LECTURE NOTES - Week 12.1

Neuroscience of Learning

Neuroscience of Learning

• Entire field of research, behavioural neuroscience, in one lesson

  • Use the reading as background

• Challenges:

  • Brains are complex

    • Made of neurons, which are about 10-20 um (micrometers) in diameter

      • About ⅕ thickness of paper

    • Neurons wok fast: 

      • Action potentials take about 1-2 ms (1/1000 second)

    • Billions of neurons (about 80-100 billion in a human)

    • In complex networks

      • Need to study large numbers to see interesting effects 

      • Synapse = connection between neurons

      • Human brain has about 1000 trillion synapses

Basics of Brains

• Brains, 2 kinds of cells: neurons and glia (we won’t discuss them)

• Focus on learning:

  • Acquiring new information

  • Remembering it

  • Using it to drive behavior

• Brains do this:

  • Information comes in from the world

    • Through special organs (eyes, ears…)

    • Sent to the brain via sensory neurons, connect organs to spinal cord

  • Brain processes the information

  • Information used to drive behavior

    • Moving muscles: spinal cord uses motor neurons

Neurons

• Designed to receive and pass on information: directional

  • Info enters at dendrites = soma = axon = synapse

  • Synapse connects axon terminal of presynaptic cell of dendrite of postsynaptic cell

Action Potentials

• What it means for a neuron to “fire”

• Action potentials are:

  • Electrical:

    • Neuron is charged (membrane potential)

    • Maintained by movement of ions in and out of the cell

  • All-or-nothing:

    • Cell has a threshold; if the potential goes above that, it fires

• Action potential starts in the dendrite and moves down

• After firing, cell enters refractory period 

  • Cannot fire again until it is over

Synapses

• Involves 2 neurons:

  • Axon terminal of presynaptic neuron

  • Dendrite of postsynaptic neuron

• Neurons don’t touch

  • Gap = synaptic cleft

  • Crossed by neurotransmitter

    • Live in synaptic vesicles

    • Released by presynaptic neuron

    • Diffuses across cleft 

    • Binds to receptors on membrane of postsynaptic neuron

• Each cell’s action potential is electric; communication between them is chemical

Synapses Process

1. Action potential arrives, coming down axon of presynaptic neuron

2. Reaches axon terminal, causes a change in the membrane, leads to some vesicles fusing with membrane, spilling neurotransmitter into synaptic cleft

3. Neurotransmitter diffuses across the synaptic cleft, and arrives at the postsynaptic neuron’s dendrite

4. On the membrane of the dendrite are receptors – proteins that bind neurotransmitters

   • When they detect a neurotransmitter, they change shape, leading to ions moving in or out of the postsynaptic cell, changing its membrane potential

5. If membrane potential changes enough, exceeds firing threshold, new action potential is formed, starts moving down the axon

   • Usually one stimulation is not enough to get to the threshold

   • Neurons have thousands of synapses: 

     • Several simulations at the same time can sum up to reach threshold

6. Repeat from #1.

Regulation

• Real neuronal communication is a lot more complex

  • Neurons can modulate the communication process:

  • Over 40 kinds of neurotransmitters, different effects

    • Excitatory (e.g., glutamate): binding to receptor increases potential of postsynaptic neuron, makes it more likely to fire

      • Excitatory synapse

    • Inhibitory (e.g., GABA): binding to receptor decreases potential of postsynaptic neuron, makes it less likely to fire

      • Inhibitory synapse

  • Different thresholds

  • Release more/less neurotransmitter on each firing

  • Postsynaptic cell can change receptor density and sensitivity 

  • Grow new synapses; kill off old ones

A Simple Behavior

• Reflex arc (e.g., knee-jerk reflex):

  • 1. Doctor hits you with hammer

  • 2. Sensory neuron (purple) detects stretching of the tendon; fires 

    • Dendrite in the knee, axon terminal in spinal cord

    • Very long axon (over a meter)

  • 3. Motor neuron (green)

    • Dendrites in spinal cord

    • Axon terminal in knee, flexes quad

      • Neuromuscular junction 

    • No involvement of ‘brain’ 

      • Involuntary, fast, fixed action pattern

    • Need to also relax the opposing muscle:

      • Sensory neuron also synapses onto inhibitory interneuron (red)

      • Inhibits the hamstring from flexing

Neurons Learning

• Learning = change in reaction to situation

  • Brain change: altering synapse strength

  • Hebbian learning 

• Synapse strength:

  • Excitability of postsynaptic neuron

  • Likelihood that one stimulation will get to threshold

  • Long-term potentiation (LTP)

LTP

• Occurs at glutamate synapses

• 2 receptor types: AMPA, NMDA

• AMPA (normal receptor):

  • Glutamate binds, ions enter cell, polarization increases, more likely to fire

• NMDA:

  • To open, needs glutamate bound + potential of cell to already be high

  • “Coincidence detector”

  • When open: causes lots of long-term changes to cell (genes transcribed):

    • More receptors at the synapse

    • More sensitive receptors

    • Synapse gets stronger

LTP Example

• Neuron A fires

  • Activates AMPA on neuron C, potential increases

  • No activation of NMDA, no long term changes

• Neurons A and B fire at same time:

  • Activate AMPA on neuron C, a lot

  • Potential increases a lot 

  • NMDA also opened

  • Both synapses (A-C, B-C) stronger

• Pavlov e.g.:

  • Neuron A represents buzzer (CS)

  • Neuron B represents food (US)

  • Neuron C drives salivating (UR/CR)