Cerebrum

The cerebrum, also known as the telencephalon or endbrain, is the largest part of the brain.

In humans, it serves as the uppermost region of the central nervous system. This massive structure is responsible for nearly all complex functions, including emotions, personality, and judgment. It also acts as the center for sensory perception and voluntary motor activities. By processing information from the senses, the cerebrum allows the brain to understand the world as a single, cohesive picture. It is the primary engine for the human experience.

The structure of the cerebrum is highly organized. It consists of two C-shaped cerebral hemispheres separated by a deep groove called the longitudinal fissure. These hemispheres are roughly symmetric, but they exhibit lateralization, meaning different sides may specialize in different tasks. The cerebrum is also contralaterally organized. This means the right hemisphere processes signals from the left side of the body, while the left hemisphere handles the right side. Scientists believe this organization occurs because of an axial twist during early embryonic development.

The cerebrum is composed of two distinct types of tissue. The outer layer is called the cerebral cortex, which is made of grey matter. In mammals, this cortex is layered, a feature known as cytoarchitecture. To increase surface area, the cortex folds into ridges called gyri and deep furrows called sulci.

Beneath this outer layer lies the white matter. The cerebrum also contains several subcortical structures. These include the hippocampus, the basal ganglia, and the olfactory bulb. Each of these parts plays a specific role in how the brain functions.

Developmentally, the cerebrum begins as part of the forebrain, or prosencephalon. In the vertebrate embryo, the neural tube divides into sections, including the prosencephalon. This forebrain further develops into the telencephalon and the diencephalon. In mammals, the dorsal telencephalon becomes the pallium, which forms the cerebral cortex. The ventral telencephalon becomes the subpallium, which forms the basal ganglia. This complex growth process ensures that the brain has the specialized regions required for advanced survival.

The cerebral cortex is divided into several functional lobes based on the skull bones above them. The frontal lobe contains the primary motor cortex, which sends signals through upper motor neurons to control voluntary movement. The parietal lobe contains the somatosensory area, which maps feelings like touch, pain, and temperature. The occipital lobe houses the main visual area, or V1, which detects edges and colors. The temporal lobe manages hearing and contains areas for speech comprehension. Additionally, a small insular lobe is tucked deep within the lateral sulcus.

Memory and language are some of the most advanced tasks the cerebrum performs. Language relies on specific regions like Broca's area for speech production and Wernicke's area for comprehension. These areas are connected by a white matter tract called the arcuate fasciculus. Memory involves different timescales and mechanisms. Short-term or working memory often involves the prefrontal cortex and the hippocampus. Long-term memory is linked to structural changes in the brain, such as increases in synaptic vesicles. The hippocampus is specifically vital for forming explicit, or factual, memories.

The cerebrum also manages the sense of smell through the olfactory bulb. In humans, this bulb is relatively small and sits under the frontal lobe. Unlike other senses, olfactory neurons send signals directly to the olfactory cortex without passing through the thalamus first. This is known as ipsilateral processing, where input is processed on the same side of the brain. Finally, the cerebrum connects to broader biological systems. While humans have highly developed cortices, other vertebrates like fish and birds have a dorsal telencephalon that lacks these complex layers.