What does the somatosensory system do?
transmits & analyzes touch or tactile information from external & internal locations on the body & head
What are the 6 somatic sensations submodalities?
1. discriminative touch
3. proprioception (position sense)
4. crude (nondiscriminative) touch
5. thermal (hot/cold) sensation
6. nociception (pain/tissue damage)
What are the 4 pathways that transmit the 6 somatic sensation submodalities?
1. posterior column-medial lemniscal pathway
2. trigeminothalamic pathway
3. spinocerebellar pathway
4. anterolateral system
What is the primary pathway that transmits discriminative touch, flutter-vibration, & proprioceptive information?
posterior column-medial lemniscal pathway (PCMLS)
What is the posterior column-medial lemniscal system (PCMLS) involved in?
perception & appreciation of mechanical stimuli
What is stereognosis?
recognition of 3-D shape; occurs in the PCMLs
What kind of proprioception do the PCMLs carry?
conscious awareness of body position in space
What is kinesthesia? What afferent tract carries this information?
limb movement in space; PCMLs
What are characteristic features of the PCMLS?
- afferent fibers with fast conduction velocities & limited number of synaptic relays
- precise somatotopic organization
What is somatotopic organization? (PCMLS)
point-for-point correspondence of an area of the body to a specific point on the central nervous system. Think sensory homunculus.
What is PCMLS frequency coding?
cell's firing rate signals stimulus intensity or temporal aspects of the tactile stimulus
What is PCMLS population coding?
distribution in time/space of the number of activated cells signals location of the stimulus as well as its motion/direction
How is the PCMLS have such a high degree of resolution?
due to inhibitory mechanisms such as feed-forward, feedback, and lateral (surround) inhibition
- basically negative selection/Darwinism for neuronal signal
How does PCMLS play a role regarding two-point discrimination?
It sharpens discrimination between separate points on the skin
- it has the ability to discriminate between two stimuli simultaneously
- it also varies widely over different parts of the body & is related to density of peripheral nerve endings
What can activation of peripheral mechanoreceptors evoke?
somatic sensations of touch
What is the mechanism of action of peripheral mechanoreceptors activation?
mechanical pressure is transduced into an electrical signal by primary afferent neuron ---> if this depolarizes the neuron to threshold, an action potential is produced and related to the CNS via PCMLS
What plays a role regarding accuracy with which a tactile stimulus is localized? (2)
depends on receptor density & receptive field size
What is a receptor density gradient?
various body parts have various density of tactile receptors
What is the receptor density gradient of digits and the perioral region?
they have increased density of tactile receptors
What is the receptor density gradient of other regions, like the back?
a decreased density of tactile receptors
What is a receptive field?
an area of skin innervated by branches of a somatic afferent fiber
Where are small receptive fields found?
areas such as fingertips, where receptor density is high!
Where are large receptive fields found?
areas with low receptor density, like the back!
What is special about small receptive fields vs. low receptive fields?
Small receptive fields provide an INCREASE in discrimination
What is special about low receptive fields vs. small receptive fields?
low receptive fields have a DECREASE in discrimination
What happens to densely innervated body parts?
they are represented by greater numbers of neurons ---> this takes a disproportionately larger part of the somatosensory cortex
What type of relationship is there between size of the receptive field and the representation of that body part in the somatosensory cortex?
an INVERSE relationship!
EX: the trunk (large receptive fields) = small representation fingers (small receptive fields) = large representation
What's special about fingertips & lips and the information they provide to the CNS?
They provide the CNS w/ most specific & detailed information about a tactile stimulus
What are the 3 things that a primary afferent fiber consists of?
1. *peripheral process* extending from the DRG (mechanoreceptor or free nerve ending)
2. *central process* extending from DRG into CNS
3. *pseudounipolar cell body* in the DRG
What does the peripheral distribution of the afferent nerves arising from each spinal level delineate?
the segmental pattern of dermatomes