390
UNIT THREE
then triggered along the axon of the motor neuron and trav-
els back to the quadriceps femoris. The muscles respond by
contracting, and the refl ex is completed as the leg extends
(f
g. 11.8)
.
The patellar refl ex helps maintain an upright posture.
For example, if a person is standing still and the knee begins
to bend in response to gravity, the quadriceps femoris is
stretched, the reflex is triggered, and the leg straightens
again. Adjustments in the stretch receptors keep the refl
ex
responsive at different muscle lengths.
Another type of refl
ex, called a
withdrawal refl
ex
(f
g.
11.9)
, happens when a person touches something painful, as
in stepping on a tack. Activated skin receptors send sensory
impulses to the spinal cord. There the impulses pass on to
interneurons of a refl
ex center and are directed to motor neu-
rons. The motor neurons transmit signals to the fl
exor mus-
cles of the leg and thigh, which contract in response, pulling
the foot away from the painful stimulus. At the same time,
some of the incoming impulses stimulate interneurons that
Refl
ex Behavior
Refl exes are automatic responses to changes (stimuli) inside
or outside the body. They help maintain homeostasis by
controlling many involuntary processes such as heart rate,
breathing rate, blood pressure, and digestion. Refl exes also
carry out the automatic actions of swallowing, sneezing,
coughing, and vomiting.
The
patellar refl
ex
(knee-jerk refl ex) is an example of a
simple monosynaptic refl ex, so-called because it uses only
two neurons—a sensory neuron communicating directly to
a motor neuron. Striking the patellar ligament just below the
patella initiates this refl ex. The quadriceps femoris muscle
group, attached to the patella by a tendon, is pulled slightly,
stimulating stretch receptors in the muscle group. These
receptors, in turn, trigger impulses that pass along the periph-
eral process (see ± g. 10.7) of the axon of a unipolar sensory
neuron, continuing along the central process of the axon into
the lumbar region of the spinal cord. In the spinal cord, the
sensory axon synapses with a motor neuron. An impulse is
White matter
Gray matter
Lateral funiculus
Dorsal root
of spinal nerve
Dorsal root
ganglion
Ventral root
of spinal nerve
Anterior
median
fissure
Posterior funiculus
Posterior median
sulcus
Gray commissure
Central canal
Anterior
funiculus
Portion of
spinal nerve
(a)
(b)
Posterior horn
Anterior
horn
FIGURE 11.6
Spinal cord. (
a
) A cross section of the spinal cord. (
b
) Identify the parts of the spinal cord in this micrograph (7.5×).
previous page 420 David Shier Hole's Human Anatomy and Physiology 2010 read online next page 422 David Shier Hole's Human Anatomy and Physiology 2010 read online Home Toggle text on/off