1.2 Brain Structure and Function

The brain is a unique organ that distinguishes from other organs in the human body. When we think about the heart, lungs, kidneys and even our skin, these are organs that can be transplanted from one person to another, yet the brain is uniquely positioned to control a myriad of functions in a single human being. Within its restricted confinement inside a skull, activity of the brain is strongly regulated by over 85 billion neurons, with each neuron capable of making about 10,000 synaptic connections. Those synaptic connections are essential given that the brain is responsible for controlling how we interpret sensory information that is conveyed through touch, temperature, and pain or how we execute voluntary movements. Additional functions can be analyzed by examining the brain externally and internally.

Overview: Gross Anatomy of External Brain Structures: Cranial Nerves, Vasculature and Drainage

Externally, the brain’s visible surface is comprised by the cerebral cortex, a thin layer of gray matter that is covered in wrinkled ridges known as “gyri” (gyrus in singular) which are separated by invaginated folds referred to as “sulci” (sulcus in singular). The cortex is divided into left and right hemispheres, both regions recognized for containing paired structures of most brain structures that function either independently or in association with each other. The left and right hemispheres are connected by the “corpus callosum”, an internal region formed by neuronal myelinated axons (Figure 4). In cases of agenesis of the corpus callosum, children may be born with impairments in cognition, vision, hearing, speech, or motor skills. These challenges can impact not only their developmental progress as they grow but also their behavior, due to difficulties or delays in the learning process.

Each hemisphere contains four lobes: frontal, parietal, temporal, and occipital. Each of these four lobes integrate, process, and regulate different functions that affect human behavior (Figure 5).

When studied independently, the lobes of the brain, while sharing some overlapping functions, also exhibit distinct roles. For instance, the frontal lobe is primarily associated with higher cognitive functions, including speech production, decision-making, reasoning, judgment, emotions, self-regulation of behavior, attention, motor control, facial expressions, spontaneity, serial tasks, and personality development. Compared to the other three lobes, the frontal lobe is the most recent to evolve. “Synaptic pruning“, the process by which excess synapses are eliminated—continues in the frontal cortex until the third decade of life. As a result, although the brain reaches approximately 95% of its adult size between the ages of sixteen and twenty, full maturation of the frontal lobe is not achieved until after age thirty. This prolonged development underscores the significance of the posterior portion of the frontal lobe, which plays a crucial role in motor control. Specifically, the precentral gyrus in this region contains descending motor fibers that are responsible for executing voluntary movements. Another crucial structure in the frontal lobe, Broca’s area (Figure 6), is essential for the production of language.

Separated from the frontal lobe by the central sulcus, and posterior to it, is the parietal lobe. This lobe is characterized for its control over complex events such as spatial orientation, regulation of temperature, touch, and pain processes. Tasks that involve visuospatial processing, left and right distinctions and self-awareness are also key functions of the parietal lobe.

Its most anterior gyrus is known as the “post-central gyrus or primary somatosensory cortex”, a region that controls involuntary functions associated with somatosensory relay signals that are delivered from the skin to the thalamus and finally, to the post-central gyrus in the cortex.

Located beneath the parietal lobe, the occipital lobe contains the primary structures responsible for processing visual information, including object identification, recognition, location, distance, color, and size. Additionally, this lobe plays a crucial role in the visual perception of body language, such as facial expressions, posture, and gestures, as well as the spatial analysis of 3D objects and visual attention. These functions are made possible by pathways that connect the ventral and dorsal parts of the occipital lobe to the temporal and parietal lobes, respectively. This communication is essential for our ability to navigate our environment, recognize the orientation of objects in space, and interpret 3D shapes, all of which are vital for tasks like manipulating objects or reading maps.

The temporal lobe, located just above the ear and below the parietal lobe, is specialized for the interpretation and processing of auditory information. It is also involved in the recognition of faces, the processing of perceptions, and the storage of verbal, visual, and autobiographical memories, which are closely linked to deeper brain structures such as the hippocampus. One key structure within the temporal lobe, Wernicke’s area (Figure 7), plays a vital role in language comprehension. This area communicates with Broca’s area in the frontal lobe via the arcuate fasciculus, a bundle of myelinated fibers.

From the external surface of the brain, other structures such as the cranial nerves and vasculature that supply and drain the brain can be examined.

CRANIAL NERVES

Cranial nerves are paired nerves that originate from the cerebrum (CN I and CN II) and, for the most part, from the brainstem (CN III–CN XII). There are a total of 12 cranial nerves (Figure 8; Table 1), each responsible for specific functions, including controlling vision, hearing, touch, smell, and taste. Additionally, several cranial nerves are crucial for regulating the contraction of muscles in the face, neck, and back. Although these nerves originate in the brain, they exit through openings or foramina in the skull to reach and innervate their target regions (Table 1).

VASCULATURE AND DRAINAGE OF THE BRAIN

The brain is protected by several mechanisms, one of which is its vasculature, which provides a constant supply of oxygen and glucose to support the highly active neurons and glial cells. Different regions of the brain have distinct metabolic needs, so increased brain activity and neuronal function lead to higher blood flow, delivering more oxygen and glucose to the active areas. This increase in blood flow is facilitated by the dilation of blood vessels, triggered by higher brain activity, which produces more nitric oxide, a lower pH, lower partial oxygen (paO2), and higher partial carbon dioxide (paCO2). In contrast, reduced blood supply can have harmful effects, leading to ischemic neuronal cell death.

The brain is supplied by two main vessels, the common carotid, and the vertebral arteries (Figure 9). The common carotid is a paired artery located on the left and right areas of the neck. It bifurcates to give rise to two main branches, the external carotid artery and the internal carotid artery. The external carotid arteries forms between the neck of the mandible and the earlobe and it supplies the face, head and neck. The internal carotid arteries travel superiorly to enter through the carotid foramen (i.e. canal) of the temporal bone in the skull and supply different brain areas as it anastomoses with the branches of the vertebral arteries (Figure 9), this latter which arises from the subclavian artery, on the left side, and from the brachiocephalic trunk on the right side.

The left and right vertebral arteries give rise to the posterior inferior cerebellar artery (PICA), which supplies the inferior part of the cerebellum, and ascend toward the brain through the transverse foramina of the cervical vertebrae, reaching the base of the skull. They then enter the cranial cavity through the foramen magnum, pierce the dura mater, and run along the anterior surface of the medulla oblongata. At the lower lateral side of the pons, the left and right vertebral arteries join to form the basilar artery, which primarily supplies the brainstem, cerebellum, and occipital lobes. Branches of the basilar artery include the anterior inferior cerebellar arteries (AICA), which supply the inferior cerebellum; the superior cerebellar arteries, which supply the superior cerebellum; the pontine arteries, which supply the pons; and the posterior cerebral arteries, which supply the occipital and temporal lobes.

The posterior cerebral arteries are particularly important because they contribute to the inferior portion of the Circle of Willis, a critical anastomotic network of arteries that ensures blood supply to both the anterior and posterior regions of the brain. The pons and spinal cord are irrigated by branches arising from the vertebral arteries. Table 2 illustrates the various arteries contributing to the Circle of Willis and the areas and structures they supply.

The venous drainage system of the brain (Figure 10) differs significantly from the blood vessels in both its drainage pathway and anatomy. Unlike blood vessels, veins lack internal valves and muscular walls. While the cerebrum, cerebellum, and brainstem are drained by veins that converge into the intracranial dural venous sinuses, the spinal cord is drained by the anterior and posterior spinal veins, which empty into the internal and external vertebral plexuses.

Removal of deoxygenated blood from the cerebrum by the dural venous system occurs within the space that is between the arachnoid and pia matter, known as the leptomeningeal or subarachnoid space. These cerebral veins are divided into superficial or deep veins. The superficial veins are comprised by the sagittal sinuses and cortical veins, whereas the deep veins are comprised by the straight sinus, transverse/lateral and sigmoid sinuses (Figure 11).

Connecting the extracranial vessels draining the scalp to the intracranial dural venous system are the emissary veins. These are also valveless structures that play a role in cooling of the brain and providing an alternate pathway in cases where the dural venous sinuses become obstructed. They are mainly formed by the junction of a venous sinus, diploic veins and superficial cranial veins (Figure 12).

Altogether, brain blood is drained in the following order before deoxygenated blood reaches the heart (Figure 13):

See Figures 11, 12 and 14 for illustration of the brain’s venous drainage system.  

VENTRICULAR SYSTEM OF THE BRAIN

The brain’s ventricular system consists of four internal cavities that are interconnected. These include two lateral ventricles, along with the third and fourth ventricles. Each of these cavities produces and secretes cerebrospinal fluid (CSF) for the brain and spinal cord. The choroid plexus, located within each ventricular cavity and composed of ependymal cells, is responsible for producing CSF and providing the brain with essential electrolytes. The left and right lateral ventricles deliver CSF to the third ventricle through the interventricular foramina, while the third ventricle sends CSF to the fourth ventricle through the cerebral aqueduct. At the midline, above and below the fourth ventricle, the lateral and median apertures open into the subarachnoid space, allowing CSF to flow from the fourth ventricle into the spinal cord and the subarachnoid space at the base of the brain. From there, it drains into the sagittal venous system (Figure 18).

Overview: Gross Anatomy of Internal Brain Structures

The understanding of human physiology and normal body functions would not be possible without the proper functioning of healthy external and moreover, internal brain structures. The external cortical surface of the cerebrum is made of grey matter formed by neuronal cell bodies, dendrites and unmyelinated fibers. The internal cortical surface of the cerebrum is made of white matter formed by myelinated fibers and oligodendrocytes.

Internal structures of the brain are encapsulated within the forebrain, the midbrain and the hindbrain (Figure 15). The forebrain, mainly comprised by the cerebrum, the thalamus and the hypothalamus. As described above, the cerebrum is made by the left and the right hemispheres, both of which are connected by the myelinated axons that that form the corpus callosum. Damage to these myelinated fibers could result in lack of movement control, vision problems or cognitive impairment. The thalamus is considered the “relay center” of the brain, as it serves as a bridge through which tracts (i.e. neural pathways) descend from or ascend to the cortex to the spinal cord with the goal of coordinating motor and sensory information. The thalamus is also involved in creating alertness, consciousness and sleep, this latter as it associated with the pineal gland that sits between the left and right side of the thalamus. The hypothalamus locates on the base or inferior surface of the thalamus and is considered a master gland given its secretion of hormones to regulate the pituitary gland. Somatic functions such as hunger, thirst, pleasure, sexual behavior, body temperature and appetite are also modulated by the hypothalamus, which connects the brain with the endocrine system.

Within the deep forebrain, structures that associate with the hypothalamus include the limbic system (Figure 16). The limbic system is recognized for its role in emotional and behavioral responses, and for connecting the unconscious roles of the brainstem with the conscious functions associated with the cerebral cortex. Structures that comprise the limbic system are the amygdala (regulate emotions such as fear), hippocampus (formation of new memories), fornix (episodic memory formation), mammillary bodies (recollective memories: recall memory from a past experience), cingulate gyrus (regulate pain and emotions; prediction of negative consequences) and parahippocampal gyrus (memory encoding and retrieval). As expected, injury to the limbic system would result in abnormal emotions (i.e. increase in anger, anxiety), lack of memory formation, encoding and retrieval), among other symptoms.

The midbrain locates below the cerebral cortex, right between the forebrain and the hindbrain. It is part of the brainstem and is highly related to visual and auditory processing and ocular movements (Figure 17). Three main structures of the midbrain are the superior and inferior colliculi, the tectum, the tegmentum and the superior, middle and inferior cerebellar peduncles. Both, the superior and inferior colliculi serve as a stop station for visual and auditory information prior to its processing at the occipital and temporal lobes. The tectum is also located on the dorsal part of the midbrain, below the colliculi, and plays a role in visual and auditory reflexes. The tegmentum, located below the tectum associate with maintaining homeostatic and reflexive pathways for movement coordination and alertness, muscle tone and posture. The cerebellar peduncles connect the brainstem with the cerebrum and the cerebellum. Within the midbrain, the cerebral aqueduct serves as a conduit to transfer CSF from the third to the fourth ventricle, which provides electrolytes and serves as a cushion to the brain and spinal cord (Figure 18). Other deep structures of the midbrain that are responsible for the control of body movements and contain large dopaminergic neurons are the red nucleus and the substantia nigra. Damage to these structures result in tremors observed in patients with Parkinson’s disease.

The hindbrain can be viewed as the superior extension of the spinal, located on the back of the head (Figure 17). It is part of the brainstem and as such, it contains the pons, the cerebellum and the medulla oblongata. Collectively, these hindbrain structures control autonomic functions that are key to survival, such as breathing, heat rate and swallowing. The pons contains axonal fibers that connect the medulla oblongata and the cerebellum to higher centers of the brain. Below the occipital lobe and dorsal to the pons and medulla oblongata, the cerebellum provides subconscious (unaware) precise timing and muscle contraction during coordinated movements such as walking and driving. It also serves as a storage center for memories of previously learned motor skills, such as skating and swimming. Anatomically, the cerebellum is like a separate cerebrum, with the outer aspect made of gray matter and the inner aspect made of white matter. It receives information from ear, muscle, tendons and joints to regulate balance and for the learning and coordination of motor movements. With respect to memory, the cerebellum is known to process procedural memory.

Finally, the medulla oblongata is directly connected to the spinal cord, and it is containing the main centers to regulate blood pressure, heartbeat, and respiration. In also controls vomiting, coughing, and swallowing.

Table 3 summarizes key internal brain structures along with their functions and pathologies and/or conditions associated with dysfunction of these areas. *** Introduce H5P and table