134 Neuroanatomy: Draw It to Know It
Muscle–Nerve Physiology ( Advanced )
Here, let’s address muscle tone, the muscle spindle, and
peripheral nerve classifi cation. We begin with muscle
tone. Re-draw our A α motor refl ex loop: draw the spinal
cord and lower extremity and then show the muscle
spindle project via a type Ia sensory aff erent to excite the
motoneuron, which projects via an A α motor nerve to
excite the extrafusal muscle fi ber. Next, label an A γ motor
neuron in the anterior gray matter and show that it proj-
ects to the muscle spindle. Th e A γ motor neurons stimu-
late muscle tone. Finally, draw a supraspinal projection
to the A γ motor neuron to show that supraspinal centers
send descending excitatory inputs to the A γ motor neu-
rons. Initially, when supraspinal input to the A γ motor
neurons is disrupted, muscle tone becomes fl accid; it is
not until days or weeks later that spasticity develops.
Now, let’s draw the muscle spindle anatomy. Begin
with the connective tissue-enclosed muscle spindle cap-
sule. Next, label the skeletal muscle fi bers outside of the
capsule as extrafusal fi bers; these fi bers produce limb
movement and they are innervated by the A α motor
nerves. Now, draw several intrafusal fi bers within the
muscle spindle; they are innervated by the A γ motor
nerves. Th en, within the central, non-contractile portion
of the muscle spindle, draw nuclei clustered together like
marbles in a bag and label this as the nuclear bag fi ber;
next, draw a row of nuclei like pearls on a chain and label
this as the nuclear chain fi ber. Both fi ber types exist
within the muscle spindle and they each are attuned to
diff erent aspects of muscle tone.
Next, let’s address the type Ia and type II sensory
aff erents of the muscle spindle. Show an annulospiral
sensory nerve ending around the non-contractile, cen-
tral portion of both the nuclear bag and nuclear chain
fi bers; the annulospiral nerve ending connects to the
type Ia sensory aff erent fi ber. Th en, show that fl ower
spray sensory nerve endings connect the type II fi bers to
the nuclear chain fi bers. However, to complicate mat-
ters, two forms of nuclear bag fi ber actually exist, bag 1
and bag 2 — the latter shares many similarities with nuclear
chain fi bers. Indicate, now, that in addition to attaching
to chain fi bers, the type II sensory aff erents also attach to
bag 2 fi bers.
Now, let’s address the eff erent innervation of the
muscle spindle. Th e A γ motor nerves terminate in either
plate or trail endings in the polar region of the muscle
spindle along the intrafusal muscle fi bers. Indicate that,
generally, plate endings lie along nuclear bag fi bers
and trail endings lie along nuclear chain fi bers and bag 2
fi bers. Th e bag 1 fi bers act when there is a change in muscle
fi ber length, during the dynamic phase, whereas the
chain fi bers and bag 2 fi bers act when muscle length is
unchanged, during the static phase.
Lastly, let’s address the classifi cation of peripheral
nerves. Two classifi cation schemes are commonly used:
the Gasser scheme, which applies to all nerve types —
motor, sensory, and autonomic, and the Lloyd scheme,
which applies to sensory nerves, only. Th e two schemes
are fairly redundant, however, and can be learned
together. We will only list the largest and smallest fi bers,
here, but the table in Drawing 8-3 is complete for refer-
ence. Label the top row of our table as the Gasser nerve
class, Lloyd nerve class, diameter, and speed (ie, conduc-
tion velocity). Make a notation that the diameter units
are micrometers and the speed units are meters per
second. List the largest fi ber type as Gasser class A α and
Lloyd class Ia and Ib; indicate that they are 12 to 22
micrometers ( μ m) in diameter and conduct at 70 to 120
meters per second (m/s). Th en, label the smallest fi bers
as Gasser class C and Lloyd class IV, and show that they
are less than 1 μ m in diameter and conduct at less than
2 m/s. Impulses slowly ascend the small unmyelinated
nerve axons, whereas they quickly ascend the large, heav-
ily myelinated nerve fi bers because impulses of myeli-
nated fi bers fi re only in the interspaces between segments
of myelin — the nodes of Ranvier; this pattern of fi ring is
called saltatory conduction.^1 , 2 , 4 , 5 , 9 – 16