HUMAN BIOLOGY

THE MUSCULAR SYSTEM 119

Review Questions

1. In a general sense, how do skeletal muscles produce
   movement?


2. In the diagram below, label the fine structure of a muscle,
   down to one of its myofibrils. Identify the basic unit of
   contraction in a myofibril.

contraction. Each sarcomere consists of an array of filaments of
the proteins actin (thin) and myosin (thick):

To shorten a sarcomere, the myosin attaches to a neighbor-
ing actin, and the actin slides over the myosin. ATP powers
this interaction, which is called the sliding filament mechanism
of muscle contraction.

section 6.4 Nerve impulses make
skeletal muscle fibers contract. They do this
by triggering the release of calcium ions
from sarcoplasmic reticulum, a membrane
system that wraps around myo fibrils in the
muscle fiber. The calcium alters proteins on
actin filaments so that the heads of myosin
molecules can bind to actin.
A neuromuscular junction is a synapse between a motor
neuron and a muscle fiber. A nerve impulse triggers the release
of a neurotransmitter called ACh into the synapse. This starts
the events that cause the fiber to contract.

section 6.5 The ATP required for
muscle contraction can come from cellular
respiration, from glycolysis alone, or from the
generation of ATP from creatine phosphate.
When muscles use more ATP than aerobic respiration can
provide, an oxygen debt may develop in muscle tissue.

section 6.6 A motor neuron and the
muscle fibers it controls form a motor unit.
When a stimulus activates enough motor
units, it produces a muscle twitch. If a series
of twitches occur close together, a sustained
contraction called tetanus develops. Skeletal
muscles normally operate near or at tetanus. Important
functional properties of whole muscles include the force they
exert (tension), muscle tone, and fatigue.

actin myosin actin

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Function of Skeletal Muscle:

Contraction (shortening) that moves the body and its parts.

Major Components of Skeletal Muscle Cells:

Myofibrils: Strands containing filaments of the contractile proteins

actin and myosin.

Sarcomeres: The basic units of muscle contraction.

Other:

Motor unit: A motor neuron and the muscle fibers it controls.

Neuromuscular junction: Synapse between a motor neuron and

muscle fibers.

Table 6.1 Review of Skeletal Muscle

ATP

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3. How do actin and myosin interact in a sarcomere to
   bring about muscle contraction? What roles do ATP and
   calcium play?


4. How does a muscle fiber incur an oxygen debt?


5. What is the function of the sarcoplasmic reticulum in
   muscle cell contraction?


6. Explain why (a) calcium ions and (b) ACh are vital for
   muscle contraction.


7. What is a motor unit? Why does a rapid series of muscle
   twitches yield a stronger overall contraction than a
   single twitch?


8. What are the structural and functional differences between
   “slow” and “fast” muscle?

self-Quiz Answers in Appendix VI

1. The and systems work together to move the
   body and specific body parts.


2. The three types of muscle tissue are , ,
   and.


3. Muscle fibers shorten when slides over.
   a. myosin, actin c. myoglobin, actin
   b. actin, myosin d. myosin, sarcomeres


4. The is the basic unit of muscle contraction.
   a. myofibril c. muscle fiber
   b. sarcomere d. myosin filament


5. Skeletal muscle contraction requires.
   a. calcium ions c. arrival of a nerve impulse
   b. ATP d. all of the above

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