Sensory receptors-body sensation 1&2

Question 1

What are the roles of sensory receptors?

Answer 1

Inform your brain about the internal and external environment

are nerve endings – many have specialized non-neural endings

convert different stimuli into frequency of Action Potentials - so they are transducers

Question 2

What are the three types of sensory receptors?

Answer 2

mechanoreceptors

proprioceptors

nociceptors

Question 3

what do mechanoreceptors do?

Answer 3

stimulated by mechanical stimuli - pressure, stretch, deformation - give us skin sensations of touch and pressure

Question 4

what do proprioceptors do?

Answer 4

are mechanoreceptors in joints and muscles. They signal information about body or limb position (touching nose)

Question 5

what do nociceptors do?

Answer 5

respond to painful stimuli (heat and tissue damage)

Question 6

Where are the sensory receptors in the body?

Answer 6

position receptors in the ear
light receptors in the eyes
sound receptors in the ear
chemical receptors in nose and tongue
touch, pressure, pain and temp receptors in skin

Question 7

What is sensory modality?

Answer 7

the stimulus type that activates a particular receptor: eg. touch, pressure, joint angle, pain

Question 8

What is an adequate stimulus?

Answer 8

is the form of energy to which a receptor normally responds.
Min amount of energy to activate receptors.
Sensory receptors are highly sensitive to one specific energy form

Question 9

What happens when sensory receptors get activated?

Answer 9

transduce energy into action potentials which involves ion channels opening or closing.

Question 10

What is a receptor or generator potential?

Answer 10

an adequate stimulus causes a graded membrane potential change (only a few mV)

Question 11

What does adequate stimulus lead to?

Answer 11

the adequate stimulus in cutaneous mechanoreceptors and proprioceptors is membrane deformation

And this activates stretch-sensitive ion channels – so ions flow across the membrane and change the membrane potential locally

Question 12

How do we generate action potentials from a stimulus?

Answer 12

stimulus activates receptor membrane, if small then is a small receptor potential and not big enough to trigger action potentials.
If stimulus bigger, local membrane potential change is enough to reach first node of ranvier and that is initiation of APs

Question 13

How do we increase frequency of action potentials?

Answer 13

Greater stimulus and greater receptor potential - which will raise threshold at first node of ranvier and will open up lots more sodium channels. Potentially more APs

Question 14

Describe the frequency coding of stimulus intensity?

Answer 14

a larger stimulus,
causes a larger receptor potential and
a HIGHER FREQUENCY of action potentials

Question 15

What would happen to stimulus of amp 40mV and and duration 4 ms?

Answer 15

exceeds threshold and generates action potentials

action potentials conducted down sensory axon

small amount of neurotransmitter released

Question 16

What would happen to longer and stronger stimulus?

Answer 16

some decay of receptor potential but stays above threshold
higher frequency of action potentials for longer period
larger amount of neurotransmitter released at synapse

Answer 18

Adequate stimulus has effect on sensory receptor and get transduction and generation of graded receptor potential.

Frequency coded action potentials are conducted down primary afferent neuron
and come to synapse where can get synaptic integration

where no. primary afferent neurons could synapse onto same second order sensory neuron and get neurotransmitter released - generates EPSPs in second order neuron

If they summate enough and reach threshold, sets off frequency coded action potentials in second order neuron

reduced frequency in second order neuron

and that takes info to brain

Question 19

What do merkel receptors do?

Answer 19

sense steady pressure and texture

Question 20

What do meissner’s corpuscle do?

Answer 20

respond to flutter and stroking movements

Question 21

What do ruffini corpuscle do?

Answer 21

respond to stretch

Question 22

What do free nerve endings of hair root do?

Answer 22

sense hair movement

Question 23

what do pacinian corpuscles do?

Answer 23

sense vibration

Question 24

How can brain process new or changing events?

Answer 24

for some mechanoreceptors: if the stimulus persists, APs persist
for others, continuous mechanical stimulation (above) causes a drop off in APs - your brain then can process new or changing events

Question 25

What is meaning of adaptation?

Answer 25

some mechanoreceptors adapt to a maintained stimulus -only signals onset of stimulus

Question 26

How is adaptation achieved?

Answer 26

the stimulus causes an above threshold generator potential
this triggers APs

generator potential declines rapidly below threshold and APs cease

So this mechanoreceptor only signals the onset of a stimulus

it responds only to a change or a novel event

Question 27

What are the Rapidly/Moderately-adapting receptors ?

Answer 27

include Pacinian corpuscles and Meissner’s corpuscles

Question 28

What are the Slowly-adapting receptors ?

Answer 28

include Merkel’s discs and Ruffini endings

Question 29

Describe rapidly/ moderately adapting receptors action potentials?

Answer 29

when stimulus starts- get firing of action potentials and then stop even if stimulus still going
when stimulus stops you then get another brief firing of action potentials

Question 30

Describe slowly adapting receptors action potentials?

Answer 30

fire action potentials throughout duration of stimulus because need to know when we are in pain

Question 31

Describe the pacinian corpuscle?

Answer 31

It comprises a myelinated nerve with a naked nerve ending

enclosed by a connective tissue capsule of layered membrane lamellae

each layer is separated by fluid : (a bit like a spongy onion)

Question 32

Describe how pacinian corpuscle detects on and off phases of mechanical stimulus?

Answer 32

First: Mechanical stimulus deforms capsule - nerve ending is stretched - ion channels open - local depolarisation causes generator potential - APs fire - brain detects stimulus ON.
Next: fluid rapidly redistributes within capsule lamellae, this spreads the stimulus impact out laterally - minimizing downward deformation
the downward force causing mechanical stretch to nerve ending stops - so APs stop firing.
As stimulus is withdrawn - capsule lamellae spring back – (think of a suction-like force - clearing a blocked drainpipe) - and AP fire again.

Question 33

How does pacinian corpuscle respond?

Answer 33

A mechanical stimulus deforms the capsule and the nerve ending
This stretches the nerve ending and opens ion channels
Na+ influx causes local depolarisation - a receptor/generator potential
APs are generated and fire where myelination begins (because regenerative Na+ channels cluster at nodes of Ranvier)

Question 34

What happens if capsule is removed from pacinian corpuscle?

Answer 34

bare nerve ending loses much of adaptation

So it continues to produce a receptor/generator potential

the capsule enhances sensory function

Question 35

How can you tell where an itch is?

Answer 35

because sensory receptors have a receptive field

Question 36

Where are the stimuli that activate somatic sensory neuron found?

Answer 36

in specific area called receptive field

Question 37

Where does touch sensitive neuron respond to pressure?

Answer 37

defined receptive field (area of skin)

Question 38

What is the order from receptive field to neurons?

Answer 38

one receptive field is associated with
one sensory neurone (a primary sensory neuron), which synapses on
one CNS neuron, (a secondary neuron in the spinal cord)

Question 39

What is our ability to tell 2 points apart on the skin measured by?

Answer 39

1) the receptive field size
2)neuronal convergence (is there a single path to the brain, or do pathways merge )

Question 40

What is divergence?

Answer 40

single stimulus detected by single neuron sends information to multiple more neurons as goes towards brain

Question 41

What is convergence?

Answer 41

the opposite of divergence
multiple stimuli in periphery through convergence of second order and then third order sensory neurons converge that signal

Question 42

Why might sensory neurons with neighbouring receptive fields may exhibit neuronal convergence?

Answer 42

possibly all synapse onto second order neuron in spinal cord

Question 43

What does convergence of 3 primary sensory neurons in skin on one secondary neuron in spinal cord allow?

Answer 43

allows simultaneous sub-threshold stimuli from the 3 primary receptive fields to sum at the secondary neuron, forming a larger secondary receptive field and initiating APs.

Question 44

How do we tell which are sensitive areas of skin?

Answer 44

lots of convergence and a large secondary receptive field
indicate a relatively insensitive area

Question 45

What does the two point discrimination test tell us about sensitivity of skin?

Answer 45

distance between points adjusted until you just perceive 2 points rather than one.
fingers and lips very sensitive : 2-point threshold ~ 2 mm

back and limbs, much less sensitive : 2-point threshold several cm.

Question 46

What is acuity?

Answer 46

The ability to locate a stimulus on the skin and differentiate it from another closeby is called acuity

Question 47

What is low acuity caused by?

Answer 47

caused by high convergence

Question 48

How does lateral inhibition work?

Answer 48

If you stick a pin in skin, the primary neuron and most local to that stimulus is the one that gets activated most.
Sensory neurons around perimeter also get activated
But sensory neuron activated the most, releases most neurotransmitter onto second order neuron.
When its activated so strongly, has branches that inhibit second order neurons that are not activated so strongly.
Inhibition of lateral neurons enhances perception of stimulus

Question 49

How does sensory info reach brain?

Answer 49

all sensory info from body goes to the brain via spinal cord and then through thalamus and then onto individual areas of somatosensory cortex

Question 50

What is the sensory homunculous?

Answer 50

The representation of neighbouring areas of our skin peripherally are projected to neighbouring areas in the somatosensory cortex.

Topographical map of sensations in the somatic sensory cortex

Most sensitive areas have largest part of cortex to innervate