P. VNormal Atrioventricular septum defect
15.6 Blood supply of the brain
The brain receives blood from two sources, the internal carotid arter- ies and the vertebral arteries . The internal carotid and vertebral inputs may be connected with each other on the under surface of the brain by a circle of small arteries termed the arterial circle (of Willis).
The venous return from the brain is generally into the nearest venous sinus of the dura mater.
The arteries and veins of the brain run within the subarachnoid space inside the cranial cavity. The arteries tend to lie deep inside the sulci of the cerebral hemispheres whereas veins are more superfi cial.
15.6.1 The arteries
If you have the opportunity to examine a cadaveric human brain, you will notice that the arteries look diff erent from those you will have encountered elsewhere in the body; they look more transparent and quite thin-walled. They are, in fact, thin-walled because the outermost fi brous coat (the tunica adventitia) is poorly developed or absent. These thin-walled arteries are more prone to rupture than arteries elsewhere (see Box 15.10 ). The cerebral arteries also diff er from arteries supplying most other organs in that they branch repeatedly before their terminal
branches enter the brain. The cerebral arteries are true end arteries , each terminal branch supplying a circumscribed area of the brain with no alternative supply (see Section 4.1.3 ). If a cerebral arterial branch should rupture or become blocked, a small area of the CNS will lose its blood supply and will die because there is no alternative anastomotic or collateral circulation. The functions carried out by the dead area of brain tissue will no longer operate (see Box 15.10 ).
The internal carotid arteries
As described in Section 12.5.2 , the common carotid arteries arise from the arch of the aorta, the right being a branch of the brachio- cephalic trunk and the left one being a direct branch (see Figure 12.10 ).
Each common carotid artery divides into external and internal carotid arteries high in the neck. Each internal carotid artery passes without branching through a bony canal in the base of the skull, the carotid canal , into the cranial cavity where they pass through the correspond- ing cavernous sinus to emerge through its roof.
The branches of the internal carotid arteries are illustrated in Figure 15.24 which should be followed as the following description is read. Each
Blood supply of the brain 135
Fig. 15.24 The arterial supply of the brain seen from below. The anterior part of the right temporal lobe and right cerebellum have been removed for clarity.
Olfactory bulb Anterior cerebral artery
Anterior communicating artery
Posterior communicating artery Posterior cerebral artery Superior cerebellar artery Internal carotid artery
Abducens nerve Facial and vestibulocochlear nerves
Anterior inferior cerebellar artery Posterior inferior cerebellar artery Basilar artery
Vertebral artery Olive Pyramid
Optic nerve Optic chiasma Optic tract Oculomotor nerve Trochlear nerve Tigeminal nerve Middle cerebral artery
internal carotid artery gives off an ophthalmic artery (not illustrated) just above the point where it leaves the cavernous sinus; these arter- ies run anterolaterally to enter the orbits through the optic canals. A posterior communicating artery may arise from the posterior aspect of the internal carotid artery; if present, it runs backwards to join the posterior cerebral artery. The internal carotid artery on each side ter- minates by dividing into the anterior and middle cerebral arteries . This division occurs below the anterior perforated substance, an area on the inferior surface of the cerebral hemisphere lateral to the optic chiasma. The anterior perforated substance is perforated by numerous small perforating arteries that arise from the terminal branches of the internal carotid arteries and supply deep structures of the brain such as the striate cortex.
As can be seen in Figure 15.24 , each anterior cerebral artery passes forwards and medially above the optic nerve before disappear- ing deeply into the longitudinal fi ssure separating the two cerebral hemispheres. Just before entering the fi ssure, the left and right arter- ies may be joined by the short anterior communicating artery. The anterior cerebral arteries then turn to run upwards and backwards above the corpus callosum, across the medial surfaces of the corre- sponding hemispheres as far as the parieto-occipital sulcus. Figure 15.25 indicates that each anterior cerebral artery supplies the medial surface of each hemisphere and overlaps about 2 cm on to the lateral surface.
Each middle cerebral artery is larger than the anterior cerebral artery. Each artery passes laterally into the lateral fissure to sup- ply the lateral surface of each cerebral hemisphere up as far as the area supplied by the anterior cerebral artery and down as far as the upper surface of the temporal lobe, an extensive area as you can see in Figure 15.25 . Although there is no obvious boundary line on
the lateral surface, the middle cerebral artery is distributed only as far as a line corresponding to the parieto-occipital sulcus on the medial surface.
The vertebrobasilar system
The vertebral arteries are branches of the subclavian arteries. Each one ascends through the neck in the foramina in the transverse proc- esses of the upper six cervical vertebrae, winds around the lateral mass of the atlas forming the articulation with the underside of the skull, and enters the cranial cavity through the foramen magnum.
The intracranial course and distribution of the vertebral arteries is illustrated in Figure 15.24 . The vertebral arteries run up alongside the medulla and join together at the lower border of the pons to form the single midline basilar artery . It ascends to the upper border of the pons where it terminates at a T-junction by dividing into the right and left posterior cerebral arteries . These arteries turn back between the inferior surface of the temporal lobes and the cerebellum and as shown in Figure 15.25 , they supply most of the temporal lobes and all of the occipital lobes. The posterior perforated substance lies above the bifurcation of the posterior cerebral arteries in the interpeduncular fossa of the midbrain (see Figure 15.10 ). Small perforating arteries enter here to supply deeper structures, in this case, the thalamus and poste- rior areas of the basal ganglia.
As you can see in Figure 15.24 , the vertebral and basilar arteries have several important branches as they travel from the foramen magnum to their termination.
The vertebral arteries give rise to:
• Spinal branches which pass downwards into the vertebral canal to
supply the spinal cord;
136 The structure of the central nervous system
• A posterior inferior cerebellar artery to the cerebellum and lat-
eral aspects of the medulla;
• Short medullary branches to the anterior part of medulla oblongata
immediately either side of the midline.
The basilar artery also has important branches:
• The anterior inferior cerebellar arteries arise from either side of
the artery just after its formation;
• Pontine branches arise as it passes over the pons together with a
labyrinthine branch to the internal ears;
• The left and right superior cerebellar arteries branch off just
before the basilar artery terminates as the posterior cerebral arteries.
Note that the anterior, inferior, and superior cerebellar arteries not only supply the cerebellum as their names imply, but also the lateral aspects of the pons.
The arterial circle (of Willis)
The complete arterial circle as shown in Figure 15.26 is formed by the anterior communicating artery between the anterior cerebral arter- ies and the posterior communicating arteries between the internal carotid arteries and posterior cerebral arteries. The communicating ves- sels are small and the circle is only complete in about 60% of people.
Even if all the communicating branches are present, they are usually too small to maintain adequate circulation to the brain if one or other of the major arteries entering the brain is suddenly blocked. They are, how- ever, capable of expanding if blockage occurs more slowly; adequate cerebral blood fl ow may be maintained even if one or more of the feed- ers into the arterial circle should become completely occluded. It is interesting that cerebral angiograms show that there is little or no fl ow through the communicating arteries in normal subjects; a tracer dye injected into the vertebrobasilar system through the subclavian artery will only enter the brainstem, cerebellum, and areas of the cerebrum supplied by the posterior cerebral arteries. Likewise, an injection into one internal carotid artery will be limited to the anterior and middle cerebral arteries on the side of the injection only.
15.6.2 The veins
The veins returning blood from the brain are extremely thin-walled because they lack a muscular layer; they also have no valves. They open into the venous sinuses of the dura mater. The cerebral veins comprise two groups, external and internal, draining the superfi cial and deep parts of the cerebral hemispheres, respectively. The veins draining the cerebellum and brainstem open into adjacent venous sinuses adjacent to these structures.
Fig. 15.26 An enlarged view of the arterial supply of the brain seen from above showing the arterial circle of Willis and branches of the vertebral and basilar arteries.
Anterior cerebral artery Anterior
communicating artery
Posterior communicating artery Posterior cerebral artery
Posterior inferior cerebral artery
Superior cerebellar artery Internal carotid artery
Anterior inferior cerebellar artery Basilar artery
Vertebral artery
Pontine arteries Labyrinthine artery
Spinal arteries Middle cerebral artery
Posterior cerebral artery Middle cerebral artery Anterior cerebral artery
Fig. 15.25 The distribution of the cerebral arteries to the cerebral hemispheres. A) Lateral view; B) Medial view.
Blood supply of the brain 137
Box 15.10 Cerebrovascular accident (CVA) or ‘stroke’
The thin-walled arteries of the brain are prone to rupture ( aneu- rysm ) as a result of degenerative changes in their walls. In addi- tion, the arterial branches that actually penetrate into the brain tissue are relatively small and are, therefore, particularly liable to thrombosis. These events are referred to as a cerebrovascu- lar accident (CVA) or ‘stroke’ in lay terminology because the cause of blood loss to a particular area of the brain is not usually known immediately. Irrespective of the actual cause of blood loss to the brain, it is important to emphasize that lesions of this sort respect neither anatomical nor functional boundaries. In other words, they can spread extensively and adversely aff ect quite wide areas.
This is more the case with aneurysm than thrombosis because blood escapes from the cardiovascular system ( extravasation ).
When blood makes direct contact with neural tissue, it is extremely toxic. The high molecular weight materials that are required for blood functions such as albumin and haemoglobin actually dehy- drate neural tissues. The initial signs and symptoms of ‘stroke’ are often far worse than the long-term results because extravasated blood is damaging tissues. Such damage is reversible; these areas may well recover when natural mechanisms of blood removal and clot resolution have taken place. On the other hand, any brain area that does not receive a blood supply starts to degenerate irrevers- ibly very rapidly. Thus the areas distal to the site of a blood clot or aneurysm in an artery will be deprived of blood and nutrients and will be unlikely to recover.
When assessing the likely consequence of a CVA, it is, therefore, important to determine which functional areas of the brain may
have been aff ected. For example, a CVA in the middle cerebral artery may possibly lead to degeneration of most of the motor, sensory and auditory cortices, and Broca’s and Wernicke’s areas, depending on how far from its origin the lesion is sited. Loss of these cortical areas would lead in turn to spastic paralysis of arms, trunk and head, sensory loss over the same areas, hearing problems, and problems interpreting or encoding language.
The arterial blood supply to the internal capsule is by perforat- ing striate arteries that branch from the middle and posterior cerebral arteries; these arteries are small and prone to throm- bosis. If the subject survives, which is more likely with a throm- bosis than with a haemorrhage, there will be some destruction of descending and ascending pathways in the internal capsule.
The actual effects will depend on the extent of the lesion and its location in the internal capsule, but motor and sensory deficits are likely. As will be described in more detail in Chapter 16 , the effects will be contralateral if ascending or corticospinal path- ways are damaged, but the effects are bilateral in most instances if corticonuclear pathways are affected by a lesion close to or in the genu.
In the brainstem, CVA in one or other of the cerebellar arteries is likely to produce some degree of cerebellar ataxia. However, these arteries also supply the lateral aspects of the brainstem and the nuclei in which cranial nerves originate or terminate will also be aff ected. The eff ect on specifi c cranial nerves depends on the level of the lesion. For example, a posterior inferior cerebellar CVA would be most likely to aff ect the lower ninth, tenth, and twelfth cranial nerves arising from the lower medulla.
16
Major sensory and motor
systems
Chapter contents
16.1 Introduction 139
16.2 General sensory pathways 139
16.3 Motor pathways 146
General sensory pathways 139
The previous chapter provided an overview of the anatomy of the CNS, concentrating on structures that can be seen during dissec- tion of the human brain and spinal cord or the study of anatomical models of these structures. Some indication of the function of diff er- ent components of the CNS has been given in Chapter 15 , but this chapter shows how the various anatomical components of the CNS are functionally linked together through sensory and motor pathways.
These pathways enable the nervous system to convey information over considerable distances, to integrate the information, and formulate functional responses that coordinate activities of diff erent parts of the body. It will be necessary to introduce some other structures in addi- tion to those described in Chapter 15 during the description of major pathways; most are not visible to the naked eye and even when seen
in microscopical sections, they require considerable practice to distin- guish them. However, they are important landmarks or relay stations in the central nervous pathways and you need to know of them for a full understanding of pathways.
As emphasized in Chapter 14 , our views of the structure and func- tion of many aspects of the nervous system are constantly subject to revision in the light of new clinical and experimental observations and methods of investigation. This applies to nerve pathways just as much as any other aspect of the nervous system. This chapter presents a sum- mary of current views on somatic sensory and motor functions and their application to the practice of dentistry. The special sensory pathways of olfaction, vision, and hearing are described in Chapter 18 in the context of the cranial nerves that form the fi rst part of these pathways.