to improve with creatine supplementation?
Some studies have demonstrated improved performance from creatine supplementation during explosive movements, such as sprinting.
Have all studies found a performance-enhancing effect of creatine supplementation?
No, other studies have failed to find a performance-enhancing effect of creatine supplementation.
What happens when extra creatine is ingested?
Ingesting extra creatine decreases the body’s own synthesis of creatine.
Is it known whether natural creatine synthesis recovers after long-term creatine supplementation?
No, it is not known whether natural synthesis recovers after long-term creatine supplementation.
What are the potential negative effects of creatine supplementation?
Creatine supplementation can cause dehydration and may cause kidney dysfunction.
What is needed to determine the long-term effects of creatine supplementation?
Further research is needed to determine both the long-term safety and the value of creatine supplementation.
During a long-term event such as a marathon race, how is most ATP produced?
Most ATP is produced aerobically.
When muscle activity continues and the supply of creatine phosphate within the muscle fiber is depleted, what is catabolized to generate ATP?
Glucose is catabolized to generate ATP.
How does glucose pass into contracting muscle fibers?
Glucose passes easily from the blood into contracting muscle fibers via facilitated diffusion.
How else is glucose produced within muscle fibers?
Glucose is also produced by the breakdown of glycogen within muscle fibers.
What is the series of reactions that quickly break down each glucose molecule into two molecules of pyruvic acid?
The series of reactions is called glycolysis.
Where does glycolysis occur, and how many molecules of ATP does it produce?
Glycolysis occurs in the cytosol and produces a net gain of two molecules of ATP.
Does glycolysis require oxygen?
No, glycolysis does not require oxygen and can occur under aerobic or anaerobic conditions.
What happens to pyruvic acid under normal conditions?
Under normal conditions, pyruvic acid enters the mitochondria and undergoes aerobic respiration to produce a large amount of ATP.
What happens to pyruvic acid during heavy exercise when oxygen is unavailable?
Under anaerobic conditions, pyruvic acid is converted into lactic acid.
What is the process by which the breakdown of glucose gives rise to lactic acid in the absence of oxygen?
The process is called anaerobic glycolysis.
How many molecules of lactic acid and ATP are produced from one molecule of glucose via anaerobic glycolysis?
Anaerobic glycolysis yields 2 molecules of lactic acid and 2 molecules of ATP.
Where does most of the lactic acid produced by anaerobic glycolysis go?
Most of the lactic acid diffuses out of the skeletal muscle fiber into the blood.
What can liver cells do with lactic acid from the bloodstream?
Liver cells can convert lactic acid back into glucose.
What does the conversion of lactic acid in the liver do to the blood?
The conversion reduces the acidity of the blood.
What is thought to cause muscle soreness during strenuous exercise?
The buildup of lactic acid in active skeletal muscle fibers and in the bloodstream is thought to cause muscle soreness.
Compared to aerobic respiration, how does anaerobic glycolysis perform in terms of ATP production?
Anaerobic glycolysis produces fewer ATPs but is faster and can occur when oxygen levels are low.
How long can anaerobic glycolysis provide energy for maximal muscle activity?
Anaerobic glycolysis provides enough energy for about 2 minutes of maximal muscle activity.
If sufficient oxygen is present, where does pyruvic acid formed by glycolysis go?
Pyruvic acid enters the mitochondria.
What does pyruvic acid undergo in the mitochondria when oxygen is present?
Pyruvic acid undergoes aerobic respiration.
What are the oxygen-requiring reactions that make up aerobic respiration?
The Krebs cycle and the electron transport chain.
What does aerobic respiration produce?
ATP, carbon dioxide, water, and heat.
What happens when oxygen is present in muscle cells?
Glycolysis, the Krebs cycle, and the electron transport chain occur.
