Cortical connectivity and sensory encoding 25/4

Question 1

What are the two main classes of cortical neurons?

Answer 1

• Principle cells, which use glutamate. They are usually pyramidal. They fall into multiple classes that are distributed across and within cortical layers. They make up 80% of cell types. Principle cell-principle cell connectivity is highly specific. Bidirectional connections between them are frequent but do not occur at a probability of 1 - suggesting that not all pairs of neurons within networks are connected and a principle neuron can belong to more than one network.
• GABAergic inhibitory interneurons. These make up the remaining 20%, and their connections are local. Connectivity between interneurons and principle cells is non specific. Their connection probabilt to a neighbouring cell is almost 100%.

Question 2

What determines connectivity between excitatory neurons in the cortex?

Answer 2

There are developmental determinants: Sharing common progenitor cells increases liklihood of connectivity and to share orientation preferences. Molecular signalling types also predict connectivity and preference.
However, it is likely that this is a starting point for the refinement of circuitry. Just after first eye opening, visual preference poorly predicts connectivity. However this becomes stronger with visual experience. Synaptically connected principle cells tend to share visual preferences, and it is likely that sensory input forms functionally specific subnetworks that repsond to modality type. This is likely underpinned by hebbian plasticity.

Question 3

What are the two main types of coding strategy?

Answer 3

Sparse coding. information is encoded at any instant by spiking of a small subset of cells.

Dense coding. most neurons are active at any moment, therefore information is encoded by variations in firing rate.

Question 4

Sparse coding strategy

Answer 4

This occurs in more superficial layers of the cortex, where inhibitory interneurons are dense. Optogenetic stimulation of an L2/3 principle cell is therefore more likely to result in inhibition of another cell than excitation.

There is a low mean firing rate, and this is believed to confer higher selectivity for sensory features.

This also increases storage capacity, as there is less overlap in representations for different stimuli.

It is not space efficient, as more neurons are required for sparse coding.

Question 5

Dense coding stragegy

Answer 5

Deeper layers of the cortex tend to show dense coding, where most neurons are active at any given moment. They respond to a large range of stimuli in a graded manner. They recieve weaker inhibition, and more excitatory input from diverse cell types and subnetworks. Optogenetic stimulation of L5 neurons results in self sustaining activity.

Question 6

How may the use of sparse and dense coding be an adaptation of the cortex?

Answer 6

The coding types of L2/3 v.s. L5 principle neurons are tailored to the cell types that they target.

It may balance the costs and benefits of each coding type. Sparse coding in a large number of phsyically smaller intracortically projecting neurons may allow efficient information storage in local synpases. Dense coding in in a smaller number of large, long-range projection cells may allow efficient broadcasting of signals to distant structures, without requiring sufficient volume.

Question 7

General definition of plasticity

Answer 7

Persistent change in a neuron, a neural circuit
or set of neural circuits that occurs in response
to its history

Question 8

What are the 6 levels at which plasticity can operate?

Answer 8

Neuron
•Ion channels
•Synapse
•Receptive field (single-unit firing)

Cortical Circuits
•Microcircuits in a cortical column
•Cortical maps, local and cross-modal
•Higher cortical functions, memory,
emotions

Question 9

What are the types of interneuron?

Answer 9

Calbindin

Parvalbumin

Question 10

What are the types of inhibition?

Answer 10

• Feedforward. A presynaptic cell excites an inhibitory interneuron which inhibits the next follower cell
• Feedback. output of a process is used as an input to control the behavior of the process itself i.e., negative feedback

Question 11

Examples of experience dependent plasticity.

Answer 11

• Highly-trained musicians have a larger cortical representation of piano tones with a concomitant increase in the size of auditory cortex.
• String players develop an enlarged representation of fingering digits in sensori-motor cortex.
• Pascual-Leone et al studied patients who had been blind for many years. MEG maps of index finger used for reading Braille. Size of map varies with holidays and even over a weekend off work.