Somatosensory

Question 1

Somatosensory System

Answer 1

– receives information on multiple types of sensation from the body
o Light, touch/pressure/pain, joint/muscle position sense (proprioception), temperature, itch/prickle

Question 2

3 Anatomical Group of Afferent Sensory Fibers

Answer 2

o Trigeminal ganglia (in brainstem) – sensory fibers from cranial nerves
o Dorsal root ganglia – sensory fibers that come into spinal cord from dermatomes/body
o Nodose ganglia – sensory fibers from vagus nerve that innervated the intestines

Question 3

Touch (and Proprioception)

Answer 3

– Aα and Aβ fibers (axons) – large diameter, myelinated, unimodal (either touching something or not); fast conduction speed due to myelination and large diameter of axons
o Unique nerve endings with specialized mechanoreceptors
o Afferents from body periphery travel via Dorsal Column – Medial Lemniscal on ipsilateral side  SYNAPSE in brainstem at nucleus cuneatus  cross to contralateral side  thalamus  primary somatosensory cortex in parietal lobe
o Afferents from cranial nerve travel via trigeminal system  enter brainstem  cross to contralateral side  thalamus  somatosensory cortex

Question 4

Mechanoreceptors in Skin

Answer 4

 Density of mechanoreceptors is greater in fingers, lips, tongue than the skin on the torso
 Receptors respond differently to constant/changing pressures; thus interpreting touch
o Superficial Skin - Merkel’s disk & Meissner corpuscle receptors
 Meissner’s – smaller receptor field; fire in response to changing pressures
 Merkel’s – smaller receptor field; fire in response to constant pressures
o Deep Skin - Pacinian corpuscle & Ruffini’s corpuscle receptors
 Pacinian – larger receptor field; respond to changing pressure and adapt rapidly
 Ruffini – larger receptor field; respond to constant pressure

Question 5

• What mechanoreceptors are important for reading Braille?
• Which mechanoreceptors detect sandpaper roughness?
• What mechanoreceptors register a pen tapped on your finger?

Answer 5

• What mechanoreceptors are important for reading Braille? – Meissner’s are key in reading bumps on paper; Merkel’s tell you that your finger is going across the paper
• Which mechanoreceptors detect sandpaper roughness? – Meissner’s & Meckel’s – same as Braille
• What mechanoreceptors register a pen tapped on your finger? – there is a vibratory stimuli and therefore all four receptors are used; vibratory stimuli especially engage Pacinian receptors

Question 6

• Why does stroking feel better than just touching?

* What part of body has the lowest touch detection threshold?

Answer 6

• Why does stroking feel better than just touching? – stroking engages more receptors and gives changes in stimulation; changing the stimulation and intensity involves both adapting and non-adapting receptors
• What part of body has the lowest touch detection threshold? Nose – slightest touch is noxious (itch/pain)

Question 7

Touch and Pressure Detection at Molecular Level

Answer 7

o Direct mechanosensation – involves mechanosensative ion channel that is tethered to extracellular “sensor” cells and to intracellular cytoskeleton; fast signaling
 Deforming the extracellular structure results in opening of the ion channel
o Indirect mechanosensation – mechanical stretch can release chemical mediators that excite sensory nerves and often release ATP that will then signal proximal neurons
 Ex: mediates sensation of bladder distension

Question 8

Pain (and Temperature) Fibers and Characteristics

Answer 8

– Aδ and C fibers (“nociceptors”) – medium and small diameter; lightly (Aδ) or unmyelinated (C); polymodal stimuli (respond to multiple stimuli)
 Slower conduction speed but they synapse immediately upon entering the spinal cord
o Bare nerve ending that are often highly fenestrative
o Type of pain that is perceived depends on activities of the pain fibers
 Aδ fiber – responds to acute danger (ex: standing on a nail); fast response (myelination)
 C fiber – 80% of fibers; cover entire body; usually quiet; slow response (unmyelinated)
• Most abundant fiber in the skin; even more than touch fibers
• Involved in classic inflammatory pain by inducing prolonged sensitivity that allows for repair (soreness after lifting; injured area hurts to touch)
• Neuropathic pain – arises from traumatic damage to nerves OR virus infection

Question 9

Pain (and Temperature) System

Answer 9

o Afferents from body periphery travel via Anterolateral Column – Spinothalamic Tract
 Enter dorsal horn and immediately SYNAPSE (allows for modulation of signal)  cross over to contra lateral side  thalamus  primary somatosensory cortex
o Afferents from cranial nerve travel via trigeminal system  enter brainstem  cross over to contra lateral side  thalamus  somatosensory cortex
o Limbic system involved with pain; responsible for the emotion behind the painful event
o Frontal cortex – assigns memory to the painful event so that you don’t do it again

Question 10

Pain Molecules

Answer 10

– trauma/damage that ruptures cells causes release of pain molecules inside the cell
o Prominent pain molecules – K+, ATP, protons, prostaglandins, serotonin, bradykinin

Question 11

Pain Receptors

Answer 11

– two major classes; most pain molecules can act on both kind of receptors
o Ion channels – open quickly; respond to ATP, H+, capsaicin
o G-Protein Coupled Receptors – slower response and more involved in sensitization rather than direct activation; respond to serotonin, bradykinin, prostaglandin, ATP

Question 12

Capsaicin Receptor (TRPV1)

Answer 12

– expressed in sensory nerves (C fibers)
 Responds to inflammatory soup
 Polymodal channel that senses multiple noxious stimuli (KEY) – capsaicin (chili peppers), heat (above 43oC), protons, itch
 Activated by capsaicin (ingredient in chili peppers), heat (above 43oC), protons
 Upon activation, releases multiple chemicals that cause sensitization of the receptor
• Causes phosphorylation of the receptor which makes the channel more sensitive and open at a lower threshold (temperature of 30oC instead of 43oC)
 Applying ice causes the receptor to turn off

Question 13

Mustard Oil Receptor (TRPA1)

Answer 13

– expressed in sensory nerves (C fibers)
 Activated by isothiocyanates (in mustard oil, wasabi, and cinnamon)
 Causes release of volatile chemicals that travel to the cornea
• Receptor responsible for crying when you cut an onion

Question 14

Hyperalgesia vs. Allodynia

Answer 14

o Hyperalgesia – increase in sensitivity/response to noxious stimuli; triggers spontaneous pain
o Allodynia – perceive innocuous/unharmful stimuli as pain (ex: showering with sunburn)

Question 15

Analgesia vs. Anesthesia

Answer 15

o Analgesia – decrease/block in response to noxious stimuli without affecting other somatosensory stimuli (specific to pain)
o Anesthesia – decrease/block all somatosensory stimuli (blocks all somatosensory/motor neurons)

Question 16

Spinal Cord and Modulation of Pain

Answer 16

– involved in modulation of pain
o Primary afferents enter spinal cord and synapse in dorsal horn  release glutamate  form LTP in spinal cord called central sensitization of transmission
o Increased transmission at the synapse = sensitization; cause upregulation of the receptor and form LTP that cause more pain

Question 17

Touch and Modulation of Pain

Answer 17

o Reflexive touch behavior in response to pain
o Gate Control Theory of Pain – stimulation of “touch” fibers reduces pain
• ex: waving hand around after hitting it with hammer
 If you engage all the receptors in an area then there is a huge increase in activity at the spinal cord resulting in less room for pain to come in
 Touch receptors do not synapse in the spinal cord but go directly to the brainstem and give off collateral branches that can travel back to spinal cord and inhibit pain fibers

Question 18

Opioids

Answer 18

– powerful analgesics
o Inhibit C fibers which are abundant in opioid receptors
o Do not work as well at Aδ fibers  therefore you still feel fast pain but not slow pain

Question 19

Temperature and Receptors

Answer 19

– sense through Transient Receptor Potential (TRP) channels
o Excitatory ion channel that causes influx of Na+ and Ca+
o 10% of the C fibers code for temperature and send information to the somatosensory cortex
o Great sense of temperature because we have a large combination of TRP receptors with different temperature set points
TRPV1 – capsaicin, heat (>43oC), itch, H+, lipids – sensory nerves
• Agonists induce hypothermia (decrease body temperature via cooling mechanisms); antagonists cause hyperthermia (increases body temperature)
TRPV2 – heat (>50oC)
TRPV3 – innocuous heat (>35-39oC)
TRPV4 – innocuous heat (>28-36oC) - activated in normal thermal body range
TRPM8 – cold (<18oC), mustard oil, wasabi – (co-expressed with TRPV1)

Question 20

Itch

Answer 20

– select group of C fibers code for itch; restricted to a subset of TRPV1 expressing neurons
o Histamine dependent and independent
o Gastrin Releasing Peptide Receptor – secondary afferent that aids in itch sensation
 Itch specific in the spinal cord