What Is Neuron?/Neuron (also called neurons or nerve cells) are the fundamental units of the brain and nervous system, the cells responsible for receiving sensory input from the external world, for sending motor commands to our muscles, and for transforming and relaying the electrical signals at every step in between
Neurons are electrically excitable cells that transmit signals throughout the body. Neurons employ both electrical and chemical components in the transmission of information. Neurons are connected to other neurons at synapses and connected to effector organs or cells at neuroeffector junctions. A typical multipolar neuron is comprised of soma or cell body, an axon, and dendrites. The axon is thought of as the part transmitting efferent signals, while the dendrites are receiving afferent signals from their surroundings.[rx]
Classification
Neurons vary in shape and size and can be classified by their morphology and function.[rx] The anatomist Camillo Golgi grouped neurons into two types; type I with long axons used to move signals over long distances and type II with short axons, which can often be confused with dendrites. Type I cells can be further classified by the location of the soma. The basic morphology of type I neurons, represented by spinal motor neurons, consists of a cell body called the soma and a long thin axon covered by a myelin sheath. The dendritic tree wraps around the cell body and receives signals from other neurons. The end of the axon has branching axon terminals that release neurotransmitters into a gap called the synaptic cleft between the terminals and the dendrites of the next neuron.
Structural classification
- Unipolar neuron
- Bipolar neuron
- Multipolar neuron
- Pseudounipolar neuron
Most neurons can be anatomically characterized as
- Unipolar: single process
- Bipolar: 1 axon and 1 dendrite
- Multipolar: 1 axon and 2 or more dendrites
- Golgi I: neurons with projecting axonal processes; examples are pyramidal cells, Purkinje cells, and anterior horn cells
- Golgi II: neurons whose axonal process projects locally; the best example is the granule cell
- Anaxonic: where the axon cannot be distinguished from the dendrite(s)
- Pseudounipolar: 1 process which then serves as both an axon and a dendrite
Other
Types of neurons: 1: Unipolar neuron, 2: Bipolar neuron, 3: Multipolar neuron, 4: Pseudounipolar neuron
Some unique neuronal types can be identified according to their location in the nervous system and distinct shape. Some examples are:
- Basket cells, interneurons that form a dense plexus of terminals around the soma of target cells, found in the cortex and cerebellum
- Betz cells, large motor neurons
- Lugaro cells, interneurons of the cerebellum
- Medium spiny neurons, most neurons in the corpus striatum
- Purkinje cells, huge neurons in the cerebellum, a type of Golgi I multipolar neuron
- Pyramidal cells, neurons with triangular soma, a type of Golgi I
- Renshaw cells, neurons with both ends linked to alpha motor neurons
- Unipolar brush cells, interneurons with unique dendrite ending in a brush-like tuft
- Granule cells, a type of Golgi II neuron
- Anterior horn cells, motoneurons located in the spinal cord
- Spindle cells, interneurons that connect widely separated areas of the brain
Functional classification
Direction
- Afferent neurons convey information from tissues and organs into the central nervous system and are also called sensory neurons.
- Efferent neurons (motor neurons) transmit signals from the central nervous system to the effector cells.
- Interneurons connect neurons within specific regions of the central nervous system.
Afferent and efferent also refer generally to neurons that, respectively, bring information to or send information from the brain.
Action on other neurons
A neuron affects other neurons by releasing a neurotransmitter that binds to chemical receptors. The effect upon the postsynaptic neuron is determined by the type of receptor that is activated, not by the presynaptic neuron or by the neurotransmitter. A neurotransmitter can be thought of as a key, and a receptor as a lock: the same neurotransmitter can activate multiple types of receptors. Receptors can be classified broadly as excitatory (causing an increase in firing rate), inhibitory (causing a decrease in firing rate), or modulatory (causing long-lasting effects not directly related to firing rate).
The two most common (90%+) neurotransmitters in the brain, glutamate and GABA, have largely consistent actions. Glutamate acts on several types of receptors, and has effects that are excitatory at ionotropic receptors and a modulatory effect at metabotropic receptors. Similarly, GABA acts on several types of receptors, but all of them have inhibitory effects (in adult animals, at least). Because of this consistency, it is common for neuroscientists to refer to cells that release glutamate as “excitatory neurons”, and cells that release GABA as “inhibitory neurons”. Some other types of neurons have consistent effects, for example, “excitatory” motor neurons in the spinal cord that release acetylcholine, and “inhibitory” spinal neurons that release glycine.
The distinction between excitatory and inhibitory neurotransmitters is not absolute. Rather, it depends on the class of chemical receptors present on the postsynaptic neuron. In principle, a single neuron, releasing a single neurotransmitter, can have excitatory effects on some targets, inhibitory effects on others, and modulatory effects on others still. For example, photoreceptor cells in the retina constantly release the neurotransmitter glutamate in the absence of light. So-called OFF bipolar cells are, like most neurons, excited by the released glutamate. However, neighboring target neurons called ON bipolar cells are instead inhibited by glutamate, because they lack typical ionotropic glutamate receptors and instead express a class of inhibitory metabotropic glutamate receptors.[rx] When light is present, the photoreceptors cease releasing glutamate, which relieves the ON bipolar cells from inhibition, activating them; this simultaneously removes the excitation from the OFF bipolar cells, silencing them.
It is possible to identify the type of inhibitory effect a presynaptic neuron will have on a postsynaptic neuron, based on the proteins the presynaptic neuron expresses. Parvalbumin-expressing neurons typically dampen the output signal of the postsynaptic neuron in the visual cortex, whereas somatostatin-expressing neurons typically block dendritic inputs to the postsynaptic neuron.[rx]
Discharge patterns[
Neurons have intrinsic electro responsive properties like intrinsic transmembrane voltage oscillatory patterns.[rx] So neurons can be classified according to their electrophysiological characteristics:
- Tonic or regular spiking. Some neurons are typically constantly (tonically) active, typically firing at a constant frequency. Example: interneurons in the neostriatum.
- Phasic or bursting. Neurons that fire in bursts is called phasic.
- Fast spiking. Some neurons are notable for their high firing rates, for example, some types of cortical inhibitory interneurons, cells in globus pallidus, retinal ganglion cells.[rx][rx]
Neurotransmitter
- Cholinergic neurons—acetylcholine. Acetylcholine is released from presynaptic neurons into the synaptic cleft. It acts as a ligand for both ligand-gated ion channels and metabotropic (GPCRs) muscarinic receptors. Nicotinic receptors are pentameric ligand-gated ion channels composed of alpha and beta subunits that bind nicotine. Ligand binding opens the channel causing an influx of Na+ depolarization and increases the probability of presynaptic neurotransmitter release. Acetylcholine is synthesized from choline and acetyl coenzyme A.
- Adrenergic neurons—noradrenaline. Noradrenaline (norepinephrine) is a release from most postganglionic neurons in the sympathetic nervous system onto two sets of GPCRs: alpha adrenoceptors and beta-adrenoceptors. Noradrenaline is one of the three common catecholamine neurotransmitters and the most prevalent of them in the peripheral nervous system; as with other catecholamines, it is synthesized from tyrosine.
- GABAergic neurons—gamma-aminobutyric acid. GABA is one of two neuroinhibitors in the central nervous system (CNS), along with glycine. GABA has a homologous function to ACh, gating anion channels that allow Cl− ions to enter the postsynaptic neuron. Cl− causes hyperpolarization within the neuron, decreasing the probability of an action potential firing as the voltage becomes more negative (for an action potential to fire, a positive voltage threshold must be reached). GABA is synthesized from glutamate neurotransmitters by the enzyme glutamate decarboxylase.
- Glutamatergic neurons—glutamate. Glutamate is one of two primary excitatory amino acid neurotransmitters, along with aspartate. Glutamate receptors are one of four categories, three of which are ligand-gated ion channels and one of which is a G-protein coupled receptor (often referred to as GPCR).
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- AMPA and Kainate receptors function as cation channels permeable to Na+ cation channels mediating fast excitatory synaptic transmission.
- NMDA receptors are another cation channel that is more permeable to Ca2+. The function of NMDA receptors depend on glycine receptor binding as a co-agonist within the channel pore. NMDA receptors do not function without both ligands present.
- Metabotropic receptors, GPCRs modulate synaptic transmission and postsynaptic excitability. Glutamate can cause excitotoxicity when blood flow to the brain is interrupted, resulting in brain damage. When blood flow is suppressed, glutamate is released from presynaptic neurons, causing greater NMDA and AMPA receptor activation than normal outside of stress conditions, leading to elevated Ca2+ and Na+ entering the postsynaptic neuron and cell damage. Glutamate is synthesized from the amino acid glutamine by the enzyme glutamate synthase.
- Dopaminergic neurons—dopamine. Dopamine is a neurotransmitter that acts on D1 type (D1 and D5) Gs coupled receptors, which increase cAMP and PKA, and D2 type (D2, D3, and D4) receptors, which activate Gi-coupled receptors that decrease cAMP and PKA. Dopamine is connected to mood and behavior and modulates both pre- and post-synaptic neurotransmission. Loss of dopamine neurons in the substantia nigra has been linked to Parkinson’s disease. Dopamine is synthesized from the amino acid tyrosine. Tyrosine is catalyzed into levadopa (or L-DOPA) by tyrosine hydroxlase, and levadopa is then converted into dopamine by the aromatic amino acid decarboxylase.
- Serotonergic neurons—serotonin. Serotonin (5-Hydroxytryptamine, 5-HT) can act as excitatory or inhibitory. Of its four 5-HT receptor classes, 3 are GPCR, and 1 is a ligand-gated cation channel. Serotonin is synthesized from tryptophan by tryptophan hydroxylase, and then further by decarboxylase. A lack of 5-HT at postsynaptic neurons has been linked to depression. Drugs that block the presynaptic serotonin transporter are used for treatment, such as Prozac and Zoloft.
- Purinergic neurons—ATP. ATP is a neurotransmitter acting at both ligand-gated ion channels (P2X receptors) and GPCRs (P2Y) receptors. ATP is, however, best known as a transmitter. Such purinergic signaling can also be mediated by other purines like adenosine, which particularly acts at P2Y receptors.
- Histaminergic neurons—histamine. Histamine is a monoamine neurotransmitter and neuromodulator. Histamine-producing neurons are found in the tuberomammillary nucleus of the hypothalamus.[rx] Histamine is involved in arousal and regulating sleep/wake behaviors.
Parts of a neuron
Neurons vary in size, shape, and structure depending on their role and location. However, nearly all neurons have three essential parts: a cell body, an axon, and dendrites.
Cell body
Also known as a soma, the cell body is the neuron’s core. The cell body carries genetic information, maintains the neuron’s structure, and provides energy to drive activities.
Like other cell bodies, a neuron’s soma contains a nucleus and specialized organelles. It’s enclosed by a membrane which both protects it and allows it to interact with its immediate surroundings.
Axon
An axon is a long, tail-like structure which joins the cell body at a specialized junction called the axon hillock. Many axons are insulated with a fatty substance called myelin. Myelin helps axons to conduct an electrical signal. Neurons generally have one main axon.
Dendrites
Dendrites are fibrous roots that branch out from the cell body. Like antennae, dendrites receive and process signals from the axons of other neurons. Neurons can have more than one set of dendrites, known as dendritic trees. How many they have generally depends on their role.
For instance, Purkinje cells are a special type of neuron found in the cerebellum. These cells have highly developed dendritic trees which allow them to receive thousands of signals.
Structural Diversity of Neurons
A number of anatomically neuron types have evolved to participate in different organismal functions.
Key Points
Neurons are electrically excitable cells that are the structural unit of the nervous system.
A typical neuron consists of a cell body and neuronal processes such as dendrites and axon.
Neurons can generally be anatomically characterized as unipolar, bipolar, or multipolar.
A number of anatomically distinct neuron types, such as sensory, motor, and interneurons, have evolved to participate in different organismal functions.
Key Terms
Neurons: Electrically excitable cells that are the structural unit of the nervous system.
dendrites: Short, tapering extensions that convey incoming messages toward the body of the neuron.
Axons: The conducting region of the neuron.
Neurons are electrically excitable cells that are the structural unit of the nervous system. A typical neuron consists of a cell body and neuronal processes such as dendrites and axons. The dendrites are short, tapering extensions that are the receptive regions and help in conveying incoming messages towards the cell body. Axons arise from a cone-shaped area of the cell body called the axon hillock. These extensions are the conducting region of the neuron. Nerve impulses are generated in the axon and transmitted away from the cell body towards the synapse. The cell body is the major biosynthetic center of the neuron. It contains neurotransmitters and other organelles needed to synthesize proteins and chemicals. The cell body is the focal point for the outgrowth of the neuronal process during embryonic development.
A number of anatomically distinct types of neurons have evolved to participate in different organismal functions. For example, sensory neurons respond to touch, sound, light, and other sensory inputs. Motor neurons receive signals from the brain and spinal cord to initiate muscle contractions and affect glands. Interneurons act as relays between neurons in close proximity to one another.
Neurotransmission at a Chemical Synapse: A signal propagating down an axon to the cell body and dendrites of the next cell
Neurons can generally be grouped according to the number of processes extending from their cell bodies. Three major neuron groups make up this classification: multipolar, bipolar, and unipolar. Unipolar neurons have a single short process that emerges from the cell body and divides T-like into proximal and distal branches. Bipolar neurons have two processes, an axon and a dendrite, that extend from opposite ends of the soma. Multipolar neurons, the most common type, have one axon and two or more dendrites.
Key Points
Neurons can be classified by the direction of the action potential or route by which information travels. Afferent neurons convey information from tissues and organs to the brain and efferent signals transmit information from the brain to effector cells in the body.
Neurons can have excitatory, inhibitory, or modulatory effects on target neurons depending on the neurotransmitter they release.
Afferent neurons convey information from tissues and organs into the central nervous system.
Interneurons connect neurons within specific regions of the central nervous system.
Efferent neurons carry information away from a brain region.
Key Terms
efferent: Efferent neurons transmit signals from the central nervous system to the effector cells (e.g. motor neurons).
afferent: Afferent neurons convey information from tissues and organs into the central nervous system (e.g. sensory neurons).
tonic or regular spiking: Neurons that are typically constantly (or tonically) active are called tonic or regular spiking.
The direction of Nerve Impulse
The functional classification of neurons is based on the direction the action potential (i.e. information) travel relative to the central nervous system. Afferent neurons convey information from tissues and organs into the central nervous system (e.g. sensory neurons). Efferent neurons transmit signals from the central nervous system (CNS) to the effector cells (e.g. motor neurons ). Afferent and efferent also refer generally to neurons that bring information to or send information from a brain region. Interneurons connect neurons within specific regions of the central nervous system.
The organization of the nervous system: Gross organization of the nervous system, with the peripheral nervous system, the spinal, and the cortical levels.
Neurotransmitter Type
Neurons are also classified by their effect on target neurons. A neuron releases a neurotransmitter that binds to chemical receptors on the target neuron. The combination of neurotransmitter and receptor properties results in an excitatory, inhibitory, or modulatory change to the target neuron. For example, the two most common neurotransmitters in the brain (released by 90% of neurons), glutamate and GABA, have opposing actions. Glutamate acts on several different types of receptors with largely excitatory effects. GABA acts on several different classes of receptors to exert inhibitory effects. Other types of neurons include excitatory motor neurons in the spinal cord that release acetylcholine and inhibitory spinal neurons that release glycine.
Major elements in neuron-to-neuron communication: Electrical impulses travel along the axon of a neuron. When this signal reaches a synapse, it provokes the release of neurotransmitter molecules, which bind to receptor molecules located in the target cell.
Firing Properties
A third, less common method of neuron classification is according to their electrophysiological characteristics. Neurons that are typically constantly (or tonically) active are called a tonic or regular spiking. Neurons that are intermittently active are called phasic or bursting. Neurons with high activity rates are classified as fast-spiking.
Surgical Considerations
General Considerations
The technical considerations regarding repairing an injured nerve are constantly evolving. Commonly cited factors influencing nerve healing potential following repair include:
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Age: the most important factor. Younger patients have more positive outcomes (compared to adults) and the recovery time is quicker
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Studies have shown that, even in the pediatric population, patients < 6 years of age recovery more quickly compared to their adolescent counterparts
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Level of injury: distal injuries have a better chance of recovery compared to more proximal injuries
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Injury pattern: sharp lacerations/transections do better than crush injuries
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Delay in repair: chronic injuries do poorly
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The technique is critical:
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Secondary tissue damage occurs with excessive handling
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Nerve endings should be properly aligned (and not under tension) during a direct repair
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Excessive suture material used during the repair risks the development of a neuroma at the site of repair
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The general consensus in the literature favors utilizing an epineurial repair
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Epineurial repair ensures the proper orientation of the repair and reduces tension at the repair site
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Traditionally, techniques also included the incorporation of a perineurial repair in addition to an epineurial repair
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Outcome studies comparing epineurial alone versus perineal and epineurial repair techniques often report equivalent results; consideration should be given to excessive suture utilization as this can result in a neuroma at the repair site
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End-to-end Repair
The gold standard for microsurgical nerve injury reconstruction techniques remains the end-to-end (ETE) direct repair of the nerve stumps without tension. Difficulty with ETE is recognized in the following scenarios:
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Chronicity of the injury (leading to limited nerve stump excursion, neural tissue degeneration, and fibrosis/scarring in the region affected)
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The distance of the nerve injury from its specific muscle targets (i.e. distance from the muscle(s) it innervates)
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Significant gaps separating the injured nerve stumps from each other — including the surgical inaccessibility of one of the nerve endings
End-to-side Repair
End-to-side (ETS) is a technique that can be utilized when the surgeon is concerned regarding the reparability and usability of the injured nerve’s proximal stump. There is some literature supporting nerve fiber regeneration and collateral axonal sprouting following ETS repair. However, the efficacy of ETS remains controversial.
ETS is most often given consideration in the following settings:
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Upper extremity nerve injuries
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Facial reanimation
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Nerve reconstruction following tumor surgeries and ablation procedures
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Neuroma formation prevention
Sutureless Repair Techniques
Sutureless techniques have been becoming increasingly popular over the last 10 to 15 years. Evolving techniques include:
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Fibrin glue
References
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