Somatosensations (finished) Flashcards
Somatosensation
the process that conveys information regarding the body surface and its interaction with the environment. Information arises from the entire body, but the term somatosensation generally excludes information coming from special senses.
Somatosensation can be subdivided into three categories:
a) Mechanoreception- also called discriminative touch
b) Thermosensation- temperature
c) Nociception- painful: mainly chemical but also mechanical and thermal
Somatosensation vs. Proprioception
- Proprioception is information regarding body movement and position→ it comes from both muscle and joint receptors and vestibular systems
- Somatosensation is information regarding the body surface and interactions with the environment
Thermoreceptors
• 2 types of thermoreceptors
1. Cold: respond to temperatures between 5-35o C AND greater than 45o C (“paradoxical cold”)
2. Warm: respond to temperatures between 30-45o C
• Temperature detection is built into the structure of the channel
• Can also respond to specific chemical stimuli (e.g. cold receptors to menthol and hot to capaicin)
Mechanoreceptors
• The actual process by which a deformation of the cell membrane causes the opening of a mechanotransduction channel is not known
• Proposed mechanisms include:
a. direct activation due to lipid tension
b. direct linkage via intracellular and extracellular proteins
c. indirect activation via a second messenger system that activates the channel
Nociceptors
- chemical stimuli that activate the TRPV1 channel include capsacin, heat (>43o C) and protons
- TRPV1 channels can be sensitized by many substances including prostaglandins, serotonin, ATP, bradykinin, and adenosine (obviously preventing sensation is a key mechanism of action for analgesic drugs-specifically NSAIDS)
- Sensitization lowers the activation threshold for the channel (e.g. the channel will open at body temp)
- Sensitization leads to hyperalgesia and allodynia (2 characteristics of inflammatory pain)
Transient receptor potential (TRP) channels
• TRP channels have been implicated in taste, smell, hearing, mechanoreception, thermosensation and nociception
• All TRP channels have 6 transmembrane domains - making them structurally similar to voltage-gated ion channels
• TRP channels incorporate the receptor protein with the channel structure (i.e., there is no need for 2nd messengers)
• TRP channels are permeable to cations, however, many of them are relatively non-selective i.e., they can admit K+, Na+ or Ca2+
o The direction and nature of ion flow therefore depends on the concentration gradients for the various ions (see the discussion of the “hair cell transduction current” at bottom of document)
o In most cases, most of the transduction current will be inward and carried by Na+ (remember the Nernst equation) resulting in depolarization
Point localization
the ability to localize a discriminative touch stimulus→ dependent upon receptive field size, which varies widely throughout the body
Two point discrimination
a way to measure somatosensory acuity by determining the physical distance required for two stimuli to be perceived as separate. Two point discrimination is tested when nerve damage is suspected, as well as when monitoring nerve regeneration
fastest to slowest signals
Mechanoreception
Mechanoreception, cold, fast pain
Mechanoreception, thermoreception, slow pain
- Explain the temperature illusion!
Temperature Perception Experiment
• Thermoreceptors are rapidly adapting receptors, which are divided into two types: cold and warm
• When you put your finger into cold water, cold receptors depolarize quickly, then adapt to a steady state level which is still more depolarized than the steady-state. Warm receptors do the opposite: hyperpolarize quickly, then adapt to a slightly hyperpolarized state.
• When you move your finger to cold to warm water, cold receptors (which are already slightly depolarized), don’t respond very strongly. Warm receptors do, and the response is stronger than normal, because they are slightly hyperpolarized. The brain perceives the warm water as hot because it is receiving more information from hot receptors than from cold.
• The opposite response is observed from the thermoreceptors in the finger that is moved from hot to cold (greater response from cold receptors than warm).
• The major point is that most receptors (including thermoreceptors) respond most strongly to a CHANGE in stimulus. Therefore a preceding experience that hyperpolarizes the receptor will cause the brain to interpret a new depolarizing stimulus as being stronger than if it “actually” is.
Acute nociceptive
Can be subdivided into:
• Fast (sharp, pricking) - well localized, transmitted by Aδ fibers
• Slow (dull, aching) - poorly localized, transmitted by C fibers
Inflammatory
Inflammatory: caused by damage or sensitization of peripheral receptors
nflammatory and neuropathic pain can lead to chronic pain syndromes that result in dynamic changes to CNS processing of nociceptive stimuli that can outlast the primary cause. These syndromes are characterized by hyperalgesia and allodynia and tend to be refractory to pharmacologic treatment
Neuropathic:
a complex pain state, characterized by peripheral and/or central neuron damage
nflammatory and neuropathic pain can lead to chronic pain syndromes that result in dynamic changes to CNS processing of nociceptive stimuli that can outlast the primary cause. These syndromes are characterized by hyperalgesia and allodynia and tend to be refractory to pharmacologic treatment
Referred Pain
• An example of how the brain perceives sensory information in the context of past experiences
• Referred pain occurs when activation of nociceptors in the viscera results in a perception of pain that is localized to the body surface (only deep pain can be referred, not superficial)
• Referred pain is presumed to occur because the information from multiple nociceptor afferents converges onto individual spinalthalamic tracts neurons
o Converges: see lecture objectives from Principles of Sensory Systems (2/7/14)
o Spinalthalamic tract: see below lecture objective 6
• The brain therefore interprets the information coming from visceral receptors as having arisen from receptors on the body surface, since this is where nociceptive stimuli originate more frequently
• The most common example is the perception of heart attack pain as pain in the inner aspect of the left arm
• The referral point is typically poorly localized and is not identical in all people
o For example, some people describe the referred heart pain as localizing to the stomach, right arm or neck
o Referral to the left arm is more common in men than in women
• Other clinically relevant examples include:
o Pain in the tip of the shoulder that actually comes from irritation of the central region of the diaphragm
• Pain in the testicle due to distention of the ureter
Diagram the gate theory of pain, including the actions of the endogenous opiate system. Explain the success of TENS by using gate theory.
Perception of pain is not simply due to activation of nociceptors, but is the outcome of modulation of both nociceptive and non-nociceptive inputs. According to the gate theory of pain, inhibitory interneurons regulate the transmission of ascending nociceptive information at the level of the second order neuron, allowing modulation of the signal (both increases and decreases in activity are possible). This modulation can explain phantom limb pain, as well as the success of TENS treatment and the actions of opioid analgesics.
• Pain can be learned!
• The basic tenet of gate theory is that ascending nociceptive signals can be suppressed by the activity of inhibitory interneurons that function as gates to decrease transmission by the second order neurons
• The inhibitory interneurons are thought to be tonically active - therefore, under normal conditions (no external stimuli), neither the C or Aβ fibres are active, the second order neuron is inhibited by the interneuron and no nociceptive signals are sent to the thalamus
• During unmodulated pain, activation of the C fibre causes excitation of the second order neuron as well as inhibition of the inhibitory interneuron, therefore nociceptive information is passed on to the thalamus (i.e., the gate is opened)
o C fibres release the neurotransmitter glutamate (as well as substance P and/or CGRP), which can cause excitation via AMPA receptors or inhibitition via mGLURs - therefore, it is the type of receptor expressed by the postsynaptic cell that determines whether excitation or inhibition occurs
• Simultaneous activation of the Aβ fibre reactivates the inhibitory neuron, decreasing the flow of nociceptive signals and modulation of the pain (i.e., the gate is closed)
o A simple example would be that rubbing your elbow decreases the pain associated with a bumped elbow
— Rubbing the skin activates large-diameter, myelinated afferents (Aβ) associated with mechanoreceptors, subsequently increasing interneuron activity
o Trans-cutaneous electrical nerve stimulation (TENS) is a treatment for pain that also activates this suppression mechanism by delivering a small current to the skin overlying a nerve
• The CNS also has the ability to block pain at the spinal cord level
o Brainstem neurons that release endorphins are part of an endogenous analgesic system that decrease pain perception due to actions that occur (in part) at the level of the second order neurons
— Endogenous opioids (endorphins, enkephalins) act on opioid receptors on both the presynaptic (primary afferent) nerve terminal and the postsynaptic cell (2nd order neuron) to WS_FTP decrease nociceptive neurotransmission
→ Presynaptic actions include a decrease in Ca2+ conductance that results in decreased neurotransmitter release
→ Postsynaptically, activation of opioid µ receptors causes an increase in K+ conductance (gK+) resulting in an IPSP
