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.

Forebrain

The forebrain (proencephalon) is the largest part of the brain, most of which is cerebrum. Other important structures found in the forebrain include the thalamus , the hypothalamus and the limbic system. The cerebrum is divided into two cerebral hemispheres connected by a mass of white matter known as the corpus callosum. Each hemisphere is split into four lobes; the frontal, parietal, occipital and temporal lobes. The surface of each hemisphere is made up of grey matter known as the cerebral cortex and is folded to increase the surface area available within the skull. The cortex has roles within perception, memory and all higher thought processes. Inside the cortex is the white matter, within which are a number of nuclei (grey matter), known as the basal nuclei. The basal nuclei receive information from the cortex to regulate skeletal movement and other higher motor functions.

The thalamus functions to relay sensory information to the cerebral cortex and the hypothalamus, regulating visceral functions including temperature, reproductive functions, eating, sleeping and the display of emotion. The limbic system describes a collection of structures within the forebrain, including the amygdala and hippocampus, also known as the 'emotional brain'. It is important in the formation of memories and in making decisions and learning.

Thalamus

The thalamus is comprised of two oval shaped structures, connected by the interthalamic adhesion. The thalamus has many functions. It is involved in processing and relaying sensory information selectively to various parts of the cerebral cortex. All senses from lower centres, including auditory, somatic, visceral, gustatory and visual systems, have input to the thalamus before being relayed to the appropriate areas within the cerebral cortex. The thalamus plays a major role in regulating arousal, levels of consciousness and levels of activity.

Hypothalamus

The hypothalamus is located on the floor of the diencephalon, beneath the thalamus. It communicates with the cerebral cortex, thalamus, pituitary gland and other areas of the brain via the infundibular stalk.

The function of the hypothalamus is mainly related to the overall regulation of the Endocrine System. It also controls and integrates the autonomic nervous system, therefore controlling heart rate, blood pressure, body temperature and gastrointestinal secretions. The hypothalamus is closely related to the pituitary gland, controlling a large proportion of the activity going to it. For a more detailed analysis of the function of this part of the brain, please use the link: Hypothalamus Anatomy and Physiology.

Pituitary

The function of the pituitary is mainly related to the production of hormones as part of the Endocrine System. For further information on the pituitary gland please use this link: Equine Pituitary Gland.

Cerebral Cortex

This cerebrum is the largest part of the brain, divided by the median fissure into two cerebral hemispheres. The cerebral hemispheres are connected by the corpus callosum. The outer layers of the cerebrum are made up of grey matter. Grey matter is formed by neurons and their unmyelinated fibres. The white matter below the grey matter of the cortex is formed predominantly by myelinated axons (myelin is white in appearance). The surface of the cerebral cortex is folded, the numerous folds are known as gyri which greatly increase the surface area. Grooves between these gyri are known as sulci; more than two thirds of the surface is within sulci. The cerebral cortex is connected to structures such as the thalamus and the basal ganglia, sending information to them along efferent connections and receiving information from them via afferent connections. Most sensory information is routed to the cerebral cortex via the thalamus. The cortex is commonly described as comprising three parts; sensory, motor and association areas.

The sensory areas receive and process information from the senses. Inputs from the thalamus are called primary sensory areas. Vision, hearing, and touch are processed by the primary visual cortex, primary auditory cortex and primary somatosensory cortex respectively. The two hemispheres of the cerebral cortex receive information from the opposite (contralateral) side of the body. There are a number of anatomical areas of the brain responsible for organising this sensory information. The parietal lobe is located within the dorsocaudal aspect of the cortex. The temporal lobes are located laterally and the occipital lobes are located in the caudal most aspect of the cortex. The frontal lobe or prefrontal association complex is involved in planning actions and movement.

The motor cortex areas of the brain are located in both hemispheres of the cortex and are shaped like a pair of headphones stretching from ear to ear. The motor areas are related to controlling voluntary movements, especially fine movements. There are two main types of connection between the motor cortex and motor neurones found in the ventral horn of the spinal cord; the Pyramidal tracts and the Extrapyramidal tracts.

• Pyramidal tract connections are direct with no synapses in the brain stem. Axons pass through the ventral aspect of the medulla oblongata. The pyramidal tracts are responsible for aspects of fine motor skills that require a degree of conscious thought and concentration.
• The extrapyramidal tracts pass through the medulla oblongata outside the ventral pyramidal tracts and have synapses within the brain stem nuclei. These synapses make it possible for signals travelling down the extrapyramidal horns to be influenced by other areas of the brain including the cerebrum. The extrapyramidal tracts are generally responsible for activation of larger muscle groups and often work in a coordinated manner to achieve smooth synchronous movements.

Limbic System

The Limbic system is made up of parts of the brain bordering the corpus collosum. The Limbic system contains areas of cerebral cortex, the cingulate gyrus (dorsally), the parahippocampus gyrus (ventrally), the amygdala, parts of the hypothalamus (mamillary body) and the hippocampus. The limbic system is principally responsible for emotions and the various types of emotion can affect the activity of the Autonomic Nervous System, facilitated by the hypothalamus.

Olfactory Bulb

The olfactory bulb is responsible for olfaction and the bulb itself is located within the rostral forebrain area, supported by the cribiform plate and the ethmoid bone. The olfactory nerves are connected directly to the limbic system.As a result, olfaction plays a central role and is particularly important in regulating/stimulating sexual behaviour.

Midbrain

The midbrain or mesencephalon represents the connection between the brain stem and the higher centres of the brain and is involved in most body systems including sleep/consciousness, vision, hearing and temperature regulation.

The midbrain is located between the diencephalon and the hind brain, or brain stem. More specifically, it can be found ventral to the cerebral cortex and between the cerebral pedicles of the diencephalon and the pons. It is a relatively short portion of the upper brain stem and connects higher brain centres with the lower centres and spinal cord.

The midbrain has a stratified structure comprising various layers including the tectum, tegmentum and cerebral peduncle. These structures are found in a dorsoventral sequence. The tectum lies dorsally to the cerebral aqueduct and it has four major rounded surface swellings; colliculi (see below). The tegmentum is the core of the midbrain and a large proportion of it is made up by the reticular formation.

Cranial Nerve Nuclei

The major cranial nerve nuclei within the midbrain are the mesencephalic nuclei of the trigeminal nerves (V), the trochlear nuclei (IV), the principle and parasympathetic oculomotor nuclei (Cranial Nerve III), the 'red nuclei' (so named due to its pronounced vascularity) and the periaqueductal grey nuclei. The periaqueductal grey nucleus is a core of grey nervous tissue located adjacent to the cerebral aqueduct. The oculomotor nerve emerges from the mid brain rostral to the pons.

Cerebral Aqueduct

Within the lumen of the midbrain lies the cerebral aqueduct, which acts as a simple passage between the spinal cord and the third and fourth ventricles.

Colliculi

The tectum (roof) has four colliculi, two rostral and two caudal.

Caudal Colliculi

The caudal colliculi are widely spaced and are joined by a substantial commissure. The caudal colliculi act as integration centres for auditory pathways. The caudal colliculi also have a further connection to the thalamus via the ipsilateral medial geniculate body. This body is effectively a swelling of the thalamus.

Rostral Colliculi

The rostral colliculi are placed closer together in comparison to the caudal. The rostral colliculi are also joined to the thalamus, but by the lateral geniculate bodies rather than the medial. The rostral colliculi help to integrate the visual pathways and also are involved in somatic reflexes that are caused by visual cues. The rostral colliculi have also been suggested to be involved in spatial integration.

Substantia Nigra

The substantia nigra is a prominent area of the mid brain and is identifiable on cross sections by its darker pigmentation. This pigmentation is due to the gradual accumulation of pigmentation of neurons and is associated with basal nuclei within the tissue. The substantia nigra nuclei are involved in the control of voluntary movement.

Crura Cerebri

These are visible on the ventral surface of the mid brain and consist of fibre tracts that are in passage between the telencephalon and the brain stem. The oculomotor nerves (see above) also emerge in this region of the mid brain, directly rostral to the pons.

Hindbrain

Brain sagittal section stem highlighted

The hind brain is also called the rhombencephalon, it provides the connection between the spinal cord and the rest of the brain. The hindbrain contains many vital structures including the Medulla Oblongata, the Pons (the link between the cerebellum, forebrain and mid-brain) and the majority of the cranial nerves III to XII. In general the brain stem governs essential functions that are carried out sub-consciously via reflexes. As well as containing numerous cranial nerves, the hind brain also contains many 'extra-pyramidal pathways' which include the reticular formation, the olivary nucleus and the pontine nuclei. Nuclei within the hindbrain are also responsible for the reflexive control of posture and eye movement.

The reticular formation is a diffuse interconnection of neurons running throughout the brainstem receiving both sensory and motor nerve tracts. This information is then passed on to higher centres in the brain such as the cerebrum. One important aspect of the reticular formation is that in order to transition from sleep to consciousness, the reticular formation is required to activate the cerebral cortex (ascending reticular activating system). It also contains cerebellar pathways and peduncles facilitating a connection from the brain stem to the cerebellum. There are also a number of 'pyramidal pathways' and afferent pathways including the cuneate and gracile pathways.

Medulla Oblongata

The medulla oblongata can be found within the myelencephalon region of the hindbrain. Nuclei in the medulla oblongata control heart activity including rate and contractility. The medulla oblongata also controls other related functions, including blood pressure and distribution of blood to different organs. In conjunction with the nuclei found in the pons, the medulla oblongata also exerts an influence on respiratory movements.

Pons

The pons can be found in the metencephalon region of the hindbrain. The pons is able to exert some influence on respiratory movements and can also influence many digestive processes.

Cranial Nerves

Cranial nerves III to XII exit from the brain stem and act to innervate parts of the head, neck, viscera and the thoracic and abdominal cavities. Although most of these nerves contain both sensory and motor fibres, the sensory fibres all have their cell bodies in ganglia outside the brainstem.

Cerebellum

The location of the cerebellum. Image courtesy of BioMed Archive

The cerebellum is located in the caudal part of the cranial cavity and is caudal to the tentorium cerebelli but dorsal to the fourth ventricle. Its gross structure is made up of a central 'vermis' surrounded by two lateral hemispheres. It is attached to the brain stem via three pairs of peduncles; a rostral pair to the midbrain, a middle pair to the pons and a caudal pair to the medulla oblongata. Its internal structure is composed of a cerebellar cortex which is made up of fissures on the surface that divide into lobules and then further sub divide into folia or leaves. There are white matter fibres running to and from this cortex, also called arbor vitae. Within the cerebellum there are various nuclei including the dentate, interpositus and the fastigial.

The generalised function of the cerebellum is to receive information regarding any movement in progress or any intended movement via inputs from the muscles, vestibular system and motor centres of the pyramidal and extrapyramidal systems. The most important function of the cerebellum is to minimise the difference between the intended and the actual movements. The cerebellum then projects corrections regarding these movements to all motor centres of the brain via feedback circuits between the pyramidal and extrapyramidal systems. It should be noted that the cerebellum cannot initiate movement.

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.

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.

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, 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 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.

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 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).

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

Facial nerve (VII)

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.

The facial nerve enters the petrosal bone via the 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 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.

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.

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.

During a clinical examination any facial paralysis, drooling or abscence of blinking may indicate a problem with the facial 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 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.

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

Glossopharyngeal nerve (IX)

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.

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.

On clinical examination, choking or dysphagia as a result of malfunctioning or absent pharyngeal reflexes would indicate a problem with the glossopharyngeal nerve.

Vagus nerve (X)

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.

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.

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.

Accessory nerve (XI)

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.

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.

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.

Hypoglossal nerve (XII)

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 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.

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.

Vasculature of the Brain

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.

Spinal Cord

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

Spinal cord tracts

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.

Meninges

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

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.