Somatosensation

Question 1

what does the somatosensory system do

Answer 1

• Tells us what the body is up to and what’s going on in the environment by providing bodily sensations such as:
  
  • Touch, temperature, pain, position in space, and movement of the joints
• Allows us to distinguish between what the world does to us and what we do to it
• It has a closer relationship with movement than the other senses do

Question 2

where and what are somatosensory receptors

Answer 2

• receptors are all over: skin, muscles, tendons, joints
• more receptors = increased sensitivity to stimulation than area with relatively fewer receptors eg hands, lips
• sensitivity to different somatosensory stimuli is a function of the kinds of receptors

Question 3

what are the two types of skin humans have

Answer 3

1. hairy skin
2. glaborous skin

Question 4

what are the properties of hairy skin

Answer 4

Relatively low sensitivity

Question 5

what are the properties of glabrous skin

Answer 5

• Skin that does not have hair follicles but contains larger numbersof sensory receptors than do other skin areas (e.g. palm ofhands/feet, lips)
• Sensitive to wide range of stimuli

Question 6

what is the two-point discrimination test

Answer 6

reveals differences in skin sensitivity across the body

Question 7

what does the two-point discrimination test

Answer 7

• fingertips most sensitive
  
  • higher density of mechanoreceptors
  • receptors with small receptive fields
  • brain devotes a lot of resources
• back, calf → very low sensitivity

Question 8

what are the 3 main types of somatosensory perception

Answer 8

• Nocioception
  
  • perception of pain and temperature
• Hapsis
  
  • perception of fine touch and pressure [e.g grasp, manipulate ]
• Proprioception
  
  • perception of the location and movement of the body

Question 9

nocioceptors

Answer 9

Nocioceptors: free nerve endings

• sharp/dull pain and heat/cold
• Damage to dendrite or surrounding cells release chemicals (e.g. prostaglandins) that stimulate dendrite and produces an action potential

Question 10

haptic receptors

Answer 10

Haptic receptors: dendrite attached to hair, connective tissue, or dendrite encased in capsule of tissue (see diagram)

• Distinguish touch, pull, vibration, indentation
• Mechanical stimulation produces action potential e.g.:
• hair receptor: bending of hair
• Ruffini’s corpuscle: stretch

Question 11

specifci types of haptic receptors

Answer 11

• light touch = steady skin indention
• light touch = changes
• pain and temperature
• stroking
• steak skin indention, skin stretch, perception of grasp objects
• vibration, rapid skin indention, deeper pressure

Question 12

proprioceptors

Answer 12

Movements stretch the receptors to mechanically stimulate dendrites and produces an action potential

Question 13

what do somatosensory receptors tell us about a sensory event

Answer 13

when it occurs, and whether it is still occurring

Question 14

what are two types of somatosensory receptors

Answer 14

• rapidly adapting receptor
• slowly adapting receptor

Question 15

what does the rapidly adapting receptor do

Answer 15

Body sensory receptor that responds briefly to the beginning and end of a stimulus on the body
- motions of object on skin
- gives good responses to repeated stimuli
- eg vibration, tickle

Question 16

what does a slowly adapting receptor do

Answer 16

Body sensory receptor that responds as long as a sensory stimulus is on the body
- signals the presence of a long sustained stimulus
- eg pain, long pinch

Question 17

3 types of somatosensation and adaption types

Answer 17

1. Nocioception [pain and hot/cold] → slow
2. Hapsis [fine touch and pressure] → slow and rapid
3. Proprioception [body awareness] → slow and rapid

Question 18

the dorsal-root ganglion neuron

Answer 18

• The dendrite and axon are continuous and carry sensory information from the skin to the CNS via the spinal cord
• The tip of the dendrite is responsive to sensory stimulation
• Each spinal cord segment has one dorsal-root ganglion on each side that contains many dorsal-root ganglion neurons
• In the spinal cord, the axons of these neurons may synapse onto other neurons or continue up to the brain

Question 19

Proprioceptive and Haptic Neurons

Answer 19

• Carry information about locationand movement (proprioception)and touch and pressure(hapsis)
• Large, well-myelinated axons(fast)

Question 20

Nocioceptive Neurons

Answer 20

• Pain and temperature information
• Small axons with little or nomyelination (slow)

Question 21

Deafferentiation

Answer 21

Loss of incoming sensory inputusually due to damage to sensory fibers; also loss of anyafferent input to a structure

Question 22

dorsal-root ganglion neurons consequence of deafferentiatiation

[patient]

Answer 22

• Patient G.O. lost sensation due to peripheral neuropathy after influenza
• Did not lose motor control, butsimple actions for prolonged periods (right) would require visual feedback
• Could not perform many daily tasks
• Afferent feedback is required forfine movements

Question 23

the dorsal spinothalamic tract

Answer 23

• Carries haptic and proprioceptiveinformation
• Axons from the dorsal-root ganglion neurons enter the spinal cord and ascend ipsilaterally until they synapse in the dorsal column nuclei (base of brain)
• Axons from the dorsal columnnuclei cross over to the oppositeside of the brain and project upthrough the brainstem as part ofa pathway called the mediallemniscus
• Axons synapse with neurons located in the (ventrolateral nucleus of the) thalamus, which projects to the somatosensory cortex and motor cortex

Question 24

ventral [or anterior] spinothalamic tract

Answer 24

• Carries nocioceptive information
• Axons from the dorsal-root ganglion neurons enter the spinal cord and synapse onto other neurons. These ‘2nd order’ neurons, in turn send their axons to the contralateral side.
• Axons from contralateral spinal cord then ascend to the brain where they join with other axons forming the medial lemniscus, eventually synapsing with neurons located in the (ventrolateral nucleus of the) thalamus
• Neurons from the thalamus then project to the somatosensory cortex

Question 25

somatosensory pathways to the brain

Answer 25

- two separate pathways convey somatosensory info - haptic-proprioceptive - nociceptive - because of this arrangement, unilateral spinal-cord damage results in distinctive sensory losses to both side of the body below the site of injury

Question 26

what are the two main somatosesnory areas in the cortex

Answer 26

* primary somatosensory cortex * secondary somatosensory cortex

Question 27

primary somatosensory cortex

Answer 27

- receives projections from the thalamus - begins the process of constructing perceptions from somatosensory info

Question 28

secondary somatosensory cortex

Answer 28

- located behind the primary somatosensory cortex - continues the construction of perceptions, projects to the frontal cortex

Question 29

the somatosensory homunculus -> history

Answer 29

- Wilder Penfield (1930s) stimulated the cortical surface with electrodes and recorded patient responses. - Wanted to locate source of seizures - Penfield’s original studies suggested that there was a single ‘somatosensory homunculus’→ representation of human body in cortex - Distinct parts of the somatosensory cortex senses different somatosensation

Question 30

effects of damage to the somatosensory cortex

Answer 30

- damage to the primary somatosensory cortex results in impairment in: - pressure sensitivity, proprioception, - hapsis [ ability to identify objects by touch], and simple movements [eg reaching and grasping]

Question 31

"Happy People Do It Daily — Crossing Makes The Contact Sensible." | Dorsal Spinothalamic Tract

Answer 31

Happy = Haptic (touch) People = Proprioception (body position) Do = Dorsal root ganglion It = Ipsilateral ascent in spinal cord Daily = Dorsal column nuclei (first synapse) Crossing = Cross over (decussate) Makes = Medial lemniscus The = Thalamus (ventrolateral nucleus) Contact = Cortex Sensible = Somatosensory & motor cortex

Question 32

"No Dumb Students Can Make Themselves Smart." | Ventral (Anterior) Spinothalamic Tract

Answer 32

No = Nociception (pain & crude touch) Dumb = Dorsal root ganglion (entry point) Students = Synapse in spinal cord (2nd-order neurons) Can = Cross to contralateral side Make = Medial lemniscus (join pathway to brain) Themselves = Thalamus (ventrolateral nucleus) Smart = Somatosensory cortex