Brain Wiring - Lecture notes Topic 7 PDF

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Brian Tansley, Carleton University...

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Human Neuropsychology

Topic 7

Brain Wiring! One of the Challenges Involved in Understanding Brain Connectivity! If we consider the number of ways in which circuits or loops of (neuronal) connections could be excited, we would be dealing with hyperastronomical numbers: 10 followed by at least a million zeros. (There are 10 followed by seventy-nine zeros, give or take a few, particles in the known universe.)""" -- Gerald Edelman, 1998. ! Connectomics! Goal: to produce simplified circuit diagrams at several spatial scales (e.g., macroscopic (brainwide), mesoscopic (circuit level), nanoscopic (synapse level)) that will further classify neurons in terms of their connectivity to each other! - heavily dependent upon automated neuroanatomic techniques and computer-based information processing (recently, Focused Ion-Beam electron microscopy has reduced the image acquisition time for a cubic millimeter of nervous tissue from ~18 years down to about 1.5 years)! - more broadly, would include the mapping of all neural connections within an"organism's"nervous system.! - The raw dataset for a single human brain is estimated to use as much memory as half the digital storage currently available on earth and cost somewhere between $4B to $12B US$! - Evidence of neuroplasticity in the connectome exists at all spatial scales! Neuroanatomic Techniques Used to Study Brain Wiring! Histological methods! - Conventional in vitro histology (various stains viewed by microscope) Intracellular staining (electrophoresis)! - Degenerative axonal tracing methods (e.g., horseradish peroxidase, rabies virus, isotopes)! - Automated axonal tracing (utilizing artificial vision algorithms to construct 3D models from 2D slices! Diffusion Tensor Imaging Tractometry!

- derived from functional Magnetic Resonance Imaging! - Involves computer-based estimation of neuronal tracts from measures of fractional anisotropy of water in axons! Connectome Spacial Scale! Synaptic scale (~10-100 x 10-9m):" neurophysiologic processes that occur at the synaptic level (e.g., long-term potentiation, synaptic pruning, etc.) (Measures of NT type and neurochemistry)! Microcircuit scale (~0.1 – 1 x 10-6m): Hebbian “reverbertory circuits” -the mean number of synapses between two neurons is proportional to their axo-dendritic overlap…(Functional measures: graded electrical potentials EPSPs and IPSPs)! Systems scale (myelinated tracts) (~0.1 - 10 x 10-2m): revealed through diffusion tractometry, fMRI, PET imaging, retrograde degeneration studies (rabies virus, etc.) White-matter tract connecting distant brain ‘modules’ (e.g., SLF, etc.) (Functional measures: evoked electrical potentials, conduction velocity, amplitude and latency)! Data Visualization Techniques Applied to Conventional Histology! Fluorescent Dyes "-- identifying neuron dendritic and axonal processes by staining with fluorescent dyes and viewing under ultraviolet illumination!

Human Neuropsychology

Topic 7

UV light has high energy per quantum, which, when absorbed into fluorescent dyes, gives up some of its energy and is re-emitted at a longer (visible) wavelength which increases the visual contrast of the stained tissue against the background in which it is embedded...! ! Diffusion Tractography! An unconstrained H20 molecule diffuses by Brownian motion where the probability of movement is equal in all directions (isotropy). Neural tracts are elongated structures that limit the diffusion of H20 more in some directions more than others (fractional anisotropy). ! E.g., myelination causes diffusion to be hindered perpendicular to the axon orientation! Following voxels high in FA and their ‘nearest neighbours’ along a pathway through a brain can trace brain tracts from origin to termination.! Tractometry! - The ‘white matter’ of the CNS connects distant neurons with one another via myelinated bundles of axons (axonal ‘tracts’).! - Over the past 20 years or so math-based algorithms developed to operate upon MR Imaging data, that make it possible to follow tracts from their origin to terminus.! - Utilizing the principle of fractional anisotropy to construct axonal pathways through the " brain ! - “constrained spherical deconvolution” is another, improved method for characterizing intravoxel behaviour reflecting microstructure of nervous tissue – one that doesn’t miss some important features that DTI does miss...! Neural Networks! Neural networks in the brain can be defined in two ways: ! - structural connectivity (what neurons connect to)! - Functional interdependence (the degree to which changes in the neural activity of one brain region results in associated changes in the neural activity of another region).! Large-scale Brain Networks ! - collections of widespread brain areas that are functionally connected (i.e., the neural activity within discrete regions is tightly coupled in time).! - Functional connectivity may be measured as long-range" synchronization of various types of brain dependent measures" such as: fMRI (Blood Oxygen Level Dependent) signaling, EEG, Positron Emission Tomography and Magnetoencephalography, etc. ! Examples of Large-scale Brain Networks.! - The Default Mode Network: a brain network whose elements are active when an individual is awake and at rest, and focused on ‘internally-oriented’ tasks such as daydreaming, envisioning the future, retrieving memories, and thinking about what others are thinking about (theory of mind). Negatively correlated with processing visual stimuli…! - Dorsal Attention Network: a brain network activated voluntary orientation of attention and/or reorientation to an unexpected visual event! - Ventral Attention Network: large-scale brain network that activates when an unexpected event occurs.! - Salience Network: network that monitors the degree to which an external stimulus (or an internal state or thought) is able to attract the individual’s attention! - Fronto-parietal Network: A large-scale neural network that initiates cognitive activity! - Lateral visual Network: A network important in activating brain processes in the presence of complex stimuli that evoke emotional responses.!

Human Neuropsychology

Topic 7

White Matter Fibre Pathways Form ‘rectilinear grids’ in the Brain!

- Cortico-cortical pathways form parallel “sheets” of interwoven paths in the longitudinal and medio-lateral axes !

- fiber pathways in simpler NSs are organized as parallel families derived from the three -

principal axes of embryonic development: the rostro-caudal, the medio-lateral (or proximodistal), and the dorso-ventral! The cerebral fiber pathways form a rectilinear three-dimensional grid that are also consistent with the three principal axes of development.! major pathways are considered ‘local condensations’ of this grid-like organization! “This architecture naturally supports functional spatio-temporal coherence, developmental path-finding, and incremental rewiring with correlated adaptation of structure and function in cerebral plasticity and evolution.” (Wedeen et al, 2013)!

Long Range Tractography! - Tracts are defined in the brain as ‘bundles’ or ‘ribbons’ of myelinated axons that carry information from one brain region to another, distal region.! - Generally, these fall into two categories: ! Intrahemispheric tracts (i.e, tracts that remain within a single hemisphere)! ! Interhemispheric tracts (i.e., tracts that cross from one hemisphere to the other (typically at the corpus callosum) connecting homotopic points (i.e., mirror image neuroanatomic structures) ! 10 Long Intrahemisphere(longitudinal) Tracts! 1.emanating from the parietal lobe: the superior longitudinal fasciculus subcomponents I, II and III; ! 2.emanating from the occipital-parietal region: the fronto-occipital fasciculus; ! 3.emanating from the temporal lobe: the middle longitudinal fasciculus and (from rostral to caudal), the uncinate fasciculus, extreme capsule and arcuate fasciculus; ! 4.emanating from the occipital-temporal region: the inferior longitudinal fasciculus ! 5.emanating from the cingulate gyrus: the cingulum bundle! The Superior Longitudinal Fasciculus! -The SLF is a long, bi-directional bundle of axons connecting distal regions of the ipsilateral cortex, including frontal, occipital, parietal and temporal lobes! -The anterior extreme consists of white matter radiations in the frontal lobe! -The posterior extreme consists of white matter radiations in posterior parietal, occipital and temporal lobes" ! -Courses through the operculum to the posterior end of the lateral sulcus!

Human Neuropsychology

Topic 7

The 4 Main Divisions of the SLF I! 1. SLF I: dorsal component and originates in the superior and medial parietal cortex, passes around the cingulate sulcus and terminates in the dorsal and medial frontal lobe (BA 6, 8 and 9) and in the supplementary motor cortex (MII)! 2. SLF II: middle component. It originates in the caudal-inferior parietal cortex and terminates in the dorsolateral prefrontal cortex (BA 6, 8 and 46) ! 3. SLF III: ventral component and originates in the supramarginal gyrus of the inferior parietal lobe and terminates in the ventral premotor and prefrontal cortex (BA 6, 44 and 46)! 4. Arcuate Fasciculus (AF): originates in the caudal superior temporal gyrus superior temporal sulcus and passes next to SLF II axons around the Sylvian fissure to terminate in the dorsal prefrontal cortex (BA 8, 46)! Functions Associated with SLF I! - SLF I connects to the superior parietal cortex which encodes locations of body parts in a body-centric coordinate system and with M II and dorsal premotor cortex, ! - Aids in regulating motor behaviour, including" (e.g., selecting amongst competing tasks)! Functions Associated with SLF II! - SLF II connects to the caudal inferior parietal cortex which controls spatial attention and visual and oculomotor functions! - Provides prefrontal cortex with parietal cortex information regarding visual space! - BA 46 (prefrontal cortex) may, in turn, provide parietal cortex with information to focus spatial attention! - Aids in selection and retrieval of spatial information! Functions Associated with SLF III! - SLF III connects the rostral inferior parietal cortex which receives information from the ventral precental gyrus! - Aids in the transfer of somatosensory information (including speech articulation) between ventral premotor cortex, BA44 (pars opercularis), supramarginal gyrus (BA 40) and the lateral inferior prefrontal cortex (BA 46)! Functions Associated with the Arcuate Fasciculus! - The arcuate fasciculus (aka arcuate fascicle) connects the superior temporal gyrus and the dorsal prefrontal cortex! - Associated with the transmission of auditory information between these two cortical areas associated with language comprehension (Wernicke’s area) and speech production (Broca’s area)! Inferior Longitudinal Fasciculus (ILF)"(aka Occipito-temporal Projections) ! - Arises in the occipital pole in extrastriate visual ‘association’ areas! - projects to lateral and medial anterior temporal regions…! - appears to mediate the fast transfer of visual signals to anterior temporal regions! - Mediating object recognition, semantic processing and the association of objects to object names! Inferior Fronto-Occipital Fasciculus (IFOF) ! Recent evidence suggests that the IFOF links the salience network to the executive control network to mediate goal-oriented behavior! Neuroplasticity of the Connectome! - Bennett, Kirby and Finnerty (2018) argue that the connectome shows neuroplastic properties at every spatial scale -- from individual synapses to large-scale networks…!

Human Neuropsychology

Topic 7

- Recent studies indicate that neural connections in mammalian brains may undergo rewiring during learning and experience-dependent plasticity.! Clinical Tractography! A means of quantifying and monitoring the effects of various forms of brain. Damage (e.g., TBI, ABI, degenerative disease, etc.)!...