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The Cerebrum

The cerebrum is the largest and most prominent part of the brain. It forms most of the brain’s visible mass and is covered by a gray outer layer called the cerebral cortex. 

Beneath this cortex lies deeper brain tissue, including white matter and subcortical nuclei.

The cerebrum is divided into two halves, known as the right and left cerebral hemispheres. These hemispheres are separated by a deep groove called the longitudinal fissure. 

Despite being separated, they communicate through a large bundle of white matter called the corpus callosum, which allows information to pass between the two sides.

The Frontal Lobe

The frontal lobe is located at the front of the brain and extends back to the central sulcus, which separates it from the parietal lobe. It is also limited below by part of the lateral fissure and, on the inner surface, by the cingulate sulcus. 

The surface of the frontal lobe is divided by several grooves, called sulci, into four main ridges, called gyri. These include the precentral gyrus and the superior, middle, and inferior frontal gyri. 

The anterior central gyrus lies just in front of the central sulcus and plays an important role in controlling voluntary movements. The superior, middle, and inferior frontal gyri run forward and are involved in more complex brain functions. The inferior frontal gyrus is further divided into parts, one of which (especially on the left side) is known as Broca’s area, which is important for speech production. 

On the lower surface, the frontal lobe rests above the eye sockets and contains the orbital gyri, which are separated by an H-shaped groove. There is also a groove for the olfactory tract, related to the sense of smell. 

On the inner surface, part of the frontal lobe forms the paracentral lobule, which connects with nearby motor and sensory areas. 

Overall, the frontal lobe is mainly responsible for motor control, planning movements, eye movement control, speech production, and higher cognitive functions such as personality, decision-making, and short-term memory.

The Parietal Lobe

The parietal lobe lies behind the frontal lobe and is separated from it by the central sulcus, although its boundaries with other lobes are less clearly defined. 

It extends back toward the parieto-occipital fissure and is separated from the temporal lobe below by part of the lateral fissure. 

Its surface is divided by the intraparietal sulcus into two main regions: the superior and inferior parietal lobules. 

In front of this area is the postcentral gyrus, which lies just behind the central sulcus and is closely connected to the precentral gyrus. 

The superior parietal lobule is located above the intraparietal sulcus and connects with the occipital lobe at the back. The inferior parietal lobule lies below and is divided into two parts: the supramarginal gyrus, which curves over the end of the lateral fissure, and the angular gyrus, which curves over the end of another groove and connects with the temporal lobe. 

On the inner surface, the parietal lobe includes a small region called the precuneus. 

Functionally, the parietal lobe is responsible for processing somatosensory information, including touch, pressure, pain, temperature, and awareness of body position and movement.

The Occipital Lobe

The occipital lobe is located at the back of the brain and is relatively small and pyramid-shaped. It has three surfaces: lateral, medial, and inferior (tentorial). 

The lateral surface is divided by grooves into upper and lower regions and connects with both the parietal and temporal lobes. 

The medial surface is especially important, as it contains the calcarine fissure, which divides the area into the cuneus above and the lingual gyrus below. 

The cuneus is a wedge-shaped region, while the lingual gyrus extends toward the front and connects with structures in the temporal lobe. 

The Temporal Lobe

The temporal lobe is located on the side of the brain, below the frontal and parietal lobes. It has superior, lateral, and inferior surfaces.

The superior surface forms part of the lateral fissure and contains the transverse temporal gyri, which are involved in processing sound.

The lateral surface is divided into three main gyri (superior, middle, and inferior temporal gyri) by two grooves called the superior and middle temporal sulci.

The superior temporal gyrus lies just below the lateral fissure and connects with parts of the parietal lobe. The middle temporal gyrus lies below it, and the inferior temporal gyrus extends further down and continues onto the lower surface of the lobe

The Subcortical Structures

Below the cerebral cortex are subcortical nuclei that support and modify cortical activity. The basal forebrain produces acetylcholine, which influences attention and cortical activity. Damage to this region is associated with Alzheimer’s disease. 

The hippocampus and amygdala are important for memory and emotional responses. The basal nuclei play a major role in controlling movement by regulating whether actions are initiated or suppressed.  

The basal nuclei operate through two main pathways. The direct pathway promotes movement by allowing signals to pass through the thalamus to the cortex. The indirect pathway inhibits movement by preventing these signals. 

Dopamine, released from the substantia nigra pars compacta, influences these pathways by stimulating the direct pathway and inhibiting the indirect pathway. This balance determines whether movement is more or less likely to occur.

The thalamus is a large group of nuclei that acts as a relay station for information traveling to and from the cerebral cortex. Almost all sensory information (such as touch, vision, and hearing) passes through the thalamus before reaching the cortex, where it is consciously perceived. 

The only sensory pathway that does not go through the thalamus is the sense of smell. Sensory signals arriving from the body or from intermediate processing centers first synapse in the thalamus, and then thalamic neurons send this information directly to specific regions of the cerebrum.

However, the thalamus does more than simply pass information along. It also processes and filters incoming signals. For example, when visual information reaches the thalamus, it is adjusted so that certain stimuli receive more attention than others. In this way, the thalamus helps determine what sensory information becomes important for conscious awareness.

The thalamus is also involved in motor functions. The cerebrum sends motor-related signals down to the thalamus, which works closely with the cerebellum and brain stem nuclei to coordinate movement. In addition, the basal nuclei send their output to the thalamus, which then relays that information back to the cortex. 

This creates a feedback loop in which the cortex and thalamus continuously influence each other. Through these connections, the thalamus plays a key role in integrating both sensory and motor information.

The hypothalamus is located just below and slightly in front of the thalamus. It is another major component of the diencephalon and consists of a collection of nuclei with essential regulatory functions. Its primary role is maintaining homeostasis, which means keeping the internal conditions of the body stable.

The hypothalamus acts as the main control center for the autonomic nervous system and the endocrine system.

The Brain Stem

The brain stem consists of the midbrain, pons, and medulla. It connects the brain to the spinal cord and controls many essential life functions. 

The midbrain processes sensory information, especially related to vision and hearing. It contains structures called colliculi, which help coordinate responses to sensory stimuli. 

The pons serves as a bridge between the brain and the cerebellum. It transmits signals between these regions and contains nuclei involved in motor control.

The medulla regulates vital functions such as heart rate and breathing. It also contains pathways that carry information between the brain and spinal cord.

The cerebellum is responsible for coordination and balance. It does not start movements but compares motor commands from the cerebrum with sensory information from the body. 

A copy of movement instructions is sent to the cerebellum, while sensory feedback comes from muscles, joints, and balance systems. If there is a difference between intended and actual movement, the cerebellum sends corrective signals. These signals help make movements smooth and accurate.

The cerebellum has a central part called the vermis and two lateral hemispheres. Its surface is covered with many curved grooves called sulci, which create a leaf-like appearance. These folds divide the cerebellum into thin layers called folia. 

Each layer contains white matter inside and gray matter on the surface. The internal white matter forms a branching pattern called the arbor vitae. 

The cerebellum is divided into many smaller parts called lobules. These are separated by deep grooves called fissures. The lobules are arranged in the central vermis and the two hemispheres. They are also organized into 10 distinct lobules (divided among the anterior, posterior, and flocculonodular lobes): the lingula (I), central lobule (II) and (III), culmen (IV) and (V), declive (VI), folium (VIIA), tuber (VIIB), pyramid (VIII), uvula (IX), nodule (X), and flocculus (HX).

Inside the cerebellum is a central core of white matter with branching fibers. Gray matter is found on the surface as the cortex and also in deeper nuclei. 

The cortex has several layers, including one with Purkinje cells, which are important for processing signals. There are also deep nuclei, such as the dentate nucleus, which help send signals out of the cerebellum.

The Cranial Nerves

The nerves attached to the brain are called cranial nerves. They are mainly responsible for the sensory and motor functions of the head and neck. 

One exception is a nerve that also connects to organs in the thoracic and abdominal cavities as part of the parasympathetic nervous system.

There are twelve cranial nerves in total. They are numbered from I to XII using Roman numerals. These nerves can be sensory, motor, or a combination of both. 

Sensory fibers come from sensory ganglia outside the skull and enter the brain, where they connect to nuclei. Motor fibers begin in the brain stem and connect to muscles in the head and neck.

The pattern of how blood vessels are distributed in the brain is very important because it affects how certain diseases or injuries develop in the nervous system.

The main arteries that supply the brain come from two sources: the internal carotid arteries and the vertebral arteries. These vessels originate from the aorta, which is the main artery carrying oxygenated blood away from the heart.

The aorta first gives branches to the heart itself, and then gives rise to the common carotid arteries. These divide into internal and external carotid arteries. The external carotid arteries supply the outer tissues of the head, while the internal carotid arteries supply the brain.

At the base of the common carotid arteries are stretch receptors that detect changes in blood pressure. When a person stands up, blood pressure may drop due to gravity. The body responds with the orthostatic reflex, increasing heart rate and raising blood pressure to maintain proper blood flow to the brain.