NEUR 0010 - Chapter 12 Flashcards
What are the four sub-senses of the somatic sensory system?
Touch, pain, temperature, body position
What is the largest sensory organ?
Skin
What kind of axon is at the heart of every mechanoreceptor? What kind of channels are here?
Unmyelinated; mechanosensitive ion channels
What kinds of skin are Pacinian corpuscles found? Ruffini’s endings? Meissner’s corpuscles? Merkel’s disc? Krause ends bulbs?
Pacinian in the dermis, Ruffini’s in hairy and glabrous, Meissner’s in glabrous ridges, Merkel’s in epidermis, Krause end bulbs in border of dry/mucous
What do Merkel’s discs in the epidermis look like?
Nerve terminal and flattened, non-neural epithelial cell
What do Krause end bulbs in the dry/mucous look like?
Nerve terminals look like knotted ball of string
Describe the receptive field sizes of Meissner’s corpuscles, Ruffini’s endings, Merkel’s discs, and Pacinian corpuscles.
Meissner and Merkel are small; Pacinian and Ruffini are larger
Which receptors are slow adapting or fast adapting: Meissner, Merkel, Pacinian, Ruffini.
Slow adapting is Merkel and Ruffini; Fast is Meissner and Pacinian
Describe how the structure of the Pacinian corpuscle allows for adaptation and sensation of vibration?
Football-shaped onion corpuscle with slippery layers: if you maintain deformation, the layers shift so that no pressure is on the nerve ending; but when you cease deformation, the layers shift again to put pressure back on the nerve ending, until they readapt to normal again
What are primary afferent neurons?
Axons bringing info from SS receptors to CNS
Where do the primary afferent neurons of the SSS enter the spinal cord?
Through the dorsal roots/dorsal root ganglia
Describe the primary afferent neurons from largest diameter to smallest.
(A-alpha from skin, Group I from muscle; sensory receptors are proprioceptors of skeletal muscle); (A-beta from skin, Group 2 from muscle; mechanoreceptors of skim); (A-delta from skin, Group 3 from muscle; sensory receptors are pain/temp); (C from skin, Group 4 from muscles; temp/itch/pain)
What axon-from-muscle Group of primary afferent neurons is not present in the skin? Describe the characteristics.
Proprioception sensory receptor afferent neurons: A-alpha, Group 1, large diameter, fast conduction
What is the relationship between axon diameter and conduction speed?
Larger diameter, faster conduciton
Out of the four groups of primary afferent neurons, which group is unmyelinated?
Group C: itch/temp/pain
How many spinal segments are there? What’s a spinal segment?
Spinal segment is the area named after the vertebrae where the nerves originate; 30 of them
Describe the breakdown of the 30 spinal segments.
8 cervical, 12 thoracic, 5 lumbar, 5 sacral
What is a dermatome?
The area of skin innervated by the right and left dorsal roots of a single spinal segment
Why does a dermatome retain residual sensation even when the dorsal root is cut?
Adjacent dorsal roots innervate overlapping areas
What are second-order sensory neurons?
Neurons that receive sensory input from primary afferents; mostly lie within dorsal horns
What are the two main branches from when the large, myelinated A-beta (Group 2) neurons enter the dorsal horn of the spinal cord?
One branch: synapses with second-order sensory neurons for reflexes; Other branch: goes straight up to the brain for perception and judgment about stimuli touching the skin
What is the dorsal column-medial lemniscal pathway?
Pathway serving touch: instead of pain/temperature
What is the ascending part of the DC-ML pathway?
Large sensory axons (A-beta) enter ipsilateral dorsal column, which carry tactile sensation info to the brain
What kinds of axons make up the ascending DC-ML pathway?
Primary sensory and second-order from neurons in the spinal gray matter
Where do the DC-ML pathway ascending axons end?
In the dorsal column nuclei: junction of the spinal cord and the medulla
When does the DC-ML path become contralateral?
After the dorsal column nuclei: axons in the DCN branch toward ventral/medial medulla and decussate
What happens to axons in the DC-ML pathway after leaving the dorsal column nuclei?
Leave the DCN for the medulla: white matter tract called the medial lemniscus
Where do medial lemniscus axons terminate?
In the ventral posterior nucleus of the thalamus, then to the SS cortex (S1)
What are the trigeminal nerves?
Cranial nerve V: somatic sensation of the face
What is the trigeminal touch pathway?
Large sensory axons of trigeminal nerves carry info to second-order neurons in the ipsilateral trigeminal nucleus; the decussate and project into medial part of VP nucleus of thalamus, then on to S1 in the cortex
Where is most of the SS cortex?
Parietal lobe
What is the primary SS cortex?
Area 3b: on the postcentral gyrus behind the central sulcus
Why is area 3b the primary SS cortex?
Receives dense input from VP of thalamus; neurons very responsive to SS stimuli but not other senses; lesions impair somatic sensation; electrical stimulation evokes SS experiences
How do the projections to Areas 1 and 2 from Area 3b differ?
Area 1 is texture, area 2 is size/shape
In what layer does thalamic input terminate in S1?
Layer IV
What is somatotopy?
The mapping of the body’s surface sensations onto a structure in the brain
Does agnosia occur on ipsi/contralateral side of damage?
Contralateral
How does mechanical damage trigger nociceptors?
Stretched mechanically gated ion channels depolarize the cell and generate action potentials; damaged cells can release proteases/ATP/K+; proteases break down kininogen to for bradykinin, which binds to receptors in nociceptors; ATP binds to nociceptors and depolarizes them
What types of SS neurons transduce pain?
Unmyelinated C fibers, lightly myelinated A-delta fibers
What are the four types of nociceptors?
Thermal, chemical, mechanical, polymodal
What is primary vs secondary hyperalgesia?
Primary occurs within damaged area tissue; secondary occurs in surrounding tissue
What are three sensitizing chemicals that modulate nociceptors?
Bradykinin, prostaglandin, substance P
Where is substance P made?
In the nociceptors
What is bradykinin?
Depolarizes nociceptors; stimulates long-lasting intracellular changes, make heat-activated ion channels more sensitive
What are prostaglandins?
Generated by lipid membrane breakdown; increase sensitivity of nociceptors to other stimuli (NSAIDs inhibit prostaglandins)
What is substance P?
Made by nociceptors, causes release of histamine and vasodilation, sensitizes nearby nociceptors for secondary hyperalgesia
What causes sharp first pain and dull second pain?
Activation of A-delta fibers, then activation of C fibers
What is the zone of Lissauer?
The region of the spinal cord that carries the small-diameter SS fibers (dorsal)
What is the substantia gelatinosa?
The outer part of the dorsal horn, where the fibers from the zone of Lissauer synapse
Where is substance P contained?
The storage grandules in the axon terminals of pain fibers
What causes referred pain?
The fact that nociceptor axons from the viscera and cutaneous enter the spinal cord by the same route: cross-talk that causes visceral pain to seem cutaneous
How do nerve endings differ between touch/pain?
Touch has specialized receptors, pain has free nerve endings
How does diameter differ between touch/pain?
Thick and fast conduction, small and slow conduction
How do the connections to spinal cord differ between touch/pain?
For touch, axons terminate in dorsal horn; for pain, axons terminate in substantia gelatinosa after zone of Lissauer
What is the spinothalamic pathway for pain?
Second-order neurons decussate IMMEDIATELY and ascending through ventral spinal cord; project up through spinal cord and through medulla/pons/midbrain to the thalamus
How does decussation differ in touch/pain’s DC-ML vs spinothalamic pathway?
For touch (DC-ML), decussate at dorsal column nuclei (junction of cord and medulla); for pain (spinothalamic), decussate immediately
What is Brown-Sequard syndrome?
The weird contra/ipsi pain/touch loss due to damage to one side of the spinal cord
What is the trigeminal pain pathway?
Info from face/head small-diameter fibers synapse on second-order sensory neurons in spinal/trigeminal nucleus of the brainstem; cross and ascend to thalamus in trigeminal lemniscus
Where can spinothalamic axons project after reaching the thalamus?
In thalamic VP nucleus, in intralaminar nuclei of thalamic, then on to other cortical areas
How can mechanoreceptors reduce pain?
Simultaneous activation of mechanoreceptors (A-beta fibers) can reduce nociceptor activity
What is the gate theory of pain?
Certain neurons of the dorsal horns (project axons up spinothalamic tract), excited by large-diameter sensory neurons AND unmyelinated pain axons; inhibited by interneuron, which is excited by large sensory and inhibited by pain
How does the periaqueductal gray matter affect pain?
Suppression: analgesia
How does the PAG’s descending pathway suppress pain?
Sends descending axons to medulla (esp. raphe nuclei for serotonin), which project to the dorsal horn and depress nociceptive activity
What are the receptors for “hot” and “cold?”
TRPV1 and TRPM8
How selective are thermoreceptors?
Depends on the receptor: some are sensitive only to certain ranges of temperature, others can tell hot or cold
What pathway is most similar to the thermoreceptor pathway?
Pain pathway
What kinds of thermoreceptors are coupled to what kinds of SS fibers?
Cold is A-delta and C, warm is C only
Where does the thermoreceptor pathway decussate?
Same as pain: immediately
How does the thermoreceptor pathway ascend?
Same as pain: through spinothalamic tract
