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==Central Nervous System==

==Central Nervous System==

===Brain===

===[[Equine Brain - Horse Anatomy|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.

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====

====Forebrain====

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'''.

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===

===[[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.

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.

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)'''.  

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)'''.  

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.

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===

===[[Vasculature of the Equine Brain - Horse Anatomy|Vasculature of the Brain]]===

====Circle of Willis====

====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:  

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:  

'''ventral petrosal sinus''' exits the '''foramen lacerum''' or '''jugular foramen''' to become continuous with the '''jugular vein'''.

'''ventral petrosal sinus''' exits the '''foramen lacerum''' or '''jugular foramen''' to become continuous with the '''jugular vein'''.

===Spinal Cord===

===[[Equine Spinal Cord - Horse Anatomy|Spinal Cord]]===

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 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 '''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.

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====

====Sensory Pathways====

[[File:Spinal cord tracts - English.png|right|200px|thumb|Spinal cord tracts]]

[[File:Spinal cord tracts - English.png|right|200px|thumb|Spinal cord tracts]]

The spinal cord contains a number of [[Sensory Pathways - Anatomy & Physiology|sensory (ascending) pathways]] or tracts contained within the [[Central Nervous System - Anatomy & Physiology#White Matter|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.  

The spinal cord contains a number of [[Sensory Pathways - Anatomy & Physiology|sensory (ascending) pathways]] or tracts contained within the [[Central Nervous System - Anatomy & Physiology#White Matter|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====

====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 - Anatomy & Physiology|"blood-brain barrier"]].  

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 - Anatomy & Physiology|"blood-brain barrier"]].  

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The vasculature of the spinal cord has a close relationship with the [[Cerebral Spinal Fluid - Anatomy & Physiology|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.

The vasculature of the spinal cord has a close relationship with the [[Cerebral Spinal Fluid - Anatomy & Physiology|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=====

=====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 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.  

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The '''ventral spinal artery''' supplies the main "core" of the spinal cord, i.e. the [[Central Nervous System - Anatomy & Physiology#Grey Matter|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.

The '''ventral spinal artery''' supplies the main "core" of the spinal cord, i.e. the [[Central Nervous System - Anatomy & Physiology#Grey Matter|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=====

=====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.   

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 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.

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====

====Lymphatics====

There are no lymphatic vessels or nodes within the spinal cord or other central nervous tissue.

There are no lymphatic vessels or nodes within the spinal cord or other central nervous tissue.

===Meninges===

===[[Equine Meninges - Horse Anatomy|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.

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====

====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.

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====

====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).

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).

====Pia Mater====

====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.

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===

===[[Equine Cerebrospinal Fluid - Horse Anatomy|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.

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====

====Production====

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.

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====

====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'''.  

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'''.