Download Neurons

Neurons
• Vary in size and structure, but have
common features:
1. Cell Body
2. Dendrites
3. Axon
• Mature neurons do not divide, but
neural stem cells can divide and form
neurons or neuroglial cells.
1. Cell Body
• Contains normal cellular structures (golgi
apparatus, mitochondria, cytoplasm, cell
membrane, etc.)
• Neurofibrils – fine threads that extend into
the axon
• Nissl bodies (chromatophilic substances)
– Membranous sacs in the cytoplasm
– Similar to rough ER
– Ribosomes on Nissl bodies synthesize ______
2. Dendrites
• Usually short and highly branched (dendr = ?)
• The main receptive surfaces for receiving
communication from axons of other neurons
3. Axons
• Arise from a slight elevation of the cell body,
called the axonal hillock.
• Conduct nerve impulses away from the cell
body
• Contains many mitochondria, microtubules,
and neurofibrils
• Originates as a single structure, but may have
branches, especially at the end to interact
with receptive surfaces of other cells
PNS Axons
• Enclosed in myelin sheaths composed of
many Schwann cells
• Myelin is a lipoprotein.
• Neurilemma sheath surrounds the myelin
sheath
• Nodes of Ranvier – narrow gaps in the myelin
sheath between the Schwann cells
Classification of Neurons
• Classification based on Structural differences:
• Bipolar neurons
• Unipolar neurons
• Multipolar neurons
• Classification based on Functional differences:
• Sensory neurons (afferent neurons)
• Interneurons (association or internuncial neurons)
• Motor neurons (efferent neurons)
Structural Differences
• Sketch the neurons below. Notes on the next
3 slides:
Structural Differences, cont…..
1. Bipolar:
– 2 processes
•
•
Axon
Dendrite
– Found in specialized parts of the eyes, nose, and
ears
Structural Differences, cont…..
2. Unipolar:
– 1 process divides into 2 branches, which
function as a single axon
•
•
1 branch (peripheral process) associated with
dendrites
Other branch (central process) enters brain or spinal
cord
Structural Differences, cont…..
3. Multipolar:
– Many processes arising from cell body:
•
•
1 axon
Many dendrites
– Most neurons whose cell bodies lie in the brain
or spinal cord are multipolar.
• Direction of impulse is ALWAYS from
dendrites to axon.
Functional Differences
1. Sensory (afferent) neurons
– From peripheral body parts to the brain or spinal
cord
– Have specialized receptor ends at the tips of their
dendrites OR
– Dendrites closely associated with receptor cells
in the skin or sensory organs.
– Most are unipolar, but some are bipolar.
Functional Differences, cont…..
2. Interneurons (association or internuncial
neurons)
– Lie entirely in the brain or spinal cord
– Multipolar and link other neurons
– Transmit impulses from one part of the brain or
spinal cord to another
3. Motor (efferent) neurons
– Multipolar
– Carry nerve impulses from brain or spinal cord
to effectors
– Stimulate muscles or glands