Chapter 13
The Peripheral Nervous System and Reflex Activity
511
13
is filled with a glycoprotein-rich basal lamina, a structure not
seen at other synapses. Te basal lamina contains
acetylcho-
linesterase
, the enzyme that breaks down ACh.
Innervation of Visceral Muscle and Glands
Te junctions between autonomic motor endings and their
effectors (smooth and cardiac muscle and glands) are much
simpler than the junctions formed between somatic fibers and
skeletal muscle cells. Te autonomic motor axons branch re-
peatedly, each branch forming
synapses en passant
(“synapses
in passing”) with its effector cells. Instead of a cluster of bulblike
terminals, an axon ending serving smooth muscle or a gland
(but not cardiac muscle) has a series of
varicosities
, knoblike
swellings containing mitochondria and synaptic vesicles, that
make it look like a string of beads (see Figure 9.26, p. 306).
Te autonomic synaptic vesicles typically contain either ace-
tylcholine or norepinephrine, both of which act indirectly on
their targets via second messengers. Consequently, visceral mo-
tor responses tend to be slower than those induced by somatic
motor fibers, which directly open ion channels.
Motor Integration:
From Intention to Effect
Outline the three levels of the motor hierarchy.
Compare the roles of the cerebellum and basal nuclei in
controlling motor activity.
How does integration in the motor system compare with in-
tegration in sensory systems? In the motor system, we have
motor endings serving effectors (muscle fibers) instead of sen-
sory receptors, descending efferent circuits instead of ascend-
ing afferent circuits, and motor behavior instead of perception.
However, as in sensory systems, the basic mechanisms of motor
systems operate at three levels.
Levels of Motor Control
Te cerebral cortex is at the highest level of our conscious motor
pathways, but it is
not
the ultimate planner and coordinator of
complex motor activities. Te cerebellum and basal nuclei (gan-
glia) play this role and are therefore at the top of the motor con-
trol hierarchy. Motor control exerted by lower levels is mediated
by
reflex arcs
in some cases, but complex motor behavior, such
as walking and swimming, depends on more complex patterns.
Currently, we define three levels of motor control: the
segmental
level
,
projection level
, and
precommand level
(Figure 13.14)
.
The Segmental Level
Te lowest level of the motor hierarchy, the
segmental level
,
consists of reflexes and spinal cord circuits that control auto-
matic movements. A segmental circuit activates a network of
ventral horn neurons in a group of cord segments, causing them
to stimulate specific groups of muscles.
Circuits that control locomotion and other specific and o±-
repeated motor activities are called
central pattern generators
the joint.
Hence, once you learn which nerves serve the various
major muscles and muscle groups, no new learning is necessary.
For example, the quadriceps, gracilis, and hamstring muscles
all cross the knee. Te nerves to these muscles are the femo-
ral nerve anteriorly and branches of the sciatic and obturator
nerves posteriorly. Consequently, these nerves innervate the
knee joint as well.
Check Your Understanding
10.
Spinal nerves have both
dorsal roots
and
dorsal rami
. How are
these different from each other in location and composition?
11.
After his horse-riding accident, the actor Christopher Reeve
was unable to breathe on his own. Which spinal nerve roots,
spinal nerve, and spinal nerve plexus were involved?
For answers, see Appendix H.
PART 3
Motor Endings
and Motor Activity
Peripheral Motor Endings
Compare and contrast the motor endings of somatic and
autonomic nerve fibers.
So far we have covered the sensory receptors that detect stimuli
and the nerves containing the afferent and efferent fibers that
deliver impulses to and from the CNS. We now turn to
motor
endings
, the PNS elements that activate effectors by releasing neu-
rotransmitters. Because we discussed that topic in Chapter 9 with
the innervation of body muscles, all we need to do here is recap.
We will follow the recap with a brief overview of motor integration.
Innervation of Skeletal Muscle
Recall that the terminals of somatic motor fibers that innervate
voluntary muscles form elaborate
neuromuscular junctions
with their effector cells (Figure 9.8, p. 287). As each axon branch
reaches its target, a single muscle fiber, the ending splits into a
cluster of
axon terminals
that branch treelike over the junctional
folds of the sarcolemma of the muscle fiber. Te axon terminals
contain mitochondria and synaptic vesicles filled with the neu-
rotransmitter acetylcholine (ACh).
When a nerve impulse reaches an axon terminal, ACh is
released by exocytosis, diffuses across the fluid-filled synaptic
cle± (about 50 nm wide), and attaches to ACh receptors on the
sarcolemma at the junction. ACh binding opens ligand-gated
channels that allow both Na
1
and K
1
to pass. Because more
Na
1
enters the cell than K
1
leaves, the muscle cell interior at
that point depolarizes. Te resulting graded potential is called
an
end plate potential
.
Te end plate potential spreads to adjacent areas of the
membrane where it triggers the opening of voltage-gated so-
dium channels. Tis event causes an action potential to propa-
gate along the sarcolemma, which stimulates the muscle fiber to
contract. Te synaptic cle± at somatic neuromuscular junctions
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