Nerves Flashcards
Nervous tissue composed of:
mostly cells
neurons
glia (supporting cells - glue)
Nucleus
group of neuron cell bodies inside the CNS
found in GRAY MATTER of CNS
Ganglion
group of neuron cell bodies outside the PNS
Two kinds:
- sensory
- autonomic
Tract
group of CNS axons projecting from one place to another
form WHITE MATTER of CNS
Nerve
group of fibers in the PNS
Afferent
towards a source
sensory
Efferent
away from a source
motor
Two areas of the spinal cord:
1) Gray Matter - cell bodies - butterfly
2) white
Nissl Stain
stains the RER of neurons
axons dont stain (no protein production)
usually more staining at the ventral horn
Typical neuron structure (in CNS)
- multipolar
- cell body with array of organelles
- multiple tapered dendrites
- single axon of uniform diameter that has collaterals that branch at right angles to form synapses at the presynaptic terminals
Axon hillock
where axon begins
Initial segment of axon
site where action potentials are generated
voltage gated ion channels here
Nissl substance (RER)
synthesizes NT or its precursors
Neuron cell body
- nissl substance
- lots of golgi
- microtubules, microfilaments, neurofilaments
Spines
- increase surface area on dendrites
- flexible and active
- contain actin in the neck, no microtubules
- lose spines as we age
- neurotransmitter vesicles can fuse with them
Axons
- no RER (Nissl)
- use mainly chemical NT
- Na/K pumps located along entire length
Anterograde axonal transport:
- transport NT from cell body to terminal(400 mm/day)
- Kinesin ATPase
Retrograde axonal transport:
- recycle substances
- destroy garbage
- 200 mm/day
- Axon terminals can take up toxins/viruses and transport them to the cell body
Glial cells in CNS
- in white matter
- oligodendrocytes: make myelin
- microglia: macrophage (smallest)
- fibrous astrocytes: CT, sometimes act like neurons
NOTE: see the same glial cells in gray matter but protoplasmic astrocytes instead
Glial cells in PNS
- satellite cells: microenvironment, insulate, allow metabolic exchange
- Schwann: make myelin
Oligodendrocytes versus Schwann
O: myelinate up to 50 segments of 50 different axons
S: myelinate only a single segment of ONE axon
Demyelination diseases:
1) multiple sclerosis: CNS myelin
2) Guillain-Barre: PNS myelin
What is myelin
membranes of the myelin glial cells wrape around and squeeze out cytoplasm
How do axons increase conduction velocity?
-increase axon diameter: more current flows through larger tube because decreased resistance
- insulation so passive current flows further down the axon
- thicker sheath = faster conduction
Nodes of Ranvier
- voltage gated ion channels of myelinated axons only here
- Na/K pumps only here (myelinated uses less energy)
- Action potentials jump from node to node
Saltatory conduction
-Action potentials jump from node to node
Regeneration
- axon degenerates distal to the cut (anterograde/Wallerian degeneration)
- cell body undergoes retrograde chromatolysis
- PNS can regenerate
- CNS cannot because no macrophages (microglia are ineffective), instead astrocytes form glial scar and oligodendrocytes undergo apoptosis
PNS regeneration:
- myelin and axonal debris cleared quickly by macrophages
- schwann cells provide a tube for axon sprout growth
- slow (1-4 mm/day)
Inhibitory molecules on myelin suppress CNS axon regeneration:
- nogo
- myelin associated glycoprotein (MAG)
- oligodendrocyte-myelin glycoprotein (OMgp)
Largest neurons in the spinal cord gray matter
ventral horn, motor neurons
Retrograde transport diseases
- polio
- tetanus
Both retrograde and anterograde transport diseases
- herpes
- rabies
Intermediate Zone:
-site of termination of the dorsal axons in the spinal cord
Why myelin conducts faster
- insulation
- decreased capacitance (charges dont try to meet outside charges)
- larger diameter
- saltatory conduction
Unmyelinated action potentials:
graded, slower
Na/K pump in action potential:
-makes the concentration gradient that powers the action potential
