Essentials of Anatomy and Physiology

sium pumpreturns them inside. Both of these pumps
are active transport mechanisms, which, you may
recall, require ATP. Muscle fibers use ATP to maintain
a high concentration of Naions outside the cell and
a high concentration of Kinside. The pumps, there-
fore, maintain polarization and relaxation until a nerve
impulse stimulates a change.

SARCOLEMMA—DEPOLARIZATION

When a nerve impulse arrives at the axon terminal, it
causes the release of acetylcholine, which diffuses
across the synapse and bonds to ACh receptorson
the sarcolemma. By doing so, acetylcholine makes the
sarcolemma very permeable to Naions, which rush
into the cell. This makes the inside of the sarcolemma

positive relative to the outside, which is now consid-

ered negative. This reversal of charges is called depo-

larization. The electrical impulse thus generated

(called an action potential) then spreads along the

entire sarcolemma of a muscle fiber. The sarcolemma

has inward folds called T tubules(transverse tubules,

shown in Fig. 7–2), which carry the action potential to

the interior of the muscle cell. Depolarization initiates

changes within the cell that bring about contraction.

The electrical changes that take place at the sar-

colemma are summarized in Table 7–1 and shown in

Fig. 7–4.

The Muscular System 145

Na+ Na+ Na+ Na+ Na+ Na+

Na+ Na+

Na+ Na+ Na+ Na+

Na+ Na+ Na+ Na+ Na+ Na+

K+ K+

K+ K+ K+ K+

K+ K+ K+ K+

ACh

A

B

C

Polarization

Depolarization

Repolarization

K+ K+ K+ K+

Figure 7–4. Electrical charges and ion concentrations
at the sarcolemma. (A) Polarization, when the muscle
fiber is relaxed. (B) Wave of depolarization in response to
stimulus of acetylcholine. (C) Wave of repolarization.
QUESTION:Which ion enters the cell during depolariza-
tion? Which ion leaves during repolarization?

Table 7–1 SARCOLEMMA—

ELECTRICAL CHANGES

State or Event Description

Resting Potential

Polarization

Action Potential

Depolarization

Repolarization

• Sarcolemma has a () charge
  outside and a () charge inside.
  •Naions are more abundant
  outside the cell; as they diffuse
  inward, the sodium pump
  returns them outside.
  •Kions are more abundant
  Inside the cell; as they diffuse
  out, the potassium pump
  returns them inside.


• ACh makes the sarcolemma very
  permeable to Naions, which
  rush into the cell.


• Reversal of charges on the sar-
  colemma: now () outside and
  () inside.


• The reversal of charges spreads
  along the entire sarcolemma


• Cholinesterase at the sar-
  colemma inactivates ACh.


• Sarcolemma becomes very per-
  meable to Kions, which rush
  out of the cell.


• Restoration of charges on the
  sarcolemma: () outside and
  () inside.


• The sodium and potassium
  pumps return Naions outside
  and Kions inside.


• The muscle fiber is now able to
  respond to ACh released by
  another nerve impulse arriving
  at the axon terminal.