Somatosensory System 1

Question 1

Name 3 typical features of a Somatosensory neuron

Answer 1

1. Pseudounipolar neurons
2. Cell body (soma) is located in dorsal root ganglion (spinal cord) or cranial nerve ganglion
3. Axon terminates in spinal cord or brian stem

Question 2

Responses of receptors can either be tonic or phasic. Describe the two responses..

Answer 2

Tonic: Are slowly adapting receptors that respond for the duration of a stimulus

Phasic: Radidly adapt to a constant stimulus and turn off. Stop responding even if stimulus continues.

Question 3

What are the 3 types of cutaneous receptors?

Answer 3

1. Mechanoreceptors
2. Thermoreceptors
3. Nociceptors

Question 4

Hair follicle receptors (RA1), name; class, receptive field, modality, ending, adaptation, location, skin type, innervation…

Answer 4

Class: mechanoreceptor

Receptive field: small

Modality: hair movement

Ending: non-encapsulated

Adaptation: rapid

Location: superficial

Skin type: hairy

Innvervation: A-beta

Question 5

Meissner’s corpuscle (RA1), name; class, receptive field, modality, ending, adaptation, location, skin type, innervation…

Answer 5

Class: mechanoreceptor

Receptive field: small

Modality: touch, vibration (50 Hz)

Ending: encapsulated

Adaptation: rapid

Location: superficial

Skin type: glabrous

Innvervation: A-beta

Question 6

Pacinian corpuscle (RA2), name; class, receptive field, modality, ending, adaptation, location, skin type, innervation…

Answer 6

Class: mechanoreceptor

Receptive field: large

Modality: pressure, vibration (250-300 Hz)

Ending: encapsulated

Adaptation: rapid

Location: deep

Skin type: all

Innvervation: A-beta

Question 7

Merkel receptors (SA1); name; class, receptive field, modality, ending, adaptation, location, skin type, innervation…

Answer 7

Name: Merkel’s disk

Class: mechanoreceptor

Receptive field: small

Modality: light touch, pressure

Ending: non-encapsulated

Adaptation: slow

Location: superficial

Skin type: all

Innvervation: A-beta

Question 8

Ruffini’s corpuscles (SA2); name; class, receptive field, modality, ending, adaptation, location, skin type, innervation…

Answer 8

Name: Ruffini’s corpuscles

Class: mechanoreceptor

Receptive field: small

Modality: stretch, pressure

Ending: encapsulated

Adaptation: slow

Location: deep

Skin type: all

Innvervation: A-beta

Question 9

Free nerve endings (mechanoreceptors); name; class, receptive field, modality, ending, adaptation, location, skin type, innervation…

Answer 9

Name: Free nerve endings

Class: mechanoreceptor

Receptive field: small

Modality: light touch

Ending: non-encapsulated

Adaptation: slow

Location: superficial

Skin type: all

Innvervation: A-delta and C

Question 10

Free nerve endings (thermoreceptors); name; class, modality, ending, adaptation, location, skin type, innervation…

Answer 10

Name: Free nerve endings

Class: thermoreceptor

Modality: Hot and cold temperature

Ending: non-encapsulated

Adaptation: rapid

Location: superficial

Skin type: all

Innvervation: C for hot and A-delta for cold

Question 11

How do thermoreceptors transduce their adequate stimulus into a neural signal

Answer 11

Transduction: via temperature-sensitive transient receptor potential (TRP) ion channels. TRP channels are non-specific cation channels (Ca2+, Na+, Mg2+)

Question 12

How do Nociceptors (pain) transduce their adequate stimulus into a neural signal?

Answer 12

Primary afferent neurons release both glutamate and substance P on to dendrites in the second order neurons in the spinal cord and brian.

Question 13

How are “first” and “second” pain signals carried?

(what type of fibres)

Answer 13

First pain; carried by A-delta type fibres

Second pain; arrives by C type fibres

Question 14

Describe the axon reflex

Answer 14

Axon reflex causes release of neuropeptides (e.g. substance P) from afferent endings that cause vasodilation of blood vessels around the injury site

Question 15

What is Hyperalegsia?

Answer 15

Abnormally heightened sensitivity to noxious stimuli

Question 16

What is Allodynia?

Answer 16

pain sensation in response to normally non-painful stimuli (e.g. touching, cooling/heating)

Question 17

The Trigeminal nerve is the principle somatosensory nerve of the head. What are its 3 branches?

Answer 17

1. Ophthalmic branch
2. Maxillary nerve
3. Mandibular nerve

Question 18

What are the three trigeminal sensory nuclei?

Answer 18

1. Mesencephalic trigeminal nucleus
2. Pontine trigeminal nucleus
3. Spinal trigeminal nucleus

Question 19

What is the dermatome?

Answer 19

A sensory region on the surface of the body that sends afferents via a given spinal nerve.

Question 20

Sensation of pain may be accompanied by:

Answer 20

1. Relef withdrawl from stimulus
2. Autonomic response
3. Emotional response

Question 21

Via which tract do secondary order neurons ascend to the thalamus?

Answer 21

via the lateral spinothalamic tract

Question 22

Explain the gate-control theory

Answer 22

When you rub the skin around a painful area e.g. stubbed toe. It activates mechanoreceptors in the skin which excite inhibitory interneurons in the brain/spinal cord. This supresses the transmission of pain signals by inhibiting the second order neurons.

Question 23

Explain Endogenous Analgesia

Answer 23

In response to stressful situations (e.g. fights) the brain can supress the transmission of pain signals.

Descending pathways from the midbrain release endogenous opioids such as enkephalin onto the synapse between the primary nociceptor afferents and the second order neurons in the dorsal horn of the spinal cord.

Question 24

Explain Central Sensitization

Answer 24

Prolonged or intense release of glutamate from primary nociceptor afferent nerve terminals onto the dendrites of secondary afferent nerves in response to painful stimulus.

Question 25

Columnar organisation

Answer 25

skin receptors in the same body region (i.e. have overlapping receptive fields).

Question 26

Hierarchial organsation

Answer 26

Cortical neurons show increase in size of receptive field