Essentials of Anatomy and Physiology

SARCOLEMMA—POLARIZATION

When a muscle fiber is relaxed, the sarcolemma is
polarized (has a resting potential), which refers to a
difference in electrical charges between the outside
and the inside. During polarization, the outside of
the sarcolemma has a positive charge relative to the

inside, which is said to have a negative charge. Sodium

ions (Na) are more abundant outside the cell, and

potassium ions (K) and negative ions are more abun-

dant inside (Fig. 7–4).

The Naions outside tend to diffuse into the cell,

and the sodium pumptransfers them back out. The

Kions inside tend to diffuse outside, and the potas-

The Muscular System 143

ACh

Muscle

fiber

Synaptic

cleft

Vesicles of

acetylcholine

Axon terminal

Mitochondria

Motor neuron

Sarcolemma

1

2

3

ACh receptor

Cholinesterase

Na+

Na+

Na+

Sarcomere

T tubule

Figure 7–2. Structure of the neuromuscular junction, showing an axon terminal adja-

cent to the sarcolemma of a muscle fiber. Schematic of events: ( 1 ) Acetylcholine is about

to bond to the ACh receptor in the sarcolemma. ( 2 ) Channel opens to allow Naions into

the muscle cell. ( 3 ) Cholinesterase inactivates acetylcholine.

QUESTION:What event opens a sodium channel in the sarcolemma?