1. Skeletal

2. Cardiac

3. Smooth

Characterized by their histological appearance, location in the body, and function

1. Myoepithelial cells

2. Myofibroblasts

3. Pericytes

4. Hair cells

Expel secretions from glandular acini

Surround blood vessels

Contract and secrete collagen

(of auditory sense organ)

Control the amplitude of sound

Extensions of the sarcolemma (muscle membrane) to which the AP travels

Activated by the invasion of the AP into the t-tubules that opens Ca++ release channels in the sarcoplasmic reticulum and sets in motion a series of molecular events that causes contraction

Occurs when the sarcoplasmic reticulum pumps Ca++ back into its lumen using Ca++-ATPase

One contraction-relaxation cycle

1. Fast twitch

2. Slow twitch

1. Develop tension quickly

2. Cannot maintain contraction

3. Have fast myosin ATPase activity

4. Large and pale cells

5. Fatigue easily

6. Glycolytic, more anaerobic metabolism

7. Least used-jumping, e.g.

1. Develop tension slowly

2. Maintain contraction for a long time

3. Have slow myosin ATPase activity

4. Cells are small and red (due to myoglobin)

5. Fatigue resistant

6. Oxidative, anaerobic metabolism

7. Most used- postural, e.g.

1. Differentiation of muscle fibers into fast twitch or slow twitch fibers is dependent on their motor innervation

2. Motor neurons have different firing frequencies and their activity imposes a differentiation scheme on the developing muscle cells

3. Myoblasts, Myotubes

Mononuclear skeletal muscle precursor cells that differentiate and proliferate in the myotome and fuse to form myotubes

1. Multinucleated cells formed by fusion of myoblasts

2. Synthesis of contractile proteins begins after myoblasts fuse with nuclei of myotubes being displaced peripherally as more contractile protein is formed

3. Innervation occurs at this stage and guides specific differentiation of contractile proteins, thus controlling for fast or slow twitch properties

1. A single motor neuron and all the muscle fibers it innervates

2. Muscles that are used in fine motor tasks have small motor units, while those involved in gross motor activity have larger motor units

1. Motor neurons are clustered in spinal cord as motor neuron pools

2. Motor neurons with lowest threshold for firing AP are recruited first (these control fatigue-resistant slow-twitch fibers that generate minimal force)

2. As stimulus to motor neuron pool increases, motor neurons with higher firing thresholds are recruited (these control intermediate fatigue resistant oxidative fast-twitch fibers)

3. With greatest stimulus to motor neuron pool, the highest threshold motor neurons that control glycolytic fast-twitch motor units are recruited

4. At this point, contraction (force) is great, but not sustainable

5. As long as not all motor units are recruited, the nervous system avoids fatigue in a sustained contraction by asynchronous recruitment-different motor units take turns maintaining the muscle tension