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 touch and proprioception.

Answer 2

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

Question 3

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

Answer 3

Slow: fire impulses for as long as stimulus is present
Fast: fire at start of stimulus + sometimes when stimulus switches off, but tend to fade in the middle

Question 4

What type of receptors are mechanoreceptors?

Answer 4

Mixture of slow + fast adapting receptors

Question 5

Describe how sensory neurons vary in their properties.

Answer 5

SIZE

CONDUCTION VELOCITY

Question 6

What are the two classifications of axons?

Answer 6

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

Question 7

What is a receptive field?

Answer 7

A region on the skin which causes activation of a single sensory neurone when activated

Question 8

What is the difference between small + large receptive fields?

Answer 8

Small: allow for detection of fine detail over a small area= precise perception
Large: allow cell to detect changes over a wider area= less precise perception

Question 9

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

Answer 9

By FREQUENCY of the APs going down the sensory fibres

Question 10

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

Answer 10

Dorsal columns

Question 11

What are the bundles of axons within the spinal cord that have come from above and below T6 called? Describe their spatial arrangement within the spinal cord.

Answer 11

Above: Cuneate Fasciculus (lateral in cord)
Below: Gracile Fasciculus (medial in cord)
Travel ipsilaterally, desiccate in medulla

Question 12

Where do the fibres in the Gracile + Cuneate tracts have their 1st synapse?

Answer 12

In the medulla
Cuneate: Cuneate nucleus
Gracile: Gracile Nucleus

Question 13

Describe what happens after the gracile + cuneate neurones synapse and the tract that they run in.

Answer 13

2nd order neurons decussate in the caudal medulla + continue up the brainstem in the contralateral MEDIAL LEMNISCUS tract

Question 14

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

Answer 14

Ventral Posterior-lateral nucleus (VPL)

Question 15

Describe the passage of the 3rd order sensory neuron.

Answer 15

Project from VPL to the primary somatosensory cortex

Question 16

What is the main sensory nerve of the face?

Answer 16

Trigeminal Nerve (CN V)

Question 17

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

Answer 17

Pons

It synapses at the trigeminal cranial nucleus

Question 18

Describe the passage of this second order neuron (from the trigeminal nerve).

Answer 18

The 2nd order neuron decussates+ joins the medial part of the median lemniscus

Question 19

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

Answer 19

Ventral Postero-medial

Question 20

What is lateral inhibition?

Answer 20

Lateral branches of primary axons stimulate interneurones that are inhibitory on neighbouring axons
Prevents the overlap of receptive fields, facilitating pinpoint accuracy in localisation of stimulus
Mediated by interneurones in dorsal horn

Question 21

What is the purpose of lateral inhibition?

Answer 21

Facilitates enhanced sensory perception (discrimination)

Question 22

Name the 3 parts of the somatosensory cortex.

Answer 22

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

Question 23

What is the posterior parietal cortex mainly involved in?

Answer 23

Spatial relationships

Question 24

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

Answer 24

Anaesthesia (complete cessation of sensation)

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

Question 25

What is the most common cause of peripheral neuropathy?

Answer 25

Diabetes mellitus

Question 26

List the 4 sensory modalities

Answer 26

Touch (light mechanical stimuli)
Thermosensation (temperature)
Nociception (noxious + potentially damaging stimuli)
Propriception (mechanical displacement of muscles + joints)

Question 27

Describe the nerve endings in nociceptors, thermoreceptors + mechanoreceptors

Answer 27

Thermo- + Nociceptors = Free nerve endings

Mechanoreceptors = Enclosed nerve endings

Question 28

Describe 3 types of sensory neurone (diameter, conduction speed, myelination, type of receptor)

Answer 28

AB: Largest diameter, fastest conduction, highly myelinated = innocuous mechanical stimuli
AD: Medium diameter, medium conduction + myelination = noxious mechanical + thermal stimulation
C fibres: Smallest diameter, slowest conduction, no myelination= noxious mechanical, thermal + chemical stimulation

Question 29

What are sensory receptors?

Answer 29

Transducers that convert energy from the environment into neuronal action potentials

Question 30

Which neurones transmit hot and cold stimulation?

Answer 30

Heat= C fibres
Cold= AD fibres

Question 31

What type of ion channels are involved in thermoreception?

Answer 31

Transient receptor potential (TRP)

Question 32

List the 5 different types of mechanoreceptor + their function

Answer 32

Meissner corpuscles: Discriminative touch
Merkel discs: Light touch, superficial pressure
Hair follicle receptors: Light touch (hairy skin)
Ruffini endings: continuous touch, pressure
Pacinian corpuscles: Vibration

Question 33

Define absolute threshold

Answer 33

Point of intensity at which the person can just detect the presence of a stimulus 50% of the time

Question 34

What does increased stimulus strength + duration result in?

Answer 34

Increased NT release

More intense response to a stimulus

Question 35

Describe tonic receptors and give an example

Answer 35

Detect continuous stimulus strength
Continue to transmit impulses to brain as long as stimulus is present
e.g. Merkel cells (slowly adapting for superficial pressure + fine touch to be perceived)

Question 36

Describe phasic receptors and give an example

Answer 36

Detect a change in stimulus strength
Transmit an impulse at the start + end of the stimulus
e.g. when a change is taking place
E.g. Pacinian receptor (Sudden pressure excites receptor, transmits a signal again when pressure is released)

Question 37

What is 2 point discrimination and what is it related to?

Answer 37

Minimum distance at which 2 points are perceived as separate

Related to size of receptive field

Question 38

Where are cell bodies for the body and face?

Answer 38

Body: Dorsal root ganglia
Face: Trigeminal nerve

Question 39

What are the 2 main types of dorsal horn neurone?

Answer 39

Projection neurones: axons project to the brain

Interneurones: axons remain in spinal cord

Question 40

How do AB fibres differ to AD + C fibres in terms of where they relay info to?

Answer 40

AD + C fibres: relay to superficial layers (lamina 1+2)

AB fibres: relay to deeper layers (lamina 3,4,5)

Question 41

What is size of somatotropin area proportional to?

Answer 41

Density of sensory receptors in that body region