Sensory System & Ascending Tracts

Question 1

What is the name of the process that converts a sensory stimulus into a receptor potential?

Answer 1

• sensory transduction

Question 2

What is adaptation?

Answer 2

• when a receptor diminishes the extent of its depolarization to a stimulus, despite the stimulus being of sustained strength

Question 3

Tonic vs. Phasic Receptors

Answer 3

• classification of sensory receptors based on their adaptability
• tonic receptors adapt slowly or not at all; useful for detecting intensity and duration of a stimulus
• phasic receptors adapt rapidly; useful for detecting changes in stimulus (on and off)

Question 4

Are receptor potentials graded potentials or action potentials?

Answer 4

• they are graded potentials

Question 5

What are major types of sensory receptors? Where is each found? What type of fiber and transmission is involved in each?

Answer 5

• Merkel disc: pressure, deep static touch (edges, braille), proprioception; found in basal epidermal layer, hair follicles; large and myelinated, phasic (adapts slowly)
• Pacinian corpuscle: vibrations and pressure; found in deep skin layers, ligaments, joints; large and myelinated, tonic (adapts quickly)
• Meissner’s corpuscle: fine/light touch, proprioception; found in hairless skin; large and myelinated, tonic (adapts quickly)
• free nerve endings: pain and temperature; found in all skin, epidermis, and some viscera; C (slow) and Adelta (fast) fibers
• hair receptor: detects hair movement and very gentle touch; tonic (rapidly adapting)
• Ruffini endings: detect deep, sustained pressure/stretch such as a massage; phasic

Question 6

Which two factors allow for acuity and discrimination of a stimulus?

Answer 6

• the receptive field and lateral inhibition

Question 7

Explain the basic pathway of afferent transmission.

Answer 7

• sensory receptors transduce receptor potential –> 1st order/primary sensory neurons receive signal (cell bodies are in the dorsal root ganglia) –> 2nd order sensory neurons –> 3rd order sensory neurons (these are in the thalamus) –> 4th order sensory neurons in the appropriate sensory area of the cerebral cortex
• (in pathways, such as somatosensory and olfactory, where the receptors are also the primary sensory neurons, there is one less step in these)

Question 8

What are the two pathways of somatosensory transmission to the CNS (ascending pathways)? What type of information does each transmit?

Answer 8

• dorsal column pathway: fine touch, pressure, 2-point discrimination, vibrations, proprioception (limb placement); 2 divisions: fasciculus gracilis for lower body and fasciculus cuneatus for upper body
• anterolateral spinothalamic pathway: pain and temperature (lateral division), crude touch and pressure (anterior division)

Question 9

Explain the pathway of the dorsal columns.

Answer 9

• 1 of 2 pathways for somatosensory transmission
• 1st order/primary neuron cell bodies in the dorsal root ganglia receive input, project into the dorsal column (the fasciculus gracilis for lower body and the fasciculus cuneatus for upper body), and ascend ipsilaterally to the medulla (synapses at the nucleus gracilis for lower body and at the nucleus cuneatus for upper body)
• 2nd order neurons in the medulla decussate as the internal arcuate fibers before ascending as the medial lemniscus through the pons and midbrain to the contralateral thalamus (VPL nucleus), 3rd order neurons here then project to the somatosensory cortex via the posterior limb of the internal capsule and the corona radiata
• mainly involves fiber types I and II (fast)

Question 10

Explain the pathway of the anterolateral spinothalamic tracts.

Answer 10

• 1 of 2 pathways for somatosensory transmission
• 1st order/primary neuron cell bodies in the dorsal root ganglia receive input and project to 2nd order neurons in the substantia gelatinosa of the ipsilateral dorsal horn
• these 2nd order neurons project to the contralateral lateral spinothalamic tract (for pain and temp) OR to the contralateral anterior spinothalamic tract (for crude touch and pressure), and then ascend to the medulla where the two tracts converge to form the spinal lemniscus
• spinal lemniscus passes through the pons and midbrain to terminate at the thalamus (VPL nucleus), 3rd order neurons here then project to the somatosensory cortex via the posterior limb of the internal capsule and the corona radiata
• pain has some additional pathways
• mainly involves fiber types III and IV (slow)

Question 11

Which nucleus of the thalamus is receives information dealing with pain, temperature, pressure, touch, vibration, and proprioception? Which receives information dealing with facial sensation and taste? Vision? Hearing? Movement?

Answer 11

• VPL (ventral posterolateral): pain, temp, pressure, touch, vibration, proprioception
• VPM (ventral posteromedial): facial sensation, taste
• LG (lateral geniculate): vision (Lateral for Light)
• MG (medial geniculate): hearing (Medial for Music)
• VL (ventral lateral): motor/movement

Question 12

How do sensory fibers project into the spinal cord?

Answer 12

• via the dorsal root and into the dorsal horn of the spinal cord
• the cell bodies of the 1st order nuclei are in the dorsal root ganglia

Question 13

Compare the dorsal tracts to the anterolateral tracts in a general sense.

Answer 13

• dorsal tracts are more modern; they are heavily myelinated and faster, and deal with the sensations of fine touch, proprioception, 2-point discrimination, and vibration
• anterolateral tracts are more primitive; they are less myelinated and slower, and deal with the sensations of pain, temperature, crude touch, and sexual feeling
• additionally, dorsal tracts have far superior spatial organization, resulting in more accurate fine point localization

Question 14

Which receptors are involved in the dorsal columns? What about in the anterolateral spinothalamic tracts? The spinocerebellar tracts?

Answer 14

• dorsal columns: Meissner (light touch), Pacinian (vibration), muscle spindles and Golgi tendon organs (proprioception)
• aneterolateral: free nerve endings (pain), thermal receptors (temp), Merkel’s (crude touch), Ruffini (pressure)
• spinocerebellar: muscle spindles, Golgi tendon organs, pressure receptors

Question 15

As they ascend up the spinal cord, from which side (medial or lateral) are fibers added to the tracts?

Answer 15

• dorsal columns gain new fibers LATERALLY as they ascend (lower half of the cord only has fasciculus gracilis, upper half will have the addition of fasciculus cuneatus)
• anterolateral tracts gain new fibers MEDIALLY as they ascend

Question 16

Which tracts make up the medial lemnisci? What about the spinal lemnisci?

Answer 16

• medial lemnisci: dorsal columns

- spinal lemnisci: lateral spintothalamic, anterior spinothalamic, AND spinotectal tracts

Question 17

Spinotectal Tracts

Answer 17

• these originate between the lateral and anterior spinothalamic tracts, ascend with them, and also form the spinal lemnisci with them
• however, they terminate at the superior colliculi of the tectum of the midbrain and deal with the spino-visual reflex

Question 18

What are the two pathways for pain? Compare the fibers of each.

Answer 18

• the fast pain pathway: Adelta fibers (type III: small, lightly myelinated); fast and sharp pain (cold temperature, touch, pressure too) from a mechanical or thermal stimulus; 2-5 um, 6-30 m/sec
• the slow pain pathway: C fibers (type IV: very small, unmyelinated); deep and dull pain (hot temperature too) from a mechanical, thermal, OR chemical stimulus; 0.4-1.2 um, 0.5-2 m/sec
• (C fibers also play a role in motor fibers of postganglionic autonomic fibers)

Question 19

What roles do histamine, bradykinin, acid, K+, and prostaglandins pain sensation?

Answer 19

• histamine, bradykinin, acid, and K+ are chemical causes of pain (so, slow pain) (capsaicin is an exogenous chemical with the same role)
• prostaglandins decrease the threshold for pain (they make it easier for pain receptors to fire)

Question 20

Why is fast pain highly localized, while slow pain is more diffuse?

Answer 20

• because the fast pain pathway has direct connections between its 1st and 2nd order neurons
• in contrast, the slow pain pathway has several chaining interneurons between the two, masking the specific origin of the signal

Question 21

Which thalamic nuclei do pain fibers project onto?

Answer 21

• both fast Adelta pain fibers and slow C pain fibers go to the VPL (ventral posterolateral) nucleus, as do all sensory afferents
• in addition, however, slow C pain fibers also project to the intralaminal nuclei, which communicates with the reticular formation (the reticular formation will keep the cortex “awake” and constantly aware of the pain, so as to force us to try and do something about it)
• (note that all sensory fibers project directly to the reticular formation as they ascend - to “prep” the cortex - but slow C pain fibers have this extra input)

Question 22

Which cerebral cortices do pain fibers terminate onto?

Answer 22

• they project to the primary somatosensory cortex (as do all sensory afferents destined for conscious awareness)
• in addition, however, pain fibers also project to the cingulate gyrus (generates the emotional response to pain) and to the insula cortex (generates the autonomic response to pain, such as sweating and throwing up)

Question 23

Why is pain decreased by extra stimuli (ie, acupuncture, massaging, etc.)?

Answer 23

• dorsal columns actually give off collateral fibers to specialized interneurons, which project onto the 2nd order pain neurons of the substantia gelatinosa in the dorsal horn of spinal cord
• these interneurons are INHIBITORY!

Question 24

What is the intrinsic analgesic system?

Answer 24

• many central nuclei (the periaqueductal gray matter, the medulla, and the reticular formation) have descending tracts that project to the spinal dorsal horns
• these release enkephalin and endorphins (endogenous opiates), which bind to opiate recepetors on afferent pain fibers, preventing them from releasing the neuromodulator substance P
• (morphine is an exogenous opiate with the same function)

Question 25

Spinocerebellar Tracts

Answer 25

• these bring sensory information (mainly proprioception) to the IPSILATERAL cerebellum for unconscious processing
• 1st order neurons (cell body in DRG) project to 2nd order neurons in dorsal horn of spinal cord, which then ascend to the ipsilateral cerebellum through 1 of 2 tracts:
• the ipsilateral dorsal spinocerebellar tract, which simply ascends and enters the inferior cerebellar peduncle
• the contralateral anterior spinocerebellar tract, which ascends on the contralateral side and enters the contralateral cerebellum via the superior cerebellar peduncle where it then projects to the other (ipsilateral) cerebellar hemisphere
• (damage to half of a spinal cord segment will thus result in bilateral ataxia, as each half has the dorsal tract of its own side as well as the anterior tract of the other side!)

Question 26

Clarke’s nuclei are only found at levels C8/T1 to L3 - what does this mean for levels above C8/T1?

Answer 26

• (Clarke’s nuclei are nuclei in the dorsal horns where the 2nd order neurons that project to the dorsal spinocerebellar tracts are found)
• Clarke’s nuclei only being present from C8 to L3 means that the dorsal spinocerebellar tracts only carry info from the trunk and lower limbs - not the upper limbs
• for levels above C8, there is actually another area in the dorsal horns called the accessory cuneate nucleus, which contains the 2nd order neurons that will project to the cuneocerebellar tracts (these are basically the upper limb’s equivalent of the dorsal spinocerebellar tracts)

Question 27

Which tract plays a role very similar to that of the spinocerebellar tracts?

Answer 27

• the spinoolivary tracts (unconscious proprioception)

- (the olives will then project to the cerebellum)

Question 28

Where are the dorsal columns located in the spinal cord?

Answer 28

• located in the posterior of the cord, between the mid-line/dorsal sulcus and the dorsal horn
• tract located medially: fasciculus gracilis (for the lower body); sacral (medial), lumbar, and some thoracic (lateral) innervation
• tract located laterally: fasciculus cuneatus (for the upper body); cervical (lateral) and some thoracic innervation (medial)

Question 29

Where are the anterolateral spinothalamic tracts located in the spinal cord?

Answer 29

• lateral spinothalamic tract is involved in detecting pain and temperature; it is located anterolaterally in the cord
• anterior spinothalamic tract is involved in detecting crude touch and pressure; it is located anterior to the ventral horn and medial to the lateral spinothalamic tract

Question 30

Why are nociceptors unique when compared to other sensory receptors? Do they adapt rapidly or slowly? Do they tend to have high or low thresholds?

Answer 30

• because they elicit the sensation AND a motivated behavioral/emotional response
• they do not adapt and have high thresholds

Question 31

What are the three categories of nociceptors and what type of pain does each detect? Which pain pathway and nerve fibers does each nociceptor type use?

Answer 31

• 1) mechanical/noxious: cutting, crushing, pinching pain
• 2) thermal: temperature extremes (usually heat greater than 45 degrees C)
• 3) polymodal and “sleeping”: respond equally to all kinds of damaging stimuli (mainly respond to the chemicals released by the damaged tissues - bradykinin)
• mechanical/noxious and thermal nociceptors use the fast pain pathway (Adelta fibers); 30 m/sec
• polymodal nociceptors use the slow pain pathway (C fibers); 2 m/sec

Question 32

Following nociceptor stimulation, when does an increased alertness occur? What about pain perception? What about pain localization? When is the context of pain generated?

Answer 32

• increased alertness once the signal reaches the brainstem/midstem (reticular formation)
• pain perception once the signal reaches the thalamus
• localization occurs in the somatosensory cortex
• contextualization occurs in the cingulate cortex and limbic system (help generate the behavioral and emotional response)

Question 33

What cell morphology do nociceptors have? Where are their cell bodies?

Answer 33

• pseudo-unipolar cellular morphology
• free nerve endings in the innervated tissue, cell body in the dorsal root ganglia (or the cranial nerve ganglia), and an axon that synapses in the dorsal horn of the spinal cord (or in the brainstem sensory nuclei)

Question 34

Which neurotransmitters do afferent pain fibers use? What is the role of each?

Answer 34

• pain fibers release substance P and glutamate (fast pathway releases only glutamate, slow releases both)
• substance P stimulates the ascending pathways for further processing of the pain
• glutamate stimulates the excitatory interneurons in the dorsal horn of the spinal cord; there are two types of receptors for glutamate (AMPA and NMDA receptors)

Question 35

What are the two types of glutamate receptors and how do they relate to pain transmission?

Answer 35

• in terms of pain transmission, the excitatory interneurons in the dorsal horn have AMPA and NMDA glutamate receptors
• AMPA receptors will propagate the pain message
• NMDA receptors will lower the threshold (sensitize) the dorsal horn; this is responsible for the exaggerated sensitivity of an injured area

Question 36

What are the four steps involved in the pain pathway?

Answer 36

• transduction (via the nociceptors) –> transmission (lateral spinothalamic tract) –> perception (processing in higher centers) –> modulation (descending input to modulate nociceptive transmission)

Question 37

What is tabes dorsalis?

Answer 37

• tabes dorsalis is a late complication of CNS syphilis and causes damage mainly to the dorsal columns and lumbosacral dorsal roots
• patients present with loss of proprioception, fine touch, and vibration and/or total loss of sensation of the lower limbs

Question 38

Which tract(s) are usually selectively damaged in syringomyelia?

Answer 38

• (syringomyelia is a cystic cavity of the spinal card, often arising from the central canal)
• due to their proximity to the central canal, the decussating fibers of the lateral spinothalamic tracts (anterior spinal commissural fibers) are usually damaged (loss of pain and temperature; preservation of fine touch, vibration, and proprioception)

Question 39

Where do lateral and anterior spinothalamic tracts decussate?

Answer 39

• they both decussate in the spinal cord
• lateral tracts decussate 1-2 spinal segments above their origin
• anterior tracts decussate 3-5 spinal segments above their origin

Question 40

What is syringomyelia? Where does is classically occur? Which tracts are usually affected?

Answer 40

• syringomyelia is a cystic cavity (a syrinx) resulting in degeneration of the spinal cord; often it arises in the central canal (hydromyelia)
• via trauma or in association with an Arnold-Chiari malformation (cerebellar tonsils herniate and obstruct CSF flow)
• this classically occurs between C8 and T1, involving the upper limbs; loss of pain and temperature with sparing of fine touch and proprioception occurs because of the proximity of the anterior white commissure to the central canal (this is where the lateral spinothalamic tracts decussate)
• (note that the syrinx can expand and destroy the anterior horns, creating LMN lesions, and the lateral horns, Horner’s syndrome)

Question 41

What is Friedrich’s Ataxia? When so patients usually present? What is is associated with?

Answer 41

• this is an inherited (A.R.) degeneration of the cerebellum and the spinal cord tracts resulting in ataxia, loss of vibration, fine touch, and proprioception, muscle weakness of lower limbs, and loss of deep tendon reflexes
• pathogenesis: GAA trinucleotide repeats in the frataxin gene of chromosome 9 involved in iron regulation/metabolism (results in the generation of free radicals and thus damage)
• presents in early childhood with staggering gait, freqeunt falls, nystagmus, dysarthria, kyphoscoliosis; is associated with hypertrophic cardiomyopathy
• (patients may also have pes cavus and hammer toes)