Types of neurons
Our nervous system is a complex network of diverse nerve cells or neurons, as well as glial cells, that work together to coordinate the system's overall function. Neurons are classified into different categories based on different criteria, such as morphology, function, connections and neurotransmitters.
Depending on the cell body morphology, as well as the number of processes extending from it, neurons can be classified as:
|Classification of neurons
|Many different classification schemes based on:
Unipolar neurons: One process, the axon; mainly found in invertebrates
Bipolar neurons: Two processes, one axon and one functioning as distant dendrite
Pseudounipolar neurons: One short axon which splits into two processes (central and peripheral); the most common sensory neurons in the PNS
Multipolar neurons: Multiple processes, a single axon and multiple dendrites
|Motor neurons, sensory neurons, interneurons (short axon or long axon)
|Afferent, efferent (or projection), local circuit neurons (or intrinsic neurons), excitatory, inhibitory, modulatory
|Glutamatergic, cholinergic, GABAergic, dopaminergic neurons
- Unipolar neurons
- Bipolar neurons
- Pseudounipolar neurons
- Multipolar Neurons
- Additional Classifications
- Clinical Relations
Unipolar neurons, often referred to as ‘true’ unipolar neurons, feature a single process extending from the cell body (soma), which then branches into dendrites or an axon. In the context of human neurophysiology, the term "unipolar" is sometimes mistakenly used in place of "pseudounipolar." True unipolar neurons are generally not found in the mature vertebrate nervous system; however, specific developmental stages may display neurons with only one process. These neurons are predominantly observed in invertebrates, where they form a prevalent neuronal population.
Bipolar neurons bear an oval shaped cell body possessing two processes: one axon and one process functioning as a distant dendrite. In humans, these neurons serve as sensory neurons and are primarily found in special sensory organs such as the olfactory epithelium, retina and vestibulocochlear apparatus.
The terminal ramifications in the periphery receive signals from the sensory organs and combine into one process that reaches the cell body. The axon transfers the signal from the cell body to the central nervous system (CNS) and distributes impulses to second order afferent neurons. Both processes exhibit axonal characteristics and can be encased in a myelin sheath which increases the speed of impulse conduction.
Pseudounipolar neurons consist of one short process, which splits into two other processes. They serve as sensory neurons and, along with bipolar neurons, constitute the entirety of the primary sensory neurons within the human peripheral nervous system (PNS). Except for the olfactory epithelium, retina and vestibulocochlear apparatus, pseudounipolar neurons are found in all sensory ganglia of cranial and spinal nerves.
Pseudounipolar neurons can be considered as variations of bipolar neurons. During development, the opposing processes of certain bipolar neurons shift around the cell body and combine into a single, short-length axon proximal to the cell body. After a brief course, the axon forms a T-shaped junction. The peripheral/distal process of the axon terminates in the periphery, where the terminal ramifications respond to a wide range of stimuli, thus functioning as distant dendrites. These tend to be the longer of the two axonal branches, however this depends on the site of innervation. For example, neurons which innervate the foot may have a lengthy peripheral branch, in contrast to cranial nerves.
The second branch, known as the central/proximal process, is usually shorter, terminating in the CNS where they distribute impulses to second order afferent neurons. In the case of cranial nerves, the central process terminates in specialized nuclei of the cerebrum and brainstem. Meanwhile, those of the sensory spinal nerves terminate in the posterior horn of the spinal cord gray matter. Nerve impulses in these neurons can pass from the peripheral to central processes without the involvement of the cell body in signal processing. The cell body mainly retains trophic functions (i.e. support, nourishment and maintenance of the neuron).
Multipolar neurons are the dominant type of neurons in vertebrates. They are characterized by multiple processes: a single axon and numerous dendrites. The dendrites originate from different regions of the cell body, displaying varying degrees of branching and directionality.
Multipolar neurons are notable for their extensive diversity, manifesting in a wide range of sizes, shapes and complexity within their dendritic tree. Their cell bodies may measure as small as 5 μm in diameter or reach as large as 100 μm, as exemplified by the giant pyramidal cells (of Betz). The cell body can take on various forms, including ovoid, spherical, pyriform or fusiform, while the axon may be short or long. Multipolar neurons can further be categorized depending on similarities in morphology and function. Some common subtypes of multipolar neurons with characteristic morphology are pyramidal, stellate, Purkinje and granule cells.
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 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 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 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.
Neurons may also be classified based on other characteristics such as function, projection type and neurotransmitter specificity.
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.
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.
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.
Many neurological and neurodegenerative diseases can selectively affect specific types or categories of neurons. This specificity often arises from the underlying biological processes and genetic factors involved in each condition.
One such example is motor neuron disease, which encompasses a group of chronic sporadic and hereditary neurological disorders. These conditions are characterized by the progressive degeneration of motor neurons exclusively.
Another example, spinocerebellar ataxia, comprises genetic disorders primarily impacting neurons in the cerebellum and spinal cord. This leads to issues related to coordination and balance. The key cell involved in this degeneration is the Purkinje cell, with other cells like granule cells, astrocytes, Golgi cells and oligodendrocytes remaining unaffected.
Sensory neuronopathies are characterized by the primary degeneration of sensory neurons in the dorsal root ganglia and trigeminal ganglion. This results in sensory impairment.
Understanding the specific neurons affected in each disease is crucial for developing targeted treatments and interventions. Ongoing research in neuroscience and genetics continues to unveil the underlying mechanisms of these diseases, offering promising avenues for more effective therapies in the future.
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