Lec28

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