Muscle Physiology

Question 1

What are the three types of muscle?

Answer 1

Skeletal muscle
Cardiac muscle
Smooth muscle

Question 2

What is the primary function of skeletal muscle?

Answer 2

Skeletal muscle is used for posture and locomotion. It enables conscious control over the contraction of our arms and legs.

Question 3

What is the responsibility of cardiac muscle?

Answer 3

Cardiac muscle is responsible for the rhythmic contractions of the heart

Question 4

In which body parts does smooth muscle cause involuntary contractions? (Examples)

Answer 4

Smooth muscle causes involuntary contractions in blood vessels, gut, bronchi, and the uterus.

Question 5

How is skeletal muscle attached to bones?

Answer 5

Muscle is attached at each end to tendons, which, in turn, attach to bone on both sides of a joint.

Question 6

What is the basic structural unit of skeletal muscle?

Answer 6

sarcomere

Question 7

What are muscle fibers composed of in skeletal muscle?

Answer 7

bundles of long, thin cells called muscle fibers
(Many fibers wrapped -> fascicle)
(Many fascicles -> muscle)

Question 8

What causes the striations within each myofibril in skeletal muscle?

Answer 8

alternating light I-bands and dark A-bands, and the Z-line in the center of each light band- these structures delineate the sarcomere

Question 9

What is the sliding filament model in muscle contraction? Steps?

Answer 9

The sliding filament model explains that the thin filaments are pulled over the thick filaments by the myosin head groups, resulting in muscle contraction.

1. Energized myosin head groups bind to actin thin filaments. (Grab)
2. Power stroke: Head groups pull in.
3. Myosin unbinds and is re-energized: lets go/release
4. Power stroke: Reach out and grabs again

Question 10

What drives the reaction in the sliding filament model during muscle contraction?

Answer 10

driven by ATP hydrolysis

Question 11

Skeletal muscles cells consiste of cylindrical bundles called ?

Answer 11

Myofibrils

Question 12

Thin filaments consist of ______. Each _____ filament is formed of ______ chains of ________ ________ subunits, twisted into a _________.

Answer 12

Actin
Actin
Two
Globular actin
Helix

Question 13

Thick filament are made of?

Answer 13

Myosin

Question 14

1 thick filament is surrounded by ___ thin filaments

Answer 14

6

Question 15

Why is a certain portion of the A band lighter than the rest? What is it called?

Answer 15

The lighter portion within the A band is called the H-zone. It appears lighter because it contains only thick filaments (myosin) without overlapping thin filaments (actin). The H-zone represents the central region of the sarcomere where there is no overlap of the crossbridges between thick and thin filaments.

Question 16

What is the M-line?

Answer 16

The M-line is a structural component in the sarcomere. It is found in the center of the H-band (lighter region) of the A-band.

Question 17

If you make a slice in the I-band, you would see _______. If you make a slice in the H-zone, you would see _______. If you make a slice in the A-band (not H-zone), you would see ________.

Answer 17

Thin filaments
Thick filaments
Overlap of thick and thin filaments

Question 18

Do all the head groups pull at the same time? And why?

Answer 18

They don’t. Each one acts independently, because if they all let go—> muscle lets go.

Question 19

Contraction of the ______ shortens the entire ______.

Answer 19

Sarcomere
Myofibril

Question 20

Explain why muscle fibers have many nuclei in a single fiber?

Answer 20

Muscle fibers are generated during development by the fusion of a large number of small precursor cells called myoblasts. Each myobast has a single nucleus, whereas the fiber is a multinucleated cell.

Question 21

What is the advantage of having many nuclei in a single muscle fiber?

Answer 21

1. Muscle fibers make a lot of proteins, which are transcribed in the nucleus. Hence, more copies of the gene= more RNA= more proteins
2. Muscle fibers are very long and thin. If there was only one nucleus, the RNA and protein has to be transported all through the length of the fiber (like neuron). Instead, there are many sites (nuclei) all through the length of the fiber.

Question 22

During contraction, does the length of thick and thin filament change?

Answer 22

No, the overlap changes

Question 23

What determines the amount of tension a muscle fiber can develop? Explain

Answer 23

The amount of tension a muscle fiber can develop depends on fiber length, and this length-tension relation reflects the degree of overlap between thick and thin filaments.

If muscle fiber length too short, the thick filaments will run in the Z-line. The muscle will have no place to go—> can’t contract —> won’t generate any force.
If fiber length too long, less overlap (thin/thick filaments) —> less contraction force.

Question 24

What drives the cross-bridge cycle in muscle contraction? Explain the steps

Answer 24

The cross-bridge cycle is driven by ATP binding and hydrolysis by the myosin head groups.

1. ATP binding to myosin head groups energizes them.
2. ATP hydrolysis to ADP+Pi. (P released is Energy). Changes conformation of head group.
3. Myosin heads bind to actin filaments, forming cross-bridges. (Receptors for head groups on the actin)
4. Triggers conformational change: Power strokes occur as myosin heads pull on actin, leading to filament sliding.
5. ADP+Pi is relaesed, allowing myosin to detach and reset for the next cycle.

Question 25

How is the contraction of voluntary muscle initiated?

Answer 25

involves the primary motor cortex firing an action potential. This signal travels to the spinal cord, where motor neurons are activated. Motor neurons then transmit the signal to muscle fibers, initiating contraction.

Question 26

What is the motor unit in muscle physiology?

Answer 26

The motor unit consists of a motor neuron and the group of muscle fibers it innervates. The number of fibers innervated by a single motor neuron can vary, ranging from 10 (e.g., extraocular muscles) to 100 (muscles of the hand) to several thousand (large flexor and extensor muscles of the leg).

Question 27

__________ muscle fibers are innervated by _______ motor neuron.

Answer 27

Multiple A Single

Question 28

What is the structure of the neuromuscular junction?

Answer 28

The neuromuscular junction consists of the presynaptic terminal, synaptic vesicles containing acetylcholine (ACh), the basement membrane with acetylcholinesterase (AChE), postsynaptic membrane with nicotinic acetylcholine receptors (nACh receptors), and the end plate with junctional folds.

Question 29

List the steps involved in neuromuscular transmission.

Answer 29

1. Action potential in motor neuron 2. Acetylcholine release at presynaptic terminal 3. Na+ influx through activated nicotinic acetylcholine receptors 4. Endplate potential 5. Fiber action potential

Question 30

What is the role of T-tubules and the sarcoplasmic reticulum in muscle contraction?

Answer 30

T-tubules are invaginations of the cell membrane that allow the action potential to penetrate into the muscle fiber, while the sarcoplasmic reticulum is a specialized endoplasmic reticulum that stores and releases calcium ions, crucial for muscle contraction.

Question 31

Describe the process of excitation-contraction coupling in muscle cells.

Answer 31

Excitation-contraction coupling involves the activation of the Ryanodine Receptor, leading to Ca2+ efflux from the sarcoplasmic reticulum. Released calcium binds to troponin on thin filaments causing a conformational change, causing tropomyosin to move away from the myosin binding site on actin. This allows the interaction between thick filament heads and thin filaments, initiating muscle contraction.

Question 32

What is a twitch in muscle contraction, and why does it lag behind the muscle action potential?

Answer 32

A twitch is the contraction of a muscle fiber in response to a single action potential. The lag is due to delays in excitation-contraction coupling. The duration of contraction reflects the time needed for calcium concentration in the muscle cell to return to baseline.

Question 33

Define muscle tension, and how is it controlled in a whole muscle?

Answer 33

Muscle tension is the force generated by a muscle. Tension in a whole muscle is controlled by recruitment (increasing the number of active fibers) and summation (additive effects of closely spaced twitches). Skeletal muscle is adapted for large force generation over a narrow operating range.

Question 34

Explain summation and how it leads to sustained contraction of muscle fibers.

Answer 34

Summation occurs when single action potentials in motor neurons, applied rapidly, cause transient twitches to add together. Motor neurons firing in bursts result in sustained contraction called tetanus.

Question 35

What is a more important mechanism than summation for increasing muscle tension, especially in prolonged contractions?

Answer 35

Recruitment of additional motor units is a more important mechanism for increasing muscle tension, allowing for sustained contractions and adapting to varying force requirements.

Question 36

How long does the premade ATP in a muscle fiber last, and what is the alternative source for ATP during initial muscle activity?

Answer 36

Premade ATP in a muscle fiber lasts for a few twitches. The transfer of a phosphate from creatine phosphate to ADP creates enough ATP for a few seconds of muscle activity.

Question 37

How are ATP levels sustained during prolonged muscle activity?

Answer 37

ATP levels during prolonged muscle activity are sustained by glycolysis and oxidative phosphorylation. Glycogen in the muscle and glucose and fatty acids from the blood provide the necessary fuel.

Question 38

In what form do muscle fibers store energy, and what is the composition of this storage?

Answer 38

Muscle fibers store energy in the form of glycogen, which is a polysaccharide comprising long chains of glucose molecules.

Question 39

Name the 3 skeletal muscle fibers and their characteristics.

Answer 39

1. Fast glycolytic fibers: -Myosin with high ATPase activity. -no myoglobin (“white muscle”) -For generation of large force over short periods of time. 2. Slow oxidative fibers: -Myosin with low ATPase activity. -Myoglobin to facilitate oxygen transport from blood (“red muscle”). -For generation of low levels of force over long periods of time. 3. Fast oxidative fibers: -Intermediate properties. “Fast” myosin and oxidative metabolism.

Question 40

What protects muscles from damage, and what is a misconception about the causes?

Answer 40

Fatigue protects muscles from damage. It is not caused by ATP depletion.

Question 41

What are some causes of fatigue in response to high-intensity, short-duration activity? (fast gly. fibers)

Answer 41

Causes may include changes in ion gradients (e.g., increased extracellular K+), reduction in pH due to lactic acid buildup

Question 42

What might contribute to fatigue during low-intensity, long-duration activity? (slow oxid. fibers)

Answer 42

-Depletion of glycogen may be important in causing fatigue - “Central command fatigue”, i.e. failure of command signals from the CNS is also a significant factor.

Question 43

How do changes in muscle physiology vary depending on the type of exercise?

Answer 43

Changes depend on the type of exercise. Low-intensity, long-duration (aerobic) exercise increases fiber mitochondria and vascularization. High-intensity, short-duration exercise causes an increase in the diameter of fast glycolytic fibers and muscle hypertrophy.

Question 44

Why does muscle soreness occur after exercise?

Answer 44

Muscle soreness is due to inflammation in response to muscle damage. Factors like insulin-like growth factor 1 released by damaged tissue may contribute to muscle changes in response to exercise.

Question 45

How does the contraction process in smooth muscle compare to skeletal muscle?

Answer 45

In smooth muscle, contraction involves myosin thick filaments pulling on actin thin filaments, similar to skeletal muscle. However, smooth muscle lacks the highly ordered structure seen in skeletal and cardiac muscle, resulting in the absence of striations.

Question 46

What activates smooth muscle contraction, and what is the role of calcium in this process?

Answer 46

Smooth muscle contraction is activated by Ca2+, released from the sarcoplasmic reticulum or entering the cell through membrane calcium channels. Ca2+ binds to calmodulin, which, in turn, activates myosin light chain kinase. The kinase phosphorylates and thus activates smooth muscle myosin.

Question 47

How is the activity of smooth muscle regulated?

Answer 47

by various extracellular signals, including hormones and neurotransmitters from the autonomic nervous system. (hormone and metabotropic receptors) These signals modulate the contraction and relaxation of smooth muscle.