MUSCULAR TISSUE PT. 2 Flashcards
Compared to anaerobic glycolysis, how does aerobic respiration perform?
Aerobic respiration is slower but yields much more ATP.
How many molecules of ATP are produced from one molecule of glucose under aerobic conditions?
30 or 32 molecules of ATP.
What are the two sources of oxygen for muscular tissue?
Oxygen that diffuses from the blood and oxygen released by myoglobin within muscle fibers.
What are the oxygen-binding proteins found in muscle and red blood cells?
Myoglobin (in muscle cells) and hemoglobin (in red blood cells).
When do myoglobin and hemoglobin bind and release oxygen?
They bind oxygen when it is plentiful and release it when it is scarce.
When does aerobic respiration supply enough ATP for muscles?
During periods of rest or light to moderate exercise, provided sufficient oxygen and nutrients are available.
What nutrients are used in aerobic respiration?
Pyruvic acid (from glycolysis), fatty acids (from triglyceride breakdown), and amino acids (from protein breakdown).
During what type of activities does aerobic respiration provide nearly all the ATP needed?
Activities lasting from several minutes to an hour or more.
What is the inability of a muscle to maintain force of contraction after prolonged activity called?
Muscle fatigue.
What mainly causes muscle fatigue?
Changes within muscle fibers.
What is the feeling of tiredness and the desire to cease activity before actual muscle fatigue called?
Central fatigue.
What causes central fatigue?
Changes in the central nervous system (brain and spinal cord).
What might be the purpose of central fatigue?
It may be a protective mechanism to stop a person from exercising before muscles become damaged.
Do all types of skeletal muscle fibers fatigue at the same rate?
No, certain types of skeletal muscle fibers fatigue more quickly than others.
What is one factor that contributes to muscle fatigue?
Inadequate release of calcium ions from the SR, leading to a decline of Ca²⁺ concentration in the sarcoplasm.
How does depletion of creatine phosphate relate to muscle fatigue?
Depletion of creatine phosphate is associated with fatigue, but ATP levels in fatigued muscle are not much lower than in resting muscle.
What are some other factors that contribute to muscle fatigue?
Insufficient oxygen, depletion of glycogen and other nutrients, buildup of lactic acid and ADP, and failure of action potentials in the motor neuron to release enough acetylcholine.
What enhances oxygen delivery to muscle tissue during prolonged periods of muscle contraction?
Increases in breathing rate and blood flow.
What happens to breathing after muscle contraction has stopped?
Heavy breathing continues for a while, and oxygen consumption remains above the resting level.
How long can the recovery period last depending on the intensity of exercise?
It may be a few minutes or several hours.
What term refers to the added oxygen taken into the body after exercise?
Oxygen debt.
What are the three ways the extra oxygen taken in after exercise is used to restore metabolic conditions?
(1) To convert lactic acid back into glycogen in the liver, (2) to resynthesize creatine phosphate and ATP in muscle fibers, and (3) to replace the oxygen removed from myoglobin.
Does glycogen resynthesis from lactic acid account for most of the extra oxygen used after exercise?
No, only a small amount of glycogen is resynthesized from lactic acid.
How is most glycogen replenished after exercise?
It is made much later from dietary carbohydrates.
What happens to most of the lactic acid that remains after exercise?
It is converted back to pyruvic acid and used for ATP production via aerobic respiration in the heart, liver, kidneys, and skeletal muscle.
What are three ongoing changes that boost oxygen use after exercise?
(1) Elevated body temperature increases the rate of chemical reactions, (2) heart and breathing muscles still work harder and consume more ATP, and (3) tissue repair processes occur at an increased pace.
What term is better than “oxygen debt” to describe the elevated use of oxygen after exercise?
Recovery oxygen uptake.
What does a single nerve impulse in a somatic motor neuron elicit in all skeletal muscle fibers with which it forms synapses?
A single muscle action potential.
Do action potentials vary in size in a given neuron or muscle fiber?
No, action potentials always have the same size in a given neuron or muscle fiber.
Does the force of muscle fiber contraction vary?
Yes, the force of muscle fiber contraction does vary.
What is a muscle fiber capable of producing compared to a single action potential?
A much greater force than the one that results from a single action potential.
What mainly determines the total force or tension that a single muscle fiber can produce?
The rate at which nerve impulses arrive at the neuromuscular junction.
What is the frequency of stimulation?
The number of impulses per second.
What factors affect maximum tension besides the frequency of stimulation?
The amount of stretch before contraction and nutrient and oxygen availability.
What determines the total tension a whole muscle can produce?
The number of muscle fibers that are contracting in unison.
How many neuromuscular junctions does each skeletal muscle fiber have?
A single neuromuscular junction.
What does the axon of a somatic motor neuron do?
It branches out and forms neuromuscular junctions with many different muscle fibers.
What is a motor unit?
A somatic motor neuron plus all of the skeletal muscle fibers it stimulates.
How many skeletal muscle fibers does a single somatic motor neuron make contact with on average?
150 skeletal muscle fibers.
How do the muscle fibers in one motor unit contract?
They contract in unison.
How are the muscle fibers of a motor unit typically arranged within a muscle?
They are dispersed throughout a muscle rather than clustered together.
What kind of motor units do whole muscles that control precise movements consist of?
Many small motor units.
How many muscle fibers per motor unit do the muscles of the larynx have?
As few as two or three muscle fibers per motor unit.
How many muscle fibers per motor unit do the muscles controlling eye movements have?
10 to 20 muscle fibers per motor unit.
Which skeletal muscles have large motor units with 2000 to 3000 muscle fibers?
The biceps brachii and the gastrocnemius muscle.
Why does the total strength of a contraction vary?
It depends on the size of the motor units and the number that are activated at a given time.
consists of a somatic motor neuron plus all of the muscle fibers it stimulates.
Motor unit
What is a twitch contraction?
The brief contraction of all muscle fibers in a motor unit in response to a single action potential in its motor neuron.
How can a twitch be produced in the laboratory?
By direct electrical stimulation of a motor neuron or its muscle fibers.
What is the record of a muscle contraction called?
A myogram.
How long do twitches of skeletal muscle fibers last?
Anywhere from 20 to 200 msec.
How long does a muscle action potential last?
1–2 msec.
What is the brief delay between the application of the stimulus and the beginning of contraction called?
The latent period.
How long does the latent period last?
About 2 msec.
What happens during the latent period?
The muscle action potential sweeps over the sarcolemma and calcium ions are released from the sarcoplasmic reticulum.
What is the second phase of a twitch contraction?
The contraction period.
How long does the contraction period last?
10–100 msec.
What happens during the contraction period?
Ca2+ binds to troponin, myosin binding sites on actin are exposed, and cross-bridges form.
When does peak tension develop in the muscle fiber?
During the contraction period.
What is the third phase of a twitch contraction?
The relaxation period.
How long does the relaxation period last?
10–100 msec.
What happens during the relaxation period?
Ca2+ is actively transported back into the sarcoplasmic reticulum, myosin binding sites are covered by tropomyosin, myosin heads detach from actin, and tension in the muscle fiber decreases.
What determines the actual duration of these periods?
The type of skeletal muscle fiber.
What type of fibers have contraction periods as brief as 10 msec?
Fast twitch fibers that move the eyes.
What type of fibers have contraction and relaxation periods of about 100 msec each?
Slow twitch fibers that move the legs.
What happens if two stimuli are applied one immediately after the other?
The muscle will respond to the first stimulus but not to the second.
What is the period of lost excitability called?
The refractory period.
What is the duration of the refractory period for skeletal muscle?
About 1 msec.
What is the duration of the refractory period for cardiac muscle?
About 250 msec
is a record of a muscle contraction.
Myogram
What happens when a second stimulus occurs after the refractory period but before the muscle has relaxed?
The second contraction will be stronger than the first.
What is wave summation?
A phenomenon where stimuli arriving at different times cause larger contractions.
What happens when a skeletal muscle fiber is stimulated at a rate of 20 to 30 times per second?
It can only partially relax between stimuli, leading to unfused (incomplete) tetanus.
What is unfused (incomplete) tetanus?
A sustained but wavering contraction due to partial relaxation between stimuli.
What happens when a skeletal muscle fiber is stimulated at a rate of 80 to 100 times per second?
It does not relax at all, resulting in fused (complete) tetanus.
What is fused (complete) tetanus?
A sustained contraction where individual twitches cannot be detected.
What causes wave summation and tetanus?
Additional Ca2+ release from the sarcoplasmic reticulum while levels are still elevated from the first stimulus.
How much larger is the peak tension during fused tetanus compared to a single twitch?
5 to 10 times larger.
How are smooth, sustained voluntary muscle contractions achieved?
By out-of-sync unfused tetanus in different motor units.
What role do elastic components play in wave summation?
They remain taut, requiring less stretching before the next contraction, contributing to stronger contractions.
Due to wave summation, the tension produced during a sustained contraction is greater than that produced by a single twitch.
wave summation
What is motor unit recruitment?
The process in which the number of active motor units increases.
Do all motor units contract in unison?
No, while some motor units contract, others are relaxed, which delays fatigue and sustains muscle contraction.
Which motor units are recruited first?
The weakest motor units are recruited first, with stronger ones added as needed.
How does motor unit recruitment contribute to smooth movements?
It prevents jerky movements by gradually increasing force rather than activating all units at once.
How does motor unit size affect muscle movement?
Small motor units control precise movements, while large motor units generate more force but less precision.
Why do small muscles have small motor units?
To allow fine control and precise movements.
When are large motor units recruited?
When a large amount of tension is needed and precision is less important.
What effect does aerobic training have on skeletal muscles?
It increases the supply of oxygen-rich blood for aerobic respiration, enhancing endurance.
What type of activities rely on anaerobic ATP production?
Activities like weightlifting, which depend on glycolysis.
What is muscle hypertrophy?
An increase in muscle size due to the synthesis of muscle proteins from anaerobic training.
Why should athletes engaged in anaerobic training consume protein?
To provide the necessary building blocks for muscle protein synthesis and muscle growth.
What is the main benefit of aerobic training?
It builds endurance for prolonged activities.
What is the main benefit of anaerobic training?
It increases muscle strength for short-term, high-intensity feats.
What is interval training?
A workout regimen that combines aerobic and anaerobic exercises, such as alternating sprints with jogging.
What is muscle tone?
A small amount of tension in a muscle due to weak, involuntary contractions of motor units.
How is muscle tone established?
By neurons in the brain and spinal cord that excite motor neurons.
What happens if motor neurons to a skeletal muscle are damaged?
The muscle becomes flaccid (limp and loses tone).
Does muscle tone produce movement?
No, it keeps muscles firm but does not generate enough force for movement.
What is an example of muscle tone in daily life?
The muscles in the back of the neck maintaining head posture while awake.
Why is muscle tone important in smooth muscle?
It helps maintain steady pressure in the digestive organs and regulates blood pressure by keeping blood vessels partially contracted.
What does hypotonia refer to?
Decreased or lost muscle tone.
What are muscles with hypotonia said to be?
Flaccid.
How do flaccid muscles appear?
Loose and appear flattened rather than rounded.
What may result in flaccid paralysis?
Certain disorders of the nervous system and disruptions in the balance of electrolytes (especially sodium, calcium, and, to a lesser extent, magnesium).
What characterizes flaccid paralysis?
Loss of muscle tone, loss or reduction of tendon reflexes, and atrophy (wasting away) and degeneration of muscles.
What does hypertonia refer to?
Increased muscle tone.
In what two ways is hypertonia expressed?
Spasticity or rigidity.
What is spasticity characterized by?
Increased muscle tone (stiffness) associated with an increase in tendon reflexes and pathological reflexes.
What is an example of a pathological reflex seen in spasticity?
The Babinski sign, in which the great toe extends with or without fanning of the other toes in response to stroking the outer margin of the sole.
What may result in spastic paralysis?
Certain disorders of the nervous system and electrolyte disturbances.
What is spastic paralysis?
Partial paralysis in which the muscles exhibit spasticity.
What does rigidity refer to?
Increased muscle tone in which reflexes are not affected.
What condition is an example of rigidity?
Tetanus.
What causes tetanus?
A bacterium, Clostridium tetani, that enters the body through exposed wounds.
What are the effects of tetanus?
Muscle stiffness and spasms that can make breathing difficult and can become life-threatening.
What does the bacteria that causes tetanus produce?
A toxin that interferes with the nerves controlling the muscles.
What are the first signs of tetanus?
Spasms and stiffness in the muscles of the face and jaws.
What are the two types of muscle contractions?
Isotonic or isometric.
What happens in an isotonic contraction?
The tension (force of contraction) developed in the muscle remains almost constant while the muscle changes its length.
What are isotonic contractions used for?
Body movements and for moving objects.
What are the two types of isotonic contractions?
Concentric and eccentric.
What happens in a concentric isotonic contraction?
If the tension generated is great enough to overcome the resistance of the object to be moved, the muscle shortens and pulls on another structure, such as a tendon, to produce movement and to reduce the angle at a joint.
What is an example of a concentric isotonic contraction?
Picking up a book from a table involves concentric isotonic contractions of the biceps brachii muscle in the arm.
What happens in an eccentric isotonic contraction?
When the length of a muscle increases during a contraction.
What happens during an eccentric contraction?
The tension exerted by the myosin cross bridges resists movement of a load and slows the lengthening process.
What is an example of an eccentric isotonic contraction?
Lowering a book back onto a table while the biceps lengthens in a controlled manner.
What type of contraction produces more muscle damage and delayed onset muscle soreness?
Eccentric isotonic contractions.
What happens in an isometric contraction?
The tension generated is not enough to exceed the resistance of the object to be moved, and the muscle does not change its length.
What is an example of an isometric contraction?
Holding a book steady using an outstretched arm.
What are isometric contractions important for?
Maintaining posture and supporting objects in a fixed position.
Do isometric contractions expend energy?
Yes.
How do isometric contractions counteract stretch?
The book pulls the arm downward, stretching the shoulder and arm muscles, and the isometric contraction of the shoulder and arm muscles counteracts the stretch.
Why are isometric contractions important?
They stabilize some joints as others are moved.
Do most activities include only isotonic or isometric contractions?
No, most activities include both isotonic and isometric contractions.
What happens in an isotonic contraction?
Tension remains constant as muscle length decreases or increases.
What happens in an isometric contraction?
Tension increases greatly without a change in muscle length.
What do skeletal muscle fibers vary in?
Composition and function.
What is myoglobin?
The red-colored protein that binds oxygen in muscle fibers.
What are red muscle fibers?
Skeletal muscle fibers that have a high myoglobin content.
Why do red muscle fibers appear darker?
Because they have a high myoglobin content.
What are white muscle fibers?
Skeletal muscle fibers that have a low myoglobin content.
Why do white muscle fibers appear lighter?
Because they have a low content of myoglobin.
What do red muscle fibers contain more of?
More mitochondria and blood capillaries.
How do skeletal muscle fibers differ in contraction and relaxation?
They contract and relax at different speeds.
How do skeletal muscle fibers differ in ATP generation?
They vary in which metabolic reactions they use to generate ATP.
How are fibers categorized as slow or fast?
By how rapidly the ATPase in their myosin heads hydrolyzes ATP.
What are the three types of skeletal muscle fibers?
Slow oxidative fibers, fast oxidative–glycolytic fibers, and fast glycolytic fibers.
Why do slow oxidative (SO) fibers appear dark red?
Because they contain large amounts of myoglobin and many blood capillaries.
Why do SO fibers generate ATP mainly by aerobic respiration?
Because they have many large mitochondria.
Why are SO fibers called oxidative fibers?
Because they generate ATP mainly by aerobic respiration.
Why are SO fibers said to be “slow”?
Because the ATPase in the myosin heads hydrolyzes ATP relatively slowly.
Why do SO fibers have a slow speed of contraction?
Because the contraction cycle proceeds at a slower pace than in “fast” fibers.
How long do SO fiber twitch contractions last?
From 100 to 200 msec.
Why do SO fibers take longer to reach peak tension?
Because their contraction cycle proceeds at a slower pace.
Why are SO fibers resistant to fatigue?
Because they are capable of prolonged, sustained contractions for many hours.
What are SO fibers adapted for?
Maintaining posture and aerobic, endurance-type activities such as running a marathon.
What are the largest fibers?
Fast oxidative–glycolytic (FOG) fibers
Why do FOG fibers have a dark red appearance?
Because they contain large amounts of myoglobin and many blood capillaries.
How do FOG fibers generate ATP?
By aerobic respiration and anaerobic glycolysis.
Why do FOG fibers have a moderately high resistance to fatigue?
Because they generate considerable ATP by aerobic respiration.
Why can FOG fibers also generate ATP by anaerobic glycolysis?
Because their intracellular glycogen level is high.
Why are FOG fibers considered “fast”?
Because the ATPase in their myosin heads hydrolyzes ATP three to five times faster than in SO fibers.
Why do FOG fiber twitches reach peak tension more quickly than those of SO fibers?
Because their speed of contraction is faster.
How long do FOG fiber twitches last?
Less than 100 msec.
What activities do FOG fibers contribute to?
Walking and sprinting.
What type of fibers have low myoglobin content, relatively few blood capillaries, and few mitochondria?
Fast glycolytic (FG) fibers
What color do FG fibers appear?
White
What do FG fibers contain large amounts of?
Glycogen
How do FG fibers generate ATP?
Mainly by glycolysis
Why do FG fibers contract strongly and quickly?
Because they hydrolyze ATP rapidly
What types of movements are FG fibers adapted for?
Intense movements of short duration, such as weight lifting or throwing a ball
Why do FG fibers fatigue quickly?
Because they rely mainly on glycolysis for ATP production
How do strength training programs affect FG fibers?
They increase the size, strength, and glycogen content of FG fibers
How much larger can FG fibers of a weight lifter be compared to those of a sedentary person or endurance athlete?
50% larger
What causes muscle enlargement in FG fibers?
Hypertrophy due to increased synthesis of muscle proteins
What are most skeletal muscles a mixture of?
All three types of skeletal muscle fibers
About how many of the fibers in a typical skeletal muscle are SO fibers?
Half
What factors affect the proportions of skeletal muscle fiber types?
The action of the muscle, the person’s training regimen, and genetic factors
Which muscles have a high proportion of SO fibers?
Postural muscles of the neck, back, and legs
Why do postural muscles have a high proportion of SO fibers?
Because they are continually active
Which muscles have a high proportion of FG fibers?
Muscles of the shoulders and arms
Why do shoulder and arm muscles have more FG fibers?
Because they are used briefly to produce large amounts of tension
Which muscles have large numbers of both SO and FOG fibers?
Leg muscles
Why do leg muscles contain both SO and FOG fibers?
Because they support the body and are used for walking and running
What is true about the muscle fibers within a particular motor unit?
All of them are of the same type
How are different motor units in a muscle recruited?
In a specific order, depending on need
If weak contractions are enough for a task, which motor units are activated?
SO motor units
If more force is needed, which motor units are recruited?
FOG fibers
If maximal force is required, which motor units are activated?
FG fibers, along with SO and FOG fibers
What controls the activation of various motor units?
The brain and spinal cord
What is the myoglobin content of Slow Oxidative (SO) fibers?
Large amount
What is the myoglobin content of Fast Oxidative–Glycolytic (FOG) fibers?
Large amount
What is the myoglobin content of Fast Glycolytic (FG) fibers?
Small amount
How many mitochondria are present in SO fibers?
Many
How many mitochondria are present in FOG fibers?
Many
How many mitochondria are present in FG fibers?
Few
How many capillaries are present in SO fibers?
Many
How many capillaries are present in FOG fibers?
Many
How many capillaries are present in FG fibers?
Few
What is the color of SO fibers?
Red
What is the color of FOG fibers?
Red-pink
What is the color of FG fibers?
White (pale)
What is the capacity for generating ATP and the method used in SO fibers?
High, by aerobic respiration
What is the capacity for generating ATP and the method used in FOG fibers?
Intermediate, by both aerobic respiration and anaerobic glycolysis
What is the capacity for generating ATP and the method used in FG fibers?
Low, by anaerobic glycolysis
What is the rate of ATP hydrolysis by myosin ATPase in SO fibers?
Slow
What is the rate of ATP hydrolysis by myosin ATPase in FOG fibers?
Fast
What is the rate of ATP hydrolysis by myosin ATPase in FG fibers?
Fast
What is the contraction velocity of SO fibers?
Slow
What is the contraction velocity of FOG fibers?
Fast
What is the contraction velocity of FG fibers?
Fast
What is the fatigue resistance of SO fibers?
High
What is the fatigue resistance of FOG fibers?
Intermediate
What is the fatigue resistance of FG fibers?
Low
What is the amount of creatine kinase in SO fibers?
Lowest amount
What is the amount of creatine kinase in FOG fibers?
Intermediate amount
What is the amount of creatine kinase in FG fibers?
Highest amount
What is the glycogen store level in SO fibers?
Low
What is the glycogen store level in FOG fibers?
Intermediate
What is the glycogen store level in FG fibers?
High
What is the order of recruitment for SO fibers?
First
What is the order of recruitment for FOG fibers?
Second
What is the order of recruitment for FG fibers?
Third
Where are SO fibers abundant?
Postural muscles such as those of the neck
Where are FOG fibers abundant?
Lower limb muscles
Where are FG fibers abundant?
Extraocular muscles
What are the primary functions of SO fibers?
Maintaining posture and aerobic endurance activities
What are the primary functions of FOG fibers?
Walking, sprinting
What are the primary functions of FG fibers?
Rapid, intense movements of short duration
What determines the relative ratio of fast glycolytic (FG) and slow oxidative (SO) fibers in each muscle?
Genetics
What does the genetically determined ratio of FG and SO fibers help account for?
Individual differences in physical performance
People with a higher proportion of FG fibers often excel in what types of activities?
Activities that require periods of intense activity, such as weight lifting or sprinting
People with a higher proportion of SO fibers are better at what types of activities?
Activities that require endurance, such as long-distance running
Does the total number of skeletal muscle fibers usually increase with exercise?
No
What can change to some extent with exercise, if not the number of skeletal muscle fibers?
The characteristics of the muscle fibers
What types of exercises cause a gradual transformation of some FG fibers into FOG fibers?
Endurance-type (aerobic) exercises, such as running or swimming
What changes occur in transformed muscle fibers after endurance exercises?
Slight increases in diameter, number of mitochondria, blood supply, and strength
What additional benefits do endurance exercises provide to skeletal muscles?
Cardiovascular and respiratory changes that improve oxygen and nutrient supply
Do endurance exercises increase muscle mass?
No
What types of exercises produce an increase in the size and strength of FG fibers?
Exercises that require great strength for short periods
What causes the increase in size of FG fibers after strength exercises?
Increased synthesis of thick and thin filaments
What is the overall result of increased FG fiber size due to strength exercises?
Muscle enlargement (hypertrophy)
What is an example of muscle hypertrophy due to strength exercises?
The bulging muscles of bodybuilders
What is an important attribute of skeletal muscles and their connective tissue attachments?
A certain degree of elasticity
How does greater elasticity affect flexibility?
It contributes to a greater degree of flexibility and increases the range of motion of a joint
What limits a relaxed muscle’s ability to lengthen when physically stretched?
Connective tissue structures, such as fasciae
How can connective tissue structures be gradually lengthened?
Through regular stretching
How often must stretching exercises be performed to improve flexibility?
Regularly—daily, if possible—for many week
Does stretching cold muscles increase flexibility?
No
What may stretching cold muscles cause?
Injury
When do tissues stretch best?
When slow, gentle force is applied at elevated tissue temperatures
What may be used as an external source of heat to elevate tissue temperatures?
Hot packs or ultrasound
What is another effective way to raise muscle temperature besides external heat sources?
10 or more minutes of muscular contraction
What heats muscle more deeply and thoroughly than external measures?
Exercise
Where does the term “warm up” come from?
From the fact that exercise heats muscles more deeply and thoroughly
When do many people stretch?
Before they engage in exercise
What should be done before stretching to avoid injury?
Warming up (e.g., walking, jogging, easy swimming, or easy aerobics)
What does strength training refer to?
The process of exercising with progressively heavier resistance for the purpose of strengthening the musculoskeletal system
What does strength training result in besides stronger muscles?
Many other health benefits
How does strength training help increase bone strength?
By increasing the deposition of bone minerals in young adults and helping to prevent, or at least slow, their loss in later life
How does strength training affect resting metabolic rate?
It increases resting metabolic rate
What is the effect of an increased resting metabolic rate?
A person can eat more food without gaining weight
How does strength training help prevent back and other injuries?
By increasing muscle mass and strengthening the musculoskeletal system
What are some psychological benefits of strength training?
Reductions in feelings of stress and fatigue
What happens as repeated training builds exercise tolerance?
It takes increasingly longer before lactic acid is produced in the muscle
What does delayed lactic acid production result in?
A reduced probability of muscle spasms
What are anabolic steroids?
Synthetic variations of testosterone, the sex hormone responsible for the development of male sexual characteristics
What are common names for anabolic steroids?
“Raids,” “juice,” “gear,” and “stackers”
How do anabolic steroids increase muscle size?
By increasing protein synthesis in muscles
What are medical uses of anabolic steroids?
To build muscle mass in patients with medical conditions such as cancer and AIDS and to treat delayed puberty
How do athletes and bodybuilders abuse anabolic steroids?
To improve physical appearance or enhance performance
What are the methods of taking anabolic steroids?
Orally, by injection directly into the muscles, or by application to the skin
How much higher are abusive doses compared to medical doses?
10 to 100 times those needed to treat medical conditions
What are some cardiovascular side effects of anabolic steroid abuse?
Changes in blood cholesterol levels, enlarged heart, high blood pressure, and blood clot formation, increasing the risk of heart attack and stroke
What are some liver-related side effects of anabolic steroid abuse?
Liver cancer, liver damage, and blood-filled cysts that may rupture and cause internal bleeding
What are some musculoskeletal side effects of anabolic steroid abuse?
Short stature, stunted growth during puberty and adolescence, and tendon injury
What are some integumentary side effects of anabolic steroid abuse?
Severe acne, oily hair and skin, baldness, and jaundice (due to liver damage)
What are some infection risks of anabolic steroid abuse?
HIV infection and hepatitis B and C due to non-sterile injection techniques and shared needles
What are some behavioral side effects of anabolic steroid abuse?
Aggression, increased irritability, extreme mood swings, delusions, and impaired judgment
What are some male-specific side effects of anabolic steroid abuse?
Atrophy of the testes, decreased testosterone levels, low sperm count, development of breasts, and increased risk of prostate cancer
What are some female-specific side effects of anabolic steroid abuse?
Atrophy of the breasts and uterus, menstrual irregularities, enlarged clitoris, sterility, growth of facial hair, and deepening of the voice
What is the principal tissue in the heart wall?
Cardiac muscle tissue
What is found between the layers of cardiac muscle fibers?
Sheets of connective tissue that contain blood vessels, nerves, and the conduction system of the heart
How do cardiac muscle fibers compare to skeletal muscle fibers in terms of arrangement?
They have the same arrangement of actin and myosin and the same bands, zones, and Z discs as skeletal muscle fibers
How do T tubules in cardiac muscle compare to those in skeletal muscle?
T tubules of cardiac muscle are wider but less abundant, with one T tubule per sarcomere located at the Z disc
How does the sarcoplasmic reticulum of cardiac muscle compare to that of skeletal muscle?
The sarcoplasmic reticulum of cardiac muscle fibers is somewhat smaller
What structures are unique to cardiac muscle fibers?
Intercalated discs
What are intercalated discs?
Irregular transverse thickenings of the sarcolemma that connect the ends of cardiac muscle fibers to one another
What structures do intercalated discs contain?
Desmosomes and gap junctions
What is the function of desmosomes in cardiac muscle?
To hold the fibers together
What is the function of gap junctions in cardiac muscle?
To allow muscle action potentials to spread from one cardiac muscle fiber to another
What connective tissue layers are found in cardiac muscle tissue?
Endomysium and perimysium
What connective tissue layer is lacking in cardiac muscle tissue?
Epimysium
How much longer does cardiac muscle tissue remain contracted compared to skeletal muscle tissue?
10 to 15 times longer
Why does cardiac muscle tissue remain contracted for a longer duration?
Due to prolonged delivery of Ca²⁺ into the sarcoplasm
Where does Ca²⁺ enter the sarcoplasm in cardiac muscle fibers?
From the sarcoplasmic reticulum and from the interstitial fluid that bathes the fibers
Why does cardiac muscle contraction last longer than a skeletal muscle twitch?
Because the channels that allow inflow of Ca²⁺ from interstitial fluid stay open for a relatively long time
How is the contraction of skeletal muscle tissue initiated?
By stimulation from acetylcholine released by a nerve impulse in a motor neuron
How is the contraction of cardiac muscle tissue initiated?
By its own autorhythmic muscle fibers
How many times per minute does cardiac muscle tissue contract and relax under normal resting conditions?
About 75 times a minute
What is a major physiological difference between cardiac and skeletal muscle tissue?
Cardiac muscle has continuous, rhythmic activity
How do mitochondria in cardiac muscle fibers compare to those in skeletal muscle fibers?
They are larger and more numerous
How does cardiac muscle generate ATP?
By aerobic respiration
What does cardiac muscle require for ATP production?
A constant supply of oxygen
What can cardiac muscle fibers use to make ATP during exercise?
Lactic acid produced by skeletal muscle fibers
What happens to cardiac muscle fibers in response to an increased workload?
They undergo hypertrophy
What is a physiological enlarged heart?
Hypertrophy of cardiac muscle in response to increased workload, as seen in athletes
What is a pathological enlarged heart?
An enlargement of the heart related to significant heart disease
How is smooth muscle tissue usually activated?
Involuntarily
What are the two types of smooth muscle tissue?
Visceral (single-unit) smooth muscle tissue and multi-unit smooth muscle tissue
Which type of smooth muscle tissue is more common?
Visceral (single-unit) smooth muscle tissue
Where is visceral smooth muscle tissue found?
In the skin, small arteries and veins, and hollow organs such as the stomach, intestines, uterus, and urinary bladder
What characteristic does visceral smooth muscle share with cardiac muscle?
It is autorhythmic
How do visceral smooth muscle fibers connect to one another?
By gap junctions
What do gap junctions allow in visceral smooth muscle?
They form a network through which muscle action potentials can spread
What happens when a neurotransmitter, hormone, or autorhythmic signal stimulates one fiber in visceral smooth muscle?
The muscle action potential is transmitted to neighboring fibers, which then contract in unison, as a single unit
How does multi-unit smooth muscle tissue differ from visceral smooth muscle tissue?
Multi-unit smooth muscle consists of individual fibers, each with its own motor neuron terminals and with few gap junctions between neighboring fibers
What happens when one visceral muscle fiber is stimulated?
Contraction of many adjacent fibers occurs
What happens when one multi-unit muscle fiber is stimulated?
Only that fiber contracts
Where is multi-unit smooth muscle tissue found?
In the walls of large arteries, airways to the lungs, arrector muscles of the hair, muscles of the iris, and the ciliary body of the eye
How long is a single relaxed smooth muscle fiber?
30–200 µm long
What is the thickness of a smooth muscle fiber in the middle?
3–8 µm
What shape does a smooth muscle fiber have?
Thickest in the middle and tapers at each end
How many nuclei does a smooth muscle fiber have?
A single, oval, centrally located nucleus
What types of filaments are found in the sarcoplasm of smooth muscle fibers?
Thick filaments, thin filaments, and intermediate filaments
What is the ratio of thick to thin filaments in smooth muscle fibers?
Between 1:10 and 1:15
Why do smooth muscle fibers not exhibit striations?
Because the various filaments have no regular pattern of overlap
What structures do smooth muscle fibers lack that are found in skeletal muscle fibers?
T tubules
What is the function of caveolae in smooth muscle fibers?
They contain extracellular Ca2+ that can be used for muscular contraction
What do thin filaments attach to in smooth muscle fibers?
Dense bodies
What are dense bodies functionally similar to in striated muscle fibers?
Z discs
Where are dense bodies located in smooth muscle fibers?
Some are dispersed throughout the sarcoplasm, others are attached to the sarcolemma
What structures attach to dense bodies and stretch from one dense body to another?
Bundles of intermediate filaments
What happens during contraction in smooth muscle fibers?
The sliding filament mechanism generates tension, which is transmitted to intermediate filaments that pull on dense bodies attached to the sarcolemma, causing a lengthwise shortening of the muscle fiber
How does a smooth muscle fiber move during contraction and relaxation?
It rotates as a corkscrew turns during contraction and rotates in the opposite direction during relaxation
connect to one another by gap junctions and contract as a single unit. Multi unit smooth muscle fibers lack gap junctions and contract independently.
Visceral smooth muscle fibers
How does the contraction speed of smooth muscle compare to skeletal muscle?
Smooth muscle contracts more slowly and lasts much longer than skeletal muscle contraction.
What ability does smooth muscle have that is greater than cardiac and skeletal muscle?
It can both shorten and stretch to a greater extent.
What initiates contraction in smooth muscle fibers?
An increase in the concentration of Ca2+ in the sarcoplasm.
Where does Ca2+ come from in smooth muscle fibers?
Both the interstitial fluid and the sarcoplasmic reticulum.
Why does smooth muscle have a slow onset of contraction?
Because it lacks T tubules, so Ca2+ takes longer to reach the filaments.
What regulatory protein binds to Ca2+ in smooth muscle?
Calmodulin
What enzyme does calmodulin activate?
Myosin light chain kinase
What is the role of myosin light chain kinase?
It uses ATP to add a phosphate group to the myosin head, allowing it to bind to actin for contraction.
Why does smooth muscle contraction last longer?
Because Ca2+ enters and exits the fibers slowly.
What is smooth muscle tone and why is it important?
A state of continued partial contraction; it helps maintain steady pressure in blood vessels and the digestive canal.
What factors regulate smooth muscle contraction and relaxation?
Nerve impulses, stretching, hormones, pH changes, oxygen and CO2 levels, temperature, and ion concentrations.
What effect does the hormone epinephrine have on smooth muscle?
It causes relaxation in airways and certain blood vessels (with β2 receptors).
What is the stress–relaxation response in smooth muscle?
When stretched, smooth muscle initially contracts, increasing tension, but then relaxes within a minute while maintaining its ability to contract.
Why is the stress–relaxation response important?
It allows hollow organs (stomach, intestines, bladder) and blood vessels to expand without significant pressure changes.
What is the main way skeletal muscle grows after birth?
Hypertrophy (enlargement of existing cells), not hyperplasia (increase in number of fibers).
What is the role of satellite cells in skeletal muscle?
They divide slowly and fuse with existing fibers to assist in muscle growth and repair.
Can skeletal muscle regenerate well?
No, only to a limited extent.
What was previously believed about damaged cardiac muscle fibers?
They could not be replaced, and healing occurred only through fibrosis (scar tissue formation).
What does new research suggest about cardiac muscle regeneration?
Under certain conditions, cardiac muscle tissue can regenerate.
How can cardiac muscle respond to increased workload?
By undergoing hypertrophy (enlargement of fibers), leading to enlarged hearts in athletes.
Can smooth muscle undergo hypertrophy?
Yes, like skeletal and cardiac muscle.
What is unique about certain smooth muscle fibers compared to skeletal and cardiac muscle?
They can undergo hyperplasia (increase in number of fibers).
What cells help form new smooth muscle fibers?
Pericytes, which are stem cells associated with blood capillaries and small veins.
In what pathological condition can smooth muscle fibers proliferate?
Atherosclerosis.
Which muscle tissue has the greatest regeneration ability?
Smooth muscle tissue.
How does smooth muscle regeneration compare to other tissues?
It regenerates better than skeletal and cardiac muscle but is still limited compared to epithelium.
What is the microscopic appearance and features of skeletal muscle?
Long cylindrical fiber with many peripherally located nuclei; unbranched; striated.
What is the microscopic appearance and features of cardiac muscle?
Branched cylindrical fiber with one centrally located nucleus; intercalated discs join neighboring fibers; striated.
What is the microscopic appearance and features of smooth muscle?
Fiber thickest in middle, tapered at each end, and with one centrally positioned nucleus; not striated.
What is the location of skeletal muscle?
Most commonly attached by tendons to bones.
What is the location of cardiac muscle?
Heart.
What is the location of smooth muscle?
Walls of hollow viscera, airways, blood vessels, iris and ciliary body of eye, arrector muscles of the hair.
What is the fiber diameter of skeletal muscle?
Very large (10–100 µm).
What is the fiber diameter of cardiac muscle?
Large (10–20 µm).
What is the fiber diameter of smooth muscle?
Small (3–8 µm).
What are the connective tissue components of skeletal muscle?
Endomysium, perimysium, and epimysium.
What are the connective tissue components of cardiac muscle?
Endomysium and perimysium.
What are the connective tissue components of smooth muscle?
Endomysium.
What is the fiber length of skeletal muscle?
Very large (100 µm–30 cm = 12 in.).
What is the fiber length of cardiac muscle?
Large (50–100 µm).
What is the fiber length of smooth muscle?
Intermediate (30–200 µm).
Are contractile proteins organized into sarcomeres in skeletal muscle?
Yes.
Are contractile proteins organized into sarcomeres in cardiac muscle?
Yes.
Are contractile proteins organized into sarcomeres in smooth muscle?
No.
How abundant is the sarcoplasmic reticulum in skeletal muscle?
Abundant.
How abundant is the sarcoplasmic reticulum in cardiac muscle?
Some.
How abundant is the sarcoplasmic reticulum in smooth muscle?
Very little.
Are T tubules present in skeletal muscle?
Yes, aligned with each A–I band junction.
Are T tubules present in cardiac muscle?
Yes, aligned with each Z disc.
Are T tubules present in smooth muscle?
No.
Are there junctions between fibers in skeletal muscle?
None.
Are there junctions between fibers in cardiac muscle?
Intercalated discs contain gap junctions and desmosomes.
Are there junctions between fibers in smooth muscle?
Gap junctions in visceral smooth muscle; none in multi unit smooth muscle.
Does skeletal muscle exhibit autorhythmicity?
No.
Does cardiac muscle exhibit autorhythmicity?
Yes.
Does smooth muscle exhibit autorhythmicity?
Yes, in visceral smooth muscle.
What is the source of Ca²⁺ for contraction in skeletal muscle?
Sarcoplasmic reticulum.
What is the source of Ca²⁺ for contraction in cardiac muscle?
Sarcoplasmic reticulum and interstitial fluid.
What is the source of Ca²⁺ for contraction in smooth muscle?
Sarcoplasmic reticulum and interstitial fluid.
What are the regulator proteins for contraction in skeletal muscle?
Troponin and tropomyosin.
What are the regulator proteins for contraction in cardiac muscle?
Troponin and tropomyosin.
What are the regulator proteins for contraction in smooth muscle?
Calmodulin and myosin light chain kinase.
What is the speed of contraction of skeletal muscle?
Fast.
What is the speed of contraction of cardiac muscle?
Moderate.
What is the speed of contraction of smooth muscle?
Slow.
What is the nervous control of skeletal muscle?
Voluntary (somatic nervous system).
What is the nervous control of cardiac muscle?
Involuntary (autonomic nervous system).
What is the nervous control of smooth muscle?
Involuntary (autonomic nervous system).
What regulates contraction in skeletal muscle?
Acetylcholine released by somatic motor neurons.
What regulates contraction in cardiac muscle?
Acetylcholine and norepinephrine released by autonomic motor neurons; several hormones.
What regulates contraction in smooth muscle?
Acetylcholine and norepinephrine released by autonomic motor neurons; several hormones; local chemical changes; stretching.
What is the capacity for regeneration in skeletal muscle?
Limited, via satellite cells.
What is the capacity for regeneration in cardiac muscle?
Limited, under certain conditions.
What is the capacity for regeneration in smooth muscle?
Considerable (compared with other muscle tissues, but limited compared with epithelium), via pericytes.
Which muscles are exceptions to being derived from mesoderm?
Muscles of the iris of the eyes and the arrector pili muscles attached to hairs.
From which germ layer are all muscles of the body derived?
Mesoderm.
How does mesoderm develop in relation to the nervous system?
It becomes arranged in dense columns on either side of the developing nervous system.
What structures does mesoderm segment into?
A series of cube-shaped structures called somites.
When does the first pair of somites appear during embryonic development?
On the 20th day of embryonic development.
How many pairs of somites are formed by the end of the fifth week?
42 to 44 pairs.
What can the number of somites be correlated to?
The approximate age of the embryo.
Into what three regions do the cells of a somite differentiate?
Myotome, dermatomal mesenchyme, and sclerotome.
What does the myotome form?
The skeletal muscles of the trunk and limbs.
What does the dermatomal mesenchyme form?
The connective tissues, including the dermis of the skin and subcutaneous tissue.
What does the sclerotome give rise to?
The vertebrae and ribs.
From what type of cells does cardiac muscle develop?
Mesodermal cells that migrate to and envelop the developing heart.
What structure does the developing heart exist as before cardiac muscle forms?
Endocardial heart tubes.
From what type of cells does smooth muscle develop?
Mesodermal cells that migrate to and envelop the developing digestive canal and viscera.
Most muscles are derived from
mesoderm
What happens to skeletal muscle mass between the ages of 30 and 50?
It undergoes a slow, progressive loss and is replaced largely by fibrous connective tissue and adipose tissue.
What percentage of muscle mass is estimated to be lost between the ages of 30 and 50?
0.1
What may be a contributing factor to the decline in muscle mass?
Decreased levels of physical activity.
What changes accompany the loss of muscle mass?
A decrease in maximal strength, a slowing of muscle reflexes, and a loss of flexibility.
Which type of muscle fiber appears to increase with aging?
Slow oxidative (SO) fibers.
What could be the reason for the increase in slow oxidative fibers with aging?
Atrophy of the other fiber types or their conversion into slow oxidative fibers.
What percentage of muscle mass is typically lost between the ages of 50 and 80?
0.4
At what age is loss of muscle strength usually perceived?
60 to 65.
Which muscles tend to weaken first with aging?
Muscles of the lower limbs.
How may muscle weakness in the elderly affect their independence?
It becomes difficult to climb stairs or get up from a seated position.
What type of exercise has been shown to be effective at any age, assuming no chronic medical conditions?
Aerobic activities and strength training programs.
How can exercise impact age-related muscle decline?
It can slow or even reverse the age-associated decline in muscular performance.
What may abnormalities of skeletal muscle function be due to?
Disease or damage of any of the components of a motor unit: somatic motor neurons, neuromuscular junctions, or muscle fibers.
What term encompasses problems at all three sites of the motor unit?
Neuromuscular disease.
What does the term myopathy signify?
A disease or disorder of the skeletal muscle tissue itself.
What is myasthenia gravis?
An autoimmune disease that causes chronic, progressive damage of the neuromuscular junction.
What does the immune system inappropriately produce in myasthenia gravis?
Antibodies that bind to and block some ACh receptors.
What does blocking of ACh receptors lead to?
A decrease in the number of functional ACh receptors at the motor end plates of skeletal muscles.
What percentage of myasthenia gravis patients have thymic abnormalities?
0.75
What happens as myasthenia gravis progresses?
More ACh receptors are lost, muscles become increasingly weaker, fatigue more easily, and may eventually cease to function.
How common is myasthenia gravis?
It occurs in about 1 in 10,000 people.
In which age group is myasthenia gravis more common in women?
Typically ages 20 to 40 at onset.
In which age group is myasthenia gravis more common in men?
Usually ages 50 to 60 at onset.
Which muscles are most often affected in myasthenia gravis?
The muscles of the face and neck.
What are the initial symptoms of myasthenia gravis?
Weakness of the eye muscles, which may produce double vision, and weakness of the throat muscles that may produce difficulty in swallowing.
What later symptoms may occur in myasthenia gravis?
Difficulty chewing and talking.
What happens when the disease progresses further?
The muscles of the limbs may become involved.
What may cause death in myasthenia gravis?
Paralysis of the respiratory muscles.
What are the first-line treatments for myasthenia gravis?
Anticholinesterase drugs such as pyridostigmine (Mestinon) or neostigmine.
How do anticholinesterase drugs work?
They act as inhibitors of acetylcholinesterase, the enzyme that breaks down ACh, raising the level of ACh available to bind with still functional receptors.
What steroid drug is used to reduce antibody levels in myasthenia gravis?
Prednisone.
What procedure can remove antibodies from the blood in myasthenia gravis patients?
Plasmapheresis.
What surgical procedure is often helpful for myasthenia gravis patients?
Thymectomy (surgical removal of the thymus).
What does the term muscular dystrophy refer to?
A group of inherited muscle-destroying diseases that cause progressive degeneration of skeletal muscle fibers.
What is the most common form of muscular dystrophy?
Duchenne muscular dystrophy (DMD).
On which chromosome is the mutated gene for DMD located?
X chromosome.
Why does DMD primarily affect boys?
Because males have only one X chromosome.
How common is DMD worldwide?
About 1 in every 3,500 male babies—about 21,000 in all—are born with DMD each year.
At what age do symptoms of DMD usually become apparent?
Between the ages of 2 and 5.
What are the early symptoms of DMD?
Falling often, difficulty running, jumping, and hopping.
By what age are most boys with DMD unable to walk?
By age 12.
What usually causes death in DMD patients?
Respiratory or cardiac failure.
By what age does death typically occur in DMD?
By age 20.
What protein is affected in DMD?
Dystrophin.
What happens to the dystrophin gene in DMD?
It is mutated, so little or no dystrophin is present in the sarcolemma.
What is the consequence of having little or no dystrophin?
The sarcolemma tears easily during muscle contraction, causing muscle fibers to rupture and die.
When was the dystrophin gene discovered?
In 1987.
What are some treatments for DMD?
Steroids to reduce inflammation and strengthen muscles, eteplirsen and golodirsen (drugs that target gene mutations), creatine supplements, range of motion exercises, braces, and mobility aids.
What is a spasm?
A sudden involuntary contraction of a single muscle in a large group of muscles.
What is a cramp?
A painful spasmodic contraction.
What are some causes of cramps?
Inadequate blood flow to muscles, overuse of a muscle, dehydration, injury, holding a position for prolonged periods, and low blood levels of electrolytes such as potassium.
What is a tic?
A spasmodic twitching made involuntarily by muscles that are ordinarily under voluntary control.
What are examples of tics?
Twitching of the eyelid and facial muscles.
What is a tremor?
A rhythmic, involuntary, purposeless contraction that produces a quivering or shaking movement.
What is a fasciculation?
An involuntary, brief twitch of an entire motor unit that is visible under the skin; it occurs irregularly and is not associated with movement of the affected muscle.
In which diseases may fasciculations be seen?
Multiple sclerosis and amyotrophic lateral sclerosis (Lou Gehrig’s disease).
What is a fibrillation?
A spontaneous contraction of a single muscle fiber that is not visible under the skin but can be recorded by electromyography.
What may fibrillations signal?
Destruction of motor neurons.
What does a comparison of electron micrographs of muscle tissue taken from athletes before and after intense exercise reveal?
Considerable exercise-induced muscle damage, including torn sarcolemmas in some muscle fibers, damaged myofibrils, and disrupted Z discs.
What indicates microscopic muscle damage after exercise?
Increases in blood levels of proteins, such as myoglobin and the enzyme creatine kinase, which are normally confined within muscle fibers.
When do skeletal muscles often become sore after a period of strenuous exercise?
From 12 to 48 hours after exercise.
What is delayed onset muscle soreness (DOMS) accompanied by?
Stiffness, tenderness, and swelling.
What appears to be a major factor in the causes of DOMS?
Microscopic muscle damage.
How do muscle fibers respond to exercise-induced muscle damage?
New regions of sarcolemma are formed to replace torn sarcolemmas, and more muscle proteins (including those of the myofibrils) are synthesized in the sarcoplasm of the muscle fibers.
Pain in or associ ated with muscles.
Myalgia
A tumor consisting of muscle tissue.
Myoma
Pathological softening of muscle tissue.
Myomalacia
Inflammation of muscle fibers.
Myositis
Increased muscular excitability and contractility, with decreased power of relaxation; tonic spasm of the muscle.
Myotonia
Permanent shortening (contracture) of a muscle due to replacement of destroyed muscle fibers by fibrous connective tis sue, which lacks extensibility. Typically occurs in forearm flexor mus cles. Destruction of muscle fibers may occur from interference with circulation caused by a tight bandage, a piece of elastic, or a cast.
Volkmann’s contracture
