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).

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, although the lobes may share some functions, they may also display unique roles. For example, the frontal lobe associates with higher level cognitive functions, these include: speech production, decision-making processes, reasoning, rationale, judgement, emotions, initiation and self-regulation of behavior, attention, control inhibition, movements, making of facial expressions, spontaneity, serial tasks, and development of personality. Compared to the other three lobes, the frontal lobe is the latest to evolve. Synaptic pruning, the process through which formed extra synapses are eliminated, is not achieved until the third decade of life in the frontal cortex. Even when the brain reaches approximately 95% of its adult size around the ages of sixteen-twenty years, full maturity of the brain is not achieved until after 30 years of age. The posterior compartment of the frontal lobe is of great relevance given the presence of the “pre-central or pre-motor gyrus”, a region that contains descending motor fibers that control the execution of voluntary movements. A critical structure of the frontal lobe that regulates the production of language is Broca’s area (Figure 6).

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.

Below the parietal lobe, is the occipital lobe, which contains the primary structures involved in the processing of visual information, whether it has to do with the identification, recognition, location, distance, color or size of an object. In addition, this lobe associates with the visual perception of body language (expressions, posture and gestures), spatial analysis of 3D objects and visual attention.

The temporal lobe, located right above the ear lobe and below the parietal lobe, is designed precisely for the interpretation, distinction and processing of auditory information, recognition of faces, processing of perceptions, verbal, visual and autobiographical memories, which associates with deeper brain structures such as the hippocampus. A structure of the temporal lobe that regulates the comprehension of language is Wernicke’s area (Figure 7). This structure communicates with the Broca’s area of the frontal lobe through the “arcuate fasciculus” which is 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 in the most part, from the brainstem (CN III-CN XII). There are a total of 12 cranial nerves (Figure 8; Table 1), each with specific roles that control what we see, hear, feel, smell and taste. In addition, several cranial nerves are also critical in determining the contraction of muscles of the face, neck and back.

VASCULATURE AND DRAINAGE OF THE BRAIN

Protection of the brain comes from several sources. It’s vasculature, for example, is a main source of oxygen and glucose for the highly active neurons and glia of the brain. Different regions of the brain have distinct metabolic demands, meaning that higher neuronal function and brain activity will result in higher blood flow and supply of oxygen and glucose in that brain area. An increase in the dilation of the blood vessels facilitates this increase in blood flow as higher brain activity produces more nitric oxide, lower pH, lower partial oxygen (paO2) and higher partial CO2 (paCO2). On the contrary, deprivation of blood supply would have detrimental effects given the resulting 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 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 arch of the aorta, left to the common carotid artery.

The left and right vertebral arteries give raise to the “posterior inferior cerebellar artery: PICA (inferiorly)” and ascend towards the brain through the transverse foramen of the cervical vertebra to the base of the skull. It then enters the cranial cavity by passing through the foramen magnum, piercing the dura matter and running on the anterior surface of the medulla oblongata. Right at the lower lateral side of the pons, both left and right vertebral arteries join to form the basilar artery, which mainly supplies that brainstem, cerebellum, and occipital lobes of the brain. Branches of the basilar artery include: “left and right anterior cerebellar arteries: AICA (inferiorly) “; left and right superior cerebellar arteries (superiorly)”: will supply the cerebellum; “left and right pontine arteries”: will supply the pons and the “left and right posterior cerebral arteries (superiorly)”: will supply the occipital and temporal lobes.

The posterior cerebral arteries are highly relevant in that they form the inferior portion of the circle of Willis, a meeting niche where critical arteries anastomose to supply the front and back of the brain. The pons and the spinal cord are irrigated by branches that arise from the vertebral arteries. Table 2 illustrates the different arteries that contribute to the circle of Willis and the brain area and associated structures that they supply.

The venous drainage system of the brain (Figure 10) is quite distinct from that of the blood vessels not only with respect to their drainage trajectory within the central nervous system, but also in their anatomy; distinct from blood vessels, veins lack internal valves and muscular walls. In addition, while the cerebrum, cerebellum and brainstem are drained by veins that join to drain into intracranial dural venous sinuses, the spinal cord is drained by anterior and posterior spinal veins that drain 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 ventricular system of the brain is comprised by four internal cavities that communicate with each other. These cavities include two lateral ventricles along with one third and one fourth ventricle, each capable of producing and secreting cerebrospinal (CSF) fluid within the brain and the spinal cord area. The presence of ependymal cells in a structure that lies within each ventricular cavity, the choroid plexus, is responsible for nourishing the brain with the produced CSF-derived electrolytes. The left and right ventricles delivers CSF to the third ventricle through the interventricular foramina, whereas the third ventricle delivers CSF to the fourth ventricle through the cerebral aqueduct area. In the midline, above and below the fourth ventricle, a lateral and median apertures open into the subarachnoid space that enables the passage of CSF from the fourth ventricle to the spinal cord and the subarachnoid space at the base of the brain to drain 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 cerebrospinal fluid (CSF) from the third to the fourth ventricle, which are brain cavities that contain ependymal cells that produce and secrete CSF (CSF protects the brain by providing electrolytes and serving as a cushion to the brain and spinal cord: Figure X). Other deep strtuctures 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 serve 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