skeletal muscle contraction Flashcards
what is the primary function of muscles
to generate force or movement in response to a physiological stimulus
all muscles transduce a chemical or electrical signal into a mechanical response
how do muscles contract (basic)
the trigger for contraction of all three types of muscle is an increase in intracellular calcium concentration
background of skeletal muscle contraction
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
what is the involvement of nicotinic Ach receptors in skeletal muscle contraction
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
how is skeletal muscle contraction initiated
generation of an action potential initiates the sequence of events leading to contraction
how is acetylcholine inactivated
acetylcholine is inactivated by acetylcholinesterase
what is the structure of muscle
whole skeletal muscle (an organ)
muscle fibre
myofibril
sarcomere
what is muscle fibre
a single muscle cell
multiple nuclei
many mitochondria
bundled together by connective tissue to form whole skeletal muscle
what is myofibril
a specialised intracellular structure
has a dark A band and a light I band
what is a sarcomere
the smallest functional unit
what is contained within a sarcomere
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
what is the composition of skeletal muscle fibres
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
what is the composition of myofibrils
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
the thick filament (basic)
special assemblies of the protein myosin
the thin filament (basic)
made up primarily of the protein actin
what is the thin filament
a relaxed skeletal muscle fibre containing actin, troponin and tropomyosin
a double stranded alpha-helical F-actin with a myosin binding site
what is tropomyosin
a thread-like coil wrapped around actin to cover the myosin binding site
what is troponin
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
what is the function of troponin T
binds to a single molecule of tropomyosin
what is the function of troponin C
binds calcium
what is the function of troponin I
binds to actin and inhibits contraction
what is the thick filament
composed of multiple myosin II molecules
what is a myosin II molecule
a double trimer containing:
2x alkali (essential) light chains
2x regulatory light chains
2x intertwined heavy chains
what is the function of the 2 alkali light chains of myosin
stabilises the myosin head region
what is the function of the 2 regulatory chains of myosin
regulates the ATPase activity of myosin
what is the composition of the 2 intertwined heavy chains of myosin
contains:
a tail
a hinge
a headw
what is the function of the tail region of the heavy chain of myosin
alpha-helices that intertwine
what is the function of the hinge region of the heavy chain of myosin
molecule opens to form 2 globular heads
what is the function of the head region of the heavy chain of myosin
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
what is the process of muscle contraction
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
what is the role of calcium in muscle contraction
calcium modulates contraction via regulatory proteins rather than interacting directly with the contractile proteins
what is the effect of an absence of calcium in muscle contraction
in the absence of calcium, these regulatory proteins act together to inhibit actin-myosin interactions, thus inhibiting the contractile process
what is the effect of a presence of calcium in muscle contraction
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
what is the composition of troponin C
2 high affinity Ca2+ binding sites
2 low affinity Ca2+ binding sites
what is the function of the high affinity calcium binding sites of troponin C
participate in binding troponin C to the thin filament
calcium binding to these sites does not change during muscle activation
what is the function of the low affinity calcium binding sites of troponin C
binding of calcium to these sites induces a conformational change in the troponin complex
what is the effect of calcium binding to the low affinity calcium binding sites of troponin C
- troponin I shifts, permitting the tropomyosin molecule to move
- via troponin T, tropomyosin is moved away from the myosin-binding site on actin and into the actin groove
- the myosin head is now able to interact with actin and engage in cross-bridge cycling
what is the cross-bridge cycle
contractile proteins convert the energy of ATP hydrolysis into mechanical energy
what are the five steps of the cross-bridge cycle
- ATP binding
- ATP hydrolyis
- cross-bridge formation
- release of Pi from myosin
- ADP release
what is the sliding filament mechanism
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
what happens to the length of the filaments during the sliding filament mechanism
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
what is rigor mortis
“the stiffness of death”
begins 3-4 hours after death and completes in ≈12 hours
how does rigor mortis occur
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.
