Somatosensory system Flashcards by Helen Jordan

Describe a primary afferent neurone

Sensory receptor in skin
Axon fibre
Cell body on stalk
Cell body in dorsal root ganglia
Terminates in dorsal horn of spinal cord

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What is the name of the area that a sensory neurone receives information from?

The receptive field

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Where is an action potential generated in a primary afferent sensory neurone?

At the peripheral end of the axon

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What are the four classifications of primary afferent axons?

Aalpha
Abeta
Adelta
C

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Which is the thickest myelinated axon?

Aalpha

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What is the classification of an unmyelinated axon?

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Where will you find Aalpha axons?

Proprioceptors of skeletal muscle

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Where will you find Abeta axons?

Mechanoreceptors of skin

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Where will you find Adelta axons?

Pain, temperature

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Where will you find C axons?

Temperature, pain, itch

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Explain the relationship between nerves and receptive fields

Each nerve axon innervates a specific receptive field

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Describe receptive fields

Can vary greatly in size

This determines precision of localisation

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What defines the size of a receptive field?

2 point discrimination

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What is 2 point discrimination?

The smallest distance between two points where you can still distinguish two stimuli

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How does sensory localisation vary over the body?

Greatly
Fingers are very sensitive
Area like the back isn’t

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What in the brain relates to the sensory localisation?

The size of the cerebral cortex for that region of the body

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What is a dermatome?

An area of skin that is innervated by afferent axon fibres

Signalling all its sensation via a signal nerve from a single spinal nerve root

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What is an adequate stimulus?

The right size and type of stimulus for that nerve ending that will stimulate a response

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Is threshold dependent on nerve endings?

Yes

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What is the line code?

The CNS interprets incoming action potentials by the specific axon they arrived via

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What is generated in the nerve ending of a sensory neurone?

A receptor potential

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Stimulus strength determines what?

The size of the graded receptor potential

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What creates the graded receptor potential?

Channels opening

Membrane depolarisation

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Firing rate is proportional to…

…stimulus strength

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What is a slow or non-adapting stimulus?

A stimulus that is always present Important when maintaining information about a stimulus is valuable e.g. amount of stretch or pain

What is a fast adapting stimulus?

Constantly changing stimulus Useful where it is important to signal a change in stimulus Also to stop paying attention to stimulus e.g. touch

Name the three kinds of cutaneous sensory receptors

Mechanoreceptors Thermoreceptors Nociceptors

What do mechanoreceptors sense?

Touch Pressure Vibration

What do thermoreceptors sense?

Temperature

What do nociceptors sense?

Noxious stimulation | Pain

Name sensory organs (mechanoreceptors) found in human skin

Ruffini's corpuscle Meissner's corpuscle Pacinian corpuscle Merkel's receptor

What classification of axon does a cutaneous mechanoreceptor have?

Abeta

The structure of sensory apparatus determines what?

Structure determines function | Function determines location

Group the mechanoreceptors of the skin by location

Superficial Meissner's corpuscle Merkel's receptor Deep Ruffini's corpuscle Pacinian corpuscle

Describe Meissner's corpuscles

``` Found in papillary dermis Rapidly adapting Constantly changing stimulus required Light touch Vibration ```

Why does the need for constantly changing stimulus make a Meissner's corpuscle effective

Example: Detect putting clothes on Do not detect wearing the clothes

Describe Merkel's receptors

``` High density in epidermis of digits and lips Lower density elsewhere (glabrous) Very low density (hairy) Slowly adapting Light touch ```

What actually is a Merkel cell?

An adapted keratocyte

What stimuli do Merkel cells respond to?

Initial indentation of skin | Sustained pressure of up to a few seconds

What do Merkel's receptors allow us to perceive?

Form and texture

Describe Ruffini's corpuscle

Responds to lateral movement or stretching of skin | Deep touch

Describe the structure of a Ruffini's corpuscle

Network of collagen fibres

Describe a Pacinian corpuscle

Found in deeper layers of dermis Rapidly adapting Stronger stimulus High frequency vibration

What is the structure of a Pacinian corpuscle

A fully encapsulated nerve ending Onion structure Relieves pressure on nerve ending

What does the activation of a Pacinian corpuscle feel like?

Vibration

Describe a hair follicle receptor

Light touch Rapidly adapting Constantly changing stimulus required

Describe a cutaneous thermoreceptor

Bare nerve endings | Slowly adapting sensory receptor

What are the two types of thermoreceptor?

Respond to cold | Respond to warm

What is important to remember about thermoreceptors?

They do not indicate absolute temperature | They are sensitive to change in temperature

How do we sense temperature?

Comparing signals from warm and cold receptors

What are thermoreceptor channels?

Non-specific cation channels | Nerve ending sensitivity dependant on which transducer channels are expressed

Describe TRPV3/4

Warm thermoreceptor Channels open 29-45 Maximum 45

Describe TRPM8

Cold thermoreceptor Channels open 8-38 Maximum 25 Also opened by menthol

Describe TRPA1

Cold thermoreceptor | Channels open <17

Cold receptors have what kind of axons?

C and Adelta

Warm receptors have what kind of axon?

What is paradoxical cold perception?

Cold receptors activated at very high temperatures | Feeling of cool at high temperatures

Describe cutaneous nociceptors

Bare nerve endings Non-adapting sensory receptors High threshold Adequate stimulus must be capable of damaging tissue

Name the 2 types of nociceptors

High threshold mechanoreceptors | Polymodal nociceptor

Describe a high threshold mechanoreceptor

Pricking pain | Adelta fibre

Describe a polymodal nociceptor

Poorly localised burning pain C fibre Sensitive to mechanical stimulus, heat and chemicals

What is proprioception?

Detecting the mechanical status of the muscle-skeletal system

What do proprioceptors provide information about?

``` Joint position Muscle length Muscle movement Acceleration Tension/force ```

Describe a muscle spindle

``` Specialised muscle fibres in a fibrous capsule Termed intrafusal fibres 1a afferents wrap around central portion Firing contributes to muscle tone Stretch sensitive = increased firing ```

Describe a golgi tendon organ

Located at the junction of the muscle and tendon Made of collagen fibrils Innervated by 1b sensory afferent neurone Sensitive to tension generated by contraction Its position is in series with the muscle