12.15.4 Muscle Structure and Action

Question 1

Muscle Structure and Action

Answer 1

• Muscle types are classified according to microstructure, control, and location or function.
• Skeletal muscles consist of many bundles of elongated muscle cells surrounded by a tough connective tissue that extends to attach the muscle to bone or other muscles.
• Skeletal muscles attach to bone at an origin (to a bone that does not move) and an insertion (to a bone that moves).
• Skeletal muscles typically span a joint between bones and operate in opposing pairs (an agonist and an antagonist) to move the bones in opposite directions.
• Muscles may act to produce movements described as flexion and extension, abduction and adduction, and circumduction among others.

Question 2

muscle

Answer 2

• Muscles are classified by microstructure into striated, smooth, and cardiac muscle types. Striated muscle shows striations in its muscle fibers, while smooth muscle lacks these. Cardiac muscle is striated, but has different cell structure than other striated muscle. Muscle can be classified based on control into voluntary and involuntary muscle types. Finally, muscle can be classified by location and function into skeletal, visceral, and cardiac types. Skeletal muscle is attached to bone; visceral muscle is not so attached.
• A skeletal muscle is formed from muscle fibers (cells) that are bundled into fascicles. The fascicles are bundled together into a muscle, which is surrounded by a tough connective tissue called the epimysium. The epimysium extends at the end of the muscle to form a tendon for attachment to a bone. A broad epimysial attachment is called an aponeurosis

Question 3

skeletal muscle

Answer 3

• Typically skeletal muscles attach to bones at an origin on a bone that does not move, and at an insertion on a bone that does move. The muscle spans a joint between the bones. An origin may also be determined as the broader attachment, while an insertion is the narrower attachment.
• Muscles work in opposable pairs to move a joint. For
  example, the biceps brachii contracts to close the elbow joint, while the triceps brachii contracts to open the elbow joint. The biceps brachii is known as the agonist, or prime mover, and the triceps brachii is the antagonist.

Question 4

muscle movement

Answer 4

• Muscle movements and the muscles that cause them have been named. Flexion is the closing of a joint, such as the elbow, and the muscle that performs a flexion is called a flexor. Extension is the opening of a joint, and is carried out by an extensor.
• Abduction is the movement of a limb away from the central axis of the body and is caused by an abductor. Adduction is the movement of a limb toward the axis of the body and is carried out by an adductor.
• The movement of a limb in a circular motion is called
  circumduction and is caused by several muscles working together.