The cerebrum (telencephalon) is the largest part of the brain and comprises the cerebral cortex and subcortical structures (e.g., basal ganglia, hippocampus). The longitudinal fissure divides the brain into two hemispheres. The cortex represents the top-outer layer of the brain, which receives its convoluted appearance from a network of gyri and sulci. Sulci separate the cerebral cortex further into a frontal, temporal, parietal, and occipital lobe. The cerebrum is the key structure involved in perception, language, and coordination. The basal ganglia are situated beneath the cortex, and they are heavily involved in motor control. The cortical hemispheres contain one of the lateral ventricles each, with the smaller third and fourth ventricles being located between the two thalami in the diencephalon and between the cerebral aqueduct and obex respectively. The four ventricles form an interconnected system that produces, drains, and is filled with cerebrospinal fluid, which plays a role in waste removal and cushioning of the brain. The meninges comprise the three protective membranes that envelop the central nervous system, i.e. the brain and spinal cord.
- The cerebral cortex receives its convoluted appearance from a network of gyri (rounded ridges on the surface of the cortex) and sulci (furrows separating the gyri).
- The longitudinal fissure divides the brain into two hemispheres.
- Deep sulci divide each hemisphere further into a frontal, temporal, parietal, and occipital lobe.
- Deep sulci separate the cerebral cortex into different lobes: frontal, temporal, parietal, and occipital
- The main sulci
- Central sulcus (of Rolando): separates the frontal and parietal lobes
- Lateral sulcus (Sylvian fissure): separates the frontal and temporal lobes anteriorly and the parietal and temporal lobes posteriorly
- Cingulate sulcus: separates the cingulate gyrus from the frontal and parietal lobes
- Parieto-occipital sulcus: separates the parietal and occipital lobes
- Calcarine sulcus: divides the occipital lobe horizontally into the cuneus (superior) and lingual (inferior) gyrus
|Overview of the frontal lobe|
|Area||Location||Motor and cognitive functions||Effect of lesion|
Primary motor cortex
Frontal eye field
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(Brodmann areas 44 and 45)
The cortical regions responsible for processing the motor functions of the different regions of the body can be mapped using a motor homunculus.
|Overview of the parietal lobe|
|Area||Location||Characteristics (sensory functions)||Effect of lesion|
Primary somatosensory cortex
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|Somatosensory association cortex|| || || |
|Visual association cortex|| || || |
|Overview of the temporal lobe|
|Area||Location||Characteristics (hearing)||Effect of lesion|
Primary auditory cortex
(Brodmann areas 41 and 42)
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|Wernicke area|| |
|Overview of the occipital lobe|
|Area||Location||Characteristics (visual functions)||Effect of lesion|
|Primary visual cortex|
|Secondary visual cortex|| |
- Processing of emotions
- Perception of body functions (interoception)
- Integration and perception of somatosensory stimuli
- Perception of gustatory and olfactory stimuli
- Modulation of autonomic functions (e.g., blood pressure, gastrointestinal motility)
- A deep subcortical structure that lies between the basal ganglia
- Largest collection of nerve fibers traveling to and from the cerebral cortex, forming the corona radiata (neuroanatomy), a collection of white matter fibers.
- Components: anterior limb, genu, and posterior limb
- Three layers of connective tissue that cover and protect the brain and the spinal cord
- Divided into (from outer to inner layer): dura mater, arachnoid mater, and pia mater
|Overview of the meningeal layers |
|Arachnoid mater|| |
|Pia mater|| |
Falces of the brain
|Overview of the falces of the brain|
|Structure||Location||Characteristics||Sites of attachment||Cerebral sinuses|
|Falx cerebri|| || |
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Spaces of the CNS
|Overview of the spaces of the CNS|
|Epidural space|| |
|Subdural space|| |
|Subarachnoid space|| |
- The basal ganglia are a group of nuclei that lie beneath the cortex.
- Distributed over the telencephalon, diencephalon, and mesencephalon (midbrain)
|Overview of the basal ganglia|
|Basal ganglia||Location||Clinical significance|
|Lentiform nucleus||Globus pallidus|
|Substantia nigra|| |
Cortico-basal ganglia-thalamo-cortical loop (CBGTC)
- A neuronal circuit between the cortices of the brain, the basal ganglia, and the thalamus
- Via this loop, the basal ganglia aid in the initiation of movement, control of skeletal muscles, and adjustment of posture.
- Two main pathways: the direct pathway and the indirect pathway.
- The balance of activity between the direct and indirect pathways is modulated by dopamine.
Functional anatomy of the basal ganglia
|Overview of the functional anatomy of the basal ganglia|
Motor excitatory part
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|Motor inhibitory part|| || |
|Pallidum||Globus pallidus internus (Gpi)|| || |
|Globus pallidus externus (Gpe)|| || |
|Subthalamic nucleus|| || |
|Substantia nigra||Pars compacta|| |
|Pars reticularis|| || |
Direct pathway of the basal ganglia (excitatory)
- Function: increase in motor activity
- Motor cortex activation → glutamate release → striatum stimulation → GABA release → inhibition of Gpi → inhibition of GABA release → thalamus disinhibition → premotor cortex stimulation → muscle activation → ↑ movement
- In addition, the subthalamic nucleus stimulates the substantia nigra (pars compacta) → dopamine release → stimulation of D1 receptor in striatum → further inhibition of Gpi → inhibition of GABA release → thalamus disinhibition → premotor cortex stimulation → muscle activation → ↑ movement
Binding of dopamine to D1 Receptors stimulates the excitatory D1Rect pathway.
Indirect pathway of the basal ganglia (inhibitory)
- Decrease in motor activity
- Modulation of the disinhibitory effect of the direct pathway
- Motor cortex activation → glutamate release → stimulation of striatum → GABA release → inhibition of Gpe → inhibition of GABA release → ↓ inhibition (activation) of subthalamic nucleus → glutamate release → stimulation of substantia nigra (pars reticularis) and Gpi → GABA release → inhibition of thalamus → inhibition of premotor cortex → deactivation of muscles → ↓ movement
- The striatum also stimulates the substantia nigra (pars compacta) → dopamine release → stimulation of D2 receptor in striatum → inhibition of GABA release → disinhibition of Gpe → GABA release → inhibition of subthalamic nucleus → ↓ stimulation of GPi → disinhibition of thalamus → ↑ movement
The indirect pathway is inhibitory.
- Dopaminergic pathways are collections of neurons that release dopamine.
- Dysfunction of dopaminergic pathways is involved in some psychiatric diseases; (e.g., schizophrenia) and movement disorders (e.g., Parkinson disease).
- Antipsychotic drugs target dopaminergic pathways.
|Overview of dopaminergic pathways|
- Consists of four interconnected cavities within the brain: the two lateral ventricles; the third ventricle, located in the diencephalon; and the fourth ventricle, located dorsal to the pons
- Involved in the production and circulation of cerebrospinal fluid (CSF)
- CSF serves as a fluid cushion and removes waste products from the brain
|Overview of the ventricular system|
|Lateral ventricles|| || |
The foramina of Luschka are the Lateral apertures and the foramen of Magendie is the Medial aperture of the fourth ventricle.
- Description: a clear and colorless fluid derived from blood plasma that provides mechanical support to the CNS and transports biochemical compounds (e.g., neuromodulators) and waste products
- CSF production: produced by choroid plexuses in the lateral, third, and fourth ventricles by filtration of plasma
- CSF flow: lateral ventricles → third ventricle (via interventricular foramina) → fourth ventricle (via cerebral aqueduct) → diffusion and active transfer into the (via foramina of Luschka and Magendie) → reabsorption in the arachnoid granulations (a group of projections of the arachnoid mater into the dural sinuses) → drainage into the
Basal cisterns (subarachnoid cisterns)
- Numerous compartments within the pia mater and arachnoid membrane, filled with cerebrospinal fluid formed by a separation of the
- Contain arteries, veins, and nerves
- Interconnected and essential for CSF circulation
- Cisterna magna
- Clinical relevance
The nervous system is primarily composed of:
- Neurons: polarized cells with the ability of transmitting signals from one cell to another
- Supporting glial cells
- For more information on the microscopic anatomy of the neurological tissue, see “.”
- Cortical neuroanatomy
- The blood-brain barrier separates the brain tissue from circulating blood and provides protection (e.g., from microorganisms, drugs)
- The blood-cerebrospinal fluid barrier separates the cerebrospinal fluid from the blood circulation.
- Makes up the majority of the cerebral cortex
- Responsible for high-level processes (e.g., sensory perception, cognition, language skills)
- Molecular layer: nerve fibers
- External granular layer
- External pyramidal layer: small to medium-sized pyramidal cells that project axons to other cortical areas
- Internal granular layer
- Termination area of thalamocortical projections (form lines of Gennari in the primary visual cortex)
- Filled with densely packed, medium-sized pyramidal and nonpyramidal cells
- Internal pyramidal layer: large pyramidal cells give rise to the axons that form the corticospinal tracts and corticobulbar tracts (pyramidal tracts to the brainstem and spinal cord)
- Multiform layer: composed of pyramidal and nonpyramidal cells of various size
- Composed of three layers
- Consists of the hippocampus and the olfactory cortex
- Description: a barrier of CNS blood vessels separating the brain from circulating blood
- Function: protects the central nervous system from microorganisms, cells, proteins, and drugs that can cause damage to the brain and other structures
- Transport mechanisms across the blood-brain barrier
Structures with no blood-brain barrier
- Structures located around the brain ventricles (circumventricular location), including:
- Allow certain molecules to affect brain function (e.g., blood-borne drugs and hormones)
- Damage mechanisms
Blood-cerebrospinal fluid barrier (BCSFB)
- Function: separates cerebrospinal fluid from the blood circulation
Structure: formed by the choroid plexus
- Modified ependymal cells (choroid epithelial cells) held by tight junctions
- Capillary endothelial cells have fenestrations
- Basal membrane
- Choroid plexus carcinoma: a rare malignant neoplasm of the choroid plexus (∼ 1% of pediatric intracranial malignancies)
Brain metabolism and homeostasis
Brain metabolism 
- The brain requires 15% of the cardiac output and ∼ 20% of the body's oxygen to function properly. 
- Main energy-demanding functions of the brain:
- Active transportation of ions
- Maintenance and restoration of membrane potentials
- Synthesis and metabolism of neurotransmitters
- Metabolic requirements vary according to regional neuronal activity and are directly related to changes in cerebral blood flow and energy substrate utilization (see “Cerebral autoregulation”).
- Energy is mainly derived from the aerobic oxidation of glucose.
- The uptake of glucose by neurons is provided by the insulin-independent GLUT3 transporter.
- Under particular circumstances, the brain has the capacity to use other energy substrates:
Brain homeostasis 
- Interactions between the brain and peripheral organs such as the liver, pancreas, adipose tissue, gut, and muscle are critical for the maintenance of energy and glucose homeostasis.
- The blood-brain barrier plays a key role by: 
- Exchanging substances between the blood and brain parenchyma
- Modulating the response to extrinsic environmental factors and physiological changes (e.g., sleep/wake cycles, aging, diet) by adjusting its permeability
- The choroid plexus contributes by maintaining a constant diffusion of nutrients to the CNS through the CSF
- For more information, see “” and “ ” in “ .”
The hypothalamus plays a key role in homeostasis by:
- Modulating energy intake and expenditure
- Modulating hepatic glucose production, insulin and glucagon secretion, and skeletal muscle glucose uptake, therefore maintaining glucose homeostasis
- Peripheral metabolic hormones and nutrients target hypothalamic neuronal circuits, providing feedback signals that help maintain this balance. (See “) ” in “ ”
- Cholecystokinin, peptide YY, and GLP-1 provide information on energy intake with meals.
- Leptin stimulates locomotor activity, skeletal muscle fatty acid oxidation, and brown adipose tissue thermogenesis, regulating energy expenditure.
- POMC-producing neurons, located in the arcuate hypothalamic nucleus, respond to neuroendocrine dysregulation. and
- Insulin acts as an adipose signal, modulating energy metabolism.
- IL-6 provides information on muscle mass and exercise and stimulates energy expenditure for active muscles.
- The brain is derived from 3 primary vesicles.
- These vesicles develop into 5 subdivisions (secondary brain vesicles)
|Overview of brain embryology|
|Primary vesicles||Brain vesicle||Derived parts of the brain||Derived fluid-filled structure|
- Migration of neurons from the ventricular zone to the cortical zone during weeks 12–24 of gestation leads to the folding of the cerebral cortex. 
Lissencephaly: failure of neuronal migration → lack of cortical sulci and gyri (smooth brain)
- Etiology: genetic mutations (e.g., of the reelin gene) or intrauterine viral infections during the first trimester of pregnancy (esp. CMV)
- Clinical features
- For more information on the embryology of the nervous system see “ ” and “ .”