7. Somatosensory system

Question 1

Define propioception

Answer 1

It allows you to be aware of the position of different parts of your body relative to the rest of your body

Question 2

Somatosensory function?

Answer 2

Ability to interpret bodily sensations
	 mechanical	
	 thermal
	 proprioceptive
	 nociceptive - encoding of noxious stimuli                   - doesn’t necessarily imply a sensation is painful

Question 3

What does the somatosensory system consist of?

Answer 3

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

Question 4

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

Answer 4

Mechanoreceptors

Question 5

Describe the structure of mechanoreceptors involved in touch and proprioception.

Answer 5

The receptor is NOT a separate entity but is actually the peripheral terminal of the peripheral axon of the primary sensory neuron.

Question 6

Describe the structure of a Pacinian corpuscle and explain how this structure relates to its function.

Answer 6

There is an axonal ending in the middle and it is wrapped around several concentric circles of epithelial cells – this allows the receptor to be very sensitive to vibration.

Question 7

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

Answer 7

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 8

Difference between mechanoreceptors and thermoreceptors/nociceptors?

Answer 8

N and T : free nerve endings

Mechanoreceptors have enclosed nerve endings

Question 9

What type of receptors are mechanoreceptors?

Answer 9

Mixture of slow and fast adapting receptors

Question 10

Describe the classification of sensory neurons?

Answer 10

alpha-beta are largest and fastest, diameter and myelin coating increases speed of transmission, they are mechanoreceptors and encode innocuous mechanical stimulation,MECHANORECEPTION REACHES SOMATOSENSORY CORTEX FIRST BEFORE NOXIOUS STIMULI

alpha-delta : smaller, still myelin covered, noxious mechanical and thermal stimulation

c fibres : smallest, no myelin covering, slowest, encode noxious mechanical, thermal and chemical stimulation

Question 11

Define sensory receptors

Answer 11

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

Question 12

Describe how sensory neurons vary in their properties.

Answer 12

They vary in SIZE and CONDUCTION VELOCITY

Question 13

What are the two classifications of axons?

Answer 13

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 14

Describe the general structure of sensory neurons that convey touch and proprioceptive information.

Answer 14

They are LARGE and have a FAST conduction velocity

Question 15

Discuss thermoreceptors

Answer 15

Alpha-δ and c fibres
free nerve endings
Transient receptor potential ion channels
4 heat activated: TRPV1-4, 1 activated by capsacin
2 cold activated: TRPM8/A1

Question 16

What is a stimulus threshold?

Answer 16

“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)’’

Question 17

Describe how neurons can code for the intensity of a stimulus.

Answer 17

Increased stimulus strength and duration = increased neurotransmitter release = greater intensity

Question 18

Two types of sensory adaption?

Answer 18

Tonic receptors

Phasic receptors

Question 19

What are tonic receptors?

Answer 19

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

Question 20

What are phasic receptors?

Answer 20

Detects change in strength
Transmit impulse at start and end of stim.
Pacinian receptor: sudden pressure excites receptor, transmits signal again when pressure is released

Question 21

What is a receptive field?

Answer 21

An area of skin that is innervated by one sensory axon and its branches

Question 22

Describe how the receptive fields in the lips and mouth vary from the receptive fields of the upper arm.

Answer 22

Lips and Mouth – high-density innervation with very small receptive fields
Upper arm – larger receptive fields and thinner innervation

Question 23

Which part of the spinal cord carries sensory axons for touch and proprioception?

Answer 23

Dorsal columns

Question 24

Define two-point discrimination?

Answer 24

Minimum distance at which two-points are perceived as seperate, related to size of the receptive field

Question 25

Where are cell bodies of face and body found?

Answer 25

Body: dorsal root ganglia

FACE : trigeminal ganglia

Question 26

What is lateral inhibition?

Answer 26

Lateral inhibition takes place in the cuneate and gracile nuclei
Each axon has lateral branches that are inhibitory on neighbouring axons
So each axon will stimulate a second order neuron and inhibit neighboring first order neurons

Question 27

What is the function of lateral inhibition?

Answer 27

Lateral inhibition prevents overlap of receptor fields, facilitates pinpoint accuracy in localisation of the stimulus, mediated by interneurons within dorsal horn, enhanced sensory perception

Question 28

What are the bundles of axons within the spinal cord that have come from above and below the waist called? Describe their spatial arrangement within the spinal cord.

Answer 28

Above the waist – Cuneate Fasciculus
Below the waist – Gracile Fasciculus 
Axons from below the waist are packed more medially in the dorsal column and above the waist are more lateral 
Lower = Medial
Higher = Lateral

Question 29

Where do these neurons synapse?

Answer 29

They synapse in the Cuneate and Gracile Nuclei in the medulla

Question 30

Describe what happens after these neurons synapse and the tract that they run in.

Answer 30

The second order neurons then cross the midline (decussation) in the caudal medulla and continue up the brainstem in the MEDIAL LEMNISCUS

Question 31

Which thalamic nucleus is responsible for relaying somatosensory information from the neck down?

Answer 31

Ventral Postero-lateral

Question 32

Describe the passage of the third order sensory neuron.

Answer 32

The third order neurone travels from the ventral postero-lateral nucleus in the thalamus to the primary somatosensory cortex

Question 33

Describe the somatotopic representation

Answer 33

Size of the somatotopic areas is proportional to density of sensory receptors in that body region
Pain temperature localisation not as precise

Question 34

What is the main sensory nerve of the face?

Answer 34

Trigeminal nerve 5

Question 35

Where does the trigeminal nerve enter the brainstem and where does it synapse with a second order neuron?

Answer 35

Pons

It synapses at the trigeminal cranial nucleus

Question 36

Which thalamic nucleus is responsible for relaying sensory information from the face?

Answer 36

Ventral Postero-medial

Question 37

Name the three parts of the somatosensory cortex.

Answer 37

Primary Somatosensory Cortex (SI) - located in postcentral gyrus of posterior parietal cortex
Secondary Somatosensory Cortex (SII)
Posterior Parietal Cortex

Question 38

What is the posterior parietal cortex mainly involved in?

Answer 38

Spatial relationships

Question 39

Damage to the touch and proprioception pathway will halt sensory information going up to the primary somatosensory cortex. What effects will this have?

Answer 39

Anaesthesia (complete cessation of sensation)

Parasthesia (sensation is there but it isn’t normal)

Question 40

Which spinothalamic pathway is responsible for pain/temp and which for crude touch?

Answer 40

Pain/temp - lateral spinothalamic tract
crude touch - medial spinothalamic tract (alpha - delta)

(light touch - dorsal column, is mediated by alpha - beta fibres meissner’s corpuscles)

Question 41

Describe the spinothalamic anterolateral pathway

Answer 41

1st order neurons terminate in dorsal horn and terminate entering the spinal cord. The 2nd order neurons deccusate immediately in spinal cord and form tract. 2nd order neurons terminate in VPLN

Question 42

Describe some features of nociceptors.

Answer 42

Polymodal – different types of nociceptor respond to different stimuli
Free nerve endings – usually just free axonal endings of neurones
High threshold – higher activation threshold than touch receptors
Slow adapting – this is good because it means you are constantly reminded of the presence of a potentially harmful stimulus

Question 43

Compare the receptive fields of nociception to those of touch.

Answer 43

Receptive fields for nociception are much LARGER because the nociceptive pathway is phylogenetically older than touch and you don’t need to be able to localise pain as well as touch.

Question 44

Describe the somatotopic arrangement of the fibres in the spinothalamic tract.

Answer 44

Lower fibres = Lateral
Higher fibres = Medial
(Opposite of dorsal columns)

Question 45

Describe the passage of the trigeminal nerve from entry into the brainstem.

Answer 45

It enters the trigeminal ganglion in the pons and then it moves DOWNWARDS along the trigeminal nucleus.
It then synapses in the lower part of the trigeminal nucleus in the medulla.
The second order neurone then decussates and joins the medial end of the spinothalamic tract.

Question 46

Which part of the trigeminal nucleus does the first order nociceptive neurones from the face synapse in?

Answer 46

Spinal Trigeminal Nucleus

Question 47

As the spinothalamic tract projects towards the primary somatosensory cortex, it gives off collateral branches. Which structures do these branches go to?

Answer 47

Brainstem (reticular formation)
Thalamus (intralaminar nuclei)
Hypothalamus
Limbic structures

Question 48

What are the two pathways that can reduce the amount of pain that you feel?

Answer 48

Central and Peripheral Inhibition Pathways

Question 49

What is the focus of the central inhibition pathway?

Answer 49

Periaqueductal Grey Matter

Question 50

Describe the arrangement and function of the central inhibition pathway.

Answer 50

Increased brain activity will increase the impulses going down the central inhibition pathway, which goes to the dorsal horn at every level.
These descending axons synapse with an interneurone and activate the interneurone.
The interneurone synapses with the first and second order nociceptive neurones and release ENKEPHALIN, which is inhibitory.
So enkephalin release will reduce the amount of information going down the spinothalamic tract hence you feel less pain.

Question 51

What type of molecule is Enkephalin?

Answer 51

Opioid

Morphine mimics the action of this central inhibition system.

Question 52

Other than a first order nociceptive neurone, what else has input into the second order nociceptive neurone?

Answer 52

Non-nociceptive neurones
Axons of non-nociceptive touch neurones will go into the dorsal horns but will also have collaterals that are capable of activating an inhibitory interneurone, which can reduce the activity of the projecting neurone and hence reduce the activity going up the spinothalamic tract.

Question 53

Describe the arrangement and function of the peripheral inhibition pathway.

Answer 53

Stimulation of touch receptors in the same area as the pain sensation will lead to increased activity of the non-nociceptive touch neurones meaning that there is increased activation of the inhibitory interneurone and hence reducing the activity going up the spinothalamic tract.

Question 54

Define hyperalgesia and allodynia

Answer 54

Allodynia: pain due to stimulus that doesn’t usually result in pain
Hyperalgesia: increased pain from a stimulus that normally provokes pain

Question 55

How can you get exacerbation of pain?

Answer 55

Wind up in the dorsal horn
If someone has chronic pain then certain peripheral nerves coming into the spinal cord will be carrying high levels of input for a long time.
The cells in the dorsal horn can lower their sensitivity or their synapses will change, which means that the information going into the spinothalamic tract is increased so this can actually increase the level of chronic pain.