Sensory Receptors 1

Question 1

What are sensory receptors?

Answer 1

Nerve endings, often with specialised non-neural structures.

Question 2

How are sensory receptors transducers?

Answer 2

They convert different forms of energy into frequency of action potentials.

Question 3

What information do they feed to the CNS?

Answer 3

Inform CNS about internal and external environments.

Question 4

What is a sensory modality?

Answer 4

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

Question 5

What is an adequate stimulus?

Answer 5

The type of energy a receptor normally responds to.

Question 6

Sensory receptors are highly sensitive to one specific energy form, but what else may activate them?

Answer 6

They can activated by other intense stimuli.

Question 7

What are mechanoreceptors?

Answer 7

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

Question 8

What are proprioceptors?

Answer 8

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

Question 9

What are nociceptors?

Answer 9

They respond to painful stimuli - tissue damage and heat.

Question 10

What do thermoreceptors detect?

Answer 10

Cold and warmth.

Question 11

What do chemoreceptors detect?

Answer 11

Chemical changes, e.g. pH, pO2, pCO2.

Question 12

What do photoreceptors do?

Answer 12

Respond to particular wavelengths of light.

Question 13

What are cutaneous mechanoreceptors and proprioceptors good examples of?

Answer 13

The principles of peripheral sensory processing.

Question 14

What does transduction in all sensory receptors involve?

Answer 14

Opening or closing of ion channels.

Question 15

What will an adequate stimulus cause?

Answer 15

A graded membrane potential change called a receptor potential or a generator potential (mV).

Question 16

What is the adequate stimulus in cutaneous mechanoreceptors and proprioceptors?

Answer 16

Membrane deformation.

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

Question 17

Is the receptor potential graded?

Answer 17

The receptor potential is graded to stimulus intensity.

Question 18

A stimulus causes local current to flow where?

Answer 18

To where the membrane has voltage-gated ion channels that generate action potentials.

Question 19

In myelinated sensory neurones, where would this be?

Answer 19

The start of myelination (before).

Question 20

What frequency of action potentials will the lowest stimulus intensity and the highest stimulus intensity produce?

Answer 20

Lowest - no action potentials.

Highest - most action potentials produced (high frequency).

Question 21

Describe frequency coding of stimulus intensity.

Answer 21

The larger the stimulus, the larger the receptor potential and the higher the frequency of action potentials in a sensory nerve.

Question 22

Aside from frequency coding of stimulus intensity, what else may convey stimulus intensity?

Answer 22

The number of receptors activated (for instance per unit area spatially).

Question 23

Skin is packed with different receptors for touch, what does the information they transmit depend on?

Answer 23

The properties of the nerve endings and of accessory, non-neuronal structures.

Question 24

What do Meissner’s corpuscles sense?

Answer 24

Flutter and stroking movements.

Question 25

What do Pacinian corpuscles sense?

Answer 25

Vibration.

Question 26

What do Merkel receptors sense?

Answer 26

Steady pressure and texture.

Question 27

What do the free nerve endings of hair roots sense?

Answer 27

Hair movement.

Question 28

What do the free nerve endings of nociceptors respond to?

Answer 28

Noxious stimuli (tissue damage/pain).

Question 29

If the stimulus persists, will the action potentials persist?

Answer 29

Only in some mechanoreceptors.

Question 30

What is adaptation in mechanoreceptors?

Answer 30

Some mechanoreceptors adapt to a maintained stimulus and only signal change - the onset of stimulation.

Question 31

Explain how adaptation in mechanoreceptors comes about.

Answer 31

- Stimulus is enough to cause an above threshold generator potential - AP triggered. - Generator potential declines rapidly and APs cease. - So only responds to onset of stimulus (a change or novel event).

Question 32

What are the two types of adaptation that can be exhibited in sensory receptors and give examples of receptors in each?

Answer 32

- Rapidly/moderately-adapting receptors (e.g. Pacinian corpuscles and Meissner's corpuscles). - Slowly adapting receptors (e.g. Merkle's discs and Ruffini endings).

Question 33

What do Ruffini corpuscles respond to?

Answer 33

Skin stretch.

Question 34

What kind of sensory receptor will not adapt and why?

Answer 34

Nociceptors are free nerve endings that detect painful stimuli that are too important to ignore.

Question 35

Describe the structure of a Pacinian corpuscle.

Answer 35

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

Question 36

How does the Pacinian corpuscle respond to mechanical stimuli (vibration)?

Answer 36

- Mechanical stimulus deforms capsule and nerve ending. - Stretching the nerve ending opens ion channels. - Sodium ion influx causes local depolarisation - generator potential. - APs generated and fire at myelinated nerve.

Question 37

How does the Pacinian corpuscle show rapid adaptation?

Answer 37

Fluid redistribution in the capsule rapidly dissipates stimulus and removes mechanical stretch of nerve ending, so APs stop firing.

Question 38

What will withdrawal of the stimulus in Pacinian corpuscles do?

Answer 38

Causes capsule to spring back - APs fire again. This allows it to detect on and off phase of the mechanical stimuli.

Question 39

In a Pacinian corpuscle what may cause adaptation to be lost? Why is this?

Answer 39

If the lamellae is removed. Bare nerve endings lose much of adaptation and continue to produce receptor potentials. So non-neural accessory structure is critical to how this sensory receptor works - enhancing sensory function.

Question 40

Sensory receptors possess receptive fields, what are these?

Answer 40

A somatic sensory neurone is activated by stimuli in a specific area called the receptive field, e.g. touch-sensitive neurone in skin respond to pressure within a defined receptive field.

Question 41

What does our ability to tell 2 points on our skin depend on?

Answer 41

Receptive field size and neuronal convergence. (It is determined by a two point discrimination test).

Question 42

What do sensory neurones with neighbouring receptive fields exhibit?

Answer 42

Neuronal convergence.

Question 43

What is neuronal convergence and what does it allow?

Answer 43

It is multiple presynaptic neurones input on a smaller number of post-synaptic neurones. Convergence of primary sensory neurones allow simultaneous sub-threshold stimuli to sum at the secondary neurone, forming a large secondary receptive field and initiating APs.

Question 44

What would indicate a relatively insensitive area?

Answer 44

Convergence and a large secondary receptive field.

Question 45

Describe the 2-point discrimination test.

Answer 45

Distance between points adjusted until just perceive two points rather than one.

Question 46

What is acuity?

Answer 46

The ability to locate stimulus on the skin and differentiate it from another close by.

Question 47

What is lateral inhibition?

Answer 47

When the information from afferent neurones with sensory receptors at the edge of stimulus are strongly inhibited, compared with information from stimulus centre.

Question 48

Why is lateral inhibition important?

Answer 48

It enhances the contrast between relevant and irrelevant information. It is a major mechanism for sharpening up or cleaning up sensory information.

Question 49

Where does lateral inhibition happen for cutaneous information?

Answer 49

Spinal cord.

Question 50

What is a major component in pathways with high precision information?

Answer 50

Lateral inhibition. Allows precise localisation to single skin hair movement.

Question 51

What does the sensory information travel?

Answer 51

To brain, relayed through thalamus to somatosensory cortex, cortical body map is distorted. Most sensitive areas occupy biggest cortical space.