Brains, neurons and neural coding

Question 1

Review the neuronal structure in terms of input and output signals

Answer 1

Inputs: synapses
Outputs: synapses
-soma, myelin sheath and axon trunk

Question 2

Describe the basic plan of the vertebrate brain

Answer 2

1. Embryonic development as a guide
2. From the 3 primary brain vesicles; forebrain, midbrain, hindbrain and spinal cord
3. Secondary brain vesicles; telencephalon, diencephalon, mesencephalon, metencephalon, myelencephalon and SP
4. We get six major divisions of adult CNS

Question 3

What are the six divisions of the adult CNS?

Answer 3

-cerebrum
-thalamus and hypothalamus
-midbrain
-pons and cerebellum
-medulla
-spinal cord

Question 4

History of Brains and neurons

Answer 4

• 1830s: development of cell theory (Schwann)
• Early 1880s-1900s: Ramon y Canal argued for neutron doctrine (cell theory applied to NS)
• Neurons output - functionally polarised
• Nature of electricity and ‘animal electricity’ worked out in parallel, from 1790s on
• Early 1900s: electrical resting potential of cells measured
• 1920s: all-or-none nature of AP (Adrian)
• 1940s-50s: theory of neuronal electrical activity worked out

Question 5

What do sensation and perception depend on?

Answer 5

Neural processing

Question 6

What is neural code?

Answer 6

• Electrical ‘Spikes’
• Jargon for action potentials; all-or-nothing response and fast
• Neural coding: individual APs do not differ from each other - no different kinds of APs for different information
• Code must be based on space, which neurons and time (precise time sequence of a series of spikes, or a spike train, in the jargon)

Question 7

Neural codes: space

Answer 7

• Neural circuits
• specific synaptic connections between nerve cells form circuits or networks
• What’s the scale?
  
  • 86 billion neurons in human brain
  • 17b in human cerebral cortex
  • 69b in cerebellum
  • up to 10k synaptic contacts per neuron
  • implies order of 10^14 synapses

Question 8

Neural codes: time

Answer 8

• time: spike trains
  
  • rate code: average spike frequency. Number of spikes over some integration time
  • temporal code: precise timing of spikes within a spike train Is important
  • Frequency = 1 / Period

Question 9

What’s the timescale?

Answer 9

Conduction and generation

Question 10

Conduction: how fast do APs travel along nerves?

Answer 10

Range: <1 ms^-1 to >100, majority <10 ms^-1

Question 11

Generation: how quickly can APs be generated (spike frequency or firing rate)?

Answer 11

• varies with cell type and species but fastest APs can take place within 1-2ms (spike plus absolute refractory period)
• this implies a maximum firing rate of 500-1000 spikes s^-1
  
  • physiological firing rates are usually rather lower