Chapter 12: The Somatic Sensory System Flashcards
What are the two major types of skin?
- hairy
2. glabrous
Outer layer of skin = […]
Inner layer of skin = […]
epidermis
dermis
Most of the sensory receptors in the somatic sensory system are […].
mechanoreceptors
Mechanoreceptors are sensitive to …
physical distortion (stretch/bend)
At the heart of each mechanoreceptor are […]. These […] have […] and their gating depends on stretching or changes in tension of the surrounding membrane.
unmyelinated axons; axon; mechanosensitive ion channels
What is the purpose of unmyelinated axons within mechanoreceptors?
axons have mechanosensitive ion channels; their gating depends of stretch/tension
Pacinian corpuscle:
mechanoreceptor of the deep skin, selective for high-frequency vibrations
List the mechanoreceptors of the skin:
- Pacinian corpuscle
- Ruffini’s endings
- Meissner’s corpuscles
- Merkel’s disks
- Krause end bulbs
Which mechanoreceptors have large field receptors? Which have small field receptors?
L: Pacinian corpuscles and Ruffini’s endings
S: Meissner’s corpuscles and Merkel’s disks
Which mechanoreceptors are rapidly adapting? Which are slowly adapting?
R: Meissner’s and Pacinian corpuscles
S: Merkel’s disks and Ruffini’s endings
Rapid adapting vs slowly adapting:
RA: respond quickly but stop firing even though the stimulus continues (P and M)
SA: generate a more sustained response during a long stimulus (M. disks and R)
At what vibrational range do Pacinian and Meissner’s corpuscles respond best at?
P: 200-300 Hz
M: 50 Hz
**lower frequencies are the smaller field mechanoreceptors
What body part has the highest resolution of two-point discrimination?
fingertips
Two-point discrimination test is…
a simple measure of spatial resolution
The primary afferent axons enter the spinal cord through the […]; their cell bodies lie in the […].
dorsal roots; dorsal root ganglia
Dermatome:
a region of skin innervated by the pair of dorsal roots from one spinal segment
Where do the primary sensory afferent axons travel to? What particular region?
enter the spinal cord through the dorsal root ganglion
How is the labeling different for the following–
Skin sensory receptor vs muscle and tendon sensory receptors:
S: A(alpha), A(beta), A (gamma), C
M/T: I, II, III, IV
**similar sizes
Skin lack which labeled group of axons? What is unique of Group C/IV?
S: the largest axons A(alpha)
C/IV: unmyelinated axons
**the rest are myelinated
What type of proprioceptors are in each category of axons:
A(alpha) or I–
no skin; proprioceptors of skeletal muscles
A(beta) or II–
mechanoreceptors of skin
Alpha(delta) or III–
pain, temperature
C or IV–
temperature, pain, itch
How many bones in each category of the vertebra?
C: 8
T: 12
L: 5
S: 5
When a dorsal root is cut, what occurs?
body does not lose all sensation in that region; residual somatic sensation present due to overlapping adjacent dorsal roots
The spinal gray matter is divided into…
- dorsal horn
- intermediate zone
- ventral horn
Second-order sensory neurons:
neurons that receive sensory input from primary afferents that lie within the dorsal horns
Dorsal column-medial lemniscal pathway purpose:
pathway serving touch
Dorsal columns carry information about…
tactile information and limb position to brain
What are dorsal columns composed of?
- primary sensory axons
2. second-order axons from neurons in the spinal gray matter
Dorsal Column- Medial Lemniscal Pathway:
Where do the axons of the dorsal column terminate?
dorsal column nuclei
**spinal cord and medulla junction
When does decussation occur along the dorsal lemniscus pathway?
when the axons from cells of the dorsal column nuclei arch toward the ventral and medial medulla
Medial lemniscus:
white matter tract
- axons of the dorsal column nuclei ascend to
- its axons synapse upon neurons of the ventral posterior (thalamus)
What is the major route by which touch and proprioceptive information ascend to the cerebral cortex?
large dorsal root axons — dorsal column [spinal cord] — dorsal column nuclei [medulla: medial lemniscus] — thalamus [VP nucleus] — primary somatosensory cortex (S1)
What is meant by the term relay nuclei?
used to describe specific sensory nuclei of the thalamus; relay stimuli from lower receptors to coordinating receptors (ex: VP nucleus)
Is the information altered from the stimuli to the cortex? Or is it simply transferred?
information is altered at every synapse; typically by inhibitory reactions to enhance the response of the stimuli
process is not a “synaptic relay”
What CN is the trigeminal nerve? What is its purpose?
CN V
supplies somatic sensation of the face
Pathway for the face to the cortex:
***decussation
large mechanoreceptors from the face — [cranial nerve V/trigeminal nerve] — principal sensory trigeminal nucleus — *** — thalamus [VP nucleus] — somatosensory cortex (S1)
Where is the region of the somatosensory cortex?
on the postcentral gyrus of the parietal lobe
**right behind the central sulcus
Area 3a vs 3b:
3a region concerned with a sense of body position rather than touch
3b is the primary somatic sensory cortex
Projections of 3b: What info do they mainly receive?
area 1– texture information
area 2– size and shape
Thalamic inputs to S1 terminate mainly in which layer? Where do these project in turn?
IV
the remainder of cells in the striate cortex layers
What is unique about the brain undergoing electrode implantation?
brain has no pain somatic receptors
no sense of pain either
Somatotopy:
mapping of the body’s surface sensations onto a structure in the brain
The somatic map is sometimes called the […].
homunculus (“little man”)
If there is a removal of a body part what occurs to the mapping of the cortex?
major rearrangement of the cortical somatotopy
What happens when the input activity of a body part is increased?
representation of stimulated body parts had expanded in comparison with the adjacent/unstimulated ones
T/F: Cortical maps are dynamic and adjust on the amount of sensory experience.
true
Ruffini’s endings found in […] are slightly smaller than […].
both glabrous and hair skin; Pacinian corpuscles
Meissner’s corpuscles are about 1/10th the size of […] and are located in the […].
Pacinian corpuscles; ridges of glabrous skin
**raised parts of your fingertips
What mechanoreceptor is being describe: Contains a cell and axon that are mechanosensitive, the two channels cooperate to activate the receptor.
Merkel’s disks
Krause end bulbs, which lie in the […] and […], the nerve terminals look like […].
regions of dry skin; mucous membrane; knotted balls of string
**ex around the lips and genitals
Hair grows from […] embedded in the skin; each […] is richly innervated by […].
follicles; follicle; free nerve endings
What is in charge of mediating goosebumps?
erectile muscles (type of hair follicle)
When the capsule (CT layers around the nerve terminal) compressed what occurs?
If the stimulus is maintained, what occurs?
When pressure is released, what occurs?
- energy transferred to the nerve terminal; deformation of the membrane leads to mechanosensitive channels to open; depolarization (if receptor potential is great enough leads to AP)
- layers slips past one another in a way that the axon terminal is no longer deformed; receptor potential dissipates
- events revers themselves; terminal depolarizes; may release AP
Single corpuscles vs naked nerve terminal:
SC– extremely sensitive to vibrating stimuli
NNT– more sensitive to steady pressure
Why is the fingertip so much better than the elbow (for example) for Braille reading?
- higher density of mechanoreceptors
- fingertips are enriched in receptor types that have small receptive fields
- more brain tissue devoted to the sensory info of each square mm of fingertip
- special neural mechanisms devoted to high-resolution discrimination
What type of axons are the slowest? What do these detect?
Group C
mediate pain and temperature sensation
To lose all sensation in one dermatome…
three adjacent dorsal roots must be cut
Large and myelinated A(beta) axons enter the dorsal horn and branch in one of two ways:
- deep part of the dorsal horn on second-order sensory neurons
- ascends straight to the brain
Agnosia:
inability to recognize objects even through simple sensory skills seem to be normal
Agnosia is due to damage of the…
Deficits are often limited to the side…
posterior parietal lobes
contralateral to the damage
Astereognosia:
cannot recognize common objects by feeling them
Neglect syndrome:
part of the body or a part of the world (entire visual field left of the center of gaze) is ignored or suppressed, and its very existence is denied
Neglect syndrome is due to…
parietal cortical lesions; typically on the R side
In general, what is the posterior parietal cortex involved in?
- SPTIAL RELATIONSHIPS: perception and interpretation
- BODY IMAGE
- COORDINATION: learning tasks of body space
Somatic sensation depends strongly on…
nociceptors
Nociceptors description:
free, branching, unmyelinated C fibers
little myelinated A delta fibers
Nociceptors function:
signal body tissue is being damaged or is at risk of being damaged
Pain vs nociception:
P– feeling, perception, of irritation/sore/stinging/aching/throbbing/miserable
N: sensory process that provides the signals that trigger pain
What are nociceptors activated by?
activated by mechanically gated ion channels
Damaged cells at the site of injury can release a number of substances that cause ion channels on nociceptor membranes to open:
- proteases
- ATP
- K+
Proteases can break down…
abundant extracellular peptide called kininogen to form another peptide called bradykinin
Bradykinin:
- DEPOLARIZATION: binds to specific receptor molecules that activate ionic conductance in some nociceptors
- SENSITIZATION: stimulates long-lasting intracellular changes (heat-activated channels more sensitive)
What occurs when your tissue oxygen levels do not meet the oxygen demand?
- cells use anaerobic metabolism to generate ATP
- release of lactic acid
- excess of H+ ions in EC
- ions activate H+ gated ion channels on nociceptors
- dull ache
What occurs when a bee stings you?
- mast cells activated due to foreign substance
- release HA
- HA binds to specific cell surface receptors; causing depolarization
- HA causes blood capillaries to become leaky (swelling/redness)
Transduction of painful stimuli occurs…
in the free nerve endings of unmyelinated C fibers and lightly myelinated A(delta) fibers
Polymodal nociceptors respond to…
mechanical, thermal, and chemical stimuli
Mechanical nociceptors are different how?
show selectivity in their responses to different stimuli
Thermal nociceptors respond to…
selective; extreme temperature
Chemical nociceptors respond to…
selective; HA and other chemicals
Perception of itch is due to
small C fibers
Hyperalgesia:
a reduced threshold for pain, an increased response to painful stimuli, spontaneous pain that follows a localized injury
Primary vs secondary hyperalgesia:
P– injury site
S– tissues surrounding are affected
What are some sensitizing chemicals?
- bradykinin
- prostaglandins
- substance P
How do prostaglandins sensitize?
increase the sensitivity of nociceptors to other stimuli
**ASA and anti-inflammatory inhibit this
Substance P:
peptide synthesized by the nociceptors
activation of nociceptor axon can lead to its secretion
How does substance P cause sensitization?
vasodilation and release of HA (from mast cells)
What triggers secondary hyperalgesia?
- substance P
2. ACTIVATION OF A(beta)– leads to cross-talk; where light touch can evoke pain
