Chapter 12- Muscles

What is the sarcolemma?

plasma membrane for muscles

Skeletal muscles are:

-multinucleated
-striated
-have typical cell organelles

Muscle fibers are striated because

-I bands: light bands (thin filaments only)
-A bands: dark banks (thick and thin)
-H zone: (thick only)
-z discs: dark lines in the middle of the I bands- determines length of sarcomere

What is a motor unit?

a single motor neuron and all the muscle fibers it innervates

What are graded contractions?

varied contraction strength due to different numbers of motor units being stimulated

What is the neuromuscular junction?

site where a motor neuron stimulates a muscle fiber

What is the motor end plate?

-area of the muscle fiber sarcolemma that contain neurotransmitter receptors

The neurotransmitter in muscle cells is always ____.

acetylcholine

The receptor at the motor end plate is always ____.

nicotinic

How are motor units controlled?

-contraction strength comes from motor unit recruitment
-finer muscle control requires smaller motor units (fewer muscle fibers)

The eye muscles have how many muscle fibers/motor units?

-2-3
-weaker, more precision

Larger, stronger muscles have how many muscle fibers/motor units?

-thousands
-powerful, less precision

What are muscle fibers made of?

-densely packed myofibrils that run the length of the muscle fiber
-contains thick and thin myofilaments
-stacked so that dark and light bands align which creates striations

What are thin myofilaments made of?

actin

What are thick myofilaments made of?

myosin

What is a sarcomere?

-basic subunit of striated muscle contraction
-area from one z-disc to the next
-forms a hexagonal pattern in 3-D

What are titin filaments?

protein that runs from the z disc to the m line; allows for elastic recoil

What is the M line?

made of protein filament in the center of each A band to hold down thick filaments

What is the sliding filament theory?

-muscle contraction caused by shortening of sarcomeres
-A bands do not change in length
-I bands shorten
-H zone shortens or disappears
-increased overlap of thick and thin filaments

What happens to the A band during muscle contraction?

do not change in length

What happens to the I bands during muscle contraction?

shorten

What happens to the H zone during muscle contraction?

shortens or disappears

Sliding is produced by several cross bridges that form between myosin and actin. How does the action of sliding work?

-the myosin head serves as a myosin ATPase enzyme, splitting ATP into ADP + Pi
-this allows the head to bind to actin when the muscle is stimulated
-release of Pi upon binding cocks the myosin head producing a power stroke that pulls the thin filament toward the center

What happens after the power stroke occurs?

-after the power stroke, ADP is released and a new ATP binds
-this makes myosin release actin and ATP is split
-the myosin head straightens out and rebinds to actin farther back
-this continues until the sarcomere has shortened

What is F-actin made of?

300-400 G-actin subunits arranged in a double row and twisted to form a helix

What blocks crossbridges?

tropomyosin

What is the troponin complex?

-troponin I inhibits binding of myosin
-troponin T binds to tropomyosin
-troponin C binds to calcium

What is the role of calcium in muscle contraction?

-when muscle cells are stimulated, Ca2+ is released inside the muscle fiber
-Ca2+ binds to troponin C, causing a conformational change in troponin and tropomyosin
-myosin binds to actin and forms cross bridges

What is the sarcoplasmic reticulum?

modified endoplasmic reticulum that stores Ca2+ when muscle is at rest

Where is more Ca2+ stored?

terminal cisternae

What are the calcium release channels in which Ca2+ is diffused out of when a muscle fiber is stimulated?

ryanodine receptors

At the end of a muscle contraction, where is Ca2+ pumped into?

Ca2+ is pumped back into the sarcoplasmic reticulum

What neurotransmitter is released from the motor neuron?

acetylcholine

Explain the steps of excitation- contraction coupling in skeletal muscle cells.

1. acetylcholine is released from the motor neuron
2. end plate potentials are produced
3. action potentials are generated (all-or-nothing event)
4. voltage-gated calcium channels in transverse tubules change shape and cause calcium channels in the sarcoplasmic reticulum to open
5. calcium is released and can bind to troponin C

What happens for excitation-contraction coupling when muscles relax?

1. action potentials cease
2. calcium release channels close
3. Ca2+-ATPase pumps move Ca2+ back into the sarcoplasmic reticulum through active transport
4. no more Ca2+ is available to bind to troponin C
5. tropomyosin moves to block the myosin heads from binds to actin