Nerve tissue, synapses, and neurotransmitters

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

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]

  • 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 12/12/2019
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