Term
| What are the 3 functionally distinct types of Somatosensory receptors? |
|
Definition
1) Mechanoreceptors (touch, stretch, vibration)- Encapsulated
2) Thermoreceptors (non-painful warming/cooling)- Free
3) Nociceptors (extremes that actually cause tissue damage)- Free |
|
|
Term
| What are the 2 structurally distinct types of somatosensory receptors? |
|
Definition
1) Free-nerve endings (Thermo or Nociceptors)
- terminal axon arborizes extensively among epithelial or connective tissue- no specialized structures.
2) Encapsulated endings (mechanoreceptors)
- Pacinian corpuscles for vibration
- Meissner's corpuscles for light touch in fingertips) |
|
|
Term
| What is the difference between rapidly adapting (RA) and slow adapting (SA) mechanoreceptors? |
|
Definition
1) RA requires changes in stimulus energy (Meissner's corpuscles and Pacinian corpuscles from mechanical and vibration)
2) SA will keep firing as long as stimulus is present (Merkel cells and Ruffini endings for mechanical and vibration) |
|
|
Term
| What is the basic structure of a Sensory Unit? |
|
Definition
1) Pseudo-unipolar fibers with central axons projecting from the DRG to the spinal cord and peripheral axons projecting to the skin
2) NO dendrites |
|
|
Term
| What types of peripheral nerve fibers exist and what is the difference in their conduction velocities and stimulus thresholds? |
|
Definition
**The faster the conduction velocity, the lower the threshold stimulus**
1) A-alpha are fastest and are found in deep muscle nerves
2) A-beta (30-70m/s), A-delta (5-30m/s) and C (0.2-2m/s) fibers are cutaneous
3) B-fibers are pre-ganglionic autonomic efferents and are slowest. |
|
|
Term
| Which receptive fields receive the greatest peripheral innervation density? |
|
Definition
You want small, specific receptive fields to resolve textured detail.
Receptive fields of individual sensory units, which are smallest and least overlapping (example is skin on fingertips). |
|
|
Term
| How is the "two-point discrimination test" used to calculate receptive field acuity? |
|
Definition
Move the two points close together on finger tip until they are perceived as being the "same point."
- The smaller the distance, the lower the measured threshold. The lowest thresholds are found in toes, finger-tips and lips.
- Highest thresholds (they are still far apart) are found in proximal leg, shoulder and back. |
|
|
Term
| What are the 5 major types of Stimulus coding? |
|
Definition
1) Quality (Pacinian corpuscles signal the presence of high-ferquency vibratory stimulus)
2) Temporal (RA and SA fibers to calculate onset and offset)
3) Threshold (certain pressure or velocity to induce response)
4) Frequency (increasing APs with increasing intensity)
5) Location (receptive field) |
|
|
Term
| What is the difference between Meissner's corpuscles and Merkel cells in terms of information coding? |
|
Definition
1) Meissner's corpuscles are RA located just beneath the epidermis. They have LOW mechanical thresholds and are coupled to edges of papillary ridges for small movement sensation
2) Merkel cells are also located just under the epidermis, but they are SA cells in the centers of papillary ridges, have HIGHER thresholds and are important for encoding shapes of small objects. |
|
|
Term
| What is the difference between Pacinian corpuscles and Ruffini endings in terms of information coding? |
|
Definition
Both are deeper in dermis than Meissner's corpuscles and Merkel cells, and they encode vibration and skin stretch, respectively.
1) PCs are RA and encode vibration
2) REs are SA and encode skin stretch (object shape) |
|
|
Term
| What is the similarity between Ruffini endings and Merkel cells? |
|
Definition
Both are SA fibers that transduce object shape information.
Ruffini endings are deep within the dermis, while Merkel cells are just below the epidermis. |
|
|
Term
True or False:
Large, rapidly conducting fibers carry information from low-threshold mechanoreceptive endings |
|
Definition
True!
These fibers (A-beta fibers) are carried on opposite sides of the spinal chord than pain and temperature fibers (A-delta and C fibers)
This is a dichotomy, because and small, slowly conducting fibers are activated by thermal and noxious stimuli. |
|
|
Term
A patient presents with elevated thresholds for two-point discrimination on the left lower half of their body.
What artery may have been damaged? |
|
Definition
This is information carried in the left posterior columns by A-beta fibers to the brain, so it is supplied by the left Posterior Spinal Artery. The lesion or infarct must be at or above T8-ish territory.
Also look for left proprioception issues. |
|
|
Term
| Where do A-delta and C-fibers in the spinothalamic system cross to the contralateral side of the body? |
|
Definition
After synapsing in the dorsal horn, they cross the midline in the anterior white commissure (during crossing, they ascend 1-2 levels).
This commissure contains crossing fibers from BOTH sides, so a lesion here would cause bilateral deficits in pain and temperature (Syringomyelia in the central canal at cervical levels affecting arms). |
|
|
Term
| What is Syringomyelia and what disease process manifest similarly, but with an acute onset? |
|
Definition
1) Degenerative disease that begins around central canal at cervical levels (damage to anterior white commissure of spinothalamic tract) leading to bilateral pain/temperature deficits in arms.
2) Similar to anterior spinal artery infarction, which supplies medial lemniscus, pyramidal tract and hypoglossal nucleus. |
|
|
Term
What types of fibers dominate each of the following Spinal chord pathways:
1) Dorsal column
2) Spinothalamic
3) Trigeminal |
|
Definition
1) A-beta
2) A-delta and C fibers
3) A-beta for touch and proprioception, and A-delta and C fibers for pain and temperature |
|
|
Term
| Where do the central processes of trigeminal ganglion cells terminate? |
|
Definition
A-beta fibers synapse on second-order neurons in
1) Principal sensory nucleus of Pons (like dorsal column)
2) Spinal trigeminal nucleus (also find A-delta and C fibers here), which extends throughout medulla into upper cervical spinal cord to merge with substantia gelatinosa (like dorsal horn in body) |
|
|
Term
| What is the cellular course of the Posterior Column/Medial Lemniscus System pathway? |
|
Definition
Cross in the caudal medulla!
1) First synapse is in gracile and cuneate nuclei of brainstem in caudal medulla
2) Second synapse is in VPN of thalamus (via medial lemniscus)
3) Third synapse is in layer IV of S1 in the Cerebral cortex (via internal capsule) |
|
|
Term
| Explain the organization of two adjacent receptive fields in the cuneate nucleus in terms of integration of excitatory and inhibitory information. |
|
Definition
Goal is to achieve functional discriminitability.
1) Central portions of receptive fields have excitatory interneurons synapsing on them.
2) The eccentric part of a receptive field contains inhibitory interneuron synapses so that information is not transmitted to the "in-between" cuneate nucleus |
|
|
Term
| Why is the movement of a cotton-tipped swab (spatiotemporally patterns stimuli) used to assess posterior column function? |
|
Definition
| The hallmark of dorsal column function is tactile information processing, which is achieved by slow-decaying IPSPs and fast decaying EPSPs (limiting excitatory activity to short period of time). |
|
|
Term
| Explain the "Functional Columnar" organization of somatosensory cortex. |
|
Definition
Horizontal columns are different! Vertical columns are "functional units"
1) Neurons within a vertically oriented region have similar or overlapping receptive fields.
2) These neurons are all the same modality
3) These neurons respond to a peripheral stimulus with short, but not identical latencies. |
|
|
Term
| What do you see when the right posterior parietal cortex is lesions? |
|
Definition
| Left-side sensory neglect. |
|
|
Term
| What types of pain are mediated by the different types of peripheral nerve fibers (i.e. A-delta, C fibers, ect.) |
|
Definition
1) A-delta is is well-locailized 'fast' pain responding to extreme heat
2) C fibers are slower, more persistent and unpleasant pain responding to thermal or chemical stimuli at moderate and intense levels (POLYMODAL) |
|
|
Term
| What is the basis for the phenomenon of experiencing severe pain from light touches to recently damaged skin areas? |
|
Definition
| Following damage, background firing rates of C fibers remain high and threshold becomes very low. |
|
|
Term
| How does transduction of pain information occur in nociceptors? |
|
Definition
SHORT- Tissue damage leads to binding of Bradykinin, 5-HT, Prostaglandins, and K+ to nerve terminals, which release Sub P and stimulate histamine release from Mast cells. Sensitized axons spread AP antidromically into peripheral branches of same axon.
1) Involves Chemical Intermediary (5HT and ACh, Bradykinin and substance P in C-fibers and solutions high in K), which are released from damaged tissue and binds nerve terminal membrane.
2) Nerve terminal releases peptides that cause histamine release from mast cells and nerve terminal SENSITIZATION.
3) Axon Reflex occurs because action potentials generated in one branch of nociceptive axon travel back to other peripheral branches of same axon. |
|
|
Term
| What is the clinical relevance of "silent' nociceptors? |
|
Definition
| These afferent fibers are connected to pain processing and are recruited to the spinal cord after prolonged tissue inflammation or irritation, leading to the development chronic pain. |
|
|
Term
| Which nerve fibers subserve Visceral pain and how is visceral pain different from skin pain? |
|
Definition
A-delta and C fibers.
- Innervation density is lower in visceral organs, and is often referred to coupled dermatomes (i.e. heart pain to left arm). |
|
|
Term
| Explain the basic structure of the posterior horn of the spinal chord. |
|
Definition
PMZ and NP cross midline and ascend in anterolateral white matter contralateral to site of spin stimulus.
1) Posteromarginal zone (PMZ) contains nociceptive A-delta and C-fibers
2) Substantia gelatinosa (SG) is deep to PMZ and contains local inhibitory interneurons, which receive input from cells located at other spinal levels via reticulospinal pathways, as well as nociceptive and non-cociceptive primary afferents (Influence NATURE of pain).
3) Nucleus proprius receives inputs from A-beta, A-delta and C-fibers as well as inhibitory inputs from SG. |
|
|
Term
| Which group of cells in the dorsal horn of the spinal chord is responsible for modulating the "nature" of an experienced painful stimulus? |
|
Definition
| The Substantia Gelatinosa, with its many inhibitory interneurons that project to Nucleus proprius and other levels of the spinal chord. |
|
|
Term
| Where do you find "wide dynamic range cells" and why are they important? |
|
Definition
Cells that respond to light touch (A-beta) , heating or cooling (A-delta) and noxious chemical, thermal and mechanical stimulation (A-delta and C fibers) that are located in the Nucleus Proprius of the base of the Dorsal Horn of the spinal chord.
Respond to visceral stimulation and may be basis of REFERRED PAIN. |
|
|
Term
| What is the molecular basis for the phenomena of 'wind-up' in the development of post-operative pain following surgery? |
|
Definition
C-fibers release glutamate that binds to NMDA receptors and lead to central sensitization of pain circuits in the spinal chord (LTP?).
Apply local anesthetics to spinal chord during surgery to prevent. |
|
|
Term
| Where do axons in the anterolateral white matter project to (after crossing following 1st order synapses in different areas of the dorsal horn of the spinal chord)? |
|
Definition
1) Spinothalamic (fast, somatotopically precise for pricking pain)
- VPN of thalamus (modality separation from medial lemniscus)
2) Spino-reticulo-diencephalic pathway (slower, diffuse with C-fiber stimulation and slow, burning pain and emotions of anxiety and suffering)
- First project to brainstem reticular formation of medulla, pons and mesencephalon and to the Superior Colliculus.
- Then to the diencephalon: thalamic nuclei other than VPN and the hypothalamus (limbic input).
** Spinal trigeminal for pain/temp on face** |
|
|
Term
| What is the neurobiological evidence for the idea that neural substrates underlying the perception of pain differ from those underlying the psychological responses to it? |
|
Definition
1) Lesions of parietal cortex lead to diminished pain sensibility
2) Lesions of the prefrontal cortex or to the cingulate gyrus produce conditions where patients are away of painful sensation, but not bothered by them. |
|
|
Term
| What is thalamic pain syndrome? Why is it an example of "neurogenic pain"? |
|
Definition
Results from lesions to posterior thalamus (perhaps infarction of posterior cerebral artery), where even gentle tactile stimulation elicits severe pain (hyperpathia).
Neurogenic pain without peripheral source of nociceptive activity. |
|
|
Term
| Why does posterior cerebral arterial infarction lead to absence of somatic sensation of the contralateral body? |
|
Definition
It supplies the Posterior Thalamus, which is the first common site of termination for cells of the posterior column/medial lemniscus system and for cells of the spinothalamic system.
Can also cause Thalamic Pain Syndrome! |
|
|
Term
| How might Central Pain Syndromes involve the Descending Pain Control Systems? |
|
Definition
SHORT- RF to Spinal Chord: Release of opiates by substantia gelatinosa neurons (reversed by Naloxone), when excited by Serotonergic reticulospinal fibers from RF nuclei, produces presynaptic inhibition at central terminals of A-delta and C-fibers.
EXPLANATION
- Brainstem reticular formation receives inputs from multiply sensory modalities and project rostrally to thalamic nuclei and on to the cerebral cortex.
- These RF nuclei can also project to spinal cord and selectively suppress nociceptive-evoked activity via reticulospinal neurons that project to the dorsal horn via the dorsolateral funiculus of the SC and use 5-HT |
|
|
