Brain Orientation Difficulties
One of the most daunting tasks for many students is the process of learning neuroanatomy, the anatomy of the nervous system. Understanding how to visualize the brain and orient oneself within it when provided with an image of a single part or region is complicated by the brain’s structural and functional complexity.
The brain is characterized by many components of various shapes and sizes clustered together in close proximity, it’s easy to get lost and confused. In particular, students tend to struggle with three major problems in understanding the structure of the brain and the relationship between its components: the first has to do with the uneven growth of structures during neurodevelopment; the second with defining the neuraxes, which indicate direction in the central nervous system, and how these differ from the axes of similar names that describe direction within the body; and the third with the curved shape of some of the brain’s inner structures and how these can appear in images.
Development of the Brain
Understanding how the brain develops can help clarify certain confusions regarding neuroanatomy. The brain develops in a series of steps with different structures growing at different rates: this results in them being layered within and/or underneath others. Understanding how and where this unequal growth and layering takes place can help to clarify, and enhance one’s memory of, how structures are oriented in relation to one another.
Development of the nervous system begins with the formation of a structure called the neural tube, which develops from a section of endoderm called the neural plate during the third week of gestation. By the fourth week, the rostral end of the neural tube is only one cell thick, but those cells soon begin to proliferate and differentiate, expanding the rostral end of the tube into three primary brain vesicles: the prosencephalon, mesencephalon, and rhombencephalon. These are the primordial forebrain, midbrain, and hindbrain respectively.
In the fifth gestational week, these three vesicles become five as the prosencephalon divides into the telencephalon (the cerebral hemispheres) and diencephalon (the thalamus, hypothalamus, and associated structures), and the rhombencephalon divides into the metencephalon (the pons) and myelencephalon (the medulla oblongata).
In the sixth week, the corpus striatum starts to form at the base of each cerebral hemisphere, and by the fourth month of development, these hemispheres have grown significantly, folding over and burying beneath them the diencephalon, mesencephalon, metencephalon, and myelencephalon. The cerebral hemispheres continue to grow until they meet in the midline, at which point they flatten along the medial surface. The cortices also end up covering the external surface of the corpus striatum, which grows at a much slower rate. This results in a region of buried cortex in the developed brain known as the insular lobe, and it can be found at the base of a lateral sulcus, or infolding, called the Sylvian fissure.
Anatomical Position and the Neuraxes
For many, describing the brain, its orientation to the body, or the orientation of structures within it requires learning a whole new language. The terminology used to describe anatomical positioning is not nearly as simple and straightforward as the colloquial language used in the everyday orientation of ourselves to our surroundings, like “forward” and “backward” or “above” and “below.” The terms used to describe orientation within our own bodies refer to specific bodily regions. If an organ or body part is rostral to a second organ or body part, it means it is closer to the nose than the second body part. If an organ or body part is caudal to a second organ or body part, it means it is closer to the tail (or tailbone, in the vast majority of humans) than the second body part. If an organ or body part is ventral to another part, it means that first part is closer to the stomach; and if it is dorsal to another part, that first part is closer to the back. The orientation of organs and body parts are typically described using two major axes:
- the rostral/caudal axis
- the ventral/dorsal axis
Although these terms seem straightforward enough, their application can be surprisingly confusing, especially in humans. This is because, rather than having a simple linear association, the cerebral hemispheres form an angle with the spinal cord in human anatomical (standing) position: this angle formed between the brain and the spinal cord results in the neuraxes (the axes used to describe positioning within the brain) differing from the body axes.
Understanding the nuances of positioning in a human is often easier if one first considers a simpler situation in which the the body axes and neuraxes match, such as in a quadruped like a cat or a dog. In a quadruped, which walks on all fours, the torso is oriented parallel to the ground. Rostral is in direction of nose, and caudal is in direction of tail, and this is true for both the head and torso: the rostral/caudal neuraxis is horizontal and continuous from the nose to the neck and down the spine through the torso to the tail, as if to follow an imaginary straight line from the nose to the tail. Similarly, in a quadruped, dorsal is in the direction of the back, and ventral is in the direction of the abdomen, and this is true for the head and torso: the ventral/dorsal neuraxis is vertical and continuous, as if to follow an imaginary straight line from the abdomen to the back at any given location along the body of the quadruped.
In contrast, the anatomical position of a human differs from that of a quadruped: a human stands upright, with the body oriented vertically instead of horizontally, making it perpendicular to the ground rather than parallel. The human head, however, still faces forward just as it does in a quadruped. Changing the orientation of the body without changing the orientation of the head leads the axes of the head to differ from the axes of the body: in a human, the axes of the head are effectively reversed compared to axes of the same title in the body.
In the head, forward to the nose is rostral, and backward toward the occiput (the back of the skull) is caudal, leading the rostral/caudal neuraxis to be closer to horizontal. In the body, however, the nose is above the tailbone, so rostral is upward toward the nose and caudal is downward toward the tailbone, making the body’s rostral/caudal axis closer to vertical, almost the opposite of the rostral/caudal neuraxis in the head.
The ventral/dorsal axis is different between the head and body in humans as well. In the head, downward toward the stomach is ventral, and upward toward the top of the head is considered dorsal, so the ventral/dorsal neuraxis in the head is closer to vertical. In the body, ventral is toward the stomach and dorsal is toward the back, so the ventral/dorsal axis in the body is horizontal.
Growth of the Brain: The Four Cs
Viewing the Brain in Two Dimensions
When the brain is imaged, it is viewed in two-dimensional (2D) slices. These are typically viewed in three different planes: the sagittal, coronal, or axial planes.
Sagittal planes are vertically oriented and divide the body into left and right sides. If the plane passes through the exact center of a body, it is called the median sagittal plane and divides the body into right and left halves, with the right side of the face and body and the right arm and leg on one side of the plane, and the left side of the face and body and the left arm and leg on the other side of the plane. Imagine a sagittal view as a 2D image slice of a patient who is facing to one side, so that the view you receive reflects the patient’s side profile.
Coronal planes are vertically oriented and divide the body into front (i.e. anterior) and back (posterior) sections. A coronal slice through the approximate middle of the body would place the face, abdominal region, and the palms of the hands in front of the plane, and the back of the head, torso, and dorsums (i.e. backs) of the hands behind the plane. Imagine a coronal view as a 2D image slice of a patient who is standing in front of you and facing you straight on.
Axial—otherwise known as transverse, or horizontal—planes are oriented perpendicularly to the body, dividing the body into upper (i.e. superior) and lower (i.e. inferior) sections. An axial slice through the approximate middle of the body would place the head, shoulders, upper torso, etc. above the plane, and the lower torso, legs, feet, etc. below the plane. Imagine an axial view as a 2D image slice of a patient who is lying down (parallel to the ground) in front of you, as if you are staring up at the patient from the foot of the patient’s bed.
2D images of the brain can be difficult to decipher for new learners unfamiliar with the nuances in the shapes of some of the brain’s internal structures. Because of the way they are shaped, two parts of the same structure—depending on the plane illustrated in the image—may appear in different locations in the image. This gives the illusion of separate structures, even though they are in fact connected by a part of the structure that simply is not visible in the image slice under consideration.
In the brain, the structures which tend to most confuse students in this way are all curved structures, and as such may be referred to as the 4 Cs. These include parts of the:
- ventricular system (the lateral ventricles)
- basal ganglia (the caudate nucleus)
- limbic system (the hippocampal formation and fornix)
- cingulate system (the cingulate gyrus)
The Lateral Ventricles
As the forebrain develops in the embryo, a pair of diverticula called the telencephalic vesicles arise. These and the cavities within them become the cerebral hemispheres and lateral ventricles. As previously noted, expansion of the cerebral hemispheres is not uniform: the corpus striatum grows very slowly compared to the rest of the telencephalon, eventually becoming covered by the cerebral hemispheres and forming the insular lobe. Since the cortical walls of each cerebral hemisphere grow significantly faster than the floors of the hemispheres, they end up expanding and curving over the diencephalon, midbrain, and hindbrain as well. This unequal growth and curvature over adjacent structures results in the hemispheres and lateral ventricles within them becoming C-shaped. As they continue to grow, the caudal ends of the hemispheres containing parts of the lateral ventricles turn ventrally and then rostrally, leading to the formation of the temporal lobes and temporal ventricular horns.
The C-shape formed by the anterior (upper curve of the C) and inferior (lower curve of the C) horns of the lateral ventricles will be captured differently in 2D slices depending on the plane: while a sagittal view may show the continuous C-shape of the lateral ventricle, certain coronal views may depict parts of the anterior horns well above parts of the inferior horns, making it appear as if the horns are disjointed and therefore two separate ventricles. Understanding that the lateral ventricles are curved with the inferior horn projecting forward below the anterior horn can help students accurately determine when they are seeing two different parts of this otherwise continuous structure in apparently different locations in an image.
The Caudate Nucleus
The basal ganglia contain a number of important structures, including the caudate nucleus, putamen, globus pallidus, subthalamic nucleus, and substantia nigra.
The caudate nucleus is a structure in the forebrain with three main components. The largest part is the head of the caudate, found in the anterior forebrain just rostral to the diencephalon, which extends backwards and narrows as it follows the body of the lateral ventricle down to the inferior ventricular horns. The narrow region of the caudate nucleus in the posterior forebrain where it comes to an end is called the tail of the caudate; the region in between the head and tail is called the body of the caudate. The body and tail of the caudate extend along the dorsolateral surface of the thalamus.
Because of its close association with the lateral ventricles and their inferior horns, curving around other internal structures, the caudate nucleus exhibits a C-shaped structure similar to the lateral ventricles. This means the caudate nucleus may also be viewed as disjointed in certain coronal sections, with the head of the caudate appearing in the superior part of the 2D slice of the brain as if it was a separate structure from the tail of the caudate, part of which can be seen in the same 2D slice but more inferiorly.
The Hippocampal Formation and Fornix
The hippocampal formation is a structure in the ventral part of the brain in the temporal lobe. Arising from the hippocampal formation is a group of fibers comprising a structure called the fornix, the outflow tract of the hippocampus, which transmits information from the hippocampal formation to the septal area. The septal area functions as an extension of the hippocampus by continuing the propagation of the information it receives to various parts of the hypothalamus, including the lateral and medial regions.
The fibers of the fornix arise from the posterior aspect of the hippocampal formation. From there, they extend posteriorly before curving upward and rostrally around the thalamus, ultimately forming a C-shaped structure between the thalamus and corpus callosum in the medial temporal lobe.
The Cingulate Gyrus
The cingulate gyrus can be visualized on the medial surface of each cerebral hemisphere, rostral to the occipital lobe. It is situated just inferior to the pre- and postcentral gyri and premotor cortices, and superior to the corpus callosum. The anterior part of the cingulate gyrus is located in the frontal region of the brain. It curves rostrally and then dorsally, before extending in the caudal direction along the superior surface of the corpus callosum.
Similar to the lateral ventricles, caudate nucleus, and hippocampal formation and fornix, the curved structure of the cingulate gyrus can result in this continuous structure appearing disjointed in certain 2D images.
Complex as it is, there are various tricks of the trade when it comes to tackling neuroanatomy. Some of the biggest initial roadblocks for students revolve around being able to visualize the brain, identify its various structures, and understand and describe how these structures are spatially related to each other.
As such, students can benefit from beginning their studies by developing an understanding of:
- the folding that occurs in the primordial brain as it develops and how this ultimately affects its structure in an adult;
- the subtleties of the language used to describe spatial relationships and directionality within the brain versus the rest of the body
- the potentially disjointed presentations of the brain’s various curved internal structures (the lateral ventricles, caudate nucleus, hippocampus and fornix, and cingulate gyrus) in 2D space
Although there is much more work to be done, identifying these difficulties and taking the time to learn about them can only provide clarity and a stronger frame of reference, making the rest of the process of learning neuroanatomy not quite a piece of cake, but significantly more efficient.