3.1.3. Central Nervous System

The brain is made of three main parts: the forebrain, midbrain, and hindbrain. The forebrain consists of the cerebral cortex, thalamus, and hypothalamus (part of the limbic system). The midbrain consists of the tectum and tegmentum. The hindbrain is made of the cerebellum, pons, and medulla. Often the midbrain, pons, and medulla are referred to collectively as the brainstem.

See the diagram below for a more detailed overview of the organization of the nervous system. Scroll past it for more information on the structures of the brain.

Forebrain

Divisions and Structures:

• Telencephalon: cerebral cortex, corpora striata (caudate nucleus, internal capsule, putamen), and rhinencephalon (e.g., olfactory bulb, hippocampus, amygdala, septal region, and cingulate cortex)
• Diencephalon: thalamus, hypothalamus, pineal gland, and posterior lobe of the pituitary (a migrated portion of the hypothalamus)

Basic Functions:

• cognition
• movement
• sensory processing
• learning
• memory
• language
• motion

Cerebral Cortex and Cerebrum

The cerebral cortex is a thin layer of tissue comprising the outermost layer of the cerebrum. The cerebral cortex is composed of neural cell bodies, which give the cortex a grey color, hence the name “grey matter”. Conversely, “white matter” is composed mainly of myelinated axons, which carry signals from neural cell bodies to other parts of the brain and body.

Together, the cerebral cortex and the cerebrum are the largest part of the human brain, associated with higher brain function, such as consciousness.

Note that the cerebral cortex is highly wrinkled. Essentially this makes the brain more efficient, because it can increase the surface area of the brain and the amount of neurons within it. We will discuss the relevance of the degree of cortical folding (or gyrencephalization) later.

A deep furrow divides the cerebrum into two halves, known as the left and right hemispheres. The two hemispheres look mostly symmetrical, yet it has been shown that each side functions slightly differently than the other. Sometimes the right hemisphere is associated with creativity, while the left hemispheres is associated with logic abilities. The corpus callosum is a bundle of axons which connects these two hemispheres.

Neocortex

The neocortex occupies the bulk of the cerebrum and cerebral cortex. It is a six-layered structure found only in mammals. It is thought that the neocortex is a recently evolved structure, and is associated with “higher” information processing in more fully evolved animals (such as humans, primates, dolphins, etc).

The neocortex is divided into four sections, called “lobes”: the frontal lobe, parietal lobe, occipital lobe, and temporal lobe.

• Frontal lobe: associated with reasoning, planning, parts of speech, movement, emotions, and problem-solving
• Parietal lobe: associated with movement, orientation, recognition, perception of stimuli
• Occipital lobe: associated with visual processing
• Temporal lobe: associated with perception and recognition of auditory stimuli, memory, and speech

Limbic System

The limbic system, often referred to as the “emotional brain”, is found buried within the cerebrum. Evolutionarily, the structure is relatively old. In other words, an animal which scientists assume to have evolved prior to humans may have a limbic system but not a neocortex.

This system contains the thalamus, hypothalamus, amygdala, and hippocampus. Below is a midsagittal view of the human brain, with several structures labelled (some limbic system structures are not visible from a midsagittal cut due to their locations):

• Thalamus: sensory processing and motor functions
• Hypothalamus: emotion, thirst, hunger, circadian rhythms, body temperature, and control of the autonomic nervous system
• Amygdala: memory, emotion, and fear
• Hippocampus: learning and memory — converting short-term memory to long-term memory, and recalling spatial relationships in the world around us

Midbrain

Divisions and Structures:

• Mesencephalon: tectum (inferior and superior colliculi) and tegmentum

Basic Functions:

• Vision
• Hearing
• Eye movement
• Motor control

Hindbrain

Divisions and Structures:

• Metencephalon: Pons and Cerebellum
• Myelencephalon: Medulla

Basic Functions:

• Monitoring and controlling body movements
• Homeostasis

Cerebellum

The cerebellum, or “little brain”, is similar to the cerebrum in that it has two hemispheres and has a highly folded surface, or cortex. This structure is associated with regulation and coordination of movement, posture, and balance. It is assumed to be much older than the cerebrum, evolutionarily. In other words, animals which scientists assume to have evolved prior to humans, for example reptiles, do have developed cerebellums. However, reptiles do not have neocortex. 

Brainstem

Structures: Midbrain (tectum and tegmentum), Pons, and Medulla

Underneath the limbic system is the brain stem. This structure is responsible for basic vital life functions such as breathing, heartbeat, and blood pressure. Scientists say that this is the “simplest” part of the human brain because the entire brains of evolutionarily older animals (for example, reptiles) resemble the human brainstem.