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

Revision as of 09:57, 22 November 2012

Central Nervous System

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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 ===[[Vasculature of the Equine Brain - Horse Anatomy|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'''.
 ===[[Equine Spinal Cord - Horse Anatomy|Spinal Cord]]===