Muscle Structure And Function Flashcards
Three Types Of Muscle Tissue:
- Skeletal
- Cardiac
- smooth
Skeletal Muscle
striated and voluntary (subject to conscious control)
* Striated refers to visible structures responsible for contraction
* Movement, posture
* Support soft tissue
* Body temperature, energy reserve
Cardiac Muscle
- Striated And Involuntary
- only found in the heart
Smooth Muscle
- not striated and involuntary.
- lines our digestive system and empties our bladder and bowels.
Myofibrils
- contain two types of protein filaments that are arranged in a regular, over-lapping pattern.
- “Striated” appearance
- Myosin - thicker filament
- actin - thinner filament
Skeletal Muscle Movement Requires 2 Basic Proteins
- Actin Filament
- Myosin (Motor Protein)
Visualizing The Sacromere Through Gel Electrogenesis
- A-band: Enriched in Myosin and Actin
(looks dark) - I-band: Absent of Myosin Filament; Actin only (looks light)
- Z-disc: Actin binding site; is the outer border of the sarcomere
Motor Proteins
- 17 classes; multiple isoforms per class
- Most common are 1, 2, and 5
- Myosin Class 2 is “Muscle Myosin”
Myosin Structure: Motor Protein
Head – ATPase activity
(Provides energy for movement)
* Tail – binds cargo; dimerization potential
* Neck – binding site for “accessory proteins”; controls position of Head
Myosin Class 2
- Class 2 (muscle) myosin exist as dimers
- 4 myosin light chains per dimer
- hexamer of myosin
- Each Myosin head has:
ATP - binding site
Actin - binding site
Thick Filaments
- Polymers of myosin brought together to form a superstructure
- 150 myosin brought together in a “bouquet”
- 2 bouquets arranged tail to tail
Thin Filaments
- are alpha actin
- are not dynamic like the cytoskeleton
- similar shape
Alpha Actin
- 2 spirals of actin chains
- Regulatory proteins Tropomyosin and Troponin can block the MyHC binding sites
Contraction
- Myosin pulls actin toward the center of each sarcomere which in turn shortens or contracts the muscle
- This shortens the sarcomere which in turn shortens the muscle
Sliding Filament Theory
Starts with Myosin and Actin interaction
1) ATP binds Myosin
2) Cross-bridge between Myosin and Actin is severed
3) ATP is hydrolysed to ADP and Phosphate
4) Myosin reaches forward and binds new Actin molecule
5) Powerstroke (Release of Phosphate from Myosin)
Actin-Myosin Interaction
1) Actin and Myosin have a strong interaction called a cross-bridge
* This connection is severed when ATP binds to the catalytic subunit of Myosin
2) ATP causes myosin to release actin; This dissociation converts ATP to ADP and Phosphate.
3) ATP hydrolysis causes myosin to “reach” forward to a new actin molecule.
4) Release of Phosphate promotes a power stroke
5) Completion of the power stroke releases ADP
Ends in Rigor Mortis
Rigor Mortis
- Post mortem rigidity
- Myosin and Actin are permanently bound because there is no ATP to release the cross-bridge
Muscle Contraction Depends ON…
INTRACELLULAR CALCIUM
made up of:
- Troponin (Ca^2+ binding sites); 3 subunits
- Tropomyosin (rope like)
Troponin 3 subunits:
-TnC
- TnC
- TnT
TnC
- Calcium sensor; has 4 Ca^2+ binding sites, 2 of which are always occupied.
Tnl
-links Troponin to Actin; facilitates inhibition
TnT
- binds Tropomyosin
Troponin In Actin
- Increased calcium causes TnC to interact with Tnl
because all of Troponin moves, TnT brings Tropomyosin with it, exposing the myosin binding site. - Resets as Ca^2+ decreases
Unitary Displacement
- Actin spirals like a staircase
- Myosin contacts Actin on the same “face” each time
- The length of Myosin corresponds perfectly to the periodicity of actin filaments.
- Myosin V walks in steps of 36 nm (average)
- seen in myosin that carry cargo, not in muscle myosins
Visual Observations Of Muscle
Muscles that are deep red are rich in myoglobin (O2 storage) * use oxidative phosphorylation for energy (rich in mitochondria)
* Called slow twitch muscles; Slower to activate, but longer contracting capacity (Type I)
Staining Of Myosin Proteins:
- Immunofluorescent staining of myosin proteins in a mixed muscle:
- Type 1 (beta): Orange
- Type 2a: Green
- Type 2b: Blue
- Type 2x/d: black
