WEEK TWELVE - REGIONAL MUSCLE ANATOMY, MUSCLE TISSUE, BEHAVIOUR OF SKELETAL MUSCLE FIBRES Flashcards
Describe skeletal muscle and the characteristics that all muscle types have in common [ECCEE]
Usually attached to bone
Voluntary - usually subject to conscious control
characteristics
- excitability [responsiveness to chemical signals, stretch, electrical changes]
- conductivity [local electrical change triggers wave of excitation that travels along muscle fibre]
- contractility [capable of being stretched]
- extensibility [capable of being stretched]
-elasticity [returns to original resting length after stretching]
Describe the structural components of a skeletal muscle fibre
bulk of sarcoplasm = taken up by myofibrils
myoblasts fuse + unfused satellite cells join –> skeletal muscle fibre
sarcolemma = tunnel like infoldings or transverse [T] tubules the penetrate cell [carry electric currents to cell interior]
- filled with myofibrils [bundles of myofilaments] + glycogen
- sarcoplasmic reticulum = smooth ER
Myosin and actin = CONTRACTILE proteins
Tropomyosin and troponin = REGULATORY proteins
- Contraction activated by release of calcium into sarcoplasm and its binding into troponin
Relate the striations of a muscle fibre to the overlapping arrangement of its protein filaments
Dark A bands [regions] alternating with lighter I bands [regions]
A band is the length of the thick filaments but also contains portions of the thin filaments
- Does NOT shorten during contractions
Define a motor unit and explain how they are used to control movement
a motor neuron and all of the skeletal muscle fibers innervated by the neuron’s axon terminals
- Each muscle fibre is supplied by only one motor neuron
- stimulation = weak contraction over large area
[effective contraction usually requires contraction of several motor units at once]
- Cell bodies of somatic lower motor neurons are found in brainstem or spinal cord in ventral horn
- Average motor unit = 200 muscle fibres/ unit
- SMALL motor units = FINE degree of control
[hand/eye movements]
[3-6 muscle fibres per neuron] - LARGE motor units = when strength/gross movement over fine motor control
Describe the structure of a neuromuscular junction
neuromuscular junction = functional connection between nerve fibre and muscle ell
ACh released from nerve fibre –> stimulates muscle cell
structure - synaptic knob is swollen [ soft triangle shape] at end of nerve fibre as it contains ACh receptors
- contains folds [sarcolemma] to increase SA for ACh receptors + contains acetylcholinerase [breaks down ACh + causes relaxation]
synaptic cleft = gap between nerve and muscle cells// pre-post synaptic cell
Explain why a cell has an electrical charge difference across its plasma membrane and, in general terms, how this relates to muscle contraction
Plas mem = polarized or charged [voltage difference between inside and outside of ell]
due to Na+ IN ECF and K+ and other anions [negative ions] in ICF
IN ICF –> anions [eg proteins, nucleic acids, phosphates, CANNOT penetrate plas mem = make the inside NEGATIVELY CHARGED compared to outer surface
Resting membrane potential = about -70mV
Electrical charge difference = makes muscle cells excitable/conductive
Explain how a nerve fibre stimulates a muscle fibre (excitation)
Plas mem = polarized or charged [voltage difference between inside and outside of cell]
Difference in charge across membrane = resting membrane potential -90mV cell
Stimulation opens ion gates in membrane
- Na+ rushes in and K+ rushed out = cell moves
towards more positive potential
- Quick up and down voltage shift = action potential
- Spreads over cell surface as nerve signal
1.generation and propagation of an action potential
2.the release of neurotransmitter
3. activation of the muscle fibre
4. calcium release
5. cross-bridge formation,
6. sliding filament mechanism
7. muscle contraction.
Explain how stimulation of a muscle fibre activates its contractile mechanism (excitation-contraction coupling)
- AP spreads over sarcolemma [skeletal muscle membrane] - enters T tubule [cavity that penetrates into centre of cardiac/skeletal muscle]
- depolarisation of t tubule membrane –> opens
voltage gated Ca2 channels in sarcoplasmic reticulum [SR] - increases calcium ions into sarcoplasm
- troponin + tropomyosin are on actin myofilament, covering myosin binding site
- Ca2 binds to troponin - moving the protein and exposing the binding site - myosin heads bind to actin myofilament creating cross bridges
Explain the mechanism of muscle contraction (sliding filament theory)
sarcomere [basic contractile unit of muscle] contains myosin and actin that overlap but are not fully engaged.
the AP triggers release of Ca+ into cytoplasm which binds to troponin, changing its shape, removing it and tropomyosin off the myosin binding site on the actin filament.
now the myosin heads are able to bind to the actin filament creating a cross bridge. ATP provides energy for a powerstroke, pulling the actin filament closer to the centre of the sarcomere. the cross bridge detaches and reattaches over and over pulling the actin closer to the sarcomere centre = muscle contraction
once the AP stimulus ceases, the ACh stops being released, and AChE breaks down the remaining ACh in the synaptic cleft. the voltage gated calcium channels close and ATPase pumps remaining Ca+ back to sarcoplasmic reticulum. ATP binds to myosin facilitating the detachment of the crossbridge from actin, which cannot be maintained without Ca+. The Ca+ also dissociated from the troponin, allowing troponin and tropomyosin to return to their original place, blocking the myosin binding site.
muscle returns to resting length.
Explain how a muscle fibre relaxes (relaxation)
- cessation of nerve stimulation by stopping release of ACh at NMJ –> breakdown of remaining acetylcholine in synaptic cleft- by acetylcholinerase
- termination of the action potential - lack of stimulation closes Ca2 voltage gated channels –> calcium ATPase pumps Ca2 back to sarcoplasmic reticulum = reducing Ca2 concentration in cytoplasm
- reformation of the troponin-tropomyosin complex
- as calcium ions dissociate from troponin –> tropomyosin moves back to position on actin filament
- prevents binding of myosin –> actin - detachment of cross-bridges
without calcium ions, cross bridges between myosin and actin cannot maintain binding
- ATP binds to myosin –> detaches it from actin - ATP needed for relaxation as well
- provides energy for Ca2 re-uptake in sarcoplasmic reticulum
- facilitates unbind of myosin from actin - muscle fibre return to the resting state ready for contraction from another signal
Explain why the force of muscle contraction depends on its length prior to stimulation
Define, list and describe the 3 phases of a muscle twitch
- latent period [2msec delay between onset of stimulus - onset of twitch response before tension in the muscle fibre begins to increase]
- AP propagating along sarcolemma and Ca+ being released from sarcoplasmic reticulum] - contraction phase
- Ca+ binding to troponin and tropomyosin moved off binding site on actin filament = formation of cross bridges
- rate of contraction depends on rate of detachment/attachment of cross bridges assisting by ATPase - relaxation phase
- Ca+ pumped back to SR by ATPase
- ATP binds to myosin, troponin and tropomyosin return to original places and cross bridge cycling ceases
- muscle returns back to resting place
Explain how successive muscle twitches can add up to produce stronger muscle contractions
If second muscle twitch happens before first twitch contraction has finished its relaxation phase = more tension will be developed in the second twitch = wave summation
a second AP stimulus creates a positive feedback cycle, releasing more Ca+ into cytoplasm activating additional sarcomeres
Distinguish between isotonic/isometric contractions and concentric/eccentric contractions
isometric = NO visible change in length upon contraction [important in postural muscle function and co-contraction of agonist/antagonist muscles]
isotonic [visible length change upon contraction]
concentric contraction - muscle SHORTENS during contraction
eccentric contraction - muscle LENGTHENS during contraction
- load generally moving in same direction as gravity
name and identify the various muscles - Mastication
TWO major muscles of mastication [chewing]
- temporalis
- arises from skull
- elevates and retracts the mandible, helping in the closure of the jaw during chewing. - masserter
- arises from cheek bone
- elevates the mandible during biting and chewing
BOTH are anterior to temporo-mandibular joint = produce the movement of TMJ elevation
