sensory systems

Question 1

how many sensory modalities?

Answer 1

5

Question 2

pathway from environment to brain

Answer 2

• peripheral sensory neurons
• spinal cord
• thalamus
• primary sensory cortex
• further cortical areas

Question 2

what are the different types of sensory modalities

Answer 2

humans:
- mechanoreceptors (hearing, touch)
- photoreceptors (sight)
- chemoreceptors (taste, olfaction)
non-human animals:
- magnetoreceptors
- electroreceptor

Question 2

route to brain for touch information

Answer 2

• Touch afferents enter spinal cord via DRG – Dorsal Root
  Ganglion
• The area of skin sending input into each DRG is called a
  dermatome
• Afferents enter brain via Ventero-posterior lateral/medial
  (VPL/M) nuclei of the thalamus

Question 3

concept of receptive fields

Answer 3

• A part of “sensory space” in which a stimulus can drive an
  electrical response in a sensory neuron
• Stimulus has to be appropriate to the type of sensory
  receptor: the modality
• The sensory space can be very straight forward like the
  skin surface
• … or more abstract like the volume, space and timing of
  a sound

Question 3

smooth regions:

Answer 3

gyri pl. (gyrus sing.)

Question 3

the folds:

Answer 3

sulci pl. (sulcus sing.)

Question 3

cortical layers

Answer 3

• Layer I:
• Almost no neuron cell bodies
• Lots of dendrites from lower layers and axons synapsing on those dendrites
• Layer II:
• Small densely-packed pyramidal neurons receiving inputs from other layers
• Layer III:
• Pyramidal neurons with outputs to other cortical areas
• Layer IV:
• Many spiny stellate (excitatory) interneurons
• Receives input from the thalamus
• Thickest layer in sensory cortex, nearly absent in motor cortex
• Layer V:
• Largest pyramidal neurons
• Outputs to brain stem and spinal cord
• Layer VI:
• Outputs leading back to the thalamus

Question 3

grey matter:

Answer 3

neuronal cell bodies

Question 4

touch receptors

Answer 4

– There are a range of tactile
receptors:
* Pacinian corpuscles
* Meissner’s corpuscles
* Merkel’s disk
* Free nerve endings
* Hair follicles
* Rufini’s ending
– All receptors adapt
* Response reduces over time to the
same stimulus

Question 4

white matter:

Answer 4

bundles of myelinated axons

Question 4

cortical collumns

Answer 4

• Micro-recordings show that the cortical neurons in the sensory areas
  appear to be roughly organized into columns.
• Oriented perpendicular to the cortical surface
• Perhaps the physiological units of computation
• Sensory inputs first activate neurons
  in layer 4
• Layer 4 neurons propagate activity to
  layers 2 and 3
• From there down to layers 5 and 6
• Recurrent pathways will send
  excitation back from layer 6 to layer 4

Question 5

receptive fields in central areas

Answer 5

• Some areas of skin have more neurons (and processing) than others
• These cortical “maps” are seen in cortex for the big senses: somatotopic,
  retinotopic and tonotopic.
• Not isomorphic with the sensory surfaces.
• The area of cortex isn’t proportion to the area of sensory space it represents
• Adjacent areas of cortex might not deal with adjacent areas of sensory space

Question 6

the homunculus

Answer 6

A visual representation
of the amount of
processing of touch
dedicated to each
part of the body

Question 7

cortical collumns in s1

Answer 7

• Within a column cells respond to stimuli from
  the same modality
• Within a column, cells respond to inputs
  originating from the same type of sensory
  receptor
• Within a column, cells respond to stimuli from
  the same area
• Adjacent columns can respond to different
  stimuli from the same region
• Therefore, the cortex has several “maps” in
  parallel

Question 8

star nosed mole, adaptations for foraging

Answer 8

• Adapted for wetland environments
• Can even smell underwater
  – Exhale ~10 small air bubbles per second
  – The bubbles are then drawn back into the nose
• Star-Nosed mole is almost blind
• 11 pairs of appendages from the nose which it uses to sweep the
  tunnel walls
• What for?
  – No grasping ability
  – No extra odour detection

Question 9

tactile cortical magnififcation

Answer 9

• The nose is a tactile appendage with 25,000 mechanoreceptors
• 100, 000 neurons from nose to brain
• Large-scale cortical magnification
• Foveation

Question 10

the nose is like an eye

Answer 10

• Eyes have peripheral and foveal vision.
• The nose is similar arrangement.
• Very specific search patterns
  – Touch – Foveate – Eat
• 12 touches/second = 25 ms to decide if something is food
  – Compare with 600ms to press brake-pedal
• The proximity of nose to teeth reduces the handling time

Question 11

special somatosensory case of whiskers in rats

Answer 11

• Mice and rats and some other rodents are nocturnal, poor availability of
  visual information but excellent sense of touch through whiskers.
• Specialization within the primary somatosensory cortex (S1) - almost 70% of
  mouse somatosensory cortex (surface area) is devoted to processing
  information from the whiskers
• Area known as barrel cortex

Question 12

organisation of barrels

Answer 12

• S1: Layer IV gets input from thalamic
  nuclei.
• Normally thalamic afferents form a
  relatively continuous distribution of
  connections in S1
• Barrel cortex: thalamic afferents form
  discrete clumps
• Barrels separated by gaps called septae

Question 13

from whisker to cortex: pathway 1

Answer 13

1. Deflection of a whisker
2. Mechanically gated ion channels
3. Sensory neurons
   I. Trigeminal ganglion
   II. Brainstem nuclei (PrV)
   III. Thalamus (VPM)
   IV. Cortex (barrels)
   In pathway 1, receptive fields of neurons
   at each stage are mainly focused on a
   single whisker.

Question 14

whisker to cortex: pathway 2

Answer 14

1. Trigeminal ganglion
2. Brainstem nuclei (SpV)
3. Thalamus (POm)
4. Cortex (barrels and septae)
   * In pathway 2 neurons have broad receptive fields
   * Axons of PoM neurons target septal regions
   * Septal regions form wide connections including to
   contralateral barrel field via corpus callosum
   * Therefore parallel processing:
   – Whisker specific
   – Broad context dependent information.

Question 15

whisking as active sensing

Answer 15

• Rodents move their whiskers in
  varying ways to actively sense the
  environment.
• Active Whisking at 3-25 whisks/second
• Can help determine the position,
  shape and texture of objects

Question 16

bat echolocation

Answer 16

1. Search phase, bats scan the
   environment with narrowband,
   long-duration sonar calls.
2. Approach phase, increase in
   bandwidth, locking beam onto
   target, and increasing rate of
   calls.
3. Capture, further decreasing
   the inter-pulse interval, until it
   intercepts the target

Question 17

human eye movements

Answer 17

* Saccadic – moving fovea over image discontinuously. * Saccades 20ms-100ms * Fixations 200ms-300ms * During saccades (10% of time) we are blind. Blur or ‘saccadic suppression‘. * Conscious or unconscious control

Question 18

active sensing - human eye movement

Answer 18

* Study by Yarbus of eye movements of a single subject looking at a picture * Subject asked different questions 1. Free examination 2. Estimate the material circumstances of the family 3. Give the ages of the people 4. Surmise what the people had been doing before visitor’s arrival 5. Remember the people’s clothes 6. Remember the positions of people and objects in the room 7. Estimate how long the “unexpected visitor” had been away from the family