Lecture 3 Part 1: Somatosensation

Question 1

what does somatic sensation originate from?

Answer 1

the activity of afferent nerve fibres whose peripheral processes ramify (branch out) within the skin

Question 2

where do the cell bodies of the afferent nerve fibres used for touch reside?

Answer 2

in the Doral root ganglia that lie alongside the spinal cord and brain stem

Question 3

what is sensory transduction?

Answer 3

The process of converting the energy of a stimulus into an electrical signal so that our brain can ‘understand’ it

Question 4

what are the three steps of sensory transduction?

Answer 4

1. A stimulus alters the permeability of cation channels in the afferent nerve endings.
2. This generates a de- polarizing current
3. If sufficient in magnitude, the receptor potential reaches threshold for the generation of action potentials in the afferent fiber.

Question 5

how does frequency coding work with sensory transduction?

Answer 5

The action potentials fire in a rate that is proportional to the magnitude of the depolarization

if dog is pressing harder, we see faster action potentials

Question 6

what are the 2 types of afferent nerves distinguished by their ‘endings’:

Answer 6

Encapsulated endings (non painful sensation) Surrounded by mechanoreceptors

Free nerve endings (pain)

Question 7

what are the two common properties of the mechanoreceptors?

Answer 7

Transduction:
* IF there is an
adequate stimulus, depolarization occurs because force produces opening of Na channels

No spontaneous activity:
* APs only produced
when adequate stimulus is present

Question 8

what are the 4 distinct functional properties of how mechanorecepters respond to stimuli

Answer 8

axon diameter

receptive field size

temporal dynamics of response

quality of somatic stimulation

Question 9

how does axon diameter affect mechanoreceptor’s response to stimuli?

Answer 9

Axon diameter determines conduction speed

thicker axons = signals travel faster

Question 10

how does receptive field size affect mechanoreceptor’s response to stimuli?

Answer 10

Every sensory neuron has a “Receptive field”: the region of skin that influences that neuron

Question 11

how is the size of the receptive field determined?

Answer 11

it depends on how widespread
he branching of its terminals are

Question 12

what is a large receptive field?

Answer 12

the same nerve is activated by different branches

the nerve can’t give accurate information about where the stimulus is within this field

Question 13

what is a small receptive field?

Answer 13

less branching

more precise information about location

Question 14

what is spatial acuity?

Answer 14

Ability to distinguish different points on the skin

Question 15

what 2 things is spatial acuity dependent on?

Answer 15

Innervation density (how many receptors in an area)

Receptive field size of the receptor (two-point discrimination)

Question 16

why is the 2 point discrimination smaller on our fingers than our thighs

Answer 16

fingers have small receptive fields so we can tell more accurately when we are touched with 2 points 1

on our thighs, the points need to be farther apart for us to be able to tell accurately if there is 1 or 2 points

Question 17

how does temporal dynamics of response affect mechanoreceptor’s response to stimuli?

Answer 17

Some afferents are slowly adapting, meaning they fire a lot when the stimulus is first added, and fire slower as the stimulus stays

others are rapidly adapting, firing rapidly only when the stimulus is first added

Question 18

what information do slowly adapting afferents provide us?

Answer 18

static stimulis

Question 19

what information do rapidly adapting afferents provide us?

Answer 19

dynamic stimulis

Question 20

how does quality of somatic stimulation affect mechanoreceptor’s response to stimuli?

Answer 20

respond to mechanical change vs temperature change

Question 21

do all mechanoreceptors go to the same place in the brain

Answer 21

after they all go to the thalamus, they go to different places in the brain

Question 22

is the entire process of sensation analyzed in the skin?

Answer 22

no. the features get analyzed in more complexity as the signal moves up the hierarchy from skin to DCML/spinothalamic nuclei to the thalamus to the primary sensory cortex

Question 23

what are the two key messages about how the primary sensory cortex is organized

Answer 23

the map shows where analysis of a particular body part occurs that goes from medial to lateral

the amount of brain devoted to a body part is related to tactile acuity (less tactile acuity in legs which makes up less brain space on sensory cortex)

Question 24

are there different channels in the brain?

Answer 24

yes, there are 4 different channels that talk to each other and they spit out more info that gets processed at higher and higher levels

Question 25

what are the the primary and secondary projections of the primary sensory cortex

Answer 25

the primary projection Is going to be the second somatosensory cortex and then it goes to the parietal lobe