The Somatosensory System

Question 1

Define Somatosensory function

Answer 1

Ability to interpret bodily sensations
o mechanical
o thermal
o proprioceptive
o nociceptive

Question 2

What does the somatosensory system consist of?

Answer 2

o Sensory receptors in skin, tissues & joints
o nerve cells & tracts in body & spinal cord
o brain centres that process & modulate sensory information

Question 3

What is referred to as somatosensory modalities?

Answer 3

Includes all conscious senses:
o touch
o proprioception
o temperature
o pain

Question 4

What are the 3 main receptors making up the somatosensory modalites?

Answer 4

o Mechanoceptor

o Thermoceptors

o Nociceptors

Question 5

Describe the nerve endings for the 3 main receptors making up the somatosensory modalities?

Answer 5

o FREE nerve endings:

• thermoceptors
• nociceptors

o ENCLOSED nerve endings
- mechanoceptors

Question 6

Nerve Fibre Classification?

Answer 6

FIBRE A:

o Alpha - proprioception (MOTOR)
o Beta - touch, pressure
o Gamma - MOTOR to muscle spindle
o Delta - pain, cold & touvh

FIBRE B:

o POSTganglionic autonomics

FIBRE C:

o Dorsal root - pain, temperature & mechanoception
o Sympathetic - POSTganglionic sympathetic

Question 7

Main SENSORY neurones?

Answer 7

o A-beta

o A-delta

o C-fibres

Question 8

Properties of the MAIN SENSORY neurones?

Answer 8

A-delta & C-fibres heavily invovled in PAIN (noxious)

A-beta heavily involved in TOUCH & PRESSURE (non-noxious)

Question 9

Speed of conduction of the MAIN SENSORY neurones?

Answer 9

A-beta = MOST QUICKLY (most abundant)

A-delta = SMALLER but myelinates

C-fibres = SMALLEST & NO myelination

Question 10

Define sensory receptor

Answer 10

Transducers

that convert energy from the environment into neuronal APs

Question 11

Fibres associated with thermoreceptors and their properties?

Answer 11

A-DELTA & C-FIBRES
(have FREE nerve endings)

Work via. TRP ION CHANNELS

Question 12

Specific TRP channels associated with thermoreceptors and what they’re activated by?

Answer 12

Transient Receptor Potential (Ion Channels)

o TRPV1-4 - HEAT activated
o TRPM8 & TRPA1/V1 - COLD activated

Question 13

Thermoreceptors that have sense overlap?

Answer 13

TRPV - heat & spice

TRPM8 - cold & menthol

TRPV1 - hear & cold

Question 14

5 example of Mechanoreceptors?

Answer 14

o Meissner’s Corpuscles

o Merkel Cells

o Pacinian Corpuscle

o Ruffni Endings

o Hair follicle

Question 15

Meissner’s Corpuscle?

Answer 15

Mechanoreceptor

o Fine, discriminate touch
o LOW frequency vibration

Question 16

Merkel Cells?

Answer 16

Mechanoreceptor

o Light touch
o Superficial pressure

Question 17

Pacinian Corpuscles?

Answer 17

Mechanoreceptor

o Detects deep pressure
o High frequency vibration & tickling

Question 18

Ruffni Endings?

Answer 18

Mechanoreceptors

o Continous pressure
OR
o Touch & stretch

Question 19

Fibres associated with mechanoreceptors?

Answer 19

A-BETA

Question 20

Define Stimulus Threshold

Answer 20

Described as the weakest stimulus detectable

The point of intensity at which the person can DETECT the PRESENCE of a STIMULUS 50% OF THE TIME (absolute threshold)

Question 21

Define Stimulus Intensity

Answer 21

Info. regarding this is conveyed by a variation of:

o Frequencies of APs generated
o Number of seperate receptors activated (recruitment)

Question 22

What does Stimulus Threshold vary with?

Answer 22

o Anatomical location

o Inter-individual differences

Question 23

Explain the Neural Adaptation phenomenon

Answer 23

If stimulus of CONSTANT STRENGTH is maintained for a period time, the FREQUENCY of APs DIMINISHES

Question 24

Tonic receptors?

Answer 24

SLOWLY adapting (e.g. Merkel cells)

o Detect CONTINOUS stimulus strength (continue to transmit impulse to brain as long as stimulus present)

Allows for (to be perceived):
o superficial pressure
o fine touch

Question 25

Phasic receptors?

Answer 25

RAPIDLY adapting (e.g. pacinian receptors)

o Detect a CHANGE in stimulus strength (transmit impulse @ start & end of stimulus)

It:
o sudden pressure excites receptor
o transmits signal again when pressure released

Question 26

Define Receptive Field

Answer 26

Region on skin which causes activation of a SINGLE SENSORY NEURONE when activated

Question 27

Difference between SMALL & LARGE Receptive Fields

Answer 27

Small R.F:
o allows for detection of fine detail over a SMALL area
o precise perception

Large R.F:
o detects changes over a WIDER area
o less precise perception

Question 28

Can Receptive Fields overlap?

Answer 28

YES

Recruitment of adjacent field can INCREASE STIMULUS INTENSITY = INCREASE APs

Question 29

Define 2-point Discrimination

Answer 29

MINIMUM DISTANCE at which TWO POINTS are perceived as separate

Question 30

What does 2-point discrimination depend on?

Answer 30

1. Peripheral mechanoreceptors (density of innervation area of receptive fields)
2. Spinal posterior column (convergence of signals)
3. Cortical function (sensory homunculus)

Question 31

Where are the sensory cell bodies for the body & face?

Answer 31

Body:
o Dorsal Root Ganglia

Face:
o Trigeminal Ganglia

Question 32

Define Lateral Inhibition

Answer 32

Activation of one neural unit INHIBITS the activation of other neural units

Prevents the OVERLAP of RECEPTIVE FIELDS - enhances sensory perception (discrimination)

Question 33

What mediates lateral inhibition?

Answer 33

Inhibitory INTERNEURONES within the dorsal horn

Question 34

2 main types that the dorsal horn neurone can be divided into?

Answer 34

o Projection neurones - project to brin

o Interneurones - remain in SC