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Anterior Cranial Fossa
- occupied by the frontal lobes of the brain and is formed by the orbital plates of the frontal, cribriform plate of the ethmoid bone, and lesser wings of the sphenoid.
- 1. Foramina of cribriform plate – conducts CN I (~20 nerves) from nasal cavity to olfactory bulbs/tracts of brain.
- 2. Crista galli – serves as an attachment site for the falx cerebri
- 3. Anterior clinoid process – attachment site for the tentorium cerebelli
Middle Cranial Fossa
- occupied by the temporal lobes of the brain and is formed by the greater wings & sella turcica of the sphenoid, & petrous and squamous part of the temporal bones.
- 1. Optic canal – conducts CN II (optic nerve) and ophthalmic artery into orbit.
- 2. Superior orbital fissure – conducts CN III, IV, V1, and VI into the orbit. Also transmits the superior ophthalmic vein which connects to the cavernous sinus.
- 3. Foramen rotundum – transmits V2
- 4. Foramen ovale – transmits V3, lesser petrosal n. & accessory meningeal a.
- 5. Foramen spinosum – transmits the middle meningeal artery & vein
- 6. Foramen lacerum – does not transmit anything; is covered by cartilage.
- 7. Sella Turcica (“Turkish saddle”) – is the area that contains the hypophyseal fossa, dorsum sellae, & posterior clinoid processes. The pituitary gland sits in the hypophyseal fossa.
- 8. Posterior clinoid process – attachment site for tentorium cerebelli
Posterior Cranial Fossa
- – is occupied by the cerebellum of the brain and is comprised mostly of the occipital bone & petrous part of the temporal bone.
- 1. Internal auditory (acoustic) meatus – transmits CN VII & VIII
- 2. Jugular foramen – transmits CN IX, X, & XI
- 3. Hypoglossal canal – transmits CN XII
- 4. Foramen magnum – transmits spinal cord, intervertebral venous plexus, vertebral arteries, spinal fibers of accessory nerve
- 5. Internal occipital protuberance – attachment site for falx cerebri, site of confluens of sinuses
- 6. Groove for sigmoid sinus – demarcates site of sigmoid sinus
- 7. Groove for transverse sinus – demarcates site of transverse sinus
- 8. Clivus – inferior to dorsum sellae and continuous w/ basilar part of occipital b. This is a feature of the sphenoid bone.
Cranial Base: Inferior View
- Special Features
- 1. Hard palate – will be studied later (with oral cavity)
- 2. Medial & lateral pterygoid plates – attachment sites for muscles of mastication
- 3. Carotid canal opening – transmits the internal carotid artery into the cranium
- 4. Styloid process – attachment site for muscles (e.g., stylohyoid, stylopharyngeus)
- 5. Stylomastoid foramen – transmits facial nerve motor root
- 6. Mastoid process – attachment site for SCM
- 7. Occipital condyle – articulates with the atlas (C1)
- 8. Mandibular fossa – articulates with the head of the mandible (details later)
- The dura mater is the tough, outer protective membrane. It has a number of folds (dural reflections) that create separation between parts of the brain.
- 1. The dura is innervated mostly by sensory branches of CN V1, V2, & V3.
- 2. Its blood supply is from the middle meningeal a. (br of the maxillary
- 3. The dura is comprised of two layers – a meningeal layer and periosteal layer.
- 4. Dural venous sinuses are formed in some locations between these two layers.
- Dural Relection that separates the cerebral hemispheres (note its anterior attachment to the crista galli & posterior attachment to the internal occipital protuberance).
- inferior to the tentorium cerebelli. It separates the two cerebellar hemispheres
Dural Relection that separates the cerebral hemispheres from the cerebellum.
anterior border of the tentorium forms the tentorial notch.
- spider web-like, thin membrane.
- Superficial to it is the subdural space (a potential space).
- Deep to it, is the subarachnoid space (containing the CSF).
- In life, the arachnoid is pushed up against the dura because of the CSF in the subarachnoid space.
most intimate covering of the brain. It follows the contours of the brain (sulci and gyri) and cannot be separated from it.
- (the olfactory nerve)
- consists of ~ 20 little nerves that extend from the nasal cavity, through the foramina of the cribriform plate, to the olfactory tracts/bulbs of the brain.
- special sensory fibers (SS) for olfaction (sense of smell).
- (the optic nerve)
- located superficial & medial to the internal carotid artery.
- It exits the cranial cavity through the optic canal as it passes into the orbit.
- special sensory fibers (SS) for sight.
- (the oculomotor nerve)
- located just lateral of the diaphragma Sellae – which is the dural covering of the sella turcica.
- It exits the cranial cavity through the superior orbital fissure to enter the orbit.
- CN III will innervate striated, skeletal eyeball muscles and is part of the cranial outflow of the parasympathetic division.
- SE, SA, and VE – para/pre fibers.
- Note: The muscles of the eyeball develop from somite-like structures and so receive SE innervation.
- (the trochlear nerve)
- located near the anterior free edge of the tentorial notch near the dorsum sellae.
- It exits the cranial cavity through the superior orbital fissure to enter the orbit.
- CN IV will innervate just one muscle of the eyeball the superior oblique.
- SE and SA fibers
- Note: It is difficult to see in this diagram and on the body!
- (the trigeminal nerve)
- located in the “trigeminal cave” which is deep to the dura covering the medial aspect of the petrous part of the temporal bone.
- Here is expands to form the trigeminal semilunar ganglion.
- This ganglion acts like a DRG and is a collection of sensory (SA) cell bodies.
- The ganglion gives rise to the 3 divisions: V1, V2, & V3. V1 & V2 are purely sensory.
- V3 is both sensory and motor (the branchial motor fibers go to muscles of mastication – which develop from branchial arch 1).
- V1 exits the cranial cavity through the superior orbital fissure. V2 exits through foramen rotundum.
- V3 exits through foramen ovale.
- SA & branchial motor (SVE) fibers.
- (the abducens nerve)
- pierces the dura near the clivus and then travels through the cavernous sinus to exit the cranial cavity through the superior orbital fissure as it enters the orbit.
- It innervates one muscle (lateral rectus m.) in the orbit which “abducts” the eyeball (hence the name “abducens”). SA & SE fibers.
- (the facial nerve)
- exits the cranial cavity by entering into the internal auditory meatus (along with CN VIII).
- Here, CN VII travels within the petrous bone in which its sensory ganglion (the geniculate ganglion) is located. Within the bone, it gives rise to the greater petrosal n. and then travels in the facial canal where it gives rise to the chorda tympani nerve (which carries SS fibers for taste to the anterior 2/3rd of the tongue).
- The facial nerve exits the skull through the stylomastoid foramen and then travels through the parotid gland to distribute the motor branches to the muscles of facial expression (which develop from branchial arch 2).
- CN VII is part of the cranial outflow of the parasympathetic division.
- SA, brachial motor fibers, VA, VE – para/pre, and SS (taste) fibers.
- (the vestibulo-cochlear nerve)
- exits the cranial cavity by entering into the internal auditory meatus (along with CN VII).
- Here, its sensory fibers are divided into two groups which are distributed to the cochlea (organ of hearing) and vestibular aparatus (organ of balance) located in the inner ear(which is located in the petrous part of the temporal bone).
- SS fibers for hearing and balance.
- (the glossopharyngeal nerve)
- exits the cranial cavity by entering into the jugular foramen (along with CN X & XI).
- It innervates a single pharyneal muscle (stylopharyngeus – which develops from branchial arch 3), supplies taste fibers to the posterior 1/3rd of the tongue, and visceral afferent fibers to middle ear cavity, oropharynx, & tongue.
- It is part of the cranial outflow of the parasympathetic division.
- branchial motor fibers, VA, VE – para/pre, and SS (taste) fibers.
- (the vagus nerve)
- exits the cranial cavity by entering into the jugular foramen (along with CN IX & XI).
- It supplies the VE – para/pre fibers to the pharynx, larynx, thorax and foregut and midgut of the abdomen.
- Hitchhking along the vagus nerve are branchial motor fibers from CN XI (called “XI via X fibers”) which will innervate muscles of the pharynx and larynx.
- The vagus also contains some taste fibers to the epiglottis and taste bud and some somatic afferent fibers from the area of the external auditory meatus.
- Thus, CN X contains branchial motor (XI via X) fibers, VA, VE – para/pre, SS (taste), and SA fibers.
- (the accessory nerve)
- exits the cranial cavity by entering into the jugular foramen (along with CN IX & X).
- It has a cranial portion and spinal portion which join briefly as they enter the jugular foramen.
- Inferior to the foramen, the cranial part joins the vagus n (these are the XI via X fibers).
- The spinal accessory part goes on to innervate the SCM and trapezius.
- The motor fibers of the spinal portion of the accessory are described as either branchial motor or somatic motor by different authors.
- We will consider them as being somatic motor (SE).
- Thus, CN XI (accessory portion) contains SA and SE fibers.
- The cranial portion (XI via X) contains branchial motor fibers.
- (the hypoglossal nerve)
- exits the cranial cavity by entering into the hypglossal canal.
- It is a motor nerve that innervates the intrinsic and extrinsic muscles of the tongue.
- CN XII contains SE fibers.
- Note: SA fibers (proprioceptive) carried by the hypoglossal nerve to the tongue may have their cell bodies in the C1-2 DRGs.
- motor fibers of certain cranial nerves that innervates those muscles of the head and neck that develop from the branchial(or pharyngeal) arches
- e.g., muscles of mastication, muscles of facial expression, muscles of the palate, larynx & pharynx.
- Cranial nerves V3, VII, IX, and XI via X contain these types of fibers.
- Note: Branchial Motor fibers are also known as special visceral efferent or SVE fibers (don’t confuse these with ANS fibers!!!).
Special Sensory (SS)
sensory fibers from 5 special senses: nose (smelling), eyes (sight), ears (hearing & balance), and tongue (taste). Cranial nerves I, II, VII, VIII, IX, and X contain these types of fibers.
Circle of Willis
- The common carotid artery divides into the internal and external carotid arteries.
- The internal carotid a. & vertebral a. supply the internal aspect of the skull.
- The internal carotid a. will enter the skull through the opening of the carotid canal on the inferior surface of the cranial base.
- The vertebral arteries enter the cranium through the foramen magnum.
- Inside the cranium, they will branch and come together to form the Circle of Willis – which is the major arterial supply to the brain.
- Arteries that directly form the Circle of Willis (from anterior to posterior):
- 1. anterior communicating a. (connects the 2 anterior cerebral arteries)
- 2. anterior cerebral a. (br of internal carotid a.)
- 3. internal carotid a. (gives rise to ant. cerebral and middle cerebral as.)
- 4. posterior communicating a. (br of posterior cerebral a.)
- 5. posterior cerebral arteries. (br of basilar a. which is formed by the
- two vertebral arteries)
Cranial Nerve Functional Components