What are the three types of muscular tissue?
Skeletal, cardiac, and smooth muscle.
Which five properties do all muscle types exhibit?
Excitability, conductivity, contractility, extensibility, and elasticity.
What characterizes skeletal muscle in control, appearance, and attachment?
It is voluntary, striated, and usually attached to bones.
What connective tissue surrounds an individual muscle fiber?
Endomysium.
What connective tissue surrounds a muscle fascicle?
Perimysium.
What connective tissue surrounds an entire muscle?
Epimysium.
What are the functional contractile units within myofibrils?
Sarcomeres.
What are myofibrils composed of?
Myofilaments.
What forms the thick filament’s central bare zone?
A middle region without myosin heads.
What binds myosin heads on thin-filament actin subunits?
Active sites on globular actin subunits.
What does calcium binding cause troponin to do during contraction?
Change shape and move tropomyosin away from actin’s active sites.
What defines a sarcomere’s boundaries?
The segment from one Z disc to the next.
Which filaments are found in the A band?
Thick filaments.
What does a motor unit consist of?
One nerve fiber and all the muscle fibers it innervates.
What is a neuromuscular junction?
The synapse where a nerve fiber meets a skeletal muscle fiber.
What does a Schwann cell do at the neuromuscular junction?
It envelops and isolates the junction.
What does the axon terminal contain at the neuromuscular junction?
Acetylcholine vesicles.
What triggers acetylcholine release into the synaptic cleft?
A nerve impulse.
What is the resting membrane potential of a skeletal muscle cell?
About −90 mV.
What happens to sodium and potassium gates during membrane depolarization and repolarization?
Sodium gates open first, then close as potassium gates open.
What is an action potential in a stimulated excitable cell?
A rapid voltage shift from depolarization followed by repolarization.
What distinguishes spastic paralysis from flaccid paralysis?
Spastic paralysis causes sustained contraction; flaccid paralysis causes limp, noncontracting muscle.
What are the four stages of skeletal muscle contraction?
Excitation, excitation–contraction coupling, contraction, and relaxation.
What does the length–tension relationship describe in muscle?
How muscle tension depends on its length before stimulation.
Why does an overly shortened skeletal muscle contract weakly?
Thick filaments butt against the Z discs.
Why does an overstretched skeletal muscle contract weakly?
Limited filament overlap permits few cross-bridges.
What minimum voltage defines the threshold for a muscle fiber?
The minimum voltage needed to generate an action potential and produce contraction.
What is a muscle twitch?
A brief cycle of contraction and relaxation caused by a threshold-level or stronger stimulus.
How does recruitment increase muscle contraction strength?
It activates more motor units as stimulus strength rises.
What happens to muscle length and tension during an isometric contraction?
Muscle length stays constant while internal tension develops.
How does a concentric contraction change muscle length while maintaining tension?
It shortens the muscle.
How does anaerobic fermentation differ from aerobic respiration in oxygen use and lactate production?
Fermentation uses no oxygen and produces lactate; aerobic respiration requires oxygen and produces no lactate.
How long can the phosphagen system support sprinting?
About 6 seconds.
What is the net ATP yield per glucose during anaerobic glycolysis?
2 ATP.
How do type I and type II muscle fibers differ in energy production and performance?
Type I fibers use aerobic ATP production to resist fatigue, while type II fibers use glycolysis and anaerobic fermentation for quick, powerful responses.
Which factors determine muscular strength?
Muscular strength depends on muscle size, fascicle arrangement, active motor-unit size, motor-unit and temporal summation, resting muscle length, and fatigue.
How are cardiac and smooth muscle cells innervated?
By the autonomic nervous system rather than somatic motor nerves.
How does cardiac muscle coordinate contractions within each heart chamber?
It synchronizes cells to contract in unison and expel blood.
What do gap junctions in cardiac intercalated discs do?
They electrically connect neighboring cardiomyocytes.
What enables cardiac muscle to contract rhythmically without nervous stimulation?
A built-in pacemaker generates electrical waves.
What structural feature is absent from smooth muscle?
Striations.
How do smooth muscle cells regenerate after injury?
They regenerate well after injury.
What is the shape and nuclear arrangement of a smooth muscle myocyte?
It is fusiform with one central nucleus.
How are thick and thin filaments arranged in smooth muscle myocytes?
They are not aligned into sarcomeres.
How do multiunit and single-unit smooth muscle differ in cell coordination?
Multiunit cells contract independently; single-unit cells couple through gap junctions and contract together.
Which stimuli can regulate smooth muscle activity?
Autonomic activity, hormones, gas levels, pH, temperature, stretch, and autorhythmicity.
What activates myosin light-chain kinase in smooth muscle?
Calmodulin bound to calcium.
How long can skeletal muscle fibers be?
As long as 30 cm.
What do tendons attach to bone matrix?
Muscle.
How does collagen in muscle connective tissue respond when tension is released?
It recoils.
How does collagen in muscle connective tissue help protect muscle?
It resists excessive stretch.
How many actin active sites does tropomyosin block?
Six or seven active sites.
What does titin prevent during muscle movement?
Overstretching of the filaments.
Where is the M line located in a sarcomere?
At the center of the H band.
How do muscle fibers in one motor unit contract, and what contraction results?
They contract in unison, producing a weak contraction over a wide area.
How many muscle fibers per neuron are in small motor units?
Three to six muscle fibers.
How many muscle fibers per neuron can quadriceps femoris and gastrocnemius motor units have?
Up to 1,000 muscle fibers.
How does the ion distribution differ across a resting cell membrane?
Sodium is concentrated outside, while potassium is concentrated inside.
How does an action potential propagate along a cell membrane?
Excitation triggers another action potential immediately ahead.
What does tetanus toxin block in the spinal cord?
Glycine release.
What effect do curare and botulinum toxin both cause?
Flaccid paralysis.
At what resting length does a muscle produce its greatest force?
Its optimum resting length.
How does muscle tone affect resting muscle length?
It helps keep resting muscles near their optimum length.
When does rigor mortis begin, peak, and diminish after death?
It begins after 3–4 hours, peaks at about 12 hours, and diminishes over 48–60 hours.
What triggers postmortem muscle cross-bridging, and what prevents relaxation?
Calcium released from deteriorating sarcoplasmic reticulum triggers cross-bridging, while ATP depletion prevents relaxation until the myofilaments decay.
What phases make up a muscle twitch?
A latent period, a contraction phase, and a relaxation phase.
Which motor units does the size principle recruit with weak stimuli?
Small motor units.
What contraction pattern results when high stimulus frequencies cause partial relaxation between stimuli?
Incomplete tetanus.
How does myokinase convert ADP into ATP?
It transfers a phosphate group from one ADP molecule to another.
Which molecule donates a phosphate to ADP during creatine kinase activity?
Creatine phosphate.
After about 40 seconds of exercise, what energy source meets most muscle ATP demand?
Aerobic respiration.
When do fatty acids become a more significant fuel during prolonged exercise?
After about 30 minutes, as glucose is depleted.
What features characterize slow oxidative fibers, and where are they common?
They are thin and rich in mitochondria, capillaries, and myoglobin, and are common in the erector spinae and soleus.
What features characterize fast glycolytic fibers, and where are they prominent?
They are thick, rich in glycogen and creatine phosphate, low in myoglobin, and prominent in eye and hand muscles, gastrocnemius, and biceps brachii.
How much tension can muscle exert per square centimeter of cross-sectional area?
About 3 or 4 kg/cm².
How does resistance training enlarge muscle fibers?
It stimulates synthesis of additional myofilaments and myofibrils, making the fibers thicker.
What does endurance training increase in slow-twitch muscle fibers?
Mitochondria, glycogen, and capillary density.
How do cardiomyocyte T tubules support contraction?
Their large size admits extracellular calcium.
How long does a cardiac muscle twitch last?
About 200–250 ms.
What proportion of cardiomyocyte volume is occupied by mitochondria?
About 25%.
How is damaged cardiac muscle repaired after a heart attack?
By fibrosis; functional cardiac muscle is not regenerated.
Where does smooth muscle obtain calcium for contraction?
From extracellular fluid through sarcolemmal calcium channels.
What anchors smooth muscle filaments in the absence of Z discs?
Dense bodies anchor the filaments.
Where does multiunit smooth muscle occur?
In some large arteries and air passages, piloerector muscles, and the iris.
What happens to smooth muscle cells during contraction?
They pucker and twist as thick filaments pull thin filaments against dense bodies and membrane plaques.
When does smooth muscle tension peak after contraction begins?
At about 500 milliseconds.
How do latch bridges sustain smooth muscle tone?
Myosin heads remain attached to actin without consuming additional ATP.
How can stretch trigger smooth muscle contraction?
Stretch opens mechanically gated calcium channels.
What is the stress–relaxation response in hollow organs?
A brief contraction followed by relaxation after stretching, allowing gradual filling without emptying.
What does smooth muscle plasticity mean?
The ability to adjust tension to the degree of stretch.
What is the anaerobic threshold?
The point at which blood lactate begins to rise above resting levels.
What fatigue property characterizes fast-twitch intermediate type IIA fibers?
They are fatigue resistant.
Answer 44 questions on Muscular Tissue.
1. Which three types of muscular tissue are found in the body?
2. Which set of physiological properties is shared by skeletal, cardiac, and smooth muscle?
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