Chapter 11 - Muscle Tissue

Question 0

What is skeletal muscle?

Answer 0

Voluntary striated muscle that is usually attached to one or more bone. The CT tissues of the muscles are continuous with the collagen of the tendons which are continuous with collagen of the bone.

Question 1

What are the universal characteristics of muscle?

Answer 1

1. Responsiveness (excitability) - when stimulated muscle cells respond with electrical changes across the plasma membrane.
2. Conductivity - stimulation of a muscle cell triggers a wave of excitation that travels along the cell.
3. Contractility - muscle cells shorten when stimulated.
4. Extensibility - muscle cells are able to stretch again between contractions.
5. Elasticity - muscle cells stretch and recoil when released.

Question 2

What are the types of skeletal muscle?

Answer 2

a. Striations - alternating light and dark bands that result from overlapping arrangements of the internal contractile proteins.
b. Voluntary - usually subject to conscious control.
c. Involuntary - usually not under conscious control.

Question 3

What are muscle fibers (myofibers)?

Answer 3

Skeletal muscle cells

Question 4

What are the sarcoplasm (cytoplasm) contents?

Answer 4

a. Myofibrils - long protein cords (myofilaments) in the sarcoplasm.
b. Glycogen - starch-like carbohydrate that provide energy.
c. Myoglobin - red pigmented molecule which stores oxygen.
d. Multiple nuclei press against the sarcolemma (plasma membrane).
e. Sarcoplasmic reticulum (SR) - smooth ER, a reservoir of Ca2+.
f. Terminal cisternae - dilated ends of the SR.
g. Transverse (T) tubules - infoldings of the sarcolemma extending from one side to the other, convey electrical signals from the surface to the interior.

Question 5

What are myofilaments?

Answer 5

Complex of contractile protein molecules

Question 6

What are the types of myofilaments?

Answer 6

1. Thick filaments - bundle of myosin proteins.
2. Thin filaments - two intertwined strands of fibrous actin made of globular (G) protein actin.
3. Elastic filaments - proteins that anchor each filament to Z disc and M line.
4. Dystrophin - protein between the outer actin and the first linking protein that pulls on CT leading to the tendon when the actin moves.

Question 8

What are the characteristics of thick filaments?

Answer 8

a. Shaft-like tail of two intertwined chains.

b. A double globular head projecting from the tail at an angle.

Question 9

What are the characteristics of striations?

Answer 9

1. A band - dark band formed by parallel thick filaments that overlap thin filaments.
2. H band - lighter region of an A band that contains thick filaments only.
3. M line - line in the middle of an H band where thick filaments are linked.
4. I band - light band of thin filaments only.
5. Z disc - dark line in the middle of the I band where thin filaments and elastic filaments are anchored.
6. Sarcomere - from one Z disc to the next (the contractile unit).
7. A muscle shortens because its sarcomere shortens and pull the Z discs closer to each other, dystrophin with the linking proteins pulls the sarcolemma.

Question 10

What are motor neurons and motor units?

Answer 10

1. Skeletal muscle never contracts unless it is stimulated by a nerve.
2. Somatic motor neurons - nerve cells with cell bodies in the brainstem and spinal cord that innervate skeletal muscle.
3. Somatic motor fiber - axon from a neuron that branches out to a number of muscle cells and stimulates each one.
4. Motor unit - a neuron and all the muscle cells it innervates.

Question 11

What are the types of motor units?

Answer 11

a. Small motor unit - 3 to 6 muscle cells per neuron, for finer control.
b. Large motor units - 1,000 muscle cells per neuron, for strength.
c. To prevent fatigue, motor units are able to work in shifts, thus the muscle as a whole can sustain long-term contractions.

Question 12

What makes up the nerve-muscle relationship?

Answer 12

A. Motor Neurons and Motor Units
B. Neuromuscular Junction
C. Electrically Excitable Cells - muscle fibers and neurons

Question 13

What is synapse?

Answer 13

The point where a neuron meets its target cell (neuromuscular junction or motor end plate for muscles)

Question 14

What makes up the synapse?

Answer 14

a. Synaptic knob - dilated tip of an axon.
b. Synaptic cleft - gap between knob and muscle fiber.
c. A Schwann cell envelops the junction and isolates it.
d. Synaptic vesicles - vesicles in knob that contain and release ACh.
e. ACh - neurotransmitter that crosses the synaptic cleft.
f. ACh receptors - proteins in the sarcolemma that bind ACh.
g. Acetylocholinesterase (AChE) - enzyme in the cleft that breaks down ACh after the cell has been stimulated.

Question 15

What are the characteristics of electrically excitable cells?

Answer 15

1. In a resting cell there are anions on the inside of the cell and more cations on the outside, thus the plasma membrane is polarized or charged.
2. In a resting cell Na+ are in the ECF and K+ are in the ICF with the anions.
3. Resting membrane potential - the voltage across the plasma membrane of a resting cell (-90 mv).

Question 16

What are the actions of stimulation?

Answer 16

a. Depolarization - Na+ gates open and Na+ diffuse in causing the inside to be positive.
b. Action potential - a rapid voltage change in the plasma membrane that is accomplished by depolarization and is self-propagating.
c. Repolarization - K+ gates open and K+ diffuses out returning the inside of the cell to negative.
d. Na+-K+ pumps exchange Na+ inside for K+ outside to prepare for another stimulation.

Question 17

What are the behaviors of skeletal muscle fibers?

Answer 17

A. Excitation
B. Excitation-Contraction Coupling
C. Contraction
D. Relaxation
E. The Length-Tension Relationship and Muscle Tone

Question 18

What happens during excitation?

Answer 18

1. A nerve signal stimulates Ca2+ channels in the neuron membrane to open and Ca2+ enters the synaptic knob.
2. Ca2+ stimulates synaptic vesicles to release ACh into the synaptic cleft.
3. ACh diffuse across the cleft and bind to receptors on the sarcolemma.
4. The receptor gate opens and creates an end-plate potential (EPP).
5. The EPP triggers Na+ and K+ gates in the sarcolemma to open creating an action potential.

Question 19

What happens during excitation-contraction coupling?

Answer 19

1. The action potential spreads out to T tubules and travels down from them.
2. In the tubule the action potential stimulates release of Ca2+ from the SR.
3. Calcium binds to troponin on the tropomyosin.
4. Tropomyosin shifts exposing the active sites of the actin.

Question 20

What happens during contraction?

Answer 20

1. ATP that has bound to the myosin is hydrolyzed to ADP + Pi activating the head and extending it back.
2. Myosin head binds to the active site on the actin forming a cross-bridge.
3. ADP + Pi are released and the myosin flexes, pulling the actin over the myosin.
4. ATP binds to myosin breaking the cross-bridge, the head is ready to repeat the process further down.
5. When one myosin head releases the actin there are many other heads on the same myosin holding on.

Question 21

What happens during relaxation?

Answer 21

1. Nerve signals stop arriving, so the knob stops releasing ACh.
2. ACh dissociates from its receptors and AChE breaks it down.
3. Ca2+ is pumped back into the SR by active transport.
4. Ca2+ dissociate from troponin and go back into the SR.
5. Tropomyosin moves back into position and myosin can no longer bind.
6. A force must pull the muscle back to resting length.

Question 22

What is the Length-Tension Relationship and Muscle Tone?

Answer 22

1. The amount of tension generated by a muscle depends on how stretched or contracted it was before stimulation.
2. A muscle cell contracted at rest may contract only a little once stimulated, thus weak contraction.
3. A muscle cell too stretched at rest cannot form cross-bridges once stimulated, thus weak contraction.
4. Muscle tone - state of partial contraction in resting muscles that maintains the optimal resting length to generate the greatest force.

Question 23

What are the behaviors of whole muscles?

Answer 23

A. Threshold, Latent Period, and Twitch
B. Contraction Strength of Twitches
C. Isometric and Isotonic Contraction

Question 24

What is Threshold, Latent Period, and Twitch?

Answer 24

1. Threshold - the minimum voltage necessary to generate an action potential and produce a contraction.
2. Twitch - low frequency stimulation with a quick cycle of contraction and relaxation.

Question 25

What are the phases of twitches?

Answer 25

a. Latent period - delay between stimulation and the twitch when tension of the elastic components occurs. b. Contraction phase - the muscle begins to move an object or load. c. Relaxation phase - muscle tension declines.

Question 26

What is the Contraction Strength of Twitches?

Answer 26

1. Increase in excitation of nerve fibers in the motor nerve stimulates more and more motor units to contract. 2. Recruitment - bringing more and more motor units into contraction. 3. Treppe - at moderate frequency of stimulation, the muscle relaxes fully between contractions, but twitches are stronger as the muscle warms up. 4. Incomplete tetanus - produced when a high frequency stimulus arrives before the previous twitch is over, and the muscle relaxes only partially. 5. Complete tetanus - muscle had no time to relax between stimuli and twitches fuse into a smooth, prolonged contraction.

Question 27

What is Isometric and Isotonic Contraction?

Answer 27

1. Isometric contraction - contraction with no change in length, but change in tension. 2. Isotonic contraction - contraction with change in length, but no change in tension.

Question 28

What are types of isotonic contractions?

Answer 28

a. Concentric contraction - a muscle shortens as it maintains tension. b. Eccentric contraction - a muscle lengthens as it maintains tension.

Question 29

What are the ATP Sources?

Answer 29

1. All muscle contraction depends on ATP which depends on O2, glucose and fatty acids. 2. Immediate energy - borrowing Pi to make ATP when O2 is used up. 3. Short-term energy - muscles obtain glucose from blood and glycogen stores and use anaerobic fermentation to make ATP. 4. Long-term energy - aerobic respiration is supported by cardiopulmonary function.

Question 30

What are the pathways of ATP synthesis?

Answer 30

a. Anaerobic fermentation - ATP is produced without O2, but yield is small and lactic acid is formed. b. Aerobic fermentation - more ATP is produced and no lactic acid is formed, but O2 is required.

Question 31

What happens to create immediate energy?

Answer 31

a. Myokinase transfers Pi from ADP to another ADP to get AMP+ATP. b. Creatine kinase transfer Pi from creatine phosphate to ADP to form creatine + ATP.

Question 32

What are the characteristics of thin filaments?

Answer 32

a. Each G actin has an active site that binds to the head of a myosin. b. Tropomyosin - protein filament that blocks the active sites when the muscle is relaxed. c. Troponin - calcium-binding protein on the tropomyosin.

Question 33

What is fatigue and endurance?

Answer 33

1. Fatigue - weakness and loss of contractivity from prolonged use of muscle. 2. Endurance is determined by one's maximum oxygen uptake (mL/min/Kg).

Question 35

What are the causes of fatigue during high-intensity, short-duration exercise?

Answer 35

a. K+ accumulation in ECF makes muscle fiber less excitable. b. ADP/Pi accumulation slows down hydrolysis of ATP. c. Lactic acid accumulation in the muscle cell lowers the pH.

Question 36

What are the causes of fatigue during low-intensity, long-duration exercise?

Answer 36

a. Fuel depletion - decline in levels of glycogen and blood glucose. b. Electrolyte loss through sweating can alter ion balance of the ECF. c. Central fatigue - central nervous system produces fewer signals.

Question 37

What is oxygen debt?

Answer 37

The difference between the resting rate of O2 consumption and the excess post exercise oxygen consumption (EPOC).

Question 38

What are the uses of O2 from heavy breathing?

Answer 38

a. Replace the body's oxygen reserves that were depleted. b. Replenish the immediate energy. c. Oxidize lactic acid to pyruvic acid and glucose. d. Accommodate the elevated metabolic rate.

Question 39

What are the physiological classes of muscle fibers?

Answer 39

1. Slow oxidative - red color with lots of O2 available, slow twitch, high endurance. 2. Fast glycolytic - white color with lots of glycogen and phosphates available, fast twitch, high fatigue. 3. Muscles are composed of both slow oxidative and fast glycolytic fibers but the proportions differ from muscle to muscle.

Question 40

What is cardiac muscle (heart)?

Answer 40

1. Cardiocyte is striated, short and thick, slightly branched, and involuntary. 2. Intercalated disc - linkage at the ends that join cardiocytes. 3. Gap junction at the intercalated disc allows each cardiocyte to stimulate its neighbors and mechanical junctions keep cells from pulling apart. 4. Damaged cells are repaired by fibrosis. 5. Autorhythmic - the heart can contract rhythmically and independently without nervous stimulation because it has a built-in pacemaker. 6. The muscle uses aerobic respiration, thus it is rich in myoglobin and glycogen and has large mitochondria.

Question 41

What is smooth muscle?

Answer 41

1. The myocytes have no striation but do have actin and myosin, fusiform shape and are involuntary. 2. Autonomic axons have swellings that release neurotransmitters into the smooth muscle and the myocytes respond. 3. They are slow to contract and relax but can remain contracted without fatigue and with minimal energy use. 4. Propels the contents of organs, changes speed of air or blood flow, regulates pupil diameter, and moves hair follicles. 5. Only types of muscle capable of mitosis and hyperplasia.

Question 42

What are the types of smooth muscle?

Answer 42

a. Multiunit smooth muscle - branches of a nerve (axon) fiber synapse with individual myocytes forming a motor unit. b. Single unit smooth muscle - myocytes are coupled by gap junctions which allow cells to stimulate each other once neurotransmitters are released into the tissue.

Question 43

What is the excitation of smooth muscle?

Answer 43

a. Autonomic nerve fibers and neurotransmitters. b. Chemicals - smooth muscles react to CO2, O2, NO, low pH. c. Temperature - cold induces contraction and warmth relaxes. d. Stretch - filling with contents with stimulate contraction. e. Autorhythmicity - some myocytes act as pacemaker cells.

Question 44

What happens during contraction and relaxation?

Answer 44

a. As myosin pull on actin the force is transferred to the plasma membrane and the entire cell shortens, puckers, and twists. b. Latch mechanism - myosin remains attached to actin for a prolonged time without consuming ATP. c. Smooth muscle tone - continual state of muscle contraction. d. Stress-relaxation response - when stretched muscles briefly contract and resist, but then relax. e. Smooth muscle can contract forcefully even when greatly stretched.

Question 45

What are the disorders of the muscular system?

Answer 45

A. Contracture - abnormal muscle shortening not caused by nervous stimulation. B. Cramps - painful muscle spasms caused by rapid firing of motor neurons. C. Delayed-onset muscle soreness - pain, etc. felt after strenuous exercise associated with microtrauma to the muscle. D. Disuse atrophy - reduction in size of muscle fibers from no use. E. Fibromyalgia - diffuse, chronic muscle pain and tenderness. F. Myositis - muscle inflammation and weakness from infection or autoimmune disease. G. Muscular dystrophy - hereditary diseases in which the muscles degenerate, weaken and are gradually replaced by fat and scar tissue. H. Myasthenia gravis - autoimmune disease in which antibodies attack the neuromuscular junctions so that the muscle fibers become less sensitive to ACh.