Somatosensory system

Question 1

Primary sensory neuron functions:
A alpha - 
A beta - 
A delta -
C fibre -

Answer 1

A alpha - Proprioception in skeletal muscle (fastest)
A beta - Mechanoreceptors of skin for touch
A delta - Receptors for pain and temperature
C fibre - Receptors for temperature, pain and itch (slowest)

Question 2

What does the size of the receptive field determine?

Answer 2

The precision of localisation.
Smaller field = more precise e.g. touch
Larger field = less precise e.g. vibration
The precision is related to the area of the cerebral cortex devoted to the region

Question 3

How can the size of a receptive field be determined?

Answer 3

2 point touch discrimination

Question 4

What is a dermatome?

Answer 4

An area of skin innervated by afferent axon fibres, supplying a single nerve root.

Question 5

What is the firing rate proportional to?

Answer 5

Stimulus strength

Question 6

For a stimulus to be felt, what is required?

Answer 6

A moving or newly applied stimulus

Adequate stimulus that passes threshold

Question 7

How does membrane depolarisation in receptors occur?

Answer 7

Depolarisation is caused by transduction channel opening to produce a graded receptor potential. A stimulus may be constant even if firing is not continuous.

Question 8

What is the adaptation of a phasic receptor and what can it detect?

Answer 8

Fast adapting receptor. Used to detect continuously changing stimuli to either signal the change or to stop paying attention to stimuli that are no longer important e.g. wearing clothes.
Can detect how fast the stimulus is changing.
Requires a changing stimulus to fire.

Question 9

What is the adaptation of a tonic receptor and what can it detect?

Answer 9

Slow or non-adapting receptor. Required when maintaining information about a stimulus is valuable e.g. stretch or pain.
Can detect the strength of a stimulus.

Question 10

What is a line code?

Answer 10

The pathway connection that allows identification of a location of the receptive field by knowing which specific axon is carrying AP to a specific cortex location.

Question 11

What are the four types of cutaneous mechanoreceptors and where are they located?

Answer 11

Meissner's corpuscles
Merkel's receptor / disk
Ruffini's corpuscle
Pacinian corpuscle
Cutaneous sensory receptor specialisations are located at the end of the neuron.

Question 12

What are mechanorecptors and their function?

Answer 12

Detect touch, pressure and vibration through their specialised sensory apparatus. They are touch receptors at the end of A beta fibres. Its structure determines function and function determines location.

Question 13

What are Meissner’s corpuscles, their location and its three functions?

Answer 13

Located within papillary dermis (upper layer of dermis under epidermis).
Rapidly adapting - requires constantly changing stimulus to fire.
Functions:
Detect light touch and vibration.
Causes adjustment of grip force - The movement of an object when first gripped causes brief firing to increase muscle tone until the object no longer moves

Question 14

What are Merkel’s recpetors (cell-neurite complex) and their location and function?

Answer 14

Located in high densities in epidermis of digits, around mouth and glabrous skin (no hair). Very low density in hairy skin.
Slowly adapting for sustained light touch and perception of texture.

Question 15

What are Ruffini’s corpuscles and what do the detect?

Answer 15

An apparatus of collagen fibres that respond to lateral movement or stretching of skin and deep touch.

Question 16

What are Pacinian corpuscles and what do they detect?

Answer 16

A fully encapsulated nerve ending within the deep dermis layers. It rapidly adapts to strong stimuli, detecting deep touch and high frequency vibration. The encapsulated ending distorts to take up pressure and relieve the transmission to give a sense of vibration.

Question 17

Where is a hair follicle receptor located and what is its function?

Answer 17

The receptor is located in the dermis, wrapped around the hair. It detects light touch through deflection of the hair. It is rapidly adapting.

Question 18

What are thermoreceptors?

Answer 18

Slowly adapting bare nerve endings.
Either respond to warmth or cooling stimuli. Poor indicators of absolute temperature but sensitive to changes. They compare and integrate the signals from both warm and cold receptors.

Question 19

How do thermoreceptors signal and what type of channels do they use?

Answer 19

Use non-specific TRP (transient receptor potential) channels that open for a specific range of temperatures.
TRPV3/4 - Warm channels for 29-45 degrees (max firing 45)
TRPM8 - cold channels for 8-38 degrees (max firing at 25)
TRPA1/ANKTM1 - Cold channels for <17 degrees

Question 20

Which type of thermoreceptor channels does menthol open?

Answer 20

Chemically opens TRPM8 to induce coolness

Question 21

What are nociceptors?

Answer 21

Non-adapting bare nerve endings with a high threshold. An adequate stimulus must be capable of causing tissue damage.

Question 22

What are the two types of receptors used for the detection of pain and what type of pain does each detect?

Answer 22

Nociceptors can be:
High threshold mechanoreceptors using A delta fibres to cause a well localised prickling pain.
Polymodal nociceptors use C fibres in response to mechanical stimulus, damaging heat and noxious chemicals, causing a poorly localised burning pain.

Question 23

What do proprioceptors of the musculoskeletal system detect? What special apparatus does it use?

Answer 23

Detect mechanical status of:
Joint position
Muscle length
Muscle movement
Acceleration 
Tension / force
Using muscle spindles and Golgi tendon organs

Question 24

What is the function of a muscle spindle?

Answer 24

Detects change in length and acceleration within a muscle. Sensitive to length as parallel to muscle fibres.
Specialised intrafusal fibres within a fibrous capsule. Ia afferents wrap around the capsule. Stretch = increased firing to alter muscle tone.

Question 25

What is the function of a Golgi tendon organ?

Answer 25

Detect muscle tension generated by contraction at the junction of a muscle and tendon, using Ib sensory afferents.