ICL 3.0: Nerve Structure & Histology

Question 1

what are the 4 regions of a neuron?

Answer 1

1. cell body
2. dendrites
3. axon
4. axon terminal

Question 2

what is the function of the cell body of a neuron?

Answer 2

it’s where the nucleus, organelles, etc. are

it’s the metabolic center

Question 3

what’s the function of the dendrites of a neuron?

Answer 3

they receive signals from other neurons

Question 4

what is the function of the axon of a neuron?

Answer 4

it carries signals to the axon terminal in the form of nerve impulses (aka action potentials)

Question 5

what is the function of the axon terminal of a neuron?

Answer 5

it houses and releases neurotransmitters for important communication between neurons and muscles

Question 6

what are the 3 classes of neurons?

Answer 6

1. multipolar neurons
2. bipolar neurons
3. pseudounipolar neurons

Question 7

what are multipolar neurons?

Answer 7

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

Question 8

what are bipolar neurons?

Answer 8

the cell body is in the middle of the axon –> the dendrites are on one side and the axon terminal is on the other

Question 9

what are pseudounipolar neurons?

Answer 9

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

Question 10

what kind of neurons are in the dorsal root ganglion?

Answer 10

pseudounipolar neurons

Question 11

what are Nissl bodies?

Answer 11

the ribosomes found in motor neurons!

Question 12

where are motor neurons found in the spinal cord?

Answer 12

the ventral horn of the gray matter

Question 13

what cytoplasmic structure can be found outside the nucleus of a neuron?

Answer 13

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

Question 14

are dendrites myelinated?

Answer 14

no

Question 15

what are the general characteristics of dendrites?

Answer 15

1. receive information
2. have a greater diameter than axons

the dendritic tree of a single neuron receives hundreds to thousands of synaptic inputs

Question 16

what is the function of dendritic spines?

Answer 16

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

Question 17

with what part of the neuron do axon terminals synapse with?

Answer 17

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

Question 18

what is the demarkation of the start of the axon of a neuron?

Answer 18

the axon hillock

Question 19

what is the function of the axon?

Answer 19

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

Question 20

what structures are located in the axon?

Answer 20

1. microtubules = transport proteins along the axon
2. mitochondria
3. neurofilaments
4. ER

Question 21

what is the node of Ranvier?

Answer 21

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

Question 22

what does myelin look like on EM?

Answer 22

sheets

it’s just layers of myelin wrapping around the axon

Question 23

what does an H&E stain of neurons look like?

Answer 23

the myelin is white

the circles are the endonerium while the dot inside the circles is the axonal cytoplasm

Question 24

what does a toluidine blue stain of a neuron look like?

Answer 24

the myelin sheets are blue while the axon is white

Question 25

what is the function of Schwann cells?

Answer 25

they myelinate peripheral nerves

Question 26

what are the 4 types of glial cells in the CNS?

Answer 26

1. oligodendrocytes
2. astrocytes
3. microglia
4. ependymal cells

just know that the cNS has its own set of cells separate from the peripheral nervous system

Question 27

what is the function of oligodendrocytes?

Answer 27

they myelinate neurons in the brain and spinal cord

one oligodendrocyte can myelinate 50-100 neurons while Schwann cells can only myelinate one axon

Question 28

what are the layers of single nerve?

Answer 28

1. nerve
2. fascicles
3. lots of neurons

Question 29

what are the connective tissue sheaths separating the layers of a nerve? what is their function?

Answer 29

1. epineurium = surrounds the whole nerve
2. perineurium = surrounds all the individual fasciales
3. endoneurium = surrounds individual neurons

the sheaths and arrangement of the vascular supply help prevent excessive stretching or compression of axons

Question 30

what are pockets/bubbles that can be seen within myeline sheaths?

Answer 30

Schmidt-Lanterman clefts

these pale eosinophilic areas are remnants of Schwann cell cytoplasm between layers of the myelin sheath

Question 31

what is Guillain-Barre?

Answer 31

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

Question 32

what does Guillain-Barre look like histologically?

Answer 32

1. large accumulations of lymphocytes, macrophages, and plasma cells around nerve fibers within nerve fascicles
2. 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

Question 33

how do Schwann cells myelinate unmyelinated axons of the PNS?

Answer 33

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

Question 34

how do Schwann cells myelinate myelinated axons of the PNS?

Answer 34

it’s one Schwann cell invaginating a single myelinated axon and then it wraps around in layers

Question 35

how are the myelin layers surrounding a single axon anchored together?

Answer 35

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

Question 36

what is a synapse?

Answer 36

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

Question 37

what does an EM of a pre and post synaptic neuron look like?

Answer 37

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

Question 38

what events happen at a synapse?

Answer 38

1. the arrival of an action potential at the axon terminal of the pre-synaptic neuron causes release of neurotransmitters
2. NTs diffuse across the synaptic cleft
3. Its activate receptors on dendrites or soma of the post-synaptic neuron
4. 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).

Question 39

what are the rosettes you seen in H&E stains of neurons?

Answer 39

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

Question 40

what is a ganglion?

Answer 40

an encapsulated cluster of neuronal cell bodies and glial cells, located outside of the central nervous system

the typical neuron is a pseudounipolar neuron

Question 41

what are satellite cells?

Answer 41

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