Sensory systems Flashcards
Which of the following alternatives describes C-fibers? Choose the single best possible answer.
A Small, myelinated fibers which carry sharp pain
B Large, myelinated fibers which carry dull diffuse pain
C Small, unmyelinated fibers which carry dull diffuse pain
D Large, myelinated fibers which carry sharp pain
E Large, myelinated fibers which carry temperature sensation
C Small, unmyelinated fibers which carry dull diffuse pain
A patient describes her pain by saying “Brushing my hair hurts and also other simple activities such as washing my face or getting a hug from my daughter are painful.” What is the medical term for this kind of sensation?
Allodynia
Your patient has had diffuse pain in the right shoulder for eight months. She describes that the pain intensity varies. On good days, when the shoulder pain is relatively mild, it stays localized to the shoulder, on days when the shoulder pain is more intense, she feels pain also in her upper arm, and sometimes, when the shoulder pain is extremely intense, she feels pain also in her lower arm and fingers. What kind of pain is this a typical description of? Choose the single best possible answer.
A Acute pain B Breakthrough pain C Projected pain D Referred pain E Phantom pain
D Referred pain
Describe processes that increase pain signal transmission at the level of the spinal cord dorsal horn. Include the role of input from peripheral nociceptors as well as descending pain pathways.
Increases pain signal:
- Sustained release of glutamate and neuropeptides increase activation of projection neurons
- Release of inflammatory mediators from glial cells
- Increased activity in excitatory interneurons, decreased activity in inhibitory interneurons
Yesterday, I was listening to music while being rotated in the horizontal plane. During the initial phase of the rotation (acceleration) the hair cells in both the auditory and vestibular systems became activated. Describe the transduction process that leads to neurotransmitter
release.
Mechanoelectrical transduction is mediated by the hair cells.
- Hair bundles are deflected toward the tallest stereocilium
- Cation-selective channels open near the tips of the stereocilia, allowing K+ ions to flow into the hair cell, depolarizing the hair cell
- Voltage-gated Ca2+ channels in the cell soma opens allowing calcium influx
- Release of neurotransmitter onto the nerve endings of the auditory nerve.
Describe the anatomical circuitry of the pupillary light reflex and include the cranial nerves involved in this circuitry.
Eye 1)
Light -> retinal photoreceptors -> ganglion cells -> optic nerve, chiasm, tract ->pretectum.
Eye 2)
Pretectum bilateral projections -> Edinger-Westphal -> ciliary ganglion neurons -> pupillary contrictor muscle.
The pupillary light reflex requires the optic nerve and oculomotor nerve.
Imagine that you by accident step on a small piece of hot barbecue charcoal (temp 60 °C).
Describe how this generates a sensation of sharp, easily localized pain. Begin at the foot and give detailed information about how the signal travels to the brain.
- Thermoreceptors responding to noxious temperatures (>43 °C) on A-delta fibres.
- Signal transmission (via action potentials) in a first-order neuron (primary afferent)
- Synapse with a second-order neuron in the dorsal horn of the spinal cord.
- These second-order neurons cross the midline and ascend to the brainstem and thalamus in the lateral spinothalamic tract (anterolateral system) terminating in the thalamus (VPL) synapsing on third-order neurons of this pathway.
- From the thalamus, axons of VPL neurons project to the primary somatosensory cortex.
- The foot area is represented on the medial surface of the cerebral hemisphere.
List two processes that contribute to central sensitization.
At least two of the following:
- Increased activity in descending excitatory pathways
- Repeated input from C and/or A-delta fibres
- Reduced activity in inhibitory interneurons in the spinal cord
- Microglial activation
- Removal of magnesium from NMDA receptors
- Protein synthesis in spinal neurons -> hyperexcitability in second-order neurons
What is the difference between nociceptive and neuropathic pain?
Nociceptive pain: Pain that arises from actual or threatened damage to non-neural tissue and is due to the activation of nociceptors
Neuropathic pain: Pain caused by a lesion or disease of the somatosensory nervous system (usually subdivided into peripheral or central neuropathic pain)
The major function of the middle ear is to match low-impedance (low resistance) airborne sounds to the higher-impedance fluid of the inner ear. Most of the energy is reflected when going from low-impedance to high-impedance. However, the middle ear compensates for this loss of energy by two different mechanisms. Describe these two processes that result in little loss of energy from the middle ear to the inner ear.
- large size difference between the tympanic membrane and the oval window.
- the mechanical advantage gained by the lever action of the three bones in the middle ear.
Humans use at least two different strategies to localize sound in the horizontal plane. Describe these two different strategies and name the region of the brain where these two different strategies take place.
- The lateral superior olive detects the intensity level difference between the two ears
- The medial superior olive detects the time difference between the two ears.
In sensory systems, contrast can be enhanced through which mechanism?
D) Lateral inhibition
In the mammalian somatosensory system, sensory information about
touch, vibration, and proprioception travels to the brain via the:
A) Medial lemniscus
Which statement/statements is/are correct
A) Projected pain is pain projected from a damaged structure in the
peripheral or central nervous system.
B) Referred pain is neuropathic.
C) Hyperalgesia is due to the increased response of nociceptive neurons to
their increased input.
D) Allodynia is due to the recruitment of a response to normally subthreshold stimuli
A) Projected pain is pain projected from a damaged structure in the
peripheral or central nervous system.
D) Allodynia is due to the recruitment of a response to normally subthreshold stimuli
Retinitis pigmentosa is a genetic disorder causing retinal degeneration.
Regarding this disorder, which statement/statements is/are correct?
A) It is caused by a defect in the pigment epithelium cells.
B) It affects peripheral vision.
C) It affects both rods and cones.
D) Vision is impaired due to an accumulation of disks from the rods.
A) It is caused by a defect in the pigment epithelium cells.
B) It affects peripheral vision.
D) Vision is impaired due to an accumulation of disks from the rods.
In the mammalian cochlea, high-frequency sounds cause the greatest displacement of:
A small portion at the stiff and basal part of the basilar membrane.
Regarding hair cells in the cochlea, which statement/statements is/are
correct?
A) They are direction-sensitive such that depolarization results when
the stereocilia are deflected in the direction of the shortest stereocilia.
B) Hyperpolarization results when the stereocilia are deflected towards
the longest stereocilia.
C) They are tonotopically organized along the basilar membrane.
D) K+ ions play a critical role in the transduction process.
C) They are tonotopically organized along the basilar membrane.
D) K+ ions play a critical role in the transduction process.
The hair cells in the utriculus and sacculus change their membrane
potential as a result of:
A) Linear acceleration.
B) Gravitation force.
D) Tilting of the head.
What is involved in the vestibulo-ocular reflex?
A) Semicircular canals.
B) Abducens nucleus.
C) Vestibular nucleus.
D) Oculomotor nucleus.
Stimulating a rod with light leads to membrane hyperpolarization. Explain the details of
phototransduction in the rods.
- Light hits the rods causing a configuration change from 11-cis to all-trans retinal
- Activation of transducin (G-protein), which activates phosphodiesterase.
- The phosphodiesterase hydrolyzes cGMP leading to a closure of the Na+ channel and hyperpolarization.
You are examining the pupillary light reflex in a patient. When you stimulate the left eye with light the pupil in the left eye decreases in diameter but NOT the right pupil. Explain the reason for this.
A problem with the oculomotor nerve.
Describe different types of nociceptors (i.e. free peripheral nerve ending of primary afferent nerve fibers that sense noxious stimuli)
Types of nociceptors:
Unimodal:
Mechanosensitive – respond to intense mechanical stimulation that threatens to damage the tissue.
Thermosensitive – respond to temperatures >42 °C or <17 °C.
Chemosensitive – respond to the H +, K +, serotonin, bradykinin etc.
Polymodal – respond to all types of nociceptive stimuli
Silent – nerve endings that are normally not responsive to mechanical or thermal stimuli,
but that are chemically activated during inflammation and then respond to mechanical and
thermal stimuli.
Describe different types of neurons in the dorsal horn that are activated by nociceptive stimulation.
Nociceptive specific (NS) neurons – are located in lamina I and have small receptive fields – receive input mostly from A-delta fibres – convey well localized (rapid) pain.
Polymodal nociceptive neurons – are also located in lamina I but have large receptive fields – receive afference mostly from C-fibers – convey more diffuse (slow) pain.
“Wide Dynamic Range” (WDR) neurons – receive input from both nociceptive and non-nociceptive afferents – lies in lamina 5 and has large receptor fields – convey more diffusely localized pain – some of them from visceral organs – possible substrate for referred pain.
There are 3 papillae on the surface of the tongue. Describe their location on the tongue and the taste or tastes that dominate for the papillae. What cranial nerve innervates these different regions of the tongue?
Circumvallate, bitter, 48%, glossopharyngeal (IX) posterior
Foliate, sour, 28%, glossopharyngeal(IX) laterally
Fungiform, sweet and salty, 24%, facial nerve (VII) anterior
Stimulus-coupled depolarization for bitter tastes and amino acids (umami) relies on the same PLCβ2/IP3/TRPM5 dependent mechanism. How do we then distinguish between umami and bitter tastes?
- at the receptor level
- The bitter tastes use T2R receptors
- umami uses T1R1 and T1R3.
There are several endogenous mechanisms that may inhibit pain signalling. Describe different mechanisms for inhibitory pain modulation at the segmental level (in the spinal cord and in the trigeminal system).
“Gate control” Activation of Aβ-fibres (sensitive to touch, vibration etc) may inhibit the transmission of pain signals at the first synapse in the dorsal horn of the spinal cord, possibly via activation of inhibitory interneurons releasing enkephalins or GABA etc).
There are several endogenous mechanisms that may inhibit pain signalling. Describe descending systems in the brainstem.
The Periaqueductal Gray (PAG) projects down to the Raphe Nuclei (containing serotonin neurons) in the medulla and to the locus coeruleus (LC, containing noradrenaline-neurons) which in turn project to the dorsal horn. Serotonin and noradrenaline are able to inhibit nociceptive signalling in the dorsal horn, either directly or via activation of inhibitory interneurons releasing enkephalins or GABA etc.
There are several endogenous mechanisms that may inhibit pain signalling. Describe important brain regions for pain modulation.
Brain regions that are important for pain modulation (both facilitating and inhibitory), especially in chronic pain, include the Anterior Cingulate Cortex (ACC), Insula (In), Prefrontal cortex (PFC) and the Nucleus Accumbens (NAC). Opioid peptides and Dopamine are important transmitter substances involved in these mechanisms.
Describe three characteristics of the fovea.
(1) There are only cones in fovea centralis
(2) they are connected 1:1 to the ganglion cells.
(3) It is a region of high visual acuity.
Describe the meaning of the term allodynia.
Allodynia = when a normally non-painful stimuli induce pain.
Give an example of two different forms of allodynia.
Two of the following: Cold-allodynia Heat-allodynia Static mechanical allodynia Dynamic mechanical allodynia
Describe the meaning of the term hyperalgesia.
Hyperalgesia = when normally painful stimuli are experienced with increased
strength.
Describe two different forms of hyperalgesia. What distinguishes them from each
other? Where along the pain pathways does the sensitization occur in the two types of
hyperalgesia, respectively?
- Primary hyperalgesia occurs very quickly in the injury area itself – is due to a sensitization of the peripheral nociceptors.
- Secondary hyperalgesia occurs in (is localized to) the intact tissue adjacent to the damaged area of the skin – is due to the sensitization of neurons at the spinal level (and probably also supraspinally) – gives increased sensitivity to, especially mechanical stimulation.
Describe processes that decrease pain signal transmission at the level of the spinal cord dorsal horn. Include the role of input from peripheral nociceptors as well as descending pain pathways.
Decreases pain signalling:
- Inhibition of “on-cells” (reduced descending excitatory signalling)
- Increased activity in ”off cells” projecting from RVM to dorsal horn cells
- Decreased activity in excitatory interneurons, Increased activity in inhibitory interneurons
- Reduced activity in GABAergic interneurons responsible for suppressing activity in “off cells”
What are the 4 types of mechanoreceptors?
Meissner’s corpuscles - low-frequency vibrations
Merkel’s disks - high tactile acuity
Pacinian corpuscles - mechanical deformation
Ruffini’s corpuscles - skin stretch and deformations within joints
