A motor neuron and all the muscle fibers it innervates.
What is a motor end plate?
A specialized region of the sarcolemma
Responsible for initiating the action potential that propagates across the myofiber causing the muscle contraction.
What is the NMJ?
the junction of the axon terminal of a motor neuron with the motor end plate
Where in the spinal cord are motor neurons found?
Name two important characteristics of the junctional folds.
Found on the muscle fiber side of the NMJ
There are different protein complexes at the crest and the bottom of the junctional folds.
Explain the 6 steps of Excitation-Contraction Coupling.
1) Action potential generated by the NMJ is propagated along the sarcolemma and down the T-tubules
2) AP triggers Ca2+ release from terminal cisternae of SR
3) Ca2+ ions bind to troponin; troponin changes shape, removing the blocking action of tropomyosin; actin active sites exposed
4) Contraction; myosin cross bridges alternately attach to actin and detach, pulling the actin filaments toward the centre of the sarcomere; release of energy by ATP hydrolysis powers the cycling process
5) Removal of Ca2+ by active transport into the SR after the action potential ends
6) Tropomyosin blockage restored blocking actin active sites; contraction ends and muscle fiber releases
What is botox?
A toxin produced by the bacterium Clostridium botulinum.
Inhibits Acetylcholine release by axon terminals
Leads to botulism that causes muscular paralysis, respiratory failure, and eventual death.
Describe Duchenne Muscular Dystrophy (DMD).
In DMD, the link between the actin cytoskeleton and the basal lamina is lost.
Muscle tissue is completely disrupted.
There is an infiltration of macrophages, adipose tissue, and connective tissue.
Name 7 characteristics of cardiac muscle.
Found only in the heart where it forms a thick layer called the myocardium
Pumps blood through the body
Short and branched striated fibers
One central nucleus per fiber
T-tubules are much larger and deeper than in skeletal muscle
What is an intercalated disk?
Junctional complex found between two adjacent cardiac fibers
Has a transverse part and a lateral part
What type of junctions are found in the transverse part of an intercalated disk?
Zonula adherens and desmosomes
What type of junctions are found in the lateral part of an intercalated disk?
What is the main source of calcium for cardiac muscle?
Where can smooth muscle be found?
Gastrointestinal tract, respiratory tract, and around blood vessels
Name 5 characteristics of smooth muscle.
Non-striated, spindle-shaped, and tapered
One central nucleus per fiber
Fibers communicate via Gap junctions
Explain how smooth muscle contracts.
Thin filaments are arranged obliquely in the cell. Some attach to the dense bodies in the cytoplasm while others attach to dense plaques in the plasma membrane.
Contractile units consist of myosin filaments connecting actin filaments to a dense body or dense plaque. They stretch across the cell between plaques.
Describe the smooth muscle of the arteries and veins.
Tunica media is made up of smooth muscle cells and elastic tissue - thicker in arteries compared to veins.
Smooth muscle controls vessel diameter.
Describe the smooth muscle of the intestine.
Longitudinal layer: muscle fibers run parallel to the long axis of the organ
Circular layer: muscle fibers run around circumference of the organ
Longitudinal and circular layers participate in peristalsis.
Describe the smooth muscle of the stomach.
Makes use of a longitudinal layer, a circular layer, and an oblique layer to acheive peristalsis