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.

A neuron or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. It is the main component of nervous tissue in all animals except sponges and placozoa. Plants and fungi do not have nerve cells. The spelling neuron has become uncommon.

Types of Neurons

  • Sensory Neurons – Sensory neurons are neurons responsible for converting external stimuli from the environment into corresponding internal stimuli. They are activated by sensory input, and send projections to other elements of the nervous system, ultimately conveying sensory information to the brain or spinal cord. Unlike the motor neurons of the central nervous system (CNS), whose inputs come from other neurons, sensory neurons are activated by physical modalities (such as visible light, sound, heat, physical contact, etc.) or by chemical signals (such as smell and taste).
  • Motor Neurons – Motor neurons are neurons located in the central nervous system, and they project their axons outside of the CNS to directly or indirectly control muscles. The interface between a motor neuron and muscle fiber is a specialized synapse called the neuromuscular junction. The structure of motor neurons is multipolar, meaning each cell contains a single axon and multiple dendrites. This is the most common type of neuron.
  • Interneurons – Interneurons are neither sensory nor motor; rather, they act as the “middlemen” that form connections between the other two types. Located in the CNS, they operate locally, meaning their axons connect only with nearby sensory or motor neurons. Interneurons can save time and therefore prevent injury by sending messages to the spinal cord and back instead of all the way to the brain. Like motor neurons, they are multipolar in structure.

Structural classification

Different kinds of neurons

  • Unipolar neuron
  • Bipolar neuro
  • 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

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.

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]
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Structures of a Neuron

In addition to having all the normal components of a cell (nucleus, organelles, etc.) neurons also contain unique structures for receiving and sending the electrical signals that make neuronal communication possible.

The structure of a neuron – The above image shows the basic structural components of an average neuron, including the dendrite, cell body, nucleus, Node of Ranvier, myelin sheath, Schwann cell, and axon terminal.

  • Dendrite – Dendrites are branch-like structures extending away from the cell body, and their job is to receive messages from other neurons and allow those messages to travel to the cell body. Although some neurons do not have any dendrites, other types of neurons have multiple dendrites. Dendrites can have small protrusions called dendritic spines, which further increase surface area for possible connections with other neurons.
  • Cell Body – Like other cells, each neuron has a cell body (or soma) that contains a nucleus, smooth and rough endoplasmic reticulum, Golgi apparatus, mitochondria, and other cellular components.
  • Axon – An axon, at its most basic, is a tube-like structure that carries an electrical impulse from the cell body (or from another cell’s dendrites) to the structures at opposite end of the neuron—axon terminals, which can then pass the impulse to another neuron. The cell body contains a specialized structure, the axon hillock, which serves as a junction between the cell body and the axon.
  • Synapse – The synapse is the chemical junction between the axon terminals of one neuron and the dendrites of the next. It is a gap where specialized chemical interactions can occur, rather than an actual structure.

Other Structures

  • Dendrites – cell bodies, axons, and synapses are the basic parts of a neuron, but other important structures and materials surround neurons to make them more efficient.
  • Myelin Sheath – Some axons are covered with myelin, a fatty material that wraps around the axon to form the myelin sheath. This external coating functions as insulation to minimize the dissipation of the electrical signal as it travels down the axon. Myelin’s presence on the axon greatly increases the speed of conduction of the electrical signal, because the fat prevents any electricity from leaking out. This insulation is important, as the axon from a human motor neuron can be as long as a meter—from the base of the spine to the toes. Periodic gaps in the myelin sheath are called nodes of Ranvier. At these nodes, the signal is “recharged” as it travels along the axon.
  • Glial Cells – The myelin sheath is not actually part of the neuron. Myelin is produced by glial cells (or simply glia, or “glue” in Greek), which are non-neuronal cells that provide support for the nervous system. Glia function to hold neurons in place (hence their Greek name), supply them with nutrients, provide insulation, and remove pathogens and dead neurons. In the central nervous system, the glial cells that form the myelin sheath are called oligodendrocytes; in the peripheral nervous system, they are called Schwann cells.

Function of Neurons

Neurons are unique in their ability to receive and transmit information. Neurons are characterized by the long processes which extend out from the cell body or soma. Dendrites receive afferent signals. Axons carry efferent signals. Neurons exist in a variety of forms including multipolar, bipolar, pseudounipolar, and Panasonic which differ primarily in their number and arrangement of axons and dendrites. The soma contains the nucleus and other organelles necessary for neuronal function. There may be one or many dendrites associated with a single neuron depending on its function and location. In addition to afferent signaling, dendrites can be involved in protein synthesis and independent signaling functions with other neurons. Axons typically end in an axon terminal at which neurotransmitters, neuromodulators, or neurohormones are released in the conversion of the electrical signal to a chemical signal which can cross the synapse or neuromuscular junction. Axonal transport is carried out by proteins such as kinesin and dynein. Neurons propagate their potentials by ion movement through voltage-gated ion channels (though calcium channels are largely voltage-independent) across their membranes. Potassium, sodium, and chloride ions are the greatest contributors to the membrane potential of the common neuron. The resting membrane potential of typical neurons is around -70 mV. As a depolarizing threshold stimulus occurs, an action potential that is consistent in amplitude is generated and travels down the axon to the terminal. Graded potentials are also important to note as they vary in strength, and lose amplitude throughout their transmission. The variety of interactions among neurons enables the transmission of impulses to perpetrate diverse functions within the body. Mature neurons are unable to divide, so their destruction may lead to a neurological deficit. However, neural progenitors that are capable of participation in neurogenesis are present in certain regions such as the dentate gyrus of the rat, and in the subependymal of rodents. There is a continuing interest in potential therapeutic uses for neural progenitor cells following injury.

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The Others Function of a Neuron

The specialized structure and organization of neurons allow them to transmit signals in the form of electric impulses from the brain to the body and back. Individually, neurons can pass a signal all the way from their own dendrites to their own axon terminals; but at a higher level, neurons are organized in long chains, allowing them to pass signals very quickly from one to the other. One neuron’s axon will connect chemically to another neuron’s dendrite at the synapse between them. Electrically charged chemicals flow from the first neuron’s axon to the second neuron’s dendrite, and that signal will then flow from the second neuron’s dendrite, down its axon, across a synapse,  into a third neuron’s dendrites, and so on.

This is the basic chain of neural signal transmission, which is how the brain sends signals to the muscles to make them move, and how sensory organs send signals to the brain. It is important that these signals can happen quickly, and they do. Think of how fast you drop a hot potato—before you even realize it is hot. This is because the sense organ (in this case, the skin) sends the signal “This is hot!” to neurons with very long axons that travel up the spine to the brain. If this didn’t happen quickly, people would burn themselves.

Nerves

In the periphery, individual nerve fibers are surrounded by delicate connective tissue called the endoneurium. The endoneurial-surrounded fibers are grouped together into fascicles which are surrounded by the perineurium. The perineurium is also made up of connective tissue that is arranged in a lamellar manner. The cells comprising the perineurium are epithelioid myofibroblasts. The outermost layer of connective tissue surrounding peripheral nerves is called the epineurium. The epineurium commonly surrounds multiple fascicles and the blood vessels supplying the nerve. The epineurium is formed by arachnoid and dura invagination as the nerves exit the vertebral canal.

There are a number of different nerve fiber types that have been classified. Some are constituents of both motor and sensory pathways, and others serve only sensory pathways.

Muscles

Muscles require innervation to contract and to maintain tone. Motor neurons in vertebrates secrete acetylcholine into the neuromuscular junction. When an action potential reaches the axon terminal of the motor neuron, voltage-dependent calcium channels open and accommodate the influx of calcium. This permits the fusion of neurotransmitter vesicles with the axon terminal membrane to cause acetylcholine release into the junction. Acetylcholine molecules then bind their receptors on the sarcolemma to cause the opening of sodium channels which results in the production of an action potential and its propagation to the myofibril to cause muscle contraction.

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Somatic nerves and autonomic nerves both act on muscles. Somatic innervation is the cause of voluntary muscle function, whereas, autonomic innervation controls the involuntary muscle contraction and relaxation, and glandular secretion enables the body to function and adapt without conscious thought. A notable example of autonomic innervation is the parasympathetic innervation of the detrusor muscle of the bladder and the internal urethral sphincter to accommodate micturition. Sympathetic innervation in the pelvic region inhibits the functions of the parasympathetic innervation with the relaxation of the detrusor muscle and constriction of the internal urethral sphincter. Somatic innervation to the external urethral sphincter is also involved in the process.

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:

  • Age: the most important factor. Younger patients have more positive outcomes (compared to adults) and the recovery time is quicker

    • Studies have shown that, even in the pediatric population, patients < 6 years of age recovery more quickly compared to their adolescent counterparts
  • Level of injury: distal injuries have a better chance of recovery compared to more proximal injuries
  • Injury pattern: sharp lacerations/transections do better than crush injuries
  • Delay in repair: chronic injuries do poorly
  • The technique is critical:

    • Secondary tissue damage occurs with excessive handling
    • Nerve endings should be properly aligned (and not under tension) during a direct repair
  • Excessive suture material used during the repair risks the development of a neuroma at the site of repair

    • The general consensus in the literature favors utilizing an epineurial repair

      • Epineurial repair ensures the proper orientation of the repair and reduces tension at the repair site
    • Traditionally, techniques also included the incorporation of a perineurial repair in addition to an epineurial repair
    • 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
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:

  • Chronicity of the injury (leading to limited nerve stump excursion, neural tissue degeneration, and fibrosis/scarring in the region affected)
  • The distance of the nerve injury from its specific muscle targets (i.e. distance from the muscle(s) it innervates)
  • 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:

  • Upper extremity nerve injuries
  • Facial reanimation
  • Nerve reconstruction following tumor surgeries and ablation procedures
  • Neuroma formation prevention
Sutureless Repair Techniques

Sutureless techniques have been becoming increasingly popular over the last 10 to 15 years. Evolving techniques include:

  • Fibrin glue
  • Nerve conduits – becoming increasingly popular for larger “gaps” at the nerve injury site

    • Conduits are typically polyglycolic acid and collagen-based
    • The literature supports conduit use — relative indications include defects over 8mm, but no greater than 2cm in distance

References

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