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

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

  • 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

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

Neurotransmitters

Substance Characteristics Location
Acetylcholine

Aspartate

  • Excitatory
  • CNS
Dopamine
  • Both excitatory and inhibitory
  • Involved in initiation of movement
  • Inhibits the secretion of prolactin by the anterior pituitary upon release by the hypothalamus
  • 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
-
last updated 10/07/2018
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