Structure of the nervous system Flashcards
What are the levels of organisation?
- Molecules (1Å)
- Synapses (1μm)
- Neurons (100μm)
- Networks (1mm)
- Maps (1cm)
- Systems (10cm)
- CNS (1m)
The cortex can be divided into four lobes:
- Frontal Lobe
- Parietal Lobe
- Occipital lobe
Temporal lobe
What is the frontal lobe associated with?
Complex cognitive and motor functions
What is the Parietal Lobe associated with?
Sensory information & integration
What is the Occipital lobe associated with?
vision
What is the Temporal lobe associated with?
auditory, higher level vision, speech
The cortex can be subdivided into specialised zones on the basis of:
- Gross anatomy (sulcal & gyral anatomy)
- Variations on cellular properties (Cytoarchitecture)
- Connectional anatomy Functional properties
These often coincide.
Gross anatomy of the frontal lobes?
- The central sulcus divides the frontal lobe from the parietal lobe
The precentral sulcus divides the rest of the frontal lobes from the ‘motor strip’
How does grey matter in the brain change throughout lifespan?
a reduction in grey matter volume across childhood and adolescence.
When do higher order association cortices mature?
Higher order association cortices mature only after lower-order somatosensory and visual cortices (Gogtay et al., 2004
Phylogenetically older regions mature earlier than…
newer regions.
How do networks work in the brain? What’s a simple example?
- The nervous system consists of neural networks which integrate and distribute information.
- The simplest neural networks can be represented as a chain of connected neurons, e.g. cells connecting retinal receptors to the cortex.
- Another example is the knee-jerk reflex arc. - Simple spinal cord networks can convert sensory information directly into a motor response independently of the brain.
What is convergence and divergence?
- Convergence: Information from several neurons is integrated to influence the firing of a few.
- Divergence: Information from a few neurons is distributed to several other neurons.
Convergence and Divergence in the visual system: Convergence from retinal receptors to the optic nerve, divergence from the nerve to the cortex.
What is serial and parallel processing?
- Serial processing - Information is processed in one place and passed to another in turn.
Parallel processing - Information is processed in several brain regions simultaneously.
Serial and Parallel processing in the visual system?
- Networks in the brain are organised
hierarchically – information is passed from once place to the next serially and transformed as it crosses various points in the network.
The brain is also parallel – information from a single source can be processed in many locations simultaneously.
What are cortical layers?
- The cortex forms the outer surface of the forebrain that covers the other forebrain structures.
It is comprised of six distinct layers (‘laminae’).
What is the Molecule layer?
· Top layer of the cortex
· Axons and dendrites
Few neurons
What is the External granular layer?
· Small pyramidal cells
· Dendrites in Layer 1
Axons to deeper layers
What is the External pyramidal layer?
· Medium/Large Pyramidal cells
· Dendrites in Layer 1
Axons to deeper layers or subcortical targets
What is the Internal granular layer?
· Stellate cells
· Small pyramidal cells
Axons stay within the cortex
What is the Internal pyramidal layer?
· Medium/large pyramidal cells
· Betz cells in motor strip
What is the Multiform layer?
· Fusiform and other neurons
Connections to thalamus
Cortical networks – inputs?
- Association Fibers (ipsilateral corticocortical)
- Commissural Fiber (contralateral corticocortical)
- Thalamocortical (specific)
Thalamocortical (non-specific)
Cortical networks – outputs?
- Association Fiber (to other cortical areas)
- Commissural Fiber (to other
contralateral cortex) - Corticostriate Fiber
- Corticorubral Fiber
Cortocopotine Fiber
Corticobulbar Fiber - Corticospinal Fiber (to the spinal cord)
Corticotectal Fiber
Corticothalamic Fiber
- Commissural Fiber (to other
What did Vernon Mountcastle (1957) do?
- Recorded firing properties of neurons in the somatosensory cortex.
- When the recording electrode was moved obliquely to the cortical surface, he found that the firing properties changed.
- The neurons responded to different sensory sub-modalities (light vs deep touch).
- Recorded firing properties of neurons in the somatosensory cortex.
- When the recording electrode was moved orthogonally to the cortex, the firing properties did not change.
- Neurons responded to only one sensory sub-modality (e.g. deep vs light touch).
As a result of his data, Mountcastle proposed the hypothesis that neurons formed vertical columns or cylinders which each responded to one sub- modality.
What did Hubel and Wiesel (1977) do?
- Recorded from primary visual cortex, mapped inputs from each eye and looked for neurons with ‘ocular dominance’ properties.
- Ocular dominance columns are organised into stripes across the cortex.
Patterns unique to individuals and the stripe intervals vary across species
What are cortical columns?
- Ocular dominance columns form distinctive ‘fingerprint’ patterns in the visual cortex
- Orientation Columns
- Neurons in the primary visual cortex contain columns of neurons which respond specifically to the orientation of a visual stimulus.
- Precise geometric relationships can be observed between ocular dominance and orientation patterns if they are superimposed.
How are cortical columns used in the cerebral cortex?
- In many regions of the cerebral cortex, neurons in these layers are organised into columnar modules that serve as information processing units.
- They form vertical columns because neurons form dense connections with other neurons above and below them, and fewer connections with neurons in the same layers.
- There are ~110 neurons in each cortical column. This number is constant across species and across brain areas within species (the primate visual system is an exception, and here columns contain ~270 neurons)
- The human cerebral cortex is composed of about 1 million cortical columns.
- David Hubel: The cortical column is “a little machine that takes care of contours in a certain orientation in a certain part of the visual field.”
What is Cytoarchitecture?
- Cytoarchitecture: How celluar properties vary across the cortex
- Reflects variability in the way information is processed
- Starting point for cortical localisation of function
Properties of cortical layers change from one part of the cortex to the next
Three (+1) main zones in the frontal lobes which are distinguished based on their cytoarchitecture ?
- Primary motor cortex
- Premotor cortex
- Prefrontal cortex
- Anterior Cingulate
What is Connectional anatomy?
- Cortical zones are connected to each other and to subcortical regions in specific ways
- There are direct projections from the premotor cortex to the spinal cord
- Prefrontal cortex – premotor cortex – primary motor cortex – spinal cord – muscles
- Lesions starting from the left impair motor functions, lesions from the right impair high level functioning
Each level of the frontal network exchanges information with the cerebellum and basal ganglia, which monitor and modify activity through independent looped connections
How does high level information reach the spinal cord to influence behaviour?
· There are no projections from the prefrontal cortex to the spinal cord
· But, there are projections from the prefrontal cortex to the premotor cortex
· There are projections from the premotor cortex to the primary motor cortex and to the spinal cord
· So, information cascades across the frontal lobes from the prefrontal cortex to the spinal cord via the premotor cortex
Therefore, the premotor cortex is a strategic location for converting ideas and goals about movement into actions
