MUSCLE PHYSIOLOGY Flashcards
what is the muscular system?
consists of muscle cells, responsible for movement in the body
what are the 3 major muscle types?
smooth muscle
cardiac muscle
skeletal muscle
where is smooth muscle found?
Lines the tubular and hollow structures in the body
what are smooth muscle cells connected by?
gap junctions
what shape are smooth muscle cells?
spindle shape with central nucleus
how are the Contractile cytoskeletal elements arranged in a smooth muscle cell?
diagonally across the cell
what are actin thin filaments anchored to in smooth muscle?
dense bodies
how are dense bodies arranged in smooth muscle?
within a network of intermediate (desmin) fibres
how are myosin fibres arranged in smooth muscle?
scattered throughout the cell within the network of intermediate fibres
what is cardiac muscle?
Striated muscle - each fibre consists of a single branched cell
what does a cardiac muscle cell contain?
centrally located nucleus and large numbers of mitochondria
how are cardiac muscle cells connected?
by specialised thickenings of the sarcolemma, the intercalated discs
what are found within intercalated discs?
fibre-like structures, the desmosomes (hold the muscle cells together), and gap junctions
what is the function of gap junctions in intercalated discs?
provide physical connections that enable electrical signals to be passed from cell to cell
what is skeletal muscle associated with?
with movement of bones in relation to one another
what is each muscle surrounded by?
by a connective tissue sheath (epimysium), beneath which can be found bundles of muscle fibres, the muscle fascicle
what is each fascicle surrounded by?
by a connective tissue layer (perimysium)
what are muscle fascicles composed of?
muscle fibres - multinucleate cells, made up of a large number of fused myoblasts
what are found along muscle fibres?
satellite cells – un-fused myoblasts that can enlarge and divide after muscle injury
what does each muscle fibre consist of?
myofibrils
what 2 myofilaments form myofibrils?
Thin actin filaments
Thick myosin filaments
what is the memb potential dependent on?
on relative permeability of membrane to Na and K
what is depolarisation associated with?
opening of ligand gated and voltage gated Na channels
what does depolarisation promote?
release of Ca from the sarcoplasmic reticulum
what does depolarisation mediate?
changes in K permeability and the Na/K pump
what are muscle fibres?
large structures – all components have to contract simultaneously
how is simultaneous contraction achieved?
because the signal to contract is distributed evenly by the T tubules
what are T-tubules?
narrow tubes that extend from the sarcolemma into the sarcoplasm of the muscle fibre, and then around myofibrils
what are T-tubules filled with?
with extracellular fluid and so allow a rapid conduction of potential changes throughout the muscle
what are T-tubules tightly bound to?
to the memb of the sarcoplasmic reticulum
what does the sarcoplasmic reticulum (SR) form?
a tubular network around each myofibril, either side of each T-tubule the SR enlarges and fuse forming large chambers, the terminal cisternae
what is a triad?
The association between an encircling T-tubule and a pair of terminal cisternae
what is the sarcoplasm of the muscle deficient in?
calcium
where is calcium stored?
in the sarcoplasmic reticulum
what happens when a muscle muscle cells fires an action potential?
potential change is conducted along the T-tubule system allowing the release of calcium from the sarcoplasmic reticulum into the sarcoplasm
how are the contractile filaments actin and myosin arranged?
into myofibrils
what does each myofibril consist of?
a number of contractile units, the sarcomeres
what is each sarcomere separated by?
by a thin line known as the Z band or Z disc
In the sarcomere, what do the thin actin filaments form?
the I band which attach the Z disc
what is found between he I bands?
a much darker band, the A band
what happens when the muscle contracts?
the actin filaments of the I band slide over the myosin filaments of the A band, so that the Z discs move closer together
what is myosin (A band) formed from?
myosin-II
what is myosin-II?
a protein consisting of two identical heavy chains, each of which is bound to a pair of light chains
what does the AA terminal of the heavy chain form?
a motor head domain
what does the carboxyl end of the heavy chain form?
forms an elongated tail
what does the elongated tail of myosin form?
an alpha helix with the second heavy myosin chain to form a dimer
what can the heavy chain dimers polymerise into?
stable bipolar filaments with free head groups at either end
what does the Actin (I band) consist of?
of a chain of globular actin molecules
what does each actin molecule have?
a binding site for a myosin head
what is the actin helix coupled to at every 6th molecule?
to two other proteins, tropomyosin and troponin
what is tropomyosin?
rod shaped molecule that binds via the troponin molecule to the groove of the actin helix, where it masks the myosin binding sites
what is troponin?
a complex of, troponin T, I, and C
what is troponin I?
binds to actin
what is troponin T?
binds to tropomyosin
what is troponin C?
binds calcium
what happens when 4 Ca2+ ions bind to troponin?
troponin goes through a conformational change moving the tropomyosin from the myosin biding site
what is muscle contraction a result of?
of muscle cell excitation
where are stored Ca2+ released from?
from the sarcoplasmic reticulum
what happens when Ca2+ binds to troponin C?
causes conformational change in the troponin complex
what happens when tropomyosin molecule moves away from actin molecule binding sites?
allows myosin heads to bind
what is muscle contraction driven by.?
by the conversion of ATP to ADP
how are skeletal muscles controlled?
under the control of the nervous system
how are synaptic terminals formed?
A single axon will branch within the perimysium and expand at their ends
what does the neuromuscular junction consist of?
Synaptic terminal + synaptic cleft + sarcolemma
what happens when the skeletal muscle becomes depolarised?
ACh travels across the synaptic cleft and binds to ACh receptors located in the sarcolemma
what does depolarisation of the skeletal muscle trigger?
opening of Na channels and the myocyte action potential. ACh is quickly broken down by AChE
what is the Motor end plate (MEP)?
region of sarcolemma rich in ACh receptors – MEP folded to increase surface area and number of ACh receptors
what is the tension generated dependent on?
on number of cross bridges formed between the actin and the myosin
what can modify tension?
Stretching a muscle
by varying the degree of overlap between the actin and myosin filaments
what is the relationship between the degree of stretch and tension?
initially positive and linear
what happens as the number of cross bridges between the actin and myosin filaments start to decrease?
this relationship breaks down until the relationship becomes negative
what is a twitch?
A single stimulation of a muscle induces a single contraction
what 3 main phases does a twitch consist of?
Latent period
Contraction phase Relaxation phase
what is a trippe?
If a 2nd stimulation arrives immediately after the end of the relaxation phase, the next contraction will be slightly bigger
Thought to be because of steadily increasing Ca conc
what is tetanus?
If the next stimulation occurs before the relaxation phase has ended the contractions will merge (summate) and become larger
what is incomplete tetanus?
Describes the summation of contractions that occurs when the 2nd stimulus arrives just after the relaxation phase has started. Subsequent stimulations applied at the same frequency will induce tension to rise until it reaches a maximum value roughly four times that achieved by Treppe
what is complete tetanus?
Achieved by increasing stimulating frequency, eliminating the relaxation phase. The muscle will contract smoothly up to the maximum tension
what are muscle fibres innovated by?
by motor neurons
how many muscle fibres does one motor neurone innervate?
hundreds or thousands of fibres
what is a motor unit?
the muscle fibres innervated by a single motor neuron
what is muscle tone?
In both smooth and skeletal muscle, some muscle cells or fibres are always in a state of contraction, even when the whole muscle is at rest. This resting tension is called muscle tone.
what does muscle tone enable?
Maintenance of posture
Diameter of tubular structures (blood vessels etc) and so resistance to flow
what is isotonic?
means equal tension tension rises and the muscle length changes
what are the 2 types of isotonic?
concentric
eccentric
what is concentric?
2kg weight on the muscle. To shorten the cross bridges must generate more than 2kg (the resistance) of tension before the weight can be moved. After generating 2kg of tension the weight will move but tension in the muscle will remain at 2kg
what is eccentric?
Tension generated is less than the resistance - weight cannot be moved - muscle lengthens. However, contraction of the muscle slows that extension. A very important mechanism – used to control muscle extension when sitting, walking etc
what are isometric contractions?
Tension generated in the muscle does not exceed the resistance to force
give examples of isometric contractions
trying to pick up a very heavy weight, or pushing against a locked door
what do isometric allow?
allow you to maintain posture
