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
Smooth muscle physiology
General features
no sarcomeres, oblique arrangement of thick and thin filaments
contraction results from crossbridge formation between thick and thin filaments
contraction activated by action potentials, graded potentials, or chemical signals
spontaneous contraction (peacemaker activity) in some smooth muscle cells
Activation of contraction in smooth muscle 1-3
- depolarization of smooth muscle cell (AP or GP)
- Ca2+ enters cytosol form ECF (through voltage gated Ca2+ channels) and/or from sarcoplasmic reticulum (through RyR Ca2+ channels)
- Ca2+ binds to calmodulin -> Ca-calmodulin
Activation of contraction in smooth muscle
4-6
- Ca-calmodulin complex activated myosin light chain kinase (MLCK)
- MLCK catalyzes phosphorylation of myosin light chain
- phosphorylation of myosin light chain -> crossbridge formation -> contraction
* when myosin light chain is dephosphorylated, crossbridge activity stops -> relaxation
Types of smooth muscle
single-unit smooth muscle
multi-unit smooth muscle
single-unit smooth muscle
extensive connections between cells via gap junctions
fewer innervation points (via varicosities of autonomic motor neurons)
multi-unit smooth muscle
multiple innervation points, little or no connection between cells
