The Forebrain (Prosencephalon)
The forebrain is a mammal’s brain’s largest and most obvious part. The outer layer is called the cerebral cortex and consists of the cerebral hemispheres, which account for two-thirds of the brain’s total mass.
Each cerebral hemisphere can be subdivided into four lobes (frontal, parietal, temporal, and occipital lobes), each associated with different functions.
Each pair has one lobe on the left side of the brain and another on the right.
The frontal lobe involves voluntary movement, reasoning, impulse control, language, and speech (Salazar, 2017). Broca’s Area is part of the left frontal lobe and is associated with speech production.
In a railroad workstation accident, Phineas Gage fell victim to a 43-inch-long and 1.25-inch-wide rod hurtling clean through his frontal lobe and out the other end of his skull (Harlow, 1868).
This damage forever changed his personality. Friends and relatives described him as a changed man who became bitter and aggressive.
The parietal lobe is mainly involved with processing the sense of touch, limb position, and spatial awareness (Adair & Meador, 2003).
Damage to the parietal lobe typically involves loss of these abilities in the parts of the body opposite the lesion. This is due to the brain’s contralateral function, meaning that the right side of the brain is responsible for activity on the left side of the body and vice versa.
The temporal lobe primarily processes auditory stimuli (Smith, 2007). A region in the temporal lobe known as Wernicke’s area comprehends the sounds we hear by translating different pitches and frequencies into meaning. The temporal lobe also communicates with the hippocampus to produce long-term memory.
The occipital lobe is located at the rear end of the cerebral cortex and is known to be the visual processing center of the brain (Bailey, 2019). Brodmann Area 17, the primary visual cortex, is in the occipital lobe – it determines the size, shape, and location of objects in the field of vision.
Damage to the occipital lobe can cause an inability to identify colors, loss of visual capacity, and even hallucinations.
The right and left hemispheres of the brain are connected by a network of nerve fibers known as the corpus callosum.
Damage to this region causes split-brain, where the hemispheres act independently from each other. Roger Sperry’s split-brain experiments in the 1960s pioneered modern-day knowledge on this condition (Lienhard, 2017).
Under the cortex are a number of other structures, including the thalamus, hypothalamus, pituitary gland, basal ganglia, hippocampus, and amygdala, some of which form the limbic system (involved in emotional behavior, motivation, and learning).
The thalamus is a sensory relay for the brain. All of our senses, with the exception of smell, are routed through the thalamus before being directed to other areas of the brain for processing.
The forebrain also features important structures in the limbic system, which controls basic emotions and psychological drives. The limbic system is made up of a number of different structures, but three of the most important are the hippocampus, the amygdala, and the hypothalamus.
A critical component of the limbic system is the amygdala, an almond-shaped set of neurons responsible for processing fear.
The connection between fear and the amygdala can be traced back to research by Heinrich Kluver and Paul Bucy in the 1930s, who removed the amygdalae in rhesus monkeys and found them to display little fear (Hayman et al., 1998).
Another important part of the limbic system is the hippocampus, which is responsible for long-term memory formation. Henry Gustave Molaison, known as H.M., had two-thirds of his hippocampi removed to treat his epilepsy in 1953.
As a result, he suffered from severe anterograde amnesia: an inability to form new memories. H.M. also had difficulty remembering events just before the lesion, a condition known as graded retrograde amnesia (Halber, 2018).
There are two other major structures of the limbic system worth mentioning: the thalamus and the hypothalamus. The thalamus is considered to be the neural railway station of the brain.
Sensory input from the body travels to the thalamus, where it is then relayed to the rest of the brain as a specific sensation. The thalamus is also involved with consciousness, alertness, and sleep (Mandal, 2019).
Located between the two halves of the thalamus is the pineal gland: a pea-shaped gland that regulates sleep. Its other functions are not yet fully understood.
The hypothalamus is located at the base of the brain near the pituitary gland, and it is involved with pleasure, food, body temperature, and sex (Seladi-Schulman, 2018). If the body temperature gets too high, the hypothalamus instructs the body to sweat.
If the body is cold, the hypothalamus will cause shivers. Irregular hypothalamic behavior can, in turn, lead to sweating and shivering when these reactions are not needed.
The hypothalamus also regulates the pituitary gland and regulates the other hormone-releasing glands in the body. For this reason, it is often referred to as the “master gland” of the endocrine system.
Midbrain (Mesencephalon)
The second area of the brain is the midbrain, which lies on top of the brainstem. The midbrain is involved in auditory and visual processing (Peters, 2017).
It is also responsible for eye movement. There are three parts to the midbrain: the colliculi, the tegmentum, and the cerebral peduncles.
The colliculi process visual and auditory signals before they are relayed to the occipital and temporal lobes.
The tegmentum is involved in movement coordination and alertness. Lastly, the cerebral peduncles play an important part in the central nervous system.
Hindbrain (Rhombencephalon)
The hindbrain is located at the back of the head and looks like an extension of the spinal cord. It contains the medulla oblongata, pons, and cerebellum (collectively known as the brainstem).
The hindbrain mostly coordinates autonomic functions that are essential to survival.
The medulla oblongata, also simply known as the medulla, is the lowest part of the brainstem and the primary connection to the nervous system. It is primarily involved in breathing, heart rate, digestion, swallowing, heart rhythms, and sneezing (Peters, 2016).
The medulla controls the automatic processes of the autonomic nervous system, such as breathing, blood pressure, and heart rate.
The pons, which is Latin for “bridge,” is located just beneath the midbrain and just above the medulla. It is a group of nerves that are involved in arousal, sleep, motor control, and muscle tone (Patel & Sugano, 2018).
The cerebellum (Latin for “little brain”) receives messages from muscles, tendons, joints, and structures in our ear to control balance, coordination, and movement, and facilitate motor learning.
The cerebellum is also thought to be an important area for processing procedural memory. Most disorders of the cerebellum are associated with serious motor impairment.
