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Nerve tissue, synapses, and neurotransmitters

Last updated: September 10, 2021

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
  • A type of ependymal cell that is in contact with blood vessels
  • Transport substances between the blood and the ventricles
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.

General

Synapses are areas where signals or action potentials are transmitted from a presynaptic to a postsynaptic structure (e.g., neurons, muscle). There are different types of synapses according to the synaptic structures:

Chemical synapses

The tetanus toxin tetanospasmin and the botulinum neurotoxin are proteolytic enzymes that cleave SNARE proteins, thereby inhibiting neurotransmitter release into the synaptic cleft.

Neuromuscular junction (NMJ)

Electrical synapses

Neurotransmitters are endogenous substances that allow communication between neurons in the nervous system.

Neurotransmitters

Overview of neurotransmitters
Substance Characteristics Location
Acetylcholine

Aspartate

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

Levels of neurotransmitter in disease processes

Levels of neurotransmitter in disease processes
Neurotransmitter Location Increased levels Decreased levels
Acetylcholine
Dopamine
Norepinephrine
  • Depression
Serotonin -
GABA -
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