Sensory Receptors Flashcards
Sensory receptors are
nerve endings - often with specialized non-neural structures.
Transducers – converting different forms of energy into frequency of Action Potentials (APs)
they inform the CNS about the internal and external environment
sensory modality
stimulus type activating a particular receptor: eg. touch, pressure, pain, temperature, light
an adequate stimulus
type of energy to which a receptor normally responds
sensory receptors
highly sensitive to one specific energy form but are activated by other intense stimuli - poke in the eye, “see stars”
Types of sensory receptors:
- Mechanoreceptors
- Proprioceptors
- Nociceptors
- Thermoreceptors = Detect cold and warmth
- Chemoreceptors = detect chemical changes eg pH
- Photoreceptors = respond to particular wavelengths of light
Mechanoreceptors
stimulated by mechanical stimuli
- pressure, stretch, deformation.
Detect many stimuli
- hearing, balance, blood pressure - also skin sensations of touch and pressure
Proprioceptors
are mechanoreceptors in joints and muscles.
They signal information about body or limb position
Nociceptors
respond to painful stimuli, tissue damage and heat
Cutaneous Mechanoreceptors and Proprioceptors are good examples of
- principles of peripheral sensory processing
- sensory receptor transduction involves ion channels opening or closing
an adequate stimulus causes a
graded membrane potential change,
- a receptor potential or generator potential (a few mV)
membrane deformation is
the adequate stimulus in cutaneous mechanoreceptors and proprioceptors
this activates stretch-sensitive ion channels – so ions flow across the membrane and change the membrane potential locally.
A stimulus triggers
ions to flow through the membrane locally.
When depolarisation reaches the area with voltage-gated ion channels (first node) - action potentials start firing
electrode at position 1 and 2 measure
change in membrane potential electrode 1 (receptor membrane) measures Receptor potentia electrode 2 (node of ranvier) Action potentials
in sensory nerve a larger stimulus causes
- larger receptor potential
- higher frequency of action potentials
this is called frequency coding of stimulus intensity
Skin is packed with different receptors for touch
- merkel receptors
sense steady pressure and texture
Skin is packed with different receptors for touch
- meissner’s corpuscle
responds to flutter and stroking movements
Skin is packed with different receptors for touch
- pacinian corpuscle
senses vibration
Skin is packed with different receptors for touch
- Ruffini corpuscle
responds to skin stretch
Skin is packed with different receptors for touch
- sensory nerves
carry signals to spinal cord
Skin is packed with different receptors for touch
- free nerve endings of nocieptor
responds to noxious stimuli
for some mechanoreceptors: if the stimulus persists
APs persist
ADAPTATION is when
some mechanoreceptors ADAPT to a maintained stimulus and only signal change – eg. the onset of stimulation
eg- We are aware of putting on our clothes, after that - continuous mechanical stimulation is not important - until we take them off!
different receptors show different extents of adaptation
Rapidly/Moderately-adapting receptors include
- Pacinian corpuscles
- Meissner’s corpuscles
Slowly-adapting receptors include
- Merkel’s discs
- Ruffini endings
Nociceptors and adaption
these are free nerve endings detecting painful stimuli - do not adapt.
it is important not to ignore painful stimuli.
The Pacinian corpuscle is the
best understood mechanoreceptor
- Comprises a myelinated nerve with a naked nerve ending
- enclosed by a connective tissue capsule of layered membrane lamellae
- each layer separated by fluid (a bit like a spongy onion)
How does the Pacinian corpuscle respond
- A mechanical stimulus deforms the capsule and nerve ending
- This stretches the nerve ending and opens ion channels
- *Na+ influx causes local depolarisation - a generator/receptor potential *
- APs are generated and fire where myelination begins
the Pacinian corpuscle shows rapid
adaptation.
First: Mechanical stimulus deforms capsule - nerve ending is stretched - ion channels open - local depolarisation causes generator potential - APs fire - brain detects stimulus ON.
Next: rapid fluid redistribution in capsule dissipates stimulus laterally
- vertical force causes mechanical stretch of nerve ending stops and so APs stop firing.
As stimulus is withdrawn - capsule springs back - AP fire again
Detects ON and OFF phases of mechanical stimulus
If lamellae are removed
much of the adaptation is lost.
The function of this sensory receptor depends on the non-neural accessory structure - the capsule - it enhances sensory function
Capsule intact
- Normal, rapidly adapting ON/OFF response
Capsule removed
- bare nerve ending loses much of adaptation
- So it continues to produce a receptor/generator potential
Sensory receptors possess
receptive fields.
a somatic sensory neuron is activated by stimuli in a specific area called the receptive field
so a touch-sensitive neuron in the skin responds to pressure within a defined receptive field
our ability to tell 2 points apart on the skin is measured by the
two point discrimination test
This ability depends on two things
1) receptive field size
2) neuronal convergence