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Muscle fiber
All muscles contain mixture of fiber types.
Among individual the same muscle can vary in its proportion of fiber types
Sprinter: more white muscle fibers, fast twitch, fatigue prone
Long distance runner: more red muscle fiber, slow twitch, fatigue resistant
Muscle Atrophy
Weakening and shrinking of a muscle May be caused
Immobilization Loss of neural stimulation
Muscle Hypertrophy
Increase total mass of the muscle
Increase fiber diameter due to increase actin and myosin filaments
More capillaries More mitochondria
MYASTHENIA GRAVIS Myasthenia gravis is a muscle weakness. Inability
of neuromuscular junction to transmit enough signals to the muscle fibers. Immunity develops against acetylcholine receptor proteins of neuromuscular junction causing paralysis
Normal receptor
Defective receptors
Rigor Mortis
Stiffening of the body beginning 3 to 4 hours after death
Deteriorating sarcoplasmic reticulum releases calcium Calcium activates myosin-actin cross-bridging and
muscle contracts, but can not relax. Muscle relaxation requires ATP and ATP production is
no longer produced after death Fibers remain contracted until myofilaments decay
Smooth Muscle Composed of spindle-shaped fibers with a
diameter of 1-5 m and lengths of several hundred m
Found in walls of hollow organs, such as stomach, urinary bladder
Not striated and involuntary Same mechanism of muscle contraction
between myosin and actin filament in smooth muscle as in skeletal muscle
Microscopic Anatomy of Smooth Muscle SR is less developed than in skeletal muscle Plasma membranes have pouch like folding
called caveoli T tubules are absent There is no troponin complex There are no visible striations No sarcomere Thin and thick filaments are present
Types of smooth muscle
Multiunit smooth muscle Single unit (unitary) smooth muscle
Types of Smooth Muscle: Multi-unit
Compose of discrete, separate smooth muscle fiber. Each fiber operates independently of the other fibers.
Each fiber can contract independently of each other
Multiunit smooth muscles are found: Attached to hair follicles, In the internal eye
muscles
Types of Smooth Muscle: Single Unit
The cells of single-unit smooth muscle, commonly called visceral muscle: Hundreds of smooth muscle contract together
as a single unit The cell membranes are adherent to one
another at multiple points, so force generated in one muscle fiber can transmitted to the next
Cell membranes are joined by gap junction to one another via gap junctions so action potentials can travel from one fiber to the next and caused the muscle fiber to contract together
Smooth Muscles: Contrasted to Skeletal Muscle
Dense body which are similar to Z line Actin and myosin absence of banding
pattern Thick and thin filaments are arranged
diagonally Myosin cross bridge arranged in
opposite direction. This allows myosin to pull actin in one direction in one side while pulling another actin on the opposite direction on the other side
Organization and Contraction of Myofilaments in Smooth Muscle
Smooth Muscles: Characteristics
Contraction Mechanism
Ca2+ is released from the SR and from the extracellular space
Ca2+ binds to calmodulin and activates it The calmodulin-calcium combination
joins with and activates myosin light chain kinase (phosphorylation enzyme)
Activated kinase transfers phosphate from ATP to myosin cross bridges
Phosphorylated cross bridges interact with actin to produce shortening
Smooth Muscle Contraction: Mechanism
Cessation of Contraction
Smooth muscle relaxes when intracellular Ca2+ levels drop
Require myosin phosphatase which splits the phosphate from myosin
Cycle stops and contraction ceases
The time required for relaxation of muscle contraction is determined by the amount of active myosin phosphate in the cell
Smooth Muscle Relaxation: Mechanism
Source of Ca ions
Sarcoplasmic reticulum is slightly developed in most smooth muscle.
Almost all Ca ions that caused contraction enter the muscle cell from extracellular fluid.
When extracellular fluid Ca ion concentration falls, smooth muscle contraction ceases.
To cause relaxation of smooth muscle, Ca pumps used to pump calcium ions out back in to extracellular fluid. This pump is slow acting.
Smooth muscle contraction last for few seconds rather than hundredths to tenths of a second as for skeletal muscle