12-1: How Muscles Work

Question 1

functions of muscular system

Answer 1

• muscles move the skeleton
• reduce the size of tubes in body
• take part in reflex reactions
• cardiac muscles helps heart pump and facilitates blood circulation

Question 2

types of muscles

Answer 2

• skeletal
• smooth
• cardiac

Question 3

structure of skeletal muscle

Answer 3

• each muscle fibre is a single cell w multi nuclei
• each fiber is made of smaller myofibrils which are made of myofilaments
• thick and think myofilaments
• thin: made of proteins: actin, troponin, tropomyosin
• thick: ordered arrangement of protein myosin
• functional unit of myofibril = sarcomere, the reason for the striated appearance

Question 4

Muscle fiber contraction and

the myoneural synapse

Answer 4

-each sarcomere is made of thick filaments of myosin and thin filaments of actin
-sarcomere contracts when actin filaments slide past myosin filaments
-• Contraction shortens the sarcomere, but does not change the length
of the actin or myosin filaments. It only pulls the actin filaments
toward the center of the sarcomere
-myosin molecules have ATP attached just before the contraction, which makes myosin non activated (low energy configuration)

Question 5

muscle contraction

Answer 5

-for myosin head to get attached to actin filaments myosin-binding sites on the actin must become exposed to the myosin head. They were previously covered with tropomyosin.
Arriving action potential causes the release of calcium ions. When calcium ions become available, they bind to the troponin complex. This causes the troponin complex to change shape, dislodging tropomyosin from the myosin-binding sites. Binding can then occur.
• At the same time, the myosin head hydrolyzes ATP to ADP and inorganic phosphate, and is now in its high-energy configuration (activated).
-The power stroke is when the myosin head bends, pulling the actin

Question 6

power stroke

Answer 6

The myosin head binds to actin, forming a cross-bridge.
• It also causes a conformational change in the myosin head, releasing
the ADP, Pi and energy from the myosin head, and the myosin turns
into its low-energy configuration again.
• This allows thin and thick filaments to slide with reference to each
other, and the same energy is then used to create the working stroke
of the cross-bridge cycle.
• When ADP and phosphate are released, the myosin head bends,
pulling the actin. This is called the power stroke.
-ATP binds to the myosin head again, causing it to detach from actin.
• When the ATP is broken down to ADP and phosphate, the myosin
head extends again. If calcium is still present, the whole series of
contractions is repeated.

Question 7

fiber contraction summary

Answer 7

• Myosin-actin interaction underlies muscle fiber contraction.
• Hydrolysis of ATP converts myosin to a high-energy form, which
allows myosin to bind to actin.
• Troponin complex, and tropomyosin are regulatory proteins, which
along with Ca+ ions, aid in muscle fiber contraction.
• Muscle contraction requires repeated cycles of binding and release.

Question 8

myoneural synapse

Answer 8

1. When action potential
arrives, it triggers the release
of the neurotransmitter
(acetylcholine) into the synaptic cleft.
2. This neurotransmitter binds to its receptors on the muscle cell,
triggering depolarization that leads to action potential in the muscle fiber itself (follow red arrows).
3a. The action potential
propagates along the
length of the muscle
fiber and spreads into
the interior of the fiber
via intracellular structures called T tubules (the T stands for
transverse = extending
across).
3b. Proteins in T tubules
intersect with the
sarcoplasmic reticulum
(which is endoplasmic
reticulum in a muscle cell),
which stores calcium ions.
When an action potential
passes down a T tubule and
reaches one of these
intersections, a protein in
the T tubule membrane
changes confirmation and
opens a Ca ion channel in
the sarcoplasmic reticulum.

Question 9

myoneural synapse 2

Answer 9

4. Ca ions bind to troponin in thin filament. This causes
the myosin-binding sites on actin to get exposed.
5. Sliding of myosin head against the actin happens as already explained previously
6. After the action
potential ends, Ca ions
are removed by active
transport back into
sarcoplasimc reticulum.
7. Tropomyosin blockage
of myosin
-binding sites is
restored, contraction ends,
and muscle fiber is
relaxed.

Question 10

types of locomotion

Answer 10

• swim, crawl, walk, run, hop, fly
• active travel
• requires energy to overcome two forces that tend to keep an animal stationary: friction and gravity.