Skeletal muscle (Dr. Houston)

Question 1

What are the three main types of muscle ?

Answer 1

Skeletal, cardiac (both striated) and smooth.

Question 2

What is a neurogenic contraction ?

Where do we find this type of contraction ?

Answer 2

A contraction initiated by APs in the motor nerves.

These are found in skeletal muscle.

Question 3

What is a myogenic contraction ?

Where do we find this type of contraction ?

Answer 3

A contraction initiated by AP in the autonomic nerves.

Cardiac and smooth muscle have a intrinsic rhythm that is modulated by APs in these nerves.

Question 4

How is a skeletal muscle contraction initiated ?

Answer 4

Nerve AP –> ACh secretion by nerve ending –> end-plate potential –> muscle AP –> depolarization of T-tubules + opening of Ca2+ channels of SR –> increase in sarcoplasmic Ca2+ –> contraction –> pump Ca2+ back into SR –> relaxation

Question 5

How does the cytosolic [Ca2+] vary between muscle contraction and relaxation ?

Answer 5

Rest: [Ca2+] = 100 nmol
Contraction: [Ca2+] = 0.1 - 1 μmol

Question 6

By what kind of nerves are skeletal muscles supplied ?

Answer 6

Myelinated motor nerves that originate from the CNS.

Question 7

How many nerve fibers can a single axon supply ?

Answer 7

Many.

Question 8

What is a motor unit ?

Answer 8

The motor neuron, its axon and all the muscle fibres supplied by the axon and its branches.

Question 9

What are the 4 connective tissues that group that group the different elements of the skeletal muscle ?

Answer 9

Epimysium: sheath of fibrous elastic tissue surrounding the skeletal muscle
Perimysium: sheath of fibrous elastic tissue surrounding a bundle of muscle fibers
Endomysium: connective tissue that ensheaths each individual myocyte/muscle fiber (made of many myofibrils)
Sarcolemma: fine transparent tubular sheath which envelops the muscle fiber and lies just underneath the endomysium

Question 10

Are myocytes single or multi-nucleated ?

Answer 10

Multi-nucleated.

Question 11

What are transverse tubules (T-tubules) ?

Answer 11

Deep invaginations of the sarcolemma. These invaginations allow depolarization of the membrane to quickly penetrate to the interior of the cell.

Question 12

What are terminal cisternae ?

What is their fct ?

Answer 12

Terminal cisternae are enlarged areas of the sarcoplasmic reticulum surrounding the T-tubules.
These act as a calcium storage site (not the same as the one utilized for bone tissue).

Question 13

What is the sarcoplasmic reticulum (SR) ?

Answer 13

The sarcoplasmic reticulum is a specialized type of smooth ER that regulates the calcium ion concentration in the cytoplasm of striated muscle cells.

Question 14

What is the H-zone ?

Answer 14

The zone of the thick filaments that is not superimposed by the thin filaments.

Question 15

What is the A-band ?

Answer 15

The A-band contains the entire length of single thick filaments.

Question 16

What is the Z-disc ?

Answer 16

It connects thin actin filaments and is in the middle of the I-band.

Question 17

What is the I band ?

Answer 17

The zone of thin filaments that is not superimposed by thick filaments.

Question 18

What is the M line ?

Answer 18

A line w/in the H-zone formed of cross-connecting elements of the cytoskeleton.

Question 19

What are the two main proteins that constitute the myofibrils ?

Answer 19

Thin actin filaments and thick myosin filaments.

Question 20

What is tropomyosin ?

Answer 20

Tropomyosin is a two-stranded alpha-helical coiled coil protein that, in its resting state, surrounds actin and blocks its myosin binding site.

Question 21

What is troponin ?

Answer 21

Troponin, or the troponin complex, is a complex of three regulatory proteins (troponin C, troponin I, and troponin T) that is able to bind calcium released in the sarcoplasm. This enables it to displace tropomyosin, thus allowing the actin molecules to bind to the myosin heads.

Question 22

What are the 4 stages of the sk muscle contraction cycle ?

Answer 22

1. Myosin heads hydrolyze ATP and become reoriented and NRGized
2. Myosin heads bind to actin, forming crossbridges
3. Myosin crossbridges rotate toward center of sarcomere (power stroke)
4. As myosin heads bind ATP, the cross bridges detach from actin
Contracction continues if ATP is available and if Ca2+ levels in sarcoplasm are high

Question 23

By what mechanism does rigor mortis occur ?

Answer 23

Rigor mortis occur when we die –> no more ATP –> the cross bridges no longer detach from actin –> muscle stays “contracted” despite death

Question 24

What is the latency between the muscle AP and the initiation of contraction ?
Do the AP and the contraction last the same time ?

Answer 24

The contractile response is initiated after the muscle action potential and lasts much longer than the action potential.

Question 25

What factor affects the force of muscle contraction ?

Answer 25

Different frequencies of stimulation affect contractile force.

Question 26

What are the 2 main types of muscle contraction ?

Answer 26

Isometric contraction: muscle length is held constant, and the force generated during the contraction is then measured.
Isotonic contraction: the force (or tone) is held constant, and the change in length of the muscle is then measured.

Question 27

What are the 2 subtypes isotonic contraction ?

Answer 27

Concentric contraction: muscle tension rises to meet
the resistance, then remains the same as the muscle
shortens
Eccentric contraction: muscle
lengthens due to the resistance being greater than the force the muscle produces

Question 28

How does contractile force change as the muscle length is increased ?

Answer 28

Contractile force increases as muscle length

is increased up to a point Lo (optimal length). As the muscle is stretched beyond Lo, contractile force decreases.

Question 29

What happens as sarcomere length decreases below 2μm ?

Answer 29

The thin filaments collide in the middle of the sarcomere, and the actin-myosin interaction is disturbed and hence contractile force decreases

Question 30

Why is their an optimal muscle length Lo ?

Answer 30

At Lo, the amount of overlap increases, and contractile force thus increases too.

Question 31

What happens at very long sarcomere length (3.65 μm) ?

Answer 31

The actin filaments no longer overlap with myosin filaments, so there is no contraction.

Question 32

What factors influence the amount of force exerted on the tendons by a muscle ?

Answer 32

• The number of active motor units
• The cross-sectional area of the muscle
• The frequency of stimulation
• The rate at which the muscle shortens
• The initial resting length of the muscle

Question 33

What are the 3 different types of muscle fibers ?

Answer 33

Type 1: Slow Oxidative (light)
Type 2A: Fast Oxidative (darkest)
Type 2B: Fast Glycolytic (Intermediate)

Question 34

How we measure the force, work and power of muscles ?

Answer 34

Newtons:  Force = mass × acceleration
Joules: Work = force × distance moved by load
 Watts: Power = work ÷ time
= (force × distance) / time
= force × velocity

Question 35

On a scale of 0 to 100, what is the optimal velocity of shorting of a muscle ?
How does this affect power ?

Answer 35

Approximately 33 (1/3 of maximal shorting velocity).
At this velocity, the power can only increase if the force is increased (Power = force × velocity of contraction)

Question 36

What are the physical characteristics of muscle cells ?

Answer 36

Myocytes = long, cylindrical and multi-nucleated..

Question 37

What are individual muscle fibers made of ?

Answer 37

They contain myofibrils made of repeating units called sarcomeres.

Question 38

What are sarcomeres ?

Answer 38

They are the fundamental contractile contractile units of myocytes.