The Peripheral Nervous System and Reﬂex Activity
III Oculomotor Nerves
Origin and course:
Fibers extend from ventral midbrain (near its
junction with pons) and pass through bony orbit, via superior orbital
ﬁssure, to eye.
Chieﬂy motor nerves (
motor to the eye);
contain a few proprioceptive afferents. Each nerve includes the
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.
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.
In oculomotor nerve
paralysis, eye cannot be moved up, down, or inward. At rest, eye
rotates laterally [
mus)] because the
actions of the two extrinsic eye muscles not served by cranial nerves
III are unopposed. Upper eyelid droops (
), and the person has
double vision and trouble focusing on close objects.
IV Trochlear Nerves
Origin and course:
Fibers emerge from dorsal midbrain and course
ventrally around midbrain to enter orbit through superior orbital
ﬁssure along with oculomotor nerves.
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
Test with cranial nerve III (oculomotor).
Damage to a trochlear
nerve results in double vision and impairs ability to rotate eye