Muscle Flashcards
Defining characteristics of Muscle tissue
- Contractility
- Excitability
- Conductivity
- Extensibility
- Elasticity
Skeletal muscle structure review
Muscle Cell (fibre) < sarcolemma < endomysium < perimysium < fascicle < epimysium (fascia) < Muscle
- T Tubules = invaginations of the sarcolemma
- Triad = T-tubules + terminal cisterna (enlarged SR near T Tubules)
- Actin = thin filament
- Myosin = thick filament
- Titin = binds myosin to the Z-disk/keeps both in alignment
All together form a sarcomere
- Troponin = protein on thin filament that promotes muscle contraction
- Tropomyosin = protein on thin filament that inhibits muscle contraction
Components of a Neuromuscular Junction
An axon synapses with one muscle fibre and forms a cluster of axon terminals.
This is a chemical synapse - excitatory and cholinergic. An AP causes the release of ACh, changing the post synaptic membrane, opening ligand gated Na+ channels and causing an AP.
Excitation-Contraction coupling
-AP arrives at axon terminal
-Voltage gated Ca2+ channels op; Ca2+ enters axon terminal
-ACh is released from axon terminal
-ACh binds to receptors on the muscle cell membrane
-Ligand-gated Na+ channels open; Na+ enters the cell
-AP travels along sarcolemma
-AP travels down T-tubules
-Voltage-gated Ca2+ channels open in SR
-Ca2+ leaves SR, enters sarcoplasm
-Ca2+ binds to troponin
-Troponin/Tropomyosin complex moves; exposing active sites on the G actin
-Myosin heads bind with G actin sites; forming cross-bridges
-ATP is used to move the myosin heads along actin myofilament
-Sarcomere shortens
What part of muscle contraction requires ATP?
1 molecule of ATP is required per myosin head per cycle of cross-bridge formation, movement, release, and reset into position.
Why does muscle relaxation require ATP?
The binding of ATP to a myosin cross-bridge is what causes the myosin to unbind from the actin and move back into position, ready to bind again. The ATP is hydrolyzed.
Where does skeletal muscle get ATP from?
-stored ATP (~2sec)
-Creatine Phosphate: used the phosphate in CP to make ATP from ADP (~15sec)
-Metabolism of Glycogen: anaerobic (~2min) and aerobic (~40min)
What is a muscle twitch and how can it increase muscle tension?
A muscle twitch is single stimulus contraction-relaxation sequence in an isolated muscle fibre.
The 3 phases are:
-Latent period
-Contraction time
-Relaxation time
Whole muscle tension can be affected by:
- The number of fibers in the muscle that are contracting
- The amount of tension developed by each contracting fiber
How can repeated stimulation of a muscle fiber during a “twitch” lead to increased muscle tension?
Treppe: Increase in peak tension with each successive stimulus after the relaxation phase is complete.
Wave Summation: When successive stimuli arrive before the relaxation phase has been completed.
Incomplete Tetanus: If stimulus frequency increases further, the tension production rises to a peak and relaxation periods are very brief.
Complete Tetanus: Stimulus frequency is so high that the relaxation phase is entirely eliminated. Eventually plateaus.
What is a motor unit?
Why is the size important?
One motor neuron and ALL of the muscle fibers it innervates.
Small motor units control a small amount of muscle fibers, resulting in more precise movement (i.e. eye muscles have on neuron control 4-6 muscle fibers)
Larger motor units like in the legs, control thousands of muscle fibers. Allowing for less precise but more powerful contractions.
How can recruitment of motor units result in greater muscle tension?
With multiple motor units active, one muscle fiber can be contracting while the previous one is relaxing. Alternating the contraction-relaxation phases between muscle fibers allows for sustained time under tension.
Length-tension relationship: why can muscle being too long or too short not generate any tension?
The tension created in a muscle fiber is related to sarcomere length.
- If there is too much overlap in the sarcomere, there is little room for the myosin to move over the actin before being blocked by the Z-line
- If there is too little overlap, it will be less efficient at forming cross bridges or may not even reach the zone of overlap at all resulting in a weak or non-existent contraction.
Types of muscle contraction
Isometric: Tension increased but muscle length doesn’t change. Ex: “Holding”
Isotonic:
- Concentric: Isotonic contraction where muscle shortens
- Eccentric: Isotonic contraction where muscle lengthens. (Tension of contraction cannot overcome opposing forces.)
Slow-oxidative muscle fibers (Type 1) (Slow) V Fast-glycolytic muscle fibers (Type IIx) (Fast) V
Fast-oxidative muscle fibers (Type IIa) (Intermediate)
Type 1:
-Lots of mitochondria, capillaries, and myoglobin.
-Dark red (“dark meat”)
Type IIx:
- Fewer mitochondria, capillaries, myoglobin
-Pale (“Light meat”)
-High in stored glycogen
-Good for anaerobic metabolism/strong contractions over short periods
Type IIa:
-Directly in between both fast and slow fibers
What is Fatigue and how and where does it develop?
Fatigue: the decreased capacity to do work
Psychologic Fatigue: Fatigue that develops in the nervous system because the brain forecasts sustained exercise will tire you out.
Can be overcome with discipline or distractions
Synaptic Fatigue:
Occurs at the NMJ when motor nerve fibers use all their ACh. (Uncommon)
Muscular Fatigue:
Occurs in the muscles due to depleted glycogen, damage to the Sarcolemma or SR, increased P from ATP -> ADP + P or decrease in pH.
