Cranial Nerves - Anatomy & Physiology

The brainstem, cranial nerves and pyramids. © BioMed Archive.

Introduction

Cranial nerves are those nerves which arise from the brain and brain stem rather than the spinal cord. Nerves arising from the spinal cord are the peripheral nerves. There are 12 pairs of cranial nerves and these pairs of nerves passage through foramina in the skull, either individually or in groups. Cranial nerves are traditionally referred to by Roman numerals and these numerals begin cranially and run caudally.

The most cranial nerve is the Olfactory nerve (I) which runs from the nasal cavity through to the olfactory bulb. The next most cranial is the Optic nerve (II) which runs from the eyes to the thalamus. Cranial nerves III to XII all exit from the brain stem and innervate the head, neck and organs in the thorax and abdomen. In order of most cranial to caudal, these include the Oculomotor nerve (III), the Trochlear nerve (IV), the Trigeminal nerve (V), the Abducens nerve (VI), the Facial nerve (VII), the Vestibulocochlear nerve (VIII), the Glossopharyngeal nerve (IX), the Vagus nerve (X), the Accessory nerve (XI) and the Hypoglossal nerve (XII).

Many of the cranial nerves with nuclei within the brain stem contain sensory and motor neurone components. The sensory fibre components have their cell bodies located in ganglia outside the central nervous system and the motor fibre element have their cell bodies within the central nervous system.

Cranial Nerves

There are three main groups of cranial nerves; special senses, innervation of the head muscles and innervation of structures originating from brancial arches. The special senses cranial nerves include the olfactory, ocular and vestibulocochlear. Cranial nerves related to the innervation of the head muscles include the oculomotor, trochlear, abducens and the hypoglossal nerves. Cranial nerves related to innervation of structures originating from branchial arches includes the trigeminal, facial, glossopharyngeal, vagus and accessory nerves.

Olfactory Nerve (I)

Olfaction is part of the special senses cranial nerve group and represents the chemical senses of olfaction (smell) and gustation (taste). When chemical substances interact with our bodies they stimulate special sensory cells which in turn generate an action potential. The resultant impulse is sent to the brain via sensory afferent fibres and it is these fibres that represent the olfactory cranial nerve. Note: an action potential is only generated where a special sensory cell for that particular chemical substance exists. The special sensory cells are found within the olfactory mucous membrane in the nasal fundus and the olfactory neurosensory cells are found within the olfactory epithelium. The olfactory epithelium contains cilia on the surface of the cells to increase the surface area for chemical interactions.

The olfactory nerve runs to the olfactory bulb which is found within the telencephalon. The olfactory nerve is a sensory nerve and is composed of many Special Visceral Afferent fibres. The fibres are formed into bundles that are referred to as 'Olfactory filaments'. The olfactory nerve passes through the Cribiform plate and is surrounded by meningeal sheets including the sub-arachnoid space. Therefore the route of the olfactory nerve represents a potential site for an infection to track towards the brain. Injury of this nerve can lead to 'anosmia', or loss of smell.

The olfactory organ in dogs is extremely well developed and species such as dogs use olfaction to orientate themselves in an environment in a way that humans do not. Olfactory cells are continuously replaced and are only viable sensory cells for between 30 - 60 days. This point is of importance as this is the only area in which nerve cells are able to be replaced or regenerated in adult animals and this area is now under research at Cambridge University for the treatment of damaged nerves and/or nerve disorders in nerves other than those involved in olfaction.

Optic Nerve (II)

The optic nerve is part of the special senses cranial nerves and represents the connection between the receptor cells of the retina and the diencephalon. The optic nerve is a sensory nerve and is composed of Special Somatic Afferent fibres. The optic nerve passes through the optic chiasm which is an area of the ventral brain where both optic nerves run in a medial direction and eventually decussate. The optic nerve runs through the optic canal.

The optic nerve can be examined clinically via a menace response and anopsia (loss of vision) can be seen in injury.

Oculomotor nerve (III)

The oculomotor nerve is part of the group of cranial nerves responsible for innervating the muscles of the head. The nerves originate from the ventral midbrain and is a motor nerve. It is composed of general somatic efferent fibres and general visceral efferent fibres. The general somatic efferent fibres of the oculomotor nerve are responsible for the motor function of four of the six external muscles of the eyeball; the 'dorsal rectus', 'medial rectus', 'ventral rectus', 'ventral oblique' and 'levator of the upper eyelid'. The general visceral efferent fibres of the oculomotor nerve are responsible for the control of pupil diameter and therefore control the 'spincter pupillae' muscle and the 'ciliaris' muscle.

The oculomotor nerve has a pre-ganglionic nucleus in the midbrain and the nerve passes through the orbital fissure, along with the trochlear, abducens and opthalmic branch (V1) of the trigeminal nerve. It synapses in the ciliary ganglion of the eye. During a clinical examination, horizontal eye movements (strabismus) or an abscent pupillary light reflex (PLR) may indicate a problem.

Trochlear nerve (IV)

The trochlear nerve is part of the cranial nerve group responsible for innervation of the muscles of the head. The trochlear nerve originates from the dorsal mid-brain and is a motor nerve. It is composed of general somatic efferent fibres and is the smallest of the cranial nerves. The nerve is specifically responsible for the 'dorsal oblique muscle' muscle of the eye and also passes through the orbital fissure.

During a clinical examination, a dorso-lateral strabismus may indicate a problem with this nerve.

Abducent nerve (VI)

The abducent nerve is part of the cranial nerve group responsible for innervation of the muscles of the head. The abducent nerve originates from the medulla oblongata and is a motor nerve. It is composed of general somatic efferent fibres which are responsible for controlling the 'lateral rectus' muscle and the 'restractor bulbi' muscle of the eye. The nerve passes through the orbital fissure and can be found within the same layer of the meninges as the opthalmic branch (V1) of the trigeminal nerve (V).

During a clinical examination, medial deviation stribismus may indicate a problem with this nerve.

Vestibulocochlear nerve (VIII)

The vestibulocochlear nerve is part of the special senses group of cranial nerves and is made up of two components; the vestibular nerve and the cochlear nerve. The vestibular nerve is responsible for balance whilst the cochlear nerve is responsible for hearing. The nerves send impulses from the inner ear which contains the vestibular apparatus and cochlear. The vestibulocochlear nerve is a sensory nerve made up of special somatic afferent fibres. It passes through the internal acoustic meatus and into the petrosal bone. The facial nerve also takes this route.

Clinical problems with the vestibulocochlear nerve would be indicated on examination by changes in hearing and/or horizontal and verticle eye movements (strabismus and nystagmus). A head tilt is also associated with this nerve.

Cranial Nerve Location and Routes

A summary of the cranial nerves is shown below:

Nerve	Attatchment to brain	Exit Foramen	Fibre Type	Location of cell bodies	Distribution
I - Olfactory	Olfactory bulb	Cribriform plate	SVS	Olfactory epithelium	Olfactory epithelium
II - Optic	Optic chiasma	Optic canal	SSS	Retina	Retina
III - Occulomotor	Midbrain	Orbital fissure and Round Foramen	GSM + GVM	Nuculi III parasympathetic nuculi III	Extraoccular mm. Constrictor pupulli
IV - Trochlear	Midbrain (dorsal)	Round Foramen	GSM	Nuc IV	Dorsal oblique mm.
V - Trigeminal	Pons	V1 - Orbital fissure V2 - Round Foramen and Orbital Fissure V3 - Oval Foramen	V1 - GSS V2 - GSS V3 - SVM	V1 - Trigeminal gangion V2 - Trigeminal ganglion V3 - Nuculi V	V1 - Eye region V2 - Upper jaw inc. teeth V3 - mm. of arch 1
VI - Abducens	Medulla oblongata	Orbital fissure	GSM	Nuc IV	Lateral rectus and retractor bulbi mm.
VII - Facial	Medulla oblongata	Stylomastoid foramen	SVM + GVM + SVS	Nuc III (Taste - gleniailate ganglion)	mm. of arch 2 Submaxillary and sublingual glands Rostral 2/3 taste
VIII - Vestibulocochlear	Medulla oblongata	Inner accoustic meatus	SSS	Vestibular ganglion	Maculae and cristae
IX - Glossopharyngeal	Medulla oblongata	Jugular foramen	G/S V M/S	Ganglia IX - SVM Nuculi ambiguous - mm. arch 3	Carotid gland Caudal 1/3 taste mm. of arch 3 Carotid body
X - Vagus	Medulla oblongata	Jugular foramen	GVM + SUM + GVS + GSS	Ganglion X - (amongst others)	Thoracic and abdominal organs mm. of arches 4-6 Pharynx and larynx External ear
XI - Accessory	Medulla oblongata	Jugular foramen	GSM	Cervical spinal cord	Trapezius m. etc
XII - Hypoglossal	Medulla oblongata	Hypoglossal canal	GSM	Medulla nuculi XII	Tongue mm.

Cranial Nerve Fibre Types

Cranial nerves are best explained by function:

• General Somatic Motor (GSM)

Motor cortex —→ Motor nuculi —→ Muscle

• General Visceral Motor (GVM)

Brainstem —→ Parasympathetic nuculi —→ Parasympathetic ganglion —→ Gland etc

• "Branchial motor" - Special Visceral Motor (SVM)

Motor cortex —→ Motor nuculi —→ Muscle

• General Somatic Sensory (GSS) and General Visceral Sensory (GVS)

Sensory cortex —→ Thalamus —→ Trigeminal nuculi —→ Trigeminal ganglion —→ Skin etc

Special sensory fibres (SSS + SVS):

Smell SVS

Priform cortex —→ Olfactory tubercle —→ Olfactory Bulb —→ SMELL

Vision SSS

Occipital cortex —→ Thalamus MGN —→ Rostral colliculus/Retinal ganglion cell —→ Bipolar cell —→ VISION

Hearing/Balance SSS

Temporal cortex —→ Thalamus MGN —→ Caudal colliculus and hindbrain —→ Spiral ganglion —→ HEARING

Taste SVS

Sensory cortex —→ Thalamus —→ Hindbrain (VII, IX, X) —→ Ganglion —→ TASTE

Cranial Nerves and their Functions

#	Name	Nuclei	Function
I	Olfactory nerve	Anterior olfactory nucleus	Transmits the sense of smell
II	Optic nerve	Lateral geniculate nucleus	Transmits visual information to the brain
III	Oculomotor nerve	Oculomotor nucleus, Edinger-Westphal nucleus	Innervates the dorsal rectus, medial rectus, ventral rectus, and ventral oblique, which collectively perform most eye movements
IV	Trochlear nerve	Trochlear nucleus	Innervates the superior oblique muscle, which depresses, pulls laterally, and intorts the eyeball
V	Trigeminal nerve	Principal sensory trigeminal nucleus, Spinal trigeminal nucleus, Mesencephalic trigeminal nucleus, Trigeminal motor nucleus	Receives sensation from the face and innervates the muscles of mastication
VI	Abducens nerve	Abducens nucleus	Innervates the lateral rectus, which abducts the eye and the retractor bulbi which forces the third eyelid across the surface of the cornea as a protective mechanism.
VII	Facial nerve	Facial nucleus, Solitary nucleus, Superior salivary nucleus	Provides motor innervation to the muscles of facial expression and stapedius, receives the special sense of taste from the anterior 2/3 of the tongue, and provides secretomotor innervation to the salivary glands (except parotid) and the lacrimal gland
VIII	Vestibulocochlear nerve	Vestibular nuclei, Cochlear nuclei	Senses sound, rotation and gravity (essential for balance & movement)
IX	Glossopharyngeal nerve	Nucleus ambiguus, Inferior salivary nucleus, Solitary nucleus	Receives taste from the posterior 1/3 of the tongue, provides secretomotor innervation to the parotid gland, and provides motor innervation to the stylopharyngeus
X	Vagus nerve	Nucleus ambiguus, Dorsal motor vagal nucleus, Solitary nucleus	Supplies branchiomotor innervation to most laryngeal and pharyngeal muscles; provides parasympathetic fibers to nearly all thoracic and abdominal viscera down to the splenic flexure; and receives the special sense of taste from the epiglottis
XI	Accessory nerve	Nucleus ambiguus, Spinal accessory nucleus	Controls muscles of the neck and overlaps with functions of the vagus
XII	Hypoglossal nerve	Hypoglossal nucleus	Provides motor innervation to the intrinsic muscles of the tongue and other glossal muscles