Anatomy: Cranial Nerves

Cranial Nerves

• Cranial Nerve I (Olfactory) and Cranial Nerve II (Optic) are not true cranial nerves
• True cranial nerves originate from the brainstem
• CN I and CN II are outgrowths/extensions of the brain/CNS

Names of Cranial Nerves

• CN I: Olfactory
• CN II: Optic
• CN III: Oculomotor
• CN IV: Trochlear
• CN V: Trigeminal
• CN VI: Abducens
• CN VII: Facial
• CN VIII: Vestibulocochlear
• CN IX: Glossopharyngeal
• CN X: Vagus
• CN XI: Accessory
• CN XII: Hypoglossal

Composition of Cranial Nerves

• Sensory: CN I, CN II, CN VIII
• Motor: CN III, CN IV, CN VI, CN XI, CN XII
• Mixed (sensory and motor): CN V, CN VII, CN IX, CN X

General Functions of Cranial Nerves

• CN I (Olfactory): Smell
• CN II (Optic): Vision
• CN III (Oculomotor): Eye movement, pupil constriction, and eyelid elevation
• CN IV (Trochlear): Eye movement, specifically of the superior oblique muscle
• CN V (Trigeminal): Facial sensation and mastication
• CN VI (Abducens): Eye movement, specifically of the lateral rectus muscle
• CN VII (Facial): Facial expression, taste (anterior 2/3 of tongue), salivation, and lacrimation
• CN VIII (Vestibulocochlear): Hearing and balance
• CN IX (Glossopharyngeal): Taste (posterior 1/3 of tongue), swallowing, and salivation
• CN X (Vagus): Parasympathetic control of the heart, lungs, and digestive tract, as well as swallowing and speech
• CN XI (Accessory): Shoulder and neck movement via the sternocleidomastoid and trapezius muscles
• CN XII (Hypoglossal): Tongue movement

Lesions of Cranial Nerves

• Lesion of CN I: Loss of smell (anosmia)
• CN I fibers enter the cranial vault via the cribriform plate of the ethmoid bone to synapse at the olfactory bulb

Optic Nerve vs. Optic Tract

• Optic Nerve (CN II): Carries visual information from the retina to the optic chiasm
• Optic Tract: Carries post-chiasmal fibers to the lateral geniculate nucleus (LGN) and other visual processing areas
• Fibers of the optic nerve decussate at the optic chiasm; nasal retina fibers cross, but temporal fibers do not
• Optic tract fibers primarily terminate in the lateral geniculate nucleus (LGN) of the thalamus, with some projecting to the superior colliculus and pretectal area

Lesions Affecting Extraocular Muscles

• CN III Lesion: Ptosis, "down and out" eye position, pupillary dilation, and loss of accommodation
• CN IV Lesion: Vertical diplopia and trouble looking down
• CN VI Lesion: Horizontal diplopia and inability to abduct the eye

Cranial Nerve IV

• CN IV is the only cranial nerve that exits dorsally from the brainstem
• CN IV has the longest intracranial course

Trigeminal Nerve Divisions

• Mandibular division (V3): Carries motor fibers for mastication
• V1 (Ophthalmic) goes to the forehead, scalp, cornea, and upper eyelid
• V2 (Maxillary) goes to the cheeks, upper lip, nasal mucosa, and palate
• V3 (Mandibular) goes to the jaw, lower lip, chin, and muscles of mastication

Sensory Information from the Face

• The trigeminal lemniscus and trigeminal thalamic tract carry sensory information from the face up toward the sensory cortex

Trigeminal Neuralgia

• Tic douloureux (trigeminal neuralgia) is a severe, paroxysmal facial pain disorder affecting CN V
• Usually triggered by mild stimuli, such as wind, chewing, or touching the face

Cranial Nerves and Salivation

• CN VII (Facial nerve) stimulates the submandibular and sublingual glands via the chorda tympani
• CN IX (Glossopharyngeal nerve) stimulates the parotid gland via the otic ganglion

Oculomotor Nerve (CN III)

• Oculomotor nucleus is located in the midbrain at the level of the superior colliculus
• Controls most of the extraocular muscles (superior, inferior, and medial rectus, and inferior oblique) and the levator palpebrae superioris for eyelid elevation
• Edinger-Westphal nucleus is the parasympathetic component
• Responsible for pupillary constriction (via the sphincter pupillae) and accommodation of the lens (via the ciliary muscle)

Medial Longitudinal Fasciculus (MLF)

• Nuclei Contributing: Oculomotor (CN III), Trochlear (CN IV), Abducens (CN VI), and Vestibular nuclei
• Function: Coordinates eye movements by linking nuclei, ensuring conjugate gaze

Red Nucleus Targets

• Fibers from the red nucleus primarily project to the spinal cord (via the rubrospinal tract) and the inferior olivary nucleus
• Involved in motor learning and facilitates flexor muscle movements

Substantia Nigra

• Purpose: Plays a crucial role in motor control via dopamine release, affecting the basal ganglia
• Location: In the midbrain tegmentum within the ventral midbrain, specifically in the basis pedunculi (crus cerebri), between the cerebral peduncles and the tegmentum

Corticospinal Fibers

• Midbrain: Fibers descend through the crus cerebri (basis pedunculi)
• Pons: Fibers break into fascicles interspersed with pontine nuclei
• Medulla: Fibers form the medullary pyramids and decussate (pyramidal decussation) before continuing as the lateral corticospinal tract

Reticular Formation

• General function: Regulates autonomic functions, consciousness, motor reflexes, and pain modulation
• Rostral reticular formation: In the upper pons and midbrain, includes the ascending reticular activating system (ARAS), regulating alertness and wakefulness
• Caudal reticular formation: In the lower pons and medulla, controls autonomic functions and motor reflexes

Midbrain Decussation

• Superior cerebellar peduncle fibers (dentatorubrothalamic tract) decussate
• Transmit information from the cerebellum to the red nucleus and thalamus

Brainstem Tracts

• The medial lemniscus carries proprioception, vibration, and discriminatory touch
• Originates from the dorsal column nuclei in the medulla, carrying somatosensory information

Basis Pontis

• Includes the corticospinal, corticobulbar, and corticopontine tracts
• Includes pontine nuclei and their transverse pontocerebellar fibers

Fourth Ventricle

• The roof of the fourth ventricle is made up of the cerebellum, superior and inferior medullary vela, and choroid plexus

Facial Colliculus

• Bulge in the floor of the fourth ventricle formed by the abducens nucleus (CN VI) and the fibers of the facial nerve (CN VII) looping around it

Inferior Olivary Nuclei

• Located in the medulla oblongata; involved in motor learning and coordination
• Receive input from the red nucleus and spinal cord, sending climbing fibers to the cerebellum

Motor Fibers in Medulla Oblongata

• Located in the pyramids
• Form part of the corticospinal tract

Dorsal Column Second-Order Neurons

• Decussation occurs in the caudal medulla
• This forms the medial lemniscus

Cranial Nerve Nuclei in Medulla Oblongata

• Vagus (CN X): Nucleus ambiguus (motor), dorsal motor nucleus (autonomic), and solitary nucleus (sensory)
• Hypoglossal (CN XII): Hypoglossal nucleus is in the medullary tegmentum near the midline

Cerebellar Peduncles

• Inputs:Inferior cerebellar peduncle and middle cerebellar peduncle
• Outputs: Superior cerebellar peduncle

Cerebellum

• Posterior fissure separates the flocculonodular lobe from the posterior lobe
• Primary fissure separates the anterior lobe from the posterior lobe
• Nodulus is the inferior part of the vermis (part of the flocculonodular lobe)
• Flocculus and nodulus are within the flocculonodular lobe

Cerebellar Divisions

• Vestibulocerebellum (flocculonodular lobe): Balance and eye movements
• Spinocerebellum (vermis and intermediate zone): Posture, gait, and limb coordination
• Cerebrocerebellum (lateral hemispheres): Planning and coordination of voluntary movements, motor learning

Fastigial Nucleus

• Projects to the vestibular nuclei and reticular formation to influence balance and posture

Cerebellar Hemispheres

• Outputs from the lateral cerebellar hemispheres synapse in the dentate nucleus before leaving the cerebellum

Interposed Nuclei

• The emboliform and globose nuclei coordinate limb movements

Cerebellar Cortex Layers

• Molecular layer: Stellate and basket cells, Purkinje dendrites and parallel fibers
• Purkinje cell layer: Single layer of Purkinje cell bodies
• Granule cell layer: Granule cells and Golgi cells

Folia of Cerebellum

• Granule cells send their axons to the molecular layer, forming parallel fibers

Cerebellar Lesions

• Cerebellar outputs decussate twice resulting in ipsilateral motor control
• Lesions cause ipsilateral deficits

Deficits Associated with Cerebellar Lesions

• Cerebellar Vermis: Truncal ataxia, dysarthria, and impaired balance
• Intermediate Part of Cerebellum: Limb ataxia and dysarthria
• Lateral Part of the Cerebellum: Impaired motor planning and coordination, delays in movement initiation, dysmetria, dysdiadochokinesia, and cognitive dysfunction in severe cases

Cerebellar Nuclei Projections

• Dentate Nucleus: Projects to the contralateral ventrolateral nucleus of the thalamus (VL) and red nucleus
• Interposed Nuclei: Project to the contralateral red nucleus and VL thalamus

Brain Arterial Supply

• Anterior supply: Internal carotid arteries
• Posterior supply: Vertebral arteries

Internal Carotid Artery Segments

• Cervical segment: Vertical portion in the neck
• Petrous segment: Bends horizontally upon entering the carotid canal in the temporal bone
• Cavernous segment: Forms an S-shaped curve as it passes through the cavernous sinus
• Intracranial segment: Pierces the dura mater and enters the subarachnoid space

Vertebral Arteries

• Course through the transverse foramina of the cervical vertebrae
• Enter the skull via the foramen magnum
• Fuse to form the basilar artery running along the basilar pons

Cerebral Arteries

• Basilar artery bifurcates to form the posterior cerebral arteries

Branches of the Internal Carotid Artery

• Ophthalmic artery: Supplies retina and orbit structures
• Posterior communicating artery
• Anterior choroidal artery: Supplies globus pallidus, putamen, thalamus, posterior limb of internal capsule
• Anterior cerebral artery
• Middle cerebral artery

Anterior Cerebral Artery Terminal Branches

• Pericallosal and Callosomarginal arteries
• Supply to the anterior medial surface of the brain, including the frontal and anterior parietal lobes, covering the medial sensorimotor cortex

Posterior Cerebral Arteries

• Supply the inferior and medial temporal lobes and medial occipital cortex

Arteries Supplying the Internal Capsule

• Anterior choroidal artery
• Lenticulostriate arteries (from middle cerebral artery)
• Recurrent artery of Heubner (from anterior cerebral artery)

Arteries and Hypertension

• Lenticulostriate arteries are susceptible to dissection due to hypertension and prone to narrowing
• Leads to lacunar infarcts or intracerebral hemorrhage

Arteries Contributing to the Insula

• Branches of the middle cerebral artery

Temporal Lobe Arterial Supply

• Inferior division of the middle cerebral artery supplies the lateral temporal lobe
• Posterior cerebral artery supplies the medial and inferior temporal lobes

Basal Nuclei Arterial Supply

• Lenticulostriate arteries supply large portions of the basal ganglia and internal capsule

Anterior Pituitary Gland

• Develops from ectoderm originating near the developing pharynx
• Forms a pouch (Rathke's pouch) that migrates toward the developing brain

Posterior Pituitary Gland

• Evaginates from the periventricular nervous tissue, making it nervous tissue (neurohypophysis)

Pituitary Gland

• Sits within the pituitary fossa in the sella turcica of the sphenoid bone
• Lateral walls are the cavernous sinuses and it is held in place by the diaphragma sellae,
• The diaphragma sellae has a small opening for the pituitary stalk

Hypothalamus Location

• Part of the diencephalon, located inferior to the thalamus
• Forms the walls and floor of the inferior portion of the third ventricle
• Hypothalamic sulcus separates the hypothalamus from the thalamus
• Located just posterior to the optic chiasm and includes the tuber cinereum and mammillary bodies

Hypothalamic Divisions

• Preoptic area
• Anterior (supraoptic) area
• Middle (tuberal) area
• Posterior (mammillary) area

Medial Forebrain Bundle

• Runs through the lateral hypothalamic region
• Carries axons transporting information to and from the hypothalamus

Nuclei for Oxytocin and Vasopressin

• Supraoptic nucleus releases oxytocin and vasopressin
• Paraventricular nucleus releases oxytocin and small amounts of vasopressin

Suprachiasmatic Nucleus

• Receives input from specialized retinal ganglion cells containing melanopsin
• Neurons convey day-night cycle information to the suprachiasmatic nucleus via the retinohypothalamic tract from the optic chiasm

Median Eminence

• The median eminence is the site where hormones are released into the hypophyseal portal system
• Arcuate nucleus projects to the median eminence to help regulate pituitary function

Mammillary Bodies

• Important for memory formation and spatial navigation
• Connected to the fornix and project to the anterior thalamic nucleus via the mammillothalamic tract

Hypothalamic Nuclei

• Anterior hypothalamic nucleus: Heat dissipation; lesion leads to hyperthermia
• Posterior hypothalamic nucleus: Heat conservation; lesion leads to hypothermia

Autonomic Nervous System Regulation

• Paraventricular, dorsomedial, lateral, and posterior hypothalamus
• These project axons inferiorly to synapse with preganglionic sympathetic (thoracolumbar) and parasympathetic (cervical/sacral) neurons

Appetite and Satiety Nuclei

• Lateral hypothalamus: Stimulates appetite; lesion leads to weight loss
• Ventromedial nucleus: Suppresses appetite; lesion leads to obesity

Water Regulation

• Osmoreceptors in the anterior hypothalamus regulate thirst
• Hypovolemia or elevated body temperature can also activate thirst
• Lesions of the lateral hypothalamus decrease water intake

Limbic System Connections

• Hippocampus → Mammillary bodies via the fornix
• Mammillary bodies → Anterior thalamic nucleus → Cingulate gyrus via the mammillothalamic tract
• Amygdala ↔ Hypothalamus via the stria terminalis and ventral amygdalofugal pathway

Limbic Connections Purpose

• Connections link emotions, memory, and autonomic/homeostatic responses
• Emotions affect the autonomic nervous system (e.g., stress-induced sweating) and immune system (e.g., increased susceptibility to infections in depression)