1. Unit 2 Notes: Muscles & Contractions

2. (1) Types of Muscle All internal organs = Muscle Skeletal Muscle:
Located on top of bones.
Helps with overall body mobility and voluntary movement.
Cardiac Muscle:
Located in the Heart.
Produces involuntary and rhythmic contraction of the heart chambers.
Smooth Muscle:
Located in all other internal organs.
Produces wave-like contractions to push solids and fluids through organs.

3. (2) Cellular Organization in Muscles
Muscles have levels of organization similar to nerves.
Levels of Organization: (Macro to Microscopic)
Muscle (The whole organ)
Fascicle (A bundle of muscle fibers)
Muscle Fiber (A single muscle cell)
Myofibril (The contractile organelle within muscle cells)
Sarcomere (The structure in the myofibril which allows for contraction)
Myofilaments Actin and Myosin (The contracting proteins in the sarcomere)

6. (3) What is a Muscle Contraction?
A muscle contraction is a sequence of steps resulting in the sliding of the myofilaments (actin and myosin) toward one another.
Steps:
Nerve impulse sent to muscle.
Neurotransmitter Ach released.
Depolarization of the muscle.
Release of Calcium from the Sarcoplasmic Reticulum.
Calcium binds to Troponin.
Actin Exposed.
Actin and Myosin slide toward one another.

7. (4) Details: Neuron to Muscle
The muscle contraction is initiated either by:
A conscious decision to move a body part.
A homeostatic response / reflex.
The nervous system will send a nerve impulse to stimulate the muscle.
The neurotransmitter Acetylcholine (Ach) is released.
Ach binds to the sarcolemma (muscle fiber membrane).
The sodium-potassium channels are opened.
Sodium & Potassium ions move in and out of the sarcolemma (Depolarization).

9. (5) Details: Depolarizing the Muscle
Sodium and Potassium ions move in and out of the channels, down the length of the sarcolemma.
The movement of the ions is driven by the polarity of the sarcolemma.

10. (6) Details: Calcium & Troponin
(A protein structure called Troponin- Tropomyosin covers Actin.)
Calcium is released from the Sarcoplasmic Reticulum (organelle), when depolarization stimulates the organelle.
Calcium binds to Troponin.
Actin is exposed.

12. (7) Details: Myosin and Actin
Once actin is exposed, Myosin will automatically proceed.
Myosin releases Inorganic Phosphate and ADP.
Myosin changes conformation.
Myosin hinges up and grabs Actin.
Myosin pulls Actin toward it (sliding toward each other).

15. (8) Muscle Relaxation
Similar to a nerve impulse, everything must “go back”:
ATP binds back to Myosin.
Myosin detaches from Actin.
Troponin and Tropomyosin cover up Actin.
Calcium moves back to the Sarcoplasmic Reticulum.
Sodium and Potassium move back in/out of the sarcolemma.