The oculomotor nerve arises from nuclei in the midbrain and controls most eye movements and the constriction of pupils. It has superior and inferior branches that innervate specific extraocular muscles. Damage to the oculomotor nerve can be detected by observing eye movements and pupils in response to light during a clinical examination.

1. TheThe oculomotor nerveoculomotor nerve is the third of twelveis the third of twelve
pairedpaired cranial nervescranial nerves. It controls most of the. It controls most of the
eye movementseye movements (cranial nerves(cranial nerves IVIV andand VIVI
also do some), constriction of the pupil, andalso do some), constriction of the pupil, and
holding the eyelid openholding the eyelid open

2. PathPath
NucleiNuclei
The fibers of theThe fibers of the oculomotoroculomotor nervenerve arise from a nucleus in thearise from a nucleus in the midbrainmidbrain, which lies in the, which lies in the
gray substance of the floor of thegray substance of the floor of the cerebral aqueductcerebral aqueduct and extends in front of theand extends in front of the
aqueduct for a short distance into the floor of theaqueduct for a short distance into the floor of the third ventriclethird ventricle. From this nucleus the. From this nucleus the
fibers pass forward through thefibers pass forward through the tegmentumtegmentum, the, the red nucleusred nucleus, and the medial part of, and the medial part of
thethe substantia nigrasubstantia nigra, forming a series of curves with a lateral convexity, and emerge, forming a series of curves with a lateral convexity, and emerge
from the oculomotor sulcus on the medial side of thefrom the oculomotor sulcus on the medial side of the cerebral pedunclecerebral peduncle..
The nucleus of the oculomotor nerve does not consist of a continuous column of cells,The nucleus of the oculomotor nerve does not consist of a continuous column of cells,
but is broken up into a number of smaller nuclei, which are arranged in two groups,but is broken up into a number of smaller nuclei, which are arranged in two groups,
anterior and posterior. Those of the posterior group are six in number, five of whichanterior and posterior. Those of the posterior group are six in number, five of which
are symmetrical on the two sides of the middle line, while the sixth is centrally placedare symmetrical on the two sides of the middle line, while the sixth is centrally placed
and is common to the nerves of both sides. The anterior group consists of two nuclei,and is common to the nerves of both sides. The anterior group consists of two nuclei,
an antero-medial and an antero-lateral .an antero-medial and an antero-lateral .
The nucleus of the oculomotor nerve, considered from a physiological standpoint, can beThe nucleus of the oculomotor nerve, considered from a physiological standpoint, can be
subdivided into several smaller groups of cells, each group controlling a particularsubdivided into several smaller groups of cells, each group controlling a particular
muscle.muscle.
A nearby nucleus, theA nearby nucleus, the EdingerEdinger--WestphalWestphal nucleusnucleus, is responsible for the autonomic functions of the, is responsible for the autonomic functions of the
oculomotor nerve, includingoculomotor nerve, including pupillarypupillary constriction and lens accommodationconstriction and lens accommodation

3. The oculomotor nerve arises from the anterior aspect ofThe oculomotor nerve arises from the anterior aspect of
mesencephalonmesencephalon (midbrain). There are two nuclei for the(midbrain). There are two nuclei for the
oculomotor nerve:oculomotor nerve:
 TheThe oculomotor nucleusoculomotor nucleus originates at the level of theoriginates at the level of the
superior colliculussuperior colliculus. The muscles it controls are the. The muscles it controls are the
ciliary muscleciliary muscle (affecting(affecting accommodationaccommodation), and all), and all
extraocular musclesextraocular muscles except for theexcept for the
superior oblique musclesuperior oblique muscle and theand the lateral rectus musclelateral rectus muscle..
 TheThe Edinger-Edinger-Westphal nucleusWestphal nucleus suppliessupplies
parasympatheticparasympathetic fibres to the eye via thefibres to the eye via the
ciliary ganglionciliary ganglion, and thus controls, and thus controls pupil constrictionpupil constriction..

4. Superior and inferior ramiSuperior and inferior rami
thethe superior branch of the oculomotor nervesuperior branch of the oculomotor nerve or theor the superiorsuperior
divisiondivision , the smaller, passes medialward over the, the smaller, passes medialward over the optic nerveoptic nerve..
It supplies theIt supplies the SuperiorSuperior rectusrectus andand Levator palpebrae superiorisLevator palpebrae superioris
TheThe inferior branch of the oculomotor nerveinferior branch of the oculomotor nerve or theor the inferior divisioninferior division ,,
the larger, divides into three branches.the larger, divides into three branches.
 One passes beneath the optic nerve to theOne passes beneath the optic nerve to the medial rectusmedial rectus..
 Another, to theAnother, to the inferior rectusinferior rectus..
 The third and longest runs forward between the inferior recti and lateralis to theThe third and longest runs forward between the inferior recti and lateralis to the
inferior obliqueinferior oblique. From the last a short thick branch is given off to the lower part of. From the last a short thick branch is given off to the lower part of
thethe ciliary ganglionciliary ganglion, and forms its short root., and forms its short root.

5. Testing the oculomotor nerveTesting the oculomotor nerve
 Eye musclesEye muscles
 Cranial nerves III, IV and VI are usually tested together. The examinerCranial nerves III, IV and VI are usually tested together. The examiner
typically instructs the patient to hold his head still and follow only with thetypically instructs the patient to hold his head still and follow only with the
eyes a finger or penlight that circumscribes a large "H" in front of the patient.eyes a finger or penlight that circumscribes a large "H" in front of the patient.
By observing theBy observing the eye movementseye movements andand eyelidseyelids, the examiner is able to obtain, the examiner is able to obtain
more information about themore information about the extraocularextraocular musclesmuscles, the levator palpebrae, the levator palpebrae
superioris muscle, and cranial nerves III, IV, and VI.superioris muscle, and cranial nerves III, IV, and VI.
 Since the oculomotor nerve controls most of the eye muscles, it may beSince the oculomotor nerve controls most of the eye muscles, it may be
easier to detect damage to it. Damage to this nerve, termedeasier to detect damage to it. Damage to this nerve, termed oculomotoroculomotor
nerve palsynerve palsy is also known by theis also known by the down n' outdown n' out symptoms, because of thesymptoms, because of the
position of the affected eye.position of the affected eye.
 Pupillary reflexPupillary reflex
 The oculomotor nerve also controls the constriction of the pupils. This canThe oculomotor nerve also controls the constriction of the pupils. This can
be tested in two main ways. By moving a finger towards a person's face tobe tested in two main ways. By moving a finger towards a person's face to
induce accommodation, as well as them going cross-eyed, their pupilsinduce accommodation, as well as them going cross-eyed, their pupils
should constrict.should constrict.
 Shining a light into their eyes should also make their pupils constrict. BothShining a light into their eyes should also make their pupils constrict. Both
pupils should constrict at the same time, independent of what eye the light ispupils should constrict at the same time, independent of what eye the light is
actually shone on.actually shone on.


6. Testing the oculomotor nerveTesting the oculomotor nerve
 Eye musclesEye muscles
 Cranial nerves III, IV and VI are usually tested together. The examinerCranial nerves III, IV and VI are usually tested together. The examiner
typically instructs the patient to hold his head still and follow only with thetypically instructs the patient to hold his head still and follow only with the
eyes a finger or penlight that circumscribes a large "H" in front of the patient.eyes a finger or penlight that circumscribes a large "H" in front of the patient.
By observing theBy observing the eye movementseye movements andand eyelidseyelids, the examiner is able to obtain, the examiner is able to obtain
more information about themore information about the extraocularextraocular musclesmuscles, the levator palpebrae, the levator palpebrae
superioris muscle, and cranial nerves III, IV, and VI.superioris muscle, and cranial nerves III, IV, and VI.
 Since the oculomotor nerve controls most of the eye muscles, it may beSince the oculomotor nerve controls most of the eye muscles, it may be
easier to detect damage to it. Damage to this nerve, termedeasier to detect damage to it. Damage to this nerve, termed oculomotoroculomotor
nerve palsynerve palsy is also known by theis also known by the down n' outdown n' out symptoms, because of thesymptoms, because of the
position of the affected eye.position of the affected eye.
 Pupillary reflexPupillary reflex
 The oculomotor nerve also controls the constriction of the pupils. This canThe oculomotor nerve also controls the constriction of the pupils. This can
be tested in two main ways. By moving a finger towards a person's face tobe tested in two main ways. By moving a finger towards a person's face to
induce accommodation, as well as them going cross-eyed, their pupilsinduce accommodation, as well as them going cross-eyed, their pupils
should constrict.should constrict.
 Shining a light into their eyes should also make their pupils constrict. BothShining a light into their eyes should also make their pupils constrict. Both
pupils should constrict at the same time, independent of what eye the light ispupils should constrict at the same time, independent of what eye the light is
actually shone on.actually shone on.
