Somatosensory neurones and receptors

Question 0

What are the common components of somatosensory pathway?

Answer 0

• contain specialised sensory endings and peripheral afferent neurones
• all synapse with second order neurones in CNS
• they decussate at some point and end in the cortical areas
• relay in thalamus ( where info integration occurs)

Question 1

What information does the somatosensory system obtain?

Answer 1

• all sensory neural information excluding special sense, eg. hearing/ vision
• there are different types of receptor in place to obtain info: exteroceptors (outside world), proprioceptors (position of body), interoceptors (internal environment)
• these receptors are important in maintaining homeostasis by detecting pain, heat, touch, pressure…

Question 2

How are the components of somatosensory pathway differ from each other?

Answer 2

• conduction velocities, level of decussation, terminal sites,number of neurones in path vary in different modalities

Question 3

What are the 3 types of sensory modalities?

Answer 3

• Thermal, mechanical and chemical

Question 4

What are primary afferent neurones?

Answer 4

• they detect the sensory stimuli and send message towards the CNS unlike sensory ganglion neurones (eg retinal ganglion cells) where they transduce and transmit

Question 5

What are sensory ganglia and where are they found?

Answer 5

• Most sensory neurones have cell bodies in dorsal root ganglia found in each spinal segment (aka spinal ganglia)

Exceptions:
- trigmeninal ganglia found in cranial nerve root ganglia; visceral afferent cell bodies in superior and inferior vagal ganglion

Question 6

What is peripheral neuropathy?

Answer 6

• It is where distal peripheral nerves are affected due to metabolic/ toxic causes. Usually has a glove and stocking disturbution
• long and small diameter axons are especially vulnerable ( larger diameter afferents are usually spared)

Question 7

What is transduction?

Answer 7

• conversion of 1 form of energy to electrical impulses (as the CNS signals by using electrical signalling)
• sensory nerve endings have specialised receptors designed to detect various stimulus modalities so when they are activated, a receptor potential (aka generator potential) is generated. If the receptor potential is high enough, ie above threshold, action potential can be triggered

Question 8

What are the main classification of afferent sensory neurones? (5)

Answer 8

• Modality, ie what stimulus they respond to
• Threshold, ie high intensity of stimulus or low
• Adaptation, ie how do they react to a maintained stimulus
• Conduction velocity, ie how fast they conduct message towards the CNS
• Receptive field, ie what region of the body do they innervate

Question 9

Different types of fibre respond to different modality. What are the different types?

Answer 9

• mechanical, chemical, thermal and multiple (by polymodal receptors)
• the modality depends on the specialised channels or receptors i nthe sensory membrane

Question 10

THRESHOLD

Answer 10

• high intensity stimulus is needed to activate a high threshold fibre. (intensity describes the stimulus; threshold describes the characteristic of the neurone not the stimulus)
• low threshold mechanoreceptors give rise to sensations such as stroke and touch; low threshold thermoreceptors give rise to sensations such as warm and cool; low chemoreceptors give rise to sensations such as taste or smell.
• high threshold fibres only respond to potentially damaging stimuli = noxious, eg hot and cold; acids; pinch
• pain would only be perceived once the info has been reached and processed by the higher centres so no pain if doesnt reach higher centres in brain

Question 11

ADAPTATION: what is it?

Answer 11

• when the maintained stimulus of constant strength is applied to a sensory receptive terminal= the firing frequency can reduce with time.
• some neurones are fast adapting while some are slow adapting

Question 12

What is the adaptation characteristics of different mechanical receptors?

Answer 12

• Maissner’s corpuscle= rapidly adapting
• Merkel’s disc= slowly adapting
• Pacinian corpuscle= very rapidly adapting
• Ruffini’s corpuscle= slowly adapting

->slowly adapting units activated by pressure/ stretch while rapidly adapting units activated by movement/ acceleration

Question 13

What are the different mechanisms of adaptation?

Answer 13

• desensitisation of the receptors that produce the sensory potential
• activity-dependent changes in the excitability of sensory nerve endings
• changes in tissue surrounding nerve endings

Question 14

How do neurones code the intensity of stimulus?

Answer 14

• 1) action potentials do not get bigger, they become more frequent
• 2) recruitment: units with a low threshold or with receptive fields closer to the stimulus are recruited and now units with a higher threshold or receptive fields which are more distant to the stimulus are recruited

Question 15

What can alter the activity of impulse conduction?

Answer 15

• A fibres are myelinated; C fibres are unmyelinated but run in Remak bundles.
• saltatory conduction takes place in myelinated fibres where conduction between nodes of Ranvier so faster

Question 16

Electrical activation of primary afferents

Answer 16

• sensory fibres can be activated by electrically stimulating them. So bypass the receptors. Microneurography can be done in clinical neurophysiology to assess how well nerves are working.

Question 17

CONDUCTION VELOCITY: which type of fibres can transmit info the quickest?

Answer 17

• A fibres are myelinated= faster than C fibres which are unmyelinated
• C and A delta fibres are nociceptive fibres so have free nerve ending; other classes of A fibres are NON-nociceptive so have encapsulated ending

Question 18

Classification of sensory receptor fibre types

Answer 18

- class 1a and 1b are A alpha fibres= myelinated, largest diameter (13-20micrometers), quickest conduction velocity (80-120 m/s). 
FOR: primary receptors of muscle spindle and golgi tendon organ

- class 2 is A beta fibres= myelinated, slower diameter, slower conduction velocity 
FOR: seondary receptors of muscle spindle and all cutaneous mechanoreceptors

- class 3 is A delta fibres, little myelination and even smaller diameter axon and slower conduction velocity
FOR: free nerve ending of touch and pressure; nociceptive of neospinothalamic tract (lateral) and cold thermoreceptors

- class 4 is C fibres, unmyelinated with smallest diameter and slowest conduction velocity. 
FOR: nocipetive of paleospinothalamic tract ( medial) and warm receptors

Question 19

What are the main types of sensory fibres which proprioceptors, mechanoceptors

Answer 19

• Proprioceptors= type 1 and 2 sensory fibres
• Mechanoreceptors= type 2 and 3 sensory fibres
• Nociceptors and thermoreceptors= type 3 and 4 sensory fibres

Question 20

RECEPTIVE FIELDS

Answer 20

• is where an adequate stimulus at that region can cause AP firing in the neurone
• for primary afferent neurones, receptive fields are at site of projection of peripheral terminal; for secondary afferents the receptive fields maybe larger or more complex

Question 21

Classification of the sense

Answer 21

• exteroceptors: provide info about the outside world eg touch, pressure, vibrationt
• proprioceptors: provide info about the muscle skeletal system eg homeostasis, organ function…
• interoceptors: provide info about the internal bodily organs eg position and movement