Cranial Nerves - Anatomy & Physiology

From WikiVet English
Jump to navigation Jump to search
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.

Hypoglossal nerve (XII)

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