Human Physiology, 14th edition (2016)

Muscle 363

terminal by an action potential causes the exocytosis of acetyl-
choline (ACh) from about 100 synaptic vesicles, in a process dis-
cussed in chapter 7 (see fig. 7.23). These ACh molecules rapidly
diffuse across the narrow 100 nm synaptic cleft that separates the
axon terminal from the motor end plate, where they bind to sev-
eral thousand nicotinic ACh receptors. This binding causes the
ACh receptor ion channels to open, producing a depolarization
known as an end plate potential that stimulates action potentials.
Action potentials lead to muscle contraction, as will be described
later in this section.
The cell body of a somatic motor neuron is located in the ven-
tral horn of the gray matter of the spinal cord and gives rise to
a single axon that emerges in the ventral root of a spinal nerve
(chapter 8, section 8.6). Each axon, however, can produce a num-
ber of collateral branches to innervate an equal number of muscle
fibers. Each somatic motor neuron, together with all of the mus-
cle fibers that it innervates, is known as a motor unit ( fig. 12.4 ).
Whenever a somatic motor neuron is activated, all of the
muscle fibers that it innervates are stimulated to contract. In
vivo, graded contractions (where contraction strength is var-
ied) of whole muscles are produced by variations in the num-
ber of motor units that are activated. In order for these graded
contractions to be smooth and sustained, different motor units
must be activated by rapid, asynchronous stimulation.

CLINICAL APPLICATION

Botulinum toxin is a potential deadly poison produced by

the bacteria Clostridium botulinum. It is selectively taken into

excitatory cholinergic nerve endings and cleaves a SNARE

protein (SNAP-25) needed for the exocytosis of the synaptic

vesicles (chapter 7; see fig. 7.23). This blocks nerve stimula-

tion of the muscles, producing a flaccid paralysis. Botulinum

toxin is now used medically in certain cases to relieve mus-

cle spasms due to excessive nerve stimulation. For example,

it is injected into an affected extraocular muscle in order to

help correct strabismus (deviation of the eye). Intramuscular

injections of Botox (a brand name for botulinum toxin), caus-

ing local muscle paralysis, are also given for the temporary

cosmetic treatment of skin wrinkles.

Clinical Investigation CLUES

Mia had Botox injections for cosmetic purposes.

• What is the action of Botox, and what cosmetic
  benefits does it provide?

Figure 12.4 Motor units. A motor unit consists of a

somatic motor neuron and the muscle fibers it innervates.

( a ) Illustration of a muscle containing two motor units. In reality,

a muscle would contain many hundreds of motor units, and

each motor unit would contain many more muscle fibers than

are shown here. ( b ) A single motor unit consisting of a branched

motor axon and the three muscle fibers it innervates (the fibers

that are highlighted) is depicted. The other muscle fibers would

be part of different motor units and would be innervated by

different neurons (not shown).

Spinal cord

Somatic motor neuron

Somatic motor neuron

Motor unit

Motor unit

Somatic

Skeletal motor axon

muscle fibers

Neuromuscular

junctions

Motor unit

(b)

(a)