Lecture 19 Muscle Physiology 2

Question 1

Neuromuscular junction

Answer 1

synpase between motor neuron and muscle fiber

motor neuron AP-> ACH release -> synaptic transmission-> EPP -> muscle AP

Question 2

Muscle action potential

Answer 2

formed along the sarcolemma, similar to axon membrane

Question 3

Excitation-Contraction Coupling

Answer 3

“calcium is the key”
T tubules
Sarcoplasmic reticulum

Question 4

T tubules

Answer 4

conduct APs deep into the muscle fiber

DHP receptors function as voltage sensors, activated by the muscle AP (depolarization)

Question 5

Sarcoplasmic reticulum

Answer 5

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

Question 6

Sequence of events in Excitation-Contraction Coupling

1-3

Answer 6

1. Muscle AP travels down the T-tubules
2. DHP receptors on the T-tubule are activated
3. Activated DHP receptor opens RyR Ca2+ channels

Question 7

Sequence of events in Excitation-Contraction Coupling

4-6

Answer 7

4. Ca2+ diffuses out of the SR into the cytosol and to the thin filaments
5. Ca2+ binds to troponin, which moves tropomyosin off the myosin bindin site
6. Myosin binds to actin -> sarcomere contracts (crossbridge cycle)
   * when APs stop, Ca2+ is pumped back into SR -> sarcomeres relax

Question 8

Motor Unit

Answer 8

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

Question 9

recruitment

Answer 9

increase in number of active motor units

Question 10

increased CNS stimulation

Answer 10

activates more motor neurons with higher thresholds

increased motor unit recruitment -> more muscle fibers activated -> increased force of contraction

Question 11

Twitch

Answer 11

single contraction relaxation cycle, evoked by a single muscle AP
latent period, contraction phase, relaxation phase

Question 12

summation

Answer 12

two or more closely spaced APs -> increased force

Question 13

tetanus

Answer 13

high frequency of APs -> maximum, sustained force

Question 14

Length-tension relationship

Answer 14

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

Question 15

Skeletal muscle mostly operates at

Answer 15

the top of the L-T curve

Question 16

Cardiac muscle operates at

Answer 16

shorter than optimal length, so increase stretch -> increase force

Question 17

Functions of ATP in muscle contraction

Answer 17

detaches the actin-myosin crossbridges
energizes the myosin head
powers the Ca2+ pump in the SR

Question 18

Sources of ATP

Answer 18

intracellular ATP stores
phospocreatine + ADP -> creatine + ATP
Glycolytic metabolism
Oxidative metabolism

Question 19

Glycolytic metabolism

Answer 19

anaerobic

glycogen -> glucose-6-P -> lactic acid, yields 3 ATP per glucose unit

Question 20

Oxidative metabolism

Answer 20

aerobic
glycogen-> gluoce-6-P -> CO2 + H20 yields about 32 ATP per glucose
triglycerides -> fatty acids -> CO2 + H20, yields>100 ATP per fatty acid

Question 21

Muscle fiber types

Answer 21

slow-twitch oxidative (SO) (“red muscle”)
fast-twitch oxidative-glycolytic (FOG)
fast-twitch glycolytic (FG) (“white muscle”)

Question 22

Slow-twitch oxidative (SO) (red muscle)

Answer 22

slow myosin ATPase, small diameter -> low power
abundant mitochondria and myoglobin
high triglyceride content
highly aerobic -> slow fatiguing

Question 23

fast-twitch oxidative-glycolytic (FOG)

Answer 23

fast myosin ATPase, intermediate diameter, intermediate power
can utilize both aerobic and anaerobic metabolism

Question 24

fast-twitch glycolytic (FG) (“white muscle”)

Answer 24

fast myosin ATPase, large diameter -> high power
high glycogen content
mostly anaerboic -> fast fatiguing

Question 25

Smooth muscle physiology

General features

Answer 25

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

Question 26

Activation of contraction in smooth muscle 1-3

Answer 26

1. depolarization of smooth muscle cell (AP or GP)
2. Ca2+ enters cytosol form ECF (through voltage gated Ca2+ channels) and/or from sarcoplasmic reticulum (through RyR Ca2+ channels)
3. Ca2+ binds to calmodulin -> Ca-calmodulin

Question 27

Activation of contraction in smooth muscle

4-6

Answer 27

4. Ca-calmodulin complex activated myosin light chain kinase (MLCK)
5. MLCK catalyzes phosphorylation of myosin light chain
6. phosphorylation of myosin light chain -> crossbridge formation -> contraction
   * when myosin light chain is dephosphorylated, crossbridge activity stops -> relaxation

Question 28

Types of smooth muscle

Answer 28

single-unit smooth muscle

multi-unit smooth muscle

Question 29

single-unit smooth muscle

Answer 29

extensive connections between cells via gap junctions

fewer innervation points (via varicosities of autonomic motor neurons)

Question 30

multi-unit smooth muscle

Answer 30

multiple innervation points, little or no connection between cells