week 3: skeletal muscle organisation, activation and deactivation-10.1, 10.2, 10.3 Flashcards
whole muscle is sheathed within
layer of connective tissue
muscle fibre diameter
10-100 micrometre
muscle fibre surrounded by
sacrolemma
how does skeletal muscle move the body
by pulling on our bones
four common properties of muscle tissue
excitability
contractility
extensibility
elasticity
excitability
ability to recieve and respond to stimulus
what do muscle tissue respond to
chemical stimulus from a nerve cell with a change in membrane potential
contractility
ability of a muscle cell to shorten when stimulated
extensibility
stretching movement of a muscle
elasticity
ability of a muscle to recoil to resting length
6 main functions of skeletal muscle
producing movement
maintaining posture and body position
supporting soft tissue
guarding body entrances and exits
maintaining body temperature
storing nutrients
what does skeletal muscle organs contain
skeletal muscle tissue
connective tissue
blood vessels
nerves
what is the muscle tissue of skeletal muscle surround by
three layers of connective tissue
- epimysium
-perimysium
-endomysium
epimysium
dense layer of collagen fibres that surrounds the entire muscle
separates muscle from nearby tissues and organs
connected to deep fascia- dense layer of connective tissue
perimysium
divides skeletal muscle into series of compartments-fascicles
contains collagen and elastic fibres as well as blood vessels and nerves
endomysium
delicate connective tissue surrounds individual skeletal mucle cells within a fascicle
loosley interconnects adjacent muscle fibres
(seperates muscle fibres from another)
flexible, elastc tissue layer contains capillary networks, myosatellite cells, stem cells
how are the collagen fibers of the perimysium and endomysium arranged
interwoven and blend into one another
at each end of the muscle, collagen fibers of the epimysium, perimysium and endomysium
come togehter to form either a bundle-tendon
or a broad sheet- aponeurosis
function of tendons and aponeuroses
usually attach skeletal muscles to bones
what happens where collagen fibre contact the bone
collagen fibers extend into bone matrix, providing a firm attachment
as a result, contraction of muscle pulls on attached bone
what does the connective tissue of the endomysium and perimysium contain
blood vessels and nerves that suply the muscle fibres
why is an extensive vascular network needed
delivers necessary oxygen and nutrients
carries away metabolic waste generated by active skeletal muscles
blood vessels and nerves in muscle
blood vessels and nerves normally enter muscles together and follow the same branching course through the perimysium
each fascicle recieves branches of these blood vessles and nerves
arterioles within endomysium
supply blood to capillary network that services the individual muscle fiber
axons
nerve fibers extending from neurons
penetrate the epimysium
branch through perimysium
enter endomysium
to innervate individual muscle fibers
multinucleate
each skeletal muscle fibre contains hundreds of nucleui just internal to the plasma membrane
genes of nuclei of multinucleate muscle fibres
control production of enzymes and structural proteins required for normal muscle contraction
the more copies of the genes,
the faster these proteins can be produced
development of skeletal muscle fibers
groups of myoblasts fuse, forming individual multinucleate skeletal muscle fibers
each nucleus in skeletal muscle fiber reflects the contribution of a single myoblast
unfused myoblasts
remain in adult skeletal muscle tissues as myosatellite cells
myosatellite cells after an injury
enlarge and divde
fuse with damaged muscle fibers
assisting in repir of tissue
skeletal muscle tissue is known as
striated muscle
striations visible with a light microscope
striations are due to
precise arrangement of actin and mysosin filaments in myofibrils
each muscle fibre contains hundreds to thousands of cyclindrical myofibrils
arrangement of actin and myosin filaments forms
functional repeating unit: sarcomere
sacrolemma
plasma membrane of a muscle fiber
surrounds sarcoplasm
has a characteristic membrane potential
in skeletal muscle fiber, a sudden change in the membrane potential is the first step that leads to contraction
transverse tubules
narrow tubes whose surface are continous with the sarcolemma (extensions of sacrolemma) and extend deep into sacroplasm
filled with extracellular fluid and form passafeways through the muscle fiber
function of T tubules
propagate action potentials generated by sarcolemma
electrical impulses travel along t tubules into cell interior
importance of T tubules
all regions of large skeletal muscle fiber must contract at the same time
signal to contract must be distributed quickly throughout interior of cell
sarcoplasmic reticulum
membrane complex
forms a tubular network around each myofibril
wherever a T tubule encircles a myofibril
the tubule is tightly bound to the membranes of the SR
terminal cisternae
on either side of a T tubule, tubules of SR enlarge, fuse and form expanded chambers: terminal cisternae
triad
combination of a pair of terminal cisternae plus a T tubule
fluid contents of triad
separate and distinct despite membranes being tightly bound
what is the SR specialised for
storage and release of calcium ions
myofibril dimensions
1-2 micrometres in diameter and as long as the muscle fibre
what is responsible for skeletal muscle fiber contraction
active shortening of myofibrils
myofibrils consist of
myofilaments: bundles of protein filaments
types of myofilaments
actin
myosin
titin
why does the entire cell shorten and pull on a tendon when myofibrils contract
myofibrils are anchored to the inner surface of the sarcolemma at each end of a skeletal muscle fiber
outer surface of the sarcolemma is attached to collagen fibers of the tendon of the skeletal muscle
what provides energy in the form of ATP for short- duration, maximum-intensity muscular contractions
mitochondria and granules of glycogen scattered among the myofibrils
mitochondrial activity and glucose breakdown by glycolysis
sacromere
repeating functional units
made up of thin and thick myofilaments
smallest functional unit of muscle fiber
how many sarcomeres does a myofibril consists of
approx 10,000 end to end
what does a sacromere contain
thin filaments
thich filamets
proteins that stabilise the positions of thick and thin filaments
proteins that regulate the interactions between thick and thin filaments
what are sacromeres seperated by
z discs
I band
thin filaments
A band
thick filaments
overlap region
thick filaments held in place by
titin
myosin molecules are
polar
because myosin molecules are polar
region in middle of thick filament with no myosin heads
titin
large protein
spans half of sacromere
spring like properties- resists overstretching of muscle preventing damage
thin filmants
composed largely of actin
G-actin
z discs contain a-actinin which anchors thin filaments
nebulin holds thin filaments together
also contain tropomyosin and troponin proteins
structure strongly conserved
G actin
globular protein
polymerises into double stranded helix filamentous form, F actin
tropomysoin and troponin involved in
regulation of muscle contraction
control whether myosin heads can form cross-bridge links with thin filaments
[Ca2+] < 0.1 μM
tropomyosin covers binding site on actin- prevents cross bridge attachement
[Ca2+] > 1μM
Ca2+ binds to Tn-c
troponin undergoes conformational change
tropomyosin pulled out of the groove
cross-bridges can now form
isoforms of Tn-C
fast and slow isoforms specific to fast and slow muscles
excitation contraction coupling
the sequence of events whereby the nerve impulse results in Ca2+ release in the fibre
link between the generation of action potnetial in sacrolemma and the start of a muscle contraction
where does the excitation-contraction coupling occur
at the triads
what happens when action potential reaches a triad
triggers the release of Ca2+ from the terminal cisternae of the sarcoplasmic reticulum
how long does the change in permeability of the SR to Ca2+ last
0.03 seconds
[Ca2+] in and around sarcomere after AP reaches triad
100 times resting level
why is the effect of calcium ion release almost instantaneous
because terminal cisternae are loacted at zones of overlpa where thick and thin filaments ineract
calcium ion binding to troponin
changes the shape of the troponin molecule
weakens the bond between troponin and actin
troponin molecule changes position rolling the attached tropomyosin strand away from the active sites
contraction cycle begins
contraction cycle
series of molecular events that enable muscle contraction
after active sights are exposed
myosin heads bind to them forming cross bridges
connection between head and tail
functions as a hinge that leads the head pivot
pivots using energy released from hydrolysis of ATP
head swings towards the M line-power stroke
pivoting is the key step in muscle contraction
calcium regulation in the muscle occurs via the
SR
SR relaxed muscle
in relaxed muscle, Ca2+ bound to calsequestrin in terminal cisternae (stored)
low Ca2+ conc in sacrolasm (app 1μM)
SR
free [Ca2+] in sarcoplasm rises (app 10μM)
diffuses to the myofibrils and binds with Tn-C
Tn-C undergoes conformational change allowing cross-bridges to form
at end of contraction,
Ca2+ pumps in SR actively transport it back to lumen of SR
why must calcium be removed from myofibrils at end of contraction
prevent further formation of actin-myosin cross bridges
steps that result in muscle activation
- AP arrives at motor end plate
- synaptic trasnmission: ACh release, diffusion and binding to voltage-gated channels, triggers AP in sarcolemma
- AP conducted into the cell via T tubule
- voltage-gated conformational change occurs in the dihydropyridine receptor (DHPR)
- conformational chnage in DHPR is transmitted to the Ryanodine receptor ( the calcium release channel) opening the channel and releasing Ca2+ into the myofibrillar space
- Ca2+ diffuses into myofibrils and binds to troponin C on the thin filament
- conformational change in troponin C alters the binding of troponin I to actin and releases the troponin-tropomyosin complex
- tropomyosin is able to move, allows mysoin head to bind
- cross-bridge cycling can occur as long as system stays activated
steps in muscle deactivation
- Ca2+ is pumped back into SR by the Ca2+-ATPase lowering myoplasmic [Ca2+]
- Ca2+ is released from troponin-C because of the lower conc
- cross-bridges released from actin, binding sites for myosin on actin get covered up by tropomyosin
- number of strongly bound cross-bridges decline, force declines
