Essential Clinical Anatomy of the Nervous System Paul Rea
One area that has proved challenging to gain a greater understanding in medicine, surgery, dentistry, related health professions and the students studying within those specialties, has been the nervous system.
All too often, resources available in the field of anatomy, neurology and the neurosciences for the study of the nervous system are frought with complex detail and pathways which can appear daunting.
Therefore, the purpose of this textbook is to provide the key facts, in an easy to access format, for each of the major components of the nervous system without over complicating the anatomy. It takes the reader through, in a step-by-step process, the basics of the nervous system and the key divisions from both the structural and functional perspective.
Then, an overview of the essential antomy and function of the brain is provided. Following on from this, each of the brain regions are dealt with in turn in a little more detail namely the forebrain, midbrain and hindbrain.
A summary on each of the major divisions on the arterial supply and venous drainage of the brain is given, followed by the key relevant facts and figures related to the spinal cord.
The final two chapters are devoted to the ascending and descending tracts that occur with the brain and spinal cord, and also the key areas of clinical relevance that communicate with other regions of the brain and in and around the head and neck.
What differs with this text is that rather than copius amounts of de-tail, the essential anatomy is presented, frequently in tables. That way the key facst about each territory can be accessed immediately without having to refer to numerous parts of the text continuously.
Another key feature of this text lies in the fact that when each area is dealt with, the pertinent clinical applications are discussed and frequent-ly hints and tips on how to examine these clinically in the patient. This means that all key information is readily accessible in the same region.
The combination of ensuring the relevant detail is provided without over-complicating the matter, combined with the clinical applications and details on how to clinically examine each region makes this text incredibly unqiue.
It is hoped this text will be the perfect revision tool for upcoming professional examinations, to have at the bedside, surgery or outpatient department, or simply to have at hand for quick access to the key anat-omy, clinical applications and reminders of how to examine a patient’s neurological system.
I really do hope you enjoy this book and find it a great companion during your studies and professional life.
Introduction to the Nervous System
OVERVIEW OF THE NERVOUS SYSTEM
The nervous system is comprised of two parts: the central nervous sys-tem (CNS) consisting of the brain and spinal cord, and the peripheral nervous system (PNS), consisting of peripheral and cranial nerves as well as the motor and sensory nerve endings of those nerves.
Central Nervous System
The cells of the brain and spinal cord are called neurons (or neurones) and are specialized cells which can electrically conduct and communicate with nearby neurons. The purpose of these cells is to allow communication of the surrounding environments to adjacent cells to help regulate the internal environment of the body, and to respond to external stimulations. Supporting “glue” called neuroglia also holds the neuronal tissue together. Neuroglia provides nutrients to the neurons, maintains electrochemical stability, but also helps to defend the nervous system environment from attack by pathogens.
Brain
The brain is a mass of convoluted neural tissue and is referred to as the cerebrum (Latin: brain). The brain is a complex organ consuming approximately 15% of cardiac output, and can only survive a few min-utes deprived of oxygen. If it is deprived of oxygen, death will ensue quickly. It is comprised of two cerebral hemispheres – left and right, and together these arise from the embryologic telencephalon. The cerebral hemispheres process information related to a wide variety of functions, and will be dealt with separately in later chapters. Between these two massive cerebral hemispheres lies the diencephalon. This is the thalamus and hypothalamus. The telencephalon and the diencephalon together form the forebrain – the first part of the brain.
Essential Anatomy and Function of the Brain
The skull is a logical consequence of the process of cephalization – an evolutionary trend in which multicellular organisms have developed an elongated, cylindrical body with a leading (front) and trailing (back) end. There are clear advantages in grouping sense organs at the front, and also in making the front end the site of ingestion (food, air and water); to cope with these sense organs, the nervous system enlarges at the front (i.e. a brain is developed) and there is a simultaneous requirement to protect these valuable acquisitions – hence the skull.
Forebrain
The forebrain is comprised of the telencephalon (cerebral hemispheres) and the diencephalon (thalamus and hypothalamus) as previously discussed. This chapter will provide summary tables as to the key functions of each of these territories. First, the telencephalon will be dealt with which will break down the cerebral hemispheres into the lobes (frontal, parietal, temporal and occipital) providing key information about the various regions and as-sociated functions. The subdivisions are classified according to Brodmann’s classification of the areas. This was a wide study which was based on the cytoarchitecture as determined by the Nissl staining method of all of the ce-rebral cortex (Brodmann, 1909). This text was in German, but a translation was developed for this and edited by Garey (2006). The key areas and their functions are detailed below. In addition, the basal ganglia and the limbic system will be dealt with in the same tabulated format. The structures of the diencephalon will be dealt with in a similar manner.
Midbrain (Mesencephalon)
The following is a summary of the main regions of the brain, detailing what comprises the hindbrain (Table 4.1).
(1) Telencephalon (cerebral hemispheres) + Diencephalon (thalamus and hypothalamus) = FOREBRAIN
(2) Mesencephalon = MIDBRAIN
(3) Metencephalon (pons, cerebellum and the trigeminal, abducent, facial and vestibulocochlear nerves) + Myelencephalon (medulla oblongata, plus the glossopharyngeal, vagus, accessory and hypoglossal nerve nuclei) = HINDBRAIN
The midbrain connects the forebrain and the hindbrain. It is found in
the tentorial notch of the dura mater. It is comprised of a ventral part, the cerebral peduncles and the tectum.
The cerebral peduncles are two large bundles which converge from the cerebral hemispheres, and is continuous with the internal capsule. The anterior part of the cerebral peduncles is called the crus cerebri and the posterior portion is referred to as the tegmentum. The superior part of each peduncle is crossed by the optic tract. The left and right optic tracts emerge from the optic chiasma, which is formed by the junction of the two optic nerves. The depression posterior to the chiasma and bounded by the optic tracts and the cerebral peduncles is called the in-terpeduncular fossa.
The interpeduncular fossa contains the tuber cinereum and the infun-dibular stem of the hypophysis, the mammillary bodies and the poste-rior perforated substance. The oculomotor nerve emerges at the superior border of the pons and the medial border of the corresponding cerebral peduncles.
Blood Supply of the Brain and Clinical Issues
6.1 GENERAL INTRODUCTION
The blood supply of the brain arises from two main arteries – the inter-nal carotid and the vertebral arteries. The left and right internal carotid and the left and right vertebral arteries merge on the undersurface of the brain. It is referred to as the Circle of Willis (or circulus arteriosus cere-bri (Willisii) (Symonds, 1955; Eastcott, 1994)). It is named after Thomas Willis who described it after extensive studies of the brain and nerves.
The arterial supply is also classified as anterior and posterior. The anterior cerebral supply arises from the internal carotid arteries and the posterior cerebral supply arises from the vertebral arteries. The posterior cerebral communicating arteries (or posterior communicating arteries) unite these two supplies. Not everyone has a complete Circle of Willis, and approximately only a quarter of the population have a fully intact Circle of Willis (Creasy, 2011; Hines and Marschall, 2012), with collat-eral pathways existing in those without one. The anterior communicating arteries unite the two anterior cerebral arteries.
6.2 INTERNAL CAROTID ARTERY
The common carotid artery arises from different points on the left and right sides of the body. On the left hand side, the common carotid ar-tery arises direct from the aorta and ascends into the neck alongside the phrenic and vagus nerves. As the common carotid artery ascends into the neck, it bifurcates at approximately the level of the third or fourth cervical vertebra into the external and internal carotid arteries. The other way to look at it clinically is that the bifurcation occurs at the level of the upper border of the thyroid cartilage. This part of the internal carotid artery can also be referred to as its cervical segment. At the point of bifurcation, and start of the internal carotid artery, it is at this point where the carotid sinus is found.
