Somatic Sensation Flashcards
Describe the different types of mechanoreceptors and the type of touch information each is tuned to detect.
- Pacinian Corpuscle - encapsulated, deep in dermis, lollipop looking, identifies pressure, large receptive field
- Ruffini’s ending - encapsulated, deep dermis, floppy looking, detect skin stretch, large receptor field
- Meissner’s corpuscle - encapsulated, upper dermis, leaf looking, detect fine discriminative touch, small receptor field, glabrous skin ie fingertips
- Merkel’s Disc - separate cells, epidermis, branches looking, detect fine touch, v small receptor field, lips & fingertips
- Krause end bulbs - polymodal (touch and temp), genitals and lips.
Describe the receptive fields and adaptation rates of the different mechanoreceptors.
Meissner’s corpuscle and Merkel disks have small receptive fields
Pacinian corpuscle and Ruffini ending have large receptive field.
Meissner & pacinian adapt rapidly
Merkel & Ruffini adapt slowly
What is meant by adaptation rate of mechanoreceptors?
Rapid adaptation is where they fire at the start and end of stiulus but not during sustained stimulus.
Slow adapting continue firing during stimulus, but these are for senses we dont want to adapt because they’re in highly sensitive areas like lips.
Describe the different mechanical forces that lead to changes in ionic current in mechanosensitive ion channels.
Forces that open mechanosensitive channels:
1. Stretching of lipid membrane
2. Extracellular protein forces
3. Intracellular cytoskeleton forces
Give a detailed account of the process by which peripheral chemical mediators lead to sensitisation of nociceptors.
- nociceptive stimulus ie splinter
- nociceptor activated
- substance P released
- acts on mast cells
- mast cells release histamine
- histamine sensitises nociceptors by moving them closer to threshold to smaller stimulus can depolarise them
- more responsive to subsequent stimuli
What is the difference between pain and nociception?
Nociception = detection of tissue damage
Pain = negative/unpleasant experience of soreness, throbbing, ache
What are some stimuli that nociceptors can respond to
- mechanical
- temperature
- O2 deprivation
- chemicals
- products of cell damage (bradykinin, ATP, K+ ion channels, histamine)
Most nociceptors are polymodal
What are the 4 sensations included in somatic sensation?
- touch
- temperature
- pressure
- pain
Describe the differences between fast and slow pain
- Fast myelinated, slow unmyelinated
- fast = A delta fibres, slow = C fibres
- Fast = higher intensity, slow = lower intensity
- fast = faster conduction, slow = slower conduction
- Fast = sharp, pricking pain, slow = dull, burning ache
- Fast = glutamate, slow = substance P
- Fast = well localised & painful, slow = poorly localised & distressing
- fast = to somatosensory cortex, slow = to reticular formation
Describe pain regulation by the brain
- gate theory - in dorsal horn, nociceptive input blocks pain signals (ie rubbings stops pain getting to brain)
- Regulation through higher brain centers (strong analgesic when critical to survival, via periaqueductal midbrain interneuron)
Explain how thermoreceptors adapt in response to hot and cold stimuli.
- there are 6 types of thermoreceptor, all free nerve endings and each detect a different range
- Adaptation is very rapid. thermoreceptor firing is rapid when temperature changes, but slows down as temperature is maintained
Describe the primary somatosensory cortex
- located on the post-central gyrus
- Areas 1,2,3a,3b
in order from central sulcus: - 3a = proprioceptive info
- 3b = S1
- 1 = higher processing
- 2 = discrimination of touch
somatosensory cortex arranged as “homunculus’ - more cortical area for areas with finer touch
What are the adjacent somatosensory cortical areas?
posterior parietal area - 5 & 7
where am I relative to the environment
Describe the mechanism of how lateral inhibition of the somatic sensory system leads to contrast enhancement.
neighbouring cells inhibit one another, therefore neurons surrounding the target are inhibited. This reduces “noise”, increases the spatial resolution of receptive fields and gives a distinct border.
What are the 2 ascending somatosensory pathways?
- touch, vibration, 2-point discrimination & proprioception - along A fibers (alpha, beta, delta) ipsilaterally up spinal cord & decussation at medulla
- pain, temperature and some touch - along C and A delta fibers, decuss at spinal cord and travel contralaterally up spinal cord
