Cranial fossa and cranial nerves

• Anterior fossa

  The anterior cranial fossa forms the roof of the orbit, contributes to the nasal cavities and has foramina related to the special senses of vision and smell (olfaction). The frontal lobes of the brain are found here. Part of the frontal air sinus can be seen inside the frontal bone (blue arrow) but this not part of the cranial fossa  - it lies within the frontal bone not inside the skull cavity. The crista galli is a bony ridge formed by the frontal bone that provides attachment for the dura mater

  
• Anterior fossa

  The cribriform plate is part of the ethmoid bone, in life it has the olfactory bulbs lying on its superior surface. The olfactory nerves project up from the nasal cavity through the cribriform plate to join the olfactory bulb (CN I).  The sphenoid bone also contributes to the anterior fossa and contains the optic canals, which allow the optic nerves (CNII) and the ophthalmic artery to pass between the orbit and the inside of the skull.

  
• Middle fossa

  The sphenoid bone also contributes to the floor of the middle cranial fossa and forms the orbital fissures, the foramen rotundum, the foramen ovale and the foramen spinosum. The trigeminal nerve forms a large ganglia that sits on the floor of the middle cranial fossa. Here it divides into its 3 divisions V1 ophthalmic, V2 maxillary and V3 mandibular. V1 is a sensory nerve that passes through the superior orbital fissure to reach the orbit. It is accompanied by CN III oculomotor, CN IV trochlear and CN VI abducent as they also use the superior orbital fissure to enter the orbit.

  
• Middle fossa

  The V2 division also carries sensory nerves which pass through the foramen rotundum to pass towards the maxilla. The V3 division carries motor and sensory fibres through the foramen ovale to the infratemporal fossa. Passing through this foramen is also the otic ganglion (actually sitting at the exit of the foramen in the infratemporal fossa), the accessory meningeal artery, the lesser petrosal nerve and emissary veins. The foramen spinosum carries the middle meningeal artery, a branch of the maxillary artery that supplies blood to the dura.

  
• Middle fossa

  The foramen lacerum allows the internal carotid artery to enter the middle cranial fossa. The temporal lobes of the brain lie in the middle cranial fossa.

  
• The internal carotid artery enters the skull via the carotid canal, it then turns away from the canal and enters the top of the foramen lacerum. It does not pass all of the way through the foramen lacerum. The artery then lies either side of the body of the sphenoid bone where it will enter the cavernous venous sinus before contributing to the circle of Willis.

  
• Posterior fossa

  The temporal bone contributes to the floors of both the middle and posterior cranial fossae, but it is the occipital bone that forms most of the floor of the posterior fossa. It borders the jugular foramen, the foramen magnum and forms the hypoglossal canals. The jugular foramen is the starting point of the internal jugular vein that will then pass down the neck

  
• Posterior fossa

  It also allows the glossopharyngeal, vagus and accessory nerves to exit the skull. The foramen magnum allows the passage of the start of the spinal cord and its meninges to leave the skull and the vertebral arteries and spinal part of the accessory nerve to enter the skull.  The hypoglossal canal allows the hypoglossal nerve to exit the skull. The cerebellum lies in the posterior cranial fossa.

  
• Sitting in the petrous part of the temporal bone is the internal acoustic (auditory) meatus. This allows the facial nerve and the vestibulocochlear nerve to exit the cranial cavity and enter the temporal bone.

  
• Cranial nerves

  from anterior (CN I olfactory) to posterior (CN XII hypoglossal)

  
• Cranial nerves

  trigeminal nerve (CN V) forms a ganglion before dividing into its 3 divisions

  
• CN I Olfactory

  •Outgrowth of telencephalon

  •Olfactory mucosa through cribriform plate to synapse in olfactory bulb

  •Olfactory tracts to prepiriform area of amygdala (primary olfactory cortex)

  •Only sensory system not relayed through thalamus

  The olfactory nerve provides the special sense of smell

  
• CN II Optic

  •Extension of the diencephalon

  •From the retina through the optic canal, form chiasm then optic tracts

  •90% fibres to lateral geniculate body (thalamus) then striate area (visual cortex), 10% along medial root (non-geniculate) unconscious regulation

  The optic nerve provides the special sense of vision

  
• CN III Oculomotor

  •Midbrain through superior orbital fissure to orbit

  •Run in lateral wall of cavernous dural venous sinus

  •Through annular ring

  •Somatic motor and parasympathetic (visceral efferent)

  •Motor to inferior oblique, superior rectus, inferior rectus and medial rectus, levator palpabrae

  •Parasympathetic to sphincter pupillae and ciliary muscle

  •Superior division = levator palpabrae and superior rectus

  
• CN III Oculomotor

  The oculomotor nerve plays an important role in the innervation of the extra-ocular muscles and changes in the shape of the lens and pupil.

  The annular ring (ring of Zinn) lies in the posterior part of the orbit and provides attachment for the recti muscles of the eye, some structures entering the orbit pass through it, some do not

  
• CN IV Trochlear

  •Midbrain through superior orbital fissure to orbit

  •Run in lateral wall of cavernous dural venous sinus

  •Outside annular ring

  •Only cranial nerve in which all the fibres cross to the opposite side

  •Only cranial nerve from the posterior surface of the midbrain – longest intracranial (dural) route of cranial nerves

  •Motor fibres to superior oblique

  The trochlear nerve only contains motor fibres to one extra-ocular muscle.

  
• CN V Trigeminal

  •Originates from the pons, forms a ganglion and divides into 3 divisions

  •CN V1 = ophthalmic enters orbit through superior orbital fissure

  •CN V2 = maxillary enters pterygopalatine fossa through foramen rotundum

  •CN V3 = mandibular through foramen ovale to inferior surface of the base of the skull

  •Motor and sensory

  •Lacrimal (CN V1), zygomatic (CN V2) lingual and auriculotemporal (CN V3) allow passage of parasympathetic fibres from other cranial nerves

  
• Ophthalmic division (CN V1) =  general sensory for eye, lacrimal gland, eyebrow, skin of forehead, nose 

  Run in lateral wall of cavernous dural venous sinus

  Maxillary division (CN V2) = general sensory for skin of cheek, lower lid, upper jaw (and teeth), side of nose, mucosa of mouth

  Run in lateral wall of cavernous dural venous sinus

  Mandibular division (CN V3) =  sensory to ear canal, parotid gland, lower jaw and teeth. Motor to muscles of mastication, anterior belly of digastric, mylohyoid, tensor tympani, veli tensor palatini

  
• Tensor tympani is a small muscle in the middle ear that attaches to the malleus (an ossicle) and dampens vibration from the tympanic membrane to prevent damage to the inner ear structures. Tensor veli palatini helps to tense the soft palate. The trigeminal nerve also supplies general sensation for the anterior 2/3rds of the tongue.

  
• CN VI Abducent

  •Originates from the pons and passes through the superior orbital fissure to reach the orbit

  •Long extradural pathway

  •Passes through cavernous dural venous sinus with internal carotid artery

  •Through annular ring

  •Motor to lateral rectus

  The abducent nerve only carries motor fibres to one extra-ocular muscle.

  
• Cavernous venous sinus

  CN III, IV and V pass in the lateral wall of the sinus but CN VI runs through the middle with the internal carotid artery.

  
• CN VII Facial

  •Originates from the pons, through internal acoustic meatus into temporal bone

  •Greater petrosal, stapedial and chorda tympani

  •Motor fibres to muscles of facial expression leave via stylomastoid foramen = temporal, zygomatic, buccal, mandibular and cervical posterior auricular

  •Stapedius, stylohyoid and posterior belly of digastric

  
• CN VII Facial

  The facial nerve is another more complex cranial nerve. In the petrous part of the temporal bone, it supplies stapedius a small muscle that attaches to the stapes and dampens its movement at the oval window to prevent damage to the inner ear. It also send the greater petrosal and chorda tympani nerves that contain parasympathetic secretomotor fibres out to join parts of the trigeminal nerve to be carried to the orbit and the oral cavity respectively. It supplies motor innervation to the muscles of facial expression and part of the suprahyoid muscles.

  
• It also carries sensory fibres – general to parts of the auricle (see ear presentations) and special sense of taste (gustation) from the anterior 2/3rds of the tongue.

  
• CN VIII Vestibulocochlear

  •Special somatic afferent

  •Vestibular apparatus and auditory apparatus  through internal auditory meatus to pons/medulla oblongata

  •Vestibular – cerebellum, CN III, IV, VI and spinal tracts

  •Cochlear – inferior colliculi, medial geniculate nucleus of thalamus to auditory cortex (area 41 temporal)

  
• CN IX Glossopharyngeal

  •Originates from the medulla oblongata and passes through jugular foramen

  •Parasympathetic to parotid, buccal and labial (lip) glands

  •Taste from posterior 1/3rd tongue

  •Motor to pharyngeal plexus and stylopharyngeus

  •Sensory to posterior 1/3rd tongue, soft palate, pharynx, tympanic cavity, membrane, pharyngotympanic tube , external ear and auditory canal

  It joins the pharyngeal plexus and provides general sensation to the soft palate (gag reflex), the pharynx and many parts of the ear.

  
• The glossopharyngeal nerve communicates via the lesser petrosal nerve with the auriculotemporal branch of the trigeminal nerve to carry parasympathetic fibres to the parotid gland, and also senses changes in pressure at the bifurcation of the common carotid artery.

  
• CN X Vagus

  •Originates from the medulla oblongata and passes through the jugular foramen

  •Cranial, cervical, thoracic and abdominal distribution

  •Motor to pharyngeal plexus, soft palate, larynx

  •Parasympathetic to thoracic and abdominal viscera

  •Sensory to dura (post cranial fossa), ear, external auditory canal, lower pharynx, laryngeal mucosa, thoracic and abdominal viscera

  The parasympathetic fibres join the oesophageal, cardiac, bronchial plexi in the thorax and supply parasympathetic innervation to the gut tube until 2/3rds of the way along the transverse colon.

  
• The vagus nerve sends a recurrent laryngeal branch that wraps around the aortic arch on the left and the subclavian artery on the right. The nerves then ascend the neck between the oesophagus and the trachea to supply the intrinsic muscles of the larynx. The vagus nerve also joins the pharyngeal plexus (motor part of gag reflex) and supplies innervation to parts of the larynx.

  
• CN XI Accessory

  •Originate in spinal cord , enter foramen magnum and join ‘cranial root’ then pass through jugular foramen

  •Motor to trapezius and sternocleidomastoid

  •‘Cranial root’ now considered part of vagus nerve (soft palate)

  The accessory nerve only contains motor fibres to supply sternocleidomastoid and trapezius. It can be seen crossing the posterior triangle of the neck. The nerve is said to have 2 parts – a spinal part that comes from the top of the spinal cord, passes into the skull via the foramen magnum and joins cranial roots from the medulla

  
• CN XI Accessory

  The final accessory nerve then passes back out of the skull via the jugular foramen. All texts agree that the ‘spinal’ part is motor to SCM and trapezius. Some textbooks will say the cranial part joins the pharyngeal plexus to supply the muscles of the soft palate. Other texts class these cranial fibres as parts of the vagus nerve which originates very close by from the  surface of the medulla. Some texts combine these ideas stating that the soft palate is innervated by ‘ the cranial part of the accessory nerves running in the vagus nerve in the pharyngeal plexus’.

  
• CN XII Hypoglossal

  •Originates from the medulla oblongata (ant. to olive) passes through hypoglossal canal

  •Enters root of tongue above hyoid bone

  •Motor to all intrinsic muscles of the tongue and all extrinsic except palatoglossus