Day 6: Nervous Tissue

Question 1

histology of neuron

Answer 1

• nucleus:
  
  • large, spherical
  • euchromatic
  • prominent nucleolus
• cytoplasm (perikaryon):
  
  • basophilic material
  • rough ER (Nissl bodies)

Question 2

axons

Answer 2

• propagates nerve impulse
• one per neuron
• long-range cytoplasmic extensions
• branched (at 90 degrees), no taper
• no RER, some have myelin
• hillock–> where AP arises

Question 3

dorsal root ganglia

Answer 3

carry sensory signals to the CNS

axons entering CNS proceed through dorsal roots of spinal cord

• cell bodies receive no synapses, single process bifurcates into central process passing into CNS and peripheral process that extends to other organs + receives sensory information

Question 4

autonomic ganglia / ventral root of spinal cord

Answer 4

autonomic ganglia: relay signals of visceral motor system

ventral root: axons leaving CNS at spinal cord

• formed by axons of motor neurons whose cell bodies are located in the CNS
• axons of anterior horn (somatic motor) cells of spinal cord innervate striated muscle, with which they form neuromuscular junctions

Question 5

spinal nerves

Answer 5

• dorsal + ventral roots merge more distally from the spinal cord
• contain a mixture of sensory axons passing to + motor axons progressing away from spinal cord

Question 6

supporting cells: CNS/PNS

Answer 6

supporting cells shared functions:

• myelination
• physical support
• control of microenvironment
• defense/immune

CNS (glia):

• oligodendrocytes
• astrocytes
• microglia

PNS:

• schwann cells (satellite cells)
• connective tissue cells (fibroblast/macrophage)

Question 7

endoneurium

Answer 7

• connective tissue within a nerve fasicle
• type III collagen fibrils encircle + run parallel to nerve fibers
• binds external laminae of Schwann cells

Question 8

perineurium

Answer 8

• connective tissue that directly encircles a nerve fasicle
• comprised of 1-5 layers of squamous cells (perineurial cells) separted by collagen fibrils
• perineurial cells form tight junctions + actively monitor microenvironment

Question 9

epineurium

Answer 9

• connective tissue that binds nerve fasicles into nerve bundles
• outermost connective tissue sheath of a peripheral nerve bundle
• comprised of dense connective tissue
• can contian adipose tissue

Question 10

anterograde (wallerian) degeneration

Answer 10

• axonal disintegration distal to the site of injury occurs in the first 24 hours
• myelin becomes pinched off from Schwann cells
• the blood-nerve barrier is disrupted along the entire lenth of the nerve, allowing influx of additional macrophage
• loss of neural input produces atrophy of muscle fibers

Question 11

retrograde reaction

Answer 11

• axonal disintegration occurs in the proximal segment of the axon (typically 1-3 internodal lengths)
• morphological change in the cell body
  
  • peripheral movement of nucleus
  • dissolution of nissl bodies (chromatolysis)
  • “stripping” of synapses from the dendrite and cell body

Question 12

PNS Regeneration

Answer 12

• After distal axon and myelin are phagocytosed, the Schwann cells divide to form long tubes (Schwann or endoneurial tube) along the remaining external lamina
• cell body begins heitened production and proximal axon sprouts
• schwann cells of the tubes secrete growth factors that attract the sprouting axons
• the tips of the axonal sprouts love laminin (glycoprotein of external lamina)
• the dominant axon sprout travels down to the target along the Schwann tubes and external lamina, to re-innervate the target muscle
• schwann cells re-myelinate
• the muscle again receives signal and normal size is restored