ch. 12 - somatic sensory system Flashcards
somatic sensation
enables our body to feel, to ache, to chill, and to know what its parts are doing
four main groups of stimuli
- touch
- temperature
- pain
- body position
touch - skin
- can be hairy or glabrous
- largest sensory organ
- protects
- prevents evaporation
mechanorecptors of the skin
- each has unmyelinated axon branches with mechanosensitive ion channels
- pacinian corpuscle
- ruffini’s endings
- meissner’s corpuscle
- merkel’s discs
- krause end bulbs
pacinian corpuscle
deep in the dermis, large enough to be seen with your naked eye
ruffini’s endings
found in both hairy and glabrous skin
meissner’s corpuscle
located in the ridges of glabrous skin (fingertips)
merkel’s discs
found in the epidermis, consists of nerve terminal and flattened non-neural epithelial cell
krause end bulbs
lie in the boarder regions of dry skin and mucous membrane, the nerve terminals look like the knotted balls of string
which mechanoreceptors have small receptive fields?
meissner’s corpuscles and merkel’s discs
which mechanoreceptors have large receptive fields?
pacinian corpuscles and ruffini’s endings
which mechanoreceptors are fast adapting?
meissner’s and pacinian corpuscles
which mechanoreceptors are slow adapting?
merkel’s discs and ruffini’s endings
follicles
- innervated by free nerve endings
- where hair grows
- bending of hair causes change in AP firing rate
which mechanoreceptor is most sensitive to 200-300 Hz vibrations?
pacinian corpuscles
which mechanoreceptor is most sensitive to 50 Hz vibrations?
meissner’s corpuscles
what happens when mechanoreceptor membrane is deformed?
mechanosensitive channels open, depolarizing receptor potential
prolonged stimulation causes…
decreased receptor potential
two point discrimination
our ability to tell the detailed features of a stimulus
* varies across the body
why are fingertips most sensitive to two point discrimination?
- more mechanoreceptors
- small receptive fields
- more brain power
- high resolution mechanisms
primary afferent axons
- bring info to brain or spinal cord
- enter at dorsal roots
- varying diameters and size correlate with the type of receptor
which primary afferent axon type is slowest?
C fibers (unmyelinated)
spinal cord
30 pairs of dorsal and ventral roots
cervical (c)
1-8
thoracic (t)
1-12
lumbar (l)
1-5
sacral (s)
1-5
spinal nerves
pass through notches in vertebral column
dermatome
area of skin innervated by both dorsal roots of a single spinal segment
what is the relationship between spinal segments and dermatomes?
one to one
what happens when dermatomes are mapped?
delineate a set of bands on the body surface
adjacent dorsal roots innervate….
overlapping areas
shingles - herpes zoster (chickenpox)
infection of dorsal root ganglion
each half of spinal gray matter is divided into…
- dorsal horn
- intermediate zone
- ventral horn
second-order sensory neurons
neurons that receive sensory input from primary afferents
A-beta axons
- large
- synapse on second-order sensory neurons
- also send axons to brian
- responsible for perception
dorsal column-medial lemniscal pathway
- for touch and vibration
- A-beta sensory axons travel to brain on IPSILATERAL side through dorsal column
dorsal column axons
- connect to dorsal column nuclei at medulla-spinal cord junction
- decussate, sensory info is now CONTRALATERAL
- ascends in medial lemniscus
medial lemniscus
travels through medulla, pons, and midbrain to ventral posters (VP) nucleus of the thalamus, which then goes to the primary somatosensory cortex (S1)
lateral inhibition
neighboring cells inhibit one another
dorsal column nuclei and thalamic nuclei are..
relays that alter info as it passes through
trigeminal touch pathway
sensation in face carried through trigeminal nerves (CNV)
trigeminal nerve
- enters the pons
- innervates face, mouth, dura mater, tongue
- synapse on IPSILATERAL trigeminal nucleus, decussates, then thalamus, then S1
some sensation comes from which other cranial nerves?
VII, IX, X
somatosensory cortex
- in partiel lobe
- most complex levels of processing
S1 - primary somatic sensory cortex
- brodmann’s area 3b
- input from VP of thalamus
- highly responsive to only somatosensory input
- lesions impair somatic sensation
- electrical stimulation causes sensation
brodmann’s area 3a
receives dense input from thalamus like 3b but focus on body position rather than touch
brodmann’s area 1
- receives info from area 3b
- mainly texture info
brodmann’s area 2
- receives info from area 3b
- emphasizes size and shape
layer IV
- where thalamic input terminates
- project to other layers
mountcastle’s idea
cells with similar characteristics in vertical columns in S1
somatotopy
mapped by S1 stimulation or by recording from S1 neurons after somatic stimulation
another name for somatotopy mapping
homunculus
features of somatotopy mapping
size of cortical sensory region determined by density and importance of input
somatotopic map plasticity
map changes based on loss or increased use
- use fMRI in humans
phantom limb
feeling for missing limb by stimulating regions whose S1 representations border those of the limb
posterior partiel cortex
- brodmann’s area 5 and 7
- large receptive fields
- elborate stimulus preferences
- integrates with visual, attention, and movement
agnosia
inability to recognize objects
neglect
part of body or world ignored
- usual left side ignored due to right hemisphere damage
posterior partiel cortex is essenital for…
- perception and interpretation of spatial relationships
- body image
- coordinated body movements
nociceptors
free, branching, unmyelinated nerve endings that signal that body tissue is being damaged or is at risk of being damaged
pain
the feeling, or perception, of irritating, sore, aching, throbbing, miserable, or unbearable sensations arising from part of the body
nociception
the sensory process that provides signals that trigger pain
types of damaging stimuli
- oxygen deprivation
- chemicals
- temperature extremes
- mechanical stress
what causes mechanically gated channels to depolarize and release AP?
being stretched or deformed
some channels open in response to…
- proteases: enzymes that digest proteins
- ATP
- K+
- bradykinin
- H+
temperature range for warmth
37-43 C
heat above what temperature causes tissues to burn?
43C
production of what leads to H+ build up in extracellular fluid?
lactic acid
what depolarizes nociceptors?
histamine
polymodal nociceptors
respond to thermal, chemical and mechanical stimuli
hyperalgesia
unusual sensitivity to skon, joints, or muscles that have already been damaged or inflamed
features of hyperalgesia
- increased intensity
- reduced threshold
- spontaneous
sensitizing chemicals
- bradykinin
- prostaglandins
- substance P
bradykinin
stimulates long-lasting intracellular changes that make heat-activated ion channels more sensitive
prostaglandins
chemicals generated by enzymatic breakdown of lipid membrane
substance P
a peptide synthesized by nociceptors; causes vasodilation and release of histamine
first pain
fast and sharp due to A-gamma fibers
second pain
dull and longer de to C fibers
primary afferent cell bodies
- in dorsal root ganglia
- branch out of lissauer
- synapse on substantis gelatinosa neurons
what is the NT for the pain afferents?
glutamate
what stimulates substance P release?
capsaicin
capsaicin
can be analgesic by depleting substance P from nerve endings
referred pain
cross-talk between visceral nociceptor activation and cutaneous sensation
spinothalamic pathway
- pain and temperature
- decussate immediately and run in ventrally in tract
- don’t synapse until thalamus
- don’t communicate until medial lemniscus
trigeminal pain pathway
- small diameter fibers in nerve synapse on neurons in spinal nucleus in brainstem
- second-order neurons decussate and connect to thalamus vis trigeminal lemniscus
afferent regulation of pain
- hyeralgesia
- reduced by A-beta fiber activity
- gate theory of pain
gate theory of pain
certain neurons of the dorsal horns, which project an axon up the spinothalamic that, are excited by BOTH large-diameter sensory axons and unmyelinated axons
descending regulation of pain
periventricular and periaqueductal great matter (PAG) neurons synapse on raphe nucleus, suppressing pain
raphe nucleus
- serotonergic
- project to dorsal horn and depress nociceptive neuron activity
opioids
poppy, morphine, codeine, heroin
opioids produce powerful analgesia as well as…
- mood changes
- nausea
- drowsiness
- mental stupor
- constipation
endorphins
endogenous opioids, expressed in pain pathways
what is the antagonist of endorphins?
naloxone
effects of endorphins
- block glutamate release
- hyperpolarize cell
what regions of the body have temperature sensitive neurons to help maintain body temperature?
hypothalamus and spinal cord
temperature sensitive spots are sensitive to…
either hot OR cold… NOT BOTH
trpv1
- above 43C
- capsaicin
trpv8
- below 25C
- methanol
when are differences in temperature rates most pronounced?
during and right after changes
cold receptors
A-gamma and C fibers
warm receptors
only C fibers
