13: Somatic Sensation Flashcards
What are the 4 sensory receptors of skin?
- Temperature (thermoreceptors)
- Position of limbs (proprioceptors)
- Touch receptors (mechanoreceptors)
- Pain (nociceptors)
Describe thermoreceptors
- Different types more active at warm or cold temperature ranges, especially responsive to changing temperature
Describe proprioceptors
- (Muscle spindles, golgi tendon organs, joint capsule) skin mechanoreceptors
Decribe touch/mechanoreceptors
- Sensitive to mechanical deformation (mechanoreceptors)
- Various types
Describe nocicepters (pain)
- Respond to extreme mechanical, temperature, and/or chemical stimuli
What are four types of skin touch receptors?
- Meissner’s corpuscles
- Pacinian corpuscles
- Merkel cell endings
- Ruffini’s corpuscles
Draw out/decribe a pacinian corpuscle
slides
What are four types of sensory coding?
- Modality
- Intensity
- Location
- Duration
What is modality?
(eg; touch vs temperatre)
-Specificity of receptors. ‘labelled line’. (slides)
What is intensity?
-Frequency of action potentials in each axon, number of axons activated
What is location?
Mapping of receptive fields of individual primary afferents to specific cortical locations (somatotopic representation)
Describe duration
- ‘rapidly adapting’ receptors respond briefly, even if stimulus is sustained. Detect movement, changing pressure.
- ‘slowly adapting’ receptors signal true duration of stimulus
Where does the anterolateral/ventral pathway cross over, and what is it used for
- Crosses over low in the spinal cord
- For general or broad sensations pain, temperature
Where does dorsal column pathways cross over, an what is it for?
- Crosses over high in the spinal cord, in medulla
- For fine, detailed pathways
What is lateral inhibition?
A fundamental mechanism to increase accuracy of sensory information
Slides
What is lateral inhibition mediated by?
Inhibitory interneurons
When is lateral inhibition used
- In somatic sensation, lateral inhibition localises sensation to a restricted area of skin
- Lateral inhibition is used wherever accurate location of a stimulus is required -or where a pattern of input needs to be discriminated (eg; vision, smell..)
Lateral inhibition results in centre-surround inhibition
Locate the somatosensory cortex
slides
Describe the somatotopic map
-
Describe pain (noiception) and its use
slides
What are the two types of nerve fibres that carries pain/noiciception?
- “C”-fibres
- “Aδ” -fibres
Describe C fibres
- Smallest diameter, unmyelinated axons
- Slow conduction velocity
- Signal ongoing damage (or potential damage)
- Polymodal
Describe “Aδ” -fibres
- Small diamter myelinated axons
- Faster conduction velocity
- Signal acute onset of painful stimulus
- Primary mechanoreceptors
Describe sensitation in pain pathways
slides
Describe the gate control model for pain
slides
What are two chemical methods of inhibiting pain
- Endogenous opiates
- Endocannabinoids
Describe endogenous opiates
- Released at synapses on pain-pathway neurons
- “natural analgesia” system, acute response to pain, stress
- Site of action of ‘centrally acting’ painkillers (morphine, codeine)
Describe endocannabinoids
- Synthesised and released by neurons (but not stored in vesicles), pain receptors, as well as centrally
- Apparently mediate some presumed effects of endogenous opiates
Describe referred pain
-Occurs when pain is poorly localised
- Feelings from viscera (gut) is referred to the body surface
- eg; heart pain to neck and arm
- activated by stretch, ischemia -except in brain
-Also, nerve compression can cause pain to be felt in region of nerve termination (slipped disc)
What is a phatom limb
Sensation “felt” in region that no longer exists
What are two causes of a phantom limb
- Ongoing activity in nerves that used to come from that part
- Invasion of cortical representation for that part by intact body regions
- Eg; cortex representation of amputated hand may become activated by inputs from face
Fibres from the face then occupy the ‘territory’ of the hand -allowing it to become its own sensory input.
- Invasion of cortical representation for that part by intact body regions
