Nerve tissue, synapses, and neurotransmitters


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).

Nerve tissue


Supporting glial cells

Structure Precursor Characteristics

Ependymal cells (ependymocytes)

and choroid epithelial cells

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


Neuronal damage

Layers of peripheral nerves


Classification of nerve fibers

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

Nerve fibers Myelinated Characteristics Conduction velocity Size
  • Yes
  • 60–120 m/s
  • 15 μm
A-beta fibers
  • Afferent: cutaneous mechanoreceptors
  • 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.


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

Neuromuscular junction (NMJ)

A type of chemical synapse between alpha motor neurons and skeletal muscle.

Electrical synapses

  • Characterized by direct flow of current through cells via gap junctions
  • Found in the heart and smooth muscle
  • No chemical synapse is required → no delay during synapsis



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


Substance Characteristics Location
  • Usually excitatory
  • Synthesized from choline and acetyl coenzyme A.
  • Two receptors
    • Nicotinic AChR
    • Muscarinic AChR


  • Excitatory
  • CNS
  • Local chemical messenger elsewhere in the body (e.g., increases natriuresis in the kidney)
  • Inhibitory
  • Involved in sleep, mood, and pain inhibition

Levels of neurotransmitter in disease processes

Neurotransmitter Location Increased levels Decreased levels
  • Anxiety
  • Depression
  • Raphe nucleus (medulla, pons)
  • Nucleus accumbens


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last updated 07/16/2020
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