Lecture 4 - Somatosensory System Flashcards
What does dermatome mean?
Skin slice
What consists the grey matter?
Cell bodies + dendrites
What consists the white matter?
A lipid containing myelinated axons
What is the dorsal root ganglion (DRG)?
Whare the neuron cell bodies are located
How is the grey matter separated?
Dorsal Horns (sensory input), Ventral horns (motor control, so the smaller the ventral horns the less tactile abilities)
What are the primary afferent that convey information from skin to the CNS?
A beta axons: touch
A delta axons: Pain or Temperature
C axons: Pain or temperature
For the mechanoreceptor (touch) axon, name its afferent type, its receptor type(s), diameter and velocity
Receptors: Meissner, Merkel, Ruffini, Pacinian
Afferent type: A beta axon
Diameter: 16-12 um
Velocity: 35-75 m/s
For the thermo- and nociceptive (pain and temperature) axon, name its afferent type, its receptor type(s), diameter and velocity
Receptors: Free nerve endings
Afferent type: A delta axon
Diameter: 1-5 um
Velocity: 5-30 m/s
For the thermo- and nociceptive (pain, temperature and itch) axon, name its afferent type, its receptor type(s), diameter and velocity
Receptors: Free nerve endings (unmyelinated)
Afferent type: C axon
Diameter: 0.2-1.5 um
Velocity: 0.5-2 m/s
Which axon is responsible for first pain?
A delta axon
Which axon is responsible for second pain?
C axon
Compare the speeds of first and second pain
First pain: Short+fast
Second pain: Long+slow
Why is second pain so much longer than first pain and occurs with a delay?
Think of the racer analogy, where A delta axons and Olympian racers that make it to the finish line much quicker than the non Olympian racers who take much longer to reach the finish line.
What are the two somatosensory tracts?
- Dorsal Column Medial Lemniscus tract (DCML)
- Spinothalamic Anterolateral tract (STT)
Where does decussation occur in the DCML?
Dorsal Column Nuclei of the medulla (ipsilateral - dorsal)
Where does decussation occur in the STT?
Dorsal Horn of the spinal cord (contralateral - ventral)
What does DCML respond to and what are its axons?
Touch, A beta axon
What does STT respond to and what are its axons?
Pain and temperature, A delta and C axons
How many synapses occur between the skin and the cortex and where?
3 synapses occur
1st synapse: DCML; DCN, STT; Dorsal Horn
2nd synapse: Thalamus (VPL nucleus)
3rd synapse: Primary somatosensory cortex
What are the roles of 1st, 2nd and 3rd order neurons?
1st order neurons: bring sensory input into CNS
2nd order neurons: Decussate after receiving info from the first order neurons
3rd order neurons: Send signal from second order neurons to the cortex
What is the topography of DCML tract?
The more caudal the axon is at first, the more medial it can go
The more rostral the axon is at first, the more lateral it can go
What is the topography of STT tract?
The more caudal the axon is at first, the more lateral it can go
The more rostral the axon is at first, the more caudal it can go
What does a spinal cord transverse look like at the upper cervical level?
Dorsal Column: CTLSSLTC (the more caudal, the more medial)
Spinothalamic Tracts: SLTCCTLS (the more caudal, the more lateral)
As you go down the spinal cord, why is there less and less white matter?
The more caudal in the spinal cord you are there are fewer axons from the levels above that have yet gotten up
The cervical level will have a lot more white matter, than the lumbar level for instance
What is Brown Sequard Syndrome?
Brown Sequard Syndrome involves ipsilateral loss of touch sensation, and contralateral loss of pain and temperature sensation (for STT tract, if the lesion is on the left, than the tract must be coming from the right towards the left)
What differentiates lesions in the white matter vs the grey matter?
Lesions in the white matter affect larger areas, whereas lesions in the grey matter only affect the localized area
What are the different somatosensory receptors
Temperature and pain: Free nerve endings
Touch: Merkel, Meissner, Ruffini, Pacinian
What is Microneurography?
The use of a stimulus probe and a recording electrode to tap around the skin, and find areas that activate action potentials (they look for receptive fields in A beta axons)
What does the oscilloscope show investigators?
“Spikes” of action potentials in the different receptive fields
What is the importance of our ability to sense vibrations?
It allows us to perceive texture
What are the properties of Meissner?
Small receptive field, responds to low frequency vibrations (light touch), 2-50 Hz
What are the properties of Merkel?
Small receptive field, responds to static indentation (light touch)
What are the properties of Ruffini?
Large receptive field, responds to skin stretch
What are the properties of Pacinian
Large receptive field, responds to high frequency vibrations, >50 Hz
Which mechanoreceptive receptive fields are slow adapting?
Merkel and Ruffini
Which mechanoreceptive receptive fields are fast adapting?
Meissner and Pacinian
What is the dynamic phase?
The dynamic phase is when the stimulus first starts
What is the static phase?
The static phase is when the stimulus is constant
What is the stimulus-response spike train of Merkel and Ruffini RFs
- Fire a lot of action potentials during the dynamic phase (when stimulus starts)
- Fire fewer action potentials during the static phase (when stimuli is constant)
What is the stimulus-response spike train of Meissner and Pacinian RFs
- Only respond during the dynamic phase
- Stop firing action potentials entirely during the static phase, even if stimulus is still there
What did Ellen Lumpkin do?
Proposed the role of Merkel cell in mechanotransduction
How can Merkel cells be compared to neurons?
A Merkel cell is not a neuron as it has no axon, and cannot fire APs, but it does have very similar properties to a neuron, like the presence of its own VgCaC, etc.
What is the first phase of mechanotransduction, when skin is pressed?
- During the initial skin deformation, A beta axons will respond to pressure, opening their ions channels,
- This lets in in ions like Na+ and ultimately causing an AP (first AP, or dynamic phase)
What is the second phase of mechanotransduction, when skin is pressed?
- Merkel cells are touch sensitive and have their own ion channels called Piezo that open in response to pressure on the skin
- This causes ions to flow into the Merkel cell, depolarizing it, and thus opening its VgCaC, which releases NTs (probably neuroepinephrine)
- Since the Merkel cell is really close to the A beta axon, the NTs get released on the axon membrane
- These NTs cause a second AP (static phase) which does not involved the direct opening of ion channels but prolongs as a result of NT action
Who discovered the Piezo ion channels in Merkel cells?
Patapoutian
What does the two point discrimination help us determine?
Tactile spatial acuity (how good a person’s sense of touch is), by using a caliper and measuring the distance between both ends before the person reports having felt two distinct points
What is special about the center of a receptive field?
The center of a receptive field is the most sensitive part, and the closer we are to it the more APs will fire
Where is there a high density of Merkel cells?
Sensitive areas like fingertips
What is the link between the two-point threshold and the size of a body part?
The bigger the body part the bigger the threshold
Large body parts: Receptive fields are larger and more spaced out (bigger space between calipers before feeling two distinct points
Small body parts: Smaller receptive fields that are more close together
What is the two-point threshold?
The minimum amount of distance required before a person reports having felt two distinct points
Why are receptive fields much smaller on fingers than forearms?
A beta axons are bigger on forearms than fingers since they innervate 3-5 Merkel cells and there is a bigger density of them areas like fingers than other parts of the body, making their receptive fields smaller (the area that each nerve fiber senses—its receptive field—is smaller)
How is Broadman’s Somatosensory cortex divided, and what is each section responsible for?
Topography (4th neuroscience rule): The somatosensory homunculus is in the post-central gyrus of the parietal lobe
3a: Proprioceptive (the ability to sense own position and movement)
3b: Light touch, small RFs
1: Light touch, large RFs
2: Proprioceptive and light touch
What allows navigation and sensation for rats?
“Barrels” each associated with a wisker
What is a homunculus?
Representation of sensation where the more sensitive body parts are larger than the less sensitive
What is an example of somatosensory plasticity from deafferentation in humans?
Phantom limb
What is an example of somatosensory plasticity from deafferentation in monkeys?
In monkeys, each digit has its own cortex, so the hand itself has a somatotopic sequence.
When digit three was removed the neurons associated with it did not go silent, but rather, they started to respond to digits 2 or 4
What are the two kinds of thermoreceptors?
Warm receptors: Fire a lot of APs when skin is warm, then adapt
Cold receptors: Fire a lot of APs when skin is cold, then adapt
How does heat reception work?
Transient Receptor Potentials (TRPV1) open momentarily as a result of heat –> heat-gated, which causes an AP and the brain interprets this signal as a indication that the skin covered by that axon’s receptive field is experiencing an increase in temperature
Why are peppers hot? Who discovered this?
Julius discovered that peppers have capaisin which also opens the TRPV1 causing the brain to infer that what is being eaten is “hot”
How does cold reception work?
Transient Receptor Potentials (TRPM8) open momentarily in response to cold –> cold-gated, which causes an AP and lets the brain interpret that the skin covered by that axon’s receptive field is experiencing coldness
Why is mint cold? Who discovered this?
Julius and Patapoutian discovered that mint has menthol which also opens the TRPM8 causing the brain to infer that what is being eaten is “cold”
What is referred pain?
Referred pain occurs when an internal organ cannot feel pain, and instead that pain is referred somewhere else
What are two things that cause referred pain?
- Convergence of visceral (sensory nerve axons carrying info from internal organs) and cutaneous (sensory nerve axons carrying info from skin) afferents onto a single dorsal horn
- Brain inference –> the skin and organ share 2nd order neuron, and the brain is not used to our organs being injured so it just infers that the injury is on the skin which is the more common event
What is the Gate hypothesis?
The Gate hypothesis helps explain why we press our skin when we are in pain
The C axons are depolarized by glutamate, and that sends a signal through the dorsal projection neuron, far from the cell
When we touch the spot that hurts, our A beta axons also fire an AP and since they have a branch on the GABAergic or Glycinergic inhibitory interneuron, an AP is fired there too
This event releases GABA or Glycine causing an IPSP (the EPSP from glutamate with IPSP from GABA or Glycine reduces the #of AP)
What are the benefits of adaptation?
- Helps us ignore constant, innocuous stimuli, thus reducing distraction
- Help us avoid saturation of neuronal firing rates, and allows us to detect change in stimulus over a large range of intensities (without it, when neuron firing rate mases out we won’t be able to tell if there is a stronger stimulus)
What can adaptation be compared to?
TARE button on a scale
What is a consequence of adaptation?
It causes to lose one-to-one mapping, as it reduces the direct correlation between firing rate and stimulus intensity (same firing rate could correspond to different intensities)
Adaptation helps us to detect changes not absolute intensities
What did Mountcastle do?
He used extracellular recordings of APs to find the column organization of within the layers of the cortex
According to Mountcastle what happens if you go up the same column vs if you go from digit to digit?
Digit to digit means different RFs, up the same column means same RFs
What is the cortical column organization?
Digit 2 and 3: Sends and receives axons from other cortical areas
–> Pyramidal cell (projecting neuron: sends signals far from cell body, excitatory, 70% of cortical neurons)
Digit 4: Receives axons from thalamus
–> Stellate cell (interneuron:sends signal close to cell body, excitatory or inhibitory, 30% of cortical neurons)
Digit 5: Sends axons to brain stem and spinal cord
–> Pyramidal cell
Digit 6: Sends axons to thalamus
–> Pyramidal cell
How do enkephalinergic neurons inhibit pain?
Release enkephalins which inhibit NTs by blocking VgCaC and opening K+ channels hyperpolarizing the postsynaptic neuron
