Sensory Receptors

Question 1

Describe the structure of cutaneous receptors

Answer 1

In cutaneous sensory receptors, the nerve endings are protected by a capsule. In many cases, the stimulus required to trigger an action potential depends on causing a membrane deformation which activates stretch sensitive ion channels.

These channels may be present in the distal tip of the afferent axon, or in specialised sensory cells that then release neurotransmitter to create a generator potential in the axon’s dendrites.

Question 2

Explain the mechanism of signal transduction in the skin

Answer 2

The transduction process in all sensory endings involves opening or closing of ion channels.

Question 3

Explain the concept of frequency coding of stimulus intensity

Answer 3

The bigger the stimulus, the greater the receptor potential, and therefore the higher the frequency of action potentials. This is the frequency coding of action potentials. High frequency equals a high stimulus intensity.

Question 4

Describe the structure and function of the muscle spindle

Answer 4

Muscle spindles monitor muscle length and the rate at which it changes. This is very important for reflexes as well and voluntary movements.
Skeletal muscle is made up of extrafusal and intrafusal fibres. Extrafusal fibres are regular muscle fibres, however, intrafusal fibres, together with their specialised motor and sensory innervation form muscle spindles. The ends of these fibres contain sarcomeres and are contractile. They are controlled by gamma motoneurones.
Muscle stretch is the adequate stimulus for these receptors. As the muscle lengthens, ion channels open and so the length is monitored.

Question 5

Describe the mechanisms of efferent control of spindle function

Answer 5

When an agonist muscle contracts, the spindle contracts also and so output is decreased, however, this causes the antagonistic muscle to lengthen and those muscle spindles increase their outflow of action potentials. All of this information allows you to know exactly what is happening around the joint position.

Question 6

Describe the role and mechanism of action of the Golgi tendon organs.

Answer 6

Golgi tendon organs monitor muscle tension. The nerve endings are intermingled with the tendons at the ends of muscles. They act as stretch receptors and monitor the stretch in a tendons. Since tendons are inelastic, the muscles must develop tension to stretch them and so, they measure muscle tension.

Question 7

What are sensory receptors and what is their function?

Answer 7

• Are nerve endings, often with specialized 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 8

Define the term “sensory modality”

Answer 8

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

Question 9

Describe the 6 types of sensory receptor

Answer 9

Mechanoreceptors: stimulated by mechanical stimuli (pressure, stretch, or deformation).
Detect many stimuli - hearing, balance, blood pressure and skin sensations of touch and pressure
Proprioceptors: are mechanoreceptors in joints and muscles that signal information related to body or limb position
Nociceptors: respond to painful stimuli - tissue damage and heat
Thermoreceptors: detect cold and warmth
Chemoreceptors: detect chemical changes e.g. pH, pO2 and pCO2
Photoreceptors: respond to particular wavelengths of light.

Question 10

Name the 4 touch receptors found on the skin

Answer 10

• Merkel receptors
• Meissner’s corpuscle
• Pacinian corpuscle
• Ruffini corpuscle – is a slowly adapting mechanoreceptor sensitive to stretch

Question 11

Describe Merkel receptors

Answer 11

Found in the basal epidermis and hair follicles. They are classified as slowly adapting type I mechanoreceptors. They are large, myelinated nerve endings. They provide information on pressure, position, and deep static touch features such as shapes and edges.

Question 12

Describe Meissner’s corpuscles

Answer 12

A type of nerve ending in the skin that is responsible for sensitivity to ejaculation touch. In particular, they have highest sensitivity (lowest threshold) when sensing vibrations between 10 and 50 Hertz. They are rapidly adaptive receptors. They are most concentrated in thick hairless skin, especially at the finger pads.

Question 13

Describe Pacinian’s corpuscles

Answer 13

Are also called lamellar corpuscles, are one of the four major types of mechanoreceptor. They are nerve endings in the skin responsible for sensitivity to vibration and pressure.

Question 14

Describe Ruffini’s corpuscles

Answer 14

Is a slowly adapting mechanoreceptor sensitive to stretch

Question 15

What is adaptation in terms of sensory signalling?

Answer 15

Some mechanoreceptors adapt to a maintained stimulus and only signal change – 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 or it responds only to a change or a novel event.

Question 16

Name some rapidly adapting receptors

Answer 16

Rapidly/Moderately-adapting receptors include Pacinian corpuscles and Meissner’s corpuscles

Question 17

Name some slowly adapting receptors

Answer 17

Slowly-adapting receptors include Merkel’s discs and Ruffini endings

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

Question 18

Describe signalling at Pacinian corpuscles

Answer 18

Made up of a myelinated nerve with a naked nerve ending, enclosed by a connective tissue capsule of layered membrane lamellae separated by fluid.
Pacinian corpuscles respond to a mechanical stimulus which deforms the capsule and nerve ending, which stretches it and causes the opening of ion channels. Na+ influx causes local depolarisation and APs are generated and fire at the myelinated nerve. • 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 (hence rapid adaptation)
Detects ON and OFF phase of mechanical stimuli.

Question 19

How can we tell between two points of contact on the skin?

Answer 19

Our ability to tell 2 points on the skin apart depends on:

1. Receptive field size
2. Neuronal convergence

Question 20

What is convergence?

Answer 20

Sensory neurons with neighbouring receptive fields exhibit neuronal convergence = multiple presynaptic neurons input on a smaller number of post-synaptic neurons.
Convergence of primary sensory neurons allows simultaneous sub-threshold stimuli to sum at the secondary neuron, forming a large secondary receptive field (dotted) and initiating APs.
So convergence and a large secondary receptive field indicate a relatively insensitive area.

Question 21

What is lateral inhibition?

Answer 21

Information from afferent neurons with sensory receptors at edge of stimulus is strongly inhibited, compared with information from stimulus at the centre.

Lateral inhibition is a major mechanism for “sharpening or cleaning up” sensory information.

Question 22

What are proprioceptors?

Answer 22

Are mechanoreceptors that signal body or limb position. They include:
o Muscle spindles – which monitor muscle length and rate of change of muscle length and so they control reflexes and voluntary movements.
o Golgi tendon organs - monitor tension on tendons. Tension is produced by muscle contraction, so monitoring muscle tension.
o Joint receptors - monitor joint angle, rate of angular movement and tension on the joint.