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[[File:Brain human normal inferior view with labels en.svg|thumb|right|400px|Ventral brain showing labelled cranial nerves, WikiCommons]]

[[File:Brain human normal inferior view with labels en.svg|thumb|right|400px|Ventral brain showing labelled cranial nerves, WikiCommons]]

==Introduction==

==Introduction==

Cranial nerves are those nerves which arise from the brain and [[Hindbrain_-_Anatomy_%26_Physiology|brain stem]] rather than the spinal cord. Nerves arising from the spinal cord are the [[PNS Structure - Anatomy & Physiology|peripheral nerves]]. There are 12 pairs of cranial nerves and these pairs of nerves passage through [[Skull_and_Facial_Muscles_-_Anatomy_%26_Physiology|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.

Cranial nerves are those nerves which arise from the brain and [[Hindbrain - Anatomy & Physiology|brain stem]] rather than the spinal cord. Nerves arising from the spinal cord are the [[PNS Structure - Anatomy & Physiology|peripheral nerves]]. There are 12 pairs of cranial nerves and these pairs of nerves passage through [[Skull and Facial Muscles - Anatomy & Physiology|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.

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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 [[Forebrain_-_Anatomy_%26_Physiology#Thalamus|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)'''.   

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 [[Forebrain - Anatomy & Physiology#Thalamus|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)'''.   

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==Cranial Nerve Fibre Types==

==Cranial Nerve Fibre Types==

Prior to a full explanation of each cranial nerve function, physiology, anatomy and particularly composition, the numerous types of nerve fibre found in cranial nerves must be explained to ensure that the following page can be better understood. Nerve fibres can undertake numerous roles within the body and their function is dictated by what type of fibre the nerve is composed of. A classification scheme is used to describe the type of fibres found within the cranial nerves and therefore provides a better understanding of the function and composition of that particular nerve. Below is a brief outline of this nerve fibre classification.

Prior to a full explanation of each cranial nerve function, physiology, anatomy and particularly composition, the numerous types of nerve fibre found in cranial nerves must be explained to ensure that the following can be better understood.  

 

Nerve fibres can undertake numerous roles within the body and their function is dictated by what type of fibre the nerve is composed of. A classification scheme is used to describe the type of fibres found within the cranial nerves and therefore provides a better understanding of the function and composition of that particular nerve. Below is a brief outline of this nerve fibre classification.

 

===Afferent Fibres - Sensory===

===Afferent Fibres - Sensory===

====General Somatic Afferent (GSA)====

====General Somatic Afferent (GSA)====

This type of fibre is used in the majority of basic senses including temperature, touch and [[Pain|pain]]. These fibres mainly supply sensation to various areas of the head including to the skin and face via the trigeminal nerve (cranial nerve V); to the skin of the [[Ear_-_Anatomy_%26_Physiology#Outer_Ear|external ear]] via the trigeminal nerve (cranial nerve V), the facial nerve (cranial nerve VII), the glossopharyngeal nerve (cranial nerve IX) and via the vagus nerve (cranial nerve X); to the mucosae of the [[Oral_Cavity_Overview_-_Anatomy_%26_Physiology|oral cavity]] via the trigeminal nerve (cranial nerve V) and to the [[Meninges_-_Anatomy_%26_Physiology|meningies]] of the brain via the trigeminal nerve (cranial nerve V) and the vagus nerve (cranial nerve X).

This type of fibre is used in the majority of basic senses including temperature, touch and [[Pain|pain]]. These fibres mainly supply sensation to various areas of the head including to the skin and face via the trigeminal nerve (cranial nerve V); to the skin of the [[Ear - Anatomy & Physiology#Outer_Ear|external ear]] via the trigeminal nerve (cranial nerve V), the facial nerve (cranial nerve VII), the glossopharyngeal nerve (cranial nerve IX) and via the vagus nerve (cranial nerve X); to the mucosae of the [[Oral Cavity Overview - Anatomy & Physiology|oral cavity]] via the trigeminal nerve (cranial nerve V) and to the [[Meninges - Anatomy & Physiology|meningies]] of the brain via the trigeminal nerve (cranial nerve V) and the vagus nerve (cranial nerve X).

=====Skin Sensation Example=====

 

''Sensory cortex —→ Thalamus —→ Trigeminal nuclei —→ Trigeminal ganglion —→ Skin etc.''

:'''Skin Sensation Example'''

 

:''Sensory cortex —→ Thalamus —→ Trigeminal nuclei —→ Trigeminal ganglion —→ Skin etc.''

====Special Somatic Afferent (SSA)====

====Special Somatic Afferent (SSA)====

The special signifies that this fibre type is found in nerves related to the special senses which include [[Eye_-_Anatomy_%26_Physiology#Vision|vision]] and [[Ear_-_Anatomy_%26_Physiology#Function_-_Hearing|hearing]]. This therefore relates to the optic nerves (cranial nerve II) and the auditory/vestibular systems which are the vestibulocochlear nerves (cranial nerve VIII).

The special signifies that this fibre type is found in nerves related to the special senses which include [[Eye - Anatomy & Physiology#Vision|vision]] and [[Ear - Anatomy & Physiology#Function_-_Hearing|hearing]]. This therefore relates to the optic nerves (cranial nerve II) and the auditory/vestibular systems which are the vestibulocochlear nerves (cranial nerve VIII).

=====Vision Example=====

 

''Occipital cortex —→ Thalamus MGN —→ Rostral colliculus/Retinal ganglion cell —→ Bipolar cell —→ [[Eye - Anatomy & Physiology|Vision]]''

:'''Vision Example'''

=====Hearing Example=====

 

''Temporal cortex —→ Thalamus MGN —→ Caudal colliculus and hindbrain —→ Spiral ganglion —→ [[Ear - Anatomy & Physiology|Hearing]]''

:''Occipital cortex —→ Thalamus MGN —→ Rostral colliculus/Retinal ganglion cell —→ Bipolar cell —→ [[Eye - Anatomy & Physiology|Vision]]''

 

:'''Hearing Example'''

 

:''Temporal cortex —→ Thalamus MGN —→ Caudal colliculus and hindbrain —→ Spiral ganglion —→ [[Ear - Anatomy & Physiology|Hearing]]''

====General Visceral Afferent (GVA)====

====General Visceral Afferent (GVA)====

This fibre type is commonly found in sensory nerves to organs and to sensory tissues related to chemicals. These include fibres found in the glossopharyngeal nerves (cranial nerve IX) and the vagus nerves (cranial nerve X).

This fibre type is commonly found in sensory nerves to organs and to sensory tissues related to chemicals. These include fibres found in the glossopharyngeal nerves (cranial nerve IX) and the vagus nerves (cranial nerve X).

=====Organ Sensory Example=====

 

''Sensory cortex —→ Thalamus —→ Cranial nerve nuculi —→ Nerve ganglion —→ Organs etc.''

:'''Organ Sensory Example'''

 

:''Sensory cortex —→ Thalamus —→ Cranial nerve nuculi —→ Nerve ganglion —→ Organs etc.''

 

====Special Visceral Afferent (SVA)====

====Special Visceral Afferent (SVA)====

This fibre type is found again in special sense organs related to the sensing of chemicals and therefore including taste and olfaction. The olfactory sense relates to the olfactory nerve (cranial nerve I) and the taste senses relate to the facial nerve (cranial nerve VII), the glossopharyngeal nerve (cranial nerve IX) and the vagus nerve (cranial nerve X). The sense of taste is performed by the gustatory organ which are cells located in gustatory papillae in the mucous membrane of the dorsal side of the tongue. Taste buds can also be found in the soft palate, pharynx, larynx, lips and cheeks.

This fibre type is found again in special sense organs related to the sensing of chemicals and therefore including [[Gustatory System - Anatomy & Physiology|taste]] and [[olfaction]]. The olfactory sense relates to the olfactory nerve (cranial nerve I) and the taste senses relate to the facial nerve (cranial nerve VII), the glossopharyngeal nerve (cranial nerve IX) and the vagus nerve (cranial nerve X). The sense of taste is performed by the gustatory organ which are cells located in gustatory papillae in the mucous membrane of the dorsal side of the tongue. Taste buds can also be found in the soft palate, pharynx, larynx, lips and cheeks.

=====Olfaction Example=====

 

''Priform cortex —→ Olfactory tubercle —→ Olfactory Bulb —→ [[Nose - Anatomy & Physiology|Smell]]''

:'''Olfaction Example'''

=====Taste Example=====

 

''Sensory cortex —→ Thalamus —→ Hindbrain (VII, IX, X) —→ Ganglion —→ [[Gustatory System - Anatomy & Physiology|Taste]]''

:''Priform cortex —→ Olfactory tubercle —→ Olfactory Bulb —→ [[Nose - Anatomy & Physiology|Smell]]''

 

:'''Taste Example'''

 

:''Sensory cortex —→ Thalamus —→ Hindbrain (VII, IX, X) —→ Ganglion —→ [[Gustatory System - Anatomy & Physiology|Taste]]''

===Efferent Fibres - Motor===

===Efferent Fibres - Motor===

====General Somatic Efferent (GSE)====

====General Somatic Efferent (GSE)====

This type of nerve fibre is mainly found in nerves supplying [[Muscles_-_Anatomy_%26_Physiology#Skeletal_Muscle|skeletal muscles]]. Motor fibres such as these undertake numerous roles within the body such as control over the external ocular muscles via the oculomotor nerve (cranial nerve III), the trochlear nerve (cranial nerve IV) and the abducent nerve (cranial nerve VI); or for example the muscles around the dorsal and ventral neck including the brachiocephalicus, trapezius, omotransversarius and the sternocephalicus which are all supplied by the accessory nerve (cranial nerve XI).

This type of nerve fibre is mainly found in nerves supplying [[Muscles - Anatomy & Physiology#Skeletal_Muscle|skeletal muscles]]. Motor fibres such as these undertake numerous roles within the body such as control over the external ocular muscles via the oculomotor nerve (cranial nerve III), the trochlear nerve (cranial nerve IV) and the abducent nerve (cranial nerve VI); or for example the muscles around the dorsal and ventral neck including the brachiocephalicus, trapezius, omotransversarius and the sternocephalicus which are all supplied by the accessory nerve (cranial nerve XI).

=====Motor Nerve Innervation Example=====

 

''Motor cortex —→ Motor nuculi —→ Muscle''

:'''Motor Nerve Innervation Example'''

 

:''Motor cortex —→ Motor nuculi —→ Muscle''

 

====General Visceral Efferent (GVE)====

====General Visceral Efferent (GVE)====

This type of nerve fibre is found in nerves supplying [[Muscles_-_Anatomy_%26_Physiology#Smooth_Muscle|smooth muscle]], [[Muscles_-_Anatomy_%26_Physiology#Cardiac_Muscle|cardiac muscle]] and glands. Sympathetic innervation for this type of nerve fibre comes directly from the cranial cervical ganglion. However, parasympathetic innervation comes from several cranial nerves including the oculomotor nerve (cranial nerve III), the vestibulocochlear nerve (cranial nerve VIII), the glossopharyngeal nerve (cranial nerve IX) and the vagus nerve (cranial nerve X). Examples of where this type of fibre functions include the pupillary sphinter via the oculomotor nerve, glands of the nasal/oral cavity via the vestibulocochlear and glossopharyngeal nerves and thoracic/abdominal viscera via the vagus nerve.

This type of nerve fibre is found in nerves supplying [[Muscles - Anatomy & Physiology#Smooth_Muscle|smooth muscle]], [[Muscles - Anatomy & Physiology#Cardiac_Muscle|cardiac muscle]] and glands. Sympathetic innervation for this type of nerve fibre comes directly from the cranial cervical ganglion. However, parasympathetic innervation comes from several cranial nerves including the oculomotor nerve (cranial nerve III), the vestibulocochlear nerve (cranial nerve VIII), the glossopharyngeal nerve (cranial nerve IX) and the vagus nerve (cranial nerve X). Examples of where this type of fibre functions include the pupillary sphinter via the oculomotor nerve, glands of the nasal/oral cavity via the vestibulocochlear and glossopharyngeal nerves and thoracic/abdominal viscera via the vagus nerve.

=====Gland Innervation Example=====

 

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

:'''Gland Innervation Example'''

 

:''Brainstem —→ Parasympathetic nuculi —→ Parasympathetic ganglion —→ Gland etc.''

==Cranial Nerves==

==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 include the trigeminal, facial, glossopharyngeal, vagus and accessory nerves.

There are three main groups of cranial nerves; special senses, innervation of the head muscles and innervation of structures originating from branchial 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 include the trigeminal, facial, glossopharyngeal, vagus and accessory nerves.

 

===Olfactory Nerve (I)===

===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. If no sensory cell exists, that chemical substance will go undetected. 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.

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. If no sensory cell exists, that chemical substance will go undetected. 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.

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The olfactory nerve runs to the olfactory bulb which is found within the [[Forebrain_-_Anatomy_%26_Physiology#Olfactory_Bulb|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 [[Skull_and_Facial_Muscles_-_Anatomy_%26_Physiology#Ethmoid_Bone_.28os_ethmoidale.29|'''Cribiform plate''']] and is surrounded by meningeal sheets including the [[Meninges_-_Anatomy_%26_Physiology#Subarachnoid_Space|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 nerve runs to the olfactory bulb which is found within the [[Forebrain - Anatomy & Physiology#Olfactory_Bulb|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 [[Skull and Facial Muscles - Anatomy & Physiology#Ethmoid Bone (os ethmoidale)|'''Cribiform plate''']] and is surrounded by meningeal sheets including the [[Meninges - Anatomy & Physiology#Subarachnoid_Space|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.

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The olfactory organ in dogs is extremely well developed and species such as canines 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.

The olfactory organ in dogs is extremely well developed and species such as canines 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)===

===Optic Nerve (II)===

The optic nerve is part of the special senses cranial nerves and represents the connection between the receptor cells of the [[Eye_-_Anatomy_%26_Physiology#The_Wall_.28retina.2C_uvea_and_sclera.29|retina]] and the [[Forebrain_-_Anatomy_%26_Physiology|diencephalon]]. The optic nerve is a sensory nerve and is composed of '''Special Somatic Afferent fibres'''. The optic nerve passes through the [[Skull_and_Facial_Muscles_-_Anatomy_%26_Physiology#Sphenoid_Bone_.28os_sphenoidale.29|'''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 [[Skull_and_Facial_Muscles_-_Anatomy_%26_Physiology#Sphenoid_Bone_.28os_sphenoidale.29|'''optic canal''']].

The optic nerve is part of the special senses cranial nerves and represents the connection between the receptor cells of the [[Eye - Anatomy & Physiology#The Wall (retina, uvea and sclera)|retina]] and the [[Forebrain - Anatomy & Physiology|diencephalon]]. The optic nerve is a sensory nerve and is composed of '''Special Somatic Afferent fibres'''. The optic nerve passes through the [[Skull and Facial Muscles - Anatomy & Physiology#Sphenoid Bone (os_sphenoidale)|'''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 [[Facial Muscles - Anatomy & Physiology#Sphenoid Bone (os_sphenoidale)|'''optic canal''']].

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The optic nerve can be examined clinically via a [[Neurological_Eye_Examination_-_Horse#Menace_Response|menace response]] and anopsia (loss of vision) can be seen in injury.

The optic nerve can be examined clinically via a [[Neurological Eye Examination - Horse#Menace Response|menace response]] and anopsia (loss of vision) can be seen in injury.

===Oculomotor nerve (III)===

===Oculomotor nerve (III)===