Brain orientation difficulties

Overview

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 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:

1. the rostral/caudal axis
2. 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 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.

The directions rostral, caudal, ventral, and dorsal change depending on the relative position of a structure to the midbrain. Above the midbrain, the orientation of the these directions is similar to that used with animals whose nervous system in linear in arrangement. As the nervous system in humans demonstrates a bend of roughly 90 degrees around of the brainstem, the orientation of these directions changes by the same degree in a clockwise fashion.

Put simply:

• Rostral: comes from the latin ‘rostrum’  which means beak, nose or snout, so this direction suggests an orientation ‘towards the beak/nose’.
  
  • Above the midbrain: rostral can be considered as anterior.
    
  • Below the midbrain: rostral should be considered as superior.
    
• Caudal (opposite of rostral): comes from the latin ‘cauda’  which means tail, so this direction suggests an orientation ‘towards the tail’, which in humans can be considered the coccygeal vertebrae.
  
  • Above the midbrain: caudal can be considered as posterior.
    
  • Below the midbrain: caudal should be considered as inferior.
    
• Ventral:  comes from the latin ‘venter’  which means belly, or stomach, so this direction suggests an orientation on the same side as the stomach, relative to the rostral/caudal (i.e. nose/tail) axis.
  
  • Above the midbrain: ventral can be considered as inferior.
    
  • Below the midbrain: ventral should be considered as anterior.
    
• Dorsal (opposite of ventral): comes from the latin ‘dorsum’  which means back, so this direction suggests an orientation on the ‘behind’ the rostral/caudal (i.e. nose/tail) axis.
  
  • Above the midbrain: dorsal can be considered as superior.
    
  • Below the midbrain: dorsal should be considered as posterior.
    

Growth of the brain: Four C's