132 Neuroanatomy: Draw It to Know It
The Muscle Stretch Refl ex
Here, we will draw a muscle stretch refl ex (aka myotatic
or deep tendon refl ex). First, draw an axial cross-section
through the spinal cord. Th en, draw a lower extremity
that is fl exed at the knee and show a knee extensor muscle
fi ber of the quadriceps muscle group at the top of the
thigh and a knee fl exor fi ber of the hamstrings muscle
group at the bottom of the thigh: we use the knee exten-
sor refl ex as our example, here. Next, denote the location
of the related motor neurons within the anterior horn of
the spinal cord. Now, draw a muscle spindle and show a
type Ia sensory fi ber project from it to the extensor motor
neuron. Th en, draw an A α motor fi ber projection from
the extensor motor neuron to the representative extra-
fusal extensor fi ber. In large muscle groups, such as the
quadriceps, a single motor neuron commands as many as
1,000 extrafusal muscle fi bers, whereas in small muscle
groups, such as the extraocular muscles, a motor neuron
commands as few as 10 extrafusal muscle fi bers. Show
that when the patellar tendon is stretched with the tap of
a refl ex hammer, the muscle spindle sends an excitatory
volley along the type Ia sensory aff erent, which excites
the extensor motor neuron and nerve, and the muscle
extensor contracts so the knee extends.^5 , 6
If the hamstrings were also activated (meaning, if the
extensor and fl exors shortened simultaneously), the
thigh would only stiff en and not move. So, now, draw a
Renshaw cell (an interneuron) in the anterior horn of
the gray matter of the spinal cord, and show that through
the inhibitory neurotransmitter glycine, the Renshaw
cell inhibits the fl exor motor neuron from fi ring.
Interneurons are the lynchpins to more complicated
spinal refl exes, as well, such as the triple fl exor refl ex,
which has the following sequential mechanics: a painful
stimulus to the bottom of the foot causes refl ex upward
fl exion of the aff ected foot (aka foot dorsifl exion) with
simultaneous fl exion of the knee and the hip. In order
for us to remain upright when we undergo the triple
fl exor refl ex, our opposite leg must bear our weight. Th us,
through a simultaneous but opposing refl ex in our oppo-
site leg , the crossed extension refl ex, our non-aff ected
foot fl exes downward and our non-aff ected hip and knee
extend.
Now, let’s show how to terminate the muscle stretch
refl ex; bear in mind that there are a number of neurobio-
logical infl uences on the relaxation of muscle contrac-
tion, which include myosin ATPase and calcium
re-accumulation into the endoplasmic reticulum, which
we will not draw, here. Delay in the relaxation phase of
the muscle stretch refl ex (aka Woltman’s sign) is observed
in symptomatic hypothyroidism. Re-draw our muscle
stretch refl ex arrangement but exclude the fl exor neuron
and its motor fi ber. Th en, label a Golgi tendon organ
where the quadriceps tendon inserts into the patella.
Next, show that a type Ib fi ber projects from the Golgi
tendon organ to the Renshaw interneuron. Now, show
an inhibitory fi ber project from the interneuron to the
quadriceps (extensor) motor neuron. Th e type Ia and Ib
fi bers fi re at the same rate, but muscle spindle fi bers have
a much lower threshold to fi re than Golgi tendon organs.
Th us, the muscle spindle fi res fi rst, and then later the
Golgi tendon organ fi res, which terminates the muscle
stretch refl ex. Th is completes our diagram.
For reference, the commonly tested muscle stretch
refl exes are the biceps, which involves the C5, C6 nerve
roots; the triceps, which involves the C7, C8 nerve roots;
the patella (drawn here), which involves the L2 to L4
nerve roots; and the Achilles, which involves the S1, S2
nerve roots.^2 , 4 , 7 – 10