2 Neuroscience2.4 Cells
2.4.1 Cell types and their anatomical and functional localisation in cortical layers
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Written by Resident Doctor Committee · Published
Classifying neurones by axon length
Classing neurones by axon length is a separate scheme from classing them by number of processes.
Class | Axon | Examples |
|---|---|---|
Golgi type I | Long, projecting | Pyramidal and Purkinje cells |
Golgi type II | Short, ending near the cell body | Cortical spiny stellate cells, many interneurones |
Amacrine | None | Some retinal amacrine cells |
Neurones of the cerebral cortex
The cerebral cortex divides into neocortex, paleocortex and archicortex, and the neocortex is the newest and largest.
Most neocortical neurones are excitatory pyramidal cells; roughly a fifth are non-pyramidal interneurones, mostly inhibitory and releasing gamma-aminobutyric acid (GABA).
Pyramidal cells are the principal excitatory glutamatergic projection neurones, with an apical dendrite rising towards the pia, basal dendrites and a descending axon; they also occur outside the neocortex.
Chandelier (axo-axonic) cells are GABAergic interneurones whose synapses, grouped in vertical cartridges, lie only on the axon initial segment, so they control each pyramidal cell's output.
In schizophrenia, post-mortem studies repeatedly find reduced GABA transporter 1 (GAT-1) labelling of chandelier cartridges in dorsolateral prefrontal cortex.
Betz cells are giant layer V pyramidal neurones of the precentral gyrus whose long axons reach spinal motor neurones; their loss is the upper motor neurone element of motor neurone disease.
Von Economo neurones are elongated bipolar cells in layer V of the anterior cingulate and fronto-insular cortex, regions for social awareness and interoception; their early loss fits behavioural-variant frontotemporal dementia.
The six neocortical layers
Layer | Name | Cells and connections |
|---|---|---|
I | Molecular | Few cell bodies; dendritic tufts and horizontal inputs |
II | External granular | Small cells; local and corticocortical connections |
III | External pyramidal | Association and commissural projections |
IV | Internal granular | Thalamic input; spiny stellate cells, prominent in sensory cortex |
V | Internal pyramidal | Output to subcortical structures, brainstem and spinal cord; Betz and von Economo cells |
VI | Multiform | Corticothalamic feedback |
Granular (sensory) cortex has a prominent layer IV, whereas agranular (motor) cortex has an indistinct layer IV and a strong layer V.
Cerebellum and striatum
The cerebellar cortex has an outer molecular and an inner granular layer, with Purkinje cells in a single row between them.
Purkinje cells are GABAergic neurones found only in the cerebellar cortex, and they carry all of its output.
Granule cells of the cerebellum are excitatory glutamatergic neurones whose parallel fibres drive Purkinje cell dendrites; they outnumber every other neurone in the brain.
Medium spiny neurones are the great majority of striatal neurones: GABAergic cells that receive nigral dopamine and cortical glutamate and project to the pallidum and substantia nigra.
In Huntington's disease, medium spiny neurones of the indirect pathway degenerate early.
Glia
Neuroglia are astrocytes, oligodendrocytes, microglia, ependymal cells and the peripheral Schwann cells; Purkinje cells are neurones, not glia.
Microglia are mesodermal, from the yolk-sac myeloid lineage, whereas neurones and all other glia arise from neuroectoderm or neural crest.
Astrocytes
Astrocytes clear synaptic glutamate through the excitatory amino acid transporters EAAT1 and EAAT2 (GLT-1), which ends the signal and protects neurones from excitotoxicity.
They convert the glutamate to glutamine with glutamine synthetase, an enzyme found in astrocytes and not in neurones.
They also buffer extracellular potassium and supply lactate to active neurones.
Their perivascular end-feet on cerebral capillaries induce and maintain the tight junctions between endothelial cells that form the blood-brain barrier.
They give healthy grey matter its structural framework and form scar tissue after injury, called gliosis.
Rosenthal fibres are elongated, beaded eosinophilic inclusions in astrocytic processes, made of glial fibrillary acidic protein (GFAP) filaments around alpha-B-crystallin, and define Alexander disease.
Microglia
Microglia are the resident macrophage-like phagocytes of the central nervous system (CNS): they survey tissue, activate after injury and engulf dead neurones and debris.
Rod cells are a classic activated form of microglia.
Microglia are the brain cells that human immunodeficiency virus (HIV) infects.
Microglia prune synapses tagged with complement C1q and C3, and excess pruning linked to C4 gene variants is one proposed route to cortical grey matter loss in schizophrenia.
Ependyma
Ependymal cells form the ciliated lining of the ventricles and central canal, and their beating cilia help circulate cerebrospinal fluid.
The choroid plexus epithelium, which is modified ependyma, secretes cerebrospinal fluid.
Myelinating cells
Myelin is made by glia, not neurones, and which glial cell makes it depends on the site.
Oligodendrocytes myelinate the CNS, each sending processes to internodes on several axons, so losing one cell leaves several bare segments.
The optic nerve is myelinated by oligodendrocytes because it is a central tract despite being a cranial nerve.
Schwann cells myelinate peripheral nerves, each wrapping one internode of one axon, and they are the target in Guillain-Barré syndrome.
After peripheral nerve injury, Schwann cells clear myelin debris and form bands of Büngner that guide regrowth, so peripheral nerves can recover.