Gross Anatomy of The Brain

This is a huge topic. It should have been published the first in my blog, but it's still not too late to do it now I guess. The gross anatomy of the brain is fundamental knowledge every neuroscientist has to know.

Fig-1: The main divisions of the central nervous system (top-left), together with major anatomical axes; adapted from [1].

The Forebrain: Telencephalon
We usually refer to this part of the brain as the cerebral hemisphere. It consists of the cerebral cortex and the subcortical region beneath it. The cerebral cortex serves as the main computational unit and contains mainly cell bodies and glass. That's why its appearance is grayish tan, also known as the gray matter. On the other hand, the subcortical region consists mainly of myelinated axons and therefore it is called white matter. The forebrain is encapsulated inside a layer of cerebrospinal fluid or CSF a colorless fluid important for mechanical and immunological protection. The ventricles also contain CSF.

1. The cerebral cortex.
This is the outermost layer of the cerebral hemisphere. In humans, it is greatly convoluted and consists of: sulci (sulcus - singular) or small grooves, fissures (large grooves), and gyri (gyrus - singular) which are bulges between adjacent sulci or fissures. What's the reason behind it? Well, by "hiding" most of the cortical areas in the grooves, we have a much bigger brain region, suggesting a more computational advantage. The thickness is ~ 3 mm with a total surface area of 0.24 m². K. Brodmann was the first person who did an excellent job identifying various cortical areas based on cytoarchitectonic features, that is, differences in cortical layer architectures.

Traditionally, the cerebral cortex is divided into 4 regions, i.e. frontal, parietal, temporal, and occipital. The frontal lobe is known for motor and executive functions. Three regions of the cerebral cortex are sensory, meaning, they receive afferent inputs from the peripherals, sensory organs:

• The primary visual cortex (V1, BA-17), on the occipital lobe.
• The primary somatosensory cortex (S1, BA-3, BA-1, BA-2), on the parietal lobe.
• The primary auditory cortex (A1, BA-41), on the temporal lobe.
• The other two are hidden, i.e. the primary olfactory cortex, near the piriform cortex, and primary gustatory cortex, near the insular cortex. 

With the exception of olfaction and gustation, sensory information is sent from the contralateral side of the body. Regions adjacent to the primary sensory area are called the association areas which are involved in more higher-order cognition and perhaps memory. Their lesions are insightful to understand their functions. Patients with damage to S1 are unable to perceive tactile sensation and recognize the object in general. Clearly, S1 sends projections to the adjacent association area. Damage to the somatosensory association cortex allows the person to sense the presence of the stimuli, but he or she is unable to recognize, call or name, or perceive the shape or contour or the objects.

The regions further away from these primary areas, e.g. around the border of temporal, occipital, and parietal lobes, are called multimodal sensory areas where multisensory integration occurs. The region of the prefrontal cortex is involved not with movement or sensory perception, but with formulating plans and strategies. To plan and make a decision, we have to depend on the current sensory inputs, past experience or memory, and action selection (sometimes called executive functions).

2. The basal ganglia and subcortical structures.
Regions immediately below the cerebral cortex form the white matter and the subcortical structures. Some of the most important structures are the amygdala, fornix, parts of the hypothalamus. Amygdala, together with parts of the cingulate gyrus and parahippocampal gyrus of the cerebral cortex, are parts of the limbic system. These structures play a role in learning and emotional expression. Immediately below the cingulate gyrus is the corpus callosum. Immediately above it is the folding or sulcus, originated from the medial wall and goes down to the rostral end beneath the prefrontal cortex. This folding, called the cingulate sulcus, separates the frontal lobe and the cingulate gyrus. Amygdala is important for emotion and recognizing the emotional reactions of others.

More discussion on basal ganglia is in a separate blog post. Large and intricate bundles of fibers called the fasciculi form what is known as the white matter. Some of these fibers pass through the nuclei of the basal ganglia forming the internal, external, and extreme capsules. This part will not be presented here.

The brain has two hemispheres separated by the longitudinal fissure. Are the two hemispheres talking to each other? Yes, and the left parietal region knows what the right one is doing; and this is the job of the corpus callosum, containing bundles of commissural fibers, i.e. neurons that connect the two cerebral hemispheres. Another type of fibers which allows different cortical regions of the same hemisphere talk to each other is called association fibers.

The Interbrain: Diencephalon
The structure of diencephalon is quite small. It is located between the telencephalon and mesencephalon around the third ventricle. There are two main components, i.e. the thalamus and hypothalamus, both contain two lobes spanning the right and left hemisphere. In the medical field, the hypothalamus has been known as a thermostat to control body temperature. The thalamus and hypothalamus are further subdivided into several regions or nuclei, each of which contains a collection of neuronal cell bodies with a distinct functional role.

Thalamus is composed mainly of projection fibers, i.e. axons of the neurons from one part of the brain that form a synapse with the neurons of another region. The hypothalamus is important in controlling the autonomous nervous system, endocrine or hormonal system, and survival-related behaviors. Pituitary gland serves as the master gland because it controls the secretion of other endocrine glands (the details are not presented here). Anterior to the pituitary gland is the optic chiasm where optic nerves from both eyes cross.

The Midbrain: Mesencephalon
Midbrain is the middle portion of the brain surrounding the cerebral aquaduct. There are two major components: the tectum and tegmentum. Tectum is the dorsal part of the midbrain containing 2 principal structures: a pair of superior colliculi (part of the visual system) and a pair of inferior colliculi (part of the auditory system). Tegmentum is situated beneath the tectum and includes several important midbrain nuclei such as the rostral end of the reticular formation, periaqueductal gray matter, red nucleus, substantia nigra, and the ventral tegmental area. The last two structures are well-known as the source of dopamine.

The Hindbrain: Metencephalon and Myelencephalon
The hindbrain consists of the cerebellum, pons, and medulla. The part on the cerebellum, in itself the 'small brain', is being presented elsewhere in this blog. The two remaining regions, pons, and medulla (or medulla oblongata), are part of the brainstem and located anterior to the cerebellum.

Pons contains part of the reticular formation and pontine nuclei. It is involved in the facial somatosenses and automatic repetitive movements such as breathing and swallowing. The medulla is the most caudal region of the brainstem, adjacent to the rostral end of the spinal cord. As a very important structure in the autonomic nervous system (ANS), the medulla controls respiration, cardiac rate, and some other reflex centers such as swallowing, coughing, and sneezing.

Fig-2: Anatomy of the brainstem with the right hemisphere of the cerebellum. Adapted from [2].

What is beyond the cortex? Well, the central nervous system (CNS) consists of both the cerebral cortex and the spinal cord. The brainstem connects the cortex with the spinal cord. A more detailed part will be for another blog entry but the gist is this:

1. The white/gray matter location is swapped as you enter the spinal column. As we know, white matter contains myelinated axons, while gray matter contains cell bodies. The center gray matter core of the spinal cord is made up of cell bodies of spinal interneurons that connect motor-motor, motor-sensory (as in reflex arc), and propriospinal neurons. Indeed, the anatomical connections are rather intricate!
2. The central core has an inverted H-shaped. It is divided into ventral (motoric) and dorsal (somatic). Mechanoreceptors send information to the sensory neurons whose cell bodies are in the dorsal root ganglion. They enter the spinal cord through the dorsal side up. On the other hand, pyramidal cells from the motor cortex enter the spinal cord through the ventral horn where they synapse with the alpha motor neurons that target a specific muscle.
3. The spinal cord is important for low-level, stereotyped reflex behavior, as well as locomotion.

Appendix: The cerebral cortex
I want to add some more information regarding the cerebral cortex. In particular, with regards to the Brodmann Areas.

Fig-3: Parts of parietal, temporal, and occipital lobes: a The area V1, V2 and V3 of the occipital lobe (BA 17, 18, and 19). b The area postcentral gyrus, superior parietal lobule (BA 5, 7), and inferior parietal lobule (consisting angular gyrus or BA 39; and supramarginal gyrus or BA 40). c The temporal lobe bounded by the lateral fissure. Adapted from [3].

Fig-4: Parts of frontal lobe: The area corresponding to prefrontal cortex with dorsolateral prefrontal cortex (dlPFC) and a more caudal part consisting: superior, middle, and inferior frontal gyrus (F1, F2, and F3 respectively). The region associated with motor functions (M1). The medial view of the frontal lobe is also shown on the right with regions more intimately related to reflective and stimulus-driven inputs. Adapted from [3].

References
[1]  Figure 17-2 (p.320), 17-3 (p.321), and 17-5 (p.325). In Kandel E.R. et. al. (2000). Principles of Neural Science 4e, McGraw-Hill.
[2]  Chapter 3 - Structure of The Nervous System. In Carlson, Neil R. (2013). Physiology of Behavior 11e, Pearson Education.
[3]  David, C. L., et. al. (2010). The Brain and Behavior: An Introduction to Behavioral Neuroanatomy, 3e. Cambridge Press.