Muscles

Question 1

What are the two types of threadlike protein myofilaments within each sarcomere?

Answer 1

Myosin (thick) filaments

Actin (thin) filaments

Question 2

Describe the Thick filament

Answer 2

Aka myosin filament
Made of myosin and contains ATP enzymes which split to create power for muscle contraction.

Extend entire length of A band.

Mid parts are smooth but ends are studded with small projections called myosin heads

Myosin heads aka cross bridges link thick and thin filaments together during contraction.

Question 3

Describe thin filaments

Answer 3

Aka Actin filament
Made of actin and some regulatory proteins which allow/prevent myosin head from binding to actin

Anchored to Z line (disc like membrane)

Light I band includes 2 adjacent sarcomere parts and only the thin filaments

Thin filaments overlap the ends of thick filaments but do NOT extend into the middle of relaxed sarcomere which makes the H zone look lighter.

During contraction, the light zones disappear because actin and myosin filaments are overlapped

Question 4

Describe Skeletal muscle appearance

Answer 4

• Multinucleated
• Oval nuclei
• Plasma membrane is called sarcolemma
• banding appearance caused by alternating light (l) and dark (A)
• light (l) band has midline dark area called Z line
• dark (A) band has center area called H zone.
• muscle composed of many myofibrils.
• myofibrils are chains of tiny contractile units called sarcomeres.
• sarcomeres are aligned like box cars in a train.
• there are 2 types of threadlike protein myofilaments within each sarcomere (myosin and actin)

Question 5

Sarcoplasmic Reticulum

Answer 5

Specialized smooth endoplasmic reticulum

Interconnecting tubules and sacs of the SR surround every myofibril

Major role is to store calcium and release it on demand when muscle fiber is stimulated to contract.

Calcium provides the final go signal for contraction

Question 6

What are the 2 special functional properties muscles have that enable them to perform their duties?

Answer 6

Irritability - ability to receive and respond to stimulus

Contractility - ability to shorten when adequate stimulus occurs

Question 7

Skeletal muscles must be stimulated by ___________ to contract.

Answer 7

Nerve impulses

Question 8

A motor neuron is also called a ___________.

Answer 8

Nerve cell

Question 9

What is a motor unit?

Answer 9

One neuron and all the skeletal muscle cells it stimulates

Question 10

What is a neuromuscular junction?

Answer 10

When a long threadlike extension of the neuron (axon) reaches the muscle, it branches into a bunch of axonal terminals, each of which forms junctions with the sarcolemma of a different muscle cell.
These junctions are called Neuromuscular Junctions.

Question 11

What is the gap called where nerve endings of muscle cell membranes never touch

Answer 11

Synaptic cleft and is filled with interstitial fluid

Question 12

What is the name of the neurotransmitter released when the nerve impulse reaches the axon terminals?

Answer 12

Acetylcholine or Ach

Question 13

What causes filaments to slide?

Answer 13

• muscle fibers are activated by nervous system
• cross bridges attach to myosin binding sites on the thin filaments
• ATP energizes it and each cross bridge attaches and detaches multiple times during contraction to create tension and pull thin filaments to center of sarcomere.
• calcium ions are required for attachment of myosin cross bridges to actin
• action potential leads to contraction and makes Sarcoplasmic reticulum release calcium ions into Sarcoplasm which reabsorbs after contraction occurs.
• during action potential acetylcholine is broken down by enzymes and creates a single nerve impulse. This prevents continuous contraction.