Sensory Physiology

Question 1

Peripheral nerves are classified by what 2 schemes?

Answer 1

1. Contribution to a compound AP recorded from an entire mixed peripheral nerve (A, B, C waves)
2. 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]

Question 2

Sensory afferent fiber types

Answer 2

A-alpha
A-beta
A-delta

C

Question 3

Which sensory afferent fiber type has largest diameter vs. smallest? How does this correlate with conduction velocity?

Answer 3

A-alpha = largest, meaning fastest conduction velocity

C fibers = smallest, meaning slowest conduction velocity

Question 4

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

Answer 4

Generator potential

Question 5

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.

Answer 5

Olfactory; thalamus

Question 6

What is the weber-fechner law

Answer 6

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]

Question 7

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?

Answer 7

Muscle — perceived intensity matches the actual intensity very closely

Cutaneous — perceived intensity may diverge from the actual intensity substantially

Question 8

_____ _____ = when a stimulus persists unchanged for several minutes without a change in position or amplitude, the neural response diminishes and sensation is lost

Answer 8

Receptor adaptation

Question 9

Difference between slowly adapting and rapidly adapting receptors

Answer 9

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

Question 10

What is the most rapidly adapting mechanoreceptor?

Answer 10

Pacinian corpuscles

Question 11

Are meissners corpuscles considered rapidly adapting or slowly adapting mechanoreceptors?

Answer 11

Rapidly-adapting

Question 12

What sensation is produced by microstimulation of Meissner’s corpuscles, and what is their relative receptive field size?

Answer 12

Sensation = tap, flutter

Receptive field size = small

Question 13

What sensation is produced by microstimulation of pacinian corpuscles, and what is their relative receptive field size?

Answer 13

Sensation = Vibration

Receptive field size = large

Question 14

_____ _____ refers to areas where individual mechanoreceptor fibers convey info from a limited area of skin

Answer 14

Receptive fields

Question 15

Where is tactile acuity (i.e., ability for 2-point discrimination) the highest vs. lowest?

Answer 15

Highest = fingertips and lips

Lowest = calf, back, and thigh

Question 16

It is not just the size of the receptive field that determines 2-point discrimination ability. What other physiological property is important?

Answer 16

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

Question 17

Primary afferent neurotransmission is controlled by pre and postsynaptic inhibitory mechanisms. Which one is probably the more powerful form? How does it work?

Answer 17

Presynaptic inhibition

Diminishes the excitatory signal by:
-GABAergic associated influx of Cl into axon —> hyperpolarization —> less Ca enters cytosol —> less NT release

Question 18

Reduced NT release from the pre-synaptic terminal via pre-synaptic inhibition serves what purpose in neurotransmission?

Answer 18

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

Question 19

3 Basic functions of cortical processing of sensory input

Answer 19

1. Initial processing of the signal
2. Integration of the initial processing into larger schemes
3. Emotional response to the processing

Question 20

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?

Answer 20

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

Question 21

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

Answer 21

III, IV

Pyramidal

Question 22

Describe columnar organization of the cortex

Answer 22

Neurons stacked above and below each other are fundamentally similar

Neuronal columns side by side are significantly different

Question 23

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

Answer 23

IV

Question 24

Main function of primary somatosensory cortex (SI)

Answer 24

Integration of information for position sense as well as size,shape discrimination

Question 25

Main function of secondary somatosensory cortex (S2)

Answer 25

Comparisons between objects, different tactile sensations, and determining whether something becomes a memory

Question 26

Functions of parieto-temporal-occipital sensory association cortex

Answer 26

High level interpretation of sensory inputs

Receives input from multiple sensory areas

Analyzes spatial coordinates of self in environment

Names objects

Question 27

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

Answer 27

Thalamus; focusing

Question 28

What are cortico-cortical projections?

Answer 28

They link primary and association areas of the sensory cortex — establishing parallel paths of sensation

Allows for simultaneous processing of multiple sensations

Question 29

What are corticofugal signals?

Answer 29

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

Question 30

What is the doctrine of specific nerve energies

Answer 30

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

Question 31

What is the law of projections

Answer 31

No matter where along the afferent pathway is stimulated, the perceived sensation arises from the origin of the sensation

Question 32

Difference between pain and nociception

Answer 32

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

Question 33

Pain characterization by nociceptor modality

Answer 33

Thermal
Mechanical
Chemical

[many subtypes of each, can be polymodal, silent nociceptors are related to phenomenon of phenotype switching]

Question 34

Describe axons of nociceptor neurons

Answer 34

Axons tend to have C fibers or A-delta fibers with free nerve endings

Question 35

Free nerve endings can be further characterized into what 2 categories?

Answer 35

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)

Question 36

Receptor types for sensing noxious stimuli

Answer 36

TRP receptors = ligand-gated non-selective cation channels permeable to Ca, Na, and/or K

[MOST involve Ca++]

Question 37

The most important TRP receptors are TRPV1, TRPA1, and TRPM8. What are these receptors sensitive to?

Answer 37

TRPV1 = capsaicin

TRPA1 = allyl isothiocyanate

TRPM8 = menthol

Question 38

Besides TRP receptors, what are other signaling modalities for sensing noxious stimuli?

Answer 38

Nav1.7 = mechanosensitive sodium channel

P2X = receptor activated by ATP

ASIC = activated by H+

SP and CGRP

Histamine

Kinins (bradykinin)

Question 39

2 types of mutations involving Nav1.7, the mechanosensitive sodium channel that senses noxious stimuli

Answer 39

One type of mutation results in pain insensitive individuals (rare)

Other mutation results in paroxysmal extreme pain disorder

Question 40

____ type fibers utilize EAAs and SP/CGRP

_____ type fibers utilize EAAs

Answer 40

C

A-delta

Question 41

The EAAs utilized by C and A-delta fibers bind to ______ receptors

Answer 41

Non-NMDA (e.g., AMPA)

IMPORTANT

Question 42

Gate control theory of pain

Answer 42

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)

Question 43

Mechanism of descending inhibition to modulate nociception

Answer 43

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

Question 44

Descending serotonergic and noradrenergic neurons activate ________ and suppress _______

Answer 44

Local interneurons; spinothalamic projection neurons

Question 45

Difference between dorsal columns, spinothalamic tract, and spinoreticulothalamic system in terms of sensory modality

Answer 45

Dorsal columns = proprioceptive and discriminative (fine touch)

Spinothalamic tract = thermal, nociceptive, and crude touch

Spinoreticulothalamic system = nociceptive

Question 46

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

Answer 46

Insular

Question 47

Damage to the insular cortex causes _______, which is a condition in which pain is experienced without unpleasantness

Answer 47

Asymbolia

Question 48

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

Answer 48

Amygdala

Hypothalamus; medulla

Question 49

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?

Answer 49

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

Question 50

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

Answer 50

Dorsal horn

Antidromic

Question 51

What is antidromic signaling?

Answer 51

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