Neuroanatomy for the Head and Neck Surgeon

28 Neuroanatomy for the head and neck surgeon Property of Taylor & Francis Group - Not for redistribution PETER C. WHITFIELD Overview and embryology 279 Optic nerve and chiasm 284 The meninges 280 Internal carotid artery 284 Topography of the brain 280 Cavernous sinus 284 Blood supply 280 Facial nerve 284 Surgical hazards 283 Lower cranial nerves, the foramen magnum Venous sinuses, meningeal vessels and and clivus 285 bridging veins 283 Summary 285 Olfactory tract 284 Further reading 285 OVERVIEW AND EMBRYOLOGY The sagittal, coronal and lambdoid sutures are of major importance, with premature fusion leading An estimated 100 billion neurones are organized to craniofacial deformity. The skull base is formed into the human brain. The 1400 g of tissue com- by endochondral ossification of cartilage leading prising the brain is arranged in a highly complex to formation of the skull base components of the − series of neural networks receiving 750mL/min 1 ethmoid, sphenoid, petrous and occipital bones. of blood. The brain develops from the cephalic The bones of the face are mainly formed from car- end of the neural tube. Three primary vesicles tilages of the first two pharyngeal arches with the form giving rise to the forebrain, midbrain and associated musculature supplied by the mandibu- hindbrain. These give rise to the cerebral hemi- lar and facial nerves, respectively. spheres, thalamus, hypothalamus, midbrain, Common disease processes include neoplas- pons, medulla and cerebellum. The brain com- tic, vascular and traumatic conditions. Surgical municates with the head and neck structures via access to the brain is achieved directly through the cranial nerves; most of these arise from the the calvarium, or via the skull base or a combi- brainstem. Connections with the limbs and trunk nation of approaches. No part of the skull, brain are via the afferent and efferent spinal cord path- or surrounding tissues is immune to pathological ways. The calvarial part of the skull, or skull vault, processes, with many conditions requiring col- is formed by membranous ossification of mes- laborative surgical teams encompassing neuro- enchyme that invests the embryonic brain. The surgery, maxillofacial surgery, otolaryngology and bones of the newborn skull are joined by sutures. plastic surgery. 279 K18222.indb 279 30/09/15 12:19 pm 280 Neuroanatomy for the head and neck surgeon THE MENINGES The brain is invested in a layer of pia mater that is adherent to the gyri and sulci. The subarachnoid space lies between the pial layer and the arachnoid. The arachnoid layer is a transparent membrane that envelops the brain. Cerebrospinal fluid (CSF) cisterns are arachnoidal sacs of CSF that are found in close proximity to the cranial nerves at the skull base. They lie between the arachnoid membrane and the pial surface. Microsurgical opening of the CSF cisterns facilitates drainage of CSF and visual- ization of the vessels and nerves in the area under consideration. TheseProperty cisterns are evident in life but are not well visualized in cadaveric specimens. The dura mater is a thick, fibrous membrane that Lateral view of the right cerebral lines the inner surface of the skull. Folds of dura Figure 28.1 hemisphere. From this view note the Sylvian form structurally important partitionsof within the Taylor (frontotemporal) fissure lying between the frontal cranial cavity. The falx cerebri is a sickle-shaped and temporal lobes. Note the cerebellar hemi- dural membrane located in the midline sagit- sphere inferiorly. (Supplied by Dr D. Hilton.) tal plane, separating the cerebral hemispheres.& Anteriorly, it is attached to the crista galli. It Francisarches over the corpus callosum to a posterior attachment the expressive component of speech. Temporal lobe at the internal occipital protuberance. At this level it functions include memory and auditory processing forms the tentorium cerebelli, dividing the cranial Groupincluding the receptive component of speech. The cavity into the supra and infratentorial compart- parietal lobe is involved in the perception of touch ments. The cerebellum lies beneath the tentorium and the integration of sensory information, and - cerebelli. The superior sagittal venous sinus lies the occipitalNot lobe is responsible for visual process- within a dural enclosed fold along the superior ing. Deeper structures include the basal ganglia, aspect of the falx cerebri. The inferior sagittal sinus thalamus forand hypothalamus. The basal ganglia lies in the free inferior margin of the falx cerebri. contribute to movementredistribution control. The thalamus is a key relay station in sensory and circuits and contrib- TOPOGRAPHY OF THE BRAIN utes to movement control. The nearby hypothalamus is a centre for autonomic nuclei that subserve The brain comprises the cerebral hemispheres which sympathetic and parasympathetic functions and is lie in the supratentorial compartment (Figures 28.1 directly connected to the posterior lobe of the pitu- and 28.2). They are connected by the commissural itary gland. The adjacent anterior pituitary is cru- fibres of the corpus callosum. The cerebellum and cial in orchestrating the body’s hormonal milieu, brainstem (midbrain, pons, medulla) are located in secreting growth hormone, thyroid stimulating the infratentorial compartment. The midbrain lies hormone, adrenocorticotrophic hormone, gonado- at the level of the tentorial hiatus acting as a conduit trophic hormones and prolactin. The cerebellum is for information between the hemispheres and the concerned with the control of posture and muscle brainstem. Most of the cranial nerves originate from coordination. the brainstem. The hemispheres are subdivided into the frontal, temporal, parietal and occipital lobes. BLOOD SUPPLY The different lobes undertake important functions; language functions are usually lateralized to The arterial blood to the brain is supplied by the dominant hemisphere. Frontal lobe functions bilateral internal carotid and bilateral vertebral include motor, personality, executive functions and arteries. These supply the anterior and posterior K18222.indb 280 30/09/15 12:19 pm Blood supply 281 Property Figure 28.2 Topography of the brain – the cerebellum and brainstem. The cerebellum has been divided in the sagittal planeof and splayed open like the pages of a book. ThisTaylor demonstrates the arboreal architecture of the cerebellum. Figure 28.3 Circle of Willis. This is a 3D comput- Note the cerebellar tonsil projecting inferiorly. erized tomography angiogram. In the posterior This impacts against the craniocervical junction& circulation, the vertebral arteries unite forming if ‘coning’ occurs. The lower panel illustratesFrancis the basilar artery. Anteriorly, the bilateral internal axial sections of the midbrain, pons and medulla carotid arteries bifurcate into the medially pro- (from left to right). In the midbrain note the jecting anterior cerebral arteries and the laterally projecting middle cerebral arteries. The distal small diameter of the Aqueduct of Sylvius, the Group pigmented substantia nigra and the cerebral segments of the anterior cerebral arteries travel peduncles projecting anteriorly. The pons is char- in the interhemispheric plane in close apposition acterized by the prominent transverse pontine to each- other. In this case the posterior commu- fibres – these are fibres connecting the motor nicatingNot arteries are not well visualized. (Supplied cortex to the cerebellum allowing the cerebellum by Dr W. Mukonoweshuro.) to modulate coordination. The medulla contains for several cranial nerve nuclei subserving autonomic redistribution functions. Compression of these during coning branch that often supplies the descending motor ultimately causes brainstem death. (Supplied by fibres located within the confines of the internal Dr D. Hilton.) capsule. The internal carotid artery then bifurcates into the medially directed anterior cerebral artery circulations, respectively. The posterior commu- and the laterally directed middle cerebral artery. nicating arteries arise from the posterior aspect of The anterior cerebral artery crosses the optic nerve the intracranial component of the internal carotid and then abruptly turns in an anterosuperior arteries and join the posterior cerebral arteries direction to supply the frontomedial aspect of the providing an anastomotic link (of variable degree) cerebral hemispheres via the frontobasal, perical- between the anterior and posterior circulations. losal and callosomarginal terminal branches. At This was eloquently described as the Circle of Willis the point of inflection, the anterior communicat- in the seventeenth century (Figure 28.3). On entry ing artery, a short (2 mm) branch, provides a direct into the skull, the internal carotid artery (Figure communication between the right and left anterior 28.4) traverses the cavernous sinus. The ophthal- cerebral vessels. This is a key component of the mic artery is a small branch that arises at the point Circle of Willis. The middle cerebral artery trav- of emergence into the subarachnoid space. The els in the frontotemporal (Sylvian) fissure. After next branch is the posterior communicating artery a few centimetres it bifurcates into end arteries followed by the anterior choroidal artery: a small supplying the bulk of the cerebral hemispheres. K18222.indb 281 30/09/15 12:19 pm 282 Neuroanatomy for the head and neck surgeon Perforating branches arise from both the anterior and middle cerebral vessels supplying the deep structures

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