Somatosensory neurones and receptors Flashcards
What are the common components of somatosensory pathway?
- 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)
What information does the somatosensory system obtain?
- 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…
How are the components of somatosensory pathway differ from each other?
- conduction velocities, level of decussation, terminal sites,number of neurones in path vary in different modalities
What are the 3 types of sensory modalities?
- Thermal, mechanical and chemical
What are primary afferent neurones?
- they detect the sensory stimuli and send message towards the CNS unlike sensory ganglion neurones (eg retinal ganglion cells) where they transduce and transmit
What are sensory ganglia and where are they found?
- 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
What is peripheral neuropathy?
- 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)
What is transduction?
- 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
What are the main classification of afferent sensory neurones? (5)
- 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
Different types of fibre respond to different modality. What are the different types?
- mechanical, chemical, thermal and multiple (by polymodal receptors)
- the modality depends on the specialised channels or receptors i nthe sensory membrane
THRESHOLD
- 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
ADAPTATION: what is it?
- 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
What is the adaptation characteristics of different mechanical receptors?
- 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
What are the different mechanisms of adaptation?
- 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
How do neurones code the intensity of stimulus?
- 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
