Psychobiology: Learning & Memory, WEEK 4

Question 1

Electrical signalling

Answer 1

• how neurons communicate when they are far apart
• Neurons are large so axons can extend to reach different parts of the neuron to communicate
• We know communication is fast due to how quick reflexes are. (diffusion of chemicals is too time consuming, signal to communicate is electrical in nature)
• signals in neurons are electrical in nature
• Electrical signal allowing comm = action potential > this needs a resting membrane potential

Question 2

Resting membrane potential

Answer 2

• form of electrical excitability > RMP is when a neuron is inactive so energy is used to hold an unstable state
• as the neuron is unstable, when it’s triggered, the built up energy releases enabling the action potential signal > RMP has capacity to start action potential

Question 3

What happens in RMP?

Answer 3

• RMP depends on the cell membrane having a lipid bilayer (insoluble)>means ions don’t pass through the fatty molecule layer > is a bit leaky some ions can pass
• Membrane is a barrier between the inside + outside of the neuron, if we alter the structure of the outside compared to the inside, ions can get through > creates a ‘excitable + unstable’ RMP.
• Cell membrane have proteins which can serve functions like ion transporter

Question 4

Sodium-Potassium exchange transporter

ion transporter

Answer 4

• In the surface of cells of neurons, there is a sodium-potassium exchange transporter
• Moves Na+ ions out of the neuron + brings K+ ions into the neuron > leads to accumulation of + ions out of the cell, partially in exchange for K+ inside the cell.
• Partial exchange as more Na+ moves out than K+ moves in as theres more -ve charge inside.
• Outside is more +ve, inside is more -ve
• RMP difference is at -70mV (- as the inside is more -ve than outside)
• Na cations tend to move into the cell due to electrostatic pressure + diffusion but the transporter is pushing it back out

Question 5

Movement of ions

Answer 5

• Ion transporter: moves specific ions using energy from the whole body which maintains the RMP.
• Electrostatic pressure: ions w/ the same charge repel each other, + ions w/ the opposite charge attract
• Diffusion gradient: Regardless of the charge, they will want to move from areas of their own high concentration of the charge to low concentration of their charge.

Question 6

Cations at rest + activating action potential

Answer 6

• Diffusion + electrostatic pressure makes Na+ ions want to go in the cell, at rest the membrane is leaky so some Na goes in > if membrane became permeable, Na ions would flood in + make the inside more +ve than outside,
• Due to this, Na channels are the first mechanism in activating action potentials
• K+ wants to leave the cell because of diffusion (too much K in the cell) but electrostatic pressure makes it stay in as there is a high conc of cations outside the cell

Question 7

Stage 1: Action potential

Answer 7

• Na channels are voltage gated, approx threshold is -55mV (thresholds are not fixed but this is the average)
• If the membrane is depolarised (less -ve) + reaches -55mV from -70mV RMP, the Na channel opens

Question 8

Stage 2: Action potential

Answer 8

• Na channels open w/ K channels > Na channels flood into the cell > membrane potential gets more +ve
• When K channels open, there is little movement of K+ due to balance from diffusion gradient + electrostatic pressure.

Question 9

Stage 3: Action potential

Answer 9

• More Na cations will be on the inside than outside so membrane potential is more +ve than outside > allows K+ ions to move out as there is no more electrostatic pressure + moves w/ diffusion
• Na channel closes when it reaches 40mV

Question 10

Stage 4: Action potential

Answer 10

• Na channel close but K channels stay open as K ions keep leaving the cell due to diffusion + electrostatic p
• This makes the membrane potential more -ve as there are less cations inside the cell > this is called membrane repolarisation

Question 11

Stage 5: Action potential

Answer 11

• K cations keep leaving the cell past original RMP > membrane potential gets more -ve past -70mV
• K channels close eventually + combination of processes lead to balance of membrane potential

Question 12

Relevance of action potentials for brain function

Answer 12

• Action potentials are basic code for info in the brain > this code is specific in nature (same shape + size)
• They follow an ‘all or nothing’ law, they either will occur or do not (nothing in between)
• Size of the action potential doesn’t contain info
• Frequency (rate) of action potentials contain info

Question 13

Transmission of the electrical signal

Answer 13

• The signal cannot jump from one neuron to another as the synaptic cleft in only 20nm but isn’t wide enough to allow it to jump
• Recipient neuron needs to be depolarised
• Membrane may depolarise to begin with as a action potential may have been generated nearby > creates localised movement of ions + membrane change which can breach threshold of excitation

Question 14

Chemical synapse

Answer 14

• Communication across the synaptic cleft is a chemical synapse

Question 15

Chemical synapses : pre-synaptic neuron

Answer 15

• Input neuron brings info to the synapse
• Terminals > end of axon are axonal terminals where there are vesicles containing neurotransmitters
• Neurotransmitters get released by the pre-synaptic neuron into the synaptic cleft > done by fusing vesicle into membrane at axonal terminal

Question 16

Chemical synapse: post-synaptic neuron

Answer 16

• Output neuron receives info at the synapse
• Receptors (many which are ion channels) > the released neurotransmitters bind to the receptor, binding opens the channels > allows ions to flow through + change voltage + conductance of output neuron > creates a post synaptic potential as potential of membrane changes

Question 17

Post-synaptic potentials (PSPs)

Answer 17

• Hypopolarisation > essentially the same as depolarisation, allows opening of cation channels, excitatory (EPSP = excitatory post synaptic potential)
• Hyperpolarisation > is inhibitory and allows opening of anion channels, (IPSP = inhibitory post synaptic potential) > becomes permeable to anion channels (Cl-) > would make RMP more -ve than -70mV > less likely to fire potential

Question 18

Why is communication within a neuron fast?

Answer 18

• Speed of action potentials are facilitated by the myelin sheath made from glial cells
• AP’s decreases while travelling under sheath due to insulating properties, but it fully regenerates at nodes of Ranvier (gaps between) > allows AP to jump from nodes to nodes
• This speeds communication through the axon but also conserves energy as AP is only regenerated between the myelin sheath + not all through the axon

Question 19

Neural integration

Answer 19

• communication between synapses allow neural integration
• where each pre-synaptic neuron may contact other different post-synaptic neurons > post-synaptic neurons can get inputs from several different pre-synaptic neuron
  > means post-synaptic neurons integrate across many inputs. > these several inputs combine to affect the probability of the post-synaptic neuron firing.
• PSP’s are small + opening of a channel isnt enough for depolarisation or hypopolarisation > means summation of many PSP’s enhance ability of post-synaptic neuron firing - likely basis of brains computing power

Question 20

Post-synaptic receptor (ligand)

Answer 20

• Ligand = chemical interacting w/ a receptor > neurotransmitter + drugs are ligands
• the binding site on the receptor is where the ligand interacts

Question 21

Binding ligands to receptors

Answer 21

• Selectivity of binding: only specific ligands fit in a binding site due to 3D shape of receptor. Some drugs bind to more than one receptor (means there are more receptor types than neurotransmitters)
• Affinity: describes how well a ligand binds to a receptor > high affinity ligand means receptors are completely bound by small amounts of ligand

Question 22

Iontropic receptors

Answer 22

• directly paired to an ion channel (receptor is an ion channel in a way) > means it has a direct effect on neuron as when ligand binds to the receptor, the ion channel opens

Question 23

Metabotropic receptors

Answer 23

• Metabotropic receptor: begins w/ ligand binding to receptor + neurotransmitter released into cleft > binds to outside of post-synaptic membrane
• This changes the 3D shape of receptor as receptor proteins span lipid bilayer of membrane
• Receptor proteins are connected to other proteins > the changed shape of receptor proteins activates G proteins
• Activation of G proteins initiates intracellular signalling cascade > results in many effects such as altering iontropic receptors to make them more/less excitable

Question 24

Receptors on pre-synaptic neurons

Answer 24

• Receptors here allow many functions such as negative feedback > release of neurotransmitter can stop its own further release to avoid wasting neurotransmitter + is stored in pre-synaptic neuron
• Another function is retrograde signalling > signal from post synaptic neuron to pre synaptic neuron

Question 25

2 broad types of neurotransmitter based on chemical nature

Answer 25

• Amino acid derivatives

- Monoamines

Question 26

Amino acid derivatives

Answer 26

• Glutamate: comes from glutamic acid + are usually seen in brains of mammals > excitatory neutrotransmitter as receptors it binds to tend to make ESPS
• Glutamate binds to at least 8 different receptors (ionotropic+metabotropic+NMDA+AMPA)
• GABA: synthesised from glutamate but most abundant inhibitory neurotransmitter > makes post-synaptic neuron less likely to fire
• Glycine: unusual as it is an amino acid itself + binds to inhibitory receptors in spinal cord > co-agonist w/ glutamate at NMDA receptors

Question 27

Monoamines

Answer 27

• Mostly bind to metabotropic receptors + found in specific groups in neurons
• Includes dopamine, seratonin and neuropeptides (eg: opiods)
• Peptides are proteins which act as neurotransmitters

Question 28

Factors affecting Na+ and K+ channels

Answer 28

• increasing Na+ channels reduce the threshold of action potential
• increasing K+ channels delays initiation of action potential
• High amplitude can reduce threshold
• High density of Na+ channels reduce threshold

Question 29

Blocking sodium channels with TTX

Answer 29

• TTX is a toxin binding to Na+ channels and blocks the flow of sodium ions through a channel
• This prevents the production of an action potential

Question 30

Blocking potassium channels with TEA

Answer 30

• TEA is a potassium-selective ion channel blocker
• TEA only affects voltage dependant changes in K+ permeability
• This increases the duration of an action potential > because K+ is responsible for cell repolarisation, when it is blocked, it takes longer to repolarise, increasing AP time