Download The sense of touch

The sense of touch
Sensory information
The “five senses” (Aristotle):
•sight
•hearing
•smell
•taste
•touch
•it’s not that simple......
•Today’s main topic: the sense of touch
•what we would now call the “somatosensory
system” (much wider than just “touch”)
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Sensory information
Somatosensation: apart from “touch”, it tells us
about our own bodies:
- muscle stretch
)
- tendon stretch
) “proprioception”
- joint position
)
- internal organs (“visceral sensation”)
- pain
- touch (including pressure/vibration/tapping)
- temperature: warm, cold, burning, freezing
Each of these has its own specific receptors and
specific pathways in the nervous system
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Sensory spots
Skin sensations are localised:
e.g. 2 x 2.5 cm on wrist
Cold
Warm
Light touch
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What’s under the sensory spots?
Sensory nerve terminals in the skin:
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Nerve terminals
are the peripheral ends of sensory neurones
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Central nervous system connections
Third order neurone
Thalamus
Second order neurone
(crosses midline)
Primary sensory neurone
(DRG neurone)
Receptor
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Signalling from sensory receptors
•Nerve impulses called “action potentials”
•Action potential frequency encodes stimulus strength
•Gradually stronger pressures applied to the same finger
area: gradually increasing action potential frequency
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The action potential
Action potential = “nerve impulse”
•each nerve action potential is the same as any
other in the same nerve fibre
•it’s the frequency of impulses that carries the
information
•“all or none” principle: if stimulus is strong
enough, we get an impulse, otherwise nothing
happens
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The action potential
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Some real action potentials
50
0
mV
-50
-100
0
Squid nerve fibre (1939)
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2
msec
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Human nerve fibre (1992)
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Somatosensory receptors are
activated by:
•Mechanical force
•Cold (and menthol)
•Gentle warmth
•Painful heat (and chillies)
•Many substances found in damaged tissue
) pain
) receptors
•This is because they contain ion channels (like those that
make action potentials), specifically activated by these stimuli
•Each receptor contains only one or a few of these ion
channels
•This is why receptors are specific for only one type of
stimulus (or a few types of stimulus, in pain receptors)
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Muscle and tendon receptors
Golgi tendon organ
Muscle spindle
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Muscle spindle
Muscle spindle:
detects muscle
stretch
!3
Muscle spindle and stretch reflex
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Golgi tendon organ
Golgi tendon organ:
detects muscle
contraction force
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Golgi tendon organ reflex
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Skin sensory receptors
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Skin sensory receptors
•Merkel disk: accurate light pressure (e.g. Braille)
•Meissner’s corpuscle: light tapping
•Pacinian corpuscle: coarse tapping/vibration
•Ruffini ending: skin stretch
•Free nerve endings: warm, cool, pain
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Skin sensory receptors
Temperature
Action potentials from
human cold receptor
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Skin sensory receptors
Pain
Nociceptors: Receptors that respond only to
actual or imminent tissue damage
We perceive nociceptor activity as PAIN
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Nociceptor activation
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First and second pain
- due to impulses in fast and slow pain fibres
C fibre (slow)
Aδ fibre (fast)
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Touch can inhibit pain:
the GATE CONTROL theory (Wall & Melzack)
TENS: Stimulate here↑
This is the basis of
Transcutaneous Electrical Nerve Stimulation
(TENS)
Widely used in pain control
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Referred pain (1)
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Referred pain (2)
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How do we localise a stimulus?
Two-point discrimination: related to brain area and receptor
density
Density of
Merkel disks
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Sensory representation in the brain
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