Types of Neurons (Nerve Cells)
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• Cells of the nervous system, called neurons,
are specialized to carry electrochemical
impulses called action potentials or nerve
impulses.
• The human brain alone has about 100 billion
neurons.
• Neurons vary in size and shape
– The smallest have cell bodies only 4 microns wide,
– The biggest have cell bodies 100 microns wide
How Neurons are similar to other cells:
• Surrounded by a cell membrane.
• Have a nucleus that contains genes.
• Contain cytoplasm, mitochondria and other
"organelles".
• Carry out basic cellular processes like protein
synthesis and energy production.
How Neurons Differ From Other Cells:
• Neurons have specialized extensions called
dendrites and axons.
– Dendrites bring information to the cell body and
axons take information away from the cell body.
• Neurons communicate with each other
through an electrochemical process.
• Neurons contain some specialized structures
(for example, synapses) and chemicals (for
example, neurotransmitters).
• Neurons are the oldest and longest cells in the
body.
• You have many of the same neurons for your
whole life.
Structure of Neurons
• Structurally consist of 4 parts:
– Dendrites
– Cell body (soma)
– Axon
– Axon terminal
Dendrites
• Project from soma
• Pick up information from surrounding
environment and transmit it (carry it) to soma.
Soma
• Contains the following organelles:
• Nucleus
• Nucleolus
• Ribosomes
• Endoplasmic reticulum (SER and RER)
• Golgi Apparatus
• Microfilaments/Neurotubules
• Mitochondria
• Cytoplasm
Axons
• Extension of soma
• Transmit impulses away from soma towards
axon terminal
Axon Terminals
• Receive impulse from axon and transmits it to
surrounding neurons
• Terminals are called
Differences Between Axons and
Dentdrites
• Axons
– Take information away from the cell body
– Smooth Surface
– Generally only 1 axon per cell
– No ribosomes
– Can have myelin
– Branch further from the cell body
• Axons grow out of the cell body
– There is a steady transport of cell components
(e.g., vesicles, mitochondria) from the cell body
along the entire length of the axon.
– This flow is driven by specialized molecular motor
proteins moving along the microtubules in the
cytoplasm within the axon.
– It may take 2 weeks or longer for material
synthesized in the cell body to reach the axon
terminals in your big toe.
• Dendrites
– Bring information to the cell body
– Rough Surface (dendritic spines)
– Usually many dendrites per cell
– Have ribosomes
– No myelin insulation
– Branch near the cell body
The Neuron
Classification of neurons by the
function
• Sensory neurons
• Motor neurons
• Interneurons
Sensory (afferent) neurons
Send information to the CNS
• In skin, muscles, joints, and organs
– Sensitive to pressure, temperature, and pain.
• In the nose and tongue
– sensitive to the molecular shapes we perceive as
tastes and smells.
• In the inner ear
– sensitive to vibration
• In the eyes
– rods and cones of the retina are sensitive to light
Sensory Neurons
Motor or efferent neurons
• Carries information from spinal cord
• Stimulate muscle cells throughout the body
Interneurons
• Provide connections between sensory and
motor neurons, as well as between
themselves.
• The neurons of the central nervous system,
including the brain, are all interneurons.
Interneuron/Association Neuron
• Interneurons: send information between sensory
neurons and motor neurons. Most interneurons are
located in the central nervous system.
Interneuron
Glial or Neuroglial Cells
Special types of connective tissue cells that help
support and protect neurons.
Types of glial cells in CNS
1.Astroycytes : Form the blood-brain barrier
2.Oligodendrites: Form the myelin sheath
around axons of the CNS. Help to hold nerve
fibers together
3. Microglia: Phagocyte cells that migrate
through the CNS removing foreign matter
and degenerated brain tissue
4. Ependymal cells: Epithelial cells that line the
brain and central canal of the spinal cord and
form cerebrospinal fluid and aid in its
circulation.
Types of glial cells in PNS
1. Schwann cells: Form myelin sheaths
around peripheral axons and are composed
of:
1. Myelin: A white fatty substance that insulates
axons. Forms myelin sheath around some
axons in the PNS
2. Nodes of Ranvier: Nonmyelinated gaps on
axons that lie between adjacent Schwann
cells.
3. Neurilemma: Outer membrane of a Schwann
cell. Plays an important role in the
regeneration of cut or injured axons.
2. Satellite cells
– Support neurons in the PNS ganglia.
• A clump of neuron cell bodies, is called a
ganglion (plural: ganglia) or a nucleus (plural:
nuclei).
• A fiber made up of many axons is called a
nerve.
• In the brain and spinal cord
– areas that are mostly axons are called white
matter
– Areas that include large number of cell bodies are
called gray matter.