PNS Physiology Flashcards
how is the PNS divided?
autonomic and somatic
cellular origins of PNS
-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
why are the neural crest cells called transient stem cells?
only present during development and they can differentiate into different types of cells
neural crest fate
-melanocytes
-glia Schwann cells
-dorsal root ganglion neurons
-sympathetic neurons
what molecules are involved in neural crest cells fate?
-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
autonomic nervous system
-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
autonomic nervous system
-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
sympathetic nervous system
-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
parasympathetic nervous system
-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
differences in functions between the sympathetic and parasympathetic nervous systems
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
enteric nervous system
-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
somatosensory system
-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
sensory neurons
-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
comparison of somatic vs autonomic organization
-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
peripheral projections
-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
skin somatosensory nerve endings
-very different nerve skin innervation patterns
-meissner’s, pacinian, merkel, and ruffini’s corpuscles with different endings that are innervated by nerves
stimuli and nerve skin endings
-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
central projections
-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
how is sensory info detected and relayed?
primary sensory neurons in DRG —> spinal cord neuron called secondary neuron —> thalamus —> cortex
what do secondary neurons do?
their axons cross the midline to go to the other side of the spinal cord to thalamus then cortex
anterolateral system
-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
dorsal column-medial lemniscal system
-processes touch and vibration
-spinal cord —> DRG synapses with secondary neurons —> ascends closer to the midline
how is sensory info mapped in the primary sensory cortex?
-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
barrel cortex
-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
whisker pattern
-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
referred pain
-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
primary sensory neurons in the DRG
-heterogeneous
-different sizes to send axons to different locations
nociceptive neurons
-cells detecting noxious stimuli capable of causing tissue injury
-if stimuli is severe enough to cause damage you activate neuron
two components of pain
sensory and emotional
ID of nociceptive neurons
-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
small-to-medium DRG neurons initiate pain
-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
nociceptors can allow us to detect pain in different modalities
- mechanical pain- pinprick
- thermal pain- <15 degrees celsius or >45 degrees celsius
- chemical pain- hydrogen, ATP, capsaicin
- electrical pain
-certain nociceptors are dedicated to each type of modality
what molecules are implicated in pain activation?
-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
transient receptor potential vanilloid 1 (TRPV1)
-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
expression of TRPV1 cloning
-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
DRG-11
-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
TRPV1 protein sequence and structure
-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
generation of TRPV1 knockout mice
-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
behavior tests for DRG neurons
-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
temperature spectrum + TRPV1 channels
-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
peripheral sensitization
-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
endogenous agents that activate or sensitize nociceptive neurons
-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
what do GPCR signaling pathways do?
-modulate ion channel activities
-activated by a ligand that will modify the ion channels to make them sensitized
bradykinin
-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
central sensitization
strengthen synaptic transmission between primary afferent neurons and second order neurons in the dorsal horn
somatosensation
-small-to-medium DRGs mediate pain/itch
-feel pain —> withdraw, avoid the tissue damage
-feel itch —> scratch to remove the irritant and suppress itch
itch
-initiated by small-to-medium DRG neurons
-unpleasant sensation to make you want to scratch
different types of itch
histamine vs non-histamine
relationship between pain and itch
antagonize each other —> suppress pain can cause itchiness
animal model for itch
-inject chemical into skin of mouse
-if the animal feels the itch it will lift its hindpaw at high frequency
animal model to distinguish pain and itch response
-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
intensity theory
-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
evidence against intensity theory
-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
labelled line theory
dedicated populations for pain and itch
evidence for labelled line theory
-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
evidence against labelled line theory
in the same study, they found that the same fiber responded to histamine and capsaicin —> changed units of theory from specific to selected
Mrg-expressing receptors expressed in subset of DRG neurons
-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
what do the MrgA3+ axons innervate?
the very apical surface of the skin
ablation of MrgA3+ neurons in DRG
-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
what happens with specific activation of MrgA3+ neurons?
evokes only itch but not pain response
what happens when you activate the 5% neurons with TRPV1?
-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
for WT mice, you inject capsaicin and yo should activate both populations —> why only pain and not itch?
-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
what happens when you topically apply capsaicin?
you feel itch initially then as it penetrates the inhibition kicks in to become pain
