Theme 1 Flashcards
What is the wiring pattern of the DCML pathway?
- Receptors in the skin
- Dorsal root ganglion cells
- Dorsal column of spinal cord
- Medulla (Gracile and Cuneate Nuclei)
- Midbrain (Medial lemniscus tract)
- Ventral posterolateral (VPL) nucleus of thalamus
- Primary somatosensory cortex
Where does the DCML decussate?
Medulla
What is the DCML pathway responsible for transmitting?
- Fine touch
- Proprioception
- Vibration sensation
- 2-point discrimination
What do the thalamus and cortex have (with regards to DCML pathway)?
A hardwired body map / pattern for sensory input
For every 1 fibre that ascends to the cortex, how many go downwards?
10
What are mechanoreceptors?
Sense organs / cells that respond to sensory stimuli
What are the two types of mechanoreceptors?
Rapidly adapting and slow adapting
How do rapidly adapting receptors work?
Fire at onset, but not continually after that even if stimulus is held
How do slowly adapting receptors work?
Fire at onset, and then keep firing (may change frequency)
What are the main mechanoreceptors in the skin?
- Meissner corpuscle
- Pacinian corpuscle
- Ruffini corpuscle
- Merkel’s disks
(+ free nerve endings)
Which receptor is Meissner corpuscle?
RA1
Which receptor is Pacinian corpuscle?
RA2
Which receptor is Ruffini corpuscle?
SA2
Which receptor is Merkel’s disks?
SA1
What is the role of Meissner corpuscle?
For textured objects moving across the skin
Detection of slippage between hand and object - important for grip
What is the role of Pacinian corpuscle?
More sensitive than Meissner - for fine textured surfaces
Produce sensation of vibration or tickle (important for skilled use of tools)
What is the role of Ruffini corpuscle?
Function not exactly known
Something to do with proprioception
What is the role of Merkel’s disks?
Stimulation produces sensation of light pressure
Role in detection of shapes, edges and rough textures
What percent of receptors in the skin are rapidly adapting?
55%
40% RA1 (Meisnner)
15% RA2 (Pacinian)
What percent of receptors in the skin are slowly adapting?
45%
15% SA1 (Merkel’s)
20% SA2 (Ruffini)
Where are Meissner corpuscle found?
Most common in hairless skin
Where are Merkel’s disks dense?
In the fingertips
What is the order of the mechanoreceptors in skin from superficial to deep?
Meissner corpuscle and Merkel’s disk at top (RA1 & SA1)
Ruffini towards top of dermis (SA2)
Pacinian deep in dermis (RA2)
What is the role of free nerve endings?
Pain and temperature sensation
Where are mechanoreceptors most dense?
Areas we are highly dependent on
eg. hands and fingers for dexterity
Evolutionary specification
What causes 2-point discrimination differences across body surfaces?
Receptor density and receptive field size
Give an example of some specialised receptors in other animals
Wind sensing for crickets and flies (why they can escape swatting)
Rat’s whisker sensitivity
What are proprioreceptors?
“Receptors of self”
Detailed and continuous information about the position of the limbs and other body parts in space
What are the two main types of proprioreceptors?
- Muscle spindles
- Golgi tendon organ
What is a muscle spindle?
A set of specialised muscle fibres in parallel to the other fibres in the muscle
What do group 1 axons do in muscle spindles?
Provide information about dynamic movement
What do group 2 axons do in muscle spindles?
Provide information about static position of limbs
Where are muscle spindles most dense?
Areas where we need to control muscles a lot eg. eyes
What is a golgi tendon organ?
Sits between tendon and muscle fibre
What do golgi tendon organs respond to?
Active contraction (not to passive stretch)
What is a dermatome?
Innervation arising from a single dorsal root
What sensations overlap in dermatomes?
Lots of overlap between segments, for touch, vibration and pressure
Pain more accurate, that’s why medics test it
What is the trigeminal nerve?
5th cranial nerve, providing sensory feedback from the head and face
What are the branches of the trigeminal nerve?
- Opthalmic
- Maxillary
- Mandibular
What is the pathway of the trigeminal system?
- Trigeminal branch
- Synapse at trigeminal ganglion
- Trigeminal complex
- Decussates
- Ventral posteromedial nucleus (VPM!! not VPL!) in thalamus
Where does the thalamus project to?
The somatosensory cortex
What are the 3 areas of the somatosensory cortex responsible for?
3a: proprioceptive
3b & 1: cutaneous stimuli
2: proprioception and tactile stimuli
How do we visualise the body map in the somatosensory cortex?
With the homunculus
Massive head, ears, lips and gigantic hands
Describe the Frostig lab stroke research?
- Caused a large stroke in the MCA
- Stimulated affected whiskers after the stroke
- If they started stimulating very soon after the stroke they could “cure” the stroke due to the brain plasticity
- If you do it too late it makes the stroke worse
How to focal seizures start?
Start small, then grow bigger. Experiments to see whether it could stay contained in local area
Why do humans need pain?
Allows us to avoid damaging stimuli
Helps us learn
What are the axons associated with pain?
- Aδ myelinated axons 20m/s
- C fibre unmyelinated axons 2m/s
SLOW
What are Aδ neurons associated with?
Aδ mechanosensitive nociceptors
Aδ mechanothermal nociceptors
What are C fibre neurons associated with?
Polymodal nociceptors
What temperature do we start to experience nociception from heat?
Around 42C starts linear increase in firing, roughly correlating to increases in pain
How can we test receptors?
Electrode into skin and then present with stimuli
What are polymodal receptors / C fibre receptors responsible for?
Wave of “second pain”
Sharp first pain from Aδ fibres, longer term pain from C fibres eg. “burning pain”
What do receptors on the neurons do (in regards to pain)
Respond to stimuli eg. heat
What is capsaicin?
Chemical in food that makes them seem hot eg, chillis
How does capsaicin make things hot?
- Binds to vanilliod receptors and vanilliod like receptors intracellularly
- These receptors detect and regulate heat
- Sensory input feeds back to brain
Where are vanilliod receptors found? (V1, TRPV1)
Aδ and C fibres
Where are vanilliod-like receptors found?
Aδ fibres
What is the pathway of pain and temperature information from the periphery travelling to the brain?
- Receptors in skin
- Decussates in spinal cord
- Ascends via anterolateral column / spinothalamic tract
- Through hindbrain and midbrain
- VPL of thalamus
What is the pathway of pain and temperature information from the face to the brain?
- Enter midbrain and through spinal trigeminal tract
- Trigeminothalamic tract
- VPM of thalamus (same as trigeminals)
- Primary somatosensory cortex
What will a spinal lesion cause?
Contralateral loss of pain and temperature sensation below the lesion
Ipsilateral loss of fine touch, vibration, proprioception, and 2-point discrimination
What causes referred pain?
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Referred pain 2
d
Visceral pain 1
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Visceral pian
2
Where does visceral GI pain ascend?
The dorsal column of the spinal cord
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viscaeral paifn
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Dorsal column lesioning
Cancer in the colon, lungs, and pelvis
Where else does pain go to in the brain that somatosensory input does not>
- Amygdala
- Hypothalamus
- Periaqueductal gray
- Superior colliculi
- Reticular formation
- Midline thalamic nuclei - - - > Anterior cingulate cortex and insular cortex
Why does pain increase heart rate and blood pressure?
Because of projections to the hypothalamus
What does the motion after effect illusion illustrate?
- Feature detection
- Adaptation
What is feature detection?
Brain has specific circuits of neurons specialised for detecting particular features of the sensory world
eg.
1. Edges
2. Colours
3. Particular orientations
4. Movement in particular directions
5. Faces
What is adaptation?
Brain is mostly interested in changes in the environment
What happens when a feature remains constant (even if it is movement)
Neural signals are dampened down (no longer as important)
Adaptation by the brain
What is the basic pathway of the visual system?
- Eye
- Lateral geniculate nucleus of the thalamus
- Cortex (Visual cortex in occipital lobe)
What is the goal of the visual system?
Build a predictive model of the external world based on incident light
Sum up the function of the visual system
- Electromagnetic radiation (light) is converted into neural impulses
- Impulses used to construct neural models that allow external features to guide movement
What is the wavelength spectrum of visible light?
Around 400nm (purple) - around 700nm (red)
What units measure the amount of light emitted?
Candela
What appears perceptually twice as bright is actually…
10x as bright (in C/dm^3)
What is the iris?
Donut shaped band of contractile tissue
What is the role of the iris?
Controls the amount of light that reaches the retina by adjusting pupil size?
What is pupillary light reflex controlled by?
Amount of light entering the eye
What happens in the dark?
- Iris relaxes
- Pupil dilates
- More light enters eye
- Sensitivity is better, but less acuity
What happens in bright conditions?
- Iris contracts
- Pupil contracts
- Less light enters eye
- Image on retina is sharper, more acuity
What happens to the pupil dilation range as we age?
It reduces, your accommodation gets worse
Harder to drive at night
What is the role of the cornea?
Helps focus incoming light
What percent of focusing light is the cornea responsible for?
75% but it’s focussing is fixed
What is the role of the lens?
To focus incoming light onto the retina
What can the lens do that the cornea cannot?
Change shape
What happens for close vision?
- Ciliary muscle contracts
- Ciliary body with inserted zonula fibres moves closer to lens
- Tension is reduced
- Lens gets fatter
What happens for distant vision?
- Ciliary muscle relaxes
- Ciliary body with inserted zonula fibres move away from lens
- Tension increases
- Lens gets flatter
What is the role of the retina?
Converts incoming light into neural signals
What is the retina?
A thin slice of sensitive tissue that lines the back of the eye
What does the retina contain?
A layer of photoreceptive cells that convert the light to neural signals
What is the name for transferring light into neural signals?
Transduction
What is the fovea?
The area of the retina with the highest concentration of photoreceptors, and therefore sharpest vision
What causes the blindspot?
Nerve supplying the eye, central artery and vein of retina
What is the macula?
A part of the retina that that processes what you see directly in front of you
Where is the fovea found?
Centre of the macula
What cells are in the fovea?
Only cones, in VERY high density
What percent of the nerve fibres in the optic nerve are supplied by foveal input?
Around 50%
How does the brain resolve the blindspot?
Uses information from around the receptors around the blindspot to fill in the gap
What is a possible reason for our photoreceptors being behind blood vessels and other cells in the eye?
Different evolutionary paths compared to octopus
Would need unfeasibly large eyeballs as receptors would need to be further back
What is the sclera?
“White of the eye”
Tough, protective layer of connective tissue
What is the choroid?
Layer of tissue between the retina and sclera
What is the role of the choroid?
Contains many blood vessels and is critical for providing oxygen and glucose to the retinal cells
What do many nocturnal animals have?
Tapetum lucidum
What causes “red eye” in photos?
The flash from camera is reflected off the blood in the choroid and back through the pupil
What is the tapetum lucidum?
A layer of reflective tissue
(increases chance of light hitting photoreceptors)
What is the trade off of tapetum lucidum?
Slightly reduced visual acuity, but worth it for nocturnal animals to see clearly
What are the 3 main layers of cells in the retina?
- Photoreceptors
- Bipolar cells
- Retinal ganglion cells
Back to front
What other cells are in the retina?
A number of interneurons, including horizontal cells and amacrine cells
Where are horizontal cells?
Linked between photoreceptors and bipolar cells
Where are amacrine cells?
Linked between bipolar cells and retinal ganglion cells
What are the two types of photoreceptors?
Rods and cones
What is the role of rods?
- Scotopic (dim light) vision
- High convergence - many rods to one bipolar cell so poor acuity but good sensitivity
- Only one type so monochromatic
- Good for peripheral vision
What is the role of cones?
- Phototopic (well lit) vision
- Low convergence - one cone to one bipolar cell so good acuity but poor sensitivity
- Three types (RGB) so responsible for colour
- Good for central vision
What happens in the dark for rods?
- Rhodopsin inactive
- Sodium channels kept open by cGMP
- Cell depolarised
- Continuous glutamate release
What happens in the light for rods?
- Rhodopsin active
- cGMP breaks down and sodium channels close
- Cell hyperpolarised
- Glutamate release reduced
What is the difference between R,G and B cones?
Sensitive to picking up light in different ranges
eg. dim blue light would be picked up more than super bright red light by a blue cone
hence three types
Why do we need two or more photoreceptors?
Differentiate between wavelength and intensity
How is colour encoded?
Differential activation of two or more photoreceptors
How does number of photoreceptor types affect better colour?
More photoreceptor types gives better colour resolving power
What is a trichromat?
Have 3 different types of cone (normal)
What % of people have colour vision deficiency?
8% of men and 0.5% of women
What causes colour blindness?
Altered sensitivity in one of the cones or a total absence
What is the most common type of colour vision deficiency?
Deuteranomoly
What is the pathology of deuteranomoly?
Sensitivity of green cones is shifted towards red
Makes resolution between green and red difficult
Why can the fovea not detect starlight? aka why do stars disappear when you look at them
No rods - so it disappears when you look at it
How does the eye scan?
- Continually scans visual field with fovea, making three fixations per second
- Visual system integrates this information to produce wide-angled, high acuity coloured perception
What do bipolar cells allow?
Different levels of sensitivity to light due to differing convergence between rods and cones
How do retinal ganglion cells?
- Input received from bipolar cells in such a way to faciliate edge detection
- They detect spots of contrasting illumination
Describe the structure of a retinal ganglion cell
Disc shaped with a “on” center, “off” surround
or “off” center, “on” surround
Receptive field
What does a large amount of retinal ganglion cells in a line allow?
Line detection or light intensity
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Explain the projection of the eyes to the brain
- Contralateral - left part of R&L side goes to right brain
- Optic Nerve
- Optic chiasm
- Superior collicus
- Visual cortex
How does the pupillary reflex work?
- Light shined in one eye
- Action potential reach both right and left pre-tectal nuclei
- Both sides of Eddinger-Westphal nucleus stimulated
- Both sides of Eddinger-Westphal nucleus generate action potentials through R&L oculomotor nerve
- Constriction
Describe visual pathways
- Projection to the brainstem accessory optic and pre-tectal nuclei for visual reflexes
- The ancient retino-tectal projection to superior colliculus orients the head and eyes towards or away from unexpected events
- Retino-genuiclate-striate pathway is the largest and provides input for complex scene analysis and object identification
Describe the retino-genuiculate-striate pathway
- Retina
- Lateral geniculate nuclei of thalamus
- lower layer IV of the primary visual cortex
How is the retino-geniculate-striate system organised?
Retinotopic - each relay within the system is organised according to spatial map of retina
Two stimulus that excite adjacent retinal regions excite adjacent neurons at all levels
What proportion of the visual cortex analyses input from the fovea?
25%
What does overlap in retinal analysis give us?
Depth perception
Motion parallax
What do parvocellular cells in the LGN respond to?
- Colour
- Fine detail
- Stationary or slow moving objects
- Scene analysis
- Object identification
(cones)
What do magnocellular cells in the LGN respond to?
- Luminance change
- On / off movement
Rods
What is the receptive field of a visual neuron?
An area of the visual field within which it is possible for a visual stimulus to influence the firing of that neuron (increase or decrease firing rate as they are spontaneously active)
How do neurons in V1 respond?
To more complicated visual stimuli than points of light
What are the two main categories of neurons in V1?
- Simple cells
- Complex cells
Describe simple cells
Antagonistic on and off regions with straight borders
Respond to bars of light in a specific orientation at a specific location in the visual field
Describe complex cells
More common than simple cells (75%)
Respond to a particular straight-edge stimulus of a particular orientation at ANY point in the visual field
How are V1 cells arranged?
Columnar structure - layers down have receptive field in same area of visual field (orientation columns prefer straight lines in same direction)
Ocular dominance columns
How does the neural signal move through V1
Neurons with simpler receptive fields to the more complex ones
What does V1 do?
Catalogs input
What does V2 do?
Relays signal
What does V3 do?
- Form
- Motion
- Depth
What does V4 do?
- Colour
- Stimulus saliency
- Attention
What does V5 do?
Motion
What does the ventral stream from V1 do?
- Travels to temporal lobe
- Scene analysis and object identification
WHAT
What does the dorsal stream from V1 do?
- Travels to parietal lobe
- Processes spatial locations
WHERE
