128 Neuroanatomy: Draw It to Know It
Know-It Points
The Spinal Canal
■ Th e vertebral column contains 33 vertebrae and 31
spinal nerves.
■ Th e spinal cord ends at the L1–L2 vertebral column
level.
■ Motor and sensory roots combine within an
intervertebral neural foramen at each level to form a
mixed spinal nerve.
■ Only the C1 through C7 nerve roots exit above their
related vertebral bodies, the rest exit beneath them.
■ Th e collection of nerve fi bers in the caudal spinal
canal is the cauda equina.
■ Th e distal, bulbous region of the spinal cord is the
conus medullaris.
■ Th e site of fl uid collection during a lumbar puncture
is the lumbar cistern, which is the subarachnoid space
below the level of the spinal cord.The Muscle Stretch Refl ex
■ Th e muscle spindle sends an excitatory volley along
the type Ia sensory aff erent, which directly innervates
the motor neuron.
■ Th rough the inhibitory neurotransmitter glycine,
Renshaw cells inhibit the antagonist refl ex from
fi ring.
■ Muscle spindle fi bers have a lower threshold
to fi re than Golgi tendon organs, which is the
basis for the termination of the muscle stretch
refl ex.
■ Th e biceps refl ex involves C5, C6.
■ Th e triceps refl ex involves C7, C8.
■ Th e patellar refl ex involves L2–L4.
■ Th e Achilles refl ex involves S1, S2.Muscle–Nerve Physiology ( Advanced )
■ A γ motor neurons project to the muscle spindles to
stimulate muscle tone.
■ Initially, when supraspinal input to the A γ motor
neurons is disrupted, muscle tone becomes fl accid;
days or weeks later, spasticity develops.
■ Skeletal muscle fi bers outside of the capsule are
extrafusal fi bers and are innervated by the A α motor
nerves.
■ I n t r a f u s a l fi bers lie within the muscle spindle and are
innervated by the A γ motor nerves.
■ Th e largest peripheral nerve fi bers are 12 to 22
micrometers in diameter and conduct at 70 to 120
meters per second.
■ Th e smallest peripheral nerve fi bers are less than 1
micrometer in diameter and conduct at less than 2
meters per second.
■ Saltatory conduction is the fi ring of impulses in the
interspaces between segments of myelin: the nodes of
R a n v i e r.