4: Somatosensory System Flashcards
What are the 4 Somatosensory Sub-Modalities?
- Touch (Mechanoreceptive)
- Temperature (Thermoreceptive)
- Pain (Nociceptive)
&&& Body Position (Proprioceptive) - motor lecture
What is a Dermatome?
-Slice/ section of the skin which has axons that enter the spinal cord at a specific spinal route
What are Free Nerve Endings Responsible For?
-Temperature & pain reception
What are Meissner Corpuscles Responsible For?
- Detects low freq vic: 2-50Hz
- Small receptive fields, rapid adaptation
- Communicants w axon in unknown way
- Specialized cell outside of the axon
What are Merkel Cells Responsible For?
- Merkel cell = neurite complex
- Detect static pressure ie. fine spatial discrimination (like brail).. feel gap b/w things
- Spatial tactile perception/ aqcuity: sensitivity to force of touch
- Small receptive field, slow adaptation
What are Ruffini Endings Responsible For?
- Skin stretch sensation
- Large receptive fields, slow adaptation
What are Pacinian Corpusles Responsible For?
- Detect high-freq vibrations: 100-200Hz
- Large receptive field
- Rapid adaptation
Why do we feel Second Pain?
-B/c of the difference in conduction times; when you touch something hot, you’ll first notice that you feel you’re touching something (A-beta), then you’ll feel first pain (A-delta) which will make you withdraw your hand & experience relief.. soon after, second pain will occur & this is when C-fibre axons reach your brain
Explain an Axons Route through the DCML.
- Comes in
- Ascends ipsolaterally
- Reaches caudal medulla & synapses
- 2nd order neuron in mechanosensory pathway receives gluta from neuron
- -Its axon decussates & ascends to thalamus
- VPL gets info from somatosensory system
- -Sends info to cortex
What Happens to a Subject Experiencing Brown-Sequard Syndrome?
- Ipsilateral loss of light touch (touch is always ipsilateral = DCML = A-beta)
- Contralateral loss of pain & temp (STT)
Explain Mechano-transduction.
Mechano-transduction = force that’s turned into an elec signal
Na Channels
- Produced in cell bod & ganglion & located there
- Cell memb contains stretch gated ion channels
- -Open in response to pressure on the cell; the cell is pressed & the membrane stretches, ion channels open b/c of the stretched memb
- Channels are open & Na goes down its conc grad & cell starts to depol
- force applied to cell = graded potential = membrane is stretched & more chans will go into the open state
- -Channels do not get wider, more just open
Weak Stimulus
-May not reach threshold, cell would stretch, ion chans would open, Na would rush in, cell = depol, charge = more but cell may not reach threshold therefore no AP w/o being over threshold
What is a Property of Receptive Fields?
-Stimuli touch closer to center will cause +AP to fire than those on periphery
- If you have two touches that occur simultaneously w/in receptive field, it can be difficult for the brain to understand this
- -Could mistake two touches for one
- –^Problem is solved through receptive field overlap
What is the Purpose of a Caliper?
- Piece of metal you can spread (like tweezers) & use to ensure that both pieces will touch your skin @ the same time
- Helps recognize the two-point discrimination threshold by body region
What are the Benefits of Adaptation?
- Ignore constant, innocuous stimuli to reduce distraction
- Avoid saturation of neuronal firing rates, allowing detection of change in stimuli intensity over a larger range of intensities
What is the Cost of Adaptation?
- Poor judgment of absolute stimulus intensity
- -Eliminates one-to-one mapping from stimulus intensity to firing rate
However, evolution has selected for adaptation as highly advantageous
- Benefit is worth the cost
- If force changes, this will affect your survival
- -More important to detect change than the exact value of a stim
Explain the 4 Areas of the Primary Somatic Sensory Cortex
3a: proprioception
3b: light touch, small RFs - meisner & merkel
1: light touch, large RFs - ruffini & pacinian
2: light touch & proprioception
What is a Homunculi?
- Where you can map bod surface onto brain
- Areas most sensitive have greatest density of receptors
- # of neurons in homunculus = proportional to relevance in the body
What is the Ratunculus?
- A rat homunculus!
- Rats = nocturnal & don’t see well- rely on whiskers & touch
- Barrels respond to particular whiskers
- -Damaged whisker = won’t see cluster of barrels
What is Use-Dependent Somatosensory Plasticity?
- The more you use something, the bigger its representation in the homunculus
- Homunculus changes shape; neurons in brain are taken over by new inputs & change shape
What is Deafferentation Plasticity?
-Lose a limb & adjacent parts in the homunculus will fill themselves in
Explain an axons route through the STT.
- Comes in
- Axons synapse in spinal cord
- Second order neurons decussate
- Synapse in VPL thal & go to layer IV of cortex
What is the Function of a Thermoreceptor?
- Respond to temp
- Have ion chans that open in response to partic stim
- Cold receptors & warm receptors
- -Both adapt & respond to change
- –We experience certain illusions b/c of this
Explain the Cold/ Warm Water Temperature Illusion.
- Cold or warm receptors have adapted in their preferred water
- -Won’t fire as much in lukewarm water
Ie. Warm receptors wont fire as much as cold receptors in lukewarm water in the warm hand
-This is b/c the warm hand has adapted, the receptors are in a state of fatigue- cold receptors are firin more than your warm receptors, so your hand must be cold
- Brain makes unconscious inferences all the time *
Why do Chile Pepper Taste so Hot?
- Capsaicin! –> activates warm receptors
- -Ion channel on warm receptor, VR1 receptor, opens (heat-gated) chan & Na will come in when they’re open
- – + temp = + open
- Capsaicin = lipophilic & will diffuse across the membrane of a free-nerve membrane & hit its binding site: don’t know why binding site exists
- -Brain makes an inference b/c it’s activated
- -Unconscious perception of warmth
- –Some are hotter than others b/c there is more capsaicin
- -Pathways = in white matter
Explain Referred Pain.
Lots of organ problems can be felt in diff parts of the body, why? Ie. heart attack & left arm
Convergence!
- Two axons synapse on the same 2nd order neuron
- Brain isn’t used to synapse from organ as much as it is from other area & infers that the pain/ excitation is from the other area (usually organ)
Caused by convergence of visceral & cutaneous (skin) afferents onto a single dorsal horn projection neuron
Explain the Gate Hypothesis.
- Fall & scrape your knee- lots of APs in nociceptors
- -They release gluta onto the STT
- -Neurons decussate & go to other side
- -You feel lots of pain
-You rub the injured area & cause APs in mechanoreceptors (ie. A-beta) to fire
- A-beta projects ipsolaterally
- -Has a collateral!
- AP reaches branch point & goes in both directions
- The one that goes down will release gluta onto an inhib local circuit interneuron (another small GABAergic neuron that has a gluta receptor & is inhibited) - will release GABA onto same STT pathway
- STT neuron receives gluta from nociceptors & GABA from interneuron/ mechanoreceptors
- -Gluta lets in Na & produces EPSP
- -GABA lets chloride in & produces IPSP
- IPSPs & EPSPs counteract w each other
Called “Gate Theory” b/c you can gate pain on/ off
Explain Descending Pain Control Pathways.
Your brain can reduce pain depending on its mental state!
- Amygdala indirectly (through midB) projects axons down to the spinal cord
- Raphe Nuclei
- -Indirectly activated by amygdala & descends
- -Syn on axon terminal of enkephalin-containing local circuit neuron
- –Axoaxonic syn from interN to c-fiber
- —Opens up K chans (inc in K conductance)
- —Closes VGCa chans (harder to open now)
- —Prevents Ca from coming in & = harder to synapse
- —–Reduces pain message!
Placebo: your brain is causing enkephalin to be released
Explain Phantom Limb.
- Neurons in homunculus randomly start to fire lots of APs; you feel pain in a limb you lost
- Is an illusion
