Exam 4: Topic 10 Somatosensory systems Flashcards
what is the rostral side of the brain? Caudal?
toward the nose; back of the head or toward the toes
which sides of the brain and spinal cord are dorsal vs ventral?
Dorsal is the upper half of the brain and toward the back side of the spinal cord; ventral is lower half of the brain and more toward the front side of the body
Coronal
left to right slices when looking directly at the brain
- from one ear to the other
sagittal
slicing the brain into two hemisphere
horizontal/transverse
slicing the brain into above and below
sensory information
how information is transformed from the external world and into the cerebral cortex
motor information
going from the cerebral cortex out to the rest of the body
what are the first approximation steps in perception?
- physical stimulus
- sensation
- perception
sensation
transformation of a physical stimulus into an electrical signal
perception
conscious awareness of the electrical signal
for vision what stimulus, receptor class, and cells are related?
light, photoreceptor, rods/cones
for hearing what stimulus, receptor class, and cells are related?
sound, mechanoreceptor, hair cells
for taste what stimulus, receptor class, and cells are related?
chemical, chemoreceptor, taste bud
for olfaction what stimulus, receptor class, and cells are related?
chemical, chemoreceptor, olfactory
for touch what stimulus, receptor class, and cells are related?
mechanical, mechanoreceptor, DRG and Cranial nerve (CN)
for pain/temp what stimulus, receptor class, and cells are related?
- mechanical/chemical, mech/chem, DRG and CN
- thermal, thermoreceptor, DRG and CN
photoreceptors
transmit light into electrical impulses
- ion channels are involved and light does something to generate an electrical current
rods vs cones
rods work in dim light and cones work in bright light
T/F there is a whole range of mechanoreceptors in our skin?
True
- Influenced by ion channels
mechanoreceptors
a pressure or stretch will trigger the neurons to have a current change
pacinian corpuscle properties
require a lot of pressure to change mechanical tension because they are deep within the skin
- have encapsulated afferent fibers and when stretched the channels open
what are properties of Merkel cells?
ABeta afferents which trigger norepinephrine release on to Beta2-adrenergic receptors
what skin layer are Merkel cells in?
the epidermis alongside the free nerve endings
what skin layer are Meissner corpuscle in?
the epidermis between the free nerve endings and Merkel cells below
what skin layer are the Ruffini corpuscle in?
the dermis
what skin layer are the pacinian corpuscle in?
the subcutaneous layer (deepest layer)
what type of ion channels are in the skin?
Piezo 1 and Piezo 2 mechanosensitive ion channels
- these are not ligand gated
what type of neuron morphology are DRG?
pseudo bi/unipolar
- one of the branches on the split axon goes to the skin and the other to the spinal cord
what type of neuron morphology are CNS neurons?
multipolar
Sensory transduction
convert stimulus into something the neurons can encode
what are the 2 properties of sensory transduction?
- quality
- quantity
quality sensory transduction
- What type of receptors have been activated
- Each type is specialized to respond to a specific type of stimulus
quantity sensory transduction
the frequency the receptors and neurons evoke electrical activity ⇒ synaptic potentials or action potentials
receptor adaptation
determines whether the receptor conveys static or dynamic properties of the stimulus
- Some mechanoreceptors adapt quickly, other do not
Mechanoreceptors threshold of activation
all mechanoreceptors are low threshold (very sensitive) but there is a still a range of thresholds
4 properties designed to convey stimuli
- modality
- location
- intensity
- timing
modality
stimulus quality ⇒ light or touch?
location
where the stimulation originates from ⇒ the body or the world
intensity
stimulus quantity ⇒ how strong the stimulus is
timing
when the stimulus stops or starts
T/F the receptive field properties of individual neurons are the keys to understanding sensation and perception?
True
Receptive field of a neuron
region in sensory space within which a specific stimulus elicits action potentials from that neuron
- Particular region of the body surface or a particular part of the visual world
when you brush on skin what is affected in the immediate surrounding area, the area just outside immediate surrounding, and larger elsewhere for a single neuron?
- if the skin is brushed in the immediate region there are many action potentials provoked
- the immediate areas surrounding area has decreased cell firing as the neuron is somewhat inhibited
- Touching the skin anywhere else has no firing
on center off surround receptive fields
- When you stimulate the center you get more action potentials relative to the background
- When you stimulate the surrounding area you get less action potentials than baseline
how does on center off surround work in the visual field? (project to the visual thalamus)
- When light is shined in the center of their field, they have a lot of action potentials
- When light is shined on the offsides of the center there are much less action potentials
what area of the body has the most fine tuned receptor fields?
the fingers
where does periphery sensory information eventually get sent to?
the neocortex
- transduction apparatus are unique in sensory systems
- Once transduction has occurred, there are equivalent styles for neural computation across the sensory systems
- There are thalamic nuclei that input sensory information
- converting the sensory stimulus into electrical impulses via specialized receptors ⇒ provides a neural code (pattern of action potential) that the brian can use
what happens if the retinal axons from the eye are routed to the auditory thalamus rather than the visual thalamus at a young age?
The animal will be able to see and it will see with its auditory cortex relatively normally if it is done with young animals
- the opposite cortexes compared to normal will be used for the functions
what does the somatic sensory system split into? Subdivisions?
- pain and temp
- mechanical => touch, vibration, pressure, tension
properties of mechanoreceptors? (3)
- encapsulated so not free ending
- axons are fast conducting
- physical deformation of skin results in depolarization of the receptor
properties of pain and temperature receptors? (2)
- free nerve endings
- axons are relatively slow conducting
for mechanosensory and pain/temp the body first neurons are what? Face first neurons?
DRG; trigeminal ganglia
T/F the neurons for touch and pain ascend to the cortex via the same pathways?
False => different
- sensory afferent go to the dorsal spinal cord and the outputs are always ventral ⇒ know dorsal and ventral
- touch and proprioception synapse and go directly up from the dorsal horn while pain and temperature synapse and then cross over and up through the ventral side
where do motor neurons connecting to muscle always exit?
via the ventral spinal cord
- efferent exits vs sensory afferents
dermatomes (4 and how many in each)
segmented arrangement of somatosensory fields according to the vertebrae on the spinal cord
- cervical (8)
- thoracic (12)
- lumbar (5)
- sacral (2)
receptor cells
enable the transduction of very detailed sensory information into neural activity
T/F in the somatosensory system receptor cells are almost always neurons?
True
- like DRG neurons in the spinal cord
T/F sensory signals on the body are transmitted to the dorsal part of the spinal cord via DRG neurons?
True
- the face is via the trigeminal ganglion
Does the pseudo monopolar neurons with axons from the periphery to the brain have dendrites?
No
- free nerve endings are the receptors but Merkel cells are a separate cell with a synaptic junction
what’s the pathway from the face vs the body for somatosensation?
- face surface => trigeminal ganglion => contralateral brainstem (pons) => ipsilateral thalamus VPM => somatosensory cortex
- body surface => DRG => spinal cord DCN (ipsilateral) => contralateral brainstem (medulla) => thalamus VPL => somatosensory cortex
T/F face and body representation go to the same brainstem area?
False they go to different areas
T/F somatosensory information is contralateral?
true
T/F DRG neurons don’t need the electrical signal to pass through the cell body?
True which is unlike most CNS neurons
what are the 3 neurons for the dorsal column medial lemniscal pathway?
- DRG
- dorsal column nuclei (DCN) => made of gracile nucleus and cutaneous
- VPL thalamus
what triggers the direct pathway from the skin via the DRG to the DCN and brain?
vibration and onset/offset
what triggers the indirect pathway for the dorsal column system?
intensity and onset/offset
what can the indirect pathway modulate?
the direct pathway
where does the indirect pathway additionally synapse?
in the postsynaptic dorsal column (PSDC) which is a branch off prior to the DCN in the spinal cord
- The speed of the indirect pathway is slower
what is the first appearance of joint encoding of specific sensory information features in a pathway?
the dorsal column system
- different types of DRG spinal neurons may converge onto the same DCN neuron projecting in the brainstem
what are the 4 types of mechanoreceptors? What do they do?
- Ruffini: skin stretch
- Merkel: light touch, edges, points, curves ⇒ fine somatosensory discrimination
- Meissner: heavy pressure, skin motion
- Pacinian: vibration
what type of receptors are free nerve endings? (3)
- Pain
- Temperature
- Chemoreceptors
T/F mechanoreceptors are considered low threshold?
True (pain is higher threshold)
- depending on how strong the bending of the skin this will influence the size of the receptor potential
- when large enough you exceed spike pressure and the DRG can transmit the signal to the brain
which mechanoreceptors are slow and sustained signals across the duration of stimulus?
Ruffini and Merkel
which mechanoreceptors are rapid and fast signals only at the start of the stimulus?
Meissner and Pacinian
slow adapting vs rapidly adapting cell properties?
slow adapting will fire a lot at the beginning of the signal and still fire throughout the rest of the signal but at longer intervals
- rapid will only fire many times at the beginning and stop firing afterward
what is the result of higher receptor density? smaller size of the receptive field?
increased discrimination for both
- there can be overlap of receptor fields, but you need tactile feedback from small receptor fields during microsurgery => If the receptor field is large you don’t need a high density of neurons
what type of mechanoreceptor has small receptor fields and high density?
Merkel
what type of mechanoreceptor has medium receptor fields and high density?
Meisner
what type of mechanoreceptor has large receptor field and low density?
Ruffini and Pacinian
which mechanoreceptor is slowly adapting in superficial layers? Deep layers?
Merkel; Ruffini
which mechanoreceptor is rapidly adapting in superficial layers? Deep layers?
Meissner; Pacinian
which mechanoreceptor has a small receptor field but is rapid?
Meisner (RA1)
which mechanoreceptor has a large receptor field but is rapid?
Pacinian (RA2)
which mechanoreceptor has a small receptor field but is slow?
Merkel (SA1)
which mechanoreceptor has a large receptor field but is Slow?
Ruffini (SA2)
what conduction speed axon type do the 4 mechanoreceptors have?
Abeta axons => 6-12 micrometers in diameter and send signals 35-75 m/s from myelination
which lobe is the somatosensory cortex in?
parietal lobe
Homunculus
predetermined and develops in the absence of sensory input but can change with experience
- There are overrepresentation of the neural territory for certain areas of the body
which are the main areas of the homunculus?
hands, tongue, lips with more tissue disproportionately
where is the face located in the brain homunculus vs the legs or feet?
the face is more toward the side of the body (ear region) while the feet/trunk are medial and dorsal to this
T/F primates have individual territory in the brain for each finger?
True
- humans have an additional amount of white matter in the brain
- if organization is bad then wiring is impossible
what happens if a monkey loses one of its fingers in adulthood?
neurons will start responding to the adjacent fingers instead and their area will take up the los fingers prior area
- referred to as experience dependent plasticity
how are mouse/rat whiskers represented int he somatosensory cortex?
- 1 cortical barrel represents 1 whisker
- Individual neurons in each barrel have a diversity of receptive field properties
- The direction of the whisker movement matters in evoking the maximal firing rate ⇒ certain neurons will respond to a particular movement of direction
what mechanosensory receptors are in the appendage of the star nosed mole?
22 appendages have merkel cell receptors for object shape and texture identification
- World fastest eater and fastest mammalian predator
- Can identify and eat its prey in 100-300 ms
what are the neurons for carrying pain and temperature? (2)
- ADelta: lightly myelinated, small diameter ⇒ 5-30 m/sec
- C fiber: unmyelinated, very small diameter ⇒ 0.5-2 m/sec
- these are free nerve endings
Nociceptor types involved in pain and temperature (3)
- Mechanosensitive: physical but higher threshold than touch
- Thermosensitive: thermal
- Polymodal: physical, chemical, and thermal ⇒ especially in second pain
Nociceptor protein channels examples for pain info (3)
piezo 2, TRPV4, ASIC (senses acids)
Thermoreceptor properties for pain and temperature? (2)
- Some are nociceptive, some are not
- All travel via the same pathway as the pain signal
TRPV1
> 43 deg C and are heat (hurt) nociceptors
TRPV3/TRPV4
warm (comfy) receptors
TRPM8
<28 deg C and are cold (pain) nociceptors
what happens with the heating of a (thermoceptor) membrane?
leads to change in conformation that opens channels
- Usually pass sodium and/or calcium ions
do thermoreceptors or nociceptors have larger thresholds for temperature?
There is a large difference between the receptors and nociceptors have higher thresholds
- Thermoreceptor increase gradually and then plateaus ⇒ it will stay on with higher heat
- The nociceptor doesn’t have action potentials at modest temperatures until it gets to threshold because it is a pain temperature receptors
what is special about TRPV channels and heat?
they put heat in chili and have intracellular binding sites and can be sensitive to acids as well
- TRPV1 can be on nociceptive and non nociceptive thermoreceptors
what are TRPV1 receptors activated by? (extracellular and intracellular)
- E: Heat ⇒ body temp and painful heat
- E: Acid (H+)
- I: Capsaicin
- I: Endovanilloids
what can lead to TRPV1 sensitization and hyperalgesia?
inflammation or other injuries
how do analgesic creams work with TRPV1
they are heating creams and cause desensitization since they hijack capsaicin binding sites
- you get adaptation and the channel will stop responding based on the TRPV1 channel
1st pain
ADelta lightly myelinated, small diameter with latency of a few hundred ms ⇒ 5-30 m/sec
which brain region(s) does the 1st pain utilize?
Ventral posterior complex via thalamus to pain localization in the somatic sensory cortex
2nd pain
C fiber unmyelinated, very small diameter latency of a few seconds but can persist for minutes, days, weeks, years ⇒ 0.5-2 m/s
which brain region(s) does the 2nd pain utilize?
Medial nuclei via brainstem and thalamus to affective dimensions of ventral medial forebrain
where do afferents always enter?
via the dorsal horn
- The DRG cells will terminate at different layers on the dorsal horn
what was the experiment done inactivating Adelta or C fibers?
determined what type of pain was felt
- if the 1st pain cell is off only the 2nd pain can persist for minutes, days, weeks, years => NMDA receptor dependent in the spinal cord
- when the 2nd pain is off only the 1st pain will be activated but then will go away after a short amount of time
what is the pathway for the 1st pain
anterolateral system => VPL nucleus => somatosensory cortex
- quick shot pathway
what is the pathway for the 2nd pain?
anterolateral system => variety of nuclei => anterior cingulate cortex and insula
- Some people can live with their pain while others have crippled chronic pain
What does the spinothalamic tract do?
Caries pain and temp info from back ⅓ of the head and rest of the body
- 1st pain pathway via anterolateral system
what are the 3 neurons in the spinothalamic tract?
- 1st order = DRG ⇒ dorsal horn (crossing over after synapse)
- 2nd order = spinal cord (decussation)
- 3rd order = VPL thalamus ⇒ different neurons than mechanosensory in the same nucleus
What does the trigeminothalamic tract do?
pain and temp info from the face
- 1st pain pathway via anterolateral system
what 3 neurons are required in the trigeminothalamic tract?
- 1st order = trigeminal ganglion (descending) => Different nucleus in brainstem vs touch
- 2nd order = medulla (decussation) => crossing over
- 3rd order = VPM thalamus ⇒ different neurons than mechanosensory in the same nucleus
Visceral pain
referred to feeling internal organ failure but feels like skin pain
- significant lack of neurons dedicated to sense pain from inside the body
- Nerve afferent from internal organs hijack the skin pain pathway
T/F Damage to internal organs may be perceived as an internal pain?
False
- Damage to internal organs may be perceived as an external (cutaneous = skin) pain
Dissociated sensory loss
contralateral loss in pain sensation from a unilateral spinal cord lesion
- it’s fairly common to get a lesion at one side of the spinal cord ⇒ consequences for pain and touch pathways
ipsilateral loss
loss of touch sensation from a unilateral spinal cord lesion
what is the pathway of pain propagation along the DRG neuron?
painful stimulus ⇒ mechanoreceptor (nociceptive = TRPV4/Piezo2) ⇒ depolarization ⇒ open VG NaV1.7 (leads to SCN9a gene) ⇒ open VGNaV1.8 ⇒ AP propagation
NaV1.7 channel
VG sodium channel at the nerve ending and
- initiated the APs
Nav1.8 chanel
VG sodium channel along the axon
- necessary for propagating the AP along the DRG neuron
cortical pyramidal neurons NaV1.6
go to the SCN8a Gene
- Unmeyelenated axons at nodes of ranvier
what do scorpion toxins do? What effect does this have?
toxins binding NaV1.7 leads to activation and increased NA+ current and pain
- Scorpion toxins have no effect on NaV1.8
what occurs when there is a mutation in the E862Q in NaV1.8 for grasshopper mice?
causes this channel to be inhibited by scorpion toxin and provides analgesia ⇒ toxin still binds NaV1.7 but the grasshopper mouse feels no pain
- allows the mouse to kill the scorpion
T/F you need both NaV1.7 and NaV1.8 to get a signal from the periphery to the brain and signal the pain?
True
- but each channel is in a specific place
Congenital insensitivity to pain (CIP)
Homozygous mutation makes the person insensitive to pain ⇒ most common cause is a mutation that affects function of NaV1.7 so you can activate the free nerve endings but cannot transmit to the brain
- People often injure themselves and don’t get treatment
Hyperalgesia
inflammatory pain ⇒ peripheral response where free nerve endings are sensitive to the inflammatory soup floating around
- Nonpainful stimulation induces pain
- Painful stimuli produce heightened pain perception
- Meant to protect the organism from further damage and promote healing
analgesia
insensitivity to pain
Analgesics
drugs that reduce pain
NSAIDS ⇒ non steroidal anti inflammtory drugs
- Aspirin, ibuprofen, acetaminophen and many others
- Target one of the COX enzymes which makes prostaglandins
T/F many of the cellular responses that lead to inflammation and wound healing also cause pain? Why?
True
- Effect may be by direct stimulation ⇒ H+ stimulation of TRPV1 (capsaicin)
- May act by stimulating pathways that modulate the response on the pain neurons
- substances bind to the free nerve ending which activate the pain signal
types of central sensitization in the CNS? (2)
- Allodynia
- Neuropathic pain
Allodynia
non painful, low threshold stimuli now produce pain
- High activity of the DRG neurons lead to alterations that reduce the threshold of the dorsal horn (2nd order) neurons
- LTP-like changes in the 2nd order neuron caused by many different, poorly understood mechanisms
Neuropathic pain
chronic pain in the absence of normally painful stimulus (LTP after an acute painful stimulus?)
- Nerve compression
-Caused by damage to the 1st or 2nd order neurons
- Cancer, AIDS, multiple sclerosis, diabetes
descending control of pain functionality
Signals from many brain regions can modify the ascending pain signal at many levels ⇒ at 1st synapse, ascending in spinal cord or brain stem, maybe even in cortex
- Endogenous opioids often involved ⇒ enkephalins, endorphins, dynorphins
- May also involve endocannabinoids
what might descending control of pain account for? (4)
- Placebo effect ⇒ it doesn’t mean the person was faking
- Situation responses ⇒ such as in the battlefield you don’t feel pain
- Cultural differences
- Mirror therapy
where does the somatosensory cortex signal down to?
first to the amygdala and hypothalamus which signal to the midbrain periaqueductal gray
- this signals to mainly the locus coeruleus and raphe nucleus (also parabrachial and medullary reticular formation nucleus)
- these all signal the dorsal horn of the spinal cord which can signal back up via the anterolateral system
where is serotonin released from? What’s its effect?
the raphe nucleus
- it is the feel good hormone
Where is norepinephrine released from? What’s its effect?
the locus coeruleus
- helps attention/arousal
Gate theory of pain
when you have a place of pain and apply touch, pressure, massage the pain can be reduced
- There are synapses on local inhibitory cells after C fibers are activated
- mechanoreceptors can block the pain signal transmission at a site of injury
how does locus coeruleus affect pain?
can presynaptically inhibit the C fibers and block pain projection from the dorsal horn projection neuron
how does norepinephrine affect pain control?
can act presynaptically and post synaptically to inhibit the dorsal horn projection neuron
how does serotonin affect descending pain?
presynaptic factor which activates a local inhibitory neuron acting on locus coeruleus neuron axons
phantom limb pain; which neurons are still firing?
the body part is gone, but the patient can still feel it and may experience pain
- 2nd and 3rd order neurons are still in communication (sometimes 1st order too) with the neocortex
why might phantom limb pain occur?
normal mechanosensation dampens the pain pathway but now there is no longer a mild block so it can be realized when it would be in check before
- There are other ways but we don’t yet know how it works
mirror therapy
patients “see” missing limb as a mirror image of intact limb
phantom motor execution (PME)
electrodes attached to the patients residual limb pick up electrical signals intended for the missing limb which are translated through AI algorithms into movements of a virtual limb in real time
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