Structure of the nervous system

Question 1

What are the levels of organisation?

Answer 1

• Molecules (1Å)
• Synapses (1μm)
• Neurons (100μm)
• Networks (1mm)
• Maps (1cm)
• Systems (10cm)
• CNS (1m)

Question 2

The cortex can be divided into four lobes:

Answer 2

• Frontal Lobe
• Parietal Lobe
• Occipital lobe
  Temporal lobe

Question 3

What is the frontal lobe associated with?

Answer 3

Complex cognitive and motor functions

Question 4

What is the Parietal Lobe associated with?

Answer 4

Sensory information & integration

Question 5

What is the Occipital lobe associated with?

Answer 5

vision

Question 6

What is the Temporal lobe associated with?

Answer 6

auditory, higher level vision, speech

Question 7

The cortex can be subdivided into specialised zones on the basis of:

Answer 7

• Gross anatomy (sulcal & gyral anatomy)
• Variations on cellular properties (Cytoarchitecture)
• Connectional anatomy Functional properties
  These often coincide.

Question 8

Gross anatomy of the frontal lobes?

Answer 8

• 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’

Question 9

How does grey matter in the brain change throughout lifespan?

Answer 9

a reduction in grey matter volume across childhood and adolescence.

Question 10

When do higher order association cortices mature?

Answer 10

Higher order association cortices mature only after lower-order somatosensory and visual cortices (Gogtay et al., 2004

Question 11

Phylogenetically older regions mature earlier than…

Answer 11

newer regions.

Question 12

How do networks work in the brain? What’s a simple example?

Answer 12

• 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.

Question 13

What is convergence and divergence?

Answer 13

• 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.

Question 14

What is serial and parallel processing?

Answer 14

• Serial processing - Information is processed in one place and passed to another in turn.
  Parallel processing - Information is processed in several brain regions simultaneously.

Question 15

Serial and Parallel processing in the visual system?

Answer 15

• 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.

Question 16

What are cortical layers?

Answer 16

• The cortex forms the outer surface of the forebrain that covers the other forebrain structures.
  It is comprised of six distinct layers (‘laminae’).

Question 17

What is the Molecule layer?

Answer 17

· Top layer of the cortex
· Axons and dendrites
Few neurons

Question 18

What is the External granular layer?

Answer 18

· Small pyramidal cells
· Dendrites in Layer 1
Axons to deeper layers

Question 19

What is the External pyramidal layer?

Answer 19

· Medium/Large Pyramidal cells
· Dendrites in Layer 1
Axons to deeper layers or subcortical targets

Question 20

What is the Internal granular layer?

Answer 20

· Stellate cells
· Small pyramidal cells
Axons stay within the cortex

Question 21

What is the Internal pyramidal layer?

Answer 21

· Medium/large pyramidal cells

· Betz cells in motor strip

Question 22

What is the Multiform layer?

Answer 22

· Fusiform and other neurons

Connections to thalamus

Question 23

Cortical networks – inputs?

Answer 23

1. Association Fibers (ipsilateral corticocortical)
   
   2. Commissural Fiber (contralateral corticocortical)
   3. Thalamocortical (specific)
      Thalamocortical (non-specific)

Question 24

Cortical networks – outputs?

Answer 24

5. Association Fiber (to other cortical areas)
   
   1. Commissural Fiber (to other
      contralateral cortex)
   2. Corticostriate Fiber
   3. Corticorubral Fiber
      Cortocopotine Fiber
      Corticobulbar Fiber
   4. Corticospinal Fiber (to the spinal cord)
      Corticotectal Fiber
      Corticothalamic Fiber

Question 25

What did Vernon Mountcastle (1957) do?

Answer 25

- 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.

Question 26

What did Hubel and Wiesel (1977) do?

Answer 26

- 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

Question 27

What are cortical columns?

Answer 27

- 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.

Question 28

How are cortical columns used in the cerebral cortex?

Answer 28

- 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.”

Question 29

What is Cytoarchitecture?

Answer 29

- 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

Question 30

Three (+1) main zones in the frontal lobes which are distinguished based on their cytoarchitecture ?

Answer 30

- Primary motor cortex - Premotor cortex - Prefrontal cortex - Anterior Cingulate

Question 31

What is Connectional anatomy?

Answer 31

- 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

Question 32

How does high level information reach the spinal cord to influence behaviour?

Answer 32

· 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