Difference between revisions of "Equine Nervous System - Horse Anatomy"

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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]]===
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====Circle of Willis====
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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:
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*Rostral Cerebral Arteries: supply the medial aspect of the cerebral hemispheres.
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*Middle Cerebral Arteries: supply the lateral and ventrolateral aspects of the cerebral hemispheres.
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*Caudal Cerebral Arteries: supply the occipital lobes.
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*Rostral Cerebellar Arteries: supply the rostral aspects of the cerebellum
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*Caudal Cerebellar Arteries: supply the caudal and lateral aspects of the cerebellum.
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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.
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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.
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====Rete Mirable====
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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.
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====Venous Sinuses====
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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
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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'''.
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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.
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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
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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.
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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
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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
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'''ventral petrosal sinus''' exits the '''foramen lacerum''' or '''jugular foramen''' to become continuous with the '''jugular vein'''.
  
 
===[[Equine Spinal Cord - Horse Anatomy|Spinal Cord]]===
 
===[[Equine Spinal Cord - Horse Anatomy|Spinal Cord]]===
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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'''.
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The '''spinal cord''' is constructed of the ''[[Spinal Cord - Anatomy & Physiology#Marginal layer|marginal layer]] ''which has axons and white matter, the ''[[Spinal Cord - Anatomy & Physiology#Mantle|mantle]]'' which contains cell bodies and grey matter and the ''[[Spinal Cord - Anatomy & Physiology#Spinal Canal|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:
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:'''cervical''' ''(C1-C6)''
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:'''cervicothoracic''' ''(C7-T2)''
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:'''thoracolumbar''' ''(T3-L3)''
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:'''lumbosacral''' ''(L3-S2)''
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:'''sacral''' ''(S3 onwards)''
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Nerves originating from the spinal cord and the segmental spinal nerves innervate the limbs.
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<br> <br>
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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)''.
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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.
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====Sensory Pathways====
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[[File:Spinal cord tracts - English.png|right|200px|thumb|Spinal cord tracts]]
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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.
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====Vasculature====
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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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<br />
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<br />
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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.
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=====Arterial Supply=====
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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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<br />
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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.
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=====Venous Supply=====
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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. 
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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.
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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.
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====Lymphatics====
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There are no lymphatic vessels or nodes within the spinal cord or other central nervous tissue.
  
 
===[[Equine Meninges - Horse Anatomy|Meninges]]===
 
===[[Equine Meninges - Horse Anatomy|Meninges]]===
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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.
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====Dura mater====
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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.
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====Subdural space====
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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).
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====Arachnoid mater====
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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 [[Cerebral_Spinal_Fluid_-_Anatomy_&_Physiology|cerebrospinal fluid]] (CSF) to enter and exit the blood stream. These protrusions adhere to the inner surface of the skull via ''calvaria'' processes.
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====Subarachnoid Space====
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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.
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====Pia Mater====
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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.
 
===[[Equine Cerebrospinal Fluid - Horse Anatomy|Cerebrospinal Fluid]]===
 
===[[Equine Cerebrospinal Fluid - Horse Anatomy|Cerebrospinal Fluid]]===
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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.
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====Production====
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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.
  
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===Clinical Links===
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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.
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*[[Equine Protozoal Myeloencephalitis]]
 
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*[[Equine Herpesvirus 1]]
 
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*[[Polyneuritis Equi]]
 
  
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==Peripheral Nervous System==
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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.
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==Introduction==
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====Circulation====
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The '''Peripheral Nervous System''' is made up of cranial and spinal nerves. Spinal nerves are named after the vertebra immediately above it, except for '''cervical vertebra'''. There are '''7''' cervical vertebrae and '''8''' cervical spinal nerves. The peripheral nervous system can be divided into the '''somatic nervous system''' and '''autonomic nervous system'''. The somatic nervous system co-ordinates body movements and also receives external stimuli. It regulates activities that are under conscious control. The autonomic nervous system subdivided into the '''sympathetic nervous system''', '''parasympathetic nervous system''', and enteric division. The sympathetic nervous system is the '''‘fight or flight’''' system which comes into role when an animal is under threat, its main neurotransmitter is '''adrenaline'''. The parasympathetic nervous system is the '''‘rest and digest’''' system which is responsible for digestion. Its main neurotransmitter is '''acetylcholine'''.
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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'''.  
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==Structure==
 
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[[Image:WIKIVETperipheralnervestructure.jpg|thumb|right|150px|Peripheral Nerve Structure. Sophie Stenner, RVC 2008]]
 
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Nerve fibres reside in a connective tissue matrix called the '''endoneurium''' and are gathered together into bundles or fascicles defined by a second connective tissue layer called the '''perineurium'''. Groups of fascicles are then gathered together in a third connective tissue layer called the '''epineurium'''. Thus, peripheral nerves have a '''three-tiered hierarchical arrangement of connective tissue'''. '''Renaut bodies''' are loose, cigar-shaped whorls of extracellular matrix within fascicles that are common in equine nerves at points of stress or compression.
 
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==Nerve Fibre==
 
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The nerve fibre consists of the impulse-carrying axon, which is surrounded by an ensheathing cell, the [[#The Schwann cell|Schwann cell]], which in turn is surrounded by an acellular basal lamina that is continuous along the length of the nerve. Nerve fibres come in various discrete diameter groups, which are reflected in their conduction velocities. The larger the diameter the more rapid the rate of impulse conduction. Particular targets or receptors are associated with axons of a particular diameter. Axons connected to muscles spindles have a large diameter (20 µm) and conduct at 120 m/s whilst the smallest myelinated fibres are about 1µm and conduct at around 6 m/s. The smallest fibres of all are the unmyelinated fibres (the high-threshold sensory afferents, or C-fibres, and post-ganglionic autonomies) and have a diameter of between 1 and 0.1 µm. These fibres do not conduct by saltatory conduction and have very slow conduction rates of around 0.5 m/s.
 
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===Axon===
 
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Axons have an outer membrane called the '''axolemma''' and within this there is the '''axoplasm''' which is continuous with the cytoplasm of the [[Neurons - Anatomy & Physiology|neuron]]. There are no ribosomes, either free or attached to endoplasmic reticulum in axons and therefore, no protein synthesis. Protein synthesis takes place within the cell body and some dendrites and all protein replacement required for the maintenance of the axon depends on proteins being imported from the cell body. A critical feature of the axon is its '''cytoskeleton''', which consists of two key elements; '''neurofilaments''' and '''microtubules'''. '''Neurofilaments''' are intermediate filaments of about 10 nm diameter, and belong to the same class as other cytoskeletal proteins such as keratin, desmin, vimentin, or GFAP of astrocytes.  Neurofilaments are formed from a triplet of polypeptide subunits of heavy (~ 200 kD), medium (~ 150 kD) and low (~ 60 kD) molecular weights. Typically, these subunits are heavily phosphorylated and are more numerous than microtubules, especially in large diameter axons, having a pivotal role in determining axon diameter. They are formed in the cell body, transported down the axon by axoplasmic transport and degraded in the terminals by Ca<sup>2+</sup> activated proteases. In other words, there is a constant turnover of neurofilament within the healthy axon. '''Microtubules''' within axons are similar to microtubules elsewhere, consisting of polymerised dimers of alpha and beta tubulin arranged as a hollow tube of about 28 nm. They are relatively abundant in smaller diameter axons, and are also synthesised in the cell body. An important component of the cytoskeleton are the '''microtubule associated proteins''' or MAP's and the tau proteins. These proteins are important in microtubule assembly and stability. Different classes of MAP's occur in the dendrites and the axons, and to some extent account for the different ultrastructural features that distinguish these two types of neuronal process. They form cross links between adjacent microtubules but also connect to neurofilaments and actin microfilaments, implying complex interactions between the various components of the axon skeleton.
 
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===Schwann Cell===
 
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Myelination in the PNS is achieved by the '''Schwann cell''', a derivative of neural crest cells, which bud off from the neuroepithelium at a very early stage of neurogenesis. During development, Schwann cells engage many small axons and as axonal diameter increases, Schwann cells eventually relate with only a single axon c.f [[Neurons - Anatomy & Physiology#Oligodendrocytes|oligodendrocytes]]. This single axon is enveloped in a trough by the Schwann cell processes that engulf it and as the processes come together, an inner '''mesaxon''' is formed. The leading-edge process continues to move over the axon forming a spiral. Myelination, an extremely complex molecular process, occurs when the cytoplasm within the process is extruded allowing the internal surfaces of the membrane to come together as the '''major dense line''', the outer membrane apposition constituting the intraperiod line. The alternating pattern of these two form the lamellae of compacted myelin. The myelin sheath is attached to, and is an integral part of the Schwann cell on which it is dependent for its maintenance.
 
  
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A single Schwann cell forms a single myelin sheath or internode and there is a reasonably constant relationship between the myelin thickness and the internodal length, which in turn is associated with axon calibre. Large axons have long, thick myelin sheaths and therefore conduct more rapidly. The internodes do not abut one another but are separated by an exposed area of axon called the '''node of Ranvier'''. If the axons remain of small diameter, then a Schwann cell will continue to associate with many axons, although none of them are myelinated. Thus, ''even unmyelinated axons retain a Schwann cell ensheathment''. These non-myelinating Schwann cells are sometimes referred to as ''Remak cells.''
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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.
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===Axoplasmic Transport===
 
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Neurons are very large cells and most of a neurons cytoplasm is present in its processes while most of the cells RNA is located in cell body (Nissl substance). These cells have therefore evolved mechanisms to transport large macromolecules and organelles up and down processes.
 
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===Anterograde Transport===
 
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Anterograde transport moves substances from the cell body to the axon. Two basic forms of anterograde transport can be recognised: '''fast anterograde transport''' and '''slow anterograde transport'''. Fast anterograde transport allows movement of all membranous organelles such as synaptic vesicles and occurs at a rate of around 400mm/day (recent evidence suggests that there are many forms of fast anterograde transport, mediated by different kinesins). Fast anterograde transport depends critically on oxidative metabolism, and is, in fact independent of the cell body. Microtubules act as a static track along which the organelles can move, driven by the ATPase '''kinesin''' which acts as a "motor" molecule. Fast anterograde transport is independent of the cell body. Anything which interferes with energy supply or cytoskeleton necessary for fast anterograde transport has profound effects on the health of the axon. Agents such as colchicine or vincristine block microtubule assembly, disrupting fast anterograde transport. '''Slow anterograde transport''' deals with cytoskeletal elements and large soluble proteins. Slow anterograde transport can be further sub-divided into a slow component, which occurs at about 2mm/day (neurofilament, rubulin, actin) and a fast component, which occurs at around 4 mm/day, transporting all other proteins (eg myosin, clathrin).
 
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===Retrograde Transport===
 
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Retrograde transport returns materials from the axon terminal to the cell body, either for degradation or restoration and reuse. As with fast anterograde transport, particles move along microtubules. The motor molecule for retrograde transport is '''dynein''' which is a microtubule-associated ATPase. The retrograde transport system is important not only for returning material to the cell body, but also provides the means whereby target-derived trophic factors, such as nerve growth factor (NGF) for dorsal root ganglion neurons, are conveyed to the cell body where they promote cell survival. Research is being undertaken into the use of trophic factors to promote cell survival during degenerative pathology. The retrograde transport system can be "hijacked" by harmful substances to gain entry to the peripheral neuron and ultimately the CNS. [[Herpesviridae|Herpes virus]], [[Tetanus - Horse|tetanus]] and heavy metals all affect the retrograde transport system.
 
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==Blood Supply==
 
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The epineurium is penetrated by the vascular supply to the nerve and this blood supply is known as the '''vasa nervorum'''. Only capillaries occur within the endoneurial compartment. The capillaries of the endoneurium are joined by tight junctions and provide a barrier to large macromolecules. This forms the basis of the blood-nerve barrier (BNB), which has similarities to the [[Blood Brain Barrier - Anatomy & Physiology|blood-brain barrier]] of the CNS. The BNB appears to be relatively weak in the sensory ganglia because fenestrations occur between endothelial cells in this location. Sensory ganglia are therefore more vulnerable to blood-borne agents.  A further "barrier" is provided by the perineurium which consists of sheets of flattened cells, connected by tight junctions and covered on both sides by a basal lamina. The only route across this structure is trans- rather than inter-cellular.
 
  
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[[Category:To Do - AP Review]]
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==Peripheral Nervous System==

Revision as of 09:55, 22 November 2012


 This article is still under construction.


Central Nervous System

Brain

Cranial Nerves

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 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

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.

Peripheral Nervous System

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