• Cells that are excitable have a plasma membrane
• Membrane potentials can be altered, by:
• Proteins in membrane, which:
• Generate graded or action potentials

1. Skeletal: attached to skeleton; longest muscle cell type; voluntary; striated (overlapping proteins)

2. Smooth: not typically attached to skeleton; in digestive tract, blood vessels, reproductive and respiratory systems; unvoluntary; not striated (cells smaller/spindle-shaped)

3. Cardiac: found only in heart; striated but fibers branch; intercalated discs (gap junctions) appear between fibers to join cells

• Excitability: can conduct AP in response to stimuli
• Contractility: can shorten & lengthen; generates a force (ex: cardiac muscle pushes blood out of heart, skeletal muscle moves skeleton, smooth muscle pushes fluids out of tube)
• Extensibility: can be stretched beyond resting length when relaxed
• Elasticity: resumes original length after stretched

• Produce Movement: such as skeleton, moving blood, or moving substance encircled by organ (food)
• Maintain Posture: only skeletal
• Stabilize Joints: keep knees locked, back straight; also skeletal
• Generate Heat: not all potential energy released from ATP is converted to kinetic energy for work; some is released as heat to help keep enzymes functioning normally

1. Muscle Cell (fiber): elongated cell; surrounded by sarcolemma (plasma membrane); contains sarcoplasm (cytoplasm) and organelles

2. Oraganelles & Proteins:

• multiple, peripheral nuclei;
• numerous mitochondria;
• myoglobin (protein that stores oxygen);
• glycosomes (glycogen storage granules);
• sarcoplasmic reticulum (modified ER) that surrounds myofibrils
• myofibrils (specialized contractile organelles, 80% vol of a cell, rod-like & run parallel to length of cell)

Consists of Myofilaments which run entire length of myofibril; 2 types:

1. Thick Filaments: made up of myosin

2. Thin Filaments: made up of actin (beads), troponin (cluster of 3 protein), and tropomyosin (thread-like)

• A-band: length of thick filament w/ thin filament overlap
• I-band: Thin filament w/out thick filament overlap
• H-zone: Area in A-band w/out thin filament 
• M-line: end to end attachments of thick filaments
• Z-line/disc: end to end attachment of thin filaments (bisects I-band)

A&I band alternate; sarcomere is region between Z-lines (functional unit)

Do not participate in contraction; act as structural support proteins for myofibrils; 2 types:

1. Titin: between z-lines and thick filament; helps provide recoil ability (curly lines)

2. Nebulin: runs along inside of thin filament (core); provides thin filament with integrity

1. Sarcoplasmic Reticulum: expands as it comes close to T-tubules; wraps around myofibrils; Terminal Cisternae- 2 expanded regions at either side of SR

2. T-Tubules: extend from plasma membrane; wraps around myofibrils in interior of cell

*Triad: 2 terminal cisternae + 1 T-Tubule; important in communication/excitation-contraction