Sensory Systems And Somatosensation

Front 1

Sensory receptors

Back 1

Referring to a specialized epithelial (in tissue detecting stimulus) or neural cell which transduces sensory stimuli. Applying term to cell itself, don't confuse with receptor proteins.

Front 2

Afferent fibers

Back 2

Fibers that send signals to the CNS

Front 3

Efferent fibers

Back 3

Fibers that send signals out from the CNS. Ex. motor neurons.

Front 4

Classification of sensory receptors

Back 4

Classified by the stimulus they detect.
Mechanoreceptors, photoreceptors, chemoreceptors, thermoreceptors, nociceptors.

Front 5

Sensory transduction

Back 5

Describes the process by which sensory receptors convert sensory/stimulus energy to membrane potentials, via ion channels.

Front 6

How does sensory transduction happen?

Back 6

Accomplished by sensory receptor ion channels which gate in response to the stimulus energy. These ion channels do not always DIRECTLY gate in response to stimuli, could be GPCR, etc.

Front 7

Receptor potential

Back 7

Describes the change in potential that occurs when a stimulus is transduced by a sensory receptor.

Front 8

Sensory transmission to CNS

Back 8

You don't always expect a receptor potential to result in transmission of a sensory signal to the CNS. Have to have a sufficient enough receptor potential created that would reach threshold level in order for an action potential to occur and transmission to CNS to happen.

Front 9

Sensory encoding

Back 9

A way to have different receptors dedicated to different stimuli. Modality, location, intensity, duration.

Front 10

Modality

Back 10

Labeled lines. Using different nerve fibers to transmit different modalities to the brain (ex. neuron dedicated to vibration NOT pressure)

Front 11

Location

Back 11

Dedicated neurons, "neural maps", or areas of the brain dedicated to one kind of stimulus. Enhanced by lateral inhibition.

Front 12

Intensity

Back 12

Firing rate (frequency of action potentials), number of receptors activated (really strong pressure in the skin might activate more receptors), different types of receptors (pressure so intense that nociceptors become involved).

Front 13

Duration

Back 13

Duration of firing for the sensory receptor, when it starts you perceive, when it stops you know the stimulus is no longer there.

Front 14

Sensory receptor adaptation

Back 14

Two major categories; phasic or tonic receptors.

Front 15

Phasic receptors

Back 15

Encode onset and offset, adapt quickly by rapidly reducing the firing rate after a stimulus is applied, even if stimulus is continuing. Ex. Pacinian corpuscles detecting vibration (oscillating very quickly).

Front 16

Tonic receptors

Back 16

Encode intensity and duration, adapt slowly by reducing the firing rate only marginally as a stimulus is applied. Ex. Merkel's receptors detecting steady pressure.

Front 17

Receptive fields

Back 17

Area of the body that, when stimulated, results in a change in firing rate or a sensory neuron. Small and large receptive fields.

Front 18

First/Second...etc Order neurons

Back 18

First-order neuron is the sensory receptor itself, synapses in a relay nucleus (in spine or brainstem) with second-order neurons.

Front 19

Small receptive fields

Back 19

One first order neuron synapses with one second order neuron. Allows advantage of discriminating location of sensation, ex. in fingertips and lips.

Front 20

Large receptive fields

Back 20

Many first order neurons synapse with one second order neuron. More conservative design, for areas where discrimination of location isn't so important, ex. skin on back.

Front 21

Lateral inhibition

Back 21

Enhances the ability to localize a stimulus, inhibitory receptive fields surround excitatory receptive fields. All converge on a common second-order neuron/afferent. The amount of 2nd order afferent firing depends on balance of excitatory vs. inhibitory input.

Front 22

Inhibitory interneuron

Back 22

First order neuron in inhibitory field generates action potential but does not send it directly to the 2nd order, it sends it to an inhibitory interneuron. The inhibitory interneuron is depolarized, generates an action potential, releases inhibitory NT onto 2nd order, decreasing the likelihood of an action potential at the 2nd order neuron.

Front 23

Mechanoreceptors

Back 23

Touch sensation, most occur in dermal layer. Nerve endings often encapsulated. Pacinian corpuscles (vibrations), Meissner's corpuscles (2pt discrimination in fingertips), hair-follice receptors (velocity and direction of movement), Ruffini's corpuscles (skin stretch and joint rotation), Merkel's corpuscles (vertical indentations).

Front 24

Thermoreceptors

Back 24

Have free, unencapsulated nerve endings, have ion channels that detect temp changes. Two major classes; cold and warm with overlapping activity.

Front 25

Temp ranges

Back 25

Around ~36 degrees that activity switches b/t the two, cold activity goes down, warm goes up.
If temp gets really high, warm receptors stop firing, but nociceptors start firing, that's why really hot temps are perceived as painful.

Front 26

Transient receptor potential vanilloid receptors (TRPVs)

Back 26

Ion channels that respond to varying stimuli, including temp.
TRPV 1 - responds to warm temp & vanilloid compounds (ex. capsacin in chili peppers)

Front 27

Nociceptors

Back 27

Have free nerve endings that respond to potentially damaging (noxious) stimuli. Different types; mechanical and polymodal.

Front 28

Mechanical nociceptors

Back 28

Ex. a thorn, a needle stick. Sharp pain, fibers of medium diameter and have some myelination. Called A-delta fibers, very fast.

Front 29

Polymodal nociceptors

Back 29

Detect variety, mechanical, chemical, temp. Fibers are narrower and unmyelinated, called C fibers, are slower at transmitting action potentials. Associated with longer duration, throbbing pain.

Front 30

Nociceptors and inflammation

Back 30

When you have tissue damages, inflammatory substances are released that activate nociceptors, generate action potentials, causing pain.

Front 31

Hyperalgesia

Back 31

After inflammation, nociceptor fibers are more sensitive and normally non-painful stimuli can become painful.

Front 32

Dorsal column pathway

Back 32

Transmits discriminative touch, pressure, vibration, and proprioceptive information.
First-order afferent enters dorsal spine, ascends in the dorsal column, and synapses with second-order afferent in the medulla (nucleus gracilis or nucleus cuneatus). The second-order afferent crosses to the contralateral side, ascends, and synapses with a third-order neuron in the thalamus. The third-order afferent ascends and synapses with a fourth-order afferent in the somatosensory cortex.

Front 33

Anterolateral pathway

Back 33

Transmits pain, temperature, and light touch information.
First-order afferent enters dorsal spine and synapses with second-order afferent in the dorsal horn. The second-order afferent crosses to the contralateral side of the spine, ascends in the anterolateral column, and synapses with a third-order neuron in the thalamus. The third-order afferent ascends and synapses with a fourth-order afferent in the somatosensory cortex.

Front 34

Localizing a spinal lesion

Back 34

If you cut all the fibers on the L side of the spine, pt will have reduced sensation of temp and pain on R side, but on ipsilateral side pt will have reduced sensation of 2pt discrimination, vibration, and proprioception. Work through picture.

Front 35

Somatosensory cortex

Back 35

Found just behind the central gyrus (postcentral gyrus), includes primary (S1), secondary (S2), and association areas.

Front 36

Primary somatosensory cortex (S1)

Back 36

Involved in direct perception of somatosensory stimuli. There is a disproportionate dedication of tissue to different areas of the body.

Front 37

Secondary somatosensory cortex (S2)

Back 37

Involved in processing of stimuli, more neural connectivity.

Front 38

Association areas of cortex

Back 38

Very high order processing, doing things like recognizing what is causing stimulus, etc.

Front 39

Referred pain

Back 39

Describes the sensation of pain on the body surface as a result of a noxious stimulus w/n an organ of the body. This occurs b/c a nociceptor innervating an organ may contact the same 2nd order afferent in the spine as a nociceptor innervating some location on the skin.