Sensory receptors 1

Question 1

What are sensory receptors?

Answer 1

• Nerve endings, often with specialised non-neural structures.
• They are transducers that convert different forms of energy into frequency of Action Potentials (APs).
• They inform the CNS about the internal and external environment.

Question 2

What is a sensory modality?

Answer 2

A type of stimulus activating a particular receptor: e.g. touch, pressure, pain, temperature, light.

Question 3

What is an adequate stimulus?

Answer 3

An adequate stimulus is the type of nergy a receptor normally responds to.

Question 4

Are sensory receptors sensitive?

Answer 4

They are highly sensitive to one specific energy form but activated by other intense stimuli, (e.g. poke in the eye).

Question 5

Types of sensory receptors

Answer 5

• Mechanoreceptors
• Proprioceptors
• Nociceptors

Question 6

Mechanoreceptors

Answer 6

Stimulated by mechanical stimuli - pressure, stretch or deformation.
Detect many stimuli - hearing, balance, blood pressure and skin sensations of touch and pressure.

Question 7

Proprioceptors

Answer 7

Are mechanoreceptors in joints and muscles that signal information related to body or limb position.

Question 8

Nociceptors

Answer 8

Respond to painful stimuli - tissue damage and heat.

Question 9

Other sensory receptors

Answer 9

• Thermoreceptors: detect cold and warmth.
• Chemoreceptors: Detect chemical changes e.g. pH, pO2 and pCO2.
• Photoreceptors: Respond to particular wavelengths of light.

Question 10

Cutaneous Mechanoreceptors and Proprioceptors

Answer 10

-Are good examples of the principles of peripheral sensory processing.
Transduction in ALL sensory receptors involves opening or closing of ion channels.
- An adequate stimulus causes a graded membrane potential change called a receptor potential or a generator potential (millivolts).

Question 11

What is the adequate stimulus in cutaneous mechanoreceptors and proprioreceptors called?

Answer 11

Membrane deformation

-this activates stretch-sensitive ion channels causing ion flow across the membrane.

Question 12

What is a stimulus?

Answer 12

Causes local current to flow to the part of the membrane with voltage-gated ion channels.

• This generates action potentials (APs).
• In myelinated sensory nerves, this is where myelination starts.

Question 13

Frequency coding of stimulus intensity

Answer 13

The larger the stimulus, the larger the receptor potential and the HIGHER THE FREQUENCY of APs in a sensory nerve.
- the number of receptors activates (for instance per unit area spatially) also reflects the stimulus intensity.

Question 14

Receptors for touch in skin

Answer 14

Their information (vibration, stretch, texture, pain) depends on the properties of nerve endings and of accessory, non-neuronal structures.

Question 15

For some mechanoreceptors, if the stimulus persists, what else persists?

Answer 15

Action Potentials (APs) persist

Question 16

Adaptation in mechanoreceptors

Answer 16

ADAPTATION - some mechanoreceptors ADAPT to a maintained stimulus and only signal change - e.g. the onset of stimulation.

-Stimulus is enough to cause an above threshold generator potential, which triggers APs
But the generator potential declines rapidly and APs cease
So the mechanoreceptor only signals the onset of a stimulus
-it responds only to a change or a novel event
-different receptors show different extents of adaptation

Question 17

Rapidly/Moderately-adapting receptors

Answer 17

include Pacinian corpuscles and Meissner’s corpuscles

-Nociceptors which are free nerve endings detecting painful stimuli - do not adapt, because it is important not to ignore painful stimuli.

Question 18

Slowly-adapting receptors

Answer 18

include Merkel’s discs and Ruffini endings

Question 19

What is the Pacinian corpuscle?

Answer 19

a myelinated nerve with a naked nerve ending, enclosed by a connective tissue capsule of layered membrane lamellae separated by fluid (like an onion)

-The Pacinian corpuscle is the best understood mechanoreceptor.

Question 20

How does the Pacinian corpuscle respond?

Answer 20

1. A mechanical stimulus deforms the capsule and nerve ending
2. This stretches the nerve ending and opens ion channels
3. Na+ influx causes local depolarisation – a generator potential
4. APs are generated and fire at the myelinated nerve

Question 21

Rapid adaptation of the Pacinian Corpuscle

Answer 21

-mechanical stimulus deforms capsule - nerve ending is stretched - ion channels open - local depolarisation – generator potential - APs fire - detect stimulus ON.

fluid redistribution in the capsule – rapidly dissipates stimulus - removes mechanical stretch of nerve ending – APs stop firing.
withdrawal of the stimulus - capsule springs back - AP fire again

Detects ON and OFF phases of a mechanical stimulus

Question 22

Are non-neural accessory structures important to some sensory receptors?

Answer 22

Non-neural accessory structures are critical to how some sensory receptors work - in general they enhance sensory function.

Question 23

What is it called when a somatic sensory neuron is activated by stimuli in a specific area?

Answer 23

The receptive field

e.g. a touch-sensitive neuron in the skin responds to pressure within a defined receptive field.

Question 24

What is the simplest case of sensory receptors and receptive fields?

Answer 24

one receptive field is associated with one sensory neurone (a primary or 1st order sensory neuron) – which synapses on one CNS neuron, a secondary or 2nd order neuron)

Question 25

What two things depends on our ability to tell 2 points on the skin apart?

Answer 25

1. Receptive field size
2. Neuronal convergence

-It is determined by a two point discrimination test

Question 26

Neuronal convergence

Answer 26

• Multiple presynaptic neurons input on a smaller number of post-synaptic neurons.
• Sensory neurons with neighbouring receptive fields exhibit neuronal convergence

Question 27

What does convergence of primary sensory neurons allow?

Answer 27

-It allows simultaneous sub-threshold stimuli to sum at the secondary neuron, forming a large secondary receptive field (dotted) and initiating APs.

Question 28

What does convergence and a large secondary receptive field indicate?

Answer 28

-Indicates a relatively insensitive area.

Question 29

2-point discrimination test

Answer 29

Distance between points adjusted until you just perceive 2 points rather than one.

Question 30

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

Answer 30

Acuity

Question 31

Lateral Inhibition explanation (important)

Answer 31

• Information from neurons with sensory receptors at the edge of a stimulus is strongly inhibited, compared with information from the centre of the stimulus.
• the contrast between relevant and irrelevant information is enhanced
• Lateral inhibition occurs in the spinal cord for cutaneous information.

Question 32

Lateral inhibition - importance

Answer 32

-Is a major mechanism for “sharpening or cleaning up” sensory information.
-Is a major component in pathways with high precision information e.g. touch and skin hair movement.
-Allows precise localisation to a single skin hair movement.
all sensory information goes to the brain.
-it is relayed via the thalamus to the somatosensory cortex
-there is a distorted body map in the somatosensory cortex
-the most sensitive areas occupy greatest cortical space