Perception: The Somatosensory System

Front 1

The experience of touch can be broken down into what two features?

Back 1

localization of touch (objective) and what did i touch/ how did it feel (subjective). The fact that there are subjective and objective components to touch distinguishes it from other senses.

Front 2

Proprioception (3)

Back 2

-location of our limbs in space
- receptors are within muscles and joints
- Proprioceptor signals synapse onto neurons in the dorsal horn of the spinal cord, then (like mechanosensory signals) go up the dorsal column, and ultimately end up in the primary somatosensory cortex (primarily in area 3a; mechanosensory input goes primarily to area 3b).

Front 3

Theory of Receptor Specificity VS The Pattern Theory

Back 3

each type of receptor not only produces a particular touch sensation but also indicates where on the skin contact occurred ; The specificity of touch sensations was believed to come not from individual receptors but through the overall pattern of activity- different types

Front 4

Mechanoreceptors; three types

Back 4

the receptors that generate the highly specific touch signals, present throughout body, terminals of bipolar neurons
1) encapsulated- capsule surrounding nerve ending
2) sensory nerve works with accessory structure ie Merkel Disc
3) free nerve endings ! for thermal and pain

Front 5

Pacinian, Meissner and, Ruffini's corpuscles

Back 5

Encapsulated mechanoreceptors
Pacinian Corpuscles- vibration- large receptive field
Meissner- light touch- small receptive field
Ruffinis- pressure- large receptive field

Front 6

merkel disc

Back 6

mechanoreceptor for detection of light touch to skin, works with an accessory structure, small receptive field

Front 7

how do mechanoreceptors work?

Back 7

- there are three types (encapsulated, receptors with accessory structures like merkels disk and freenerve endings

- when you pressure skin, you get nerve impulses. nerves have mechanically gated ion channels and when you put pressure, this mechanically deforms membranes and this allows ions to flow in

Front 8

Dorsal root Ganglion (DRG) Neurons (3)

Back 8

-located right next to spinal cord, DRG neurons are capable of producing action potentials near the peripheral branch of the mechanoreceptors rather than at the axon hillock
- the peripheral branch of the bipolar neurons proceeds to skin as a free or encapsulated nerve ending and the central branch carries touch info to spinal cord
- the cell body is located in the DRG

Front 9

What determines the speed at which afferent fibres conduct action potentials

Back 9

the larger the diameter and the more myelinated the neuron the faster the nerve signal transmission
- free nerve endings (pain and temp) are associated with slower fibres

Front 10

what are the 4 types of fibres and describe them

Back 10

type Myelination Diameter speed
Αα Heavy 15-20um 70-170 m/s
Αβ medium 5-15um 40-70 m/s
Aδ Light 1-5um 10-35 m/s
C None .2-1.5um .5-2 m/s

Front 11

SA vs FA fibers

Back 11

slow adapting fibers fire continuously as long as stimulus is maintained (merkel and Ruffini), whereas fast adapting fibers' firing subsides quickly during continuous application (Meissner and Pacinian)

Front 12

Temporal Resolution

Back 12

a system with high temopral resolution can differentiate stimuli that are alternated rapidly. SA fibers have low temporal resolution and therefore are best able to transmit tactile info like steady pressure as opposed to vibration

Front 13

receptive fields of mechanoreceptors

Back 13

- receptive field refers to the area/amount of area of the skin that is covered by that receptor
-those located in superficial skin layer have small fields whereas those located deeper have larger ones

Front 14

Spatial Resolution

Back 14

the smaller the receptive field the greater the spatial resolution because the more bits of info can be captured per unit space. Receptors with small receptive fields like Merkel are good at seeing finer spatial differences.

Front 15

dorsal root vs ventral root (2)

Back 15

- dorsal afferent to spinal cord, ventral efferent.
- Cell bodies of somatosensory neurons are located in DRG and motor cell bodies are located in ventral horn of spinal cord

Front 16

Bell-magendie law

Back 16

how the dorsal and ventral roots have different functions but are part of the same fibre bundle

Front 17

Dorsal root Ganglion; Ventral Horn

Back 17

a collection of neurons in the dorsal root where the cell bodies of the somatosensory neurons are located. In the ventral root, these cell bodies are in the spinal cord itself, in an area called the ventral horn

Front 18

dermatome

Back 18

all somatosensory info is sent to CNS through spinal and cranial nerves. the entire body surface can therefore be divided into discrete areas represented by a single nerve. Each area is called a dermatome or a dermatomal map

Front 19

Describe the routes taken by different afferent fibres after they enter the spinal cord

Back 19

- large, myelinated fibers (Aα and Αβ) which carry tactile and proprioceptive signals, branch out soon after entering and either synapse at dorsal horn or go up the spinal cord through the dorsal column. they terminate in the DORSAL COLUN NUCLEI, which then cross over and enter the medial lemniscus of thalamus. this is called the DORSAL COLUMN-MEDIAL LEMNISCUS pathway
- small, non-myelineated fivers (Aδ and C) which carry pain and temp info synapse in the dorsal horn and then cross over the spinal cord. Once there, they enter fibre tracts that transmit to higher NS levels. These tracts are called the ANTEROLATERAL PATHWAY

Front 20

Modality- segregaiton

Back 20

because there are two pathways - dorsal column- medial lemniscus and anterolateral, this allows for the independent transmission of different types of touch signals which is important for richness and diversity of experience

Front 21

internal capsule

Back 21

the fibre bundle carrying the thalamic neurons from the Medial Lemniscus to the cerebral cortex.

Front 22

Somatotopic Representation

Back 22

neural processing in somatosensory system occurs in an orderly manner resulting in a spatial representation of the body within a neural structure

Front 23

S-1

Back 23

primary somatosensory cortex
- divided into 4 sections
- Hierarchical structure (heaviest input to 3a and 3b then they project to Areas 1,2)
- somatosensory map is contralateral and non-linear (each body part's representation is a different proportion ie. lips are big part)
- next to the primary motor cortex located in the precentral gyrus

Front 24

What does it mean to say that the neural connections between the somatosensory and motor cortices are homotopic?

Back 24

a given part of the motor cortes receives projections from the same part on the body map of the somatosensory cortex. This is important for somatosensory feedback which guides movements

Front 25

Esthesiometer

Back 25

a machine created by Max Von Frey that used horse hairs to measure touch intensity

Front 26

Two-Point Limen; law of outward mobility

Back 26

the smallest separation of two points applied simultaneously to the skin that can still be discriminated. Hands, fingers and face are very good at it.; the two-point limen increases 20 fold when you move from the shoulders to the fingers.

Front 27

what two factors affect the ability of a region to discriminate two different points of touch

Back 27

- Receptive field of mechanoreceptor- smaller it is, the greater the ability
- the density of the mechanoreceptor- the denser, the greater the ability

Front 28

compass test; Point localization Error Test

Back 28

tests developed by Weber. a) probe body parts to determine variations in threshold, b) where person had to identify where they were touched

Front 29

Hyperacuity

Back 29

the term given to spatial discrimination thresholds that are much lower than the traditional two-point limen- for example like determining whether two bars are more or less separated than another pair

Front 30

What factors affect the time adaptation takes to occur and why? (5)

Back 30

- the greater the area of stimulus, the faster the adaptation
- the greater the intensity, the slower the adaptation
- less sensitive areas of the body adapt quickest
- we also become less inclined to notice a stimulus if it is uninteresting or not dynamic enough
- Adaptation is not due to reduced electrical discharges by mechanoreceptors (even when you are not conscious something is stil touching you they are still firing), but something that happens more centrally: the CNS appears to stop “paying attention” to stimuli that stop changing

Front 31

Vibrotactile Stimulation

Back 31

a vibrating, tactile stimulus. vibrating produces different effects. the two-point limen for example is lower if object is or are vibrating.

Front 32

Duplex Theory; its modification

Back 32

two independent channels mediate the temporal nature of tough- one operated only at low frequencies and one only at high; a threshold stimulus is detected by the system most sensitive to it: i) Meissners corpuscles channel produces a sensation of flutter at low frequ. ii) Pacinian corpuscles produce sensation of vibration in high frequ. iii) Merkel's disc produces sensation of pressure of near-static stimuli, and iv) Ruffini receptors are also for pressure but can provide a buzz-like sensation as well

Front 33

physiological zero

Back 33

where we feel no hot or cold. very different from absolute zero. The greater the deviation from this mark the greater the feeling of cold/hot
- 30-36 degrees celcius

Front 34

Thermoreceptors (3)

Back 34

-unencapsulated nerve terminals.
-The warm receptors are C fibres (slow, unmyelinated) and the cold ones are Aδ fibres (quicker, lightly myelinated)
-slow changes in temperature produce little change in neural firing but quick ones do

Front 35

Spatial Summation

Back 35

the larger the area stimulated by temperature, the greater the sensation experience. Ie jumping into a cold lake vs just dipping toe in

Front 36

Opponent Process Τheory

Back 36

warmth and cold are different dimensions of the same sensory experience and adaptation to s temp merely shifts the null point on the continuum.

Front 37

Golgi tendon organ

Back 37

encapsulated structure located at the junction of muscle and tendon. free nerve fibres are intertwined around it and are A Alpha type. they are among the fastest and most heavily myelinated nerves in the PNS

Front 38

Corollary Discharge/ Efference Copy

Back 38

when the motor centre of brain produces a voluntary movement, a copy of the neural command is sent to the perceptual centres of the brain so that it knows what limb movements have just been initiated. This is also a useful way of comparing how well we made the actual movement with what movement we expected to make so that we can make adjustments to unexpected changes in real time.

Front 39

haptic perception

Back 39

the combination of tactile and kinesthetic sensory experiences- ie when manipulating an object in you hands. Active expolration helps us because it engages attention and adds additional info that is useful for deducing 3 dimensional properties

Front 40

Peripheral Neuropathy

Back 40

loss of haptic perception due to degeneration of the large myelinated fibres of the PNS

Front 41

noxious stimuli are transmitted by what fibres?

Back 41

small, unmyelinated (A delta and C)

Front 42

referred pain

Back 42

pain that arrises in deeper structures is actually felt elsewhere. for example intestinal pain can be felt on the skin surface

Front 43

Gate control theory

Back 43

non-noiceptive influences from the brain or large afferent fibres (ie. expectation or stress) can influence the activity of the interneuron, thereby gating the transmission of pain signals

Front 44

Size of cortical receptive field is______ to the proportion of cortex. Explain.

Back 44

ize of cortical receptive field is usually inversely related to proportion of cortex devoted to that part of the body (cells in areas like the fingertips that have a relatively large proportion of cortex devoted to them have very small receptive fields, while areas like the back that have a relatively small proportion of cortex devoted to them have cells with relatively large receptive fields).

Front 45

Activity in the Ventral Horn is controlled by what 3 sources?

Back 45

1. descending fibres (info coming down spinal cord from brain tells you what to do)
2. Somatosensory afferents from the dorsal root that modulate spinal cord reflexes (sensory nerurons don't just go up, they also make synapses with motor neurons that can make you do automatic functions without having input from brain)
3. Descending projections from the brainstem that modulate the excitability of spinal circuitry

Front 46

Why was the Braille system successful and the Moon system not?

Back 46

problem is that as you drag your finger you create a vibration, making the symbols less discriminable. The symbols must be highly distinguishable. the braille system is more efficient at this, moon looked like smudges

Front 47

what causes phantom pain?

Back 47

Dorsal horn neurons become hyperactive when sensory input is removed (ie leg removed)
This is an example of central pain where the nociceptive signals are not being generated at the peripheral nerve terminals but by the activity of neurons at a more central site (brain or spinal cord).

Some cases of phantom pain may be due to plastic changes in central areas of the brain whereby regions that formerly received pain input from the “missing” body part now receive somatosensory input from neighboring body parts, which the brain still interprets as pain information.

Front 48

Univarite and Multivariate Concepts of Pain

Back 48

The univariate concept of pain:
-Experience of pain is measured in terms of its overall integrated impression without consideration of the various sensory or emotional dimensions that make up the experience.

The multivariate concept:
-This approach assesses the multiple dimensions of pain, such as the location, duration, intensity, and quality of pain.

Front 49

3 Methods of dealing with Pain

Back 49

1) pharmacology (anti inflammatory drugs, opioids)
2) anethetic, surgical, neurostimulatory (local anesthetics, lobotomy, TENS transcutaneous electrical nerve stimulation) tries to non-invasively stimulate Aa and Ab fibres to activate inhibitory neurons according to the Gate Theory of Pain
3) Psychological ( placebo effect, behavioural therapy)