Chapter 5 The Muscular System 163
Generating Action Potentials
How does the motor neuron communicate
with the muscle cells in the motor unit to
stimulate them? As Figure 5.5 shows, a long, thin
fi ber called an axon connects the motor neuron
cell body with the muscle fi bers included in the
motor unit. Close to the fi bers, the axon branches
into axon terminals which in turn branch out
to individual muscle fi bers. The link between
each axon terminal and muscle fi ber is called
the neuromuscular junction. The axon terminal
and fi ber are separated by a tiny gap known as
the synaptic cleft, which is fi lled with interstitial
fl uid (Figure 5.6 on the next page).
When a nerve impulse reaches the end
of an axon terminal, a chemical called a
neurotransmitter discharges and diffuses across
the synaptic cleft to attach to receptors on the
muscle fi ber sarcolemma. The neurotransmitter
that stimulates muscle is called acetylcholine
(a-SEE-til-KOH-leen).
The effect of acetylcholine is to make the
sarcolemma temporarily permeable. Channels
open that allow positive sodium ions (Na+) to
rapidly invade the fi ber at the same time that
positive potassium ions (K+) rush out of the
fi ber. Because more Na+ enters than K+ exits,
the net effect is the creation of a positive charge
inside the muscle fi ber.
This reversal of electrical charge is known
as depolarization. Depolarization triggers the
opening of additional channels in the fi ber
membrane that allow only entry of additional
Na+. This fl ood of positive ions into the fi ber
generates an electrical charge called an action
potential.
Contractions of the Sarcomeres
Glucose stored in the form of glycogen
within the muscle cell provides the energy for
creating the action potential. Phosphocreatine
within the cell enables the transfer of energy to
the protein fi laments actin and myosin. Actin
and myosin are contractile proteins that reside
in functional units called sarcomeres (SAR-
koh-mairz) inside the muscle fi ber. The release
of calcium ions (Ca++) triggers the sliding of
Spinal cord
Motor neuron
cell bodies
Motor unit A
Motor unit B
Nerve
Muscle
Axons
Muscle fibers
Axon terminals at
neuromuscular junctions
Figure 5.5 Each motor unit includes a motor neuron and all the muscle fibers it activates. How does a motor
neuron activate muscle fibers?