The Somatic Sensory System (Prof. Fitzgerald) Flashcards
(34 cards)
What are the fcts of the somatic nervous system ?
- perception of events of the skin surface
- pressure and pain in joints and internal organs
- position of limbs in space
- sensory guidance of mvnt (feedback control)
What info is provided by the somatosensory system ?
The modality of the stimulus, spatial info, info about intensity and frequency as well as the history of the receptor.
What are the different modalities of a stimulus ?
- touch and pressure (static and dynamic)
- thermal (cool and warm)
- itch (mechanical and chemical)
- noxious stimuli (pain) – mechanical, thermal, chemical
What imp info (other than modality of the stimulus) does the somatic sensory syst provide ?
- spatial location of the stimulus (where on body surface)
- spatial discrimination of stimulus (how far apart)
- intensity and duration of stimulus (strength + duration)
- nb and freq of stimuli
- history (normal skin, damaged skin, numb skin etc.)
How is the somatosensory system anatomicaly organized ?
- primary sensory neurons
- spinal chord (EXCEPT trigeminal system in the brainstem for the head and neck)
- ascending spinal tracts to brainstem + thalamus
- somatosensory cortex
- other brain regions: motor + emotional response
- descending spinal tracts for feedback control
Where are the peripheral axons, cell bodies and central axons of primary sensory neurons ?
Peripheral axons in the skin, cell bodies in the dorsal root ganglion, central axons in the spinal chord.
What are the four main types of mechanoreceptors in the skin and what are they sensitive to ?
- Merkel receptors in the epidermis are sensitive to pressure (0.3-3Hz, slow pushing)
- Meissner’s corpuscles just below the epidermis are sensitive to flutter (3-40Hz)
- Ruffini endings situated aprox in the middle of the dermis are sentive to stretching (15-400Hz)
- Pacinian corpuscules in the subcutaneous fat below the dermis are sensitive to vibration (10-500Hz, v rapid vibration at the upper range)
What do the different type of mechanoreceptors in the skin show ?
What about nociceptors ?
That mechanoreceptors have specialized anatomically recognizable endings in the skin.
Nociceptors, on the other hand, are nerve endings that are depolarized by tissue damaging stimulation: mechanical, thermal or chemical.
Nociceptors DO NOT have anatomically recognizable endings.
What kind of channels are expressed in touch receptor endings ?
MS (low threshold)
What kind of channels are expressed in pain receptor endings ?
TRPA1, TRPV1, TRPV2, MS (high threshold), ASICs
What kind of channels are expressed in hot temp receptor endings ?
TRPV3, TRPV4
What kind of channels are expressed in cold temp receptor endings ?
TRPM8
Do dorsal root ganglions contain homogenous population of sensory neurons ?
Absolutely not !
Different fctnal populations of neurons occupy the dorsal root ganglion (large A(beta) fibre axons, small unmyelintaed C fiber axons, medium thinly myelinated A(delta) axons).
These cells express different proteins that reflect their different fcts (ions channels, neurotransmitters, receptors, developmental markers).
What are the main different axon sizes, levels of myelination, conduction velocities and fcts ?
A(alpha) fibres: thickly myelinated, 13-20μm wide, 80-120m/s
A(beta) fibres: medorately myelinated, 6-12μm wide, 37-73m/s, touch and pressure
A(delta) fibres: thinly myelinated, 1-5μm wide, 5-35m/s
C fibres: unmyelinated, 0.2-1.5μm wide, 0.5-2m/s, pain/itch and temperature
How do different primary sensory neurons respond to different stimuli ?
By different patterns of APs (frequencies + durations)
“Each dorsal root ganglion has axons in one peripheral nerve”.
Why is this outrageously false ?
Each dorsal root ganglion has axons in several peripheral nerves, so each dorsal root carries information from wide overlapping skin areas
What are dermatomes ?
Dermatomes are the areas innervated by each dorsal root ganglion.
What are the four spinal segments and which parts of the body do these segments respectively innervate ?
Cervical: upper limbs
Thoracic: trunk
Lumbar: lower limbs
Sacral: pelvic
“One dorsal horn neuron will receive inputs inputs from one primary sensory neuron”.
Why is this outrageously false ?
One dorsal horn neuron will receive inputs from many primary sensory neurons. This a direct consequence of the dorsal root ganglion containing axons in several peripheral nerves.
How are sensory neurons organized in the spinal chord ?
Each type of sensory neuron has a different pattern of terminals and synapses in the spinal cord.
Good luck learning those… because the somatosensory ‘circuits’ formed by primary sensory neuron synapses and dorsal horn neurons are complex.
What is a receptive field (RF) ?
The receptive field of an individual sensory neuron is the particular region of the sensory space (e.g., the body surface, or the visual field) in which a stimulus will modify the firing of that neuron.
What are RFs for dorsal horn neurons ?
These are the areas of skin that evoke AP for particular neurons when stimulated.
How do RFs vary ?
Receptive fields may be a single modality e.g. touch,
or several modalities e.g. touch, pinch, heat.
Receptive fields may very small (e.g. 1mm2) or very large (e.g. the sole of the foot).
How are the RFs of neighbouring neurons organized ?
Each neuron has a receptive field that is adjacent to its neighbouring neuron, this allows the elaboration of a somatotopic map in the somatosensory cortex.
