The central nervous system (CNS) is composed of the brain and the spinal cord. The brain consists of the cerebrum, brainstem, and cerebellum. The brainstem is made up of the midbrain, pons, and medulla.
The CNS contains the brain and spinal cord, while the PNS includes nerves.
The cerebrum is made of two cerebral hemispheres connected by the corpus callosum. It contains:
Each hemisphere is divided into four lobes by deep fissures called sulci:
Because the brain grows rapidly within a relatively small cranial cavity, brain tissue bulges between sulci, forming gyri. Gyri can atrophy in Alzheimer’s disease, some cases of psychosis, and frontotemporal (Pick’s) dementia.
Key sulci:
The cerebrum can be further divided into:
The primary motor cortex is located in the precentral gyrus of the frontal lobe (Brodmann area 4). It executes simple movements of contralateral muscles.
The premotor cortex lies anterior to the primary motor cortex in the frontal lobe (Brodmann area 6). It’s associated with more complex movements and movement planning. It has two regions:
The supplemental motor cortex is located on the lateral and medial sides of the frontal lobe. It’s involved in complex movements, such as performing movements in a sequence (e.g., dancing).
The cerebral cortex can be described as containing three types of processing regions: primary, association, and integration areas. The primary cortical areas are where sensory information is initially processed, or where motor commands emerge to go to the brain stem or spinal cord. Association areas are adjacent to primary areas and further process the modality-specific input. Multimodal integration areas are found where the modality-specific regions meet; they can process multiple modalities together or different modalities on the basis of similar functions, such as spatial processing in vision or somatosensation.
The primary somatosensory area is located in the postcentral gyrus of the parietal lobe (Brodmann areas 3, 1, and 2). S1 ultimately receives sensory input from the VPL and VPM nuclei of the thalamus and processes pain, temperature, touch, vibration, and position from contralateral body areas.
The secondary somatosensory area is located in the inferior part of the parietal lobe (Brodmann area 40). It receives inputs from S1 and the thalamus. It’s involved in:
The somatosensory association cortex is located in the superior parietal lobule (Brodmann areas 5 and 7). These areas are involved in complex associations. Damage causes tactile agnosia (inability to recognize an object even though the object can be felt when it is touched).
The primary visual cortex lies in the occipital lobe (Brodmann area 17), surrounding the calcarine sulcus.
The visual association areas surround the primary visual cortex and are involved in recognition of form, color, and movement.
The primary auditory cortex is located in the superior temporal gyrus of the temporal lobe (Brodmann area 41). The auditory association area is Brodmann area 42. Their function is interpretation of sound.
Broca’s area is located in the inferior frontal gyrus of the frontal lobe in the dominant hemisphere (Brodmann areas 44 and 45). It was originally thought to be involved in language production, but it has also been associated (partly) with language comprehension.
It’s involved in:
Hypoactivity in Broca’s area is seen in stammering/stuttering.
Wernicke’s area is located in the left posterior-superior temporal lobe, approximately Brodmann areas 39 and 40. Traditionally, it has mainly been associated with speech comprehension (understanding).
A lesion in this area causes Wernicke’s aphasia (fluent or receptive aphasia). The ability to understand the meaning of spoken words is affected, but talking (language production) is not impaired, so speech may seem nonsensical.
The body is represented upside down in the cerebral cortex:
The neocortex is the most recently evolved area of the cerebral cortex, and it makes up the majority of the human cerebral cortex. It’s arranged in six layers, from superficial to deep:
Layer I
Molecular layer. It is deficient in cells but has an interconnected network of dendrites and axons called the neuropil.
Layer II
External granular layer. It is composed of inhibitory granule cells.
Layer III
External pyramidal layer. It has smaller pyramidal cells.
Layer IV
Internal granular layer. It is rich in stellate cells, which receive inputs from the thalamus in the primary sensory cortex.
Layer V
Internal pyramidal layer. Larger pyramidal neurons in layers V and VI form the corticospinal and corticobulbar tracts. A “Betz cell” is a large pyramidal cell that has a long dendrite at the apex that extends all the way up to layer I.
Layer VI
Multiform or fusiform layer. It forms association and projection fibers.
Glial cells provide support to neurons. They include microglia, astrocytes, and oligodendrocytes.
Microglia are like monocytes and are involved in phagocytosis. They are scavenger cells of the CNS. They fuse to form multinucleated giant cells in HIV.
Astrocytes are multifunctional and are involved in:
GFAP (glial fibrillary acidic protein) and glutamine synthetase are markers for astrocytes.
Oligodendrocytes lay down myelin in the CNS. (Remember that Schwann cells lay down myelin in the peripheral nervous system; they are injured in GBS.) Oligodendrocytes have a “fried egg” appearance on histology. They are injured in multiple sclerosis, leukodystrophies, and progressive multifocal leukoencephalopathy.
Ependymal cells line the ventricles of the brain. Choroid epithelial cells line the villi of the choroid plexuses and secrete CSF.
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