Lecture 16: Synaptic Transmission, Learning and Memory Flashcards
Hypothesised that memories are formed by
connections between neurons
Memories and the brain
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 …
How are memories stored in the brain
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
The process of information transfer at synapses is called ……which involves
synaptic transmission which involves a change in the membrane potential
synapse communication changes
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
Electrical synapses allow
allows the rapid propagation of a signal in the form of ions
membrane potential
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
measuring membrane potential
micro electrode made of glass which is filled with salt solution which allows measurement of the electrical potential across the membrane with a voltmeter
What causes the resting membrane potential
- Phospholipid membrane is impermeable to ions.
2. There is an uneven distribution of ions between inside and outside of the cell
Ions and membrane potential
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
Potassium ion and neuron
Conc outside in mM 5
Conc inside in mM 100
ration out:in = 1:20
Sodium ion and neuron
Conc outside in mM 150
Conc inside in mM 15
ration out:in = 10:1
Calcium ion and neuron
Conc outside in mM 2
Conc inside in mM 0.0002
ration out:in = 10000:1
Chlorine ion and neuron
Conc outside in mM 150
Conc inside in mM 13
ration out:in = 11.5:1
What causes the uneven distribution of ions?
Sodium-Potassium pump (within the physical membrane)
Calcium pump
Sodium-Potassium Pump
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
Calcium pump
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
Depolarised
If the membrane potential becomes more positive than it is at the resting potential, the membrane is said to be depolarized.
Hyperpolarised
If the membrane potential becomes more negative than it is at the resting potential, the membrane is said to be hyperpolarized.
Change in membrane potential allows
communication between neurons
Influx of NEGATIVE ions
Hyperpolarisation
Inhibits the neuron
IPSP
chlorine for example
Influx of POSITIVE ions
Depolarisation
Excites the neuron
EPSP
via the influx of sodium ions for example
Action potentials
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”
Membrane potential and action potential
membrane potential changes rapidly during an action potential
Membrane potential changes during an action potential (steps) …
- Stimulus moves membrane
potential to threshold - Opens voltage-gated Na+ channel Na+ flows in (action potential)
- Na+ channels close and voltage- gated K+ channels open
- K+ flows out until K+ equilibrium potential reached
- 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
Summation
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
Spatial summation
- 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
Temporal summation
- One input neuron strongly activated
- If summation reaches threshold the neuron will fire an action potential
multiple which occur shortly after one another
Synaptic transmission summary …
screenshot on desktop
Chemical synapse on EM
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
Electrical synapse used for and look like on EM
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
Electrical synapse
• 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
Neurite
any projection from the cell body of a neuron
Gap junction components
connexin to connexon to gap junction
Chemical synapse
- 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.
How do nerve cells communicate steps
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)
Learning and memory is a dynamic process underpinned by
release of neurotransmitter - activation of postsynaptic receptors - trafficking of receptors to the PSD - local translation of new proteins - altered gene expression
Electrical synapses use
gap junctions/ion flow
chemical synapses use
neurotransmitters
