The central nervous system consists of the brain and the spinal cord. The peripheral nervous system consists of the extensions of neural structures beyond the central nervous system and includes somatic and autonomic divisions.
The brain is composed of 3 main structural divisions: the cerebrum, the brainstem, and the cerebellum (see the images below). At the base of the brain is the brainstem, which extends from the upper cervical spinal cord to the diencephalon of the cerebrum. The brainstem is divided into the medulla, pons, and midbrain. Posterior to the brainstem lies the cerebellum.
The cerebrum is the largest component of the brain. It is divided into right and left hemispheres. The corpus callosum is the collection of white matter fibers that joins these hemispheres.
Each of the cerebral hemispheres is further divided into 4 lobes: the frontal lobe, the parietal lobe, the temporal lobe, and the occipital lobe. The medial temporal lobe structures are considered by some to be part of the so-called limbic lobe.
Briefly, the frontal lobe is distinguished from the parietal lobe posteriorly by the central sulcus (see the image below). The frontal lobe and parietal lobes are divided inferiorly from the temporal lobe by the lateral sulcus. The parietal lobe is distinguished from the occipital lobe by the parieto-occipital sulcus on the medial surface.
The cerebrum is further divided into the telencephalon and diencephalon. The telencephalon consists of the cortex, the subcortical fibers, and the basal nuclei. The diencephalon mainly consists of the thalamus and hypothalamus. The telencephalon of the cerebrum is disproportionately well-developed in humans as compared with other mammals.
Cortex and subcortical fibers
The outermost layer of the cerebrum is the cortex, which has a slightly gray appearance–hence the term “gray matter.” The cortex has a folded structure; each fold is termed a gyrus, while each groove between the folds is termed a sulcus. Cortical anatomy is discussed in greater detail below.
Below the cortex are axons, which are long fibers that emanate from and connect neurons. Axons are insulated by myelin, which increases the speed of conduction. Myelin is what gives the white appearance to these fibers of the brain–hence the term “white matter.”
The neocortex is the most phylogenetically developed structure of the human brain as compared with the brains of other species. The complex pattern of folding allows an increased cortical surface to occupy a smaller cranial volume. The pattern of folding that forms the sulcal and gyral patterns remains highly preserved across individuals. This enables a nomenclature for the cortical anatomy.
The left and right cerebral hemispheres are separated by the longitudinal cerebral fissure. The principal connection between the 2 hemispheres is the corpus callosum. Each cortical hemisphere can be divided into 4 lobes: frontal, temporal, parietal, and occipital. The frontal lobe can be distinguished from the temporal lobe by the lateral sulcus (Sylvian fissure). The frontal lobe can be distinguished from the parietal lobe by the central sulcus (Rolandic fissure). The parieto-occipital sulcus, which is visible on the medial aspect of the hemisphere, divides the parietal and occipital lobes. Within the lateral sulcus is another cortical surface referred to as the insula.
The frontal lobe can then be further divided into the superior, middle, and inferior frontal gyri, which are divided by the superior and inferior frontal sulci, respectively. The inferior frontal gyrus forms the frontal operculum, which overlies the lateral sulcus. The frontal operculum can be divided into 3 triangular gyri: the pars orbitalis, the pars triangularis, and the pars opercularis, in order from anterior to posterior. The precentral gyrus is the gyrus immediately anterior to the central sulcus.
Similarly, the temporal lobe is divided into the superior, middle, and inferior temporal gyri, which are separated by the superior and inferior temporal sulci. On the inferior surface of the temporal lobe just lateral to the midbrain the parahippocampal gyrus can be identified, with the collateral sulcus lying lateral. Between the parahippocampal gyrus and the inferior temporal gyrus lies the occipitotemporal gyrus, also known as the fusiform gyrus.
Within the parietal lobe, the superior temporal sulcus is capped by the angular gyrus. Just above this, the lateral sulcus is capped by the supramarginal gyrus. Just below the angular gyrus, the lateral occipital gyrus caps the inferior temporal sulcus.
Evolutionarily, the brainstem is the most ancient part of the brain. Structurally, it can be divided into the medulla oblongata, pons, and midbrain. These three structures are briefly described below. Cross-sectional anatomy of the brainstem is rather complex, given the multiple traversing pathways and cranial nerve nuclei (see the image below). [1, 2, 3]
The medulla oblongata, or simply medulla, is continuous with and superior to the cervical spinal cord. There are several external anatomic features of the medulla that can be visible grossly. Ventrally, the pyramids and pyramidal decussation is visualized just below the pons. These are the descending corticospinal tracts. Just lateral to the pyramids, the rootlets of the hypoglossal nerve can be seen as they exit the brainstem. Lateral to the rootlets of the hypoglossal nerve is the inferior olive. Dorsolateral to the inferior olive, the rootlets of the 9th and 10th cranial nerves (glossopharyngeal and vagus) exit.
Dorsally, 2 pairs of protrusions are visible, which are the gracile tubercles medially and the cuneate tubercles just lateral to those. These represent the nuclei where sensory information from the dorsal columns is relayed onto thalamic projection neurons. Just superior to these protrusions is the floor of the fourth ventricle, which bears several characteristic impressions. The vagal trigone is the dorsal nucleus of the vagus nerve (cranial nerve X) and lies inferiorly, just below the hypoglossal trigone.
Superior to the medulla lies the pons, the ventral surface of which has a characteristic band of horizontal fibers. These fibers are the pontocerebellar fibers that are in turn projections from the corticopontine fibers. They cross to enter the contralateral middle cerebellar peduncle and thus enter the cerebellum.
On either side of the midline, there are bulges that are produced by the descending corticospinal tracts. At the pontomedullary junction, the 6th cranial nerve (abducens) can be seen exiting the brainstem. Laterally, but anterior to the middle cerebellar peduncle, the fifth cranial nerve (trigeminal) is seen exiting the brainstem. Below the middle cerebellar peduncle, the seventh and eighth cranial nerves (facial and vestibulocochlear) can be seen exiting. Dorsally, the pons forms the floor of the fourth ventricle.
The midbrain, also termed the mesencephalon, is the superiormost aspect of the brainstem. Ventrally, the midbrain appears as 2 bundles that diverge rostrally as the cerebral peduncles. Between the cerebral peduncles, the third cranial nerve (oculomotor) can be seen exiting. The fourth cranial nerve (trochlear) exits dorsally and is unique in this regard. It then courses anteriorly against the cerebral peduncles.
The posterior aspect of the midbrain has 2 pairs of characteristic protrusions, the superior and inferior colliculi. The superior colliculi are involved in mediating the vestibulo-ocular reflex, whereas the inferior colliculi are involved in sound localization.
The cerebellum occupies the posterior fossa, dorsal to the pons and medulla. It is involved primarily in modulating motor control to enable precisely coordinated body movements. Similar to the cerebrum, which has gyri and sulci, the cerebellum has finer folia and fissures that increase the surface area.
The cerebellum consists of 2 hemispheres, connected by a midline structure called the vermis. In contrast to the neocortex of the cerebrum, the cerebellar cortex has 3 layers: molecular, Purkinje, and granular. There are 4 deep cerebellar nuclei: the fastigial, globose, emboliform, and dentate nuclei, in sequence from medial to lateral. The afferent and efferent pathways to and from the cerebellum exist within the 3 cerebellar peduncles.
In children, the cerebellum is a common location for tumors such as juvenile pilocytic astrocytomas and medulloblastomas. In adults, the posterior fossa is a very common location for metastatic tumors but also a common location for tumors such as hemangioblastomas. Another pathology of the posterior fossa can occur when the cerebellar tonsils descend below the foramen magnum; this is termed a Chiari Imalformation.
The meninges consist of 3 tissue layers that cover the brain and spinal cord: the pia, arachnoid, and the dura mater (see the image below). The pia along with the arachnoid are referred to as the leptomeninges, whereas the dura is referred to as the pachymeninx.
The innermost of the 3 layers is the pia mater, which tightly covers the brain itself, conforming to its grooves and folds. This layer is rich with blood vessels that descend into the brain.
Outside the pia mater, which tightly contours the brain, is the arachnoid mater. The arachnoid mater is a thin weblike layer. Between the pia mater and the arachnoid mater is a space called the subarachnoid space, which contains cerebrospinal fluid (CSF). This space is where the major arteries supplying blood to the brain lie. If a blood vessel ruptures in this space, it can cause a subarachnoid hemorrhage. The arachnoid cap cells can give rise to meningiomas, a usually benign tumor.
The outermost meningeal layer is the dura mater, which lines the interior of the skull. The dura mater is composed of 2 individual layers, the meningeal dura and the periosteal dura. For the most part, these layers are fused; venous sinuses can be found in areas of separation. The tentorium cerebelli is a dura mater fold that separates the cerebellum from the cerebrum. The falx cerebri is a fold that separates the left and right cerebral hemispheres.
Between the arachnoid mater and the dura mater is the subdural space. If bleeding occurs in the space underneath the dura mater, it is called a subdural hematoma. If bleeding occurs outside the dura but underneath the skull, this is called an epidural hematoma.
The brain is bathed in cerebrospinal fluid (CSF), which is continuously produced and absorbed. The ventricles are CSF-containing cavities within the brain. The structures that produce CSF are contained within the ventricles and are called the choroid plexuses. CSF is produced at a rate of about 450 mL/day, although at any given time about 150 mL can be found within the CSF spaces. Thus, the volume of CSF in most adults is turned over about 3 times per day.
The brain has 4 ventricles (see the image below). Within the cerebral hemispheres are the lateral ventricles, which are connected to each other and to the third ventricle through a pathway called the interventricular foramen (of Monro). The third ventricle lies in the midline, separating deeper brain structures such as the left and right thalami. The third ventricle communicates with the fourth ventricle through the cerebral aqueduct (of Sylvius), which is a long narrow tube.
From the fourth ventricle, CSF flows into the subarachnoid space around both the brain and the spinal cord. From the subarachnoid space, CSF is then absorbed into the venous system. Arachnoid granulations or villi are structures projecting into the superior sagittal sinus that release CSF back into the venous system.
Hydrocephalus is a condition in which production of CSF is disproportionate to absorption. This is most commonly caused by impaired absorption resulting from obstruction of the CSF circulatory pathways, in which case it is termed obstructive hydrocephalus. This also occurs when the absorption of CSF is impaired, in which case it is termed communicating hydrocephalus. Rarely is hydrocephalus caused by increased CSF production.
Arteries supply blood to the brain via 2 main pairs of vessels: the internal carotid artery and the vertebral artery on each side. The internal carotid artery on each side terminates into the anterior cerebral artery, the middle cerebral artery, and the posterior communicating artery. The vertebral arteries on each side join to form the basilar artery. The basilar artery then gives rise to the posterior cerebral arteries and the superior cerebellar arteries.
The basilar artery, the posterior cerebral arteries, the posterior communicating arteries, and the anterior cerebral arteries, along with the anterior communication artery, form an important collateral circulation at the base of the brain termed the cerebral arterial circle (of Willis). These vessels lie within the subarachnoid space and are a common location for cerebral aneurysms to form.
Venous return to the heart occurs through a combination of deep cerebral veins and superficial cortical veins. The veins then contribute to larger venous sinuses, which lie within the dura and ultimately drain through the internal jugular veins to the brachiocephalic veins and then into the superior vena cava.
The cellular structure of the brain is composed primarily of neurons and their support cells, which are broadly termed glial cells. The 3 principal types of glial cells are astrocytes, oligodendrocytes, and microglia. These glial cells can give rise to glial tumors, such as astrocytomas, oligodendrogliomas, and glioblastomas, which are among the most common primary brain tumors.
When examined histologically, the neurons of the cortical gray matter demonstrate a laminar pattern. The neocortex contains 6 distinct layers, in contrast to the evolutionarily older paleocortex and archicortex, which typically contain 3 layers. The specific cytoarchitectural patterns of the cortex are not uniform throughout the cerebral cortex, and their variation was mapped by the German physician Korbinian Brodmann and presented in 1909. The so-called Brodmann areas represent cytoarchitectural differences across different brain regions, and the numbering scheme developed by Brodmann is still used to refer to distinct areas of the cortex.
Layers of neocortex
See the list below:
I: The molecular layer is the outermost layer of the cortex, which lies adjacent to the pial surface
II: The external granular layer is a dense layer of primarily inhibitory granule cells; this layer serves mainly to establish intracortical connections
III: The external pyramidal layer contains smaller neurons than its deeper counterpart; this layer provides projections to association fibers and commissural fibers.
IV: The internal granular layer is the principal input layer of the cortex, with input derived largely from the thalamus
V: The internal pyramidal layer is typically the largest layer within the cortex, containing large pyramidal cells; it is one of the principal output layers of the cortex, projecting to subcortical and spinal pathways; in the motor cortex, cells of this layer are termed Betz cells
VI: The fusiform layer contains cells that form association and projection fibers
White matter tracts connect both nearby and distal brain structures and can be distinguished according to the types of connections they mediate.
Projection fibers connect structures over the longest distances, such as the corticospinal projections from the motor cortex to the anterior horn cells of the spinal cord. Association fibers connect structures within the same hemisphere, such as the arcuate fasciculus, which connects the temporoparietal receptive speech areas with the frontal speech areas. Commissural fibers connect homologous structures in the left and right hemispheres, the most notable example being the corpus callosum.
Diffusion tensor imaging has emerged recently as a magnetic resonance imaging tool that provides exceptionally detailed white matter tractography in both normal and pathologic anatomy.
Our current understanding of functional localization in the cortex (see the image below) is derived from several sources, which include insights from patients with lesions involving specific areas of the cortex, awake mapping of the cortex during brain surgery, and functional imaging studies such as functional magnetic resonance imaging (MRI) and positron emission tomography (PET) in healthy volunteers.
Some of the earliest contributions to modern language mapping can be traced to the work of neurologist Paul Broca, who studied the language deficits in patients with stroke. Broca’s area, as it is termed, is a region of the frontal operculum, which also overlaps with Brodmann area 44 and 45. Three overlapping names describe this region, which is responsible for speech production. Selective damage to this region leads to difficulty speaking but typically with preserved comprehension.
In contrast, Wernicke’s area refers to the posterior aspect of the superior temporal gyrus, which overlaps with Brodmann area 22. This region is generally responsible for speech comprehension, and selective injury to it can lead to impaired understanding with preserved speech production.
Additionally, language function is hemispherically dominant. This means that Broca’s and Wernicke’s aphasia typically result from damage to the hemisphere that is dominant for language. In right-handed individuals, the left hemisphere is nearly always dominant for language. However, among left-handed individuals, the left hemisphere is dominant for speech in only 70%. Bilateral representation occurs in 15% of left-handed people, and right-hemisphere language representation occurs in 15% of left-handed people.
The primary motor and sensory cortex have been mapped extensively through intraoperative stimulation in awake patients. Early work performed by neurosurgeon Wilder Penfield in Montreal led to the conceptualization of the homunculus, which is the somatotopic representation of the body in both the primary motor and primary sensory cortex (see the image below).