Anatomy

Circle of Willis

The circle of Willis is an anastomotic network of arteries connecting the anterior and posterior pathways of cerebral blood.

The circle of Willis consists of:

• Anterior cerebral artery

• Anterior communicating artery

• Internal carotid artery

• Posterior communicating artery

• Posterior cerebral artery

These are all paired arteries, with the exception of the anterior communicating artery.

The anterior cerebral arteries are branches of the internal carotid artery that supply the medial and superior surfaces of the frontal lobe along with superior surfaces of the parietal lobe. They are connected by the anterior communicating artery.

The middle cerebral arteries are the terminal branches of the internal carotid artery that supply the lateral surfaces of the brain and temporal lobes. They are directly connected to the anterior cerebral artery and to the posterior cerebral artery by the posterior communicating artery.

The posterior cerebral arteries arise at the bifurcation of the basilar artery. They supply blood to the inferior surface of the brain and occipital lobe, and they communicate with the circle of willis via the posterior communicating arteries.

The basilar artery is an arterial branch thatconnects the vertebral arteries and the circle of willis. It supplies blood to the posterior aspect of the circle of willis and to the pontine arteries.

The anterior inferior cerebellar arteries branch from the basilar artery between the level of the pons and medulla and supply blood to the pons and parts of the cerebellum.

The posterior inferior cerebellar arteries are the largest branches of the vertebral artery and supply the medulla and parts of the cerebellum.

Cranial Nerves

CN I is the olfactory nerve, which is a sensory nerve that mediates the sense of smell. The nerve travels to the olfactory bulb, which contains the smell receptors.

CN II is the optic nerve, which is a sensory nerve that mediates sight.

CN III is the oculomotor nerve, which is a motor nerve.

The muscles innervated by the oculomotor nerve include:

• Medial, inferior, and superior rectus

• Inferior oblique

• Ciliary muscle

• Sphincter muscle of the eye

• Levator palpebrae superioris

CN IV is the trochlear nerve, which is a motor nerve that supplies the superior oblique muscle.

CN V is the trigeminal nerve, which is both motor and sensory. It is responsible for sensation of the face and motor innervation to the muscles of mastication, along with somatosensation from the anterior 2/3 of the tongue.

CN VI is the abducens nerve, which is a motor nerve supplying the lateral rectus muscle of the eye.

CN VII is the facial nerve, which is bothmotor and sensory.

Functions that are mediated by the facial nerve include:

• Taste for the anterior ⅔ of the tongue

• Muscles of facial expression

• Stapedius muscle

• Stylohyoid muscle and the posterior belly of the digastric muscle

• Lacrimal,sublingual, and submandibular gland innervation

CN VIII is the vestibulocochlear nerve, which is a sensory nerve that mediates hearing and balance.

CN IX is the glossopharyngeal nerve, which is both motor and sensory.

Functions that are mediated by the glossopharyngeal nerve include:

• Taste for the posterior ⅓ of the tongue

• Sensation of the pharynx

• Stylopharyngeus muscle

• Parotid gland innervation

• Carotid baroreceptors

CN X is the vagus nerve, which is both motor and sensory.

Functions that are mediated by the vagus nerve include:

• Sensation of the trachea, esophagus and viscera

• Laryngeal and pharyngeal muscles

• Parasympathetic visceral control down to the proximal ⅔ of the colon.

CN XI is the spinal accessory nerve, which is a motornerve supplying the sternocleidomastoid and trapezius muscles.

CN XII is the hypoglossal nerve, which is a motor nerve supplying the intrinsic muscles of the tongue.

Normal Eye Function

Neurovascular integrity is important for maintaining normal eye function. The cranial nerves involved with vision are II, III, IV, and VI. A common way of remembering the nerve-function association is LR6SO4AO3, which is short for lateral rectus VI, superior oblique IV, and all others III.

Occlusion or any other vascular pathology affecting the retinal artery or vein will disrupt blood flow to the retina, and thus eye function.

The optic nerve (CN II) is responsible for vision.

The trochlear nerve (CN IV), which controls the superior oblique muscle, is responsible for depression, abduction and intorsion of the eyeball.

The abducens nerve (CN VI), which controls the lateral rectus muscle, is responsible for eye abduction.

The oculomotor nerve (CN III) is responsible for all other eye function not controlled by CN II, IV, and VI.

The medial longitudinal fasciculus maintains conjugate gaze, which is when one eye abducts and the contralateral eye adducts simultaneously (i.e. CN VI and CN III work together). Lesions affecting the MLF cause MLF syndrome.

Because the temporal visual fields cross at the optic chiasm and the nasal visual fields do not, several visual field defects are possible depending on the specific site of neuronal injury.

A lesion of the optic nerve produces anopia, which is complete vision loss in the affected eye.

A lesion of the inner, crossing fibers at the optic chiasm produces bitemporal hemianopsia, which is loss of the temporal visual fields.

A lesion of the optic tract produces homonymous hemianopsia, which is a loss of ipsilateral nasal and contralateral temporal fields.

A lesion in the temporal lobe to Meyer's loop produces upper quandrantopia, which is a loss of the upper ipsilateral nasal field and upper contralateral temporal field.

A lesion in the parietal lobe produces lower quandrantopia, which is a loss of the lower ipsilateral nasal field and lower contralateral temporal field.

A lesion in the occipital lobe produces homonymous hemianopsia with macular sparing, which is loss of the ipsilateral nasal and contralateral temporal fields with the exception of the direct center field of vision.

Spinal Cord

The dorsal columns are a spinal cord tract that contain the neurons responsible for:

• Proprioception

• Two point discrimination

• Vibratory sense

The dorsal columns are located medially in the posterior aspect of the spinal cord.

The first-order neurons of the dorsal column pathway enter at the ipsilateral dorsal hornandascend in the cuneate and gracile fasciculi to the cuneate and gracile nuclei where they synapse.

The second-order neurons of the dorsal column pathway originate in the cuneate/gracile nuclei and decussate at the level of the medulla. They then continue ascending as the medial lemniscus and synapse in the thalamus.

The spinothalamic tract is a spinal cord tract that contains the neurons responsible for pain and temperature sensation.

The spinothalamic tracts are located in the anterior and lateral aspectof the spinal cord.

The first-order neurons of the spinothalamic tract have cell bodies in the dorsal root ganglion. Free nerve endings in the periphery enter the dorsolateral tract of lissauer and ascend or descend ipsilaterally one or two spinal levels before synapsing.

The second-order neurons of the spinothalamic tract decussate in the ventral white commissureand ascend in the now contralateral lateral spinothalamic tract. These second-order neurons will then synapse in the VPL of the thalamus.

The corticospinal tract is a spinal tract that contains the neurons responsible for voluntary movement of striated muscle.

There are two corticospinal tracts. The main corticospinal tract is located in thelateral aspect of the spinal cord while a moreminor corticospinal tract is located in the anterior aspect of the spinal cord.

The first-order neurons of the lateral corticospinal tract descend from the internal capsule and midbrain, continue through the pyramidal decussation, descend in the contralateral side, and synapse in the ventral horn.

The second-order neurons of the lateral corticospinal tract exit through the ventral horn where they synapse on the intended muscle.