Lecture 19 Muscle Physiology 2 Flashcards Preview

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Flashcards in Lecture 19 Muscle Physiology 2 Deck (30):
1

Neuromuscular junction

synpase between motor neuron and muscle fiber
motor neuron AP-> ACH release -> synaptic transmission-> EPP -> muscle AP

2

Muscle action potential

formed along the sarcolemma, similar to axon membrane

3

Excitation-Contraction Coupling

"calcium is the key"
T tubules
Sarcoplasmic reticulum

4

T tubules

conduct APs deep into the muscle fiber
DHP receptors function as voltage sensors, activated by the muscle AP (depolarization)

5

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

6

Sequence of events in Excitation-Contraction Coupling
1-3

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

7

Sequence of events in Excitation-Contraction Coupling
4-6

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

8

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

9

recruitment

increase in number of active motor units

10

increased CNS stimulation

activates more motor neurons with higher thresholds
increased motor unit recruitment -> more muscle fibers activated -> increased force of contraction

11

Twitch

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

12

summation

two or more closely spaced APs -> increased force

13

tetanus

high frequency of APs -> maximum, sustained force

14

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

15

Skeletal muscle mostly operates at

the top of the L-T curve

16

Cardiac muscle operates at

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

17

Functions of ATP in muscle contraction

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

18

Sources of ATP

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

19

Glycolytic metabolism

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

20

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

21

Muscle fiber types

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

22

Slow-twitch oxidative (SO) (red muscle)

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

23

fast-twitch oxidative-glycolytic (FOG)

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

24

fast-twitch glycolytic (FG) ("white muscle")

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

25

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

26

Activation of contraction in smooth muscle 1-3

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

27

Activation of contraction in smooth muscle
4-6

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

28

Types of smooth muscle

single-unit smooth muscle
multi-unit smooth muscle

29

single-unit smooth muscle

extensive connections between cells via gap junctions
fewer innervation points (via varicosities of autonomic motor neurons)

30

multi-unit smooth muscle

multiple innervation points, little or no connection between cells