Sensory pathways: somatosensory system

Question 1

What can we use to salvage limbs

Answer 1

Free tissue transfer

Question 2

What is important to remember about noicieptors

Answer 2

they encode noxious stimuli- not just pain

Question 3

Summarise what is meant by somatosensory function

Answer 3

ability to interpret bodily sensations i.e. Mechanical, thermal, proprioceptive, nociceptive (noxious, not necessarily painful) - system consists of sensory receptors in skin/tissues/joints, nerve cells/tracts, and brain centres that process and modulate sensory information
 These include all the conscious senses other than vision, hearing, balance, sense and smell.
Gives you a lot of information- can identify where your body is touched, amount of pressure, whether it’s sharp or a threat- without looking at it

Question 4

List the main sensory modalities

Answer 4

mechanical - touch (light mechanical stimuli)
thermal- detection of temperature
proprioceptive- detection of the mechanical displacement of muscles and joints- positional sense
nociceptive - noxious or potentially damaging stimuli

Question 5

What does the somatosensory system consist of

Answer 5

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

Question 6

What re each of the senses referred to as

Answer 6

Modality, and modality refers to the type of information encoded

Question 7

What is important to remember about sensory neurones

Answer 7

‘’Individual axons of sensory nerves have modified terminals’’

Question 8

Describe some of the modifications of sensory neurones

Answer 8

Free nerve endings:
thermoreceptors &
nociceptors

Enclosed nerve endings:
mechanoreceptors

Question 9

1. What receptors are responsible for the sensory modalities of touch and proprioception?

Answer 9

Mechanoreceptors

Question 10

How do sensory neurones vary in their properties

Answer 10

size and conduction velocity

Question 11

What are the classifications of sensory neurones based on

Answer 11

Anatomical = based on axon diameter (labelled using LETTERS)
Physiological = based on conduction velocity (labelled using ROMAN NUMERALS)
As axon diameter and conduction velocity are related, there is a lot of overlap in the classifications

Question 12

Describe A- alpha fibres (group 1)

Answer 12

Large diameter (13-20 micrometres)
Fast conduction velocity ( 80-120m/sec)
Proprioceptors of skeletal muscle, also somatic motor
Myelinated

Question 13

Describe A-beta fibres (group 2)

Answer 13

Medium diameter (6-12 micrometres)
Medium conduction velocity (35-75 m/secs)
mechanoreceptors of skin- innocuous mechanical stimulation
Myelinated

Question 14

Describe A-delta fibres (group 3)

Answer 14

Small diameter ( 1-5 micrometers)
Relatively slow conduction velocity (5-30 m/secs)
Myelinated
noxious mechanical (pain) and thermal stimulation

Question 15

Describe C fibres (group 4)

Answer 15

Small diameter (0.2-1.5 micrometres)
Slow conduction velocity (0.5-2 m/secs)
Unmyelinated
noxious mechanical, thermal and chemical stimulation
itches too

Question 16

Define the term ‘receptor’

Answer 16

“sensory receptors are transducers that convert energy from the environment into neuronal action potentials

Question 17

Describe the activation of receptors

Answer 17

The receptor membrane depolarizes in response to its modality stimulus, causing a generator potential. If sufficient, this causes the axon to depolarize to its threshold level and produce an action potential. Because the axon recovers after its refractory period, a long-lasting generator potential will cause the axon to fire a train of impulses whose frequency will be proportional to the magnitude of the generator potential. Essentially, all stimuli are encoded as analogue signals and the sensory systems function as analogue-digital ‘translators’.

Question 18

What is the ultimate role of thermoreceptors

Answer 18

To allow us to detect small changes in temperature both in our environment and inside our bodies

Question 19

What is important to remember about thermoception

Answer 19

Thermoreceptors are not evenly distributed throughout the body
One area may be sensitive to cold- but an adjacent area sensitive to heat
 Temperature detection is most sensitive on the face and chest.

Question 20

What does the sensitivity of the thermoreceptor depend on

Answer 20

Depends on the ion channel type
Transient receptor potential (TRP) ion channels 
4 heat activated:
TRPV1-4 (also capsaicin), 3 -camphor 
2 cold activated 
TRPM8 ( mint) 
TRPA1 (horseradish, garlic, cinnamon)

Question 21

At what temperature if TRPV1 activated by

Answer 21

43 degrees Celsius- described as painful in psychosis clinical experiments

Question 22

What can also activate cold ion channels

Answer 22

Wasabi- 15 degrees Celsius

Question 23

What is important to remember about these agents that can bind to thermoreceptors and describe their potential in clinical use

Answer 23

They bind to ion channels for a long period of time
Why the spice sensation lasts for a while
Decreasing the ability for other stimuli- to have a painful response- so can be used as a topical analgesic

Question 24

Describe the thresholds for temperature pain

Answer 24

<32 degrees- painful cold
32- neutral
37-43- warmth
>43 -painful hot

Question 25

Summarise the thermoreceptors

Answer 25

Aδ- (cold) and C-fibres (heat)

Free nerve endings

Question 26

How many mechanoreceptors are there and how do they differ

Answer 26

5

Structure, size of receptive fields, tactile information they transmit and location

Question 27

What are the different types of mechanoreceptors

Answer 27

Meissner’s Corpuscle: fine touch discrimination and low frequency vibration
Merkel cells: light touch and superficial pressure
Pacinian Corpuscle: deep pressure and high frequency vibration (putting hand on speaker) and tickling
Ruffini endings: continuous pressure/touch and stretch- monofilament can squash the tissue underneath the finger and activate these receptors
Hair follicles- Light touch (hairy skin)

Question 28

What type of axons are the mechanoreceptors

Answer 28

A-beta

Question 29

What type of axons are the noicieptors

Answer 29

Sharp pain (Pin prick)- A-delta
Thermal (cold)-A-delta

Dull pain, ache- C
Thermal (heat)- C

Question 30

What is meant by the stimulus threshold

Answer 30

“A threshold is the point of intensity at which the person can just detect the presence of a stimulus 50% of the time (absolute threshold)’’
When a subliminal stimuli is applied- use monofilament- apply more pressure each time

Question 31

Summarise what is meant by stimulus threshold

Answer 31

 Stimulus threshold – described as the weakest stimulus detectable;
o Also known as – adequate stimulus required to elicit a specific response or reflex.
o Also known as – the minimum stimulus that is detected >50% of the time.
o Varies in location to anatomical location and with individual (personal) differences.

Question 32

What is meant by stimulus intensity

Answer 32

How quickly the neurone fires
increased strength and duration of the stimulus leads to greater NT release and greater intensity - encoded by frequency of neurone firing

timulus intensity discrimination:
o Information regarding stimulus intensity is conveyed by a variation of:
 Frequencies of APs generated.
 Number of separate receptors activated (recruitment).
o The final relationship between stimulus intensity and ultimate sensory discrimination may be linear or logarithmic.

Question 33

4. What is the difference between slow adapting and fast adapting receptors?

Answer 33

Slow adapting receptors continue firing impulses for as long as the stimulus is present
Fast adapting receptors tend to fire at the start of the stimulus and sometimes when the stimulus switches off but they tend to fade in the middle

Question 34

Describe how the adapting fibres work together

Answer 34

These two categories work in harmony to send different information about the same stimulus. The different signalling depends either on the linkage of the receptor to its incident energy or on a property called adaptation (i.e. a decline in receptor responsiveness even though the stimulus is still present). As a general rule, slowly adapting receptors signal the magnitude or location of a stimulus, whereas rapidly adapting receptors signal its rate of change and duration.

Question 35

Essentially, what does the term ‘adaptation’ refer to

Answer 35

ability of a receptor to stop firing while a stimulus is in place

Question 36

Summarise tonic receptors

Answer 36

Detect continuous stimulus strength
Continue to transmit impulses to the brain as long the stimulus is present
Keeps the brain constantly informed of the status of the body
e.g. Merkel cells
Slowly adapt allowing for superficial pressure and fine touch to be perceived.

Tonic receptors – do not adapt or adapt very slowly

Question 37

Give an example of tonic receptors in use

Answer 37

Sitting on a chair- only notice it when you first sit on it- but tonic receptors are continuously relaying information to your brain without you realising it- like a backup on a computer checking for viruses and ensuring that everything is working properly

Question 38

Summarise phasic receptors

Answer 38

Detect a change in stimulus strength
Transmit an impulse at the start and the end of the stimulus
e.g. when a change is taking place
The pacinian receptor
Sudden pressure excites receptor
Transmits signal again when pressure is released
Adapt quickly

Question 39

Describe the passage of stimulus activation

Answer 39

Stimulus
Receptor potential generated
Action potentials in sensory neurone

Question 40

Summarise cutaneous thermoreceptors

Answer 40

Thermoreceptors are slowly adapting and are poor indicators of absolute temperature. Cold receptors increase their discharge frequency in response to decreasing temperature whereas warm receptors increase their discharge frequency in response to increasing temperature

Question 41

Summarise what is meant by the term receptive field

Answer 41

The receptive field is the region on the skin which causes activation of a single sensory neuron when activated’’
Largely a function of the branching characteristics of the afferent within the skin; smaller arborisations result in smaller receptive fields.

Question 42

Compare the small and large receptive fields

Answer 42

Small receptive fields allow for the detection of fine detail over a small area. Precise perception

Large receptive fields allow the cell to detect changes over a wider area (less precise perception)

Question 43

Describe the regional differences in receptive fields

Answer 43

The fingers have many densely packed mechanoreceptors with small receptive fields
There are systematic regional variations in the average size of afferent receptive fields that reflect the density of afferent fibres supplying the area.
The receptive fields in the areas with dense innervation (fingers, lips, toes) are relatively small compared to those in the forearm or back, that are innervated by a smaller number of afferent fibres.

Question 44

Explain what is meant by two-point discrimination

Answer 44

Minimum distance at which two points are perceived as separate
Related to the size of the receptive field (region on the skin which causes activation of a single sensory neuron)

Regional differences in receptive field size and innervation density are the major factors which limit the spatial accuracy with which tactile information can be sensed. Thus, measures of two-point discrimination-the minimum inter stimulus distance required to perceive two stlmmutaneously applied stimuli as distinct- vary dramatically across the skin surface

Question 45

Summarise two-point discrimination

Answer 45

 The ability to detect that two stimuli are distinct from each other.
 Two-point Threshold – the minimum distance required between 2 stimuli in order to perceive that they are two separate stimuli.
 Depends on – peripheral mechanoreceptors, spinal posterior column and cortical function.
 Magnitude varies – back is 65mm, fingers are 2mm.
 Related to – density of innervation, area of receptive field and sensory homunculus.

Question 46

Summarise somatosensory dermatomes

Answer 46

Each spinal nerve has a specific dermatome on the skin
Mapping system to localise sensation

C5- Clavicles
T4- nipples
T10 -umbilicus

Question 47

Where are the key cell bodies of sensory neurones found

Answer 47

dorsal root ganglia (body) trigeminal ganglia (face)

Question 48

Describe the pathway of sensory information from the face

Answer 48

Cutaneous mechanoreceptor information from the face is conveyed centrally by a separate set of first-order neurones that are located in the trigeminal ganglion at the level of the pons
Corresponds to the dorsal column nuclei
Decussate at the pons
Second-order neurones cross the midline and ascend to the ventral posterior medial nucleus of the thalamus by way of the trigeminal leminiscus
Neurones in the VPM, send their axons to the ipsilateral cortical areas SI and SIII