Lecture 19 Muscle Physiology 2 Flashcards
Neuromuscular junction
synpase between motor neuron and muscle fiber
motor neuron AP-> ACH release -> synaptic transmission-> EPP -> muscle AP
Muscle action potential
formed along the sarcolemma, similar to axon membrane
Excitation-Contraction Coupling
“calcium is the key”
T tubules
Sarcoplasmic reticulum
T tubules
conduct APs deep into the muscle fiber
DHP receptors function as voltage sensors, activated by the muscle AP (depolarization)
Sarcoplasmic reticulum
Stores Ca2+ and releases it to myofilaments ryanodine receptors (RyR) in the SR membrane are Ca2+ channels RyR of the SR interact w/ DHP receptors of the T tubule Ca2+ released through RyR channels activates muscle contraction Ca2+ pumps in the SR membrane actively transport Ca2+ back into the SR
Sequence of events in Excitation-Contraction Coupling
1-3
- Muscle AP travels down the T-tubules
- DHP receptors on the T-tubule are activated
- Activated DHP receptor opens RyR Ca2+ channels
Sequence of events in Excitation-Contraction Coupling
4-6
- Ca2+ diffuses out of the SR into the cytosol and to the thin filaments
- Ca2+ binds to troponin, which moves tropomyosin off the myosin bindin site
- Myosin binds to actin -> sarcomere contracts (crossbridge cycle)
* when APs stop, Ca2+ is pumped back into SR -> sarcomeres relax
Motor Unit
one motor neuron + all the muscle fibers it stimulates
small motor units (1:10) fine control, less force per unit
large motor units (1:2000) coarse control, greater force per unit
recruitment
increase in number of active motor units
increased CNS stimulation
activates more motor neurons with higher thresholds
increased motor unit recruitment -> more muscle fibers activated -> increased force of contraction
Twitch
single contraction relaxation cycle, evoked by a single muscle AP
latent period, contraction phase, relaxation phase
summation
two or more closely spaced APs -> increased force
tetanus
high frequency of APs -> maximum, sustained force
Length-tension relationship
sarcomere length = 2.0 - 2.2 um (‘resting length)
maximal number of crossbridges -> maximal tension
length >2.2 um - overlap decreases -> tension decreases
length < 2.0 um - interference between filaments -> tension decreases
Skeletal muscle mostly operates at
the top of the L-T curve
Cardiac muscle operates at
shorter than optimal length, so increase stretch -> increase force
Functions of ATP in muscle contraction
detaches the actin-myosin crossbridges
energizes the myosin head
powers the Ca2+ pump in the SR
Sources of ATP
intracellular ATP stores
phospocreatine + ADP -> creatine + ATP
Glycolytic metabolism
Oxidative metabolism
Glycolytic metabolism
anaerobic
glycogen -> glucose-6-P -> lactic acid, yields 3 ATP per glucose unit
Oxidative metabolism
aerobic
glycogen-> gluoce-6-P -> CO2 + H20 yields about 32 ATP per glucose
triglycerides -> fatty acids -> CO2 + H20, yields>100 ATP per fatty acid
Muscle fiber types
slow-twitch oxidative (SO) (“red muscle”)
fast-twitch oxidative-glycolytic (FOG)
fast-twitch glycolytic (FG) (“white muscle”)
Slow-twitch oxidative (SO) (red muscle)
slow myosin ATPase, small diameter -> low power
abundant mitochondria and myoglobin
high triglyceride content
highly aerobic -> slow fatiguing
fast-twitch oxidative-glycolytic (FOG)
fast myosin ATPase, intermediate diameter, intermediate power
can utilize both aerobic and anaerobic metabolism
fast-twitch glycolytic (FG) (“white muscle”)
fast myosin ATPase, large diameter -> high power
high glycogen content
mostly anaerboic -> fast fatiguing
