Equine Nervous System - Horse Anatomy

Central Nervous System

Brain

The brain is responsible for co-ordinating, integrating and controlling the rest of the nervous system. The brain is divided into several parts. Based on phylogenetic development, it can be divided into the forebrain, midbrain and hindbrain. Based on gross anatomy, it can be divided into the cerebrum, cerebellum and brainstem. The brain is enclosed within the cranial cavity of the skull.

Cranial Nerves

Cranial nerves 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. TheOlfactory 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.

Olfactory Nerve (I)

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 Cribiform plate and is surrounded by meningeal sheets including the sub-arachnoid space. The olfactory nerve terminates at the 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.

Optic Nerve (II)

The optic nerve represents the connection between the receptor cells of the retina and the 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.

The visual pathway' involves three consecutive neurons:

• The first order neuron is the bipolar cells of the retina, which are known as rods and cones.
• The second order neuron is the ganglion cells of the retina and axons within the optic nerve. 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 (cross). In the horse, approximately 85-88% of fibres decussate. The optic nerve then runs through the optic canal.
• 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.

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).

The optic nerve can be examined clinically via the menace response and pupillary light reflex (PLR). Anopsia (loss of vision) can be seen, especially associated with shear injury to the nerve after head trauma.

Oculomotor nerve (III)

The oculomotor nerve is part of the group of cranial nerves responsible for innervating the muscles of the head. The nerve originates 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 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.

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 absent 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 midbrain and is a motor nerve. It is composed of general somatic efferent fibres and is the smallest of the cranial nerves.

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 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.

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

Trigeminal nerve (V)

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).

Opthalmic nerve (V1)

The opthalmic nerve is a sensory nerve composed of general somatic afferent fibres. It passes along the cavernous sinus and exits via the 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.

• 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.
• 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.
• The infratrochlear nerve innervates the medial aspects of the eyelids, third eyelid and frontal sinus.
• Nasociliary nerves, which carry parasympathetic fibres from the oculomotor nerve to the iris, also provide sensory innervation to the globe.

Maxillary nerve (V2)

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 round foramen before entering the 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.

Mandibular nerve (V3)

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 masticatory muscles, the ventral throat and muscles of the palate. The mandibular nerve further branches into the masticatory nerve, the masseteric nerve and the temporal nerve. The mandibular nerve provides sensory branches called the buccal nerve, the auriculotemporal nerve, and then itself divides into two smaller branches; the lingual nerve and the inferior alveolar nerve. The lingual nerve receives sensory taste fibres and also connects some sensory taste fibres to parasympathetic salivary glands via the 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.