Chapter 13
The Peripheral Nervous System and Reflex Activity
495
13
Medial rectus
muscle
Superior
orbital fissure
Superior
rectus
muscle
Levator
palpebrae
muscle
Inferior
oblique
muscle
Ciliary
ganglion
Parasympathetic
motor fibers
Inferior
rectus muscle
Oculomotor
nerve (III)
Pons
Superior
orbital fissure
Pons
Trochlea
Superior
oblique
muscle
Trochlear
nerve (IV)
III Oculomotor Nerves
(ok
0
u-lo-mo
9
tor)
Origin and course:
Fibers extend from ventral midbrain (near its
junction with pons) and pass through bony orbit, via superior orbital
fissure, to eye.
Function:
Chiefly motor nerves (
oculomotor
5
motor to the eye);
contain a few proprioceptive afferents. Each nerve includes the
following:
Somatic motor fibers to four of the six extrinsic eye muscles
(inferior oblique and superior, inferior, and medial rectus
muscles) that help direct eyeball, and to levator palpebrae
superioris muscle, which raises upper eyelid.
Parasympathetic (autonomic) motor fibers to sphincter pupillae
(circular muscles of iris), which cause pupil to constrict, and to
ciliary muscle, controlling lens shape for visual focusing. Some
parasympathetic cell bodies are in the ciliary ganglia.
Sensory (proprioceptor) afferents, which run from same four
extrinsic eye muscles to midbrain.
Clinical testing:
Examine pupils for size, shape, and equality.
Test pupillary reflex with penlight (pupils should constrict when
illuminated). Test convergence for near vision and subject's ability to
follow objects with the eyes.
Homeostatic Imbalance
In oculomotor nerve
paralysis, eye cannot be moved up, down, or inward. At rest, eye
rotates laterally [
external strabismus
(strah-biz
9
mus)] because the
actions of the two extrinsic eye muscles not served by cranial nerves
III are unopposed. Upper eyelid droops (
ptosis
), and the person has
double vision and trouble focusing on close objects.
IV Trochlear Nerves
(trok
9
le-ar)
Origin and course:
Fibers emerge from dorsal midbrain and course
ventrally around midbrain to enter orbit through superior orbital
fissure along with oculomotor nerves.
Function:
Primarily motor nerves; supply somatic motor fibers to (and
carry proprioceptor fibers from) one of the extrinsic eye muscles, the
superior oblique muscle, which passes through the pulley-shaped
trochlea.
Clinical testing:
Test with cranial nerve III (oculomotor).
Homeostatic Imbalance
Damage to a trochlear
nerve results in double vision and impairs ability to rotate eye
inferolaterally.
Table 13.2
(continued)
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