Lecture 25: Muscle Physiology Flashcards
functions of muscles
- generate force, which can lead to generating motion
- restrain movement, by keeping the boy in place
- surround the visceral organs, blood vessels, respiratory channels, and glands and influence their activities
- form sphincters that control the passage of material out of tubular structures
- produce heat for thermoregulation
- produce electrical energy
muscle types
- skeletal
- smooth
- cardiac
skeletal muscle
- responsible for voluntary movements
- attached to bones
- striated
smooth muscle
- found in walls of internal organs like the stomach and intestines
- involved in involuntary movements
- non-striated
cardiac muscle
- found in the heart
- controls involuntary contractions to pump blood
- striated
skeletal muscle: extensor and flexor
groups of muscles in the forearm that help control wrist and hand motion
tricep
muscle on the back of the upper arm, crucial for straightening the arm
bicep
a muscle on the front part of the upper arm, critical for bending the arm
axis of rotation
fixed point (elbow joint) around which the arm moves
muscle structure hierarchy
- whole muscle contains bundles of muscle fibers
- a single muscle fiber contains multiple myofibrils
- sarcomeres are the repeating units within myofibrils, responsible for muscle contraction
myosin
thick protein filaments within sarcomere that slide past actin during contraction
actin
thin protein filaments within sarcomere that slide past myosin during contraciton
sarcomere
the functional unit of muscle contraction composed of actin, myosin, and structural components
troponin and tropomyosin
regulate the binding of actin and myosin for contraction
sarcoplasmic reticulum
stores calcium ions needed for muscle contraction
muscle contraction process
- signal initiation: a motor neuron generated an action potential in response to a stimulus from the nervous system
- signal transmission to the neuromuscular junction: the action potential travels down the motor neuron axon to the neuromuscular junction; at the NMJ, the motor neuron releases the neurotransmitter acetylcholine (ACh) into the synaptic cleft
- signal reception and muscle fiber activation: acetylcholine binds to receptors on the plasma membrane (sarcolemma) of the muscle fiber; this binding generates an action potential in the muscle fiber’s sarcolemma
- propagation of signal inside the muscle fiber: the muscle action potential travels along the sarcolemma and enters the muscle fiber via T-tubules; this signal reaches the sarcoplasmic reticulum
- release of calcium ions: the action potential triggers the sarcoplasmic reticulum to release calcium ions into the muscle fiber’s cytoplasm
- calcium binds to troponin: calcium binds to troponin and causes tropomyosin to move away, exposing binding sites on actin filaments
- cross bridge formation: the myosin heads bind to exposed sites on the actin filaments
- power stroke: using energy from ATP, myosin heads pull the actin filaments toward the center of the sarcomere; this shortens the sarcomere, resulting in muscle contractions
- ATP binding and cross bridge detachment: a new ATP molecule binds to the myosin head, causing it to release from the actin filament; myosin then hydrolyzes ATP to reset to its original position for another cycle
- end of contraction: once the stimulation stops, calcium ions are actively pumped back into the sarcoplasmic reticulum; tropomyosin re-covers the actin binding sites, preventing further cross bridge formation
- return to resting state: the muscle fiber relaxes, and the sarcomere returns to its original length, ending contraction
myosin binding sites blocked, muscle cannot contract
- tropomyosin covers the myosin binding sites on actin filaments
- troponin complex holds tropomyosin in place, preventing interaction between actin and myosin
- muscle cannot contract because myosin heads cannot bind to actin
myosin binding sites exposed, muscles can contract
- calcium ions bind to the troponin complex, causing a conformational change
- tropomyosin shifts, exposing the myosin binding sites on actin
- the muscle is now ready for contraction, allowing actin and myosin to interact
cyclical interactions between actin and myosin
- rigor is a transient state: myosin is tightly bound to actin in the absence of ATP; this is where the muscle is locked in place
- ATP binding dissociates myosin from actin: ATP binding to myosin causes it to release from actin, initiating the muscle contraction cycle
- the myosin ATPase hydrolyzes ATP to ADP, and energy is transferred to the cross bridge, ADP and Pi remain bound to myosin, the myosin head moves to the cocked position and loosely binds a G-actin
- when Ca2+ is present, the cross-bridge attaches tightly to the G-actin and goes through another cycle
- the myosin head releases Pi as it swivels in the power stroke, it moves the thin filament 10 nm toward the center of the sarcomere
- myosin unbinds ADP after the power stroke and stays attached to actin in rigor
aerobic metabolism
requires O2 and includes glycolysis, the citric acid cycle, and the electron transport chain, it produces ATP, CO2, and H2O
anaerobic metabolism
occurs without oxygen, relying on glycolysis and converting pyruvate into lactate and CO2, it produces less ATP than aerobic metabolism
slow-twitch oxidative muscle fibers
smaller diameter and darker color due to higher myoglobin content, fatigue resistant, efficient for prolonged activities, abundant mitochondria and myoglobin which support aerobic metabolism
fast-twitch glycolytic muscle fibers
larger diameter, pale color due to lower myoglobin levels, specialized for short bursts of intense activity but fatigue quickly, lower mitochondria content and reliance on anaerobic metabolism
compare two species of insectivorous lizards in the Namib Desert: E. lugubris and E. lineocellata
E. lugubris: wide forager, 3.83 grams, max sprint speed of 1.58 m/sec, endurance (minutes at 0.5 km/hr) is >30, maximum rate of O2 consumption is 3.22 ul g-1 h-1, and maximum lactate formation is 2.12 mg g-1 min-1
- slower sprints, higher endurance
E. lineocellata: wait ambusher, 3.27 grams, max sprint speed of 2.63 m/sec, endurance (minutes at 0.5 km/hr) is 7, maximum rate of O2 consumption is 2.49 ul g-1 h-1, and maximum lactate formation is 2.56 mg g-1 min-1
- faster sprints, lower endurance
slow oxidative characteristics
myosin ATPase activity: slow
speed to reach peak tension: slow
duration of twitches: long
rate of Ca2+ uptake by the endoplasmic reticulum: slow to intermediate
resistance to fatigue: high
number of mitochondria: many
myoglobin content: high
color: red
diameter of fiber: small
number of surrounding capillaries: many
levels of glycolytic enzymes: low
ability to produce ATP using oxidative phosphorylation: high
fast glycolytic characteristics
myosin ATPase activity: fast
speed to reach peak tension: fast
duration of twitches: short
rate of Ca2+ uptake by the endoplasmic reticulum: high
resistance to fatigue: low
number of mitochondria: few
myoglobin content: low
color: white
diameter of fiber: large
number of surrounding capillaries: few
levels of glycolytic enzymes: high
ability to produce ATP using oxidative phosphorylation: low
why is resistance to fatigue high in SO and low in FG?
SO fibers: utilize oxidative phosphorylation, a highly efficient process requiring oxygen to produce large amounts of ATP over extended periods, the ample ATP supply supports sustained contractions without quickly depleting resources, making these fibers fatigue resistant
FG fibers: rely primarily on glycolysis, which generates ATP rapidly but in lower quantities, this process produces lactate, leading to acidic conditions that impair muscle function and result in quicker fatigue
why is number of mitochondria many in SO and few in FG?
SO fibers: rely heavily on oxidative phosphorylation, an aerobic process that occurs in mitochondria to generate ATP efficiently over extended periods, to sustain their fatigue resistant function in endurance activities SO fibers have a high density of mitochondria
FG fibers: primarily depend on glycolysis, an anaerobic process that produces ATP rapidly without involving mitochondria, as FG fibers focus on short bursts of intense activity, they do not require as many mitochondria
why is myoglobin content high in SO and low in FG?
SO fibers: rely heavily on aerobic metabolism, which requires oxygen to produce ATP efficiently; myoglobin, a protein that binds and stores oxygen within muscle fibers, ensures a continuous supply of oxygen for oxidative phosphorylation
FG fibers: function primarily through anaerobic metabolism which does not rely on oxygen; as a result, they have less need for myoglobin
why is SO red and FG white
SO fibers: high levels of myoglobin, a protein that stores oxygen and gives the fibers their red appearance; myoglobin contains iron, which binds oxygen and creates the reddish hue
FG fibers: low myoglobin contents leads to paler, whitish appearance
why is the diameter of SO small and diameter of FG large?
SO fibers: designed for endurance and aerobic metabolism; their smaller diameter allows oxygen and nutrients to diffuse more efficiently from capillaries to the center of the fiber, ensuring sustained energy production during prolonged activities
FG fibers: specialized for short bursts of power and anaerobic metabolism; their larger diameter provides more space to pack contractile proteins (actin and myosin), enabling stronger, quicker contractions
why is the number of surrounding capillaries many for SO and few for FG?
SO fibers: depend heavily on aerobic metabolism, which requires a consistent supply of oxygen; a dense network of capillaries ensures oxygen delivery to support prolonged energy production
FG fibers: rely mainly on anaerobic metabolism, which doesn’t depend on oxygen; therefore, fewer capillaries are needed for their function
why is the level of glycolytic enzymes low for SO and high for FG?
SO fibers: primarily rely on aerobic metabolism to produce ATP; this process occurs in the mitochondria and does not require glycolytic enzymes; consequently, the level of glycolytic enzyme is low in SO fibers
FG fibers: depend on anaerobic metabolism to generate ATP quickly during high intensity, short duration activities; high glycolytic enzyme levels enable rapid ATP production through this pathway
why is ability to produce ATP using oxidative phosphorylation high for SO and low for FG?
SO fibers: rely primarily on aerobic metabolism, where oxidative phosphorylation occurs in the mitochondria to produce ATP; this process is efficient and supports sustained energy production which is crucial for long duration, low intensity activities
FG fibers: depend mostly on anaerobic for ATP production; since this pathway does not utilize oxygen, FG fibers have limited reliance on oxidative phosphorylation
hematocrit levels connected to activity
- endurance group shows the highest hematocrit levels, likely indicating enhanced oxygen carrying capacity to sustain prolonged physical exertion
- in contrast, the sprint group has hematocrit levels comparable to the control group, reflecting their reliance on anaerobic energy systems rather than oxygen dependent pathways
endurance training impact
graph a
- endurance training significantly increases endurance levels
graph b
- sprint speed decreases across all groups, with the most substantial drop observed in the sprint group
- this demonstrates the trade off in physiological adaptations: endurance training enhances prolonged activity capabilities but may compromise peal sprint performance
