HUMAN BIOLOGY

the MUSCULar SYSteM 111

hoW does the nervous system control the

contraction of muscle cells?

• Nerve impulses spark the release of calcium ions from a
  membrane system around a muscle cell’s myofibrils.


• Nerve impulses pass from a neuron to a muscle cell across
  neuromuscular junctions.

takE-homE mEssagE

motor neuron Type of

neuron that carries nervous

system signals to skeletal

muscles.

neuromuscular junction

Sites where endings of

neuron axons come close

to the cell membranes of

muscle fibers.

neurotransmitter A chemi-

cal messenger that carries

signals from a neuron to

a receiving cell across a

synapse.

Botox is potentially quite dangerous, since it blocks

the release of ACh. Yet each year millions of people

around the world receive Botox injections for cosmetic

and medical purposes. Research this topic on the

website of the National Institutes of Health (www

.nih.gov) to learn more about both the potential

health dangers of Botox and how it is used in medical

treatments.

ThiNk OuTsiDE ThE bOOk

Axon ending of

motor neuron

Muscle fiber

Muscle fiber receptor for ACh

Synapse

Vesicles containing ACh molecules

Each year in the United States

about 2 million people have injec-

tions of Botox to smooth out facial

wrinkles. Made by the bacterium

Clostridium botulinum, Botox blocks

the release of ACh, so the muscle

contractions that produce wrinkles

stop for a while. The muscle-relaxing

effect lasts four to six months and

can have side effects, such as droopy

eyelids. Botox also is used to treat

disorders. For example, it may relieve

abnormal muscle contractions that

trouble stroke patients. Only a physi-

cian can legally prescribe Botox.

neurons signal muscle cells
at neuromuscular junctions

A motor neuron has long extensions called axons that carry
nerve impulses. The nerve impulses that stimulate a skele-
tal muscle fiber arrive at neuromuscular junctions. These
are places where the branched endings of axons come close
to muscle fiber membranes, as you can see in Figure 6.9B
and 6.9D, and in Figure 6.11. Between the neuron endings
and each muscle cell is a gap called a synapse. A type of
chemical messenger, a neurotransmitter called ACh (for
acetylcholine), carries the signals from a motor neuron
across the gap.
The signaling between a neuron and a muscle cell takes
place in steps. As these steps take place, calcium ions from
the extracellular fluid flow inside the axon endings, and
vesicles in each ending release ACh. If enough ACh binds
to receptors on the muscle cell membrane, the events that
cause the muscle cell to contract may get under way. ACh
can excite or inhibit muscle and gland cells, as well as some
cells in the brain and spinal cord.

Figure 6.10 Animated! Proteins uncover and then cover
sites on actin where myosin can attach. Calcium ions must
bind to the protein troponin and then release it in order for the
sliding filament mechanism of muscle contraction to operate.
(© Cengage Learning)

Figure 6.11 Ach carries signals across a neuromuscular junction. It is

a type of chemical messenger called a neurotransmitter.

(© Cengage Learning)

Nerve impulse Calcium released

Myosin binds

to actin

Actin Tropomyosin Troponin

Myosin

head

Troponin in actin
binding site

1

Troponin moves,
twisting tropomyosin
and uncovering the
site on actin. Now
myosin can attach
at the site.

2

Troponin covers
the actin binding
site again

3

Nerve impulse stops No calcium

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