Techniques in Neuroanatomy+ Brain maps Flashcards
Are we born with conscious vision or learned?
• Conscious vision is learned perception – learn how to interpret things in the environment in first couple of years of life
o Light input to the retinal is actively assembled by the visual pathways to create perception of the visual world
o Visual perception is not a passive reconstruction but an active interpretation of the visual world. This requires learning
What are the 4 basic features of the visual system and are they all processed at the same time?
• The visual system language consists of 4 basic features
o Orientation of the objects in visual world (for form vision and perception of shapes)
o Colour
o Motion
o Depth
• Each basic feature of the visual system language is processed separately but ends up as unified perception through visual cortex
What does conscious vision require?
• Conscious vision (image formation) requires:
o An accurate representation of the visual field (the outside world) on the retina (receptive structure)
o A point-to-point representation-mapping-of the retina to the primary visual nuclei of the thalamus and then to visual cortical regions
Is all vision conscious? Describe aspects of unconscious vision and the function of unconscious vision
• Not all light reception produces an image leading to conscious vision
o Don’t need an accurate map or point-to-point representation
• Reception of light stimulus also interacts to change lens and pupil shape, postural reflexes, vestibular function at fundamental anatomical levels
• Many of these interactions are not consciously perceived
• The function of unconscious vision is to coordinate body position and movement to react to and interact with the visual world
• Reception of light stimulus also influences hypothalamic circuitry that sets circadian rhythms for sleep and diverse behaviours related to daily light/dark cycles
• Unconscious vision serves reflexive and homeostatic functions
Describe how retinal output influences reflexive body posture
Retinal output is a sensory arm of reflexive body posture
• Reflexive response to activity in the visual field
• Key players superior colliculus, pulvinar, pretectal nuclei
Describe how retinal output aids in setting circadian rhythm
Retinal output is a sensory arm of reflexive body posture
• Reflexive response to activity in the visual field
• Key players superior colliculus, pulvinar, pretectal nuclei
What is the function of the pupil?
• Pupil- opening that allows light to enter the eye and reach the retina
What is the function of the iris?
• Iris- surrounds the pupil: two muscles that can vary the size of the pupil
What is the function of the cornea?
• Cornea-Responsible for ¾ of focusing of light-glassy transparent external surface
What is the function of the sclera?
• Sclera- forms the tough wall of the eyeball
What is the function of the lens and how is it controlled/what are the mechanics of its control for different lines of sight?
• Lens- varies the focus with ciliary muscle
o Lens shape-determined by tone of ciliary muscle (controlled by the oculomotor nerve)
Relaxes for far vision (lens is flatter)
Contracts for near vision (lens is rounder and more interactive)
o Lens and cornea act to produce a clear image of the visual world on the retinal photoreceptor layer
Draw the human eye
Timestamp: 7:56pm at 28/09
Describe the visual light spectrum
400-700nm
Describe properties of rods in terms of light preference, colour and acuity
o Rods Most sensitive to dim light Do not convey the sense of color • Do not respond to specific wavelengths Lower intensity vision- lower acuity
Describe properties of cones in terms of colour, light preference and acuity
o Cones Work better in bright light Responsible for acute detail Respond to both black and white (intensity) and colour-wavelength specific • Green cones have green filters • Red cones have red filters • Blue cones have blue filters
What is acuity?
• Acuity- a measure of how much detail a sensory system can resolve
What does acuity depend on in the retina?
• In the retina, acuity depends on:
o Type of cell present at retinal locations- rod or cone
o Number (numerical density) of receptor cells (rod or cone) present at retinal location
Density of sensory receptors and the size of receptive field determines resolution of sensory systems
• Greater density and smaller receptive field means greater resolution
o Amount of convergence built into the retinal circuits: of 125 million receptors, about 1.25 million ganglion axons have output to the thalamus
But convergence not uniform across retina
Is visual acuity constant in the retina?
No
Where is the area of the retina with the highest acuity and why?
o Highest acuity in the fovea
o In peripheral retina (mainly rods), there is about 1:1000 convergence of photoreceptors onto each retinal ganglion cell which sends output to thalamus
High convergence of rods in the peripheral retina
o In central retina (fovea, mainly cones), there is a 1:1 correspondence of photoreceptors onto each retinal ganglion cell which sends output to the thalamus
Low convergence of cones in the central retina
Adaptations of fovea that serve high acuity vision-
• Peak density of cone photoreceptors in the fovea
• Absence of rods
• Local absence of retinal blood vessels
• Absence of inner retinal layers in path of fovea
o Other retinal cell layers move out of the way of cones
o Cones connect to other layers at an angle rather than straight connections
• Low convergence of receptors to ganglion cell output
How is the retina moved?
o Eye moves retina around using oculomotor nerve, trochlear nerve and abducens nerve so central retina is positioned on target
What is the optic disk?
• Retina-
o Optic disk- where optic nerve comes out. It is a blind spot. Where the blood vessels enter the eye
What is the fovea in terms of description and function
o Center of retinal axes- the fovea. Doesn’t have a lot of blood vessels in the fovea
Fovea centralis (fovea) lies at the centre of the visual axis (set at 0). The fovea is responsible for majority of high acuity vision
What is the macula?
o Macula- spot and the center of the retina
No blood vessels
How many rods are there in the human retina?
100 million rod
Where are rods the most dense?
Reach peak density about 7-8mm from the fovea at the rod ring and then density decreases
Are absent from the fovea
How many cones are there in the human retina?
About 4.6-5 million cones in the average human retina: 20% in central 6mm
Where are cones the most dense in the human retina?
Cones outnumber rods in the central retina: about 7mm ring of fovea
Maximum spatial density is at the fovea: 100000-325000 per mm2
A region of elevated cone density surrounds the fovea and extends into the nasal retina (green area)
Describe the layers of the retina from top to bottom and what the function of each of them is
o Layers (from top to bottom)
Ganglion cells- output to the thalamus. Axons from ganglion cells assemble to form the optic nerve
Amacrine cells- lateral inhibition at the level of the retinal ganglion cell: where convergence occurs
Bipolar cells- connect photoreceptors to retinal ganglion cells
Horizontal cells- lateral inhibition at the level of the photoreceptors: where convergence occurs
Photoreceptors (rods and cones)
What is the optic nerve made of?
Made of axons of ganglion cells
Which retinal cells fire action potentials?
Ganglion cells
What does a single retinal ganglion cells receive input from?
• A single retinal ganglion cell (RGC) can receive input from
o A single cone
o Thousands of rods
What is the receptive field of the ganglion cell?
• This cluster of cells activating a ganglion cell is the receptive field of the ganglion cell
How does the degree of convergence vary across retinal topography?
• The degree of convergence varies across the retinal topography
o High convergence (may rods to one retinal ganglion cell)
Peripheral retina- less axons
o Low convergence (one cone to one retinal ganglion cell)
Every cone in the fovea gets projection to one retinal ganglion cell
Fovea-more axons
How does an amacrine cell work and what does it input/project to?
- Integration cell- sum up input from photoreceptors
- Receive input from bipolar cells and project laterally to influence surrounding ganglion cells, bipolar cells and other amacrine cells
How does a horizontal cell work and what does it input/project to?
- Integration cell-sum up input from photoreceptors
- Receive input from photoreceptors and project neurites laterally to influence surrounding bipolar cells and photoreceptors
What parts does a photoreceptors have?
• Have 4 parts o Outer segment (stack of membranous disks)-absorbs light due to photopigment Rods- long cylindrical outer segment Cone- short, tapering outer segment o Inner segment o Cell body o Synaptic terminal
Describe the pathway of signal from photoreceptor to the brain
Pathway through retinal cell layers: photoreceptor (transduction of light)->synapse at bipolar cell-> synapse at ganglion cell (output layer)-> go to the brain
• Light must travel through all the layers of the retina to reach the receptors
• The action potential travels from photoreceptors to retinal ganglion cells
From what retinal cell layers does light have to travel from and to?
• Direction of light goes from ganglion cells to photoreceptors
Describe how a ganglion cell on centre works in centre-surround inhibition
On centres- when light hits the centre of the receptive field, there is MORE action potential firing. When light hits the periphery of the receptive field (the receptive field is off in an on centre cell), INHIBITION of action potential firing
Describe how a ganglion off centre works in centre-surround inhibition
Off centres- when light hits the centre of the receptive field, there is LESS action potential firing. When light hits the periphery of the receptive field (receptive field is on in an off centre cell), EXCITATION of action potential firing
Describe how signal sharpening work in the visual system
o Sharpening the signal
‘On’ ganglion cell has maximum firing when the light is in the centre of the receptive field – they are turned on by light
• If light or no light on centre and periphery of receptive field, then have very low action potential firing rate
• If light in centre and dark in periphery, very high action potential firing rate
Light shined on an ‘off’ ganglion cell will cause it to fire fewer action potentials- it will fire more action potentials if a small dark spot covers the receptive field centre
How does centre-surround inhibition work
o When the edge enters the surround region of the receptive field without encroaching the centre, there is a decrease in the cell’s firing rate
o As the dark area begins to include the center, however, the partial inhibition by the surround is overcome, and the cell response increases
When darkness/light covers the entire receptive field center- 100% excitation of the cell
When dark/light area fills the entire surround, the centre response is cancelled
When a portion of the surround is in darkness, there is only partial inhibition
Describe what targets ganglion cells project to and the function of each target
• Each ganglion cell projects to one of the following locations bilaterally
o Lateral geniculate nucleus (of the thalamus) then to the cortex-CONSCIOUS
Geniculate neurons project to visual cortex to enable visual perception
o Midbrain (superior colliculus)-UNCONSCIOUS
Collicular neurons integrate visual and auditory information: reflexively direct gaze
Don’t need a cortex for this
o Pretectal nucleus (midbrain)-UNCONSCIOUS
Has projection to oculomotor nerve nucleus (light reflex: CNIII, for change in lens shape and pupil diameter) and to guide eye movement reflexively
o Pulvinar (of the thalamus)-UNCONSCIOUS
Helps to stabilise retinal image, maintain direction of gaze during head movement, role in saccades, visual attention
o Suprachiasmatic nucleus (of the hypothalamus)-UNCONSCIOUS
Light reception synchronises circadian rhythm to the diurnal (day/night) cycle
Describe the pathway from the optic nerve to the brain
o Retinal ganglion cells project through the optic nerve
o Optic nerves exit the left and right eyes at the optic disks, travel through the fatty tissue behind the eyes and pass through holes in the floor of the skull
o Optic nerves from both eyes combine to form the optic chiasm
o Nasal ganglion cells decussate in the optic chiasm-partial decussation but unilateral temporal projections (no decussation for temporal)
o Optic tracts are formed under pia of lateral surface of diencephalon
o Each visual field maps to the contralateral lateral geniculate nucleus
Can also peel off to form synaptic connections with cells in the hypothalamus, and another 10% or so continue past the thalamus to innervate the midbrain or other nuclei
o Optic radiation: the lateral geniculate projects onto the gyri surrounding the calcarine sulcus of the occipital lobe
What area is the primary visual cortical area?
• Primary visual cortical area (V1, area 17 in occipital lobe)
Where are the central visual field and peripheral visual field in relation to each other?
o The central visual field represented most posteriorly: the peripheral visual field more anteriorly
Is the projection upright or inverted?
o The projection is inverted- image is upside down relative to the world
What is retinotopy
Retinotopy- organization whereby neighboring cells in the retina feed information to neighboring places in their target structures
Which has more cortical space: fovea or periphery?
Fovea
What are two fundamental principles of retinotopy and mapping of visual field?
• Two dimensional surface of the retina is mapped onto the two-dimensional surface of the subsequent structure
• Mapping of the visual filed onto a retinotopically organised structure is often distorted because visual space is not sampled uniformly by the cells in the retina
o Fovea is magnified in the retinotopic map
• When the retina is stimulated by a point of light, the activity in the visual cortex is a broad distribution with a peak at the corresponding retinotopic location due to overlap of receptive fields so that a point of light can activate many cells in the retina and many more cells in the target structure
Where is the primary visual cortical area?
o On medial surface of occipital lobe
Describe the lcoation and function of the visual association cortical areas
• Association visual cortical areas (V2-5)
o Visual information is also received by visual association areas, each with a specific role in visual perception V2-V5 (color appreciation, depth, motion)
o In occipital lobe
Why is visual anatomy efficient?
• Efficiency of retinal design- concentration of cones at the fovea
o Save of brain space
• Circuitry to center the fovea on visual target- moving eyes and head to position the eyes and center the fovea
• Cortical processing to reconstruct cohesive visual perception
• These strategies effectively increase information about the visual world without increasing size of ganglion output to the brain and thus increasing perception in available cortical territory
Describe the muscles and nerves responsible for efficient vision (besides the eye)
o Cranial nerves to move the eyes (cranial nerves III, IV and VI)
o Innervation of postural muscles in the head and neck
o Motor planning areas (frontal eye fields in the M2 supplementary motor)
o Vestibular areas- monitor head position to keep visual target
o Cerebellum-eye tracking, head position, balance
What are 4 techniques to understand the structure of the nervous system and what they do?
• Histological staining
o Used to reveal neuron morphology (many at once)
• Intracellular injection (cell filling)
o Used to reveal neuron morphology (one at a time)
• Immunohistochemistry
o Used to reveal specific neuron or glia biochemistry (many at a time)
• Neuronal tract tracing
o Used to reveal connectivity between regions of the nervous system
How does neuronal tract tracing work (procedure)
Tracer molecules are injected into nervous system areas
Tracer is taken up by the neuron and transported along the axon by axoplasmic transport
• Different tracers are preferentially transported in the anterograde or retrograde direction
• Inactivated (non-pathogenic) viruses can also be used to trace connections
Take sections through nervous system
• Tissue collected following appropriate survival time for the tracer to be transported
Process the CNS for histology
Visualise the tracer
• Analyze using fluorescent light
• Immunohistochemistry with specific antibodies for the tracers
Make a map of the location of the tracer
What are some common tracer molecules?
- Phaseolus vulgaris agglutinin
- Horseradish peroxidase
- Inactivated viruses
- Carbocyanine fluorescent dyes (e.g. Dil)
What kind of tracer is phaseolus vulgaris agglutinin and what does it allow/its limitations? How is it visualised?
• Phaseolus vulgaris agglutinin
o Anterograde tracer -cell body to axon terminal
Injection into area containing cell bodies and dendrites
o Must be conjugated to a colour molecule
o Tracer is visualised using immunohistochemistry in post-experimental histology
o Allow for single fibre tracing
o Don’t cross synapses
o Not influenced by action potential
What kind of tracer is horseradish peroxidase and what is its function and limitations? How is it visualised?
• Horseradish peroxidase
o Retrograde tracer -axon terminal to cell body
Injection into area containing terminal fields
o Must be conjugated to a colour molecule
o Tracer is visualised using immunohistochemistry in post-experimental histology
o Allow for single fibre tracing
o Don’t cross synapses
o Not influenced by action potential
Give examples of inactivated virus tracers and what their use is
• Inactivated viruses o Rabies virus o Herpes virus o Used for trans-synaptic tracing Virus invades and replicates in neurons following central/peripheral inoculation and then infects synaptically connected nerve cells
What are considerations for tracers?
Tracer considerations- • Injection protocols • Survival time • Visualisation protocols • Areas sampled after experimental considerations
What are the 3 protein categories of the neuronal cytoskeleton and each of their roles/composition?
o Protein composition
Neurofilament-intermediate filament class
• Provides structural support
Microtubules- composed of tubulin subunits
• Support movement of proteins and organelles along axons
Actin filaments- microfilaments
• Contractile properties- growth cone extension and dendritic spine formation
Why is axonal transport necessary and where does it occur?
o The axon and terminals of a neuron do not contain ribosomes, the protein-producing organelles of the cell, in any substantial quantity
o Therefore, the neuron requires a form of transport with which to ferry essential cell components
o Axonal transport occurs within the neurons in the CNS and PNS
What is axoplasmic transport?
o Axoplasmic transport-movement of materials along the axon
Does axoplasmic transport require energy?
Yes
Is axoplasmic transport dependent on axon potentials?
No
What can be carried in axoplasmic transport?
o Cargo: proteins, vesicles, organelles, foreign substances, toxins, some viruses
what cargo makes axoplasmic transport slow
Slow- mainly proteins
What cargo makes axoplasmic transport fast?
Fast-mainly vesicles and organelles
What is anterograde axoplasmic transport?
o Cell body to axon terminal: anterograde
What is the speed of anterograde axoplasmic transport?
Slow: 0.1-10mm/day or fast: 400nm per day
• Slow- for gradual structural repair and replacement of the subunits of the cytoskeleton
• Fast- for small membrane vesicles
What is the motor protein for anterograde axoplasmic transport?
Anterograde motor protein- kinesin
What does anterograde axoplasmic transport carry?
Carries neurotransmitter enzymes, vesicles and organelles in transport vesicles
What does axoplasmic transport speed depend on?
• Rate of transport-reflects molecules that either bind poorly with motor protein or that pass in and out of vesicles during transport
In what area do you inject an aterograde tracer?
Injection of tracer in ganglion or selected areas of the brain
What is retrograde axoplasmic transport?
o Axon terminal to cell body: retrograde
What speed does retrograde axoplasmic transport occur?
Fast: up to 400-500mm per day depending on cargo and type of neuron
What motor protein does retrograde axoplasmic transport use?
Retrograde motor protein- dynein
What is the use and cargo of the retrograde axoplasmic transport?
Caries protein building blocks of neurofilaments, subunits of microtubules, materials taken up by endocytosis and other small molecules and proteins
In what area do you inject a retrograde tracer?
Injection of peripheral organs
Describe the whisker barrel fields of rodents and their maps
• Whisker barrel fields-
o The whisker barrel receptors are the somatosensory areas for the whiskers
Sensory input and motor output to the whiskers
o They have very specific somatosensory innervation
o Alphanumeric designation in the map, a bit like the dermatomes for humans
o Highly reproduceable
Whisker plan is the same in all populations and in all brain areas of the individual mouse (although there may be some rotation variation)
What is the function of whisker barrel fields in rodents?
o The barrel fields for the rodent whiskers map very specifically from sensory receptors to the trigeminal brainstem nuclei (equivalent to the cuneate and gracile nuclei or the upper and lower body respectively) thalamus (ventroposterior) to cortex (Primary somatosensory)
o Barrel receptors are used to sense the environment
Make map of world based on olfaction
Describe the procedure for anterograde tracer use in rat whisker barrel fields and what will happen to that tracer
o Anterograde tracer in rat whisker barrel fields-
Anterograde tracer is injected into the trigeminal ganglion (CNV) of the rat
The tracer is found in the brainstem CNV nucleus and in the terminals of CNV axons
• The tracer is taken up by the cell bodies CNV ganglion and transported in an anterograde direction-> towards the terminals of the CNV axons
• The tracer will travel along the axons the trigeminal nerve (CNV) to the axon terminals in the brainstem CNV (trigeminal) nucleus
• The tracer will not travel to the thalamus or cortex as it cannot cross the synapse
Where is the facial nucleus and what does it contain? What is its use in rodents?
• Facial nucleus
o Akin to the ventral horn
o Contain LMNs that travel in the facial nerve (from facial nucleus) to facial muscles
o Travels in tracts similar to the corticospinal tract or corticobulbar tract
o Rodents move their whiskers to sense their environment
How can the facial nucleus be inspected using anterograde tracers in rodents?
o Inspection
Anterograde tracer (virus)
Injection site- ventral (inferior) motor cortex
Herpes, rabies are common viral tracers, can cross the synapse
Virus appears in the terminals of pyramidal neurons
Where does knowledge of brain functions come from?
Mostly animal studies
• But can come from accidental lesions in the brain (trauma, strokes, neurodegenerative diseases)
Describe the Phineas Gage case study in 1850
1850s- Phineas Gage
• Metal rod through his orbital cortex and gyrus rectus: his behaviour changed and he became disrespectful and reckless
Describe what Broca learned by describing his patients
1860s- Broca’s stroke patients (such as Leborgne)
• Leborgne showed a motor language deficit: language mechanics poor due to left inferior frontal lobe
• Language function maps to the left inferior frontal lobe
What is the reasoning behind using case studies to infer damage?
o Study reasoning
Loss of or damage to brain areas results in functional deficits
Conclude the function lost is the function of the brain region damaged
Replicate lesion in animals, test hypothesis for function
What are the disadvantages of using lesion case studies to infer brain function?
o Caveats:
The function lost is not exactly the original function due to overlapping functions, compensation, gaps in ability to test function
Some functions are more obscure, especially true for non-motor and no-sensory cortex
Lesions are messy, cross brain functional boundaries
Some areas don’t lesion commonly, biasing how much we understand about cortical differences
Are MRIs photographs?
o MRIs are not photographs, they are generated images
What does MRI measure and what is the key principle of MRI?
o MRI- quantifies protons in particular areas of the brain
o Key to MRI- making protons jump from higher-energy state to low-energy state and vice versa
o Data is collected about proton behaviour in large magnet (which brain is put in) with radiofrequency application
o Every time protons relax, they generate current recorded by machine which generates a recognisable image
o Images are reconstructed graphics from 3 dimensional data (CGI)
o Manipulate radiofrequency application and timings to yield a wealth of data
o Data can be manipulated to extract and accentuate particular tissue features
o Expect new techniques, new data analyses and new paradigms for testing
Describe the magnets used in MRIs and what effects magnets have on MRI scans
Magnet:
• 1.5-4 Tesla commonly used
• 7-9 Tesla- very expensive
• Up to 9 Tesla custom built
• Greater resolution with stronger magnets
• But a bigger magnet means a lot more noise (motion artefacts)
o Even pulse of heartbeat can affect resolution at this level
Describe the purpose and appearance of a T1 MRI scan
T1 image- anatomical image
• Grey matter is grey
• White matter is white
• Cerebrospinal is black
Describe the purpose and appearance of a T2 MRI scan
T2 image-doctor’s image • Useful for pathological changes • Many lesions are wet- show up brightly • CSF- white • Grey matter- grey • White matter- black
Describe the physics/process behind MRIs
Protons spin in tissue randomly
Apply magnet
Protons align with magnet (z axis)
Protons are wobbling around the z axis
Turn on a radiofrequency coil perpendicular to the magnet- this adds energy to the protons
Protons wind down to the XY plane
Protons align in the new plane and precess (wobble) in phase
Turn off radiofrequency coil
Protons dephase T2 faster and realign with original magnet
Protons eventually align vertically (Z axis) T1 slower
Spinning protons induce a current in the coil - this emits a radio signal that can be picked up by a radio receiver: stronger the signal. the more hydrogen atoms between the poles of the magnet
How do you construct an MRI 3D image?
To get 3D image, need 3 dimensional planes, so radiofrequency pulse will be varied in XY direction and magnet strength will be varied in Z direction (which produces the brain slices)
Why is type of tissue able to be determined in MRI scans?
o Tissue variables affect the behaviour of protons
How does tissue variability affect proton behaviour?
Proton behaviour depends in what material or molecule they are in when in tissue
Proton movement is variable depending on different tissue
• Protons are more constrained in fat and looser in water
o Hence more constrained in myelin vs cerebrospinal fluid
Time course of protons returning to native orientation
• Protons align/spin differently depending on what tissue they’re in
How protons behave when relaxing
• How fast they relax depends on tissue type
What is resonance frequency?
• Resonant frequency- the frequency at which the protons absorb energy
What are the problems with using MRIs to compare populations?
• Caveats in comparing populations
o Everyone has same general structure but they’re not always in the same place and the brains are different sizes- hard to account for variation between individuals in comparing structural difference
Have to normalise the different sizes and measure them and compare them
o Hard to define brain norms
o Hard to account for development and ageing differences between people
What is tractography?
• Tractography- tracing tracts by visualising fibres
What is the knowledge of brain connectivity based on?
• Brain connectivity in humans-the connectome (connectivity of the brain)- has been, and still is, largely based on lesion studies, dissection, and homology with non-human primates or rodents (tracing studies)
o Human connectome studies with DTI in normal brain, development, neurodegenerative disease and psychiatric conditions hard as cannot experiment on humans
What is the concept of diffusion tensor imaging and how is it visualised
o Diffusion of protons depends on freedom of movement in tissue
Protons move differently in different brain compartments
• The diffusion matrix (tensors) can be used to generate a 3D image of the tracts
o Outline squares in the matrix where protons can’t move much: these will be the fibre tracts
o In the myelin of oligodendrocytes, there is directionality: can tell moving from anterior to posterior/ superior or inferior
Hence, DITs can show which direction the fibres are projecting in
In diffusion tensor imaging, how do protons move in CSF and why?
CSF • Isotropic o Moves in all directions • Water • High diffusivity
In diffusion tensor imaging, how do protons move in grey matter and why?
Grey matter
• Isotopic
• Low diffusivity (move in any direction but constrained-in smaller space)
• Lots of water in cells but also lipid, cell membranes
Membranes restrict movement
In diffusion tensor imaging, how do protons move in white matter and why?
White matter
• Anisotropic
o Constrained direction- can only move in very narrow space
• High diffusivity
• Mostly myelin but also axonal membranes
o In fibre tracts, combination of movement in axonal membrane and surrounding oligodendrocyte membrane (myelin) restricts movement of protons
What is fractional anisotropy?
o FA fractional anisotropy- scalar value between zero and one that describes the degree of anisotropy of a diffusion process- a value of zero means that diffusion is unrestricted and value of one means that diffusion is fully restricted
What is the principle of functional MRI and how does it make images?
• Takes advantage of the fact that oxyhemoglobin has a magnetic resonance different from that of deoxyhemoglobin
• Measure change in blood flow (level of oxygenation of blood cells) in different regions of the brain
• When doing scans, subtract basal level from stimulated level during task
o Make slower, high resolution T1 scan (anatomical)
Resolution of scan A is much greater than B and C
o Make fast low resolution baseline BOLD scan T2 depending on study design, statistics, model, baseline (as thoughts are fast)
o Make fast low resolution ‘task’ BOLD scan T2 depending on study design, statistics, model and baseline
o Subtract scan B from scan C
Depends on time frame
Size of pixels
o Superimpose statistically significant differences in signal intensity on high resolution brain image in A
What happens to blood flow/oxygenation at low neuronal activity?
o Low neuronal activity
Basal level blood flow
Basal level blood oxygenation
What happens to blood flow/oxygenation at high neuronal activity?
o High neuronal activity
Increased blood flow
Increased blood oxygenation
More oxygen and more blood flow= brighter signal
What is BOLD?
• Blood Oxygen Level Dependent contrast
o Detects differences in blood oxygenation as a proxy for neuronal activity
o Measures oxyhemoglobin: deoxyhemoglobin ratio
What are the disadvantages of looking at fMRIs for thought processes?
o Caveats-
Thoughts are non-specific
Baseline is non-specific
Pixels are hard to superimpose on one another (hard to superimpose A with B and C)
Study design- looking at brain as whole or only where activity was expected
What is the area of cortex dedicated to each sensation proportional to?
• Area of cortex dedicated to each sensation is proportional to the sensory input from the sense organ
What are brain maps and how is information organised in these maps?
• Brain maps refer to the way in which the brain represents and appreciates the external and internal environment (the body)
• Information is organised depending on:
o The type of info and relative weight of the information
o Characteristics of resident neurons
o Anatomy of connections
What are 2 types of neural connections?
o Mapped: well-defined in their projection
o Non-mapped: diffuse and general in their projections
Identify 2 non-mapped neuronal systems
- Arousal systems
* Visceral relays
Describe the role and location of the arousal system
• Arousal systems o Part of the ascending reticular activating system Gross action required Wake up, alert cortex Put cortex to sleep Raise responsivity of cortex Mood setter The system depends on neurotransmitter-specific projections: serotonin, dopamine, noradrenalin, acetylcholine from the brainstem and forebrain
Describe the role of the visceral relay system
o Gross action required- rhythmic contraction of gut: mass action reaction
Detailed point-to-point not necessary
o Parasympathetic and sympathetic system involved in this non-map system (part of autonomic nervous system)- general appreciation
What are the roles of non-mapped neuronal systems?
Serve to signal general state: • Overall feeling, emotional mood • Wake up cortex or put to sleep • Regulate general state of cortex- awake, dozy • Signal emergency to cortex • Broadcast output Sums up information for gross action
What is the role of the cerebral cortex?
• Responsible for higher order functions such as language, planning, conscious sensory perception, emotional perception and multimodal integration
What is the total surface area of the cerebral cortex and how is it distributed?
• Total surface area: 2.2 m2
o About 1/3 on the surface
o About 2/3 hidden in the banks of the sulci
What are the 3 ways in which the cortex is organised and what is the role/description of each organisation?
o Laminar- input and output to different parts of the brain (local processing of each layer)
o Regional- different types of information (e.g. visual auditory)
o Columnar- cortical units (roughly 100-200um2) with specific connectivity/receptive field
How many layers does the neocortex have?
6 layers
How many layers does the archicortex have?
3 layers
Are all layers in the laminar cortical organisation the same?
The cortical layers differ between regions: laminar thickness, cell size, density of input/output
Describe features of layer I in the neocortex
Layer I- mainly tufts of apical dendrites and intracortical axons: no neurons-thin white matter layer
Describe roles of layer II and III of the neocortex
Layer II and III connections with ipsilateral and contralateral cortex
• Layer II- ipsilateral cortex
• Layer III- contralateral cortex
o Corpus collosum contains axons from layer III
Describe the role of layer IV of the neocortex and its inputs, as well as an example of such an input and its effect of layer IV size
Layer IV- line of Genari (myelin-rich): major input from thalamus
• Sensory areas have major input to layer IV: in primary sensory cortex, super thick layer IV
• Primary visual cortex has enlarged layer IV with input from the thalamic lateral geniculate nucleus
Describe the role of layer V of the neocortex and an example of a circuit it is responsible for
Layer V-output to basal ganglia, brainstem, spinal cord
• Layer V gives rise to corticospinal tract : in primary motor cortex, there is a super thick layer V made of Betz cells
Describe the role of layer VI of the neocortex
Layer VI- output to thalamus and other subcortex
Describe the regional organisation of the neocortex and the role of each region
Primary sensory areas receive sensory input via thalamus
Primary motor- output specifically mapped motor commands to brainstem and spinal cord
Secondary sensory areas process sense-specific (modality-specific) information- more integrated and complex
Secondary motor areas process motor information at a more complex, combined level
Higher order association areas- combine multiple sensory information, integrate emotional and homeostatic information, complex learning
What are Brodmann’s areas and what are the based on?
- Delineated more than 50 regions (52) based on cytoarchitectonic differences between cortical regions: that is, cytoarchitectural differences in the cortical layers, including neuronal density, size, laminar depth by looking at hundreds of cortical sections stained with Nissl’s or Golgi’s methods
- At that time, he didn’t know what each region did, but many of the areas turned out to have clear functions
What are the disadvantages of Brodmann’s areas and are they still used today?
• Brodmann’s designations (numbers) are slowly being phased out, but more than 100 years later they still stick until a clear new nomenclature can be devised
o Brodmann areas do not always correspond to named gyri
o Brodmann designations are not completely satisfactory for comparing homologous structures in non-humans
Describe and draw the lobe division of the cerebral cortex
• Each hemisphere is subdivided into lobes: o Frontal lobe o Parietal lobe o Temporal lobe o Occipital lobe o Limbic lobe Cingulate Hippocampal/rhinal areas of the temporal lobe TImestamp: 1:47pm on 16/10/2019
What is the role of the insular cortex?
• Insular cortex
o Taste, pain, visceral sensation and disgust (from uncouth social behaviours or other people looking disgusted)
What is the role of primary cortical areas?
o Primary cortical areas receive direct sensory information from thalamic regions (sensory) or project directly to lower motor neurons (motor)
o Have a prominent and dense input from thalamus
Where are primary cortical areas situated in comparison to sensory receptors?
o Contralateral to sensory receptor except for olfactory cortex (mostly ipsilateral) and visual cortex (maps receptors that take stimulus from opposite side of world-some are contralateral (nasal) or temporal are ipsilateral)
What is amount of cortex
mapped related to?
o Amount of cortex mapped is related to density of receptors
Describe the type of mapping of primary cortical areas and give an example
o Topographically mapped
E,g, the tonitopic A1 tone map from cochlea via CNVIII, auditory nuclei and the medial geniculate nucleus
Describe the location and Brodmann area of the primary somatosensory cortex
Location: Post-central gyrus (parietal)
Brodmann area: 1,2 and 3
Describe the location and Brodmann area of the primary motor cortex
Location: Pre-central gyrus (frontal)
Brodmann area: 4
Describe the location and Brodmann area of the primary auditory cortex
Location: Transverse gyrus of Heschel (superior temporal lobe)
Brodmann area: 41
Describe the location of the primary taste cortex
Location: Insular cortex
Describe the location and Brodmann area of the primary visual cortex
Location: Banks of the calcarine sulcus area (occipital lobe)
Brodmann area: 17
What does the somatosensory association area integrate and where is it?
o Somatosensory association area integrates and interprets sensations
S2- posterior to S1 and is important in object recognition from touch and position sensation.
What does the visual association area process and where is it?
o Visual association areas process various aspects of vision, combines into perception
V2-5- Occipital and inferior temporal cortex. Colour, motion, complex images.
What does the auditory association area process and where is it?
o Auditory association area distinguishes sounds, sounds in sequence
A2- Planum temporale (temporal plane) around A1 in superior temporal lobe, area 42, part of language area ‘Wernicke’s area’: processes timing and rhythm of sounds
What does the premotor area process and where is it?
o Premotor area process learned, patterned skilled movement
M2-Anterior to M1- programs and plans movements and serves gross motor movement coordinating many muscles
Where is Broca’s area, what does it do and what does damage to it do?
o Broca’s area- anatomically the inferior frontal gyrus, triangular gyrus, operculum
Plans motor speech
Adjacent to M1 cortex for lips, tongue, larynx
Damage to Broca’sarea causes changes in mechanics of speech
Where is Wernicke’s area, what does it do?
o Wernicke’s area
Anatomically the planum temporale. Adjacent to A1
Damage results in fluent aphasias -understanding is poor
How are Wernicke’s area and Broca’s area connected?
Arcuate fibres
How were brain maps obtained?
• How maps were obtained
o Penfield 1930s-1950s
o Penfield divided brain into different regions with pieces of paper stimulated brain with electrode and asked the person what they felt and what they were doing/feeling- mapped these
o Everyone has a slightly different map in terms of size
What is the role of higher order association areas and what do they receive?
o Responsible for more complex cognitive tasks- receive 2nd and 3rd order sensory information from primary and secondary association areas and integrate this sensory input to form a highly complex picture of sensory stimuli in the environment
What does damage to association cortices do?
o Damage to association cortices results in complex changes such as alterations in personality or the awareness of self
What is the role of the prefrontal cortex?
o Prefrontal cortex
Planning and decision making, categorisation
• Complex motor activity
o Apraxia- inability to properly execute a learned skilled movement coming from lesions of pre-motor areas
• Effortful processing-dorsolateral prerontal cortex
Working memory
Cortical attention
• Essentially appropriate sensory selection
• Classically identified as executive
Emotional regulation and expression, impulse control
What is the role of the orbital and ventromedial (gyrus rectus) prefrontal cortex?
Orbital and ventromedial (gyrus rectus) prefrontal cortex
• Decision making relative to emotional state
• Social decision making
o Active in choosing social behaviours and looking at what social behaviours other people perform
• Shows increased activity with recognition of emotion from voice and facial expression in others especially fear and anger
• Shows increased activity seen when attempting to suppress/control emotional behaviour
o Suppressing/activating emotions -trouble controlling emotions
What is the result of orbito and ventral prefrontal damage?
• Orbito and ventral prefrontal damage
o Damage- reduced emotions, reduced ability to judge consequence of actions, reduced ability to perceive disapproval or reward
What is the role of the parietal cortex and what does damage toit do?
Sensory integration and attention, visual/body tracking
• Multisensory input required to perceive where your body fits into the spatial world
• Combines and integrates somatosensory, visual, temporal (auditory/verbal) information
Damage leads to neglect syndrome or spatial agnosia
• Neglect syndrome- don’t pay attention to part of the world
What is the role of the limbic areas and archicortex?
o Limbic areas and archicortex
Memory, (hippocampus) , emotional memory formation (amygdala), empathy, motivation, error correction
What is the role of the arcuate fasciculus?
• Arcuate fasciculus (arc-shaped)- connects Broca’s and Wernicke’s language areas
What is the role of the Cingulum bundle?
• Cingulum-(C-shaped) bundle connecting the cingulate cortex with the limbic cortex
What is the role of the occipital fasciculus?
• Occipitofrontal fasciculus- major bundle of axons from the occipital lobe to the rest of the cortex
What is the role of the corpus callosum?
• Corpus callosum- connects the two hemispheres.
What is the role of the coronal radiata?
• Coronal radiata- fans out from the internal capsule, which carries fibres to and from the subcortex
What is the role of optic radiations?
• Optic radiations- highly myelinated bundles of axons connecting with the lateral geniculate nucleus to the primary visual cortex
