The Muscular System
The human body has 656 individual muscles
327 antagonistic muscles and two unpaired muscles as the Orbicularis oris muscle and the Diaphragm

Functions of Muscle
1. Movement: skeletal muscles are attached to bones by tendons. Because skeletal muscles Cross joints, when these muscle contract, the bones they attach to move this allows for various body motions, such as walking or waving your hands.
2. Stability : muscles are Holding your bones tightly together so it Stabilize joints
3. Control of body openings and passages Valve-like structures called sphincters around various body opening and passages. These sphincters control the movement of substance into and out of the passages. Example: A urethral sphincter prevents or allows urination
4. Heat production: when muscle contract heat is released. So it helps the body maintain a normal temperature. The normal body temperature is between 98.6 °F to 99.8°F
Types of muscle tissue:
1. Skeletal muscles : Under conscious control

Contraction of the skeletal muscles
Movement of the muscles occurs as a result of two major events; neural stimulation and contraction of muscle protein. The skeletal muscle must be stimulated by nerve impulses to contract. A single muscle contraction is called a muscle twitch.
Skeletal muscles respond only to neurotransmitter acetylcholine at the neuromuscular junction. Acetylcholine causes skeletal muscle to contract.
At the nerve ending, the chemical neurotransmitter acetylcholine is released. ACH diffuses across the synaptic cleft and attaches to receptors on the muscle fibers.
Following contraction, muscles release the enzyme Acetylcholinesterase. Which Breaks down acetylcholine? Acetylcholinesteras allows muscle to relax

2. Smooth muscle: Not under conscious control
There are two types of smooth muscle
1- Multiunit smooth muscle is found in the iris of the eye and the wall of the blood vessels
2- Visceral smooth muscles
Found in the wall of the hollow organs such as the stomach, intestine, bladder.
Contractions produce movement of organ contents. Example: Peristalsis – rhythmic contraction that pushes substances through tubes of the body 3. Cardiac muscle
This pumps blood from the heart into the blood vessels
Self-exciting – does not need nerve stimulation to contract
Nerves speed up or slow down contraction
Cardiac muscle responds to two Neurotransmitters
1- Acetylcholine – slows heart rate.
2- Norepinephrine – speeds up rate.

Characteristics of Muscles
All muscles, whether they are skeletal, smooth or cardiac, have four common characteristics
1. Contractibility: contractibility is the capacity for the muscle to shorten in response to a nerve stimulus
2. Excitability or irritability: is a characteristic for both muscle and nervous cells. it is the ability to respond to certain stimuli by producing electrical signal called action potentials
3. Extensibility: excitability is the ability to be stretched
4. Elasticity: elasticity is the ability to return to original length when relaxing

Attachments and Actions of Skeletal Muscles
Muscles must be attached to bones for leverage Muscles only pull, never push
Attached to bone by tendons
Muscles are attached at both ends
Attachment sites
Origin – an attachment site for a less movable bone
Insertion – an attachment site for a more moveable bone

Body Movements
1. Flexion – bending a body part
2. Extension – straightening a body part
3. Abduction – moving a body part away from the anatomical position
4. Adduction – moving a body part toward the anatomical position
5. Pronation – turning the palm of the hand down
6. Supination – turning the palm of the hand up

Muscle fatigue: is a condition in which a muscle has lost its ability to contract. It is usually develop because of an accumulation of lactic acid. During periods of vigorous exercise, the blood is unable to transport enough oxygen for the complete oxidation of the glucose in the muscles. This causes the