Lecture 1 - The Neuron and Neurotransmission

8/12/2019 Lecture 1 - The Neuron and Neurotransmission

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Psychobiology

Dr. Lee Hogarth

1. The neuron and neurotransmission

Ren Descartes 1596 - 1650A balloonist theory of neural communication
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The neuron

Luigi Galvani 1737 - 1798

Galvani was slowly skinning a frog at a table where he had been conducting experiments

with static electricity by rubbing frog skin. Galvani's assistant touched an exposed sciaticnerve of the frog with a metal scalpel, which picked up a charge. At that moment, they sawsparks and the dead frog's leg kick as if in life. The observation made Galvani the firstinvestigator to appreciate the relationship between electricity and animation. This findingprovided the basis for the new understanding that electrical energy (carried by ions), andnot air or fluid as in earlier balloonist theories
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The neuron

Alessandro Volta 1745 - 1827

Volta was working on electrochemistry (the creation of electrical charges at the interface of

chemical solutions and metal conductors). He created the first battery in order to confirmthat Galvanis effect on frog leg contraction was produced by electricity (not heat orsomething else).

Voltas first battery
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The neuron


Batteries work by a chemical reaction causing a build up of ions at the anode (negative

terminal). This creates a potentialdue to the higher density of ions there. Ions repel eachother so they will flow down a conductor if attached to the anode if it is attached to terminalwith a lower ion density (cathode or earth).

Voltas first battery
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The neuron


The battery could now be used to demonstrate that electricity formed part of the biological

machinery underpinning movement, and presumably also thought.
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The neuron

1. Immerse a formaldahyde fixed brain slice in 2% aqueous solution of potassium dichromate for 2 days2. Dry the slice with filter paper.

3. Immerse the block into a 2% aqueous solution of silver nitrate for another 2 days.4. Dehydrate quickly in ethanol, clear and mount on slide.4. Use microscope to examine neuron.

Camillo Golgi 1843 1926Nobel prize 1906

Golgi method for staining brain cells

The black reaction consisted in fixing silver chromate particles to the neuron membraneresulting in a stark black deposit which provides clear and well contrasted picture of theneuron against a yellow background. The ability to visualize separate neurons led to theeventual acceptance of the neuron theory.
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The neuron

Golgi method for staining brain cells

Drawing of a Purkinje cell in the cerebellum, bySantiago Ramn y Cajal, clearly demonstrates thepower of Golgi's staining method to reveal fine detail.
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The neuron

Different staining techniques reveal the distribution of different neuron types in the brain.


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Lots of different types of brain cells.

The neuron


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Dendrites receive information fromother cell terminals. The cell body contains the machinerythat keeps the cell alive and functioning. The axon hillock is where theexcitatory and inhibitory input from

other cell terminals are summated todetermine whether the cell fires or not. The myelin sheath is a fatty layer thatspeed electrical conduction down theaxon. The nodes of Ranvier allow theelectrical charge to jump to each node,speeding conduction. The terminal buttons pass theelectrical signal to the dendrite of thetarget cell upon which it makes contact.

The neuron


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The action potential

Andrew Huxley 1917 2012Nobel Prize in 1963Giant squid axon


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Andrew Huxley 1917 2012Nobel Prize in 1963

Placing the electrode tip inside the action reveals a negative charged internal state equal to-70 millivolts (mV). This means there are more positively charged ions outside the cell.


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The action potential Placing the electrode tip inside the action reveals a negative charged internal state equal to-70 millivolts (mV). This means there are more positively charged ions outside the cell.

Na+ = sodium ions K+ = potassium ions

It is now known that the

resting potential of -70 mV iscaused by a higherconcentration of positivelycharged sodium ions (Na+)outside the cell, which havebeen pumped out of the cellby a sodium pump to create

the electrical potential.Inside the cell are morepositively charges potassiumions (K+) which areimportant for repolarization.


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Watch videohttp://www.youtube.com/watch?v=HnKMB11ih2o


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Synaptic communication

The synaptic cleft is aspace between theterminal buttons and thedendrites of the targetcell. The action potentialmay continue into thenext cell through aprocess known asneurotransmission orsynaptic communication.Changes in

neurotransmission at thesynaptic cleft are thebasis for learning, whichwe will explore in a laterlecture.


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Synaptic communicationThree different forms of synaptic connection


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EPSP = Excitatory post-synaptic potential.

IPSP = inhibitory post-

synaptic potential.

Two basic types of neurotransmitters


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nA = nanoamp


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NeurotransmittersGlutamate is a key

excitatory neurotransmitter.

GABA is a key inhibitoryneurotransmitter.

Whether this cell fires

reflects the summation ofthe excitatory (glutamate)and inhibitory (GABA)inputs .

Thus, changes in thestrength of these synapticconnections throughlearning determine thebehaviour of the animal orhuman and is the organic

basis for psychology


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NeurotransmittersA circuit model of the

chicken retinal cellorganisation allowingdetection of change in thevisual field. It is importantto understand how singlesynaptic weights change

with learning. However, tounderstand the biologicalbasis of psychology, it ismore important tounderstand how thespecific organisation ofcircuits contributes to theadaptive behaviour of theanimal.


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Neurotransmitters

PET imaging of glutamatereceptors in the brain.

PET imaging of GABAreceptors in the brain.

Positron emissiontomography (pet) involves

binding a radioactiveatom to a molecule thathas the same shape as aneurotransmitter and

injecting this into theperson. When theneurotransmitter binds toits receptor the radioactiveatom is released and thisis detected by the scanner

and translated into theimages shown.These images show thatglutamate and GABA aredensely distributedthroughout brain tissue.


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Neuromodulators

In contrast to neurotransmission, neuromodulationoccurs when the neurotransmittor molecule is notpumped back into the presynaptic cell by thereuptake transporter, but instead leaks into theextra-synaptic space, to modulate the activationlevel of neighbouring cells (either inhibitory orexcitatory) and thus the propensity of theseneighbouring cells to fire in response to direct

synaptic input. Modulation sets the tone of thebrain, whereas neurotransmission plays a direct rolein information signalling.


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Neuromodulators
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Lecture 1 - The Neuron and Neurotransmission

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