Muscle Contraction 2 Flashcards
Nerve cell components:
-Soma (cell body):
houses the nucleus
-Dendrites:
multiple short branched processes (receive signals from cells, conduct to soma)
-Axon (nerve fiber):
single, much longer process (sends signals to other cells)
Motor units
-muscles are stimulated by motor neurons
-motor units: a single motor neuron and all muscle fibers innervated by it
Neuromuscular junction:
-axons have axon terminals
–contain synaptic vesicles filled with acetylcholine (Ach)
-axon terminals form a neuromuscular junction (NMJ) with a muscle fiber but DO NOT touch
How do we get muscles to contract?
2 phases
-Phase 1 (excitation): nerve stimulus; action potentials
-Phase 2 (contraction): contraction of muscle fibers
excitation-contraction coupling
Excitation-contraction coupling
-process by which an AP causes a muscle fiber to contract
-electrical excitation of the sarcolemma leads to mechanical contraction of muscle fibers
-all-or-none phenomenon: once stimulated, muscle fibers contract to full extent, or not at all
Excitation-contraction coupling steps: 1->8
- Action potential starts in brain or spinal cord
- AP arrives at axon terminal, releases Ach, binds to receptors on sarcolemma
- Binding opens chemically gated ion channels; Na+ flows into muscle fiber
- Na+ influx causes voltage gated ion channels to open= wave of depolarization
- AP propagated down sarcolemma, T-tubules
- Triggers Ca2+ release from SR
- Ca2+ enables actin-myosin contraction
- Sliding filament theory
Role of Ca2+ in muscle contraction summary:
-arrival of AP at axon terminal opens voltage gated Ca2+ channels
–Ca2+ enters axon terminal, releasing Ach from vesicles
-ACh binding to sarcolemma, opens Na+ channels
–depolarizes sarcolemma, T-tubules (AP)
-AP arrives at SR from T-tubule
–cause mass release of Ca2+ into sarcoplasm
-Ca2+ binds to troponin on thin filament
–troponin-Ca2+ complex moves tropomyosin
-myosin binds to actin, cross bridge is formed, contraction can occur
Cross bridge movement: 4 steps
- Cross-bridge formation
energized myosin head attaches to an actin filament, forming a cross bridge. high-energy position
2.power stroke
ADP and Pi are released and the myosin head pivots and bends, changing to its bent low-energy state. As a result, it pulls the actin filament toward the M line
3.Cross-bridge detachment
After ATP attaches to myosin, the link between myosin and actin weakens, and the myosin head detaches (cross-bridge breaks )
4.Cocking of myosin head
As ATP is hydrolyzed to ADP and Pi, the myosin head returns to its prestroke high-energy, or cocked position
How does a muscle relax after contraction?
-AP ends, electrical stimulation of SR stops
-Ca2+ pumped back into SR
–stored until next AP arrives
-requires ATP (active process)
-Without Ca2+, troponin and tropomyosin return to resting conformation
–covers myosin-binding site
–prevents actin-myosin cross bridging
Energy for muscle contraction:
-ATP!
-Binds to myosin head
–ATPase on myosin head
–ATP-> ADP+Pi+energy
-necessary for muscle contraction and relaxation
-3 sources
How do we get energy for muscle contraction and relaxation? 3 forms
-creatine phosphate
-anaerobic respiration
-aerobic respiration
ATP from creatine phosphate
-Creatine + phosphate (PCr)
muscles store 3x more PCr than ATP
-stores energy at rest, first source used a onset of activity
-PCr donates its phosphate to ADP= 1 ATP
rapid synthesis
short duration (15s)
does not require oxygen
ATP from anaerobic respiration
-glucose metabolized to pyruvate
-generates 2 ATP
-source of ATP when O2 unavailable or need more ATP faster than aerobic respiration provides
-mod fast synthesis
-mod long duration (2 mins max)
-does not require oxygen
ATP from aerobic respiration
-glucose or triglycerides are broken down into various intermediate products
–32 ATP from glucose
–100+ ATP from fat
–CO2 and H2O by products
-occurs in mitochondria
-slow synthesis (3 multi-step process)
-long duration
-requires O2
most efficient way to make ATP
