Week 2: Sensory Systems Flashcards
(29 cards)
What are some examples of sensory detectors(i.e. receptors)
- mechanoreceptors (hearing/touch) e.g. hair cells in ear.
- photoreceptors (sight) e.g. rods and cones
- chemoreceptors (taste/olfaction)
- magnetoreceptors
- electroreceptors
What does having lots of sensory neurons at the periphery allow for
Allows for categorisation of the when and what of the sensory signals. Each sensory neuron has a specific where and what which is that neuron’s receptive field.
Define receptive field
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
Describe the route from the peripheral sensory neurons to the cortex
- Thalamus is a major site for initial
processing of sensory information
Peripheral sensory neurons -> Spinal Cord -> Thalamus -> Primary sensory cortex -> Further cortical areas - Sensory information from the periphery first enters the thalamus.
- Thalamus is formed of “nuclei”. Groups of cell bodies all dealing with similar things
- The thalamus sends projections to the cortex.
- Thalamus is a major site for initial
processing of sensory information.
What are the folds and smooth regions of the brain called and what makes up the white and gray matter of the brain?
Folds = suici (or sulcus in singular)
Smooth regions = gyri (sing. gyrus)
White matter = bundles of myelinated axons (fatty sheets of myelin hence white)
Grey matter = neuronal cell bodies
The cortex is a sheet of…
Folding increases…
even depth/thickness
the surface area of large brained animals
What are the names of the stain used to see structures in the brain
Nissel stain - shows cell bodies
Golgi -shows the entire architecture of a small proportion of cells.
Weigert stain - shows myelin therefore axons.
Describe the cortical layers of the brain
1) Layer 1: outside layer
- almost no neuron cell bodies
- lots of dendrites from lower layers and axons synapsing on those dendrites.
2) Layer 2:
- Small densely-packed pyramidal neurons receiving inputs from other layers
3) Layer III:
* Pyramidal neurons with outputs to other cortical areas
4) Layer IV:
* Many spiny stellate (excitatory) interneurons
* Receives input from the thalamus
* Thickest layer in sensory cortex, nearly absent in motor cortex
*5) Layer V:
* Largest pyramidal neurons as signal must travel long distance.
* Outputs to brain stem and spinal cord
6) Layer VI:
* Outputs leading back to the thalamus
What connects the two halves of the brain
The corpus collosum
Describe the cortical colums
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
How do sensory inputs travel through the layers
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
How do receptors (especially tactile) adapt
Their response reduces over time to the same stimulus
Do touch receptive fields vary in size?
Yes they vary in size across the body
Define acuity
Acuity is your ability to discern detail in a sensory stimulus
* A small 2PD means a high acuity (2 point discrimination scale)
* The touch system has more (and smaller) RFs for areas with high acuity
Route to the brain for touch information
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
Describe receptive fields in central areas
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(sound).
* 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.
How do columns in the brain relate to each other
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
What are rapid and slow signals used for in the brain columns
Rapid signals - texture detection
Slow signals - pressure and shape detection
How are star-nosed moles adapted for foraging?
Adapted for wetland environments
* Can even smell underwater
- their nose is a tactile appendage with 25,000 mechanoreceptors.
- 100,000 neurons from nose to brain
– Exhale ~10 small air bubbles per second
- they have foveation - higher density of photoreceptors in fovea of the eye.
- large-scale cortical magnification (2 levels)
– 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
How are star-nosed moles’ nose like an eye?
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.
How does the size and density of sensory receptors/cortical magnification correspond to animal’s specialisations and needs?
Larger areas of the brain devoted for sensory information for that modality. It would be bigger and cover a larger area depending on the animal’s needs and behaviour eg. star-nosed mole has more sensors for their nose, platypus has more for their beaks, raccons for their hands/dexterity, etc.
Cortical magnification reflects behaviour.
Describe the special somatosensory case of whiskers
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
- Pattern of whisker follicles arranged in rows and same pattern is seen in layer 4 of s1 cortex.
Describe the organisation of barrels
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.
Layer IV patterns
* Visualized with Nissel
stain shows rings of high
cell density
* Middle is thalamic axons
What are the two pathways from the whiskers to the cortex
1) Pathway 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.
2) Pathway 2:
1. Trigeminal ganglion
2. Brainstem nuclei (SpV)
3. Thalamus (POm)
4. Cortex (barrels and septae)
In pathway 2 neurons have broad receptive fields so less specific.
* 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
