Types of neurons

Pyramidal neurons

Pyramidal neurons are characterized by their cell body, whose shape resembles a teardrop or rounded pyramid. The dendrites emerge either from the top of the pyramid (apical dendrite) or from the base (basal dendrites). Each neuron usually possesses a single apical dendrite that is longer than the basal dendrites and extends numerous dendritic branches.

Pyramidal neurons can be found in the cerebral cortex, mainly in layers III and V (external and internal pyramidal layer respectively). They are also common in subcortical structures, such as the hippocampus and amygdaloid body. Due to the diversity of pyramidal neurons, there are subtypes of pyramidal neurons. For example, giant pyramidal cells (of Betz) are the largest example of pyramidal cells; they are located in the motor cerebral cortex and are considered as the origin for the pyramidal tract controlling voluntary movements (upper motor neurons).

Stellate cells

Stellate cells are small multipolar neurons found mainly in the internal granular layer (cortical layer IV). These neurons have many local dendrites with equal lengths (isodendritic) that radiate uniformly in all directions and a short arbored axon which does not extend beyond the cerebral cortex.

They mainly receive input from the thalamus and are considered to be high-fidelity translators of thalamic input, maintaining strict topographic organization and accurately and efficiently convey sensory information received from the thalamus to other parts of the cerebral cortex. They are also common in the cerebellar cortex, spinal cord and reticular formation.

Purkinje cells

Purkinje cells are located in the cerebellum, between the molecular and granular layers. They have large pear-shaped cell bodies and characteristic fan-shaped dendritic trees which fill the molecular layer. They are the only projection (efferent) neurons of the cerebellar cortex (all other neurons in the cerebellum are intrinsic) and their axons terminate in the cerebellar nuclei. They have an inhibitory role, using gamma-aminobutyric acid (GABA) as a neurotransmitter.

Granule cells

Granule cells are small oval-shaped multipolar interneurons. They exert different functions and neurochemical characteristics depending on their location. They can be found in the cerebral cortex, the granular layer of the cerebellum, the olfactory bulb and the dentate gyrus.

Function

From a functional point of view, neurons can be classified into:

• Motor neurons facilitate the transmission of signals from the CNS to effector organs, such as muscles and glands.
• Sensory neurons receive input from the periphery and convey it to the CNS.

However, most neurons in the CNS can be classified as neither motor nor sensory; since they integrate, combine, process and further transmit the signals received towards other brain regions, they can be characterized as short axon (a.k.a. local circuit) or long axon (projection, association or commissural) interneurons.

Projection type

Regarding their connections, nerve cells of every CNS area can further be classified into:

• Afferent neurons receiving information from other brain areas.
• Efferent (or projection) neurons that are considered the principal neurons of every brain region, extending their axons beyond the borders of the specific area establishing connections with neurons of other regions in the CNS.
• Local circuit neurons (or intrinsic neurons) which have shorter axons and connect with other neurons in their close proximity, exerting their role as mediators between other neurons of the same CNS area. These neurons are also called short axon interneurons.

Effect on other neurons

On the basis of their effect on other neurons, nerve cells can also be classified into:

• Excitatory Neurons: Excitatory neurons facilitate the transmission of signals that induce depolarization in neighboring neurons. This depolarization increases the likelihood of generating an action potential, subsequently activating the neurons.
• Inhibitory Neurons: Constituting a relatively small fraction of the neural population, inhibitory neurons are distinguished by their diverse expression of molecular markers and firing properties. They form intricate circuits that provide inhibition for a wide array of stimuli while also regulating the activity of excitatory neurons. Their connectivity and recruitment play crucial roles in processes such as sensation, movement and cognition.
• Modulatory: Modulatory neurons release neurotransmitters or neuromodulators to influence the activity of other neurons. They modify the sensitivity or responsiveness of neurons to other signals and do not directly stimulate action potentials. They play a crucial role in regulating neural circuits and shaping overall brain function.

However, the effect of each neuron does not only depend on its neurotransmitter but also on the kind of receptors that postsynaptic membranes express, occasionally making this classification inaccurate.

Neurotransmitters

Neurons can also be categorized according to the neurotransmitters which they release. Some common types are the glutamatergic, cholinergic, GABAergic and dopaminergic neurons.

• Glutamatergic neurons produce and secrete glutamate, which is the main excitatory neurotransmitter of the CNS. Pyramidal neurons are principally categorized as glutamatergic.
• Cholinergic neurons are located both in the PNS and the CNS and secrete acetylcholine.
• GABAergic neurons, such as Purkinje cells and many interneurons, are inhibitory neurons. Their neurotransmitter is GABA, the main inhibitory neurotransmitter of the CNS.
• Dopaminergic neurons produce and release the monoamine neurotransmitter dopamine and are mainly located in the midbrain, hypothalamus and olfactory bulb.

It is important to recognize that there are many different classifications, which can sometimes present overlaps or variations in terminology. Researchers and neuroscientists continue to refine and update these classifications to reflect the evolving understanding of the nervous system. It is important to approach such classifications acknowledging their limitations.