Nerve tissue, synapses, and neurotransmitters

Summary

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

Neurons

Supporting glial cells

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

Myelin

Neuronal damage

Layers of peripheral nerves

References:[1][2]

Classification of nerve fibers

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

Nerve fibers Myelinated Characteristics Conduction velocity Size
A-alpha-fibers
  • Yes
  • 60–120 m/s
  • 15 μm
A-beta fibers
  • Afferent: cutaneous mechanoreceptors
  • 30–60 m/s
  • 8 μm

A-gamma fibers

  • Efferent: muscle spindles (gamma motoneurons)
  • 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

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

References:[2]

Neurotransmitters

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

Neurotransmitters

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

Aspartate

  • Excitatory
Dopamine
  • CNS
  • Local chemical messenger elsewhere in the body (e.g., increases natriuresis in the kidney)
Endorphins
Enkephalins
GABA
Glutamate
  • Excitatory
Glycine
  • Inhibitory
Norepinephrine
Serotonin
  • Involved in sleep, mood, and pain inhibition

Levels of neurotransmitter in disease processes

Neurotransmitter Location Increased levels Decreased levels
Acetylcholine
Dopamine
Norepinephrine
  • Anxiety
  • Depression
Serotonin
  • Raphe nucleus (brain)
-
GABA
  • Nucleus accumbens
-

References:[3]

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last updated 03/28/2020
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