1. Muscular System by: Daniel Gable
An overview of the muscular system

2. Muscles move skeletal parts by contracting
Muscles always contract
Muscles must be attached in antagonistic pairs so each muscle can work against each other
Skeletal muscle is attached to the bones and is responsible for their movement, it is characterized by a hierarchy of smaller and smaller parallel units
A skeletal muscle consists of a bundle of long fibers running the length of the muscle

3. Each fiber is a single cell with many nuclei, reflecting its formation by the fusion of many embryonic cells. Each fiber itself a bundle of smaller myofibrils arranged longitudinally. The myofibrils in turn are composed of two kinds of myofilaments. Thin filaments consist of two strands of actin and one strand of regulatory protein coiled around one another, while thick filaments are staggered arrays of myosin molecules. Each repeating unit is a sarcomere, the basic contractile unit of the muscle.

4. Interactions between myosin and actin generate force during muscle contractions
When a muscle contracts the length of each sacomere is reduced
This is known as the sliding-filament model where neither the thin or thick filaments change length but rather slide past each other longitudinally
The myosin head is the center of bioenergetic reactions that power muscle contractions
Muscle cells typically only store enough ATP for a few contractions

5. Calcium ions and regulatory proteins control muscle contraction
A skeletal muscle contracts only when stimulated by a motor neuron
When at rest the myosin binding sites on the actin molecules are blocked by the regulatory protein tropomyosin
The troponin complex controls the position of tropomyosin on the thin filament
When calcium concentration in the cytosol falls, the binding sites of actin are covered, and contraction stops
Calcium concentration in the cytosol of the muscle cell is regulated by the sarcoplasmic reticulum, a specialized endoplasmic reticulum
The action potential spreads deep into the interior of the muscle cell along infoldings of the plasma membrane called T (transverse) tubules

6. Diverse body movements require variation in muscle activity
Muscles act in twitches and if the rate of stimulation is fast is enough, the twitches will blur into one smooth and sustained contraction called tetanus
A motor unit consists of a single motor neuron and all the muscle fibers it controls
Tension in a muscle can be progressively increased by activating more and more of the motor neurons controlling the muscle, a process called recruitment of motor neurons

7. Fast and Slow Muscle Fibers
Fast muscle fibers are used for short, rapid, powerful contractions
Slow muscle fibers, often found in muscles that maintain posture, can sustain long contractions
Mygoglobin binds oxygen more tightly than hemoglobin so it can effectively extract oxygen from the blood

8. Other Types of Muscle
Vertebrate cardiac muscle is found in only one place-the heart
The junctions between cardiac muscles contain specialized regions called intercalated discs, where gap junctions provide direct electrical coupling among cells
Smooth muscle lacks the striations of skeletal and cardiac muscle because the actin and myosin filaments are not all regularly arrayed along the length of the cell.