The brain is the most complex organ of the body and is responsible for cognition as well as the coordination of most of the body's vital processes. Together with the spinal cord, it forms the central nervous system. The brain is protected by the meninges and the neurocranium. Physiologically, the brain functions to control both conscious and unconscious processes by generating action potentials that travel through axons connected to the spinal cord and the peripheral nervous system.
For information on the blood supply of the brain, see.
- 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.
|Area||Location||Characteristics (motor functions and cognition)||Effect of lesion|
(Brodmann area 4)
(Brodmann area 6)
(Brodmann area 8)
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(Brodmann area 44 and 45)
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|Area||Location||Characteristics (sensory functions)||Effect of lesion|
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|Somatosensory association cortex|| || || |
|Visual association cortex|| || |
|Area||Location||Characteristics (hearing)||Effect of lesion|
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|Wernicke's area|| || |
|Area||Location||Characteristics (visual functions)||Effect of lesion|
|Primary visual cortex|| |
|Secondary visual cortex|| || |
|Anterior limb|| |
|Posterior limb|| |
The meninges are 3 layers of connective tissue that cover and protect the brain and the spinal cord. They are divided into the dura mater, arachnoid mater, and pia mater (from outer to inner layer). In addition, the meninges, particularly the dura mater, fold to form septae which include the falx cerebri, falx cerebelli, and tentorium cerebelli.
|Dura mater|| || |
|Pia mater|| || |
Falces of the brain
|Falx cerebri|| || |
|Falx cerebelli|| || || |
|Tentorium cerebelli|| || || || |
|Subthalamic nucleus|| |
|Substantia nigra|| |
- The basal ganglia aid in the initiation of movement and control of skeletal muscles via this loop
- There are two main pathways: the direct pathway and the indirect pathway.
- The balance of activity between the direct and indirect pathways is modulated by dopamine.
Direct pathway of the basal ganglia (excitatory)
- Function: increase in motor activity
- Pathway: motor cortex → glutamate release → stimulates striatum → GABA release → inhibits globus pallidus internus (GPi), which usually inhibits thalamus → disinhibition of thalamus → stimulates premotor cortex → activates muscles → ↑ movement
- Additionally, the subthalamic nucleus stimulates substantia nigra → dopamine release → stimulates D1 receptor in striatum (further inhibits internal globus pallidus) → see above
Indirect pathway of the basal ganglia (inhibitory)
- Function: decrease in motor activity
- Pathway: motor cortex → glutamate release → stimulates striatum → GABA release → inhibition of globus pallidus externus (GPe) → less inhibition of subthalamic nucleus → stimulates substantia nigra and globus pallidus internus (GPi) → inhibition of thalamus → inhibition of premotor cortex → deactivates muscles → ↓ movement
- substantia nigra → dopamine release → stimulates D2 receptor in striatum → inhibits GABA release → disinhibition of globus pallidus externus (GPe) → GABA release → inhibition of subthalamic nucleus → decreases stimulation of GPi → disinhibition of thalamus → ↑ movement
- Pathway: from the substantia nigra (pars compacta) in the midbrain to the caudate nucleus and the putamen
The nervous system is primarily composed of the following parenchyma: neurons and supporting (glial) cells. Neurons are specialized cells that are polarized and have the ability to pass signals from one cell to the other. For more information on the microscopic anatomy of the nerve tissue, see the learning card on .
Layers of the cerebral cortex
Most of the brain is composed of six layers. These areas are referred to as the neocortex. The hippocampus and the olfactory cortex are only composed of three layers and together are referred to as the allocortex. In addition, the cerebral cortex is divided into 47 Brodmann areas.
- The neocortex is formed by the following layers (from external to internal)
- Molecular layer
- External granular layer
- External pyramidal layer
- Internal granular layer → termination area of thalamocortical projections (form lines of Gennari in the primary visual cortex)
- Internal pyramidal layer → area where axons of the corticospinal and corticobulbar tracts form
- Multiform layer → composed of polymorphic cells
- Function: protects the central nervous system from microorganisms, cells, proteins, and drugs that can cause damage to the brain and other structures.
- Structure: made up of four components
- Transport mechanisms: Substances can be transported across the blood-brain barrier via
- Function: separates cerebrospinal fluid from the main circulation
- Structure: It is formed by the choroid plexus.
The CNS is derived from 3 primary vesicles: the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain). These go on to develop into 5 subdivisions: the telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon.
|Primary vesicles||Brain vesicle||Derived parts of the brain||Derived fluid-filled structure|
|Myelencephalon|| || |
For more information on the embryology of the nervous system see , and .