Sensory Physiology Flashcards
Peripheral nerves are classified by what 2 schemes?
- Contribution to a compound AP recorded from an entire mixed peripheral nerve (A, B, C waves)
- Fiber diameter, myelin thickness, and conduction velocity (class I, II, III, and IV)
[the two schemes are related because the conduction velocity determines a fiber’s contribution to a compound AP]
Sensory afferent fiber types
A-alpha
A-beta
A-delta
C
Which sensory afferent fiber type has largest diameter vs. smallest? How does this correlate with conduction velocity?
A-alpha = largest, meaning fastest conduction velocity
C fibers = smallest, meaning slowest conduction velocity
An appropriate stimulus applied to a somatosensory receptor produces a _____ _____ that, when large enough, leads to action potentials that can be carried over a considerable distance into the CNS
Generator potential
Receptors are the sites of convergence and divergence. information from all sensory systems except ______ are relayed through the ______ on its way to the cerebral cortex.
Olfactory; thalamus
What is the weber-fechner law
The number of active receptors increases with increased intensity of the stimulus
[so the greater the intensity of the stimulus, the easier it is to interpret differences in stimuli]
The weber-fechner law applies to various types of stimuli such as weight, muscle, cutaneous, sound, and vision.
What is the difference between muscle and cutaneously sensed stimuli in terms of perceived intensity?
Muscle — perceived intensity matches the actual intensity very closely
Cutaneous — perceived intensity may diverge from the actual intensity substantially
_____ _____ = when a stimulus persists unchanged for several minutes without a change in position or amplitude, the neural response diminishes and sensation is lost
Receptor adaptation
Difference between slowly adapting and rapidly adapting receptors
Slowly adapting = receptors that respond to prolonged and constant stimulation
Rapidly adapting = receptors that respond only at the beginning or end of a stimulus; only active when the stimulus intensity increases or decreases
What is the most rapidly adapting mechanoreceptor?
Pacinian corpuscles
Are meissners corpuscles considered rapidly adapting or slowly adapting mechanoreceptors?
Rapidly-adapting
What sensation is produced by microstimulation of Meissner’s corpuscles, and what is their relative receptive field size?
Sensation = tap, flutter
Receptive field size = small
What sensation is produced by microstimulation of pacinian corpuscles, and what is their relative receptive field size?
Sensation = Vibration
Receptive field size = large
_____ _____ refers to areas where individual mechanoreceptor fibers convey info from a limited area of skin
Receptive fields
Where is tactile acuity (i.e., ability for 2-point discrimination) the highest vs. lowest?
Highest = fingertips and lips
Lowest = calf, back, and thigh
It is not just the size of the receptive field that determines 2-point discrimination ability. What other physiological property is important?
Degree of convergence/divergence
—many primary sensory neurons converging onto a single secondary neuron creates a very large secondary receptive field. The two stimuli will be perceived as a single point because both stimuli fall within the same receptive field. When fewer primary neurons converge, secondary receptive fields are much smaller. The two stimuli activate separate pathways and are percieved as distinct stimuli
Primary afferent neurotransmission is controlled by pre and postsynaptic inhibitory mechanisms. Which one is probably the more powerful form? How does it work?
Presynaptic inhibition
Diminishes the excitatory signal by:
-GABAergic associated influx of Cl into axon —> hyperpolarization —> less Ca enters cytosol —> less NT release
Reduced NT release from the pre-synaptic terminal via pre-synaptic inhibition serves what purpose in neurotransmission?
Improves the brain’s ability to localize the signal
—this occurs because the primary neuron (the one most stimulated at the mechanoreceptor) will inhibit its neighbors; this process can repeat all the way to the cortex, further narrowing the signal
3 Basic functions of cortical processing of sensory input
- Initial processing of the signal
- Integration of the initial processing into larger schemes
- Emotional response to the processing
Cortical responses to sensory input occur in part d/t response mapping (i.e., sensory homunculus) however these somatotopic maps are not fixed. How might the map change?
Continual input to one part of the somatosensory system can magnify its corresponding map
Restricted input from a body surface decreases the portion of the cortex devoted to that region
The cortex has 6 layers, consisting of input projections as well as ascending and descending output projections.
Layers _____ and _____ are enlarged in the primary sensory cortex and are the main site of termination of axons from the thalamus
The main output neurons are the ____ cells
III, IV
Pyramidal
Describe columnar organization of the cortex
Neurons stacked above and below each other are fundamentally similar
Neuronal columns side by side are significantly different
Each column deals with one sensory modality in one part of the body. Sensory info arrives at its column in layer ____ via the thalamus. Neighboring columns receive sensory info from the same part of the body but a different sensory modality
IV
Main function of primary somatosensory cortex (SI)
Integration of information for position sense as well as size,shape discrimination
Main function of secondary somatosensory cortex (S2)
Comparisons between objects, different tactile sensations, and determining whether something becomes a memory
Functions of parieto-temporal-occipital sensory association cortex
High level interpretation of sensory inputs
Receives input from multiple sensory areas
Analyzes spatial coordinates of self in environment
Names objects
Primary sensory cortex sends projections back down to subcortical structures, most often back to the ______. Descending corticothalamic axons are present in greater number than ascending thalamocortical axons. This permits ______ activities
Thalamus; focusing
What are cortico-cortical projections?
They link primary and association areas of the sensory cortex — establishing parallel paths of sensation
Allows for simultaneous processing of multiple sensations
What are corticofugal signals?
Transmitte back from cortex to lower relay stations in thalamus, medulla, or SC
Controls the intensity of sensory sensitivity
TYPICALLY INHIBITORY AND SUPPRESSES SENSORY INPUT
What is the doctrine of specific nerve energies
No matter where along the afferent pathway is stimulated, the sensation that will occur is determine by the nature of the sensory receptor in the periphery connected to that pathway
What is the law of projections
No matter where along the afferent pathway is stimulated, the perceived sensation arises from the origin of the sensation
Difference between pain and nociception
Pain = unpleasant sensory and emotional experience associated with actual or potential tissue damage
Nociception = neural process of encoding noxious stimuli (stimulus that is damaging or threatens damage to tissues)
NOT ALL NOCICEPTIVE STIMULI ARE INTERPRETED AS PAIN
Pain characterization by nociceptor modality
Thermal
Mechanical
Chemical
[many subtypes of each, can be polymodal, silent nociceptors are related to phenomenon of phenotype switching]
Describe axons of nociceptor neurons
Axons tend to have C fibers or A-delta fibers with free nerve endings
Free nerve endings can be further characterized into what 2 categories?
Peptidergic = expresses neuropeptides like substance P and CGRP. Most visceral afferents and half of cutaneous afferents (chronic inflammation and visceral pain)
Non-peptidergic = does not express neuropeptides. Other half of cutaneous afferents (diabetic neuropathy)
Receptor types for sensing noxious stimuli
TRP receptors = ligand-gated non-selective cation channels permeable to Ca, Na, and/or K
[MOST involve Ca++]
The most important TRP receptors are TRPV1, TRPA1, and TRPM8. What are these receptors sensitive to?
TRPV1 = capsaicin
TRPA1 = allyl isothiocyanate
TRPM8 = menthol
Besides TRP receptors, what are other signaling modalities for sensing noxious stimuli?
Nav1.7 = mechanosensitive sodium channel
P2X = receptor activated by ATP
ASIC = activated by H+
SP and CGRP
Histamine
Kinins (bradykinin)
2 types of mutations involving Nav1.7, the mechanosensitive sodium channel that senses noxious stimuli
One type of mutation results in pain insensitive individuals (rare)
Other mutation results in paroxysmal extreme pain disorder
____ type fibers utilize EAAs and SP/CGRP
_____ type fibers utilize EAAs
C
A-delta
The EAAs utilized by C and A-delta fibers bind to ______ receptors
Non-NMDA (e.g., AMPA)
IMPORTANT
Gate control theory of pain
In the absence of input from C fibers, tonically active inhibitory interneuron suppresses the pain pathway
With strong pain, C fiber stops inhibition of the pathway, allowing a strong signal to be sent to the brain
Pain can be modulated by simultaneous somatosensory input (i.e., rubbing the spot that hurts to ease the pain)
Mechanism of descending inhibition to modulate nociception
Periaqueductal gray activated by opiates, EAA, and cannabinoids
Descending projections travel to locus ceruleous (NE) and raphe nucleus (5HT)
NE and 5HT released into dorsal horn and activate inhibitory neurons
Local inhibitory interneurons release opiates like enkephalin
Opiates activate mu receptors on presynaptic and postsynaptic terminals of C fiber
Results in reduction of SP from C-fiber and reduces nociception
Descending serotonergic and noradrenergic neurons activate ________ and suppress _______
Local interneurons; spinothalamic projection neurons
Difference between dorsal columns, spinothalamic tract, and spinoreticulothalamic system in terms of sensory modality
Dorsal columns = proprioceptive and discriminative (fine touch)
Spinothalamic tract = thermal, nociceptive, and crude touch
Spinoreticulothalamic system = nociceptive
Nociception is widely distributed in the brain. SI and S2 do receive input from nociceptors and play a role in localization of pain. The ____ cortex is particularly important in interpretation of nociception because it processes info about the internal state of the body, conributes to autonomic response to pain, and integrates all signals related to pain
Insular
Damage to the insular cortex causes _______, which is a condition in which pain is experienced without unpleasantness
Asymbolia
Lesions in any single area in a pain pathway alters the experience of pain but does not abolish it completely.
The ____ is important in the emotional component of pain.
Visceral input travels with autonomic nerves to the _____ and ______, integrating physiological changes associated with visceral pain
Amygdala
Hypothalamus; medulla
It is important to understand that stimuli adequate to activate nociceptors are not the same for all tissues. What is the difference in terms of skin, joints, muscle, and viscera?
Skin = thermal, mechanical, and chemical. Superficial wounds. Fast pain (sharp) and slow pain (dull+achy)
Joints = mechanical and chemical. Usually dull and achy. Associated with muscle spasm
Muscle = mechanical and chemical. Injury or ischemia. Both fast and slow pain.
Visceral = mechanical and chemical. internal organs. Poor localization, sensitive to distension. Referred pain
Referred pain occurs because the brain requires some experience to localize pain, visceral pain is not experienced enough in early life to train the brain to localize it. Afferents converge in the ____ ____ of the SC.
______ signaling further diffuses visceral pain across multiple organs
Dorsal horn
Antidromic
What is antidromic signaling?
A noxious stimulus leads to APs in nociceptive fibers that propagate not only to the CNS but also antidromically into peripheral branches. These antidromic APs lead to the release of neuropeptides such as substance P. CGRP and NKA. These substances can stimulate epidermal and immune cells, or lead to vasodilation, plasma extravasation, and smooth muscle contraction
