Sensory Pathways: Touch and Proprioception

Question 1

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

Answer 1

Mechanoreceptors

Question 2

Describe the structure of mechanoreceptors involved in touchand proprioception.

Answer 2

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

Question 3

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

Answer 3

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 4

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

Answer 4

Slow adapting receptors continue firing impulses for as long as thestimulus 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 5

What type of receptors are mechanoreceptors?

Answer 5

Mixture of slow and fast adapting receptors

Question 6

Describe how sensory neurons vary in their properties.

Answer 6

They vary in SIZE and CONDUCTION VELOCITY

Question 7

What are the two classifications of axons?

Answer 7

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 8

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

Answer 8

They are LARGE and have a FAST conduction velocity

Question 9

What is a receptive field?

Answer 9

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

Question 10

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

Answer 10

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

Question 11

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

Answer 11

It is coded by the FREQUENCY of the action potentials going down the sensory fibres

Question 12

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

Answer 12

Dorsal columns

Question 13

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

Answer 13

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 14

Where do these neurons synapse?

Answer 14

They synapse in the Cuneate and Gracile Nuclei in the medulla

Question 15

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

Answer 15

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

Question 16

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

Answer 16

Ventral Postero-lateral

Question 17

Describe the passage of the third order sensory neuron.

Answer 17

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

Question 18

What is the main sensory nerve of the face?

Answer 18

Trigeminal Nerve (CN V)

Question 19

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

Answer 19

Pons It synapses at the trigeminal cranial nucleus

Question 20

Describe the passage of this second order neuron.

Answer 20

The second order neuron crosses the midline (decussation) and joins the medial part of the median lemniscus

Question 21

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

Answer 21

Ventral Postero-medial

Question 22

What is lateral inhibition?

Answer 22

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

Question 23

What is the purpose of lateral inhibition?

Answer 23

Improves the resolution of localising the stimulus

Question 24

Name the three parts of the somatosensory cortex.

Answer 24

Primary Somatosensory Cortex (SI) Secondary Somatosensory Cortex (SII) Posterior Parietal Cortex

Question 25

What is the posterior parietal cortex mainly involved in?

Answer 25

Spatial relationships

Question 26

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

Answer 26

Anaesthesia (complete cessation of sensation) Parasthesia (sensation is there but it isn’t normal)

Question 27

What is the most common cause of peripheral neuropathy?

Answer 27

Diabetes mellitus

Question 28

What are examples of the 2 types of nerve ending?

Answer 28

Free nerve endings - noiciceptors Enclosed nerve endings - mechanoreceptors

Question 29

Describe the different kinds of sensory neurones

Answer 29

Aα - Biggest and fastest, Propiception Aβ - 2nd biggest and 2nd fastest, mechanoreceptors Aδ - 3rd biggest 3rd fastest, pain and temperature C - smallest and slowest, temperature, pain, itch.

Question 30

What are the types of thermoreceptor?

Answer 30

Aδ - 2 Cold activated TRPM8 & TRPA1 C - 4 Heat activated TRPV1-4

Question 31

What are the characteristics of thermoreceptors?

Answer 31

Free nerve endings, Transient receptor potential (TRP) ion channels

Question 32

What are the types of mechanoreceptor? What are they stimulated by?

Answer 32

• Meissner’s corpuscle - Fine discriminative touch, low frequency vibration
• Merkel cells - light touch and superficial pressure
• Pacinian corpuscle - Deep pressure, high frequency, vibration and tickling
• Ruffini endings - Continuous pressure or tough and stretch

Question 33

Define Stimulus threshold

Answer 33

‘’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 34

Define Stimulus intensity

Answer 34

The amount of neurotransmitter released as a direct result of the strength and duration of the stimulus

Question 35

Define adaptation

Answer 35

How the sensory organs transmit sensation and how the brain interprets that

Question 36

Define receptive field

Answer 36

‘’The receptive field is the region on the skin which causes activation of a single sensory neuron when activated’’ 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 37

Define 2 point discrimination

Answer 37

Minimum distance at which two points are perceived as separate Related to the size of the receptive field

Question 38

Draw the different mechanoreceptors

Answer 38

Question 39

Define lateral inhibition

Answer 39

Interneurons in the dorsal horn inhibit lateral neurones within the dorsal horn of the spinal cord This is because receptive fields can overlap Lateral inhibition helps facilitate pinpoint accuracy in the location of stimulus and discrimination

Question 40

What are the 2 types of receptor adaptation?

Answer 40

Tonic receptors - slow adapting detect constant stimulus strength transmit impulses to the brain as long as the stimulus is present e.g. merkel cells Phasic receptors - fast adapting detect a change in stimulus strength trasmit impulses at the beginning and end of a stimulus e.g. pacininan corpuscle

Question 41

What is the dorsal column system?

Answer 41

Innocuous mechanical stimuli Fine discriminative touch Vibration Aβ fibers enter via the dorsal horn and enter the ascending dorsal column pathways Information conveyed from lower limbs and body (below T6) travel ipsilaterally along the gracile tract Information conveyed from upper limbs and body (above T6) travel ipsilaterally along the cuneate tract

Question 42

What is the pathway of 1st order dosal column neurones?

Answer 42

1st order neurones synpase in the medulla Fibers in the Gracile Tract have their first synapse in the Gracile Nucleus Fibers in the Cuneate Tract have their first synapse in the Cuneate Nucleus

Question 43

What is the pathway of 2nd order dosal column neurones?

Answer 43

Second order axons decussate (cross the midline) in the caudal medulla Forms the contralateral medial lemniscus tract 2nd order neurons terminate in the thalamus The axons of the second order neurons terminate in the ventral posterior lateral nucleus of the thalamus (VPL).

Question 44

What is the pathway of 3rd order dosal column neurones?

Answer 44

3rd order neurons from the VPL project to the somatosensory cortex

Question 45

What is the spinothalamic pathway?

Answer 45

Pain and temperature sensations ascend within the lateral spinothalamic tract Crude touch ascends within the anterior spinothalamic tract

Question 46

What is the pathway of 1st order spinothalamic pathway neurones?

Answer 46

Primary afferent axons terminate upon entering the spinal cord Second order neurons decussate immediately in the spinal cord and form the spinothalamic tract

Question 47

What is the pathway of 2nd order spinothalamic neurones?

Answer 47

2nd order neurons terminate in the ventral posterior nucleus of the thalamus

Question 48

What is the pathway of 3rd order spinothalamic neurones?

Answer 48

3rd order neurons from the VPL project to the somatosensory cortex

Question 49

How do you assess damage to the sinal columns?

Answer 49

2 point discrimination - assess dorsal root column Temperature and pain tests assess the spinothalamic tract