Exam I (Somatic sensation) Flashcards
What is transduction?
Where a stimulus is changed into an electrical signal
What are the different types of stimuli?
Mechanical, chemical, change of temperature and electromagnetic
What are the four attributes of a stimulus?
Modality, location, intensity and Duration (timing)
What are the two systems used for nerve fiber classification?
Erlanger’s and Lloyd’s
What is the speed of conduction dependent on for a nerve fiber?
Diameter and myelination
How does myelination increase conduction velocity?
six fold
What are the diameters of the different nerve classifications in Erlanger’s system?
A(alpha) - 8-20 microns A(beta) - 5-12 microns A(gama) - 2-8 microns A(sigma) - 1-5 microns B - 1-3 microns C - < 1 micron
Erlanger’s system is used in what?
Motor nerves and skin afferents
What types of nerve fibers represent motor fibers under the Erlanger classification system?
A(alpha) and A(gamma)
What types of nerve fibers represent skin afferent fibers under the Erlanger classification system?
A(beta) , A(sigma) and C
What are the diameters of the different nerve classifications in Lloyd’s system?
I - 12-20 microns
II - 4-12 microns
III - 1-4 microns
IV - < 1 micron
What is Lloyd’s system used for?
Afferents from receptors in muscle and spinal joints
Opening of Na+ ion channels is indicative of what?
A change in receptor potential
What is the relationship between the frequency of an AP and the negativity of a receptor potential?
The less negative the receptor potential the faster the frequency of AP into the CNS.
What is the labeled line principle?
the specificity of nerve fibers transmitting only one modality of sensation
The type of sensation felt is dependent on what?
The termination point in the CNS
What is adaptation?
When the firing rate of a neuron decreases over time due to a sustained stimulus
What are examples of slow-adaptive receptors?
Merkel’s discs and Ruffini end organs (corpuscles)
What are merkel’s discs?
Punctate receptive fields
How are Ruffini end organs stimulated?
by stretching the skin
What are the types of rapid or phasic-adaptive receptors?
Pacinian corpuscles and Meissner’s corpuscles
Both types of rapid-adaptive receptors respond to what type of stimulation?
Vibration
What is the optimum stimulus rate of Pacinian corpuscles?
250 Hz
What is the optimum stimulus rate of Meissner’s corpuscles?
30-40 Hz
What is the mechanism of adaptation?
Membrane adaptation is thought to be due to the entry of Ca++ ions during APs
- Ca++ opens a K+ channel increasing permeability of the membrane for K+..taking membrane away from threshold
What are the most heavily innervated spinal joints?
Cervical joints
Describe type I mechanoreceptors
- Outer layers of joint capsule
- Fire at a degree proportional to joint movement or traction
- low threshold
- dynamic-fire with movement
- slow adapting
- tonic effects on lower motor neuron pools
Describe type II mechanoreceptors
- Deeper layers of joint capsule
- low threshold
- rapidly adapting
- completely inactive in immobilized joints
- functions in joint movement monitoring
- phasic effects on lower motor neuron pools
Describe type III mechanoreceptors
- recently found in spinal joints
- very high threshold
- slow adapting
- joint version of GTO
Describe type IV receptors
- Nociceptors
- very high threshold
- completely inactive in physiologic normal joint
- activation with joint narrowing, increased capsular pressure and chemical irritation
Information transmitted to the brain from mechanoreceptors in fingers allows us to:
- feel the shape and texture of objects
- play musical instruments
- type on computer keyboards
- palpate and perform adjustments
- perform a multitude of tasks using our hands
What happens after tactile information is fragmented by receptors?
It must be integrated (interpreted) by the brain
Why is tactile information important?
It gives us the ability to recognize objects placed in the hand on the basis of touch alone.
How is tactile information obtained?
Through palpation
What is Stereognosis?
The ability to perceive form through touch
What does stereognosis test?
The ability of the dorsal column-medial lemniscal system to transmit sensations from the hand
Also the ability of cognitive processes in the brain where integration occurs
How does the brain interpret an object if most objects are larger than the receptive field of any receptor in the hand?
The objects stimulate a large population of sensory nerve fibers. Each of which scans a small portion of the object. The brain reconstructs the pattern.
Does a single sensory axon or class of sensory axons signal all relevant information?
No. The CNS constructs a coherent image of an object from fragmented information conveyed in multiple pathways
What are the categories of sensory modalities?
Pressure receptors, cold receptors, warmth receptors and nociceptors
Getting wet stimulates what receptors?
pressure and temperature receptors
Being tickled stimulates what receptors?
Gentle stimulation of pressure receptors
What receptors are stimulated by an itch?
Gently stimulation of nociceptors
How do humans detect extremely fine textures?
When fine textures are stroked on the fingerpad skin, the fingerprint ridges vibrate and cause Pacinian corpuscles to respond enabling the dection of the microtexture.
Depth of indentation and the change in curvature of the skin surface are encoded by discharge rates of what?
Slow adapting afferent fibers
Velocity and the rate of change in skin surface curvature are encoded by discharge rates of what?
Both slow adapting and rapidly adapting afferent fibers
Meissner’s corpuscles are found in what type of skin?
Glabrous (non hairy)
What are the characteristics of Meissner’s corpuscles?
Concentrated in the fingertips
Signals edges, register sideways shearing of the skin
Density declines with age
Where are pacinian corpuscles found?
Subcutaneous tissue
What are the characteristics of Merkel’s discs?
Slow adapting
Punctate receptive fields
senses curvature of an object’s surface
What are the characteristics of Ruffini end organs?
Slow adapting
Activated by stretching of the skin (even at some distance away from receptor)
Which receptors are deep in the skin (large receptive field)?
Pacinian Corpuscles and Ruffini Endings
Which receptors are superficial (small receptive field)?
Meissner’s corpuscles and Merkel’s discs
What is the somatotopic homunculus based on?
the density of receptors
What is lateral inhibition?
Where a first order neuron synapses it excites a second order neuron as well as local interneurons (surround inhibition)
Describe the dorsal column-medial lemniscal system
- Most aspects of touch, proprioception
- First order neurons synapse in the brain stem
- 2nd order neurons of both pathways cross to other side and ascend, synapsing in the thalamus
Describe the Anterolateral system
- Sensations of crude touch, nociception, temperature, tickle, itch and sexual sensations
- First order neurons synapse in the dorsal horn of the spinal cord
- 2nd order neurons of both pathways cross to other side and ascend, synapsing in the thalamus
What is protopathic and what are its characteristics?
Primitive feeling
- Old (phylogenetically)
- synapse in the cord
- Contralateral in the cord
- INITIATE ACTIONS
- small fibers
- Pain, temperature, tickle, itch, sexual sensations
What is epicritic and what are its characteristics?
Precise objective information
- New (phylogenetically)
- Synapse in brain stem
- Ipsilateral in cord
- MODIFY ACTIONS
- Large fibers
- Encapsulated mechanoreceptors
- Provide feedback used by the brain
What is the clinical significance of spinal cord lesions?
Hemisection of the cord
- Loss of vibration/proprioception – ipsilateral
- Analgesia - contralateral
Describe the early stages of cortical processing
Dynamic properties of central neurons and receptors are similar (eg rapidly adapting cutaneous receptors connected to rapidly adapting 2nd and 3rd order neurons)
Describe the later stages of cortical processing
The central nerve cells have complex feature detecting properties and integrate various sensory inputs
What are the three different types of neurons in BM area 1 and 2?
Motion sensitive neurons, Direction-sensitive neurons and Orientation-sensitive neurons
Describe motion sensitive neurons
Respond well to movement in all directions but not selectively to movement in any one direction
Describe direction-sensitive neurons
Respond much better to movement in one direction than in another direction
Describe orientation-sensitive neurons
Respond best to movement along a specific axis
Describe BM 5
Integrates tactile information from mechanoreceptors in skin with proprioceptive inputs from underlying muscle and joints
Describe BM 7
receives visual, tactile and proprioceptive inputs
-integrates stereognostic and visual information
What is the posterior parietal cortex?
BM 5 and 7
Describe the secondary somatic sensory cortex
- located in the superior bank of the lateral fissure
- projections from S-1 are required for function of S-II
- projects to the insular cortex, which innervates regions of temporal lobe believed to be important in tactile memory
What are the characteristics of thermoreceptors?
- Slow adapting
- Discharge spontaneously under normal conditions
- Active over a wide range of temperatures
- Discharge phasically when skin temperature changes rapidly
What are the two types of thermoreceptors?
Cold and Warmth
When are the two types of thermoreceptors inactivated?
- Cold- when the skin is warmed
- Warmth- when the skin is cooled
What are nociceptors activated by?
Mechanical, thermal and chemical stimuli
What are the sensations of pain?
Pricking, burning, aching, stinging and soreness
What is the difference between pain and nociception?
Pain is the perception of an unpleasant sensation. Nociception provides information about tissue damage that is often extremely painful.
Why is pain hard to define and treat clinically?
It is highly subjective
What is pain?
Conspicuous sensory experience that warns of danger
What is the least differentiated sensory receptor?
Nociceptors
What is hyperalgesia?
Increased sensitivity to pain
- repeated heating
- axon reflex may cause spread of hyperalgesia in periphery
- sensitization of central nociceptor neurons as a result of sustained activation
what does potassium from damaged cells do to nociceptors?
activates
Characteristics of Fast nociceptive pathways?
- A delta fibers
- glutamate
- neospinothalamic
- mechanical, thermal
- good localization
- sharp, pricking
- Most terminate in VB complex of thalamus
Characteristics of Slow nociceptive pathways?
- C fibers *
- substance P/ glutamate
- Primary paleospinothalamic
- polymodal/chemical
- poor localization
- dull, burning, aching
- Diffuse termination;
- Reticular formation
- tectal area of mesen.
- Periaqueductal gray
Describe C-fiber nociceptors (first order)
Secrete both substance P and glutamate
Synapse primarily in substantia gelatinosa (lamina II & III)
Describe Glutamate
Effects are transient and short acting
-Fast component of C-fiber
Descrive Substance P
Released more slowly and concentration builds over seconds-minutes responsible for longer lagging predominant effect (slow pain)
What are the different nociceptive pathways?
Spinothalamic-major -neo- fast (A delta) -paleo- slow (C fibers) Spinoreticular Spinomesencephalic Spinocervical (mostly tactile) Dorsal columns- (mostly tactile)
What are the cardinal signs of inflammation?
Redness
Heat
Swelling
Pain
What is rubor?
redness
What is Calor?
Heat
What is tumor?
swelling
What is dolar?
Pain
What is the peripheral pain control mechanism?
Gating theory
What is the gating theory
Involves inhibitory interneuron in cord impacting nociceptive projection neurons
- inhibited by C-fibers
- Stimulated by A alpha and beta fibers
- TENS
Describe the Central pain control mechanism
Direct electrical stimulation to the brain
->Analgesia
Nociceptive control pathways descend to cord
Endogenous opiods
Endogenous cannabinoids
Enkephalin projections to Raphe
Endogenous opioids
Periaqueductal gray
Serotonin projections to the cord
Endogenous opioids
Raphe N.
Inhibitory interneurons in the cord
Endogenous opioids
Release encephalin which can cause pre-synaptic inhibition of incoming C & A delta fibers
Inhibit second order projection neurons
BM area that forms muscle stretch receptors
3a
BM area that forms cutaneous receptors
3b
BM area that forms deep pressure receptors
2
BM area that forms rapidly adapting cutaneous receptors
1
what does serotonin from platelets do to nociceptors?
activates
What does bradykinin from plasma kininogen do to nociceptors?
activates
What does histamine from mast cells do to nociceptors?
activates
What do prostaglandins and leukotriends from ARA damaged cells do to nociceptors?
Sensitize
What does substance P from the primary afferent do to nociceptors?
Sensitize
Sensory innervation of most of the head and face
Trigeminal N. (V)
muscle receptor that responds to stretch
Muscle spindle
Muscle receptor that responds to tension
GTO
Where are muscle spindles located?
within the belly of the muscle parallel with extrafusal fibers
Where are GTOs located?
at the junction of the muscle and tendon
Innervated by two types of myelinated afferent fibers
Muscle spindles
What are the two types of myelinated afferent fibers that innervate muscle spindles?
group Ia (large diameter) and group II (small diameter)
innervated by group Ib afferent fibers
GTOs
what happens when a spindle is activated by stretch of a muscle?
muscle contraction
What happens when a GTO is stimulated by either a stretch or a contraction of a muscle
inhibition of muscle contraction
muscle spindle that is most responsive to muscle shortening?
group II
muscle spindle that is most responsive to muscle lengthening?
group Ia
afferent innervation of GTO?
group Ib
afferent innervation of muscle spindle?
Group Ia and II
efferent innervation of GTO?
none
Efferent innervation of muscle spindle
y fibers
Relationship to extrafusal fiber of GTO
series
relationship to extrafusal fiber of spindle
parallel
primary GTO stimulus
contraction (tension)
reflex response of GTO
inhibition of extrafusal fibers
reflex response of spindle
contraction of extrafusal fibers
innervates intrafusal fibers
gamma (y) motor system
mesencephalic area appears to regulate rhythmic gate in what structure?
reticular formation
What system fascilitates gamma motor neuron antigravity control?
vestibular system
Over skeletal muscle, sensory afferent activating gamma motor neurons
cutaneous sensory receptors
increased signal strength transmitted by progressively greater number of fibers
spatial summation
increased signal strength by increased frequency of firing with the same number of fibers
temporal summation
