ANS-205 Midterm 2-Muscle System

Excitation

Depolarization (influx of Na+)

Relaxation

Repolarization

Skeletal Muscle

1. Striated muscle attached to bones of the skeleton

2. Calcium binding protein is troponin

3. Sources of calcium: sarcoplasmic reticulum

4. Excitation occurs by the acetyl choline binding to acetyl choline receptor

Cardiac Muscle

1. Striated muscle found only in heart

2. Calcium binding protein is troponin

3. Sources of calcium: sarcoplasmic reticulum and Ca2+ ion channels in T-tubules

4. Excitation occurs by pacemaker cells depolarizing

Smooth Muscle

1. Non-striated muscle found in internal organs and tubes

2. Calcium binding protein is calmodulin

3. Sources of calcium: Sarcoplasmic Reticulum and Ca2+ ion channels in caveoli

4. Excitation occurs by autonomic transmitters, hormones, mechanical stretch, or paracrine signals.

T-tubules

Present in skeletal and cardiac but not smooth muscle

Caveoli

Smooth Muscle

Flexor

Brings bones together

Extensor

Moves bones away

Abductors

Muscle that moves limb away from body

Adductors

Muscle that moves limb towards body

Anatomy of Muscle Fiber

Plasma membrane= sarcolemma (generates action potentials)

Cytoplasm= sarcoplasm, contains myoglobin (stores oxygen, glycogen,

MYOFIBRILS (contractile units of muscles fibers)

Anatomy of Myofibril

Anatomy of Myosin

Bulkier than actin, but less abundant. Myosin heads have a strong connection to actin, but if they were together all the time the muscles would always be contracted so the tropomyosin acts as a hinderance.

Anatomy of Actin

Thinner than myosin and more abundant than myosin.

DHP Receptor

1. On inside of T-tubule and has foot-like mechanism on SR that when lifted Ca2+ ions flow out.

2. Voltage sensitive protein (shape of receptor changes through depolarization)

Sarcoplasmic Reticulum

1. Wrapped around a myofibril that covers actin and myosin sarcomeres.

2. Source of Calcium

Sarcomere structure

1. A band-anisotropic region of myosin and actin filaments

2. I band- isotropic region of only actin filaments and shrinks during contraction

3. H zone- Holler zone, region of no actin. Shrinks during contraction and can disappear at full contraction.

4. M line- holds myosin together

5. Z line or Z disk- criss-cross pattern allows actin to be highly flexible

Skeletal Muscle Excitation

1. Neuron receives stimuli (this stimuli can be mechanical, electrical, or chemical).

2. Sodium (Na+) channel opens and depolarization occurs. A synapse forms between two neurons and sodium is able to enter motor neuron.

3. Depolarization continued all the way down the motor neuron to the terminal end. Acetyl and choline formed into one neurotransmitter called acetyl choline by ester bond.

4. Sodium (Na+) that arrives at terminal end of axon (the depolarization process) continues and the influx of Na+ (sodium) opens the calcium ion channels in the neuron.

5. Calcium from ECF flows into the axon. The abundance of calcium leads to vesicles destabilizing and terminal end vesicles migrating to the cell membrane to dump their content.

6. Vesicles merge with neuron cell membrane and acetylcholine that was contained in the vesicles is emptied in the synaptic cleft.

7. Acetylcholine binds to acetyl choline receptors which acts as a dual Na+, K+ ion channel. This opens Ash receptor as an ion channel.

8. Na+ (sodium) flows into skeletal muscle cells. It depolarizes it which excites it.

Skeletal Muscle Contraction

1. Na+ ions enter the sarcolemma through dual ion channel.

2. DHP lifts its foot since it senses the charge changing due to Na+ entering cell. This opens the calcium channel allowing Ca2+ ions to flood the cytosol out and Ca2+ diffuses down concentration gradient.

3. Calcium in cytosol binds to Ca2+ sensitive protein (troponin)

4. The Ca2+ that binds changes the shape of tropomyosin which lifts tropomyosin from where myosin heads bind to actin.

5. ATPase on myosin heads breaks an ATP, attaches the phosphate to it, and stores the energy in the myosin head.

6. Myosin heads bind to actin and make actin stand closer to M line. Myosin heads flex and unfelt through stored energy from clearing ATP and more actin strand. At the end, ADP falls off myosin head as well.

Skeletal Muscle Relaxation

1. Potassium diffuses out of ICF

2. Break ester bond in ACh. Once bond is broken, the compounds are no longer a neurotransmitter.

3. ATPase Pump

Smooth Muscle Excitation

1. Autonomic neurotransmitters-Neurotransmitter binds to a receptor which opens a channel for Na+ to travel into a cell.

2. Hormones- A hormone binds to a hormone receptor which opens the sodium ion channel

3. Mechanical Stretch-a food bolus physically stretches the channel to allow sodium to enter.

4. Paracrine signals

Paracrine signals

Gastrin + secretin = hormones that bind to gastric receptors and secrete receptors

Histamine = immune protein released to protect organism with immune cells

Nitric Oxide = Inhibits depolarization and produced by simple squamous cells and immune cells

Hyperpolarization

Depolarization is skipped and we go straight to relaxation

Smooth Muscle Contraction

1. Sodium enters smooth muscle cell through sodium channel by the four methods above.

2. Upon depolarization, calcium ion channels in sarcoplasmic reticulum and caveoli opened to flood cytosol with calcium ions.

3. Calcium that enters through caveoli bind with calmodulin.

4. MLCK was formerly inactive, but binds with calmodulin which activates it.

5. ATP is recruited and is broken into ADP and PO4.

6. MLCK is a catalyst in phosphorylating myosin heads. The free PO4 from the hydrolyzed ATP gets added to myosin heads (phosphorylation).

7. Myosin engages with actin.

8. Contraction ends when dense bodies move closer together since there is no H-zone or I-band.

Smooth Muscle Relaxation

1. Repolarization: Potassium leaks out of ECF and ATPase pump takes sodium back to ECF and potassium back to ICF.

2. Return Ca+ in ICF to original sources

Cardiac Muscle Excitation

1. Pacemaker cells depolarize the atria.

2. Atria and ventricles do not contract at the same time, so depolarization of ventricles is delayed from atria.

Cardiac Muscle Contraction

1. Sodium ions enter the sarcolemma through dual ion channels opened by ACh bound to ACh receptors.

2. DHP senses the change in charge and lifts its "foot" which opens calcium ion channels allowing the calcium to flood the cytosol out of the SR and ECF. Ca2+ diffuses down the concentration gradient.

3. Calcimine cytosol released by SR binds to troponin.

4. The Ca2+ that binds changes the shape of tropomyosin which lifts tropomyosin from where myosin heads bind to actin.

5. ATPase on myosin heads breaks an ATP, attaches the phosphate to it, and stores the energy in the myosin head.

6. Myosin heads bind to actin and make actin stand closer to M line. Myosin heads flex and unfelt through stored energy from clearing ATP and more actin strand. At the end, ADP falls off myosin head as well.

Cardiac Muscle Relaxation

1. Repolarization: Potassium leaks out of ECF and ATPase pump takes sodium back to ECF and potassium back to ICF.

2. Return Ca+ in ICF to original sources

Skeletal Muscle vs. Smooth Muscle vs. Cardiac Muscle