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| | ===[[Equine Cranial Nerves - Horse Anatomy|Cranial Nerves]]=== | | ===[[Equine Cranial Nerves - Horse Anatomy|Cranial Nerves]]=== |
| − | Cranial nerves 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 & 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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| − | 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. The'''Olfactory nerve (I)''', '''Optic nerve (II)''' and '''Vestibulocochlear nerve (VIII)''' are sensory nerves. The , '''Oculomotor nerve (III)''', '''Trochlear nerve (IV)''','''Abducens nerve (VI)''','''Accessory nerve (XI)''' and '''Hypoglossal nerve (XII)''' are motor nerves. Finally, the '''Trigeminal nerve (V)''', '''Facial nerve (VII)''','''Glossopharyngeal nerve (IX)''', and '''Vagus nerve (X)''' are mixed sensory and motor nerves.
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| − | ====Olfactory Nerve (I)====
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| − | The olfactory nerve is involved in the conscious perception of smell. Primary afferent cell bodies are located within the olfactory epithelium of the nasal mucosa on ethmoturbiate bones,rather than in a ganglion like the other cranial nerves. Projections from these cell bodies are the olfactory nerve fibres. 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]]. The olfactory nerve terminates at the [[Forebrain - Anatomy & Physiology#Olfactory_Bulb|olfactory bulb]]. The horse also has nerves which arise from the nasal septum that course into the olfactory bulb, along with the '''vomeronasal nerve''' arising from the '''vomeronasal organ'''. Secondary neurons within the olfactory bulb project through the olfactory tracts to synapse with third order neurons in the medial forebrain bundle, amygdala, septal nuclei and habenular nuclei.
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| − | In the horse, special consideration must be given to diseases of the guttural pouch when considering cranial nerve dysfunction. The Glossopharangeal (IX), Vagus (X)and Accessory (XII) nerves are located in the medial compartment of the guttural pouch. The Facial (VII) nerve runs along the lateral compartment. The Mandibular nerve (V2) has limited contact with the dorsal wall of the lateral compartment.. Guttural pouch mycosis commonly results in paresis of cranial nerves IX,V and XII as well as erosion of the internal carotid artery. Rarely, there is involvement of cranial nerves VII and VIII.
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| − | ====Optic Nerve (II)====
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| − | The optic nerve represents the connection between the receptor cells of the [[Eye - Anatomy & Physiology#The Wall (retina, uvea and sclera)|retina]] and the [[Forebrain - Anatomy & Physiology|forebrain]]. It is not a true nerve, but represents an extension of the brain. The optic nerve is sesory, and is composed of '''Special Somatic Afferent fibres'''.
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| − | The '''visual pathway'''' involves three consecutive neurons:
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| − | *The first order neuron is the bipolar cells of the [[Eye - Anatomy & Physiology#The Wall (retina, uvea and sclera)|retina]], which are known as rods and cones.
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| − | *The second order neuron is the ganglion cells of the retina and axons within the optic nerve. 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 (cross). In the horse, approximately 85-88% of fibres decussate. The optic nerve then runs through the [[Skull and Facial Muscles - Anatomy & Physiology#Sphenoid Bone (os_sphenoidale)|'''optic canal''']].
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| − | *The third order neuron has its cell body in the lateral geniculate nucleus in the diencephalon. Its axon projects to the visual cortex, which is mostly the contralateral occipital cortex, in the '''optic radiation'''. The occipital lobe is where visual processing takes place at a conscious level.
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| − | The nerve is also involved in modulation of '''parasympathetic tone to the iris'''. The first and second order neuron pathways are the same as those responsible for vision, however after synapsing with the lateral geniculate nucleus axons involved in modulation of parasympathetic tone synapse with a third order neuron in the '''pretectal nucleus'''. Most axons from the pretectal nucleus then decussate back to synapse in the parasympathetic component of the '''Occulomotor nerve (III)''' in the ipsilateral eye (because it has crossed once at the optic chiasm and then again at the pretectal nucleus).
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| − | The optic nerve can be examined clinically via the [[Neurological Eye Examination - Horse#Menace Response|menace response]] and [[Neurological_Eye_Examination_-_Horse#The_pupillary_light_reflex_(PLR)|pupillary light reflex (PLR)]]. Anopsia (loss of vision) can be seen, especially associated with shear injury to the nerve after head trauma.
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| − | ====Oculomotor nerve (III)====
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| − | The oculomotor nerve is part of the group of cranial nerves responsible for innervating the [[Skull and Facial Muscles - Anatomy & Physiology#Facial_Muscles|muscles of the head]]. The nerve originates from the ventral [[Midbrain - Anatomy & Physiology|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 [[Eye - Anatomy & Physiology#Around_the_Eye|external muscles of the eyeball]]; the 'dorsal rectus', 'medial rectus', 'ventral rectus', 'ventral oblique' and 'levator palpebri superioris' (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. These fibres control pupillary constriction via the parasympathetic component of the nerve.
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| − | The oculomotor nerve has a pre-ganglionic nucleus in the midbrain and the nerve passes through the [[Skull_and_Facial_Muscles_-_Anatomy_%26_Physiology#Major_Foramen_and_Canals|'''orbital fissure''']], along with the trochlear, abducens and opthalmic branch (V1) of the trigeminal nerve. It synapses in the ciliary ganglion of the eye.
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| − | During a clinical examination, horizontal eye movements (strabismus) or an absent [[Neurological_Eye_Examination_-_Horse#The_pupillary_light_reflex_(PLR)|pupillary light reflex (PLR)]] may indicate a problem.
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| − | ====Trochlear nerve (IV)====
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| − | The trochlear nerve is part of the cranial nerve group responsible for innervation of the [[Skull and Facial Muscles - Anatomy & Physiology#Facial_Muscles|muscles of the head]]. The trochlear nerve originates from the dorsal midbrain and is a motor nerve. It is composed of '''general somatic efferent fibres''' and is the smallest of the cranial nerves.
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| − | After leaving the dorsal midbrain, its axons decussate (cross) and then run in a rostral direction through the cavernous sinus before exiting the skill via the [[Skull and Facial Muscles - Anatomy & Physiology#Major Foramen and Canals|'''orbital fissure''']]. In the horse, it may also exit via a seperate trochlear foramen. Finally, it runs to innervate the 'dorsal oblique muscle' muscle of the contralateral eye.
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| − | During a clinical examination, a dorso-lateral strabismus may indicate a problem with this nerve.
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| − | ====Trigeminal nerve (V)====
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| − | The trigeminal nerve is part of the cranial nerve group responsible for innervation of structures originating from branchial arches. The trigeminal nerve nuclei is in the area of the '''pons''' and '''medulla oblongata''' and is the nerve of the 1st branchial arch. The trigeminal nerve provides sensory innervation of cutaneous elements of the face, cornea, mucosa of the nasal septum and mucosa of the oral cavity. It also provides motor fibres to structures also associated with the 1st branchial arch, which are the muscles of mastication (''temporalis'', ''masseter'', ''medial and lateral pterygoids'' and ''rostral digastricus''. There are three primary branches of the trigeminal nerve; the '''Opthalmic nerve (V1)''', the ''' Maxillary nerve (V2)''' and the '''Mandibular nerve (V3)'''.
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| − | =====Opthalmic nerve (V1)=====
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| − | The opthalmic nerve is a sensory nerve composed of '''general somatic afferent fibres'''. It passes along the cavernous sinus and exits via the [[Skull and Facial Muscles - Anatomy & Physiology#Major Foramen and Canals|'''orbital fissue''']]. As it enters the orbit of the eye, it splits further into the '''lacrimal nerve''', the '''frontal nerve''', the '''nasociliary nerve''' and the '''infratrochlear nerve'''.
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| − | * The '''lacrimal nerve''' containes postganglionic parasympathetic fibres from the pterygopalatine ganglion that innervate the lacrimal gland. The lacrimal nerve also contains general somatic afferents that provide sensation to the lateral part of the upper eyelid.
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| − | *In the horse, the '''frontal nerve''' exits the medial aspect of the orbit via the '''supraorbital foramen''', becoming the '''supraorbital nerve''', and innervates the upper eyelid and forehead.
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| − | *The '''infratrochlear nerve''' innervates the medial aspects of the eyelids, third eyelid and frontal sinus.
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| − | * '''Nasociliary nerves''', which carry parasympathetic fibres from the oculomotor nerve to the iris, also provide sensory innervation to the globe.
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| − | =====Maxillary nerve (V2)=====
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| − | The maxillary nerve is a sensory nerve composed of '''general somatic afferent fibres'''. The maxillary nerve passes along the cavernous sinus and exits through the [[Skull and Facial Muscles - Anatomy & Physiology#Major Foramen and Canals|'''round foramen''']] before entering the [[Skull and Facial Muscles - Anatomy & Physiology#Major Foramen and Canals|'''alar canal''']]. It also runs across the wall of the '''pterygopalatine fossa''' and enters the '''infraorbital canal''' via the '''maxillary foramen'''. Whilst in the infraorbital canal, the maxillary nerve branch then branches further into the '''infraorbital nerve''' which supplies sensory fibres to the upper dental arcade. On exiting the infraorbital canal via the infraorbital foramen, the maxillary nerve branches again into the '''zygomatic nerve''' and '''pterygopalatine nerve''' supplying sensory fibres to the palate, lower eyelid, upper lip, nasal planum, and dorsal face.
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| − | =====Mandibular nerve (V3)=====
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| − | The mandibular nerve is a mixed sensory '''general somatic afferent fibres''' and motor '''general somatic efferent''' nerves. The mandibular nerve passes through the '''foramen lacerum''' in the horse. It provides motor branches to the [[Mastication|masticatory muscles]], the [[Larynx - Anatomy & Physiology#Intrinsic Musculature|ventral throat]] and [[Tongue - Anatomy & Physiology#Muscles|muscles of the palate]]. The mandibular nerve further branches into the '''masticatory nerve''', '''masseteric nerve''' and the '''temporal nerve'''. The mandibular nerve provides sensory branches called the '''buccal nerve''', '''auriculotemporal nerve''', and then itself divides into two smaller branches; the '''lingual nerve''' and the '''inferior alveolar nerve'''. The auriculotemporal nerve carries sensory information from the middle ear, temporal area and portions of the guttural pouch. The '''lingual nerve''' receives sensory taste fibres and also connects some sensory taste fibres to parasympathetic salivary glands via the [[Tongue - Anatomy & Physiology#Innervation|'''chorda tympani''']]. Via the chorda tympani branch, the mandibular branch supplies sensory fibres related to taste to the rostral 2/3 of the tongue. The lingual branch of the glossopharyngeal nerve supplies sensory fibres to the caudal 1/3 of the tongue.
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| − | ====Abducent nerve (VI)====
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| − | The abducent nerve is part of the cranial nerve group responsible for innervation of the [[Skull and Facial Muscles - Anatomy & Physiology#Facial_Muscles|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'' and ''retractor bulbi'' muscles 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).
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| − | During a clinical examination, medial strabismus may indicate a problem with this nerve.
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| − | ====Facial nerve (VII)====
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| − | The facial nerve is part of the cranial nerve group responsible for the innervation of structures originating from the branchial arches. It originates from the '''medulla oblongata''' and from the second branchial arch. It has a common dura sheet with the opthalmic (V1) branch of the trigeminal nerve. The facial nerve is of a mixed composite, made up of a number of different fibre types. It has a '''general somatic efferent fibre''' within the ear canal, a '''general visceral efferent fibre''' acting under parasympathetic control to some salivary glands, lacrimal glands, nasal cavity and palate, a '''special visceral afferent fibre''' providing taste to the rostral 2/3 of the tongue and finally it has a '''general somatic efferent fibre''' supplying motor function to the muscles of facial expression and ''caudal digastricus''.
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| − | The facial nerve enters the petrosal bone via the [[Skull and Facial Muscles - Anatomy & Physiology#Temporal Bone (os_temporale)|'''internal acoustic meatus''']] along with the vestibulocochlear nerve. The facial nerve also runs inside the '''facial canal'''. There are a number of intermediate branches which separate from the main facial nerve inside the facial canal including the '''greater petrosal nerve''', '''stapedial nerve''' (motor) and the '''chorda tympani'''. These then emerge via the [[Skull and Facial Muscles - Anatomy & Physiology#Major Foramen and Canals|'''stylomastoid foramen''']] at the caudoventral aspect of the skull. The chorda tympani of the facial nerve represents the '''special visceral afferent fibre''' supplying taste to the rostral 2/3 of the tongue.
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| − | There are also numerous external branches of the facial nerve once the facial nerve has left the facial canal. These include the '''internal auricular nerve''', the '''auriculopalpebral nerve''', the '''rostral auricular nerve''', the '''palpebral nerve''', the '''dorsal buccolabial nerve''', the '''ventral buccolabial nerve''', the '''ramus colli''', the '''digastric nerve''', the '''stylohoid nerve''' and the '''caudal auricular nerve'''.
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| − | The facial nerve supplies motor innervation to the muscles of facial expression. These are superficial flat, thin muscles that originate from bony areas of fascia and then radiate out around the skin. They may also often from sphincters such as around the mouth and eye.
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| − | During a clinical examination any facial paralysis, drooling or abscence of blinking may indicate a problem with the facial nerve.
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| − | ====Vestibulocochlear nerve (VIII)====
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| − | 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 [[Ear - Anatomy & Physiology#Vestibular Receptors and Balance|vestibular apparatus]] and [[Ear - Anatomy & Physiology#The Cochlea|cochlea]]. 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.
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| − | Clinical problems with the vestibulocochlear nerve would be indicated on examination by changes in hearing and/or strabismus and [[Vestibular System Examination|nystagmus]]. A head tilt is also associated with this nerve.
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| − | ====Glossopharyngeal nerve (IX)====
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| − | The glossopharyngeal nerve is part of the group of cranial nerves responsible for innervation of structures derived from the branchial arches. This nerve innervates structures related to the third branchial arch. It is also part of a group, together with the vagus and accessory nerves, that passes through the '''jugular foramen''' which is termed the '''vagus group'''. The glossopharyngeal nerve has cell bodies that are referred to as '''nucleus ambiguus'''. The glossopharyngeal nerve originates from the '''medulla oblongata''' and has several branches including the '''pharyngeal nerve''', the '''lingual nerve''' and the '''tympanic branches'''.
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| − | The glossopharyngeal nerve is composed of many fibre types including '''general somatic efferent fibres''' that innervate the stylopharyngeus muscle; the '''general visceral afferent fibres''' that provide sensory information from the carotid body, the pharynx and the middle ear; the '''general visceral efferent fibres''' that provide parasympathetic innervation to the parotid and zygomatic salivary glands; the '''special visceral afferent fibres''' that provide taste caudal to the tongue and finally the '''general somatic afferent fibres''' that provide sensory information from the external ear. The '''lingual branch''' of the glossopharyngeal nerve provides '''general somatic afferent fibres''' and '''special visceral afferent fibres''' to the caudal 1/3 of the tongue.
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| − | On clinical examination, choking or dysphagia as a result of malfunctioning or absent pharyngeal reflexes would indicate a problem with the glossopharyngeal nerve.
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| − | ====Vagus nerve (X)====
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| − | The vagus nerve is part of the group of cranial nerves responsible for innervation of structures derived from the branchial arches. It is also part of a group, together with the glossopharyngeal and accessory nerves, that passes through the '''jugular foramen''' which is termed the '''vagus group'''. The vagus nerve innervates structures related to the fourth branchial arch. The vagus nerve has cell bodies that are referred to as '''nucleus ambiguus'''.
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| − | The vagus nerve is composed of many different types of nerve fibre including '''general somatic efferent fibres''' supplying motor function to the muscles of the larynx, pharynx, palate and oesophagus; '''general visceral afferent fibres''' to the base of the tongue, pharynx and larynx; '''general visceral efferent fibres''' for parasympathetic supply of the thoracic and abdominal viscera; '''special visceral afferent fibres''' supplying taste to regions of the epiglottis and palate and finally '''general somatic afferent fibres''' to the external ear and the dura mater. The vagus nerve also supplies '''general somatic afferent fibres''' and '''special visceral afferent fibres''' to the root of the tongue.
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| − | There are many functional components of the vagus nerve including the heart, larynx, pharynx and many other viscera. On clinical examination any changes related to gag reflexes, blood pressure or heart rate, changes in 'voice' (dysphonia) or inspiratory dyspnoea may indicate a problem with the vagus nerve.
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| − | ====Accessory nerve (XI)====
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| − | The accessory nerve is part of the group of cranial nerves responsible for innervation of structures derived from the branchial arches. It is also part of a group, together with the glossopharyngeal and vagus, nerves that passes through the '''jugular foramen''' which is termed the '''vagus group'''. The accessory nerve supplies structures related to the fourth branchial arch. The accessory nerve has cell bodies that are referred to as '''nucleus ambiguus''' and originate in the '''medulla oblongata'''. The cranial root of the accessory nerve actually contributes to the vagus nerve and to the striated muscles of the pharynx, larynx, palate and oesophagus.
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| − | However, the accessory nerve also contributes to the cervical spinal cord and spinal root through the '''foramen magnum'''; providing innervation to muscles of the neck. The spinal root of the accessory nerve branches into the '''dorsal branch''' and the '''ventral branch'''. The dorsal branch innervates the ''brachiocephalicus'', ''trapezius'' and ''omotransversarius'' muscles of the dorsal neck. The ventral branch innervates the ''sternocephalicus'' muscle.
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| − | During clinical examination any difficulties in turning the neck or muscle atrophy around the dorsal and ventral neck may indicate a problem with the accessory nerve.
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| − | ====Hypoglossal nerve (XII)====
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| − | The hypoglossal nerve is part of the group of cranial nerves responsible for the control of muscles of the head. It is in part a cervical nerve due to its caudal position on the [[Hindbrain - Anatomy & Physiology|brain stem]]. The nerve is composed of '''general somatic efferent fibres''' which control the intrinsic and extrinsic muscles of the tongue (together with other nerves including the lingual nerve, facial nerve, lingual branch of the glossopharyngeal nerve and the vagus nerve). The nucleus of the nerve is located within the ''medulla oblongata'' of the brain stem and it passes through the '''hypoglossal canal'''.
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| − | During a clinical examination any deviation of the tongue may indicate a problem with this nerve. Deviation of the tongue is always to the side of the lesion initially.
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| | ===[[Vasculature of the Equine Brain - Horse Anatomy|Vasculature of the Brain]]=== | | ===[[Vasculature of the Equine Brain - Horse Anatomy|Vasculature of the Brain]]=== |
Central Nervous System
Circle of Willis
Blood is supplied to the brain from a ventral arterial supply in all species; from a circle of arteries called the Circle of Willis (also called the cerebral arterial circle or arterial circle of Willis) which lies ventrally to the hypothalamus where it forms a loose ring around the infundibular stalk. Blood is supplied to the brain by the internal carotid artery in horses. The Circle of Willis is made up of five main pairs of vessels:
- Rostral Cerebral Arteries: supply the medial aspect of the cerebral hemispheres.
- Middle Cerebral Arteries: supply the lateral and ventrolateral aspects of the cerebral hemispheres.
- Caudal Cerebral Arteries: supply the occipital lobes.
- Rostral Cerebellar Arteries: supply the rostral aspects of the cerebellum
- Caudal Cerebellar Arteries: supply the caudal and lateral aspects of the cerebellum.
The arrangement of the Circle of Willis means that if one part of the circle becomes blocked or narrowed (stenosed), or one of the arteries supplying the circle is stenosed, blood flow from the other blood vessels can continue to provide a continuous supply of blood to the brain.
The main blood supply to the circle is via the paired internal carotid arteries and the basilar artery. The basilar artery receives blood from the ventral spinal artery and the vertebral artery (the vertebral artery is a branch of the subclavian artery running through the vertebral foramina of C1 - C6). The maxillary artery does not contribute to the arterial circle in the horse, but it does supply the meninges. In horses, the vertebral artery can also supply the internal carotid artery via the occipital artery but this can be bypassed so that the vertebral artery can directly supply the internal carotid artery via a ramus to the internal carotid directly from the vertebral artery.
Rete Mirable
The brain is particularly susceptible to increased blood temperature and to protect the brain from any potential heat stress a number of species have developed protective mechanisms with the ability to selectively cool the brain. This protective system is often referred to as the Rete Mirable. The Rete Mirable is a complex network of arteries and veins lying very close to each other and depends on a countercurrent blood flow between the arterioles and venules (blood flowing in opposite directions). It exchanges heat, ions, or gases between vessel walls so that the two bloodstreams within the rete maintain a gradient.
Venous Sinuses
Venous sinuses drain the brain, meninges, and surrounding bone as well as participate in cerebrospinal fluid resorption; they are arranged into two systems. The dorsal system is within the dura
mater of the cranium and drains the cerebral cortex, the cortex of the cerebellum, the deeper telencephalon, part of the diencephalon, and the tectum of the midbrain. The ventral (basilar) system lies on the floor of the cranial vault and drains the brainstem. The dorsal and ventral systems have minimal connection between them, but each communicates with the extracranial venous system. The dorsal system begins where several dorsal cerebral veins converge in the area of the crista galli of the cribriform plate.
The dorsal sagittal sinus arises from this convergence and runs caudally along the dorsal midline; surrounded by the falx cerebri as it lies against the skill bones. Along its course, it receives cerebral veins, meningeal veins, and diploic veins from the skull.
The dorsal sagittal sinus of the horse, is incompletely divided by a septum and bifurcates rostral to the osseous tentorium. Just before reaching the osseous tentorium, the dorsal sagittal sinus
receives a single sinus that drains the medial cortex, corpus callosum, basal ganglia, and part of the diencephalon. The transverse sinuses run ventrally from the osseous tentorium to the retroarticular foramen, where they exit to join the extracranial venous system. The transverse sinuses receive the dorsal petrosal sinuses, which mainly drain the rhinencephalon. They also receive veins from the caudal cerebrum, dorsal midbrain and the meninges. The transverse sinuses are connected via the communicating sinus without directly joining to the dorsal saggital sinus in the horse.
The ventral sinus system contains the cavernous sinuses, basilar sinus, and ventral petrosal sinus. The cavernous sinuses lie on either side of the pituitary gland on the floor of the cranial vault. They are joined across the midline by the cranial and caudal intercavernous sinuses to encircle the pituitary. This circle of sinuses around the pituitary has connections through the orbital fissure, the optic foramen, and the oval foramen to peripheral veins. The internal carotid artery lies within this sinus system in the horse. Caudally, the cavernous system
communicates with the basilar sinus, which lies on the floor of the occipital bone, and the ventral petrosal sinus, which lies within the dura mater in the caudal part of the cranial vault. The
ventral petrosal sinus exits the foramen lacerum or jugular foramen to become continuous with the jugular vein.
The equine spinal cord demonstrates relatively few species specific features, other than its size. The spinal cord of a 500Kg horse is approximately 2 metres long. As in other species, it is surrounded and protected by the meninges and lies within the vertebral canal. The end of the spinal cord, known as the conus medullaris, extends relatively caudally in the horse; reaching the first sacral vertebra. It then becomes what is known as the filum terminale, which extends the spinal cord to reach the fourth sacral segment. Both the conus medullaris and the filum terminale, as well as the associated spinal nerves, form the cauda equina. In adult horses, the cauda equine begins at the lumbosacral junction.
The spinal cord is constructed of the marginal layer which has axons and white matter, the mantle which contains cell bodies and grey matter and the spinal canal. This canal conducts sensory information from the peripheral nervous system (both somatic and autonomic) to the brain, conducts motor information from the brain to various effectors and acts as a minor reflex center. The spinal cord can be divided to several regions:
- cervical (C1-C6)
- cervicothoracic (C7-T2)
- thoracolumbar (T3-L3)
- lumbosacral (L3-S2)
- sacral (S3 onwards)
Nerves originating from the spinal cord and the segmental spinal nerves innervate the limbs.
The forelimb nerves include the suprascapular (C5-6), the musculocutaneous (C5-7), the ulna/median (Originates from the brachial plexus, which is formed from C5-T1) and the radial (C5-T1).
The hindlimb nerves include the obturator (L2-4), the femoral (L2-4) and the sciatic (L4-S3). The sciatic nerve branches to the tibial nerve and the peroneal nerve.
Sensory Pathways
The spinal cord contains a number of sensory (ascending) pathways or tracts contained within the white matter. These pathways allow sensory information such as pain, touch, temperature or kinaesthesia (conscious proprioception) to be passed through the spinal cord and on to higher levels of the brain.
Vasculature
It is important to note that there is no direct vasculature to the spinal cord but instead there are a number of choroid plexuses that act as an exchanger between the vasculature of the spinal cord/brain and the fluid surrounding these structures. This distinction is referred to as the "blood-brain barrier".
The vasculature of the spinal cord has a close relationship with the cerebrospinal fluid (CSF) within the subarachnoid space. This CSF effectively forms a water jacket that buoys up the spinal cord and protects it from external influences. Therefore it is extremely important that the CSF has the appropriate properties in order to undertake this role. The vasculature of the spinal cord therefore has to provide the appropriate level of oxygen, pressure, pH and nutrients to maintain homeostasis of the spinal cord. As the CSF also performs this role within the skull, the vasculature of the brain has an important relationship with every aspect of the ventricles and subarachnoid space within the central nervous system.
Arterial Supply
The spinal cord is supplied by three main arteries that run along its length; the Ventral Spinal Artery, and paired Dorsolateral Spinal Arteries. The ventral spinal artery is the largest and follows the ventral fissure of the spinal cord. The dorsolateral arteries run close to the groove from which the dorsal nerve roots arise. Together with these three main arteries, the spinal cord is also supplied by branches from regional arteries including branches in the cervical, intercostal, lumbar and sacral regions. These regional arteries enter the spine at the intervertebral foramina, often accompanying the roots of spinal nerves. These regional arteries also form plexuses into which the three main longitudinal arteries run. The number and type of arteries that enter the spine from regional branches varies with species and also between individuals.
The ventral spinal artery supplies the main "core" of the spinal cord, i.e. the grey matter. It also partially supplies the white matter via the ventral fissure, although the majority of the white matter is supplied by radial branches of the dorsolateral arteries. There are also a number of anastamoses between both sets of arteries.
Venous Supply
Along the length of the spinal cord runs the vertebral venous plexus which drains the blood from the vertebrae and surrounding musculature. This venous plexus gives rise to veins that then leave the vertebrae via the intervertebral foramina and then go on to join the major venous channels of the neck and the trunk; namely the vertebral, cranial caval, azygous and caudal caval veins.
The venous plexus consists of paired channels within the epidural space that lie in a ventral position to the spinal cord. Each side of the pair is connected to its opposing plexus around the vertebrae resulting in a ladder-type pattern of venous vessels. The connections between each side are via the intervertebral foramina and these vessels are in close proximity to the spinal nerves.
The veins around the plexuses have no valves and can theoretically pass blood in either direction. The vessels are able to adjust their size/pressure to compensate for intrathoracic pressure. This intermittency of flow causes an increased risk of septic or neoplastic disease within the vertebral column. Where blood is impeded or where flow may become temporarily held stagnant, this may allow tumor seeds or micro-organisms to settle within tributaries.
Lymphatics
There are no lymphatic vessels or nodes within the spinal cord or other central nervous tissue.
The meninges are layers of tissue surrounding the central nervous system (CNS). Meningitis is the inflammation of these layers. Gaps and spaces between the meninges are named.
Dura mater
The Dura mater is the outer most layer and is made up of a dense fibrous connective tissue. The space in the vertebral canal ouside the dura mater is the epidural space. In the cranium, the dura layer is fused with the periosteum and therefore is in effect single layer without an epidural space. The dura contains a number of folds throughout its coverage of the brain including the falx cerebri, a midline fold between cerebral hemispheres, the tentorium cerebelli, an oblique fold between the cerebrum and cerebellum and the diaphragma sellae which forms a collar around the neck of the pituitary and forms the roof of the hypophyseal fossa. This layer and these associated folds all provide structural support to the brain and prevent the brain from undergoing excess movement within the skull. Where the dura mater folds between brain tissues it splits into two distinct layers that are separated by large blood filled spaces called venous sinuses. Venous sinuses are directly connected to the venous system and venous blood from vessels supplying the brain return to the heart via these sinuses.
Subdural space
The subdural space lies between the dura mater and the next meningial layer, the arachnoid mater. The subdural space is narrow potential space, where the two meningeal leayers lie in close proximity; but do not meet. The subdural space is thought to contain only lymph-like fluid. The meningeal layers can move apart in the event of injury or increased pressure; for example pooling of blood in the subdural space (subdural haematoma).
Arachnoid mater
This is the middle meningial layer and lies between the dura mater and the pia mater, the innermost meningeal layer. The arachnoid mater is a delicate structure and is constructed with non-vascular connective tissue. This layer also has small protrusions through the dura mater into the previously mentioned venous sinuses called Arachnoid villus and these allow cerebrospinal fluid (CSF) to enter and exit the blood stream. These protrusions adhere to the inner surface of the skull via calvaria processes.
Subarachnoid Space
The subarachnoid space lies between the arachnoid mater and pia mater. Both meninges are connected via a fine network of connective tissue filaments (spider web-like) which run through the space, originating from the arachnoid mater. This space also contains cerebrospinal fluid (CSF) from ventricular system. The largest parts of this space are called the cisterns, which are used for the collection of CSF. For example there is a cerebellomedullary cistern around the foramen magnum.
Pia Mater
This is the innermost layer and is firmly bound to the underlying neural tissue of the brain and spinal cord. The inner surface of the brain facing this meningial layer is lined with ependymal cells. The pia mater is highly vascular and is formed from delicate connective tissue. It also contains arteries and veins, but not venous sinuses.
Cerebrospinal fluid (CSF) surrounds the brain and spinal cord. It helps cushion the central nervous system (CNS), acting in a similar manner to a shock absorber. It also acts as a chemical buffer providing immunological protection and a transport system for waste products and nutrients. The CSF also provides buoyancy to the soft neural tissues which effectively allows the neural tissue to "float" in the CSF. This prevents the brain tissue from becoming deformed under its own weight. It acts as a diffusion medium for the transport of neurotransmitters and neuroendocrine substances.
Production
CSF is a clear fluid produced by dialysis of blood in the choroid plexus. Choroid plexi are found in each lateral ventricle and a pair are in the third and fourth ventricle. Further production also comes from the ependymal cell linings and vessels within the pia mater.
Edendymal cell production of CSF is via ultrafiltration of blood plasma and active transport across the ependymal cells. The ependyma is connected via a series of tight junctions preventing molecules passing between cells. The ependyma also sits on a basement membrane to provide support to the ependymal cells and provide further protection against blood perfusion. In areas of the brain where there are choroid plexi, the endothelium of the plexus vessel sits immediately adjacent to the basement membrane of the ependymal cells. Of the total CSF production, 35% is produced within the third ventricle of the brain, 23% via the fourth ventricle and 42% from general ependymal cell filtration.
CSF has a very low protein constituent, with only albumin being present together with a very low level of cellularity. The biochemistry of CSF includes high concentrations of sodium and chloride and very high concentrations of magnesium. Concentrations of potassium, calcium and glucose are low.
Circulation
Once produced, CSF is then circulated, due to hydrostatic pressure, from the choroid plexus of the lateral ventricles, through the interventricular foramina into the 3rd ventricle. The lateral ventricles are paired and are located in the cerebral hemispheres. The 3rd ventricle is located in the diencephalon and surrounds the thalamus. CSF then flows through the cerebral aqueduct (aqueduct of Sylvius or mesencephalic aqueduct) into the 4th ventricle. The 4th ventricle is located in the hindbrain. From the 4th ventricle the CSF may flow down the central canal of the spinal cord, or circulate in the subarachnoid space. The central canal of the spinal cord is in direct communication with the 4th ventricle. Most CSF escapes from the ventricular system at the hindbrain Foramen of Luschka (lateral apertures) into the subarachnoid space. Once in the subarachnoid space, the CSF may enter the cerebromedullary cistern (a dilation of the subarachnoid space between the cerebellum and the medulla) and then circulate over the cerebral hemispheres. CSF also flows down the length of the spinal cord in the subarachnoid space. Another dilation of the subarachnoid space occurs caudally due to the dura and arachnoid meninges continuing on past the end of the spinal cord. This gives rise to the lumbar cistern.
Large amounts of CSF are drained into venous sinuses through arachnoid granulations in the dorsal sagittal sinus. The dorsal sagittal sinus is located between the folds of dura, known as the falx cerebri, covering each of the cerebral hemispheres. Arachnoid granulations contain many villi that are able to act as a one way valve helping to regulate pressure within the CSF, and these arachnoid villi push through the dura and into the venous sinuses.
Peripheral Nervous System
