1. Muscle Tissue
Ch. 9

2. Three types of muscle tissue
1. Skeletal:
- Voluntary
- Multi-nucleated cells with nuclei on the periphery
- Found usually attached to long bones
- Short contractions
- Quick twitch
- Many mitochondria.
2. Smooth muscle:
- Involuntary
- Spindle shaped cells
- No striations
- Single ovoid nucleus per cell
- Found in blood vessels
- walls of hollow organs and GI tract and is associated with peristalsis; long contractions; slow twitch

3. 3. Cardiac muscle:
- Involuntary
- striated
- Single nucleus per cell
- Intercalated discs
- Found only in the heart.

4. Skeletal Muscle

5. Smooth Muscle

6. Cardiac Muscle

7. Muscle tissue is derived from the mesoderm layer (middle germ layer)
Characteristics of muscle tissue:
1. Excitability (irritability): ability to respond to certain stimuli
2. Contractibility: ability to shorten and thicken
3. Extensibility: able to stretch and extend without damage
4. Elasticity: ability to return to original shape after being stretched

8. Functions of muscle tissue
1. Movement: muscle pushes against bone for locomotion, the heart pumps blood through the blood vessels, urinary bladder empties
2. Thermogenesis: 85% of body heat is produced by skeletal muscle contraction
3. Maintenance of posture: constant contractions to keep us sitting or standing straight.
4. Stabilize joints: as muscles pull on bone they help to strengthen joints.

9. Organization of muscle tissue
Muscles are covered in connective tissue wrappings called epimysium
Bundle of muscles cells are termed a fascicle. This fascicle is covered in perimysium
Muscle (cell) fiber is covered in endomysium.
Muscle fibers are composed of elongated myofibrils
Basic contracting unit of a myofibril is sarcomere

11. Microscopic anatomy of a skeletal muscle fiber
Muscle cell is also called a muscle fiber
Muscle fiber contains large number of rod shaped myofibrils.
Myofibrils contain the contractile units called
sarcomeres.
Structures:
a. Sarcolemma: plasma membrane
b. Sarcoplasm: cytoplasm containing stored sugars (glycogen), mitochondria and myoglobin (O2 binding pigment in muscle).

15. c. Sarcoplasmic reticulum - smooth ER
forming interconnecting tubules surrounding myofibrils.
d. Transverse (T) tubules - tubules running between sarcoplasmic reticulum and penetrating deeply into cell; conducts "stimulus" into cell.
e. Terminal cisternae - terminal portions of sarcoplasmic reticulum adjacent to transverse tubules.
f. Triads: one T-tubule and two terminal cisterna

16. Sarcomere anatomy:
1.      A bands: area overlapping myosin and actin filaments
2.      I bands: contains actin filaments only
3.      Z discs (lines): separates sarcomeres and anchors the thin filaments.
4.      H zone: part of the A band that contains only myosin fibers.
5.      M line: center of the H zone that holds the myosin fibers in place.

20. Myofibril filaments:
Thick filaments: myosin containing a tail and two (globular proteins) heads. Heads interact with thinner filaments called actin.
Thin filaments: composed of actin.
- tropomyosin: two strands of protein that spiral around the actin filament.
- troponin: contains three subunits that helps bind calcium.
Sliding filament theory of contraction
1954: Hugh Huxley
The sacromeres shorten and the distance between Z lines is reduced

26. Motor unit: motor neuron (somatic nervous system) plus all of the muscle fibers it innervated.
Point of innervation is termed neuromuscular junction.
When motor neuron fires, all muscle fibers innervated by that motor neuron will contract

28. Anatomy of the neuromuscular junction:
Terminal axon: axonal ending of the motor neuron
Synaptic cleft: space between the terminal axon and the sarcolemma of the muscle fiber
Synaptic vesicle: vesicles located in the terminal axonal bud contain the neurotransmitter acetylcholine (ACh)
Motor end plate: section of sarcolemma that is folded upon itself. Millions of ACh receptors are found in the folds.

30. Initiation of the muscle contraction
I) Nerve impulse reaches the terminal axon and Ca++ is allowed to enter via voltage gated calcium channels. Ca++ floods in from the extracellular fluid.
II) Calcium triggers vesicles of ACh to fuse with the axonal membrane and release (exocytosis) Ach into the synaptic cleft.
III) ACh binds to ACh receptors on the sarcolemma and creates an impulse that travels throughout the sarcolemma and down T-tubules
IV) To prevent excessive contraction an enzyme, acetylcholinesterase, breaks down ACh into acetic acid and choline thus stopping the flow of Ach into the binding sites located on sarcolemma.

31. V) Resting sarcolemma is polarized (voltage)
(-) on the inside, (+) on the outside
Na+ normally is not allowed to enter cell
VI) When ACh binds to receptors, gated ion channels allow Na+ to flood into the cell while K+ diffuses out. More Na+ is pumped in than K+ leaving. This creates an electrical charge across the membrane:
depolarization
VII) Depolarization sets off an action potential (propagation) down the membrane surface.

32. VIII) Repolarization is resetting the membrane surface back to normal.
Na+ channels close while K+ channels remain open (K+ continues pump outside).
The refractory period is when the muscle fiber is insensitive to further stimulation until repolarization is complete.
Action potentials are considered an all or none response because once initiated, they are unstoppable.

33. Excitation-Contraction coupling: Electrical impulse does not act directly on the myofibrils. They stimulate CA++ to be released from within muscle cell.
From motor neuron the action potential travels along to axon terminal; here ACh is released causing depolarization of the motor end plate; action potential propagates along sarcolemma down T tubules.
Action potential triggers Ca++ release from terminal cisternae of sarcoplasmic reticulum (calcium ion gates open).
Calcium ions bind to troponin causing troponin to change shape and expose actin active sites.
Contraction occurs
Calcium levels decrease (due to change in permeability) and tropomyosin blockage is restored

35. Muscle twitch:
Myogram can record the phases of contraction of a muscle fiber.
Single action potential acting on a motor unit is a muscle twitch.
1. Latent period: muscle tension begins to increase, but contraction has not occurred.
2. Contraction period: muscle shortens demonstrating actin/mysoin activity
3. Relaxation period: Muscle fiber returns to resting state.