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85 Cards in this Set

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What three steps are shared by all of the senses?
1) physical stimulus
2) set of steps transforming the stimulus into nerve impulses
3) response to this signal in the form of perception or conscious experience of sensation
What are the four attributes of a stimulus that yield a sensation?
Modality
Location
Intensity
Timing
What is meant by "modality of a stimulus"?
A general class of stimulus, determined by the type of energy transmitted by the stimulus and the receptors perceiving the stimulus.
What are the 3 components of a sensory system?
Sensory receptor
Central pathways
Target areas in the brain
How is the location of a stimulus indicated to the CNS?
The CNS recognizes the set of sensory receptors that are active, against their topographical distribution.
What are the 10 recognized sensory modalities?
Vision
Hearing
Balance
Taste
Smell
Touch
Proprioception
Temperature
Pain
Itch
What is a sensory receptor?
a cell that transforms the stimulus energy into electrical energy
What is a receptor potential?
the electrical signal produced by a sensory receptor
What is stimulus transduction?
the process by which stimulus energy is converted into an electrical signal
What is receptor specificity?
a property that means the receptor is optimally selective for a single stimulus
What is an adequate stimulus?
the unique stimulus that activates a receptor at a low energy level
What is a labeled line code?
receptor specificity means that the receptor's axon is a modality-specific line of communication to the CNS
What are the four classes of sensory receptor in humans? Give an example or two of each.
Mechanical: proprioceptors, mechanoreceptors
Chemical: chemoreceptors
Thermal: thermoreceptors
Electromagnetic: photoreceptors.
How does the depolarizing receptor potential compare to the excitatory postsynaptic potential?
The mechanism is similar: the stimulus energy opens channels sensitive to that energy which increases conductance of the channel's ion. Thus, greater intensity or longer duration results in higher amplitude depolarization.
What are examples of submodalities of the different classes of receptors?
Taste: sweet, sour, salty, bitter
Vision: color, shape, movement
Touch: temperature, texture, rigidity
Why are receptors said to be "tuned" to an adequate stimulus?
A receptor responds optimally to a narrow range of intensities of a single type of energy.
What happens when a receptor is activated by a strong stimulus of a type to which it is normally not sensitive? For example, when someone receives a blow to the eye.
The photoreceptors of the retina (in this case) will perceive the strong physical stimulus as a flash of light rather than a mechanical stimulus because of the labeled line coding.
What is meant by "the receptive field of a receptor"?
It is the area within the receptive sheet where stimulation excites the cell, or the area innervated by a receptor cell.
What is the relationship between receptor density and spatial resolution in the somatosensory and visual systems?
The greater the receptor density, the higher the level of resolution possible. The fingertips of the somatosensory system have high density receptors and spatial discrimination is acute. The fovea of the retina has the greatest density of photoreceptors and is where vision is most acute.
What is the principle underlying the spatial distribution of receptors for hearing, taste, and smell?
The receptors for hearing are organized based on the spectrum of sound, not the location of sound (which is determined through analysis of input from both ears).
The receptors for taste and smell are located on different parts of the nose and tongue based on chemical sensitivity.
What is the relationship between stimulus intensity, the amplitude of the receptor potential, and sensory neuron action potential firing frequency?
Stronger stimuli evoke larger amplitude receptor potentials and generate higher frequency action potential firing of the sensory neuron.
What is another way, besides stimulus intensity, that the CNS is alerted to the size/strength of a stimulus?
The number of receptors activated or the size of the responding receptor population.
What is meant by "receptor adaptation"?
When a stimulus persists without change of position or amplitude, the intensity of the signal diminishes
What is the difference between slowly and rapidly adapting receptors to a stimulus?
Slowly adapting receptors signal stimulus magnitude almost continuously for minutes.
Rapidly adapting receptors are active only when the stimulus intensity is increasing or decreasing.
Describe the serial organization of sensory pathways.
The receptors project to first order sensory neurons, which project to second order sensory neurons, and so forth to higher-order neurons. There is a pattern of convergence, and relay nuclei preprocess the information that is forwarded to the cortex.
What is meant by "the receptive fields of higher order neurons of a sensory pathway"?
The receptive field of higher order neurons is the receptive field of all the sensory receptors that converge upon it. Successively higher level receptive fields are larger and more complex than lower levels.
How do feed-forward and feed-back inhibition within a sensory relay nucleus sharpen the contrast between stimuli?
Feed-forward inhibition is the inactivation of adjacent afferent neurons by the most active afferent neuron.
Feed-back inhibition is similar but the most active relay neuron sends a recurrent collateral axon to cause an inhibitory effect on adjacent relay signals. This sharpens the signal by reducing adjacent "noise".
What happens in distal inhibition?
Inhibitory neurons are activated from distant sites, like the cortex, to control the flow of information from the relay neurons. This type of inhibition is different from the feed-forward and feedback because it is not related to the intensity of the sensory-evoked response.
What are the four major modalities of somatic sensibility?
Discriminative touch: recognizes size, shape, and texture of objects and their movements on skin
Proprioception: recognizes static position and movement of limbs and body
Nociception: recognizes tissue damage or chemical irritation (pain and itch)
Temperature sense: recognizes warm or cold
What is the common class of sensory neurons that convey information about somatic sensibility?
Dorsal root ganglion neurons.
What is the morphology of the dorsal root ganglia (DRG) neurons?
They are pseudo-unipolar cells with the soma in the DRG and an axon projection peripherally to terminate in a sensory receptor and centrally to terminate at a relay with a second-order sensory neuron or interneuron.
Are A and C fibers myelinated?
A: Yes
C: No
What are the axon diameters of the various A fibers and the C fiber?
Aalpha: 12-20 um
Abeta: 6-12 um
Adelta: 1-6 um
C: 0.2-1.5 um
What are the velocities of the A fibers and C fiber?
Aalpha: 72-120 m/s
Abeta: 36-72 m/s
Adelta: 4-36 m/s
C: 0.4-2.0 m/s
What part of a DRG neuron determines its sensitivity to a particular stimulus?
the properties of the nerve terminal
What are the two types of peripheral endings of DRG neurons?
1) bare ending
2) encapsulation by a non-neural structure
What are the two classes of somatic sensation?
Epicritic sensations
Protopathic sensations
What is an epicritic sensation?
fine aspects of touch, mediated by encapsulated receptors
What are the 4 types of epicritic sensation?
1) Topognosis: detection of gentle contact of the skin and localization of the position touch
2) Determination of vibratory sensation including frequency and amplitude
3) Resolution of spatial detail like texture and two-point discrimination
4) Stereognosis: recognition of object shape grasped in hand.
What is protopathic sensation?
includes pain and temperature sensation as well as itch and tickle; sensations are mediated by bare nerve endings
What is glabrous skin?
hairless, and characterized by a regular array of ridges formed by folds of the epidermis containing a dense matrix of mechanoreceptors
What are the 4 major types of mechanoreceptors?
Meissner's corpuscles
Merkel discs
Pacinian corpuscles
Ruffini ending
What kind of stimulus do each of the 4 mechanoreceptors detect?
MC: stroke, flutter
MD: pressure, touch
PC: vibration
RE: stretch
What fiber types are associated with the 4 mechanoreceptors?
Aalpha and Abeta
Of the 4 mechanoreceptors, which are superficial and which are deep?
MC and MD: superficial
PC and RE: deep
Of the 4 mechanoreceptors, which are slow adapting and which are rapid adapting?
MC and PC: rapid
MD and RE: slow
Of the 4 mechanoreceptors, which have small receptive fields and which have large receptive fields?
MC and MD: small
PC and RE: large
What is meant by "two-point discrimination"?
The ability to distinguish the spacing of two points touched simultaneously.
Where are the thresholds for two-point discrimination greatest and smallest on the body?
Smallest threshold (most sensitive): fingertips, face, lips
Greatest threshold (least sensitive): back, limbs, and belly
What is the relationship between the two-point discrimination of a particular region and touch receptor density or receptive field size?
The touch receptor density is greatest, and the the receptive field size smallest, in areas of acute two-point discrimination.
What is the range of temperatures that activate cold and warm receptors?
Cold: 5-40 degrees centigrade
Warm: 29-45 degrees centigrade
What kind of primary afferent is associated with cold and warm receptors (not including thermal nociceptors)?
C fibers
Describe the activity of cold and warm receptors at normal (34 degrees centigrade) skin temperatures.
The cold and warm receptors are active; firing action potentials are at a slow, steady rate.
How does the firing rate of cold receptors change when the temperature of the skin is cooled from 40 to 5 degrees centigrade?
The rate of firing will gradually increase until the temperature reaches 25, then the rate of firing will gradually decrease.
How does the firing rate of warm receptors change when the temperature of the skin is warmed from 29 to 50 degrees centigrade?
The rate of firing increases gradually until the temperature reaches 45, then rapidly decreases to a rate of zero.
What type of stimulus activates nociceptors?
Stimuli that can damage tissue
How does direct activation of nociceptors differ from indirect activation of nociceptors?
Direct activation is caused by some noxious stimuli, while indirect activation is caused by chemicals released from traumitized tissues.
What are some substances that can excite nociceptors indirectly?
Histamine
K+ from injured cells
Bradykinin
Substance P and other related peptides
Acidity
ATP
Serotonin
ACh
What are the three types of nociceptors?
Mechanical nociceptors
Thermal nociceptors
Polymodal nociceptors
What activates mechanical nociceptors? What primary afferent fiber is associated with them?
Strong tactile stimulation, such as a pinch
Adelta primary afferent fibers
What activates thermal nociceptors? What primary afferent fiber is associated with them?
Extremes of temperature and strong mechanical stimuli
Adelta (burning pain) and C (freezing pain) primary afferent fibers
What activates polymodal nociceptors? What primary afferent fiber is associated with them?
They respond to a variety of mechanical, thermal, and chemical stimuli.
C primary afferent fibers
What temperature thresholds activate hot and cold nociceptors?
Above 45 and below 5 degrees centigrade
What is proprioception?
The sense of position and movement of one's own body and limbs without using vision.
What are the two types of proprioception?
Limb-position sense (senses stationary position of the limbs)
Kinesthesia (senses movement of the limbs)
What are the three types of mechanoreceptors in muscles and joints that provide information used for proprioception? Where is each found?
1) Muscle spindle fibers are found within muscle and sense muscle stretch
2) Golgi tendon organs are found in the tendon and sense tendon tension
3) Joint capsule receptors sense join flexion or extension
What receptors, other than those found in muscles and joints, provide information about proprioception?
Stretch-sensitive receptors (RE, MC, field receptors)
Cutaneous proprioception (important for lip movements and facial expression)
What is the contribution of each type of primary afferent to the compound action potential?
Aalpha and Abeta generate the large initial spike
Adelta generate a smaller secondary spike (after 3 ms)
C fibers generate a small, delayed bump (after 38 ms)
How does the compound action potential change in diabetes or multiple sclerosis?
In diabetes, large diameter fibers degenerate. In MS, myelin on large fibers degenerates. Both cause a reduction in the compound action potential peak, a slowing of conduction velocity, and selective loss of sensation information carried by large sensory fibers.
What is a dermatome?
A superficial sensory area of skin innervated by a single dorsal root
Where are receptors that send information to each of the 4 Brodmann areas in S-I found?
BA 3b and BA 1 receive information from the skin
BA 3a and BA 2 receive proprioceptive information from the muscles and joints.
What is the direction of information transfer among the Brodmann areas of S-I?
The thalamus projects primarily to BA 3a and BA 3b, but also to BA 1 and BA 2.
BA 3a and BA 3b project to BA 1 and BA 2.
Which BAs in S-1 transmit information to S-II and the posterior parietal cortex?
All four BAs project to S-II and the parietal cortex.
What is the function of the connection between S-II and the insular lobe/temporal lobe?
It is believed to be important for tactile memory.
What types of information are integrated into BA 5 and BA 7 by the posterior parietal cortex?
BA 5 integrates tactile information from mechanoreceptors in the skin with proprioceptive information from underlying muscles and joints. It also integrates information from the two hands.
BA 7 integrates stereognostic information with visual information.
What is the connection between the posterior parietal cortex and the motor areas of the frontal lobe?
The posterior parietal cortex projects to the motor areas of the frontal lobe and plays an important role in sensory initiation and guidance of movement.
How does the size of the receptive field of somatic sensory cortical neurons change as you go from BA 3b to BA 1 to BA 2 to BA 5?
The receptive field gets progressively larger, from one finger tip in BA 3b, to four fingertips in BA 1, to four fingers in BA 2, to eight fingers in BA 5. This occurs due to the convergence of several sensory afferents, causing receptive field enlargement.
What is the basic functional unit of the cortex?
the cortical column
Describe the circuitry within a functional unit of the somatic sensory cortex.
Thalamic afferents terminate mainly on stellate cell neurons in layer IV. The stellate cell axons project vertically toward the surface of the cortex and relay the thalamocortical input to the parallel apical dendrites and axons of adjacent pyramidal cells (somas in layers II, III, and V). The axons of pyramidal cells project to deeper layers of the cortex, other cortical regions, and subcortical regions. Layer IV also projects back to the thalamus.
What two qualities are shared by the neurons within a cortical column of the somatic sensory cortex?
Location and modality
What is the relationship between the innervation density of an area of the body and the size of the region of the somatic sensory cortex that is devoted to processing information from that area?
The areas with greater innervation density on the body are represented by relatively larger areas of the somatic sensory cortex devoted to them.
Compare the size of the areas devoted to processing somatosensory information from the lips, back, thigh, hand, etc.
The area devoted to the lips is much larger than the area dedicated to the trunk. Likewise for fingers and toes.
How can the somatotopic map of the somatic sensory cortex be changed by the degree to which a part of the body is used?
Experience and repetitive usage can develop a larger proportion of cortical neurons devoted to sensory inputs from those areas.
How does lateral inhibition enhance two-point discrimination?
Each receptive field sends inhibitory signals to surrounding areas. When two nearby receptive fields are being activated by two stimuli, their inhibitory zones overlap and summate. This enhances the separation of the two active populations.
What results in tabes dorsalis syndrome? What lesion causes it? What are the consequences for somatosensory function?
Tabes dorsalis syndrome is a late consequence of syphilis. It entails the destruction of the large-diameter neurons in the DRG. This causes degeneration of myelinated afferent fibers in the dorsal column. The result includes severe deficits in touch and position sense, but little loss of temperature and nociception.