Download BIOL 424 Muscle 2 I. Classification of Skeletal Muscle Fibers A. Slow

BIOL 424
Muscle 2
I. Classification of Skeletal Muscle Fibers
A. Slow twitch/Type I
1. sustained contractions
2. resistance to fatigue
3. increased oxidative capacity for aerobic respiration
a. rich capillaries
b. numerous mitochondria
c. high myoglobin
B. Fast twitch/Type II
1. thicker than slow twitch
2. adapted for anaerobic respiration
a. increased glycogen storage
b. glycolytic enzymes
3. Subtypes (mammals)
a. IIB : regular fast-twitch
- contract rapidly
- fatigue quickly
- glycolytic
b. IIA : intermediate
high oxidative capacity
resist fatigue
II. Response to exercise
A. Increase endurance ---> increase mitochondria
B. Hypertrophy
1. thickening of muscle fibers
2. synthesize myofilaments
3. myofibers may split
C. Hyperplasia
1. increased myoblasts
2. may be incorporated into mature muscle cells
3. usually for damage repair
III. Cardiac Muscle
A. Striated
1. actin and myosin
2. sarcomeres
B. Interconnected branches
1. gap junctions
electrical contacts
2. myocardium ---> single functional unit
a. all myocardial cells electrically joined
b. full contraction of all fibers
C. Self-generated action potentials
- other muscles are neurogenic
IV. Smooth Muscle
A. No striations
1. actin and myosin
2. no sarcomeres
3. myosin filaments can be very long
4. capacity for contraction when stretched
5. lacks T-tubules or extensive SR
B. Locations
1. surrounding lumina
2. blood vessels
3. GI tract
C. Contraction
1. usually initiated by increased cytoplasmic Ca++
not sufficient for sustaining contraction
2. influx of extracellular Ca++
a. sustains contraction
b. Ca++ gates
3. Ca++ binds to calmodulin
a. forms complex with caldesmon
caldesmon binds to actin in absence of Ca++
covers the myosin binding sites
b. activates myosin light chain kinase
4. Ca++ binds to light chains of myosin
D. Graded potentials
1. cells can have graded depolarizations
> depolarization >influx of Ca++ > contraction
2. action potential not always necessary
skeletal muscle CELLS normally all-or-none
E. Most function as single-unit smooth muscles
1. numerous gap junctions between cells
2. can generate electrical activity and contraction in response to stretch
3. tonic smooth muscles: remain contracted for long periods
4. phasic smooth muscles: contract and relax frequently
F. Stimulus
1. skeletal muscle has neurotransmitter receptors only at NJ
2. smooth muscle surface covered with receptors
3. many smooth muscle cells respond to hormones