Neuroanatomical terminology and conventions

The formal names given to parts of the body are agreed internationally and have been published as the Terminologia Anatomica (1998; Thieme) by the Federative Committee on Anatomical Terminology (Fig. 1.1). The names of many structures in the nervous system have a Latin or Greek origin and are unique to neuroanatomy. Often the name is descriptive of some perceived physical characteristic, such as shape (e.g. ‘hippocampus’, meaning seahorse) or colour (e.g. ‘substantia nigra’, meaning black substance). Some structures are known by eponyms, usually in recognition of the person who first described them or whose work on them was particularly prominent (e.g. circle of Willis; foramen of Monro). Many of the latter names are considered to be anachronistic, but some remain in common usage.

In anatomy generally, the location and relationships of structures are described with reference to three orthogonal planes: sagittal or median, horizontal or transverse (called axial in radiology) and coronal or frontal (Fig. 1.1). Directions and relationships are designated as medial or lateral, superior or inferior and anterior or posterior, with reference to the orientation of these planes. There is, however, an additional terminological complication when describing the brain and spinal cord, as explained below.

In neuroanatomy, the positional/directional terms: rostral, caudal, dorsal and ventral, are also commonly used. These terms have their origin in embryology and mean, respectively, towards the head end (rostral), tail end (caudal), back (dorsal) and belly (ventral). If the long axis of the brain and spinal cord were to remain in a straight line during embryological development then, in the adult, rostral would simply equate to superior, caudal to inferior, dorsal to posterior and ventral would equate to anterior. For all intents and purposes, this is what happens with the spinal cord but the long axis of the brain, on the other hand, undergoes considerable distortion and, in particular, the brainstem becomes flexed at several points (Fig. 1.12). In neuroanatomy, therefore, it is customary to use the terms that are common to all anatomy when describing a position in space and to reserve the terms ‘rostral, caudal, dorsal and ventral’ for the description of location and direction relative to the long axis of the nervous system.

In neuroanatomy, horizontal or transverse sections through the spinal cord and lower part of the brain (brainstem) are usually depicted/orientated with dorsal at the top and ventral at the bottom (Fig. 1.2A). The convention in neuroradiology, on the other hand, is that axial images are orientated as if looking from the subject’s feet towards the head, with anterior at the top of the image. In sections that contain the brainstem, therefore, the dorsal aspect of the brainstem is towards the bottom of the image and the ventral aspect is towards the top. This convention means that left and right are also reversed (Fig. 1.2B).

The basic structural and functional unit of the nervous system is the nerve cell or neurone (Figs 1.3, 1.4), of which the human nervous system is estimated to contain about 10¹⁰. The functions of the neurone are to receive and integrate incoming information from sensory receptors and other neurones and to transmit information to other neurones or non-neural structures that are under neural control (effector organs). Neuronal structure is highly specialised to fulfil these functions. Each neurone is a separate physical entity with a limiting cell membrane. Information is passed between neurones at specialised regions called synapses where the membranes of adjacent cells are in close apposition (Fig. 1.3).

Central and peripheral nervous systems

At a simple anatomical level, the nervous system (Fig. 1.5) is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The central nervous system consists of the brain and spinal cord, lying within the protection of the cranium and vertebral column, respectively. It is the most complex part of the nervous system, containing the majority of nerve cell bodies and synaptic connections. The peripheral nervous system constitutes the link between the CNS and structures in the periphery of the body, from which it receives sensory information and to which it sends controlling impulses. The peripheral nervous system consists of nerves joined to the brain and spinal cord (cranial and spinal nerves) and their ramifications within the body. Spinal nerves serving the upper or lower limbs coalesce to form the brachial or lumbar plexus, respectively, within which fibres are redistributed into named peripheral nerves. The PNS also includes many peripherally located nerve cell bodies, some of which are aggregated within structures called ganglia.

Afferent neurones, efferent neurones and interneurones

Nerve cells that carry information from peripheral receptors to the CNS are referred to as afferent neurones (Fig. 1.6). If the information they carry reaches consciousness, they are also called sensory neurones. Efferent neurones carry impulses away from the CNS and if they innervate skeletal muscle to cause movement, they are also called motor neurones. The vast majority of neurones, however, are located entirely within the CNS and are referred to as interneurones. The terms ‘afferent’ and ‘efferent’ are also commonly used to denote the polarity of projections to and from structures within the CNS, even though the projections are entirely contained within the brain and spinal cord. The projections to and from the cerebral cortex, for example, are referred to as cortical afferents and efferents, respectively.

Grey and white matter, nuclei and tracts

The CNS is a highly heterogeneous structure in terms of the distribution of nerve cell bodies and their processes (Fig. 1.7). Some regions are relatively enriched in nerve cell bodies (e.g. the central portion of the spinal cord and the surface of the cerebral hemisphere) and are referred to as grey matter. Conversely, other regions contain mostly nerve processes (usually axons). These are often myelinated (ensheathed in myelin), which confers a paler coloration – hence the term white matter.

Nerve cell bodies with similar anatomical connections and functions (e.g. the motor neurones innervating a group of related muscles) tend to be located together in groups called nuclei. Similarly, nerve processes sharing common connections and functions tend to follow the same course, running in pathways or tracts (Fig. 1.7 and see Fig. 1.23).

The process of formation of the embryonic nervous system is referred to as neurulation. During the third week of embryonic development, the dorsal midline ectoderm undergoes thickening to form the neural plate (Figs 1.8, 1.9). The lateral margins of the neural plate become elevated, forming neural folds on either side of a longitudinal, midline depression, called the neural groove. The neural folds then become apposed and fuse together, thus sealing the neural groove and creating the neural tube. Some cells from the apices of the neural folds become separated to form groups lying dorsolateral to the neural tube. These are known as the neural crests. The formation of the neural tube is complete by about the middle of the fourth week of embryonic development.

As development continues, a longitudinal groove, the sulcus limitans, appears on the inner surface of the lateral walls of the embryonic spinal cord and caudal part of the brain (Fig. 1.10A). The dorsal and ventral cell groupings thus delineated are referred to as the alar plate and the basal plate, respectively. Nerve cells that develop within the alar plate have predominantly sensory functions, while those in the basal plate are predominantly motor.

Further development also brings about the differentiation of grey and white matter. The grey matter is located centrally around the central canal, with white matter forming an outer coat. This basic developmental pattern can still easily be recognised in the adult spinal cord (Fig. 1.10B).

Further differentiation distinguishes seven neuronal cell groupings within the alar and basal plates (Fig. 1.11). These are arranged in discontinuous longitudinal columns, based upon their anatomical connections and physiological roles:

During embryonic development, the rostral portion of the neural tube undergoes massive differentiation and growth to form the brain (Fig. 1.12). By about the fifth week, three so-called primary brain vesicles can be identified: the prosencephalon (forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain). The longitudinal axis of the developing CNS (neuraxis) does not remain straight but is bent by a midbrain or cephalic flexure, occurring at the junction of midbrain and forebrain, and a cervical flexure between the brain and the spinal cord.

Some of the names of the embryological subdivisions of the brain are commonly used for descriptive purposes and it is, therefore, useful to know the parts of the mature brain into which they subsequently develop (Table 1.1). Of the three basic divisions of the brain, the prosencephalon or forebrain is by far the largest. It is also referred to as the cerebrum. Within the cerebrum, the telencephalon undergoes the greatest further development and gives rise to the two cerebral hemispheres. These consist of an outer layer of grey matter (the cerebral cortex) and an inner mass of white matter, within which various groups of nuclei lie buried (the largest being the corpus striatum). The diencephalon consists largely of the thalamus, which contains numerous cell groupings and is intimately connected with the cerebral cortex. The mesencephalon, or midbrain, is relatively undifferentiated (it still retains a central tube-like cavity surrounded by grey matter). The metencephalon develops into the pons and overlying cerebellum, while the myelencephalon forms the medulla oblongata (medulla). The medulla, pons and midbrain are collectively referred to as the brainstem (Fig. 1.13).

Table 1.1

Embryonic development of the brain

As the brain develops, its central cavity also undergoes considerable changes in size and shape, forming a system of chambers or ventricles (Fig. 1.13 and see Fig. 1.22), which contain cerebrospinal fluid.

Parallels have been drawn between the embryological development of the brain and the major changes that the brain has undergone during ascent of the phylogenetic, or evolutionary, scale from simple to more complex animals. While this is certainly an oversimplification, the concept does have the educational merit of introducing some of the principal parts of the brain, and their relationships to one another, in a graphic and memorable way (Fig. 1.13).

The simplest of chordate animals (e.g. amphioxus), from which the vertebrates evolved, possess a dorsal tubular nerve cord that is reminiscent of the neural tube of the developing mammalian embryo. During phylogeny, the rostral end of the tubular nervous system has undergone enormous modification and change; consequently, the adult human brain bears little obvious similarity to its evolutionary ancestors.

Regional specialisation has been an important theme in the evolution of the brain and this is especially obvious in relation to the senses and in movement control. Long ago in phylogeny, centres devoted to these functions developed as expansions or outgrowths from the dorsal aspect of the simple tubular brain (Fig. 1.13). In form, they consisted of an outer cortex of nerve cell bodies with an underlying core of nerve fibres. Bilaterally paired centres developed in relation to the senses of smell, vision and hearing, and a symmetrical, midline centre developed in association with vestibular function and the maintenance of equilibrium. Each of these centres underwent subsequent evolutionary change, but this was most evident in the rostral, ‘olfactory’, part of the brain, which developed into the massive cerebral hemispheres (Figs 1.14, 1.15). During this process, known as prosencephalisation, the cerebral hemispheres came to take on an executive role in many areas of brain function. For example, the highest level for the perception and interpretation of input from all sensory modalities eventually became localised in the cortical surface of the cerebral hemispheres, as did the highest level for motor control. This is reflected by the fact that only a small proportion of the adult human cerebral hemisphere remains devoted to olfactory function.

The process of prosencephalisation meant that the other integrative centres became progressively subservient to the cerebral hemispheres. For example, those for vision and hearing underwent relatively little further development and fulfil largely automatic, reflex functions in the human brain. They may still be identified, however, as four small swellings on the dorsal surface of the midbrain: the corpora quadrigemina or superior and inferior colliculi (Figs 1.13–1.15). The motor centre near the caudal end of the brain developed into the cerebellum (Figs 1.13–1.15), which retains a central role in the maintenance of equilibrium and the coordination of movement.