A&P Sarcomeres

Sarcomere

Main contractile unit of muscle fiber in the skeletal muscle

Sarcomere Composition

Composed of protein filaments (myofilaments) that include mainly the thick filaments (myosin), and thin filaments (actin). Bundles of myofilaments are myofirbrils

Myofilaments - myofibrils - myofibers (muscle fiber) - fasicles - muscle

3 Types of Proteins Associated with Sarcomeres

1. Structural

2. Contractile

3. Regulatory

Structural Proteins

Connects the fibers and filaments in place and/or to each other

Titin and Dystrophin

Contractile Proteins

Attach to the M Line of the sarcomere
Actin (thin filaments)
Myosin (THICK filaments)
Contain binding sites for ATP and Actin

Regulatory Proteins

Turns the process of contraction "on" or "off"
Tropomyosin and Troponin
TNI - binds Actin
TNT- binds Tropomyosin
TNC - associated with Calcium binding and release

Structure of Sarcomere

Dark and light bands visible under the microscope
Banding pattern due mainly to the arrangement of thick and thin myofilaments in each unit

5 Parts of Sarcomere

1. A Bands (Anisotropic)

2. I Bands (Isotropic)

3. Z Disc

4. M Line

5. H Zone

A (Anisotropic) Bands

Dark bands that contain whole thick filaments (myosin)

I (Isotropic) Bands

Light bands that contain only the thin filaments (actin)
Located between the two thick filaments

Z Disc

An area that traverses the I bands and marks the point of the connection between the 2 neighboring actin filaments.

*Sarcomere can be described as the structure between 2 Z discs

M Line

Marks middle of sarcomere and contains the protein called myomesin

H Zone

The area between the M line and Z disc
Contains only myosin

Which is the only unaffected part of sarcomere in a contracted state?

A (Anisotropic) Band

4 Steps to Initiate Muscle Contraction

1. Stimulus

2. Acetylcholine "plug in" to receptors

3. Electrical signal initiated

4. Ca++ stored

Stimulus

Causes initiation of electrical impulse in a motor neuron
Travels through motor neuron till it reaches Axon Terminal which releases neurotransmitter, Acetylcholine from Motor End Plate

Briefly changes signal from Electrical to Chemical Because the chemical is the only thing that can get across the Synapse (space) between nerve cell (neuron) and muscle cell. We call the space a Neuromuscular Junction
Electrical Signal must meet the threshold of -70mv "All or None" Principal

Acetylcholine "plugs in" to Receptors

Acetylcholine "swim" across synapse and "plugs into" receptors on the Sarcolemma of the muscle cell.
Initiates a chain reaction that causes Na+ channels to open in muscle cell and flood it with +Na ions (normally a negative environment)
Depolarization (reverses the polarity)

Electrical Signal Initiated

Electrical signal initiated by Na+ rushing in, continues into the T-tubules (transverse tubules) starting a contraction
Signal travels via t-tubules till it reaches Sarcoplasmic Reticulum of muscle cell

Ca++ Stored

Ca++ is stored in structures within the Sarcoplasmic Reticulum called the Terminal Cistern
These open and release Ca++ which causes rotation of Actin filaments and exposure of the Myosin Binding sites allowing for Cross Bridge formation

*Sliding Filament Theory (Actin and Myosin "sliding over" one another)

Contraction is now in full swing and Ca++ is needed to maintain it and the strength of it

Events of Contraction Cycle

1. Myosin heads attach to newly exposed Actin Binding sites to form the "cross bridge"

2. Power Stroke of myosin head "walking along" actin occurs

3. ATP needed to detach the myosin head from actin to destroy the cross-bridge so muscle can start to relax

4. ATP has to be hydrolyzed into ADP and Phosphorus to create energy for the action

How to End a Contraction

1. Close Na+ channels in an effort to restore a negative environment again intracellularly (Repolarization)

2. Stop the release of Ca++ from cisterns within the Sarcoplasmic Reticulum

3. Enzyme Acetylcholinesterase needed to "unplug" acetylcholine from binding sites of the Sarcolemma

*Before a contraction can occur again there is a REFRACTORY period

3 Sources of Energy for Contraction

1. Phosphorylation

2. Anaerobic Respiration

3. Aerobic Respiration

Phosphorylation

ATP broken into ADP+Phosphate
Creatine Phosphate
15 seconds work of energy
Occurs in places like Liver, Kidneys, and Pancreas
ex. standing up out of a chair

Anaerobic Respiration

Glycolysis (breakdown of glycogen)
Mainly in liver and muscles
30-40 seconds
Converts to Pyruvic Acid which becomes Lactic Acid
Creates an Oxygen debt
Produces 6 ATP molecules, but consumes 4 in the process, so nets 2 ATP

Aerobic Respiration

Needs O2 to occur
Gets oxygen from stored sources such as: Myoglobin, Hemoglobin
Nets 36-38 ATP molecules