HUMAN BIOLOGY

110 Chapter 6

A Signals from the nervous

system travel along spinal

cord, down motor neuron.

B Endings of

motor neuron

terminate next to

muscle cells.

C Signals travel along muscle cell’s plasma membrane

to sarcoplasmic reticulum around myofibrils.

D Signals trigger the release of calcium ions from sarcoplasmic reticulum

threading among the myofibrils. The calcium allows actin and myosin

filaments in the myofibrils to interact and bring about contraction.

part of one muscle cell

section from

spinal cord

motor

neuron

section from a skeletal muscle

Axon of neuron

Neuromuscular

junction

Plasma

membrane

of muscle fiber

T tubule

Myofibrils

Z line Z line

muscle’s outer sheath

(connective tissue)

how the nervous system Controls muscle Contraction

n skeletal muscles move the body and its parts in response to

signals from the nervous system.

n Link to the Endomembrane system 3.7

Calcium ions are the key to contraction

The nervous system controls the contraction of skeletal

muscles, sending commands to muscle fibers by way of

motor (“movement”) neurons. A motor neuron travels to a

muscle and issues signals that trigger or halt contraction of

the sarcomeres in the muscle’s fibers (Figure 6.9A and 6.9B).

Figure 6.9 Animated! Signals from the nervous system stimulate contraction of skeletal muscle. (© Cengage Learning)

When nerve impulses arrive at a muscle fiber, they

quickly spread. Eventually they reach small extensions of

the cell’s plasma membrane. These “T tubules” connect with

a membrane system that laces around the fiber’s myofibrils

(Figure 6.9C and 6.9D). The system, called the sarcoplasmic

reticulum (SR), is a version of the endoplasmic reticulum

described in Chapter 3. SR takes up and releases calcium

ions (Ca^11 ). An incoming nerve impulse triggers the release

of calcium ions from the SR. The ions diffuse into myofibrils,

and when they reach actin filaments the stage is set for

contraction.

Two proteins on the surface of actin filaments have

important roles in muscle contraction (Figure 6.10). One of

them, called troponin (tropo- means “turn or change”), has a

rounded shape. It attaches to the actin filament and also to

the second protein, called tropomyosin (“myosin changer”),

which winds along the actin filament. Importantly, in a rest-

ing muscle fiber troponin covers up the sites where myosin

can link up with actin. This changes when incoming calcium

binds to troponin. Then the troponin moves, twisting tropo-

myosin away from the actin binding sites. Myosin now can

attach to the sites, and muscle contraction can occur.

When nerve impulses stop, calcium is actively trans-

ported back into the SR. Tropomyosin covers the binding

sites on actin again, myosin can’t bind to actin, and the

muscle fiber relaxes. Notice the importance of calcium in

these events. Its central role in muscle contraction is one

reason why mechanisms of homeostasis that maintain

proper blood levels of calcium are so important.

6.4

A Signals from the nervous

system travel along spinal

cord, down motor neuron.

B Endings of

motor neuron

terminate next to

muscle cells.

C Signals travel along muscle cell’s plasma membrane
to sarcoplasmic reticulum around myofibrils.

D Signals trigger the release of calcium ions from sarcoplasmic reticulum

threading among the myofibrils. The calcium allows actin and myosin

filaments in the myofibrils to interact and bring about contraction.

part of one muscle cell

section from

spinal cord

motor

neuron

section from a skeletal muscle

Axon of neuron

Neuromuscular

junction

Plasma

membrane

of muscle fiber

T tubule

Myofibrils

Z line Z line

muscle’s outer sheath
(connective tissue)

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).