Parts of a neuron
The nervous system consists of two main cell types, neurons and supporting glial cells. The neuron (or nerve cell) is the functional unit of both the central nervous system (CNS) and the peripheral nervous system (PNS). The basic functions of neurons can be summarized into three main tasks: receiving signals, integrating these signals and transmitting the signals to target cells and organs. These functions reflect in the microanatomy of the neuron. As such, neurons typically consist of four main functional parts which include the:
- Receptive part (dendrites), which receive and conduct electrical signals toward the cell body
- Integrative part (cell body/soma), containing the nucleus and most of the cell's organelles, acting as the trophic center of the entire neuron
- Conductive part (axon), which conducts electrical impulses away from the cell body
- Transmissive part (axon terminals), where axons communicate with other neurons or effectors (target structures which respond to nerve impulses)
Neurons are categorized into different types based on their unique morphologies and functions. This article will focus on the typical multipolar neuron, the primary neuronal type found in the CNS, and explore its parts and functions in greater detail.
|Cell body (soma)
Description: Spherical or polygonal central component of a neuron
Function: Integration and signal processing, protein synthesis, metabolic activities
Components: Nucleus (DNA), cytoplasmic organelles (endoplasmic reticulum (smooth and rough), Golgi apparatus, microtubules, mitochondria, lysosomes), axon hillock
Description: Specialized, cone-shaped region of the cell body which forms the initial segment of the axon
Function: Site for initiation of action potentials (‘trigger zone’ of the neuron)
- Devoid of large cytoplasmic organelles (Nissl bodies and Golgi apparatus),
- Contains high density of voltage-gated sodium channels
Description: Tree-like, short, tapering processes of varying shape
Function: Reception of synaptic signals and translation into electrical events
Components: Similar to the cell body, neurotransmitter receptors, dendritic shaft, dendritic spines
|Axon (nerve fiber)
Description: Single long process arising from the axon hillock
Function: Conduction of electrical impulses away from the cell body
- Axolemma (cell membrane), axoplasm (cytoplasm), myelin sheath, myelin sheath gaps (nodes of Ranvier), terminal arborizations, terminal boutons, microtubules, intermediate filaments
- Devoid of endoplasmic reticulum and ribosomes
- Cell body
- Clinical relations
The cell body of a neuron, also known as the soma, is typically located at the center of the dendritic tree in multipolar neurons. It is spherical or polygonal in shape and relatively small, making up one-tenth of the total cell volume.
The functionality of the neuron is highly dependent on its cell body as it houses the nucleus, which contains the genetic material (DNA) of the cell as well as various cytoplasmic organelles. These organelles include the endoplasmic reticulum (both smooth and rough), which clusters with free ribosomes to form what is known as chromatophilic substances ( Nissl bodies) and are involved in the protein synthesis of enzymes, receptors, ion channels and other structural components. Additionally, the cell body contains the Golgi apparatus and microtubules, involved in the packaging and transport of proteins; mitochondria, involved in energy production; and lysosomes involved in the waste management of the cell.
The axon hillock refers to an anatomically and functionally distinct area of the cell body which serves as the origin of the axon. It is cone-shaped and devoid of large cytoplasmic organelles such as chromatophilic substance (Nissl bodies) and Golgi apparatus. The axon hillock contains a high density of voltage-gated sodium channels, allowing it to serve as a critical site for the initiation of action potentials. It also supports neuron polarity by separating the receptive/integrative parts from the conductive/transmissive parts, providing directionality in the flow of information from the dendrites to the cell body, axon and axon terminals.
Dendrites are tree-like processes extending from the cell body of the neuron and contain organelles similar to those in the cell body. The highly branched structure of dendrites provides an increased surface area for receiving information from other neurons at specialized areas of contact called synapses. Dendrites primarily consist of dendritic shafts, which serve as the main structural branches.
These are lined with numerous tiny protrusions called dendritic spines, which serve as sites for the initial processing of synaptic signals via membrane embedded neurotransmitter receptors; they translate the chemical messages received into electrical events, which travel down the dendrites. There are approximately ten trillion of these structures present across all dendrites of neurons in the human cerebral cortex (for 16-20 billion neurons), therefore they greatly increase the area available for synaptic events.
The axon of a neuron is also known as a nerve fiber. The membrane of an axon is known as the axolemma, while the cytoplasm is also referred to as axoplasm. Bundles of axons in the CNS form a tract, while in the PNS, they are referred to as fascicles (which, when bound together with connective tissue, form nerves). Axons originate from the axon hillock and conduct electrical impulses, in the form of action potentials, away from the cell body through a process of sequential depolarization and repolarization. Unlike dendrites that form a complex network with many tapering branches, the axon of a neuron is usually a single, long process that can extend for a considerable distance before it branches and terminates. The length of axons varies and can sometimes exceed a meter, such as in some peripheral nerves like the sciatic nerve, which extends from the spinal cord to the feet.
Axons typically terminate as fine branches called terminal arborizations; each of which is capped with a terminal bouton. These specialized structures contain synaptic vesicles that store neurotransmitters to be released into the synaptic cleft (a small gap at a synapse between neurons where nerve impulses are transmitted by a neurotransmitter) when an action potential reaches the axon terminal.
Axons can be enveloped in an insulating layer of lipids and proteins called the myelin sheath. This sheath protects the axon and prevents the loss of electrical charge (ions) during the transmission of action potentials along the neuron, increasing the speed of impulse transmission. The myelin sheath is formed by specific types of glial cells, namely oligodendrocytes in the CNS and Schwann cells (neurolemmocytes) in the PNS. The myelination of PNS axons involves many Schwann cells, each of which participates in the formation of the myelin sheath of a single axon, by wrapping around it multiple times. Not all axons are covered by myelin. In the PNS, multiple nonmyelinated axons can go through a single Schwann cell, without myelin sheath production. In contrast, an oligodendrocyte can myelinate multiple axons in the CNS, due to its arm-like processes twisting around them.
The outermost nucleated cytoplasmic layer of Schwann cells overlying the myelin sheath is called the neurolemma. This structural characteristic aids in the regeneration of damaged peripheral axons when the corresponding cell body remains intact. In contrast, CNS neurons, which lack a neurolemma, exhibit limited regenerative capacity.
Nerve fibers are classified into groups, based on their myelination; group A neurons are heavily myelinated, group B are moderately myelinated, and group C are nonmyelinated. Along myelinated axons, evenly distributed gaps known as myelin sheath gaps (commonly referred to as nodes of Ranvier) allow electrical impulses to jump from node to node. This propagation pattern is referred to as saltatory conduction. Myelin sheath gapsare more numerous in axons of the PNS compared to those of the CNS.
Since axons lack endoplasmic reticulum and ribosomes, proteins and organelles needed for its growth are synthesized in the cell body and then transported to the axon via axonal transport. This is facilitated by microtubules and intermediate filaments that provide cytoskeletal "tracks" for transportation. The microtubule arrangements overlap, providing routes for simultaneous transport of different materials to and from the cell body.
Damage of different parts of the neurons is linked to specific disorders of the nervous system. Structural degeneration of dendrites and dendritic spines in the hippocampal pyramidal neurons underlies the initial pathology of Alzheimer's disease. It alters neuronal electrical properties leading to hyperexcitability, the electrophysiological hallmark of the disease.
Axonal demyelination refers to the loss or damage of the myelin sheath surrounding nerve fibers due to genetics, various pathogens, or autoimmune diseases, disrupting the neuron’s conductive ability. This can lead to a wide range of diseases including multiple sclerosis and Guillain-Barré syndrome. Multiple sclerosis is an immune-mediated disease; self-antibodies attack the myelin sheath in the CNS mainly, leading to its destruction and leaving behind a scar in the white matter. Multiple sclerosis involves a wide array of symptoms such as double vision, visual loss, muscle weakness, coordination problems, and sensory disturbances. Guillain-Barré syndrome is also considered a demyelinating disease of the PNS resulting from an autoimmune reaction. It presents with sensory symptoms or motor deficits. In axonal demyelination, the myelin sheath may regenerate if the underlying cause is addressed, allowing for improved nerve conduction. Extensive myelin loss is usually followed by axonal degeneration.
Axonal degeneration involves damage or deterioration of the axon itself. This can occur with or without concurrent demyelination. Axonal insult due to traumatic injury or disease can result in anterograde degeneration of the axon’s distal end, known as Wallerian degeneration. Retrograde degeneration (Wallerian-like) can also be involved, affecting the neuron's cell body and the proximal part of the axon. Once axonal degeneration occurs, the ability of the affected axon to conduct nerve impulses is severely compromised or lost altogether. Axonal regeneration depends on the integrity of the cell body; if it is compromised, regeneration is precluded, but if the injury is confined to the axon's end, regeneration proceeds at an approximate rate of 1mm per day.
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