Dendrite

​​​​​​​​​​​​​​Dendrite
n., plural: ​​​​​​​​​​​​​​dendrites
[ˈdɛndɹaɪt]
Definition: Branching neuron projection for signal reception

Dendrite Definition

Dendrites are the protoplasmic projections from the neuron cells that receive the electrochemical signals from other neuronal cells or nerve cells and further, propogate them to the neuron cell body or the soma. They are also the branching projections of the cytoplasm of non-neuronal cells, such as dendritic cells of the immune system and certain skin cells (e.g. melanocytes and Merkel cells). In the next sections, we will focus more on the dendrites of the neurons.

A neuronal cell usually receives information as a chemical signal at the dendrite and sends it to the soma and eventually to the axon as an electrical impulse. The electrical impulse signal then travels to the synapse (neuronal synapse is the junction wherein one neuron terminates while the other neuronal cell starts), wherein two neuronal cells exchange the information in the form of a chemical signal. Thus, dendrites are the recipients of the synaptic inputs from axons.

Axons are single, long protrusion that propagates the signal away from the cell while the dendrites are the collection of protrusions that collects the signal from the other neurons through synapse and propagate it towards the cell body.

Video: Neuron parts, including dendrites:

Structure

Briefly, let us understand the structure of neurons. Neurons are the building blocks of the nervous system, that receive the signals/information from one part of the body and transmit it to the other part of the body. There are sensory neurons throughout the body that collect sensory stimuli from the sensory receptors of the body and send them to the brain to process them. While the motor neurons transmit the signals from the brain to the muscles. All individual neurons have these three fundamental parts:

1. Cell body: The cell body or soma is the nucleus of the neuron cell that contains, the endoplasmic reticulum, Golgi bodies, mitochondria, and other cellular components.
2. Axon: Axon is the single, protoplasmic tubular projection arising from the cell body of the neuron. It functions to carry information away from the cell body towards the axon terminals and eventually transmit it to another neuron. Axons are longer than dendrites and have almost the same diameter throughout their length. Axons function to send the electrochemical signals. Axons have voltage-gated Sodium ion channels that propagate the electrical signals along their length to reach the synapse.
3. Dendrites: Dendrites are the protoplasmic projections found at the terminal end of the neuron and they function to receive the cellular signals from the other neurons through neuronal synapse. (see Figure 1).
   • Dendrites are an integral part of the mammalian central nervous system.
   • Dendrites are shorter structures that taper off towards the ends.
   • Each dendrite is about 2 µm long and usually 5 – 7 dendrites are present and usually form a complex interwoven structure near the neuron which is referred to as a dendritic tree.
   • Distal dendrites can have multiple shapes, mushroom-shaped, cup-shaped, tapering, etc.
   • Dendrites receive the electrochemical signals.
   • Dendrites contain various cytoskeletal elements, Golgi bodies, smooth endoplasmic reticulum, and ribosomes. The smooth endoplasmic reticulum is found throughout the dendrite structure and is responsible for the calcium ion regulation in the dendrite. The cytoskeleton structure of dendrites is made up of actin filaments, neurofilaments, and microtubules (long and thin structures having 24 nm diameter and having a longitudinal orientation towards the axis of the dendrite). Microtubules form the railway track or information track for the transmission of the signals along the dendrite’s length. Microtubule polarity in the dendrite is different from the axon polarity.
   • Dendrites possess both excitatory as well as inhibitory synapses. Synapses are the junction of signal inputs for neurons.
   • Additionally, dendrites may carry out protein synthesis and can have independent signaling in association with other neurons. Ribosomes present in the dendrites are involved in the protein synthesis.
   • All the dendrites possess dendritic spines at the synapses. Spines are the dendritic protrusions (not more than 2 µm) and usually have a bulbous end attached through a narrow neck. Dendritic spines help to increase the surface area of the neuron and thus increase the susceptibility and detectability of the dendrites. According to the compartmental modeling of dendrites, dendritic spines are the subcellular dendritic compartments.

Dendrite Development

Dendritic morphogenesis i.e., the dendrite growth and differentiation is affected by several factors that include:

• Modulation of sensory input
• Body temperature
• Environmental pollutants
• Use of drugs

Although not much is known about dendrite development and differentiation, a Synaptotropic Hypothesis has been proposed that postulates the dendritic arbor development mechanism. According to this hypothesis, sensory signals/stimulus from presynaptic neurons to a postsynaptic cell affects the course of synapse formation at the axon and dendrite junction.

Types Of Dendritic Patterns

Dendrites emanating multiple branching is also referred to as Dendritic arborization or dendritic branching. The process of dendritic branching involves multiple steps, wherein the dendrites branch to form synapses with the axons. Dendrite branching acquires different shapes, densities, and morphology. This branching morphology and density of the dendrites is dependent upon the type of the dendrite and its function. Dendritic malformation can lead to neurological disorders.

Depending upon the dendrite morphology and dendrite branch, dendrites can be:

• Adendritic – no branches of dendrites, thus the structure of the dendrite is not like a tree.
• Spindled – two branches of dendrites are formed from two opposite ends of the neuronal cell body,  e.g., bipolar neurons.
• Spherical – dendritic branches appear all around the neuronal cell body thus giving a spherical appearance to the dendritic tree, e.g., cerebellar granule cells.
• Laminar – the dendritic branches radiate in a planar fashion from the neuronal cell body, e.g., retinal ganglion cells, retinal horizontal cells, and retinal amacrine cells.
• Cylindrical –  the dendritic branches protrude in all directions in a disc-like or cylindrical fashion,  e.g., pallidal neurons.
• Conical – the dendritic branches protrude in a conical fashion from the neuronal cell body, e.g., pyramidal cells.
• Fanned – the dendritic branches protrude in a flat fan-like shape from the neuronal cell body,  e.g., Purkinje cells.

Dendrites Function

Dendritic cells function to receive the signals, process them, and propagate or transfer them to the neuronal cell body or soma.

• Receive Information

The multiple branches or protrusions of the dendrite form a dendritic tree that connects at one end with the neuronal cell body or soma and terminates into small projections. The small projections form the synaptic contacts with the axons. Synapses are the information /signal transfer junctions. At the synapse, two neuronal cells are involved in the transfer of information of the signals.

One of the neurons at the axon terminal is the pre-synaptic upstream neuron that transmits the signal as neurotransmitters while the other neuron is the post-synaptic downstream neuron (dendrite) that detects or receives the biochemical signals.

The most common neurotransmitters are- GABA, serotonin, dopamine, glutamate, and norepinephrine. Post-synaptic neurons/dendrites are equipped with neurotransmitter detectors that recognize/detect the biochemical neurotransmitter signals by binding to them. Thus, if a post-synaptic neuron/dendrite is devoid of specific receptors then the signal will not be recognized or detected by the nervous system.

For example, if serotonin is released from the pre-synaptic neuron (axon) and the post-synaptic neuron (dendrite) is devoid of serotonin receptors then the effect of serotonin release will not be observed by the nervous system of the body. Research shows that electrical properties of the dendrite also affect the signal receiving and its transmission.

• Process Information

On binding the neurotransmitter to the receptors, a signaling pathways cascade is initiated that results in the processing of the signals. Depending upon the neurotransmitter released either excitatory (e.g., glutamate) or inhibitory (e.g., GABA) and the receptors involved, initiates the opening of the ligand-gated ion channels. Depending upon the signals, these channels either permit the entry of ions like sodium ions (Na⁺), Calcium ions (Ca²⁺), and Chloride ions (Cl^–), OR, the exit of ions like potassium ions or K⁺, takes place.

The excitatory neurotransmitters and their eventual binding to their corresponding receptor initiate the ligand-gated ion channels (sodium and calcium channels) to permit the entry of positive ions like Na⁺ and Ca²⁺. Correspondingly, there is an exit of K⁺ from the cells. Once a sufficient amount of positive ions enters the cell to develop the positive potential, depolarization of the dendritic membrane occurs.

The generation of net positive charge due to the entry of positive ions results in the generation of the synaptic excitatory potential (EPSP). Once the positive charge reaches the threshold value, it results in the generation of action potential at the axon hillock, across the cell membrane.

While in the case of inhibitory neurotransmitter release, the chloride ion ligand-gated channels get activated resulting in the influx of Cl^– ions into the cells. Correspondingly, K⁺ ions also move out of the cell. This movement of the negatively charged ions results in a reduction of neuron membrane potential or post-synaptic inhibitory potential (IPSP). The IPSP leads to the hyperpolarization of the neuronal membrane.

Synaptic plasticity is the term used to refer to the changes that can occur at synapses to communicate the signals received. Hence, synaptic plasticity contributes to the memory and learning ability of a person.

• Transfer Information

The transfer of the signal from the dendrite to the soma or neuronal cell body and eventually to the axon requires the generation of total EPSP above a threshold value.

In the resting condition, the membrane potential of the neuronal cell membrane is around -65mV, indicative of more negative ions outside the cell as compared to the inside of the cell. Inside the cells, a small number of positive ions (K⁺) and more negative ions (Cl^–) are present while outside the cell more positive ions (Na⁺ and Ca2⁺) and fewer negative ions (Cl^–) are present, which results in negative internal cell membrane potential. This is due to the presence of more negative ions present within the cell.

When EPSP is generated, more positive ions enter the cell resulting in the reduction of potential from -65mV to -64 mV. However, when the sum of all the EPSP crosses the threshold action potential of -55 mV, the neurons fire up and propagate the signal to the neuronal cell body or soma and eventually to the axon. (see Figure 2)

In cases, wherein the net action potential does not cross the threshold action potential, the signal is lost and no response from the nervous system is observed.

Dendrites Malfunction

Dendritic cells are the basic cellular unit that functions to receive the information /signal and transmit it to the neuronal cell body and eventually to the axon. Hence, any malfunctioning or malformation of the dendritic branches can lead to neurological disorders. The malfunctioning of the dendrites can vary in degree and severity depending upon the extent of the morphological disinformation of the dendritic branching, genesis of dendrites, abnormal dendritic development, and loss of dendritic functioning.

Some of the most widely known neurological disorders due to dendritic malfunctioning are autism, depression, schizophrenia, anxiety, Down syndrome, and Alzheimer’s. In patients with autism, reduced dendritic branching in CA1 and CA4 hippocampal neurons is seen.

In patients with schizophrenia, reduced expression of glutamate receptors in CA3 hippocampal neurons and alteration in the dendritic arborization are seen.

Patients of Alzheimer’s Disease and Down Syndrome exhibit a reduced number of dendrites.

Suppression of the dendrite growth in the CA1 region can lead to recurrent seizures and eventually childhood epilepsy.

Exposure to nicotine and cocaine at the prenatal stage has been found to alter the dendritic arborization which can eventually lead to loss of short-term spatial memory.

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References

1. Chen, I., & Lui, F. (2019). Neuroanatomy, neuron action potential.
2. Sala, C., & Segal, M. (2014). Dendritic spines: the locus of structural and functional plasticity. Physiological reviews, 94(1), 141-188.
3. · Paus, T. (2023). Tracking development of connectivity in the human brain: Axons and dendrites. Biological Psychiatry, 93(5), 455-463.
4. · Ma, S., & Zuo, Y. (2022, May). Synaptic modifications in learning and memory–a dendritic spine story. In Seminars in cell & developmental biology (Vol. 125, pp. 84-90). Academic Press.

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