| Brain cell | |
|---|---|
 Illustration of aneuron,microtubules shown as they enter theaxon, and seen enclosed in amyelin sheath. | |
| Anatomical terms of microanatomy |
Brain cells make up thefunctional tissue of the brain. The rest of the brain tissue is the structuralstroma that includesconnective tissue such as themeninges,blood vessels, and ducts. The two main types of cells in thebrain areneurons, also known as nerve cells, andglial cells, also known as neuroglia.[1] There are many types of neuron, and several types of glial cell.
Neurons are theexcitable cells of the brain that function by communicating with other neurons andinterneurons (viasynapses), inneural circuits andlarger brain networks. The two main neuronal classes in thecerebral cortex are excitatory projection neurons (around 70-80%) and inhibitory interneurons (around 20–30%).[2] Neurons are often grouped into a cluster known as anucleus where they usually have roughly similar connections and functions.[3] Nuclei are connected to other nuclei bytracts ofwhite matter.
Glia are the supporting cells of the neurons and have many functions of which not all are clearly understood, but include providing support and nutrients to the neurons. Glia are grouped intomacroglia—astrocytes,ependymal cells, andoligodendrocytes, and much smallermicroglia which are themacrophages of thecentral nervous system. Astrocytes are capable of communication with neurons involving a signaling process similar toneurotransmission, calledgliotransmission.[4]
Brain cell types are the functionalneurons, and supportingglia.
Neurons, also called nerve cells, are the functionalelectrically excitable cells of the brain. They can only function in collaboration with other neurons andinterneurons in a neural circuit. There are an estimated 100 billion neurons in the human brain. Neurons arepolarised cells that are specialised for the conduction ofaction potentials also called nerve impulses.[1] They can also synthesise membrane and protein. Neurons communicate with other neurons usingneurotransmitters released from theirsynapses, and they may be inhibitory, excitatory orneuromodulatory. Neurons may be termed by their associated neurotransmitter such as excitatorydopaminergic neurons and inhibitoryGABAergic neurons.[5]
Cortical interneurons only make up around a fifth of the neuronal population but they play a major role in modulating cortical activity needed for cognition and many aspects of learning and memory. Cortical interneurons vary in shape, molecular make-up, and electrophysiology; they function collectively to maintain the balance between excitation and inhibition in the cortex primarily through the use ofGABA. Disruption of this balance is a common feature of neuropsychiatric disorders such asschizophrenia. A cause of the disruption can occur inprenatal development through theexposure to chemicals and environment.[6]
In thecerebral cortex different neurons occupy the differentcortical layers and include thepyramidal neurons androsehip neurons. In thecerebellumPurkinje cells and interneuronalGolgi cells predominate.
Glial cells are the supporting cells of the neurons. The three types of glial cells areastrocytes,oligodendrocytes, andependymal cells, known collectively asmacroglia, and the smallerscavenger cells known asmicroglia. Glial stem cells are found in all parts of the adult brain.[1] Glial cells greatly outnumber neurons and apart from their supporting role to neurons, glia – astrocytes in particular have been acknowledged as being able to communicate with neurons involving a signalling process similar toneurotransmission calledgliotransmission.[4] They cannot produce anaction potential as generated by a neuron but in their large numbers they can produce chemicals expressing excitability that exert an influence on neural circuitry.[7][4] The star-like shape of the astrocyte allows contact with a great many synapses.[7]
Microglia account for about around 5–10% of cells found within the brain.[8]