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* Contractility
* Excitability
* Extensibility
* Elasticity

Synergists:  muscles that tend to work together to accomplish specific movement (eg. Brachialis and biceps brachii work to flex forearm)

Antagonists:  muscles works in opposition to another muscle (eg. Triceps brachii works to extend forearm)

Origin = head - Most stationary end of muscle

Insertion - End of muscle attached to bone undergoing greatest movement

Belly of muscle is between origin and insertion

Epimysium
Separates muscle from surrounding tissues and organs

Perimysium
Divides muscle into compartments (fasicles)

Endomysium
Surrounds individual muscle fibers

Size:  enormous
Diameter:  100μmLength:  up to 12in
Multinucleate
100s of nuclei, just internal to Plasma Membrane
Sarcolemma (Plasma Membrane)
Surrounds sacroplasm (cytoplasm)
Contains numberous myofibrils
Has transmembrane potential due to unequal distribution of (+) and (-) charges across membrane

Sarcoplasmic Reticulum (SR)
Related to SER:  membrane complex
Tightly bound to T-tubules
Forms tubular network around each myofibril
Enlarge, fuse and form expanded chambers on either side of T-tubules = terminal cisternae
Pair of terminal cisternae + T-tubules = triad
Ca2+ actively pumped from sarcoplasm into terminal cisternae of SR
Resting muscle
Sacroplasm:  low [Ca2+]
SR:  1000x free Ca2+ and 40000x free + bound Ca2+
AP leads to release of Ca2+ from SR into sarcoplasm
Ca2+ diffuses into individual contractile units

Composed of 2 Types of Protein Fibers (myofilaments):
Thin:  composed primarily of actin
Thick:  composed primarily of myosin
1-2 μm in diameter, as long as fiber
responsible for fiber contraction
Within and scattered about are:
Mitochondria and granules of glycogen (storage form of glucose)

Actin - Resembles 2 strands of pearls twisted together. Has active site for Myosin

Tropomysin - filaments located along groove between twisted strands of actin myofilament. Covers active/attachment site for myosin

Troponin - molecules attached at specific intervals along actin myofilament. Binds to Ca2+

Resemble bundles of minute golf clubs

2 subunits twisted around each other; each with tail and head

Tail points toward M line

Head projects outward toward nearest thin filament

Myosin head interacts with exposed attachment sites on thin filament during contraction:  cross-bridge:  fx as hinge, lets head pivot at base

Highly ordered repeating units of actin and myosin myofilaments along the myofibrils:  smallest functional unit of muscle fiber, smallest unit that can contract

2 T-tubules encircle each sarcomere
Extends from one Z disk to another Z disk
Each has
Dark bands:  A bands
Light bands:  I bands

- Thick filaments
- Thin filaments
- Proteins that stabilize positions of filaments
- Proteins that regulate interactions between filaments

M line
Center portion of each thick filament

H band
On either side of M line
Contains only thick filaments, no thin

Zone of Overlap
Area of overlap of thin and thick filaments
Ca2+ released from SR into this area

A: I Band
B: A Band
C: H Zone
D: Zone of Overlap
E: OOPS
F: Sarcomere
G: M Line
H: Z Line
I: Titin
J: Thin Filament
K: Thick Filament

Length of actin and myosin do not change, only the bands/zone change size
H and I bands get smaller
Zones of Overlap get larger
Z lines move closer together
Width of A band remains constant

Thin filaments slide toward center of each sarcomere, alongside thick filaments

Sliding occurs in every sarcomere along myofibril:  myofibril gets shorter

Myofibril attached to sarcolemma at each Z line and at either end of muscle fiber, thus when myofibril gets shorter so does the entire muscle fiber

Presynaptic terminal
Enlarged axon terminal of nerve

Synaptic cleft
Space between presynaptic and postsynaptic terminals

Postsynaptic terminal
Muscle fiber membrane
Convoluted to increase surface area

Synaptic vesicles contain Neurotransmitter (NT) = Acetylcholine (Ach)
NT = Molecule released by presynaptic terminal and stimulates (or inhibits) a postsynaptic cell
Ach always stimulates a skeletal muscle fiber

Ca2+ channels open in reaction to Action Potential traveling down the Motor Neuron Plasma Membrane, which allows Ca2+ into the Presynaptic Terminal. This then causes Exocytosis of Ach into the Synaptic Cleft.

Ach diffuses across Synaptic Cleft to bind with Ach Receptors on the Postsynaptic Terminal. This causesNa+ Channels to open, letting Na+ to enter sarcoplasma, which causes an Action Potential in the sarcolemma, triggering muscle contraction.

1.  AP travels along axon to NMJ
2.  Ca2+ channels open, Ca2+ enters presynaptic terminal
3.  Ach released from synaptic vesicles in presynaptic terminal
4.  Ach diffuses across synaptic cleft, binds to receptor sites on Na+ channels in muscle postsynaptic membrane, Na+ channels open
5.  Na+ diffuse into muscle cell initiating AP which travels along sarcolemma and T tubules

6.  AP in T tubules cause SR to release Ca2+
7.  Ca2+ binds to troponin (of thin myofilament).  Causes tropomyosin-troponin complex to move, exposing myosin binding sites on actin
8.  Muscle contraction requires energy.  ATP bound to myosin are broken down into ADP + P, releasing energy, which is briefly stored in the “cocked” myosin head

9.  Myosin heads bind to actin forming cross-bridges.  P are released from myosin heads
10.  Heads of myosin bend, causing actin myofilaments to slide across surface of myosin myofilaments (Power-stroke)
11.  ADP molecules are released from myosin heads
12.  ATP can bind to myosin, breaking cross-bridge.  Myofilaments return to resting state.  Myosin head is “cocked” (ATP broken down to ADP + P + energy)
13.  If Ca2+ is still bound to troponin and ATP is available, steps 8 – 11 repeat and muscle continues to contract