Lecture 3 Part 1: Somatosensation Flashcards
what does somatic sensation originate from?
the activity of afferent nerve fibres whose peripheral processes ramify (branch out) within the skin
where do the cell bodies of the afferent nerve fibres used for touch reside?
in the Doral root ganglia that lie alongside the spinal cord and brain stem
what is sensory transduction?
The process of converting the energy of a stimulus into an electrical signal so that our brain can ‘understand’ it
what are the three steps of sensory transduction?
- A stimulus alters the permeability of cation channels in the afferent nerve endings.
- This generates a de- polarizing current
- If magnitude is sufficient, the receptor potential reaches threshold for action potential
how does frequency coding work with sensory transduction?
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
what are the 2 types of afferent nerves distinguished by their ‘endings’:
Encapsulated endings (non painful sensation) Surrounded by mechanoreceptors
Free nerve endings (pain)
what are the two common properties of the mechanoreceptors?
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
what are the 4 distinct functional properties of how mechanorecepters respond to stimuli
axon diameter
receptive field size
temporal dynamics of response
quality of somatic stimulation
how does axon diameter affect mechanoreceptor’s response to stimuli?
Axon diameter determines conduction speed
thicker axons = signals travel faster
what is receptive field size?
Every sensory neuron has a “Receptive field”: the region of skin that influences that neuron
how is the size of the receptive field determined?
it depends on how widespread the branching of its terminals are
what is a large receptive field?
the same nerve is activated by different branches
the nerve can’t give accurate information about where the stimulus is within this field
what is a small receptive field?
less branching
more precise information about location
what is spatial acuity?
Ability to distinguish different points on the skin
what 2 things is spatial acuity dependent on?
Innervation density (how many receptors in an area)
Receptive field size of the receptor (two-point discrimination)
why is the 2 point discrimination smaller on our fingers than our thighs
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
how does temporal dynamics of response affect mechanoreceptor’s response to stimuli?
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
what information do slowly adapting afferents provide us?
static stimulis
what information do rapidly adapting afferents provide us?
dynamic stimulis
how does quality of somatic stimulation affect mechanoreceptor’s response to stimuli?
respond to mechanical change vs temperature change
do all mechanoreceptors go to the same place in the brain
after they all go to the thalamus, they go to different places in the brain
is the entire process of sensation analyzed in the skin?
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
what are the two key messages about how the primary sensory cortex is organized
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)
are there different channels in the brain?
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
All of the following may contain cells that form part of the pathway for discriminative touch and conscious proprioception EXCEPT:
-Cuneate nucleus
-Ventral/Anterior horn grey matter
-Primary sensory cortex
-VPM of thalamus
Ventral/Anterior horn grey matter
true or false:
The ability to distinguish different points on the skin (two-point discrimination) is dependent on: i.) the number of receptors in a given area (innervation density) and ii.) the size of the receptive fields of these receptors
true
true or false:
Free nerve endings and encapsulated nerve endings (mechanoreceptors) primarily differ in their connections with second order neurons in the central nervous system.
false
true or false
Somatic sensation originates from the activity of efferent nerve fibers whose peripheral processes ramify within the skin.
false
true or false:
The primary role of slowly adapting touch receptors is to obtain information about dynamic aspects of a stimulus (e.g., onset and offset).
false
