CH 4: Neural Conduction & Synaptic Transmission

Question 1

Define MEMBRANE POTENTIAL - describe how it’s recorded.

Answer 1

• = Difference in electrical charge b/w the inside & outside of a cell
• Recorded by placing an electrode tip inside the neuron & another outside neuron in extracellular fluid

Question 2

Define RESTING POTENTIAL.

Answer 2

• Steady membrane potential of -70mV

- -70mV indicates that the potential inside the resting neuron is 70mV less than the outside

Question 3

Define ION CHANNELS.

Answer 3

• Pores on neural membrane
• Each channel specialized to pass either Na+ or K+
• -> maintains uneven distribution of Na+ & K+

Question 4

Describe the resting membrane potential & its ionic basis.

Describe the 2 factors that allow more Na+ outside and more K+ inside neuron.

Answer 4

• Resting membrane potential = -70mV

1. Na+ channels are closed thus ^Na+ outside than inside
2. K+ channels are open but few K+ exit cell bc (-) resting potential (-70mV - opposites attract)

Question 5

Describe 2 reasons why theres more pressure on Na+ to enter resting neurons.

Answer 5

1. Electrostatic pressure: -70mV inside - opposites attract

2. Random motion pressure: Na+ ^likely to move down its [ ] gradient (go where there’s less of it)

Question 6

List the 2 types of postsynaptic potentials.

Answer 6

1. Depolarization

2. Hyperpolarization

Question 7

Describe how DEPOLARIZATION is conducted.

Answer 7

• NT binds to postsynaptic receptor on postsynaptic neuron & decreases the resting membrane potential (ie. -70mV –> -67mV)
• aka EPSPs

Question 8

Define EPSPs.

Answer 8

Excitatory postsynaptic potentials = ^likelihood that neuron will fire

Question 9

Describe how HYPERPOLARIZATION is conducted.

Answer 9

• NT binds to postsynaptic receptor & ^ resting membrane potential (ie. -70mV –> -72mV)
• aka IPSPs

Question 10

Define IPSPs.

Answer 10

Inhibitory postsynaptic potentials = decrease likelihood that neuron will fire

Question 11

Describe how postsynaptic potentials summate, & how action potentials are generated.

Answer 11

• Whether a neuron fires depends on balance b/w excitatory & inhibitory signals reaching its axon.
• ACTION POTENTIALS (AP) are generated when sum of depolarizations & hyperpolarizations reaching the axon at any time is sufficient to depolarize the membrane to a level = THRESHOLD OF EXCITATION = ~65mV
• AP’s are all-or-nothing response
• -> Stimulating a neuron more intensely does not ^speed or amplitude of resting action potential
• As a neuron is stimulated, it becomes less polarized until the threshold of excitation is reached & firing occurs

Question 12

Define INTEGRATION.

Answer 12

• Combining a number of individual signals into 1 overall signal

Question 13

Define SPATIAL SUMMATION.

Answer 13

• Integration of signals that originate at diff sites on the neuron’s membranes
• ie) EPSP + EPSP = greater EPSP
• ie) IPSP + EPSP = 0
  etc.

Question 14

Define TEMPORAL SUMMATION

Answer 14

• Integration of neural signals that occur at diff times at the same synapse

Question 15

Define VOLTAGE-ACTIVATED ION CHANNELS

Answer 15

• Open/close in response to changes in level of membrane potential

Question 16

Explain the ionic basis of an action potential.

Answer 16

• Resting neuron = ~impermeable to Na+ bc those that do pass in are passed out
• When EPSP excitation:
  –> membrane potential of axon is depolarized
  –> voltage-activated Na+ channels open wide
  –> Na+ rush into cell
  –> drive membrane potential from -70 to +50mV
  –> K+ driven out
  –> Na+ channels close
  = end of rising phase of action potential & beginning of depolarization by continued efflux of K+
• Once achieve depolarization:
  –> K+ channels close
  –> many K+ flow out of neuron
  –> neuron = hyper-polarized

Question 17

Define ABSOLUTE REFRACTORY PERIOD

Answer 17

• Brief period after initiation of an AP during which it’s impossible to elicit another AP in the same neuron

Question 18

Define RELATIVE REFRACTORY PERIOD

Answer 18

• After absolute refractory period when a higher-than-normal amount of stimulation is necessary to make a neuron fire

Question 19

Explain how the refractory period is responsible for 2 important characteristics of neural activity.

Answer 19

(1) Responsible for how AP’s travel along axons in only 1 direction
- bc portions of axon over which the AP has just travelled are left momentarily refractory, an AP can’t reverse direction

(2) Responsible for how the rate of neural firing is related to the intensity of stimulation
- If neuron under HIGH level of continuous stimulation, it fires & then fires again ASAP as absolute refractory period is over

Question 20

Describe how AP’s are conducted along UN-MYELINATED axons.

Answer 20

• Once AP generated, it travels passively along atonal membrane to the voltage-activated Na+ channels which have yet to be open
• The arriving electrical signal opens these channels
• -> allows Na+ into neuron
• -> generate full blown AP at this portion of membrane
• -> signal then conducted passively to net Na+ channels
• -> AP actively triggered
• -> repeated full blown AP triggered in all terminal buttons

Question 21

Describe how AP’s are conducted along MYELINATED axons.

Answer 21

• Ions pass through the axonal membrane only at the NODES OF RANVIER (=gaps b/w adjacent myelin segments)
• -> Na+ channels ^[ ] at nodes of Ranvier
• -> AP conducted passively along axon to next node
• -> another full blown AP elicited, etc.
• Myelination ^speed of axonal conduction
• Passive conduction = instant = signal jumps along axon from node to node

Question 22

Explain the shortcomings of the Hodgkin-Huxley model when applied to neurons in the mammalian brain.

Answer 22

• Model provided simple intro to what we understand about ways in which neurons conduct signals
• Problem = simple neurons & mech of model do NOT rep the variety, complexity & plasticity of many neurons in mammalian brain
• -> Model based study on squid motor neurons = large & ^simplistic than mammalian neurons

Question 23

Define AXODENDRITIC Synapses

Answer 23

• Synapses of axon terminal buttons on dendrites

Question 24

Define AXOSOMATIC Synapses

Answer 24

• Synapses of axon terminal buttons on somas (cell bodies)

Question 25

Define DIRECTED Synapses

Answer 25

• Synapses at which the site of NT release & site of NT reception are in close proximity

Question 26

Define NON-DIRECTED Synapses

Answer 26

• Synapses at which the site of release is at some distance from the site of reception

Question 27

Describe how SMALL NTs are synthesized & packaged in vesicles.

Answer 27

• Syn in cytoplasm of terminal button
• -> Packaged in synaptic vesicles by button’s Golgi complex
• NT-filled vesicles are stored in clusters next to presynaptic membrane

Question 28

Define & describe how LARGE Its are synthesized & packaged in vesicles.

Answer 28

• Large NTs = NEUROPEPTIDES = short AA chains = short proteins
• Assembled in ribosomes in cytoplasm
• -> packaged in vesicles by cell body’s Golgi complex
• -> transported by microtubules to terminal buttons
• Neuropeptide-filled vesicles are larger than small NT-filled vesicles

Question 29

Define EXOCYTOSIS

Answer 29

• Process of NT release

Question 30

Explain the process of NT exocytosis.

Answer 30

• Neuron at rest: small NT-filled synaptic vesicles congregate near sections of presynaptic membrane that are rich in voltage-activated Ca2+ channels
• Neuron stimulated by AP
• -> Ca2+ channels open
• -> Ca2+ enter button
• -> cause synaptic vesicles to fuse w/ presynaptic membrane & enter their contents into synaptic cleft
• -> small NTs released in a pulse each time an AP triggers momentary influx Ca2+ through presynaptic membrane
• -> neuropeptides (large NTs) release gradually IRT general ^ in level of intracellular Ca2+

Question 31

Explain 2 ways that NTs are removed from a synapse.

Answer 31

(1) REUPTAKE = once NTs released, are imm drawn back into presynaptic buttons via transported mech
- most common mech for deactivated a released NT

(2) ENZYMATIC DEGENERATION = enzyme break down NT in synapse

Question 32

Define GAP JUNCTIONS

Answer 32

• Narrow spaces b/w adjacent cells that are bridged by fine, tubular cytoplasts-filled protein channels

Question 33

Describe the role of glia & gap junctions in synaptic transmission.

Answer 33

• Connects the cytoplasm of 2 adjacent cells
• -> allows electrical signals & small molecules to pass from one cell to the next
• aka electrical synapses
• -> transmits signals ^rapidly than chemical synapses

Question 34

Name the 3 classes of NTs

Answer 34

(1) Conventional small-molecule NTs:
- AA’s
- Monamines
- Acetylcholine

(2) Unconventional small-molecule NTs

(3) Large-molecule NTs:
- Neuropeptides