Nerve tissue, synapses, and neurotransmitters

Last updated: January 11, 2022

Summarytoggle arrow icon

Nerve tissue consists of neurons, which are excitable cells that transmit information as electrical signals, and glial cells (e.g., oligodendrocytes, Schwann cells, astrocytes, microglial cells), which perform a variety of nonsignaling functions such as forming myelin to provide support and insulation between neurons, phagocytosing and removing cellular debris, removing excess neurotransmitters, and forming the blood-brain barrier. Oligodendrocytes myelinate neurons in the central nervous system (CNS), while Schwann cells myelinate neurons in the peripheral nervous system (PNS). Myelin sheaths increase the conduction velocity of signals across axons. Inflammation and loss of the myelin sheath are the underlying pathologic processes in multiple sclerosis (CNS) and Guillain barre syndrome (PNS). Neurons are composed of dendrites, cell bodies, axons, and axon terminals. Based on their conduction velocity, diameter, and myelination, nerve fibers (axons) are classified into large, myelinated fibers with fast conduction velocity (group A); small, myelinated fibers with slow conduction velocity (group B); and small, unmyelinated fibers with slow conduction velocity (group C). Neurons communicate through the transmission of action potentials across junctions between them called synapses. Synaptic transmission can be chemical or electrical. Chemical synaptic transmission is the transfer of signals through the release of neurotransmitters (e.g. acetylcholine, dopamine, norepinephrine) from presynaptic terminals to postsynaptic receptors. Electrical synaptic transmission is the transfer of electrical signals through gap junctions. Alterations in neurotransmitter levels have been observed in various neurological diseases, including Parkinson disease (decreased dopamine), schizophrenia (increased dopamine), depression (decreased dopamine, norepinephrine, and serotonin), and Alzheimer disease (decreased acetylcholine).

General

  • Nerve tissue is the main tissue component of the nervous system and is primarily composed of neurons and supporting glial cells.
  • The nervous system is divided into two main components:

Neurons

Supporting glial cells

Cells of nerve tissue
Structure Precursor Characteristics
Astrocytes
Microglia

Ependymal cells (ependymocytes) and choroid epithelial cells

Tanycytes
Oligodendrocytes
Schwann cells

Each myelinating SchwONE cell insulates only ONE axon.

Glial cells guard the axons of the nerve cells as COPS: CNS axons are myelinated by Oligodendrocytes; PNS axons are insulated by Schwann cells.

Myelin

Neuronal damage

Layers of peripheral nerves

  • Endoneurium
  • Perineurium
    • Layer of connective tissue around nerve fascicles
    • Contains the blood-nerve barrier
    • Clinical significance: important layer in microsurgery during limb salvage surgical procedures
  • Epineurium

Nerve fibers are classified based on their conduction velocity, diameter, and axon characteristics.

Classification of nerve fibers
Nerve fibers Myelinated Characteristics Conduction velocity Size
A-alpha fibers
  • Yes
  • 60–120 m/s
  • 15 μm
A-beta fibers
  • 30–60 m/s
  • 8 μm

A-gamma fibers

  • 2–30 m/s
  • 5 μm
A-delta fibers
  • Afferent: pain (e.g., thermal, mechanical )
    • Free nerve endings
    • Responsible for the withdrawal response to pain (e.g., rapidly moving the hand when burned)
  • 3 μm
B fibers
  • Moderately
  • 3–15 m/s
  • < 3 μm
C fibers
  • Afferent: pain (e.g., chemical, thermal, mechanical)
  • 0.25–1.5 m/s
  • 1 μm

C fibers have a slow conduction velocity due to their small diameter and lack of myelination.

Neurotransmitters

Neurotransmitters are endogenous substances that allow communication between neurons and, usually, induce a change in the target cell. There are two types of neurotransmitters:

Overview of neurotransmitters
Neurotransmitter Site of action Characteristics
Acetylcholine

Aspartate

  • Excitatory
Dopamine
Endorphins
Enkephalins
GABA
  • Inhibitory
  • Is mainly synthesized from glutamate via the enzyme glutamate decarboxylase
Glutamate
Glycine
  • Inhibitory
Norepinephrine
Epinephrine
Serotonin
  • Involved in sleep, mood, and pain inhibition
  • Inhibitory

Neurotransmitter receptors (neuroreceptors)

Overview of neurotransmitter receptors [5]
Ionotropic receptors Metabotropic receptors
Characteristics
  • Allosteric binding site
  • The receptor molecule is also an ion channel (channel-linked).
Receptor opening time
  • Fast
  • Slow
Response
  • Short-lasting postsynaptic potentials (PSPs) or fast PSPs
  • Long-lasting postsynaptic or slow PSPs
Target effect location
  • Within the immediate region of the receptor
  • Effects can be more widespread and persistent throughout the cell
Receptor examples

Ion channels [5]

  • Definition: transmembrane proteins with a narrow pore that selectively permits particular ions to permeate the membrane
  • Functions
    • Give rise to selective ion permeability changes
    • Detect the electrical potential across the membrane
    • Involved in changing of local transmembrane potentials
  • Types of channels

Clinical significance of neurotransmitter levels

Overview of the clinical significance of the neurotransmitter levels
Neurotransmitter Site of action Associated conditions
Increased levels Decreased levels
Acetylcholine
Dopamine
Norepinephrine
  • Depression
Serotonin
GABA
  • N/A

Definitions

General

Chemical synapses

A type of synapse that transmits signals between neurons separated by a cleft via a chemical neurotransmitter

Overview

  • Composed of a presynaptic membrane, a synaptic cleft (the space between a presynaptic and postsynaptic neuron), and postsynaptic membrane
  • Most neurotransmitters (e.g., GABA, glutamate, glycine) undergo the following steps: synthesis, storage, release, reuptake, and degradation

Mechanism (presynaptic and postsynaptic receptor interactions)

  1. Neurotransmitter synthesis
    • Occurs in the presynaptic neuron
    • A precursor amino acid accumulates into the neuron.
    • The precursor is metabolized sequentially and yields a mature transmitter.
  2. Neurotransmitter storage
    • Vesicles filled with neurotransmitters are stored in the presynaptic terminal and to be released in response to stimulation of the neuron
    • Synaptophysin is a major synaptic vesicle protein that is thought to play a role in synaptic vesicle formation and maintenance [7]
  3. Neurotransmitter release
    • Action potentials in the presynaptic cell trigger the opening of voltage-gated Ca2+ channels in the presynaptic membrane, permitting Ca2+ influx.
    • Ca2+ binds to synaptotagmin (a protein anchored in the vesicle membrane), which initiates the vesicle docking to the presynaptic membrane and formation of SNARE complex. [8]
    • SNARE complex
    • During SNARE complex formation, the vesicle membrane and the presynaptic membrane merge, causing neurotransmitters to be released into the synaptic cleft.
  4. Neurotransmitter binding and recognition by target receptors
  5. Termination of the action of the released transmitter
  6. Postsynaptic potentials (PSPs)

Certain proteolytic enzymes, e.g., tetanus toxin and botulinum toxin, can cleave SNARE proteins, thereby inhibiting neurotransmitter release into the synaptic cleft and, thus, causing spasms and paralysis.

Neuromuscular junction (NMJ)

Electrical synapses

  • Definition: substances that enhance neuronal survival and differentiation
  • Overview [5][10][11]
    • Neurons compete for survival-promoting agents during their development
    • NTFs ensure a match between the requirement for appropriate target innervation and the number of surviving neurons
    • Functions include regulation of nervous system development (e.g., cell proliferation, migration, differentiation, dendritic and axonal growth, synaptogenesis, synaptic plasticity), of regressive events (e.g., cell death or survival, axon and synapse elimination) after injury, and of synaptic competition and by modifying both synaptic transmission and structure
Overview of NTFs (trophic and growth factors) [5][11]
Family Examples Functions

Neurotrophins

Neuropoietic cytokines and interleukins
  • Motor neuron survival
  • Immunoregulation
Tissue growth factors
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  9. Hammond C, El Far O, Seagar M. Neurotransmitter release. Elsevier ; 2015 : p. 145-169
  10. Neuroscience. https://www.ncbi.nlm.nih.gov/books/NBK11117/. Updated: January 1, 2001. Accessed: January 7, 2022.
  11. Marios Politis, Flavia Niccolini. Serotonin in Parkinson's disease. Behav Brain Res. 2015; 277 : p.136-145. doi: 10.1016/j.bbr.2014.07.037 . | Open in Read by QxMD

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