Blood supply to brain audio
 

 

Blood supply to the CNS audio/interactive audios

SPINAL CORD

This supplied by two arteries

  • Anterior spinal artery

  • Posterior spinal artery

They provide radicular arteries, which enter the spinal cord substance

 

BRAIN

      The brain is supplied mainly by 2 arteries

(1)     internal carotid artery

(2)     vertebral artery

These 2 arteries form an arterial circle of Willis at the ventral aspect of the brain.

The internal carotid artery consists of the following parts.

(1)   cervical part

(2)   intrapetrosal portion found in the petrosal sinus

(3)   intracavernous portion found in the cavernous sinus

(4)   supraclinoid portion found above the anterior clinoid process

Most of its branches to the brain arise from the supraclinoid

     The vertebral artery from the neck region tansverse the foramina transversaria (except that for the 1st cervical vertebra) of the upper 6 cervical vertebra in order to approach the basis pontis where the two vertebral arteries join to form the basilar artery.

     The basilar artery communicates rostrally with the branches of the internal carotid artery to form the arterial circle of Willis which is the main supply of blood to the brain.

Arterial circle

     The 2 terminal branches of the internal carotid artery are the anterior cerebral artery and the continuation of the internal carotid artery known as the middle cerebral artery

     The two anterior cerebral arteries communicate in front by an anterior communicating artery which completes the circle by joining the posterior cerebral branch of the arterial circle of Willis.

     The anterior and middle cerebral arteries supply the following structures by their branches.

(1)     frontal lobe

(2)     upper part of the temporal lobe

(3)     parietal lobe

(4)     anterior portion of the basal ganglia

(5)     anterior portion of the thalamus

The posterior cerebral artery supplies

(1)    posterior part of the basal ganglia

(2)    posterior portion of the thalamus

(3)    occipital lobe

(4)    medioinferior portion of the temporal lobe.

The arterial circle is formed at the base of the brain to support the blood supply to the brain. It is formed by 

  • Anterior communicating artery

  • Posterior communicating artery

  • Proximal part of anterior cerebral artery

  • Proximal pat of middle cerebral artery

  • Proximal part of posterior cerebral artery

It has branches, which are circular (called circular branches) and they are divide into four groups

  • Anterolateral group

  • Anteromedial group

  • Posterolateral group

  • Posteromedial group

The most important clinically is the anterolateral group which forms the lateral striate artery called the artery of cerebral hemorrhage because it supplies most of the internal capsule and is most easily damaged by hypertensive stroke causing pressure on the internal capsule  and loss of blood supply to the posterior limb of the internal capsule, thereby damaging the corticospinal tract. The contralateral side of the lesion will be affected and paralyzed. This artery of cerebral hemorrhage also supplies  the region surrounding the posterior limb of internal capsule such as the thalamus and puts pressure on the same as a result of the hemorrhage. 

Some of the branches that supply the thalamus may also supply the internal capsule and hence may cause paralysis if damaged. They are 

  • Thalamogeniculate artery, which supplies the thalamus and the medial and lateral geniculate bodies.

  • Anterior choroidal artery, which supplies the posterior edge of the internal capsule and may cause problems with affection of the internal capsule.

Internal carotid artery

This artery begins at the level of the upper border of the thyroid cartilage. It is separated from the external carotid artery by the following structures

  • Pharyngeal branch of X

  • Styloid process

  • Stylopharyngeus

  • Syloglossus

  • Glossopharyngeal nerve

It moves upwards from this level and is separated from the internal jugular vein by the vagus nerve.

It proceeds upwards to reach the base of the skull, snubbing the foramen lacerum to enter the carotid canal. It emerges at the middle cranial fossa where it projects upwards above the clinoid process to form a siphon that projects at right angles to its proximal end. It finally divides into two terminal branches.

  • Anterior cerebral artery 

  • Middle cerebral artery

It is divided into four parts along its course

The supraclinoid part gives off the following branches

  • Anterior communicating artery - interconnects the anterior cerebral arteries

  • Anterior choroidal artery- supplies the posterior edge of the internal capsule

  • Posterior communicating artery

  • Anterior cerebral artery

  • Middle cerebral artery

Anterior cerebral artery

This artery supplies the inner aspect of the cerebral hemispheres. Its branches include

  • Medial striate artery

  • Orbital branch for the orbital plate

  • Callosomarginal artery which supplies the corpus callosum

  • Pericallosal artery

Middle cerebral artery

This supplies mainly the external part of the bran and has the following branches

  • Orbitofrontal artery

  • Anterior parietal artery

  • Posterior parietal artery

  • Posterotemporal artery

  • Anterotemporal artery

  • Rolandic artery

  • Prerolandic artery - supplies the central sulcus and hence when damaged,  affects the motor area.

 

 

The visual fields can be divided into.

(1)   Temporal portion

(2)   Nasal portion.

The temporal portion of the visual field projects on the nasal portion of the retina. Information then proceeds from this nasal portion through the optic nerve to the optic tract.

      Information from the nasal visual field projects onto the temporal retina.   At the optic chiasm, fibers carrying impulses from the 2 fields of the retina decussates partially.  

      Lateral fibers proceed along the same line while medial fibers cross to the other side.   Hence the optic tract consists of contralateral fibers from the nasal retina carrying impulses from the temporal field and ipsilateral temporal fibers carrying information from the nasal field.

      The optic tract then moves around the brain stem to terminate in the lateral geniculate body. 2nd order neurons move from the lateral geniculate body as optic radiations within the sublenticular part of the internal capsule.

      Some fibers proceed to the superior colliculus to form the brachium of the superior colliculus.   The superior colliculus subserves primitive visual reflexes.

      The fibers of the optic radiation then terminate in the visual cortex to form the geniculocalcarine tract that course along the bank of the calcarine sulcus to terminate in the area 17 of the medio-occipital cortex.   This is the visual cortex.

      Clinically blockage can occur in any part of the tract.  A complete transection of the tract before the optic chiasm causes total blindness of the eye of that side.

            A lateral section at the optic chiasm will lead to an ipsilateral nasal hemianopsia.   A section posterior to the optic chiasm at the optic tract causes a left sided homonymous hemianopsia if the section is on the right side.

            A section across the optic chiasm in the sagittal plane leads to bitemporal heteronymous hemianopsia.   This usually occurs in pituitary tumor because of the close relationship between the hypophysis cerebri and the optic chiasm. 

The ventricles of the brain have the following divisions

(1)   2 lateral ventricles

(2)   3rd ventricle

(3)   4th ventricle which communicates with the central canal of the spinal medulla

Lateral ventricles

     The 2 lateral ventricles have the following divisions

(1)    Anterior horn projects into the frontal lobe

(2)    Posterior horn projects into occipital lobe

(3)    Inferior horn projects into temporal lobe

The anterior horn has a roof made up of the corpus callosum and a lateral wall made up of the caudate nucleus.  It projects into the body of the lateral ventricle (at the trigone of the lateral ventricle).  The trigone of the lateral ventricle communicates with the body, the occipital horn and the temporal horn of the lateral ventricle.

            The temporal horn has a medial wall made of the hippocampus.  Its dorsolateral extreme is formed by the amygdaloid nuclear complex.

            The occipital horn has a roof made of the tapetum.  The lateral ventricles communicates with the 3rd ventricle by the interventricular foramina.

3rd ventricle

            The 3rd ventricle has a lateral wall made up of the thalamus.  It communicates with the midbrain which surrounds the cerebral aqueduct.  This aqueduct links with 4th ventricle.

4th ventricle

            The 4th ventricle has a roof made up of the superior and the inferior medullary velum.  The floor of the 4th ventricle is rhomboid in shape and is therefore called the rhomboid fossa.  The apex of the fossa caudally is called the obex.  Rostral to the obex is the area posterior which is followed by the 2 trigones- the hypoglossal and the vagal trigones which is then followed by the facial colliculus.

 

            At the rostral extreme of the medulla, the 4th ventricle opens into two lateral apertures called the foramina of Luschka.

 

MENINGES 

            The meninges are the coverings of the structures of the central nervous system eg spinal cord and brain

            The meninges are usually divided into cranial and spinal meninges depending on whether they cover the brain or the spinal cord.  They can be divided into 3

(1)   dura mater

(2)   arachnoid mater

(3)   pia mater

The dura is also called the pachymeninges.   It is the thickest of all the coverings of the neuraxis.  It is divided into 2 portions – the more superficial part of which is attached to the cranial periosteum – this is the cranial dura.  The cranial dura is followed by a deeper dura directly covering the brain substance.  The space that separates the dura and the arachnoid mater is the subdural space.  This space is important clinically because of the possibility of  hemorrhage as a result of a tear in the branches of the arteries within the space.  This type of hemorrhage is known as the subdural hemorrhage.  Hemorrhage may also occur clinically into the space above the dura by a tear in the branches of the meningeal arteries that supply the dura –it is then called epidural hemorrhage.

      The arachnoid and the pia are thin and are applied to the surface of the brain very closely.  They are together called the pia-arachnoid complex or the leptomeninges.  This leptomeninges is very thin but there also exists a potential space between the two mater called the subarachnoid space.

      Cerebrospinal fluid (CSF) is contained within the subarachnoid space.  This space also contains subarachnoid granulations which allow for the flow of CSF in a unidirectional fashion.  The CSF flows through the arachnoid granulations into the main blood stream by entering intravenous sinuses around the brain.

      The subarachnoid space communicates with the ventricular system of the brain through 2 lateral foramina of Luschka and one median foramen of Magendie.  CSF therefore moves freely within the ventricular system, communicating with the subarachnoid space.

      Hemorrhage may occur in the subarachnoid space and this is then called subarachnoid hemorrhage.

      Cisterns are spaces containing large amounts of CSF placed between the pia-archnoid complex and the dura.

      The largest cistern in the brain is the cisterna magna placed between the cerebellum and the medulla oblongata.  This is the cerebellomedullary cistern.

      Pontine cistern is placed anteriorly between the 2 halves of pons.

      The superior cistern is placed around the posterolateral part of the midbrain.  Other cisterns are the interpeduncular cistern around the cerebral peduncles, chiasmatic cistern around the optic chiasm and the lumbar cistern in the spinal cord.  These cisterns are important because they can be used in outlining the brain in radiological imaging and in pneumoencephalography.

 

 

 

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