Muscle physiology

Question 1

What is a sarcomere?

Answer 1

It is the most basic functional unit of a muscle. Sarcomeres are what make up myofibrils, which in turn make up muscle fibres.

Question 2

Describe the A band of a sarcomere

Answer 2

It is the central Area consisting of thick filaments, as well as portions of thin filaments (in the zone of overlap)

Question 3

What are the three subdivisions of the A band? describe these subdivisions

Answer 3

M line: Most central part of the the A band. Consists of proteins that connect one thick filament to the neighbouring thick filaments.

H zone/band: Lighter region on either side of the M line.

Zone of overlap: Where thin filaments are situated between thick filaments.

Question 4

Describe the I band

Answer 4

it is the sides lateral to the A band, and consists only of thin filaments and no thick filaments

Question 5

What are the features or structures found in the I band

Answer 5

Z line: It marks the boundaries between two adjacent sarcomeres.
Titin: An elastic protein

Question 6

What is titin? Describe its function as well as where it is found

Answer 6

Titin is an elastic protein that s found between the Z line and the thick filament.
-Its function is to help keep thick and thin filaments in proper alignment, and to help the sarcomere to return to its resting length after contraction.

Question 7

What are the four proteins found in the thin filaments?

Answer 7

F actin, nebulin, tropomyosin and troponin

Question 8

Describe F actin

Answer 8

F actin is a twisted strand that is made up of two rows of individual globular proteins called G actin. Each G actin has an active site for myosin to bind

Question 9

Describe nebulin

Answer 9

nebulin is a long strand that is between the two rows of G actin molecules. Its function is to hold the F actin strands together.

Question 10

Describe tropomyosin

Answer 10

Tropomyosin is a protein that covers the active sites on G protein thus preventing actin-myosin interaction. It is bound to troponin to form the troponin-tropomyosin complex.

Question 11

Describe troponin

Answer 11

It is a globular protein that consists of three subunits:
The first unit binds to tropomyosin
The second subunit binds to one G actin molecule
The third one has a receptor that binds 2 Ca++ ions

Question 12

Describe the thick filaments

Answer 12

Thick filaments consist of myosin molecules. The myosin molecules have myosin heads which are projections that interact with actin in the thin filaments during a contraction

Question 13

What is a cross-bridge

Answer 13

A cross bridge is when myosin binds to actin during a contraction.

Question 14

Explain the sliding filament theory

Answer 14

When the myosin head in the thick filament interacts with the actin in the thin filament, resulting in the actin sliding towards the center of the sarcomere and resulting in the shortening of the sarcomere

Question 15

What is the intercellular connection between the neuron and a muscle fiber?

Answer 15

neuromuscular junction

Question 16

What are the steps of muscle contraction?

Answer 16

ACh released, binding
to receptors
Action potential reaches T tubule

Question 16

What are the steps of muscle contraction?

Answer 16

1. ACh released, binding to receptors
2. Action potential reaches T tubule
3. Activate DHP receptor
4. DHP receptors causes RyR receptor to open Ca++ release channels in SR
5. Calcium channels in the Sarcoplasmic reticulum releases Ca2+
6. Active site exposure, cross-bridge formation
7. Contraction begins

muscle contraction is initiated by arrival of Ca++ in the sarcomere (at the zone of overlap).

Question 17

What are the steps of muscle relaxation?

Answer 17

1. ACh broken down by AChE
2. Sarcoplasmic reticulum recaptures Ca2+
3. Active sites covered, no cross-bridge interaction
4. Contraction ends
5. Relaxation occurs, passive return to resting length

Question 18

Explain the steps of contraction occuring at the molecular level

Answer 18

1. Ca++ binds to troponin 
2. Troponin activates & changes shape, causing displacement of tropomyosin
3. Active sites on actin are exposed, and energized myosin heads bind to exposed active sites
4. Crossbridge formed
5. Power stroke occurs (sliding filament mechanism)
6. Myosin heads remain in rigor state till a new ATP binds and terminates crossbridge
7. Myosin heads go back to initial state before the next power stroke is initiated

Question 19

What is the difference between isometric and isotonic contraction?

Answer 19

Isometric:
Occurs when the muscle does not shorten during contraction.
The muscle contracts against a force transducer without decreasing the muscle length.

Isotonic:
Occurs when the muscle shortens during contraction but the tension on the muscle remains constant throughout the contraction.
The muscle shortens against a fixed load

Question 20

What are the characteristics of fast twitch fibres?

Answer 20

Type II fibres, white muscle
Large fibres with extensive SR for rapid release of Ca2+ to initiate contraction.
Less extensive blood supply because oxidative metabolism is secondary important
Fewer mitochindria, because oxidative metabolism is secondary importance.
Large amounts of glycolytic enzymes for rapid release of energy by glycolysis
Less myoglobin, hence the name “white muscle”

Question 21

What are the characteristics of slow twitch fibres?

Answer 21

Type 1 fibers 
Smaller fibers
Innervated by smaller nerves
More extensive blood supply
Greatly increase numbers of mitochondria
Fibers contain large numbers of myoglobin

Question 22

What is hypertrophy?

Answer 22

Increase in muscle mass caused buy the increase in contractile proteins (actin and myosin) in each muscle fibre, thus increasing the size of the muscle fibre.

Question 23

What is muscle atrophy?

Answer 23

Decrease in muscle mass due to muscle inactivity

rate of contractile proteindegradation > rate of replacement

Question 24

What is hyperplasia

Answer 24

A condition in which there is an increase in the number of muscle fibres

Question 25

How do muscles adapt to being stretched or shortened for a long time?

Answer 25

Stretched: When muscles are stretched to greater than normal length, new sarcomeres are added at the ends of the muscle fibers, and attach to tendons.

Shortened: muscles that continually remain shorten to less than its normal length, sarcomeres at the end of the muscle fibers can actually disappear