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98 Cards in this Set

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Somatosensory System refers to
general sensation:
touch, proprioreception, pain and temperature
EXTERORECEPTION
perception of external stimuli

receptors found near body surface (in skin and special sense organs)
INTEROCEPTION
perception of internal stimuli

receptors found within the body (in viscera, blood vessels, glands, serous membranes, etc...)
PROPRIOCEPTION
perception of body position and movement

within muscles, tendons, joints and the vestibular apparatus
various types of perceptions or sensations, (receptors are usually specific for a particular... )
MODALITY
(hearing, seeing, tasting, etc..)
adequate stimulus
type of stimulus, to which a particular receptor is responsive

generally- individual receptors respond to only one modality of stimuli
MODALITY: Vision
STIMULUS:
RECEPTOR TYPE:
STIMULUS: PHOTONS
RECEPTOR TYPE: PHOTORECEPTORS
MODALITY(ies):
STIMULUS: Specific Molecules
RECEPTOR TYPE:Chemoreceptor
TASTE and SMELL
Modalities that involve perception of:
MECHANICAL FORCE (as stimulus)
via (MECHANORECEPTORS)
TOUCH, PROPRIOCEPTION, HEARING
Receptor Type involved in Temperature perception
Thermoreceptors
Nociceptors are involved in what sensory modality?

therefore, receptive to what type of stimuli?
PAIN

receptive to chemicals released by tissue damage
2 basic functions of sensory receptors
1) detect energy of a physical stimulus
2) convert energy to transmissible form in nervous system (ACTION POTENTIAL)
[SENSORY TRANSDUCTION]
How is a photoreceptor stimulated?
alteration of membrane permeability by absorption of photons by rods and cones in the retina
How is a chemoreceptor stimulated?
alteration of receptor membrane permeability via opening of ion channels:
-due to specific ligand binding or
-to a second messenger G-protein
How is a mechanoreceptor stimulated?
ion channels are linked to the cytoskeleton and open when mechanical force is applied,

thereby altering membrane permeability
How is a thermoreceptor stimulated?
not completely clear how temperature causes ion channels to open (in order to change thermoreceptor membrane permeability)
How is a nocireceptor membrane permeability altered?
chemicals released by damaged cells are though to bind to the ion channels directly or through a second messenger
changes in the permeability of the receptor cell membrane results in what type of potential?
a local graded potential called the RECEPTOR or GENERATOR POTENTIAL
(sim to an EPSP)
EPSP
excitatory post-synaptic potential
what must happen for a receptor to tranduce the signal (generate an action potential)
RECEPTOR POTENTIAL must reach a certain threshold (can be accomplished by summation)
Where is the ACTION POTENTIAL GENERATED in:
-Multipolar neurons
-Pseudounipolar neurons
-in multi-polar neurons: generated at AXON HILLOCK

-in pseudo-unipolar neurons: generated at the INITIAL SEGMENT (1st Node of Ranvier)of the DENDRITE (aka peripheral axon)
(TYPICAL of 1st order sensory neurons)
Most 1st order sensory neurons are what type?
PSEUDOUNIPOLAR (peripheral nerves)

-Receptor-
-Io Segment (AP generated)of Dendrite (Peripheral Axon)
-Cell Body
-(Central) Axon
what is a labeled line?
chain of neurons carrying one type of modality
What information must a receptor and the generated AP convey about a given sensory stimuli?
-MODALITY (touch, proprioception, etc..)
-INTENSITY
-DURATION
-LOCATION
INTENSITY of a stimulus is represented by what characteristic of the RECEPTOR POTENTIAL?
represented by Amplitude of the receptor potential
INTENSITY of a stimulus is encoded in ACTION POTENTIAL by its:
FREQUENCY
(whereas intensity is encoded by amplitude in RECEPTOR POTENTIAL)
Increase in Intensity of stimulus results in:
?? in AMPLITUDE of RECEPTOR POTENTIAL
?? in FREQUENCY of AP
results in
Increase in amplitude of Receptor Potential
and
Increase in frequency of Action Potential
SOFT TOUCH vs. HARD TOUCH
-degree of receptor membrane deformation
-# of ion channels open
-degree of depolarization (amplitude of receptor potential)
-frequency of AP
-slightly/significantly
-small#/large#
-small/large
-low/high
Basis for different "Pain Thresholds" in different individuals
-receptors have different thresholds
-people have varying numbers of high and low threshold receptors
process by which receptors extract information about DURATION of a stimulus?
ADAPTATION: when the response of a receptor to a constantly applied stimulus diminishes with time (receptor "adapts" and stops generating receptor potentials)
receptors that respond maximally when the stimulus is first applied, but response quickly decreases if stimulus is maintained...are called:
RAPIDLY ADAPTING (phasic) RECEPTORS
receptors that generate receptor potentials as long as the stimulus is applied
SLOWLY ADAPTING (tonic) RECEPTORS
type of receptor that communicates information from DYNAMIC STIMULUS
Rapidly adapting receptor
(communicates information when stimulus changes)
type of receptor that communicates information from STATIC stimulus
Slowly adapting receptor
(communicates information the entire time the stimulus is applied)
RECEPTIVE FIELD
region of the body within which a stimulus will cause a response in a particular receptor

smaller the receptive field, the more localized the sensation
Test used to measure receptive field size
TWO-POINT Discrimination Test (measures the smallest distance required to perceive two poitns as separate and distinct)
receptor density
the density of receptor fields,

Fingers> Thumb > Face > Palm > Belly> Calf
Sensory Axons are classified by what feature?
their conduction velocity
(fastest>slowest)
Aa(Ia)>Aa(Ib)>Ab(II)
>Ad(III)> C(IV)
the larger the nerve fiber the thicker the?
the faster the?
thicker the myelin sheath
the faster the fiber conducts

largest myelinated fibers
v >100m/sec

smallest unmyelinated fibers
v <1 m/sec
what modalities are carried in the largest, myelinated fibers?
proprioception(muscles)>
proprioception(tendons)>
what modalities are carried in medium sized myelinated fibers?
(6-12micrometers)
proprioception (from joints)
touch, pressure, vibration >>
what modalities are carried in the smallest myelinated and unmyelinated fibers?
myelinated: pain (sharp, well-localized) temperature, crude touch

smaller, unmyelinated: pain (dull, poorly localized) temperature
how/where are somatosensory axons segregated by modality?
proximal to the cell body (in dorsal roots or within the spinal cord)

segregated by modality into PARALLEL PATHWAYS
clinical implications of parallel pathways in nerve compression and traumatic nerve injuries
partial compression of a dorsal root could completely disable axons carrying touch sensation, but spare axons carrying pain sensations
locations of cell body of:
1st, 2nd, 3rd order neurons
1st: periphery (has specialized receptor end)
2nd: cell body in spinal cord or brainstem
3rd: cell body in the thalamus
pathways that carry unconscious information generally have how many neurons?
two
1st order neurons travel in what type of nerves?
PERIPHERAL NERVES, either:
-Spinal Nerves (connect to spinal cord via dorsal and ventral roots)
-Cranial Nerves (connect to brain, except CNXI)
cutaneous nerve domain
area of skin that cutaneous nerves supply (cutaneous branches of spinal or cranial nerves)

eg: sensory loss to cutaneous nerve domain = likely damage to a peripheral nerve branch
dermatome
innervation of the skin by a particular spinal nerve

eg: herpes zoster = dermatomal pattern
only area of skin that does not receive sensory innervation from spinal nerves
anterior 2/3 of head (including anterior 1/2 scalp and face, not including angle of jaw)

area supplied by CNV
Major pathway that carries tactile and proprioceptive information:
DORSAL COLUMN- MEDIAL LEMNISCUS
(DC-ML) SYSTEM
tactile and proprioceptive information from the anterior 2/3 of head travels via:
TRIGEMINAL MECHANOSENSORY SYSTEM
what travels on the Spinothalamic tract (STT)?
small amount of tactile information ("crude-touch")
dorsal spinocerebellar and cuneocerebellar tracts carry what type of information?
carry unconscious sensory information
what is the supposed function of the descending projections in the somatosensory system?
modification of ascending information

MODULATORY PATHWAYS
Path of 1st order neuron in DC-ML System
Peripheral Axon
-travels via spinal nerve to enter spinal cord through dorsal root
-ascends dorsal column to caudal medulla (in gracile(<T6)or cuneate(>T6) tract)
-synapses with 2nd order neuron in gracile or cuneate nuclei at spino-medullary jxn
where do first and second order sensory axons synapse?
in CUNEATE NUCLEUS (if from area above T6) or GRACILE NUCLEUS (if from area below T6)

at spino-medullary junction
where are the cell bodies of second order neurons of DC-ML system?
in the DORSAL COLUMN NUCLEI:
CUNEATE NUCLEUS
and
GRACILE NUCLEUS
path of 2nd order axons in DC-ML system
-leave DORSAL COLUMN NUCLEI
-travel vetro-medially, to form INTERNAL ARCUATE TRACT
-become MEDIAL LEMNISCUS as they DECUSSATE in the caudal medulla, then ascend through the medulla, pons and midbrain
-terminate in VPL NUCLEUS of the thalamus (synapse w/ 3rd order)
sensory information from lower body follows what path in DC-ML system?
-ascends GRACILE TRACT to caudal medulla
-caudally in most VENTRAL portion of MEDIAL LEMNISCUS
-rostrally in most LATERAL portion of ML
sensory info from upper body is found in what regions of the brainstem as it ascends in DCML pathway?
In dorsal column, ascends via CUNEATE TRACT
In caudal part of MEDIAL LEMNISCUS fibers located DORSALLY
In rostral part of ML fibers located MEDIALLY
3rd Order Neurons of DC-ML System
-Cell bodies located in VPL nucleus (of the ventral posterior complex of the thalamus)
-axons pass through posterior limb of the INTERNAL CAPSULE and project into the PRIMARY SOMOTOSENSORY CORTEX (SI)
PRIMARY SOMATOSENSORY CORTEX
cortex of the POSTCENTRAL GYRUS and posterior part of the PARACENTRAL LOBULE
sensory homounculus
map illustrating the somatotropic organization represented by a caricature of a "little man"
amount of sensory cortex devoted to a particular region is determined by:
the sensitivity of the region (DENSITY of receptors)

NOT size of region
where does sensory information go after it reaches SI?
transported to higher order cortical centers for further processing and integration.

these centers include the secondary somoatosensory cortex (SII) and the associated cortices
Phantom sensations perceived by amputees are a result of what process (also results from increased stimulation of specific pathways)
constant remapping of the cortex. regions that are no longer receiving info (due to limb amputation) are remapped to be used by neighboring areas (violinists- left finger area of cortex, more extensive than right)
1st Order Neurons of Trigeminal Mechanosensory System
-peripheral axons travel in 3 divisions of CNV
-cell bodies located in TRIGEMINAL (semilunar) GANGLION
-axons enter pons and terminate in PRINCIPLE (chief) SENSORY NUCLEUS of the TRIGEMINAL BRAINSTEM COMPLEX
3 divisions of TRIGEMINAL BRAINSTEM COMPLEX
*PRINCIPLE NUCLEUS (mechanosensory info)
*SPINAL NUCLEUS (pain and temperature information)
*MESENCEPHALIC NUCLEUS (proprioceptive info from the muscles of mastication)
2nd Order Neurons of Trigeminal Mechanosensory System
-cell bodies in principal nucleus
-axons cross midline within the pons and then ascend in the ML (as the TRIGEMINAL LEMNISCUS/TRIGEMINOTHALAMIC TRACT) to the VPM nucleus of the thalamus
3rd order neurons of Trigeminal Mechanosensory System
same as those in DC-ML system

project to the SOMATOSENSORY CORTEX (specifically the face area)
SPINOCEREBELLAR PATHWAYS
pathways for proprioceptive information that travels directly to the cerebellum
Examples: DORSAL SPINOCEREBELLAR TRACT and the CUNEOCEREBELLAR DTRACT
1st Order neurons of SPINOCEREBELLAR PATHWAYS

from THORACIC, LUMBAR and SACRAL REGIONS
-enter spinal cord via dorsal roots
-synapse w/ 2nd order neurons at the dorsal NUCLEUS OF CLARKE (column of nuclei bt C8-L3)
2nd Order neurons of SPINOCEREBELLAR PATHWAYS

from ALL REGIONS
ascend to the cerebellum in the dorsolateral column as the DSCT (dorsal spinocerebellar tract)
1st Order neurons of SPINOCEREBELLAR PATHWAYS

from below L3
ascend in the ipsilateral gracile tract, before synapsing (in Dorsal Nucleus of Clarke, doesn't begin until L3)
1st Order neurons of SPINOCEREBELLAR PATHWAYS

from cervical region
ascend to the medulla in the cuneate tract and synapse with 2nd order fibers in the LATERAL (accessory) CUNEATE NUCLEUS
2nd Order neurons of SPINOCEREBELLAR PATHWAYS

from the cervical region
travel with the DSCT to the cerebellum (forming the CUNEOCEREBELLAR TRACT, as opposed to
disease or lesion affecting a peripheral nerve will demonstrate what pattern of sensory loss?
cutaneous nerve domain
disease or lesion affecting a dorsal root or spinal nerve will demonstrate what pattern of sensory loss?
dermatomal pattern

(remember however, dermatomes overlap)
incomplete lesions or compression injuries might only effect
certain modalities (due to PARALLEL PATHWAYS: grouping of axons by modality)
TACTILE MECHANORECEPTORS

5 types
Meissner's Corpuscles
Pacinian Corpuscles
Merkel's Disks*
Ruffini's corpuscles
Hair follicle receptor*
(NOT encapsulated, the others are)
mechanoreceptor located in glabrous skin between dermal papillae, abundant in lips, palms, fingertips, soles of feet
MEISSNER'S CORPUSCLE

(fine touch pressure (dynamic))
RAPIDLY ADAPTING
encapsulated
mechanoreceptor
mechanoreceptor located in deep dermis, subcutaneous tissue, interosseous membranes, messenteries of viscera
PACINIAN CORPUSCLE

(deep pressure, vibration)
RAPIDLY ADAPTING
encapsulated
mechanoreceptor located at the epidermal-dermal junction of skin
MERKEL'S DISK
(fine touch pressure (static))

SLOW ADAPTING
UN-ENCAPSULATED
mechanoreceptor
mechanoreceptor located in the dermis of skin, ligaments, tendons
RUFFINI'S CORPUSCLE
(stretch)

SLOW ADAPTING
ENCAPSULATED
mechanoreceptor
mechanoreceptor found in hairy skin
HAIR FOLLICLE RECEPTOR
(touch, hair movement)

PRIMARILY RAPIDLY ADAPTING
UN-Encapsulated
MECHANORECEPTOR
Mechanoreceptors for Proprioception
*MUSCLE SPINDLES
*GOLGI TENDON ORGANS
*JOINT RECEPTORS
All mechanoreceptors for tactile information are associated with what type of axons?
ABeta
Muscle Spindles
-location
-morphology
-fxn
-adaption rate
-associated axons
mechanoreceptors for proprioception
-in skeletal muscles
-sense muscle length
-SLOW AND RAPIDLY ADAPTING types
-associated with Type Ia AND II axons
GOLGI TENDON ORGANS
-location
-morphology
-fxn
-adaption rate
-associated axons
-located in tendons
-encapsulated
-sense muscle tension
-slow adapting
-associated iwth Ib axons
JOINT RECEPTORS
-location
-morphology
-fxn
-adaption rate
proprioreceptive mechanoreceptors in joints
-both free and encapsulated
-sense joint position
-rapidly adapting
"Stocking-and-glove" pattern of sensory loss can be seen in what type of nerve damage?
PERIPHERAL NEUROPATHY caused by toxins (alcohol, heavy metals), metabolic disturbances (DM), vit. deficiencies (B12)

affects longest, most metabolically active neurons first (those that travel from hands and feet!)
Damage to DC-ML system will impair what?
TACTILE ABILITY
(with the exception of crude touch and proprioception)
what kind of deficit is associated with damage to dorsal columns of one side of spinal cord?
SENSORY DEFICIT will be:
-IPSILATERAL (same side as damage)
-at/inferior to level of lesion (BROWN-SEQUARD)
what kind of deficit is associated with damage to dorsal columns on both sides?
POSTERIOR COLUMN SNYDROME
deficit will be bilateral at level of lesion and inferior
Lesions to what structures will cause contralateral sensory deficits?
MEDIAL LEMNISCUS
THALAMUS
or POSTcentral GYRUS
lesion of medial lemniscus on the right side within the rostral medulla will cause what type of deficit?
mechanosensory loss to LEFT side of body

contralateral deficit
3 ways to test for descriminative touch sensations/abilities:
-Two-point discrimination (compass)
-Stereognosis (id object by touch)
-Graphesthesia (recognize writing on skin)