ICL 3.0: Nerve Structure & Histology Flashcards
what are the 4 regions of a neuron?
- cell body
- dendrites
- axon
- axon terminal
what is the function of the cell body of a neuron?
it’s where the nucleus, organelles, etc. are
it’s the metabolic center
what’s the function of the dendrites of a neuron?
they receive signals from other neurons
what is the function of the axon of a neuron?
it carries signals to the axon terminal in the form of nerve impulses (aka action potentials)
what is the function of the axon terminal of a neuron?
it houses and releases neurotransmitters for important communication between neurons and muscles
what are the 3 classes of neurons?
- multipolar neurons
- bipolar neurons
- pseudounipolar neurons
what are multipolar neurons?
your stereotypical neurons and they make up most of the neurons in the CNS
they have an array of dendrites and single, long axon –> long axons are more susceptible to damage
some have short axons – commonly called interneurons; they help process and integrate neuronal information with a local brain region
what are bipolar neurons?
the cell body is in the middle of the axon –> the dendrites are on one side and the axon terminal is on the other
what are pseudounipolar neurons?
the cell body looks like a budd of of the axon
so there’s a single axon from the cell body that bifurcates; one axonal branch directed to the periphery & other centrally
what kind of neurons are in the dorsal root ganglion?
pseudounipolar neurons
what are Nissl bodies?
the ribosomes found in motor neurons!
where are motor neurons found in the spinal cord?
the ventral horn of the gray matter
what cytoplasmic structure can be found outside the nucleus of a neuron?
rough ER
you can see the rough ER on an EM but on LM, stacks of rough ER are what make up the Nissl bodies
are dendrites myelinated?
no
what are the general characteristics of dendrites?
- receive information
- have a greater diameter than axons
the dendritic tree of a single neuron receives hundreds to thousands of synaptic inputs
what is the function of dendritic spines?
they are small projections along the dendrites that increase receptor surface area
since dendrites receive information from axon terminal of other neurons, the dendritic spines increase the SA and allow for even more synapsing to happen
with what part of the neuron do axon terminals synapse with?
axon terminals don’t all have to synapse with just the dendrites; they can synapse with other areas of the neuron too like the cell body
what is the demarkation of the start of the axon of a neuron?
the axon hillock
what is the function of the axon?
the initial segment of an axon is where an action potential is generated!
they are capable of generating action potential, propagation and synaptic transmission
axons can be myelinated or unmyelinated but they are all the thinnest part of the neuron
what structures are located in the axon?
- microtubules = transport proteins along the axon
- mitochondria
- neurofilaments
- ER
what is the node of Ranvier?
it’s the part of the axon where there’s no myelin but there are a lot of sodium channels which is what helps with the depolarization of the membrane and the propagation of the action potential
what does myelin look like on EM?
sheets
it’s just layers of myelin wrapping around the axon
what does an H&E stain of neurons look like?
the myelin is white
the circles are the endonerium while the dot inside the circles is the axonal cytoplasm
what does a toluidine blue stain of a neuron look like?
the myelin sheets are blue while the axon is white
what is the function of Schwann cells?
they myelinate peripheral nerves
what are the 4 types of glial cells in the CNS?
- oligodendrocytes
- astrocytes
- microglia
- ependymal cells
just know that the cNS has its own set of cells separate from the peripheral nervous system
what is the function of oligodendrocytes?
they myelinate neurons in the brain and spinal cord
one oligodendrocyte can myelinate 50-100 neurons while Schwann cells can only myelinate one axon
what are the layers of single nerve?
- nerve
- fascicles
- lots of neurons
what are the connective tissue sheaths separating the layers of a nerve? what is their function?
- epineurium = surrounds the whole nerve
- perineurium = surrounds all the individual fasciales
- endoneurium = surrounds individual neurons
the sheaths and arrangement of the vascular supply help prevent excessive stretching or compression of axons
what are pockets/bubbles that can be seen within myeline sheaths?
Schmidt-Lanterman clefts
these pale eosinophilic areas are remnants of Schwann cell cytoplasm between layers of the myelin sheath
what is Guillain-Barre?
a demyelinating peripheral neuropathy
the disease is characterized clinically by weakness in the distal limbs that rapidly advance to affect proximal muscle function = “ascending paralysis
it literally eats away at the myelin of the peripheral nerves; the macrophages are breaking down the myelin and since the myelin is attached to the axons the axons will also degenerate
it can lead to life threatening respiratory paralysis
what does Guillain-Barre look like histologically?
- large accumulations of lymphocytes, macrophages, and plasma cells around nerve fibers within nerve fascicles
- large segments of myelin are damaged (i.e. segmental demyelination), leaving the axons exposed to extracellular matrix
myelin is damaged/stripped by the T cell-mediated immune response induced by activated macrophages
the whole thing looks like total chaos with lymphocytes everywhere causing inflammation and the axon looks all wavy and weird
how do Schwann cells myelinate unmyelinated axons of the PNS?
a single Schwann cell can invaginate multiple axons
unmyelinated axons are “engulfed” by Schwann cell cytoplasm but are not wrapped tightly in multiple layers of myelin –> the axons fit into ‘grooves’ on the surface of the cell
large Schwann cells in the PNS may have 20 or more grooves, each containing one or more axons
how do Schwann cells myelinate myelinated axons of the PNS?
it’s one Schwann cell invaginating a single myelinated axon and then it wraps around in layers
how are the myelin layers surrounding a single axon anchored together?
P0, MBP and PMP22
these are both transmembrane proteins that anchor the individual layers of myelin together
mutations in genes that encode these proteins making myelin susceptible for premature breakdown causing demyelinating diseases
what is a synapse?
the junction between a presynaptic neuron to a postsynaptic neuron
it can also occur between an axon and an effector tissue like a muscle or gland
what does an EM of a pre and post synaptic neuron look like?
slide 25
the presynaptic neuron is filled with circles that are vesicles filled with neurotransmitters
the postsynaptic terminal is empty but has some darkness at the edges that represents the receptors
what events happen at a synapse?
- the arrival of an action potential at the axon terminal of the pre-synaptic neuron causes release of neurotransmitters
- NTs diffuse across the synaptic cleft
- Its activate receptors on dendrites or soma of the post-synaptic neuron
- depending on the type of receptor activated, the post-synaptic neuron is either excited or inhibited (i.e. made more likely or less likely to initiate an action potential in its axon).
what are the rosettes you seen in H&E stains of neurons?
they’re the axon terminals that are packed with neurotransmitters that will then go and bind to the receptors on muscle to initiate a contraction
for each muscle fiber, there is only one rosette of neuromuscular junctions
what is a ganglion?
an encapsulated cluster of neuronal cell bodies and glial cells, located outside of the central nervous system
the typical neuron is a pseudounipolar neuron
what are satellite cells?
they provide nutrients and create ionic balance for the ganglion
ganglion are an encapsulated cluster of neuronal cell bodies and glial cells, located outside of the central nervous system
