10 & 11 Skeletal muscles physiology

Question 1

A thin filament consist of 2 strands of polymerized actin molecules + _______ + _________.

Answer 1

Troponin (ball shape)

Tropomyosin (worm shape)

Question 2

Myosin binding site on each G-actin molecules are blocked by __________ in resting state.

Answer 2

Tropomyosin

Question 3

Thick filament are made up multiple _____________.

Each myosin consists of 2_________,2________ and 2__________.

Answer 3

myosin molecules ;

2 intertwined heavy chains +
2 regulatory light chains (RLC) +
2 essential light chains (ELC)

Question 4

In the heavy chains of the myosin molecule in thick filaments, it is divided into head, neck and tail.

The myosin head = a site for __________ + a site for _________.

The ______ means the devoid of the myosin head.

Answer 4

binding actin;
binding and hydrolyzing
ATPase

Bare zone

Question 5

What is the basic unit of striated muscles?

It is bordered by 2 ______?

Answer 5

sarcomeres;

Z-lines (for attachments of actin filaments)

Question 6

\_\_\_\_\_\_ = actin filament only, attached to Z line;
\_\_\_\_\_\_ = myosin only; attached to M line ;
\_\_\_\_\_ = Actin + myosin

Answer 6

I band (light band);
H zone;
A band

Question 7

M line serves as an attachment site for myosin filaments. They are thick filaments with attachment of accessory proteins.

________ attaches to Z disk and helps align the actin filaments of the sarcomere.

_________ extend along each thick filament from the M line to Z disk to anchor myosin filaments in proper stiffness of muscle, provide muscle with elasticity, prevent ______.

Answer 7

Nebulin;

Strands of titin;
over-stretching

Question 8

____________ represents an invagination of cell membrane. Allows action potentials to propagate rapidly into interior of muscle fiber.

Two __________ of sarcoplasmic reticulum flanked on either side of T-tubule, forming a triad.

Answer 8

T- tubule (Transverse tubule);

Terminal cisternae

Question 9

Sarcoplasmic reticulum represents an intracellular store of Ca2+ needed for muscle contraction.
What will happen next when there is an arrival of membrane depolarization?

Answer 9

Ca2+ is released from the terminal cisternae of SR to the cytosol where it initiates cross-bridging between actin and myosin and hence muscle contraction.

High density of Ca2+ -ATPase (SERCA) is located along the longitudinal tubules of SR for Ca 2+ reuptake into SR for relaxation of muscle.

Question 10

Troponin and tropomyosin are called __________ since they control the initiation and termination of the cross-bridge cycle between actin and myosin

Answer 10

regulatory proteins

Question 11

Troponin is a heterotrimer of TnT (binds ________), TnC (binds _______) and Tnl (binds actin to _________)

Answer 11

tropomyosin;
Ca2+;
inhibit its interaction with myosin

Question 12

In relaxed muscle, tropomyosin blocks _________ on actin, prevent cross-bridge cycle.

Following release from SR, Ca2+ bind to __________, causing a conformational change in _________ to push the tropomyosin aside to expose the myosin-binding site.

Answer 12

myosin-binding sites;

troponin (TnC);
troponin complex

*Excitation-contraction coupling in skeletal muscle is a lot faster than smooth muscle as it only involves Ca2+ bind to troponin C (TnC) to produce a conformational change in troponin complex to push tropomyosin aside to expose the myosin binding site

Question 13

List the events happened in the neuromuscular junction.

Phase 1 and 2

Answer 13

(PHASE 1)
1. Arrival of action potential at the axonal terminal of motor nerve opens voltage-gated Ca2+ channels

2. Ca2+ influx triggers the release of acetylcholine-containing synaptic vesicles
3. Ach diffuses across the synaptic cleft and binds to muscle-type nicotinic cholinergic receptors on the motor end plate.
4. Opening of non-selective cationic channels
5. Muscle-type nicotinic cholinergic receptors are ligand-gated ion channels located at the tip of junctional folds.
   Net Na+ entry (> K+ efflux) causes depolarization to generate end plate potential (EPP)
6. EPP spreads to extra-junctional membrane where voltage-gated Na+ channels are located, it triggers the generation of action potential.(PHASE 2)
7. AP propagates down the sarcolemma and down T tubules, cause the release of Ca2+ from intracellular store to trigger muscle contraction
8. Ach is removed by acetylcholinesterase, which is attached to the basal lamina of the synaptic cleft.

Question 14

Full process of Phase 2 involves ______ receptor and _____ receptor.
What are their functions?

State the events happen after that.

Answer 14

Dihydropyridine receptor (L-type Ca2+)
- act as a voltage sensor in skeletal muscle

Ryanodine receptor

• DHP receptors are physically coupled with ryanodine receptors
• Arrival of AP sensed by DHP receptors leads to opening of ryanodine receptors to release Ca2+ from SR

Then Ca2+ is uptaken by troponin > conformation change in tropomyosin > actin filaments slide on myosin filaments > force production

Question 15

As the muscle contracts, the sarcomere together with the H zone and I band shorten, while the Z disks move
closer together. Why does the length of A band doesn’t change?

Answer 15

Sliding of thin actin filaments along the thick

myosin filaments toward the M line. Myosin don’t move

Question 16

A term that describes the temporary rigidity of muscles after death is?

Answer 16

Rigor mortis

Question 17

Rigor mortis

A. Following the depletion of ATP, no ATP to release the cross-bridge of myosin head to actin filament (rigor state)

B. Ca2+ leaks from SR/ fails to get removed by SERCA, thus enables binding of myosin and actin

C. After 24 hours, proteolytic degradation of cross bridges by lysosomal enzymes

Answer 17

A and B

C: 72 hours

Question 18

State the processes of skeletal muscle relaxation.

Answer 18

1. Ending Ach release and Ach removed by AChE.
2. SERCA pumping Ca2+ back into SR, free cytosolic Ca2+ decreases
3. Ca2+ dissociates from binding to troponin, tropomyosin slides back to block the myosin binding site, terminating the cross-bridge cycle
4. Inside SR, Ca2+ is sequestered by binding to a low-affinity Ca2-binding protein calsequestrin. Thus no concentration gradient is formed.

(Unlike smooth muscle, extracellular Ca2+ is not important)

Question 19

What does isotonic and isometric contraction means respectively?

Answer 19

Isometric:
Fixed muscle length; sarcomere shortens while stretching elastic element, allowing muscle to remain in the same length;

Isotonic:
Same force, different lengths of muscles (because the elastic elements has already stretched, entire muscle must shorten)

Question 20

______ and connective tissues that attaches muscle to bone are in series with sarcomere.
_____ are in parallel with sarcomere.

Answer 20

Tendons;

Titins

Question 21

The elastic elements in series are stretched more in _________ contraction.

The elastic elements in parallel help ________________.

Answer 21

Isometric;

help return muscle to resting length

Question 22

A motor unit consist of a motor neuron and all muscles fibre its innervate.

Motor unit differ in _______ and ________________.

Answer 22

size;

recruitment threshold

Question 23

What is size principle of motor unit?

Answer 23

Motor unit with low threshold will be recruited first. As stimuli increases, the motor unit with
high threshold and bigger size will be recruited

> summation of muscle tenson

Question 24

Which of the following are correct?

A. Twitch =Contraction of a muscle to AP

B. Latent period = the
time required for excitation-contraction coupling before cross-bridge

C. Duration of twitch is greater than AP, hence multiple AP and force of contraction could be summated.

Answer 24

B and C only

A: Twitch = a complete response (Contraction and relaxation) of a muscles to AP

Question 25

__________ occurs when a rapid succession of twitches add together to produce a rather sustained
tension and yet it is composed of distinguishable twitches.

Answer 25

Incomplete tetanus

Question 26

____________ is reached when the muscle tension reaches a maximum steady level with no distinguishable
twitches.

Answer 26

Complete tetanus

Question 27

5 Factors determining the tension developed in skeletal muscle?

Answer 27

1. Fatigability, energy metabolism affected by types of muscle fibers
2. Frequency summation of AP, higher Ca2+, more repeated cross-bridge to occur. > tetanus
3. Number of sarcomeres in parallel (increase muscle force)
4. Length of individual sarcomere (optimal overlapping)
5. Recruitment of more motor units.
   - CNS can elicit asynchronous recruitment of motor units to avoid fatigue – different motor units take turns in maintaining the muscle tension through contracting and relaxing at slightly different times.

Question 28

What are the 3 sources of ATP to supply muscle activity?

Answer 28

1. ATP and phosphocreatinine provide immediately available sources of energy for short duration (10-20 seconds)
2. Glycolysis provides limited supply of energy due to accumulation of lactic acid
3. Aerobic respiration can supply energy to muscle activity to prolonged periods.

Question 29

Muscle spindles are composed of intrafusal muscle fibers (contractile at both ends + innervated by motor nerve fibers)

The 2 types of intrafusal fibers are:

__________: larger, nuclei accumulated in central bag region
__________: smaller, nuclei arranged in rows

Answer 29

Nuclear bag fibers;

Nuclear chain fibers

Question 30

Central portion of both nuclear bag and nuclear chain fibers are innervated by ________ sensory nerve endings of type ____ fibres

Secondary sensory nerve endings of type ___ fibers primarily innervate the ____________. (slower conduction velocity)

Answer 30

primary;
Ia;

II;
nuclear chain fibers

Question 31

What are the main difference between Type 1a and Type II fibers?

Answer 31

Type Ia is for rate of change

Type II is for degree of change

Question 32

When extrafusal muscle fibers contract under the command via ____ motor neurons, _____ motor neurons innervating the intrafusal muscle fibers are co-activated.

Answer 32

alpha;
gamma

(alpha-gamma coactivation)

Question 33

_________ are stretch receptors arranged in series with the muscle fibers at their junction with the muscle tendons.

Each ___________(same) is composed of free nerve endings that wind between collagen fibers

Answer 33

Golgi tendon organs

Golgi tendon organ

Question 34

Golgi tendon organs

A. senses the tension when the muscles contract. * Isometric > Isotonic

B. In contrast to muscle spindle, they do not sense the passive stretch of relaxed muscles, which are absorbed mainly by muscles itself

C. Active stretch must be sensed to prevent overstretching

Answer 34

All correct

Question 35

What is a Golgi tendon reflex?

Answer 35

Sensory input of tension during isometric contraction.
When excessive load is given to the muscles, the sensory input is relayed to an inhibitory interneuron to cause reflex relaxation to protect the muscles.

Question 36

What can elicit a Golgi tendon reflex?

Answer 36

• cramping: stretching of muscles can elicit Golgi tendon reflex