Somatosensory Flashcards
Somatosensory System
– receives information on multiple types of sensation from the body
o Light, touch/pressure/pain, joint/muscle position sense (proprioception), temperature, itch/prickle
3 Anatomical Group of Afferent Sensory Fibers
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
Touch (and Proprioception)
– 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
Mechanoreceptors in Skin
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
- What mechanoreceptors are important for reading Braille?
- Which mechanoreceptors detect sandpaper roughness?
- What mechanoreceptors register a pen tapped on your finger?
- 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
- Why does stroking feel better than just touching?
* What part of body has the lowest touch detection threshold?
- 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)
Touch and Pressure Detection at Molecular Level
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
Pain (and Temperature) Fibers and Characteristics
– 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
Pain (and Temperature) System
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
Pain Molecules
– trauma/damage that ruptures cells causes release of pain molecules inside the cell
o Prominent pain molecules – K+, ATP, protons, prostaglandins, serotonin, bradykinin
Pain Receptors
– 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
Capsaicin Receptor (TRPV1)
– 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
Mustard Oil Receptor (TRPA1)
– 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
Hyperalgesia vs. Allodynia
o Hyperalgesia – increase in sensitivity/response to noxious stimuli; triggers spontaneous pain
o Allodynia – perceive innocuous/unharmful stimuli as pain (ex: showering with sunburn)
Analgesia vs. Anesthesia
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)
Spinal Cord and Modulation of Pain
– 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
Touch and Modulation of Pain
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
Opioids
– 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
Temperature and Receptors
– 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)
Itch
– 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
