Nerve tissue, synapses, and neurotransmitters

Abstract

Nerve tissue

The nervous system is primarily composed of neurons and supporting glial cells.

Neurons

Glial cells and supporting cells

Structure Precursor Characteristics
Astrocytes
  • Radial glia from neuroepithelium (from neuroectoderm)
  • Most abundant type of glial cell
  • Have a large number of projections
  • Physical support → “scaffolding” of the CNS
  • Provides extracellular potassium buffer
  • Removes excess neurotransmitters
  • Glycogen reserve
  • Contain bundles of intermediate filaments
  • Foot processes form part of the blood-brain barrier
    • Form the glial-limiting membrane
  • Proliferate and hypertrophy after CNS injury (reactive gliosis)
    • Form an astroglial scar
Microglia
  • Smallest type of glial cells
  • Phagocytic cells: proliferate and migrate to damaged CNS to remove cellular debris
  • Pericytes are microglial cells that form part of the blood-brain barrier
  • Target cells for the HIV-1 virus
    • Infected cells fuse to form multinucleated giant cells

Ependymal cells (ependymocytes)

and choroid epithelial cells

  • Neuroepithelium (from neuroectoderm)
Tanycytes
  • Radial glia from neuroepithelium (from neuroectoderm)
  • A type of ependymal cell that is in contact with blood vessels
  • Transport substances between the blood and the ventricles
Oligodendrocytes
  • Neuroepithelium (from neuroectoderm)
Schwann cells
  • Myelinates axons of the PNS
  • Each cell can myelinate one single internodal segment for one single axon
  • Unmyelinated axons are covered by Schwann cell cytoplasm
  • Can phagocytose debris after injury
Myelin
  • Insulating layer of modified plasma membrane that wraps around axons of nerve in a spiral fashion
  • Increases the conduction velocity of signals traveling down axons
Node of Ranvier

Neuronal damage

  • Responses to damage
    • Cellular swelling
    • Peripherally located nucleus
    • Spread of Nissl substance throughout the cytoplasm of the neuron
    • The distal injured part of the neuron undergoes Wallerian degeneration.

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
  • Afferent: muscle spindles
  • Efferent: alpha motoneurons
  • 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
  • No
  • Afferent: pain (e.g., chemical, thermal, mechanical)
  • 0.25–1.5 m/s
  • 1 μm

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

Aspartate

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

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]

  • 1. Kaplan. USMLE Step 1 Anatomy Lecture Notes 2016. Kaplan Publishing; 2015.
  • 2. Le T, Bhushan V,‎ Sochat M, Chavda Y, Zureick A. First Aid for the USMLE Step 1 2018. New York, NY: McGraw-Hill Medical; 2017.
  • 3. Marios Politis, Flavia Niccolini. Serotonin in Parkinson's disease. Behav Brain Res. 2015; 277: pp. 136–145. doi: 10.1016/j.bbr.2014.07.037.
last updated 02/11/2019
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