Skeletal Muscle Physiology 1 Flashcards
3 types of muscle?
Skeletal
Cardiac
Smooth
Different ways of categorising smooth muscle by appearance and innervation?
Striated muscles:
Skeletal
Cardiac
Voluntarily controlled (somatic NS):
Skeletal
Involuntarily controlled (ANS):
Cardiac
Smooth
What is striation?
Seen under a light microscope as alternating dark bands (myocin thick filaments) and light bands (actin thin filaments) in striated muscles
Physiological functions of skeletal muscles?
Maintenance of posture
Purposeful movement in relation to external environment
Respiratory movements
Heat production
Contribution to whole body metabolism
What is a MOTOR UNIT?
A single α-motor neuron and all the skeletal muscle fibres it innervates
Variation in the no. of fibres per motor unit?
Muscles which serve fine, precise movements, e.g: external eye muscles, intrinsic hand muscles and those for facial expression, have fewer fibres per motor unit
For muscles where power is more important than precision, e.g: thigh muscles, there are 100s-1000s muscle fibres per motor unit
Levels of organisation in a skeletal muscle?
The muscle is the whole organ that consists of muscle fibres/cells
Each cell contains specialised intracellular organelles called myofibrils,, which are, in turn, made up of alternating thick and thin filaments
Actin and myocin filaments, within each myofibril, are arranged into sarcomeres (functional units of the muscle)
Compare the appearance of skeletal and cardiac muscle?
Both are striated
Compare the initiation & propagation of contraction in skeletal and cardiac muscle?
Skeletal muscle:
- Neurogenic initiation of contraction
- Motor units
- Neuromuscular junctions
- NO gap junctions
Cardiac muscle:
- Myogenic (pacemaker potential) initiation of contraction
- No neuromuscular junction
- Gap junctions present
Compare excitation contraction coupling in skeletal and cardiac muscle?
In skeletal muscle, Ca2+ is released entirely from the sarcoplasmic reticulum
In cardiac muscle, Ca2+ from ECF and sarcoplasmic reticulum (Ca2+ induced Ca2+ release)
Compare graduation of contraction in skeletal and cardiac muscle?
Skeletal muscle:
- Motor unit recruitment
- Summation of contractions
Cardiac muscle: Frank-Starling mechanism
How can an action potential transmitted in an α-motor neuron cause muscle contraction?
Excitation contraction coupling is the process whereby the surface action potential results in activation of the contractile mechanism of the muscle fibre
Describe Ca2+ release in skeletal muscles
Ca2+ is released from the lateral sacs of the sarcoplasmic reticulum when the surface a.p spreads down the transverse T-tubules (invaginations of the membrane)
In skeletal muscles, what is the neurotransmitter at the neuromuscular junction?
Acetycholine
Label the image
What is the functional unit?
Smallest component of an organ that is capable of performing all the function of that organ, e.g: nephrons in the kidneys and sarcomeres in skeletal muscles
Where are sarcomeres found?
Between two Z-lines (connect the thin filaments of 2 adjoining sarcomeres)
Four zones of sarcomeres?
A-band: thick filaments along with portions of thin filaments that overlap in both ends of thick filaments
H-zone: lighter area within middle of A-band where thin filaments do not reach
M-line: extends vertically down middle of A-band within the centre of H-zone
I-band: consists of remaining portion of thin filaments that do not project in A-band
Describe the sliding filament theory
Muscle tension is produced by sliding of actin filaments over myocin filaments, muscle shortens and produces force
What does force generation depend upon?
ATP-dependent interactions between thick and thin filaments power cross-bridge formation; ATP is required for both contraction and relaxation Ca2+ is required to switch on cross-bridge formation
Describe how cross-bridge formation occurs
ATP binds to the myocin head; it is broken down into ADP + Pi, by an enzyme (ATP Mg2+)
When Ca2+ is present, it moves the troponin-tropomyosin complex from the actin binding site
What is rigor complex?
No ATP present in death, so relaxation of muscles cannot occur
Why is ATP required during relaxation of smooth muscles?
To release cross bridges
To pump Ca2+ back into the sarcoplasmic reticulum
Steps in nervous stimulation of skeletal muscle contraction?
- ACh released into NMJ and binds to motor end plate
- a.p is stimulated and is propagated across the surface membrane and down T-tubules
- a.p in T-tubule triggers Ca2+ release from the sarcoplasmic reticulum
- Ca2+ ions released from the lateral sacs bind to troponin on actin binding sites; troponin-tropomyosin complex is physically moved
- Myocin cross-bridges attach to actin and bend, pulling actin to the centre of the sarcomere (energy is provided by ATP)
- Ca2+ is actively taken up by the SR when there is no longer an a.p (ATP required)
- Ca2+ no longer bound to troponin, so tropomyosin covers actin binding site again; this ends contraction and actin slides passively back to its original resting position
