PNS Physiology

Question 1

how is the PNS divided?

Answer 1

autonomic and somatic

Question 2

cellular origins of PNS

Answer 2

-neural crest cells give rise to entire PNS during embryogenesis
-neural tube —> forms the spinal cord and neural crest cells arise from the dorsal region of the neural tube

Question 3

why are the neural crest cells called transient stem cells?

Answer 3

only present during development and they can differentiate into different types of cells

Question 4

neural crest fate

Answer 4

-melanocytes
-glia Schwann cells
-dorsal root ganglion neurons
-sympathetic neurons

Question 5

what molecules are involved in neural crest cells fate?

Answer 5

-cascade of transcription factors like neurogenins (NgI) —> determine cell fate so if you knock this out, the neuronal cell cannot be differentiated or express it in the muscle cell it can turn into a neuron
-once cell fate is determined, need neuron to survive with a set of genes called neurotrophins and receptors
-need them to extend their exons to target regions —> need axon guidance molecules like netrins, which are attractive or repulsive
-once axon arrives they need glutamate receptors to refine the targets

Question 6

autonomic nervous system

Answer 6

-controls visceral functions like heart rate, digestion, salivation, perspiration, dilation of pupil, urination, and sexual arousal
-essential for maintaining homeostasis
-divided into parasympathetic and sympathetic divisions
-functions largely under the level of consciousness

Question 7

autonomic nervous system

Answer 7

-axons innervate cardiac muscle and smooth muscles as well as glands to regulate basic functions
-relays visceral sensory info to the CNS by inducing the release of hormones mediating energy, metabolism, and cardiovascular functions
-major neurotransmitters are acetylcholine and epinephrine

Question 8

sympathetic nervous system

Answer 8

-2 neuron network- one neuron (sympathetic pre-ganglion neuron) is located in the thoracic or lumbar segments of the spinal cord with the cell body in the spinal cord
-the axons extend out and synapse with post-ganglionic neurons, cell bodies aggregate to form ganglions
-post-ganglionic neurons extend axons to different organs
-connect spinal cord/CNS through ganglion to tissues

Question 9

parasympathetic nervous system

Answer 9

-2 neuron network- pre-ganglionic neuron resides either close to brain/brainstem or lateral horns of the spinal cord
-pre-ganglionic neurons extend their axons outside the CNS and synapse with parasympathetic ganglions that are located near organs

Question 10

differences in functions between the sympathetic and parasympathetic nervous systems

Answer 10

sympathetic:
-most active in time of stress —> fight/flight so dilate pupils, inhibits salivation, relaxes bronchii to breathe more, and shut down digestion
parasympathetic:
-rest and digest to conserve the body’s energy

Question 11

enteric nervous system

Answer 11

-unlike the parasympathetic and sympathetic nervous systems, it is made up of 2 plexuses along wall of GI tract
-2 networks of neurons embedded: submucuous plexus that is close to the lumen of tract and myenteric plexus
-contain local sensory neurons which detect changes in tension of gut and chemical environment
-called second brain since it has a network of neurons in gut that control GI movement

Question 12

somatosensory system

Answer 12

-responsible for receiving stimuli from external and internal environments
-under conscious control
-DRG and trigeminal ganglions —> initiate and mediate somatosensation
-processes info like touch, temperature, and pressure
-main neurotransmitter is glutamate

Question 13

sensory neurons

Answer 13

-cell bodies of primary sensory neurons are in the dorsal root ganglion (DRG)
-31 pairs of DRGs along spinal cord and each ganglion innervate to certain regions of the body
-primary sensory neuron is the first neuron to detect sensation
-2 axons: pseudounipolar so from the cell body it extends a small stump out where one goes to the peripheral and the other axon goes to the CNS to detect sensory info and send it to the spinal cord

Question 14

comparison of somatic vs autonomic organization

Answer 14

-for autonomic, you have two neurons with pre-ganglionic cell bodies in the CNS, extend axons to autonomic ganglions, and synapse with post-ganglionic neurons and their axons go to the peripheral target
-for sensory, DRG/trigeminal only have one axon that extends to the peripheral target and CNS —> DRG controls sensory info for body and back of head then the trigeminal sends axons to face

Question 15

peripheral projections

Answer 15

-each DRG sends axons to certain segments of the body called dermatomes which are an area of skin innervated by a DRG
-map dermatome by recording nerves and seeing where they innervate
-adjacent dermatomes overlap —> each dorsal root sends axon to skin that can branch out and at the boundary there is overlap by the axons

Question 16

skin somatosensory nerve endings

Answer 16

-very different nerve skin innervation patterns
-meissner’s, pacinian, merkel, and ruffini’s corpuscles with different endings that are innervated by nerves

Question 17

stimuli and nerve skin endings

Answer 17

-end organ can allow nerve to sense different types of sensation especially touch or mechanical sensors
-if nerve associated with merkel/ruffini is stimulated, slow adapting (nerve fires AP during duration of indentation on skin)
-if nerve associated with meissner/pacinian, rapidly adapting (fire different patterns and they only fire when the AP is on and off)
-combination of slowly/rapidly adapting can feel quality and quantity of touch
-large group of nerve endings without association that send axons to skin —> sense pain and temperature

Question 18

central projections

Answer 18

-spinal cord is the first relay point of somatic sensory info
-axons come into the spinal cord, DRG axons go out to different regions
-dorsal side of spinal cord has dorsal horn and ventral side has ventral horn
-dorsal horn controls pain, temperature, touch, and itch
-ventral horn controls muscles and movement

Question 19

how is sensory info detected and relayed?

Answer 19

primary sensory neurons in DRG —> spinal cord neuron called secondary neuron —> thalamus —> cortex

Question 20

what do secondary neurons do?

Answer 20

their axons cross the midline to go to the other side of the spinal cord to thalamus then cortex

Question 21

anterolateral system

Answer 21

-processes pain and temperature
-spinal cord —> through DRG, which synapses with secondary neurons —> cross midline and ascend to thalamus and the axons of the thalamus go to the cortex

Question 22

dorsal column-medial lemniscal system

Answer 22

-processes touch and vibration
-spinal cord —> DRG synapses with secondary neurons —> ascends closer to the midline

Question 23

how is sensory info mapped in the primary sensory cortex?

Answer 23

-somatotopically- adjacent areas in the body are represented by adjacent areas in the cortex
-the representation of the perioral region is disproportionately large in humans
-sensory homunculus

Question 24

barrel cortex

Answer 24

-part of somatosensory cortex in rodents
-sensory info from whiskers is represented here
-discrete areas of layer IV form anatomically distinguishable structures
-found barrel structures where one single barrel contains ~2500 neurons arranged in ring-like structure and processes tactile input from single whisker
-# of barrels = # of whiskers on contralateral side and arranged pattern corresponding to topology of whiskers
-another strain of mice had extra ring of barrels and showed extra row of whiskers

Question 25

whisker pattern

Answer 25

-pattern maintained throughout central pathways of trigeminal system
-allows organism to know what type of sensory info is received and where on face this info is being sensed

Question 26

referred pain

Answer 26

-when people have a heart attack, they feel the pain in their left arm or chest
-people think convergency of 2 DRG neurons into the same projection neurons in the dorsal horn
-internal organ has issue and the person may feel it in their skin —> one DRG may innervate the heart and another sends to skin and it sends axon to the same spinal cord neuron

Question 27

primary sensory neurons in the DRG

Answer 27

-heterogeneous
-different sizes to send axons to different locations

Question 28

nociceptive neurons

Answer 28

-cells detecting noxious stimuli capable of causing tissue injury
-if stimuli is severe enough to cause damage you activate neuron

Question 29

two components of pain

Answer 29

sensory and emotional

Question 30

ID of nociceptive neurons

Answer 30

-tease nerve out of skin and do recordings
-at the same time do stimulation on skin
-increase force of pressure
-with the blunt probe, no response
-with the pinprick from a needle, the nerve started firing APs
-with serrated forceps, even more APs are fired

Question 31

small-to-medium DRG neurons initiate pain

Answer 31

-large neuron (40 microns) has a thick axon that is wrapped with myelin —> very fast conduction velocities and is involved with light touch and body position
-medium neuron has a smaller axon with thinner myelin and slower conduction velocity and is involved in initial sharp pain
-small neuron make up the majority of DRG neurons and they are un-myelinated axons with slow conduction velocities and involved with delay burning pain

Question 32

nociceptors can allow us to detect pain in different modalities

Answer 32

1. mechanical pain- pinprick
2. thermal pain- <15 degrees celsius or >45 degrees celsius
3. chemical pain- hydrogen, ATP, capsaicin
4. electrical pain
   -certain nociceptors are dedicated to each type of modality

Question 33

what molecules are implicated in pain activation?

Answer 33

-voltage gated Na, K, Ca channels- allow these neurons to propagate APs
-ligand-gated cation channels- very diverse and activated by specific pain stimuli like acid, ATP, and temperature
-G protein-coupled receptors are activated by specifical molecules and modulate ion channel activities —> opioid receptors inhibit the channel and prostaglandin receptors potentiate channel

Question 34

transient receptor potential vanilloid 1 (TRPV1)

Answer 34

-molecular sensor for heat greater than 45 degrees celsius
-culture DRG neuron —> take it out and disassociate them and culture them then give them capsaicin
-found that capsaicin activates a subset of the DRG neurons with Ca influx
-use the Ca dye to look at activation —> kow that Ca channel in DRGs of nociceptors

Question 35

expression of TRPV1 cloning

Answer 35

-express into heterologous cells and these cells usually do not have any capsaicin channels so they put cDNA into the cells for expression cloning
-generate DRG cDNA library- collect mRNA from DRG and convert them to cDNA
-express different pool of cDNAs in heterologous cells
-monitor capsaicin activation by calcium imaging

Question 36

DRG-11

Answer 36

-in DRG-11 there are certain neurons that light up after capsaicin addition
-narrow it down to one gene VR1 or TRPV1
-express gene and the majority of cell expression respond to capsaicin

Question 37

TRPV1 protein sequence and structure

Answer 37

-structure qualifies to every feature of ion channels
-has pore domain that opens for ions to flow in —> recording of the channel found there was inward current when capsaicin is added and if you use heat there is also inward current
-expressed in small diameter nociceptors
-found that it has threshold of 42 degrees celsius to open in response to temperature increase, which correlates well with human psychophysics

Question 38

generation of TRPV1 knockout mice

Answer 38

-found that for moderate heat (45 degrees celsius), many WT neruons responded however in knockout mice no response
-for higher temps, certain neurons respond in both WT and knockout —> may be another ion channel controlling for higher temperatures

Question 39

behavior tests for DRG neurons

Answer 39

-von frey test- single filament hair used to poke hindpaw of mice
-different strands of filaments —> as the filament strains increase, so does pressure
-found in the WT and knockout, no difference in mechanical force so TRPV1 is not involved in mechanical pain
-thermal pain: at 48 degrees celsius, knockout take a little bit longer to withdraw tail and on hot plate, if they feel pain they jump as the temperature increases —> not completely gone in knockout
-TRPV1 single channel responds above 42 or 45 but many other channels compensate

Question 40

temperature spectrum + TRPV1 channels

Answer 40

-over temperature spectrum, different TRPV1 channels cover different temperatures
-TRPV2 can activate over 55 and may explain why knockout mice still respond to temps over 55
-TRPM8 can be activated at lower than 20 degrees

Question 41

peripheral sensitization

Answer 41

-injury- pain becomes exaggerate
-touch an injury site and it feels painful —> nociceptors become sensitized under chronic pain sites so normally less or non-painful stimuli can become stronger
-increased expression of ion channels so detect more pain
-increased sensitivity of ion channels
-reduced inhibitory channels
-mainly mediated by G-coupled receptors

Question 42

endogenous agents that activate or sensitize nociceptive neurons

Answer 42

-when there is a lesion/injury, the nerve site can release factors including ATP proton acid, lipids, serotonn —> these can activate their GPCRs expressed on nerve endings to sensitize nerves
-sensitization from bradykinin, prostalglandins, substance P, CGRP, and NGF
-edema and vasodilation are from substance P and CGRP

Question 43

what do GPCR signaling pathways do?

Answer 43

-modulate ion channel activities
-activated by a ligand that will modify the ion channels to make them sensitized

Question 44

bradykinin

Answer 44

-give neuron without adding bradykinin, neuron is activated at threshold of 42 degrees celsius
-add bradykinin, there is larger current and threshold shift from 42 to 35
-this means that added bradykinin could open the TRPV1 channels with normal body temp

Question 45

central sensitization

Answer 45

strengthen synaptic transmission between primary afferent neurons and second order neurons in the dorsal horn

Question 46

somatosensation

Answer 46

-small-to-medium DRGs mediate pain/itch
-feel pain —> withdraw, avoid the tissue damage
-feel itch —> scratch to remove the irritant and suppress itch

Question 47

itch

Answer 47

-initiated by small-to-medium DRG neurons
-unpleasant sensation to make you want to scratch

Question 48

different types of itch

Answer 48

histamine vs non-histamine

Question 49

relationship between pain and itch

Answer 49

antagonize each other —> suppress pain can cause itchiness

Question 50

animal model for itch

Answer 50

-inject chemical into skin of mouse
-if the animal feels the itch it will lift its hindpaw at high frequency

Question 51

animal model to distinguish pain and itch response

Answer 51

-inject chemical into cheek to distinguis pain and itch
-pain it wipes, grooms with both paws, and scratch with hindpaw
-inject histamine they scratch a lot but without wiping
-inject pain no scratching but a lot of wiping

Question 52

intensity theory

Answer 52

-single neuron can mediate pain and itch but depends on the stimuli —> stimuli strong you feel pain and stimuli weak you feel itch
-human skin has small spot that can give rise to pain and itch —> hypothesized that it was the same neuron

Question 53

evidence against intensity theory

Answer 53

-increased frequency of electrical stimuli showed an increase in feeling itchiness but no change from itch to pain
-in another test to look at pain, they lowered the frequency for pain and felt less pain but no transition to itch

Question 54

labelled line theory

Answer 54

dedicated populations for pain and itch

Question 55

evidence for labelled line theory

Answer 55

-after histamine injections, people reported itch and found that there was a specific subset of G-fibers respond to histamines
-this type of C-fiber is an itch specific fiber

Question 56

evidence against labelled line theory

Answer 56

in the same study, they found that the same fiber responded to histamine and capsaicin —> changed units of theory from specific to selected

Question 57

Mrg-expressing receptors expressed in subset of DRG neurons

Answer 57

-respond to itchy substances
-made MrgA3-GFP-Cre line (only expressed in 5% of DRG neurons)
-saw a neuron in live animals and applied different stimuli —> responded to CQ and histamine (itchy-like substance)
-also responded to capsaicin

Question 58

what do the MrgA3+ axons innervate?

Answer 58

the very apical surface of the skin

Question 59

ablation of MrgA3+ neurons in DRG

Answer 59

-express toxin to kill these neurons for experiment
-add CQ —> they scratch less and a lot of the itch sensation is reduced
-for pain it is completely normal
-specific

Question 60

what happens with specific activation of MrgA3+ neurons?

Answer 60

evokes only itch but not pain response

Question 61

what happens when you activate the 5% neurons with TRPV1?

Answer 61

-knock out TRPV1 and only use it in 5% neuron specific ones
inject capsaicin:
-WT mice have a lot of wiping and no scratching
-knockout for TRPV1 they do not feel pain
-mice with TRPV1 and 5% Mrg, they just scratch but do not feel pain

Question 62

for WT mice, you inject capsaicin and yo should activate both populations —> why only pain and not itch?

Answer 62

-inhibitory interneuron in spinal cord
-activate both populations and turn on inhibitory interneuron which blocks the itch sensation to only feel pain
-itch specific neurons in DRG but with inhibitory mechanisms from pain to itch to modify itch sensation

Question 63

what happens when you topically apply capsaicin?

Answer 63

you feel itch initially then as it penetrates the inhibition kicks in to become pain