Lecture 16: Synaptic Transmission, Learning and Memory

Question 1

Hypothesised that memories are formed by

Answer 1

connections between neurons

Question 2

Memories and the brain

Answer 2

Connections of an ensemble of neurons is strengthened when memories are formed - connections are strengthened and these will represent an engram (memory trace) in the brain

Memory is not static, goes through the process of consolidation over time and can be retrieved and then reconsolidated and then retrieved again etc

encoding phase is an active state

encoding - consolidation - retrieval - reconsolidating - retrieval …

Question 3

How are memories stored in the brain

Answer 3

release of neurotransmitter - activation of postsynaptic receptors - trafficking of receptors to the PSD - local translation of new proteins - altered gene expression

rapid, intermediate, long term changes (have rapid, intermediate and long term changes that underpin the storage of memories in the brain)

This rapid response needs to be transformed into a long-lasting response because we want our memories to last for a long time

Question 4

The process of information transfer at synapses is called ……which involves

Answer 4

synaptic transmission which involves a change in the membrane potential

Question 5

synapse communication changes

Answer 5

electrical signal propagated down an axon which results in neurotransmitter release and this neurotransmitter activates the second neuron and the cycle continues

this is a way that information can be transferred between neurons

Question 6

Electrical synapses allow

Answer 6

allows the rapid propagation of a signal in the form of ions

Question 7

membrane potential

Answer 7

Definition: electrical potential difference across the membrane
• Nerve cells are “polarised”
the reference point is the outside the cell (neurons are therefore said to be inside negative)

• Sometimes Vm is “at rest”
The resting membrane potential
-65 mV (range −30, −90 mV)

• Other times it is not
e.g. during an action potential

Question 8

measuring membrane potential

Answer 8

micro electrode made of glass which is filled with salt solution which allows measurement of the electrical potential across the membrane with a voltmeter

Question 9

What causes the resting membrane potential

Answer 9

1. Phospholipid membrane is impermeable to ions.

2. There is an uneven distribution of ions between inside and outside of the cell

Question 10

Ions and membrane potential

Answer 10

Potassium high concentration inside at a ratio of 1:20 by contrast sodium and chloride ions are concentrated outside of neurons at 10:1 or 11.5:1 respectively, calcium is highly concentrated outside the cell at a ratio of 10000:1 which is important because changing the level of calcium inside the cell is one of the most important ways to changes the response/activity of a neuron

Question 11

Potassium ion and neuron

Answer 11

Conc outside in mM 5
Conc inside in mM 100
ration out:in = 1:20

Question 12

Sodium ion and neuron

Answer 12

Conc outside in mM 150
Conc inside in mM 15
ration out:in = 10:1

Question 13

Calcium ion and neuron

Answer 13

Conc outside in mM 2
Conc inside in mM 0.0002
ration out:in = 10000:1

Question 14

Chlorine ion and neuron

Answer 14

Conc outside in mM 150
Conc inside in mM 13
ration out:in = 11.5:1

Question 15

What causes the uneven distribution of ions?

Answer 15

Sodium-Potassium pump (within the physical membrane)

Calcium pump

Question 16

Sodium-Potassium Pump

Answer 16

Is an integral membrane protein
• Concentrates K+ inside the neuron and Na+ outside

• If Na+ levels are high “inside” the pump breaks down ATP to produce energy
• This drives the pump
• The pump drives ions against their concentration gradients

Sodium leaves cells and potassium enters the cell

Question 17

Calcium pump

Answer 17

Actively transports Ca2+ out of the cytosol across the cell membrane.

Additional mechanisms methods of calcium homeostasis include:
• intracellular calcium-binding proteins and
• organelles - endoplasmic reticulum and mitochondria

It is very important to control the amount of calcium within the cell

Question 18

Depolarised

Answer 18

If the membrane potential becomes more positive than it is at the resting potential, the membrane is said to be depolarized.

Question 19

Hyperpolarised

Answer 19

If the membrane potential becomes more negative than it is at the resting potential, the membrane is said to be hyperpolarized.

Question 20

Change in membrane potential allows

Answer 20

communication between neurons

Question 21

Influx of NEGATIVE ions

Answer 21

Hyperpolarisation
Inhibits the neuron
IPSP

chlorine for example

Question 22

Influx of POSITIVE ions

Answer 22

Depolarisation
Excites the neuron
EPSP

via the influx of sodium ions for example

Question 23

Action potentials

Answer 23

crucial to neuronal communication

(nerve impulse, spike, discharge)

“Information is encoded in the frequency of action potentials of individual neurons as well as in the distribution and number of neurons firing action potentials in a given nerve.”

Information flows via electrical signals (charged atoms)

An important method of conveying information over distances

Signals do not diminish over distance; they are signals of fixed size and duration

Occur when “changes” meet “threshold”

Question 24

Membrane potential and action potential

Answer 24

membrane potential changes rapidly during an action potential

Question 25

Membrane potential changes during an action potential (steps) ...

Answer 25

1. Stimulus moves membrane potential to threshold 2. Opens voltage-gated Na+ channel Na+ flows in (action potential) 3. Na+ channels close and voltage- gated K+ channels open 4. K+ flows out until K+ equilibrium potential reached 5. Na+/K+ pumps return membrane to resting potential From a resting membrane potential of about -65mV, the membrane depolarises and repolarises over a very short period of time there may be previous stimulation that do not reach threshold and these are known as failed initiations

Question 26

Summation

Answer 26

Neurons are not simply relay stations neurons integrate multiple inputs of information which is called summation Combine spatial and temporal information from both IPSPs and EPSPs The action potential released at the trigger zone/axon hillock is a result of the summation of the EPSPs and IPSPs summating to reach a threshold Depolarizing/stimulatory and hyperpolarizing/inhibitory inputs Membrane potential in the spike- initiation (trigger) zone moves above threshold to fire an action potential

Question 27

Spatial summation

Answer 27

* Multiple input neurons * Summating EPSPs generated simultaneously at many different synapses on a dendrite * If summation reaches threshold the neuron will fire an action potential

Question 28

Temporal summation

Answer 28

* One input neuron strongly activated * If summation reaches threshold the neuron will fire an action potential multiple which occur shortly after one another

Question 29

Synaptic transmission summary ...

Answer 29

screenshot on desktop

Question 30

Chemical synapse on EM

Answer 30

Presynaptic component can be identified by the many synaptic vesicles and can characterise the post synaptic component by an electron dense region known as the postsynaptic density

Question 31

Electrical synapse used for and look like on EM

Answer 31

Used for fast communication between nerve cells Can see it looks quite different, can see a point of contact between the two neurons but no evidence of neurotransmitter vesicles and we see no darkening which represents the post synaptic density What we can see are the multiple mitochondria which tells us that the processes going on in these terminals require energy

Question 32

Electrical synapse

Answer 32

• Electrical current flows from one neuron to the next • At specialized sites called gap junctions Gap junctions formed by difference subunits, formed by connexons which are made up of connexins, gap junctions allow electrical current to flow and allow neighbouring neurons to become highly synchronised • Which allow ionic current to pass equally well in both directions • Very fast (fast information transmission) Electrical synapses are often found between motor neurons and are often involved in fast responses such as reflexes • Neighbouring neurons are highly synchronized

Question 33

Neurite

Answer 33

any projection from the cell body of a neuron

Question 34

Gap junction components

Answer 34

connexin to connexon to gap junction

Question 35

Chemical synapse

Answer 35

* Most synaptic transmission in the CNS occurs at chemical synapses * via the release of neurotransmitter * The molecular events at chemical synapses will be the key focus of the following lectures.

Question 36

How do nerve cells communicate steps

Answer 36

1 - action potential reaches axon terminal and depolarises membrane 2 - voltage-gated Ca2+ channels open and Ca2+ flows in 3 - Ca2+ influx triggers synaptic vesicles to release neurotransmitter 4 - neurotransmitter binds to receptors on target cell (in this case, causing positive ions to flow in)

Question 37

Learning and memory is a dynamic process underpinned by

Answer 37

release of neurotransmitter - activation of postsynaptic receptors - trafficking of receptors to the PSD - local translation of new proteins - altered gene expression

Question 38

Electrical synapses use

Answer 38

gap junctions/ion flow

Question 39

chemical synapses use

Answer 39

neurotransmitters