Muscular System

1. Produce movement
2. Maintain posture
3. Support soft tissues (abdominal wall and floor of pelvic cavity)
4. Guards entrances and exits to the body
5. Helps maintain body temperature

Bartholomew, Martini. Essentials of Anatomy and Physiology. (2000) pg. 168



Neuromuscular Junction
Step 1: An action potential causes the neuron to release Acetylecholine (ACh).

Step 2: The ACh molecules diffuse through the synaptic cleft and bind with ACh receptors on the muscle cell.

Step 3: The now-activated receptors on the muscle cell change the local membrane of the muscle cell to allow sodium ions to rush in.

Step 4: This change in voltage causes an action potential, which spreads throughout the muscle cell including its actin and myosin bundles.

Step 5: An enzyme known as Acetylechoninesterase (AChE) breaks down ACh and restores the synaptic cleft to its original ACh-free state.

Frequency of Muscle Stimulation

A Twitch is a single contraction/relaxation sequence in a single muscle fiber. A single muscle fiber either contracts at full force or not at all. A twitch consists of a latent period (the time it takes ACh to cross the synaptic cleft and actin and myosin to engage), a contraction phase (tension rises as myosin pulls in actin), and a relaxation phase (myosin disengages from actin).

Incomplete Tetanus occurs when a muscle fiber twitches very quickly. The relaxation phase in the muscle fiber are cut short by increasing contraction phases. (Area 'B' in diagram)

Complete Tetanus occurs when a muscle fiber twitches so quickly that there is no relaxation phase. Complete tetanus is the most common type of skeletal muscle contraction.

When I muscle contracts it can go through incomplete tetanus and complete tetanus as well. *This can be confusing: Both a single muscle fiber and a whole muscle, made up of tens of thousands of muscle fibers can undergo incomplete and complete tetanus. The graphs look the same for both a single muscle fiber and a whole muscle*
The amount of muscle fibers the nervous system activates is known as recruitment. The only way to control force in a muscle (due to a muscle fiber's all-or-none contraction) is to control the number of fibers contracting at any one time. Imagine the difference in the numbers of muscle fibers required to gently pick up a paper cup. Now, imagine increasing the recruitment of muscle fibers to smash the paper cup in your merciless iron grasp!

Isotonic vs. Isometric
An Isotonic contraction occurs when the muscle maintains the same tension and shortens in length. Picking up a book and running are isotonic contractions.

An Isometric contraction occurs when the muscle has increasing tension but does not shorten in length. Attempting to pick up a freight train, maintaining posture.


Muscles can use aerobic respiration (burning sugar using oxygen). This functions at a low to moderate level of exertion.
As exertion increases, the muscle fiber cannot get enough oxygen and begins to use anaerobic respiration (burning sugar without using oxygen), this produces lactic acid.
A muscle can also use its reserves of a chemical called creatine phosphate to regenerate ATP during muscular activity (think of this as a battery).

Oxygen Debt

After muscular activity, your body will use more oxygen than normal. This process, known as oxygen debt, uses the oxygen to turn lactic acid back into sugar and then break that sugar down aerobically. The energy from this process can also be used to regenerate creatine back into creatine phosphate (recharge that battery!).

Fast vs. Slow Muscle Fibers

Fast fibers (white meat) contract quickly, produce powerful contractions, and fatigue quickly.

Slow fibers (dark meat) contract more slowly, produce less powerful contractions, but fatigue slowly. Slow muscle fibers have increased blood flow to them, have more mitochondria (the organelle responsible for aerobic respiration), and have myoglobin (which binds more oxygen acting as another battery).
Fast Muscle
Slow Muscle

Origins, Insertions and Actions
Rectus Abdominis

Erector Spinous

Pectoralis Major


Latissimus dorsi


Biceps brachii

Triceps brachii