L1: Neurons & Neural Networks I: Regulation of Signalling

Question 1

Describe how the Na+/K+ ATPase Pump establishes a concentration gradient within the cell

Answer 1

• Na/K+ ATPase pump uses ATP to actively pump 3 Na+ ions and 2 K+ ions out and into the cell, respectively
• maintaining a more depolarised internal environment

Question 2

what role do the sodium and potassium channels play in maintaining membrane potential?

Answer 2

• Na+ channels permit rapid influx of sodium into cell upon opening, with resultant depolarisation (more positive)
• K+ channels permit rapid efflux of Na+ out of cell upon opening with resultant hyperpolarisation (more negative)

Question 3

what two forces are ions under during the establishing of the membrane potential?

Answer 3

• electrostatic force (dependent on charge)
• force of diffusion (dependent on concentration)

Question 4

describe the direction of travel within a neuron

Answer 4

• dendrites receive incoming signals
• incoming signals converge at cell body (summation effect, if strong enough AP propagates along axon)
• conveyed along axons to transmit signal in form of action potential (AP) to target neuron

Question 5

describe the simple neural networks (refer to notes for image)

Answer 5

• e.g. sensory terminals in skin responding to temp/touch
• info classically relayed in a primary afferent via dorsal root ganglion (if for body, below the neck) or trigeminal ganglion (above the neck)
• info relayed via spinal cord and conveyed up to brain through secondary afferent/motor tracts
• most info ends up at thalamus, main sensory nuclei within diencephalon, relayed through a number of different nuclei
• output and response (e.g. motor response)

Question 6

what is the function of the generator potential?

Answer 6

• function to initiate action potentials in neuronal axon
• associated with non-specific cation channels (letting in any positive ion)

Question 7

what does it mean that the generator potential is modality specific?

Answer 7

within the skin there are different receptors that respond to different modalities so can understand if it’s e.g. chemoreceptors/mechanoreceptors/thermoreceptors etc.

Question 8

the generator potential is modality specific for what?

Answer 8

mechanoreceptors (high or low threshold)

thermoreceptors (hot or cold)

chemoreceptors

polymodal receptors (nociceptors)

Question 9

is the generator potential ‘all-or-nothing’ or graded?

Answer 9

graded potentials

Question 10

is the generator potential transient?

Answer 10

yes but can be sustained

Question 11

outline the strengths of the generator potential

Answer 11

localised: info on location of stimulus (where is the stimulus)

graded: info on intensity of stimulus (how strong is stimulus)

Question 12

outline the limitations of the generator potential

Answer 12

• generated by specific modality but doesn’t convey modality specific info

(i.e. once converted into AP it goes into system and if system goes wrong brain can misinterpret info being from wrong place e.g. touch is painful when it shouldn’t be (allodynia))

Question 13

Draw an action potential graph and label all (9) steps

Answer 13

1. resting membrane potential
2. depolarising stimuli
3. depolarisation reaches threshold: voltage-gated sodium channels (NaV) open and Na+ ions enter neuron
4. rapid Na+ entry depolarises neuron further
5. NaV channels inactivate and slower (0.5mS) potassium channels (Kv) open
6. K+ ions moves out of neuron
7. Kv channels remian open and more K+ ions leave neuron, hyperpolarising it
8. Kv channels close, some K+ enters cell through leak channels
9. Normal membrane potential

Question 14

Describe the function and the nature of the action potential

Answer 14

function: carry signal from point to point

• highly specific interaction between NaV and Kv (voltage gated Na+ and K+ channels)
• an ‘all-or-nothing’ event
• very brief event (~2-5ms)

Question 15

outline the strengths of an action potential

Answer 15

• signal size is maintained over distance and branches
• versatile: frequency and pattern encoding

Question 16

outline the limitations of an action potential

Answer 16

• membrane must be hyperpolarised to start
• system has to be re-primed (refractory period)

Question 17

what is excitatory postsynaptic potential (EPSP)?

Answer 17

• postsynaptic potential that makes the postsynaptic neuron more likely to fire an AP

(depolarises postsynaptic cell, bringing membrane potential closer to threshold and so firing an AP)

Question 18

what is inhibitory postsynaptic potential (IPSP)?

Answer 18

• synaptic potential that makes postsynaptic neuron less likely to generate an AP
• (a temporary hyperpolarisation of postsynaptic neuron caused by flow of negatively charged ions (Cl-) into postsynaptic neuron)

Question 19

Are EPSPs graded or all or nothing?

Answer 19

graded

Question 20

the amplitude of EPSPs depend on what factors?

Answer 20

• amount of NT released
• number of receptors
• state of receptors

Question 21

Decay of EPSP depend on what factors?

Answer 21

• dissociation of ligand
• diffusion and uptake (e.g. glutamate)
• desensitisation (e.g. AMPA-type glu receptors
• enzymatic breakdown (e.g. Ach)

Question 22

How do EPSPs engage in spatial summation?

Answer 22

• occurs when subthreshold EPSPs, occurring simultaneously at different points along postsynaptic membrane combine to cause depolarisation that reaches threshold of excitation

Question 23

How do EPSPs engage in temporal summation?

Answer 23

occurs when multiple subthreshold EPSPs from one neuron occur close enough in time to combine and trigger action potential at axon hillock (postsynaptic potentials last for ~ 4milliseconds, while AP last for 2 ms)

Question 24

What two factors are essential for neurotransmitter release?

Answer 24

• depolarisation of terminal
• presence of Ca2+ ions in extracellular fluid

Question 25

EPSPs contribute to what and so lead to what?

Answer 25

EPSPs contribute to depolarisation leading to generation of action potential

Question 26

how fast do EPSPs occur?

Answer 26

• normally fast but slow EPSPs occur
• Fast EPSPs (~10-100ms): associated with activation of ligand-gated non-specific (Na+,K+, Ca2+) cation channels .g. glutamate (AMPA, NMDA), ATP (P2X), Ach (nicotinic)

Question 27

what are the strength of EPSPs?

Answer 27

• versatile:
• different transmitters can act on same postsynaptic cell using different receptors
• different receptors/ion channels can be regulated independently
• independent postsynaptic and presynaptic control of ‘synaptic strength’

Question 28

what are the limitations of EPSPs?

Answer 28

• metabolically expensive (e.g. making NTs)
• vulnerable to chemical interference (e.g. drugs) - not always bad, can module how neurons signal and increase e.g. serotonin

Question 29

function and nature of IPSPs?

Answer 29

• function: prevent depolarisation and inhibit the generation of the action potential (opens anion channels e.g. Cl-)
• graded (not ‘all-or-nothing’)

Question 30

are IPSPs fast or slow? describe..

Answer 30

• normally fast (but slow IPSPs do occurr)
• Fast IPSPs (~10-500ms): activation of ligand-gated (Cl-) anion channels by GABA(subscript A) receptors or glycine

Question 31

strengths of IPSPs

Answer 31

• versatile:
• different transmitters can act on the same postsynaptic cell using different receptors (e.g. mixed EPSP & IPSP)
• different receptors/ion channels can be regulated independently
• independent postsynaptic and presynaptic control of ‘synaptic strength’

Question 32

limitations of IPSPs

Answer 32

metabolically expensive and vulnerable to chemical interference (e.g. drugs and toxins)

Question 33

Neural networks allow for information to be passed through the nervous system as a result of what?

Answer 33

neuronal convergence and divergence

Question 34

what is convergence?

Answer 34

one postsynaptic neurons receives signals from multiple presynaptic neurons so it can integrate different inputs to make a decision

Question 35

what is divergence?

Answer 35

when one presynaptic neuron communicates with multiple postsynaptic neurons coordinating their action in a network

Question 36

divergence in networks is controlled by what?

Answer 36

interneurons

Question 37

what is lateral inhibition?

Answer 37

• when a neuron excites inhibitory neurons which then inhibit neighbouring neurons in a network
• e.g. used in retina to provide edge enhancement i.e. important in visual acuity

Question 38

what is feedforward excitation?

Answer 38

one neuron activates a second neuron which may go on to excite a third and so on (activates a pathway)

Question 39

what is feedback excitation?

Answer 39

when neuron activated by any of its downstream neurons

• once first neuron in circuit stimulated, circuit is self-perpetuating until neuron is switched off (way to maintain a persistent long-standing response to a brief stimulus)

Question 40

what is feedforward inhibition?

Answer 40

• neuron stimulates an inhibitory neuron which inhibits the next neuron
• used to shut down a pathway

Question 41

what is feedback inhibition?

Answer 41

• neuron inhibited any of its downstream neurons
• purpose is to terminate/inhibit excitation in a pathway after an effect is achieved

(very common used to maintain homeostasis)

Question 42

how do feedforward excitatory circuits lead to muscle contraction (e.g. knee-jerk response)?

Answer 42

• brain sends out signals to two feedforward excitatory circuits that control extensor and flexor muscles simultaneously
• these two circuits inhibit each other via inhibitory interneurons so their activation occurs alternatively (not simultaneously)
• producing contraction in one group of muscles and relaxation in the other