Chapter 2

Essential Anatomy and Function of the Brain

Abstract

This chapter provides details as an overview of the brain. First, it will provide a summary of the components of the skull for orientation and provide an overview explanation for the position of key components of the brain. Then the basic facts about the brain and its composition including key facts and figures, as well as an introduction to the key terminology used in relation to neural tissue will be provided. It also introduces, in a little more detail, the key cellular components of the nervous system, namely the neuron and the supporting glial tissue.

It introduces the term forebrain, midbrain and hindbrain. In addition, it also discusses the individual components of these areas of the brain according to the classification of telencephalon, diencephalon, mesencephalon, myelencephalon and metencephalon. It briefly introduces the components of this classification. A summary table provides a quick glance at the major regions of the brain, and their related functions. In addition, the chapter also discusses the anatomy of the ventricular system, which circulates the cerebrospinal fluid, cushioning the brain.

Keywords

brain anatomy neuron forebrain midbrain hindbrain

2.1. Introduction

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. The skull is composed of a number of individual bones, but is best appreciated as a totality. It can conveniently be divided into three parts. (1) The cranium which is in turn divided into (a) the neurocranium, essentially a rounded container for the brain and special senses, and (b) the viscerocranium, the irregularly shaped part at the front which provides a skeletal framework for the nasal and oral cavities. (2) the mandible The neurocranium consists of the rounded vault of the skull and a rather irregular floor, the base of the skull. The bones of the vault are large and easily identified. It is comprised of the frontal bone, two parietal bones and the occipital bone and the two temporal bones at the side. The bones meet (articulate) at wavy lines called sutures. These are the coronal, sagittal and lambdoid sutures. These are immobile, fibrous joints. The newborn baby has gaps between many of the bones called fontanelles (the "soft spots") where the bony plates have yet to grow and meet. The bones of the vault actually develop within a membranous sheet and it is this sheet which fills the interval. The anterior fontanelle is a particularly large, diamond-shaped space which does not fully close until two years after birth. If the baby is dehydrated, the surface of the head appears to sink in at this point. The frontal, parietal and much of the occipital bones are covered in life by the scalp, which can be moved backwards and forwards by the occipito-frontalis muscle. The scalp has a rich blood and nerve supply and is such a dense, taut structure that, when wounding occurs, the cut ends of blood vessels tend to pull apart. Thus, scalp wounds bleed profusely and injury often appears worse than it really is. The sides of the neurocranium are chiefly composed of the temporal bone. This is an extremely complex bone and it is chiefly the flattened squamous part which can be seen from the outside of the skull (a projection from this makes a contribution to the zygomatic arch (cheek bone). In life the temporal bone gives origin to (and is covered by) the temporalis muscle. The occipital bone can be followed downwards to form part of the base of the skull. This ventral part of the bone has two important features (i) the foramen magnum ("big hole") through which the spinal cord and brain are continuous and (ii) the rounded occipital condyles, by which the skull articulates with the first cervical vertebra - the atlas. The floor of the interior of the skull of the neurocranium has three levels or "steps", the highest at the front and the lowest at the back. These are the floors of the three cranial fossae (anterior, middle and posterior). The anterior cranial fossa houses the frontal lobes of the brain and is chiefly formed by the frontal bone, especially the orbital plate (which, as its name implies, forms the roof of the orbit). Behind this is the lesser wing of the sphenoid and, in the midline, the cribriform (sieve-like) plate of the ethmoid bone; this is perforated by many holes which transmit olfactory nerves from the nasal cavity which lies beneath it. The floor of the middle cranial fossa is chiefly formed by the sphenoid bone (which lies in the central part of the fossa) and the temporal bone (which lies at the side). For the sphenoid bone, it comprises the greater wing and the body; the latter has a concave region where the pituitary gland is situated called - rather fancifully - the sella turcica ("turkish saddle"). There are a number of foramina in (or at the edges of) the sphenoid bone which transmit cranial nerves. On each are: (1) the optic canal (optic nerve and ophthalmic artery); (2) the superior orbital fissure (ophthalmic veins, nerves to extraocular muscles, ophthalmic division of trigeminal); (3) the foramen rotundum (maxillary division of trigeminal); and (4) the foramen ovale (mandibular division of trigeminal). The floor of the posterior cranial fossa is chiefly formed by the occipital bone. Also present are: (1) the jugular foramen (through which the glossopharyngeal (IX), vagus (X) and accessory (XI) nerves leave the skull, together with the sigmoid venous sinus which forms the internal jugular vein) and (2) the hypoglossal canal (through which the hypoglossal nerve leaves the skull). The brain is at the center of the nervous system for all invertebrate animals and most invertebrate ones too. The anatomy of the human body has been studied since the Egyptian times (1700BC). The first major piece of scientific written work was the Edwin Smith Papyrus and was a surgical text of 48 cases. It included details on injuries, clinical examination, diagnosis, treatment and also prognosis. This was such a significant text because it did not depend on supernatural belief, but rather allowed for a comparative anatomical examination of these features (NIH, 2014). Since that time, there have been many major contributors to anatomical knowledge and understanding throughout the passage of time. Hippocrates of Cos, during the fourth century BC, provided more evidence for our understanding of the musculoskeletal system and endeavored to enhance our appreciation of the works of the kidney. Then into the fourth century BC, Aristotle changed our viewpoints on many aspects of science due to the philosophical presentation of his thoughts and understanding. Moving into the third century BC, Herophilos was then viewed as the first anatomist and performed his research on cadavers. He was the first to develop our understanding of the brain as it is today anatomically. He differentiated between the cerebrum and cerebellum, and hypothesized that each part of the brain performed different roles. He also was the first individual to try to determine the function of the optic and oculomotor nerves, and related them to a role in vision, something true to this day. Nowadays, and with the advance of the field of neuroscience, our understanding of the nervous system has been enhanced due to a variety of techniques employed in the laboratory, e.g. confocal microscopy, stereotaxic injections, tract-tracing studies and immunocytochemistry. The human brain has been defined as an incredibly complex organ, and we are still gaining new evidence in relation to its functions, and what happens pathologically. The "average" human adult brain comprises approximately 2% of body weight, and its weight can range from 1.2 to 1.4 kg. The brain is an incredibly demanding organ, and is essential for life and function of our body. As such, it consumes an enormous amount of the circulating blood volume. Approximately one-sixth of all cardiac output passes through the brain at any one time, and it uses about one-fifth of all the oxygen in the body when we are at rest. The brain is our most complex organ and controls and regulates our body, responds to stress and threat, and controls higher cognitive functions. It maintains body temperature, allows us to interpret the special senses, and to socially interact. It ensures the body works optimally in the environment we are in both protecting and nurturing the human body.

2.1.1. Neurons

The brain is composed of two main cell types - neurons and supporting glial cells. Neurons are cells which are electrically excitable and transmit information...