skeletal muscle Flashcards
what are the 5 functions of skeletal muscle
- body movement
- maintenance of posture
- protection and support
- regulating elimination of materials
- heat production
how do muscles generate body movements
through contractions
which 5 skeletal muscles contract to maintain posture
- trunk
- pelvis
- legs
- neck
- head
how do skeletal muscles protect the internal organs within the abdominopelvic cavity
- skeletal muscles are arranged in layers within the walls of abdominal cavity
- and the floor of the pelvic cavity
how do skeletal muscles regulate passage of material
contraction and relaxation of sphincters
describe sphincters
circular muscle bands
why are skeletal muscles responsible for heat production
energy is required for tissue contraction and heat is always produced by this energy
what are the
5 characteristics of skeletal muscles
- excitability
- conductivity
- contractility
- extensibility
- elasticity
what is the definition of excitability as it relates to skeletal muscle
the ability of a cell to respond to a stimulus
what does the stimulus cause in excitable cells
it causes a local change in the rmp (resting membrane potential) by triggering movement of ions across the plasma membrane
when is the response of skeletal muscle cells to the movement of ions across the plasma membrane of excitable cells
its receptors bind to a neurotransmitter, which is released from a motor neuron
why is muscle excitability important
it ensures that the muscle can conduct or transmit electrical impulses
what is conductivity of a muscle
it involves an electrical signal being propagated along the plasma membrane as voltage-gated channels open during an action potential
what is the role of electrical signals in conductivity
they functionally connect the plasma membrane of the muscle cell to the interior of the muscle cell
why is conductivity necessary for in skeletal muscle
to propagate the electrical signal across the muscle
when is contractility of muscle displayed
it’s exhibited when contractile proteins within skeletal muscle cells slide past one another
what does contractility of muscle allow
muscle cells to cause body movement and to perform the other functions of muscles
what is extensibility of muscle
it’s the lengthening of a muscle cell
how is the lengthening of muscle possible
the contractile proteins slide past one another to decrease their degree of overlap
when is muscle extensibility exhibited
when we stretch our muscles
what is elasticity of muscles
the ability of a muscle cell to return to its original length following either shortening or lengthening of muscles
what is elasticity of muscle cells dependent upon
the release of tension in the springlike connecting protein associated with contractile proteins
what else can muscle fibers be referred to as
myofibers
is a skeletal muscle a tissue or organ
organ
what are the 4 components of skeletal muscle
- skeletal muscle fibers
- connective tissue layers
- blood vessels
- nerves
how are muscle fibers being binded
within a fascicle
what are the 3 layers of connective tissue within the muscles
- epimysium
- perimysium
- endomysium
what connective tissue is epimysium made of
dense irregular connective tissue
what connective tissue is perimysium made of
dense irregular connective tissue
what connective tissue is endomysium made of
areolar connective tissue
what forms the tendon/aponeurosis of a skeletal muscle
epimysium, perimysium and endomysium collectively extend past the muscle fibers to form it
what’s the difference between a tendon and aponeurosis
- tendon is a thick, cordlike structure composed of dense regular connective tissue
- aponeurosis is a thin, flattened sheet of dense regular connective tissue
what is another connective tissue component not present within the skeletal muscle
deep fascia
what else can the deep fascia be referred to as (2)
visceral or muscular fascia
what connective tissue is the deep fascia composed of
expansive sheet of dense irregular connective tissue external to the epimysium
what are the 4 functions of deep fascia
- separates individual muscles
- binds together muscles with similar functions
- contains nerves, blood vessels and lymph vessels
- fills space between muscles
what is the deep fascia internal to
superficial fascia
what is the superficial fascia composed of (2)
- areolar connective tissue
- adipose connective tissue
what does the superficial fascia do
separates muscle from skin
how far do the blood vessels in skeletal muscle reach
extend through epimysium and perimysium to reach the endomysium
what are the smallest blood vessels in skeletal muscle
capillaries
what is skeletal muscle innervated by
motor neurons
what is the role of motor neurons in skeletal muscle
control skeletal muscle contraction
how far does the motor neuron axons extend through skeletal muscle
they extend through all 3 connective tissue layers to almost make contact with an individual muscle fiber
what is the junction between the axon and the muscle fiber called
neuromuscular junction
what are the primary cells forming a skeletal muscle
skeletal muscle fibers
what is the cytoplasm in skeletal muscle called
sarcoplasm
what is the typical diameter for skeletal muscle fiber
between 10 and 500 micrometers
what is the length of skeletal muscle typically
100 micrometers to 30 centimeters
what are embryonic muscle cells termed
myoblasts
what do myoblasts do to reach their length
they fuse to form single skeletal muscle fibers during development
why are skeletal muscle cells multinucleated
when myoblasts fuse, each myoblast nucleus contributes to the eventual total number of nuclei in the fiber
what are satellite cells
when myoblasts don’t fuse with muscle fibers during development
what may satellite cells do if a skeletal muscle cell is injured
stimulated to differentiate, fuse with damaged cell and assist it to a limited extent in repair and regeneration
what is the plasma membrane of a skeletal muscle fiber called
sarcolemma
what are T(transverse)-tubules
deep invaginations of the sarcolemma
what channels are located within both the sarcolemma and t-tubules
voltage-gated
- Na+
- K+
what composes 80% of the volume of a skeletal muscle fiber
myofibrils
what structure in skeletal muscle fibers is similar to smooth er
sarcoplasmic reticulum
what do terminal cisternae serve as
reservoirs for calcium ions
2 terminal cisternae and a centrally located t-tubule form what structure
triad
what do the t-tubule membranes within the triad contain
voltage-sensitive calcium ion channels
what do the terminal cisternae membranes contain
calcium ion release channels (ryanodine receptors)
where does the connection occur between the electrical signals propogated and the release of calcium from the SR
the calcium ion release channels
what else is embedded within the SR membrane
calcium ion pumps which move calcium from the cytosol into the SR
how is calcium stored in the SR
bound to specialised proteins
- calmodulin
- calsequestrin
how do calcium pumps function in the SR
active transport; they maintain low cytosol levels of Ca
these pumps return Ca+ to the terminal cisternae of the SR after its release to initiate muscle contraction
what are myofilaments
contractile proteins that are bundled within myofibrils
what are the 2 types of myofilaments
thick and thin
how many myosin protein molecules make up thick filaments
200 to 500
what does the myosin head of thick filaments contain
- a binding site for actin of thin filaments
- catalytic atpase site where atp molecule splits into adp + pi
what is the diameter of thick filaments
11nm
what is the diameter of thin filaments
5 - 6nm
what are thin filaments primarily composed of
2 strands of actin protein twisted to form a helix
what does each G-actin molecule of thin filaments have
a myosin binding site
what happens to the myosin head and binding site during muscle contraction
the head attaches to the binding site
what are the 2 regulatory proteins within thin filaments
- troponin
- tropomyosin
what does troponin in thin filaments contain
the binding site for calcium ion
what is the length of sarcomeres
2 micrometers
what are sarcomeres
repeating cylindrical units
what is each sarcomere composed of
overlapping thick and thin filaments
what are the 4 regions in which thick and thin filaments overlap within a sarcomere
- I band
- A band
- H zone/band
- M line
where can I bands be located
extending from both directions of a Z disc and are bisected by a Z disc
what do I bands contain
only thin filaments
what causes the I bands to disappear during maximal muscle shortening
thin filaments being pulled parallel along the thick filaments
where can the A band be found on the sarcomere
central region
what does the A band in the sarcomere contain
the entire thick filament
what overlaps the thick filament in the A band
thin filament on each end of the A band
does the A band change in length during muscle contraction
no
where can the H zone be located on the sarcomere
the most central portion of the A band in a resting sarcomere
does the H zone have thin filament overlap
no
what happens during maximal muscle shortening in the H zone
this zone disappears as thin filaments are pulled past thick filaments
what is the M line in a sarcomere
a thin transverse protein meshwork structure in the center of the H zone
what does the M line serve as
an attachment site for the thick filaments and keeps them aligned during contraction and relaxation
what causes muscle fiber striations
repeating light and dark bands of overlapping myofilaments which are caused by size and density difference between thick and thin filaments
what are the 2 other structural and functional proteins in muscle fibers
- connectin
- dystrophin
where can connectin be found
extending from the z discs to the m line through the core of each thick filaments
what does connectin do
stabilises the position of thick filaments and maintains thick filaments alignment
what shape can connectin be
springlike to return the sarcomere to its normal position
‘muscle elasticity’
what is dystrophin
part of a protein complex that anchors myofibrils adjacent to the sarcolemma proteins
where do the proteins of the sarcolemma extend to
the connective tissue of the endomysium
what is the role of dystrophin
links internal myofilament proteins of a muscle fiber to external proteins
how many mitochondrion does skeletal muscle fibers have approximately
300
what is myoglobin
a reddish, globular protein
what does myoglobin do
binds oxygen when the muscle is at rest and releases it for use during muscle contraction
what are somatic motor neurons
nerve cells that transmit electrical signals from the brain/spinal cord to control skeletal muscle activity
how are numerous skeletal muscle fibers innervated
the axon of each motor neuron divides into many individual branches
what is a motor unit
a single motor neuron and the skeletal fibers it controls
what determines the degree of control of the motor unit
the size of the motor unit
are the motor neurons innervating the eye muscles large
no, they’re small for greater control
what is the strength of contraction of the stimulation of a motor unit
weak over a wide area
where can a neuromuscular junction typically be found
in the mid region of the skeletal muscle fiber
what are the 3 parts of a neuromuscular junction
- synaptic knob
- motor end plate
- synaptic plate
what is the synaptic knob of a motor neuron
an expanded tip of an axon which enlarges and flattens to cover a large surface area of the sarcolemma
what does the synaptic knob cytosol house
numerous synaptic vesicles filled with neurotransmitter molecules, acetylcholine
what are the 3 points to note about the synaptic knob
- Ca2+ pumps embedded within the plasma membrane of the synaptic knob - establishes a concentration gradient with more Ca2+ outside of the knob than inside before the arrival of the electric signal
- voltage-gated Ca2+ channels embedded within the plasma membrane of the synaptic knob - channels open, allows Ca2+ to flow into the knob, triggering exocytosis of vesicles containing ACh
- the vesicles with ACh are normally repelled from the synaptic knob plasma membrane
what is the motor end plate
a specialised region of the sarcolemma of a skeletal muscle fiber
how does the motor end plate increase the membrane surface area covered by the synaptic knob
it has numerous folds and indentations
what does the motor end plate have
chemically gated ion channels, ACh receptors
what does the binding of ACh do
opens the ACh receptor channels, allowing Na+ entry into the muscle fiber and K+ out
what is the synaptic cleft
an extremely narrow fluid filled space separating the synaptic knob and the motor end plate
what enzyme resides in the synaptic cleft
acetylcholinesterase (AChE)
what is the role of acetylcholinesterase
they quickly break down ACh molecules following their release into the synaptic cleft
at rest, relative to the interstitial fluid, is the cytosol negative or positive in comparison
negative
what is the RMP of the skeletal muscle fibers at rest
-90mV
how is the RMP of the skeletal muscle fiber maintained at rest
by leak channels and Na+/K+ pumps
what is the primary function of Na+/K+ pumps in skeletal muscle fibers
maintain the concentration gradients for Na+ and K+ with more Na+ outside the cell and more K+ inside the cell
in the relaxed position, which acetylcholine receptors are closed (3)
- within the motor end plate
- voltage-gated Na+ channels
- voltage-gated K+ channels
in the sarcolemma and T-tubules
where are Ca2+ ions stored in the relaxed position
within the terminal cisternae of the sarcoplasmic reticulum
where do the events of skeletal muscle contraction occur at (3)
- neuromuscular junction
- sarcolemma, t-tubules, sarcoplasmic reticulum
- sarcomeres
what is the first physiological event of skeletal muscle contraction
muscle fiber excitation by a somatic motor neuron
what are the 3 steps to muscle fiber excitation at the neuromuscular junction
- Ca2+ entry at synaptic knob
- release of ACh from the synaptic knob
- binding of ACh to ACh receptor at motor end plate
what does the nerve signal trigger at the synaptic knob
the opening of voltage-gated Ca2+ channels, allowing flow of Ca2+ into the knob, binding with membrane proteins (synaptotagmin) exposed on external surface of synaptic vesicles
what does the binding of Ca2+ to synaptic vesicles cause
exocytosis of ACh into synaptic cleft, at 300 vesicles per nerve signal (thousands of ACh in each vesicle)
what causes excitation of skeletal muscle fiber
ACh binding to ACh receptors on the motor end plate
how many nerve signals are propagated per second
10 to 40
what is the second physiologic event of muscle contraction
excitation-contraction coupling
what are the 3 events that occur during excitation-contraction coupling
- development of end-plate potential at motor end plate
- initiation and propagation of an action potential along the sarcolemma and T-tubules
- release of Ca2+ from the sarcoplasmic reticulum
what is the end-plate potential (EPP)
the minimum voltage change in the motor end plate that can trigger opening of voltage-gated channels in the sarcolemma to initiate action potential
when ACh receptors open, does more Na+ flow in than K+ diffuse out or the other way around
more Na+ flows in than K+ diffuses out
when is an end-plate potential produced
when there is sufficient gain of positive charge to change the RMP from -90mV to -65mV
what 2 events encompasses an action potential
- depolarization
- repolarization
what is depolarization
inside of sarcolemma becomes positive due to influx if Na+
what is repolarization
returning of inside of sarcolemma to its rel. negative RMP due to K+ flowing out
what is the change in RMP during depolarization
-65mV to 30mV
how long does the propagation of depolarisation reach
to the end of the skeletal muscle fiber
when do the voltage-gated K+ channels (sarcolemma and T-tubules) open
immediately after the opening of voltage-gated Na+ channels
what is the change in RMP during repolarization
30mV to -90mV
what does repolarization allow
the skeletal muscle fiber to propagate a new action potential when stimulated again by a motor neuron
what does the refractory period include
period of time taken for depolarisation and repolarisation
what cannot occur during the refractory period
a new action potential
what 2 things occur when the action potential reaches the sarcoplasmic reticulum
- stimulating a change to voltage-gated channels (dihydropyridine receptors) in T-tubule membrane
- cause change in Ca2+ release channels (ryanodine receptors) in the terminal cisternae of the SR, causing them to open, allowing Ca2+ to diffuse out of cisternae into the cytosol
what does the third physiologic event involve (2)
- binding of Ca2+
- crossbridge cycling
what are the 4 things that happens during Ca2+ binding prioir to crossbridge cycling
- Ca2+ binds to a subunit of globular troponin
- inducing a conformational change in troponin
- the entire troponin-tropomyosin complex is moved, exposing myosin binding sites of actin
- crossbridge cycling is initiated
what are the 4 steps to crossbridge cycling
- crossbridge formation
- power stroke
- release of myosin head
- resetting of myosin head
how are crossbridges formed
‘cocked’ myosin heads attach to exposed myosin binding sites of actin
describe a power stroke of myosin heads
the head swivels/ratchets
what does the myosin head power stroke achieve (2)
- pulling the thin filament past the thick (towards the centre of the sarcomere)
- ADP and Pi are released so ATP binding sites become available again
what causes the release of myosin head
ATP binds to the ATP binding site on the myosin head
what resets the myosin head
myosin ATPase splits ATP into ADP + Pi which provides the energy to reset the head
what causes the sarcomere to shorten
the repetitive steps in crossbridge cycling
what happens to the H zone, I band and Z discs when muscle contracts
- H zone disappears
- I band narrows, may disappear
- Z discs move closer together
what are 2 examples of muscle paralysis caused by toxins
- tetanus
- botulism
what bacterium causes tetanus
clostridium tetani
what does the toxin produced by C. tetani do
blocks the release of glycine, causing excessive muscle contractions
what bacterium causes botulism
clostridium botulinum
what does the toxin produced by C. botulinum do
prevents the release of ACh
