skeletal muscle contraction

Question 1

what is the primary function of muscles

Answer 1

to generate force or movement in response to a physiological stimulus
all muscles transduce a chemical or electrical signal into a mechanical response

Question 2

how do muscles contract (basic)

Answer 2

the trigger for contraction of all three types of muscle is an increase in intracellular calcium concentration

Question 3

background of skeletal muscle contraction

Answer 3

contracts in response to neuromuscular synaptic transmission
a skeletal muscle cell has a single neuromuscular junction where acetylcholine receptor are concentrated
Ach released from the pre-synaptic nerve terminal binds to nicotinic Ach receptors at the NMJ

Question 4

what is the involvement of nicotinic Ach receptors in skeletal muscle contraction

Answer 4

these receptors are non-selective cation channels that open in response to Ach binding, resulting in a depolarisaiton known as an end plate potential (EPP)
if the EPP exceeds threshold for activating voltage-gated Na+ channels, an action potential is generated

Question 5

how is skeletal muscle contraction initiated

Answer 5

generation of an action potential initiates the sequence of events leading to contraction

Question 6

how is acetylcholine inactivated

Answer 6

acetylcholine is inactivated by acetylcholinesterase

Question 7

what is the structure of muscle

Answer 7

whole skeletal muscle (an organ)
muscle fibre
myofibril
sarcomere

Question 8

what is muscle fibre

Answer 8

a single muscle cell
multiple nuclei
many mitochondria
bundled together by connective tissue to form whole skeletal muscle

Question 9

what is myofibril

Answer 9

a specialised intracellular structure
has a dark A band and a light I band

Question 10

what is a sarcomere

Answer 10

the smallest functional unit

Question 11

what is contained within a sarcomere

Answer 11

Z line - border of sarcomere
thick filament - myosin protein
thin filament - actin protein
H zone - no overlap of thick and thin filaments
M line - support system for thick filament
A band - thick and thin filaments, H zone and M line
I band - thin filament from two adjacent sarcomeres

Question 12

what is the composition of skeletal muscle fibres

Answer 12

striated by a highly organised internal arrangement
a single skeletal muscle cell is relatively large, elongated and cylinder shaped
≈ 10-100µm in diameter and up to ≈75cm in length

Question 13

what is the composition of myofibrils

Answer 13

specialised contraactile elements that extend the entire length of the muscle fibre
muscle fibre can contain 100s to 1000s of myofibrils
each myofibril consists of a regular arrangement of cytoskeletal elements - the thick and thin filaments

Question 14

the thick filament (basic)

Answer 14

special assemblies of the protein myosin

Question 15

the thin filament (basic)

Answer 15

made up primarily of the protein actin

Question 16

what is the thin filament

Answer 16

a relaxed skeletal muscle fibre containing actin, troponin and tropomyosin
a double stranded alpha-helical F-actin with a myosin binding site

Question 17

what is tropomyosin

Answer 17

a thread-like coil wrapped around actin to cover the myosin binding site

Question 18

what is troponin

Answer 18

a heterotrimer of troponin T, troponin C and troponin I
each heterotrimer of troponin interacts with a single molecule of tropomyosin, which in turn interacts directly with 7 actin monomers

Question 19

what is the function of troponin T

Answer 19

binds to a single molecule of tropomyosin

Question 20

what is the function of troponin C

Answer 20

binds calcium

Question 21

what is the function of troponin I

Answer 21

binds to actin and inhibits contraction

Question 22

what is the thick filament

Answer 22

composed of multiple myosin II molecules

Question 23

what is a myosin II molecule

Answer 23

a double trimer containing:
2x alkali (essential) light chains
2x regulatory light chains
2x intertwined heavy chains

Question 24

what is the function of the 2 alkali light chains of myosin

Answer 24

stabilises the myosin head region

Question 25

what is the function of the 2 regulatory chains of myosin

Answer 25

regulates the ATPase activity of myosin

Question 26

what is the composition of the 2 intertwined heavy chains of myosin

Answer 26

contains: a tail a hinge a headw

Question 27

what is the function of the tail region of the heavy chain of myosin

Answer 27

alpha-helices that intertwine

Question 28

what is the function of the hinge region of the heavy chain of myosin

Answer 28

molecule opens to form 2 globular heads

Question 29

what is the function of the head region of the heavy chain of myosin

Answer 29

the cross-bridges between the thick and thin filaments of the sarcomere the heads of the heavy chains each possess a binding site for actin and a site for hydrolysing ATP

Question 30

what is the process of muscle contraction

Answer 30

a cycle in which myosin II heads bind to actin, these cross-bridges become distorted, and finally the myosin heads detach from actin energy comes from the hydrolysis of ATP an increase in intracellular calcium concentration triggers contraction by removing the inhibition of cross-bridge cycling upon stimulation the intracellular calcium concentration may rise from its resting level of <10-7M to >10-5M the subsequent decrease in intracellular calcium concentration is the signal to cease cross-bridge cycling and relax

Question 31

what is the role of calcium in muscle contraction

Answer 31

calcium modulates contraction via regulatory proteins rather than interacting directly with the contractile proteins

Question 32

what is the effect of an absence of calcium in muscle contraction

Answer 32

in the absence of calcium, these regulatory proteins act together to inhibit actin-myosin interactions, thus inhibiting the contractile process

Question 33

what is the effect of a presence of calcium in muscle contraction

Answer 33

when calcium binds to one or more of these proteins, a conformational change takes place in the regulatory complex that releases the inhibition of the contraction

Question 34

what is the composition of troponin C

Answer 34

2 high affinity Ca2+ binding sites 2 low affinity Ca2+ binding sites

Question 35

what is the function of the high affinity calcium binding sites of troponin C

Answer 35

participate in binding troponin C to the thin filament calcium binding to these sites does not change during muscle activation

Question 36

what is the function of the low affinity calcium binding sites of troponin C

Answer 36

binding of calcium to these sites induces a conformational change in the troponin complex

Question 37

what is the effect of calcium binding to the low affinity calcium binding sites of troponin C

Answer 37

1. troponin I shifts, permitting the tropomyosin molecule to move 2. via troponin T, tropomyosin is moved away from the myosin-binding site on actin and into the actin groove 3. the myosin head is now able to interact with actin and engage in cross-bridge cycling

Question 38

what is the cross-bridge cycle

Answer 38

contractile proteins convert the energy of ATP hydrolysis into mechanical energy

Question 39

what are the five steps of the cross-bridge cycle

Answer 39

1. ATP binding 2. ATP hydrolyis 3. cross-bridge formation 4. release of Pi from myosin 5. ADP release

Question 40

what is the sliding filament mechanism

Answer 40

cross-bridge interaction between actin and myosin brings about muscle contraction by means of the sliding filament mechanism the thin filaments on each side of the sarcomere slide inward, over the stationary thick filaments - the thin filaments pull the Z lines closer together, so the sarcomere shortens as all sarcomeres throughout the muscle fibres' length shorten simultaneously, the entire fibre shortens

Question 41

what happens to the length of the filaments during the sliding filament mechanism

Answer 41

do not shorten A band - determined by thick filaments so width stays the same I band - thin filaments not overlapping thick, so width decreases H zone - within A band, thick filaments not overlapping thin, so width decreases Z lines - distance between them decreases

Question 42

what is rigor mortis

Answer 42

"the stiffness of death" begins 3-4 hours after death and completes in ≈12 hours

Question 43

how does rigor mortis occur

Answer 43

following death, intracellular calcium concentration begins to rise this calcium lets the regulatory proteins move aside, letting actin bind myosin cross-bridges that were already charged with ATP prior to death dead cells cannot produce ATP, so actin and myosin, once bound cannot detach during the next several days, rigor mortis gradually subsides as proteins involved start to degrade.