Synaptic Transmission

Question 1

Function of a synapse:

Answer 1

enables a neuron to transmit a signal/impulse to another cell

Question 2

Two types of synapses:

Answer 2

• electrical
• chemical

Question 3

Advantage of electrical synapse and advantage of chemical synapse:

Answer 3

• electrical synapses transmit the signal faster but chemical synapses allow for more biological flexibility (different types of neurotransmitters that can be used)

Question 4

The synapse

Answer 4

Question 5

Electrical synapses:
- occur at?
- channels are made of?
- process?
- motion of ions is

Answer 5

• occur at gap junctions
• **gap junction channel = 2 connexon
• ***connexon is formed from six connexin subunits (12 connexin total), which spans both the pre and post synaptic membranes
• the action potential from the pre-synaptic cell generates a postsynaptic potential in the postsynaptic cell
• a postsynaptic potential has a low amplitude and may not generate an action potential; may require neural integration of many signals
• gap junctions are bidirectional and hence ions move in both directions

Question 6

Synaptic Integration:

Answer 6

• neurons summate all the signals received before generating an action potential; most important integration occurs as dendritic integration

Question 7

Synaptic Integration:

Answer 7

Question 8

Connexon

Answer 8

Question 9

Electrical Synapse

Answer 9

Question 10

Chemical Synapse:
- process
- locations

Answer 10

• not directly via gap junctions, but propagated via neurotransmitters (small molecules)
• over synaptic cleft = 30nm wide
• active zones = sites of neurotransmitter release
• from an axon to a dendrite, to closer to the soma, to another axon which both travel to a dendrite

Question 11

Chemical Synapse:

Answer 11

Question 12

Location of chemical synapses:

Answer 12

Question 13

Neurotransmitters are

Answer 13

small molecules that diffuse across the synaptic cleft and bind to receptor targets on the post-synaptic membrane affecting the movement of ions through the semi-permeable membrane

Question 14

Neurotransmitters Chharacteristics:

Answer 14

• synthesised in a neuron
• concentrated in the presynaptic termini
• acts on specific targets on the postsynaptic cell
• if administered exogenously then mimics the action of endogenously released ligand
• is removed from the synaptic cleft in recycling

Question 15

Events at the Synapse Order:

Answer 15

• axonal transport
• axonal action potential
• neurotransmitter release
• neurotransmitter clearance

Question 16

Events at the Synapse: Axonal Transport:

Answer 16

• neurotransmitter is synthesised in
  axonal cytosol
• stored in vesicles
• action potential reaches axon
  terminal

Question 17

Events at the Synapse: Arrival of Action Potential:

Answer 17

• action potential arrives
• Ca2+ voltage gated ion channels
  open
• influx of Ca2+
• changes proteins on vesicles and
  presynaptic membrane
• vesicle fusion is initiated

Question 18

Events at the Synapse: Neurotransmitter Release:

Answer 18

• docked vesicles are held in position
  at the active zone by specialised
  proteins called v-snares and t-snares
• vescicles fuse with the membrane
• an increase in Ca2+ causes their
  conformation to change and
  exocytosis to commence
• exocytosis is very rapid (less than
  0.2ms) because Ca2+ enters the
  active zone and the local
  concentration rises rapidly

Question 19

Events at the Synapse: Neurotransmitter Clearance:

Answer 19

1) Enzymatic degradation in the
synaptic cleft
2) Uptake into glial cells with
subsequent enzymatic degradation
3) Absorption into the presynaptic cell
for repackaging
4) Absorption into the presynaptic cell
for degradation

Neurotransmitters must be cleared from the synaptic cleft to prevent over activation of ion channel targets on the postsynaptic membrane

Question 20

Neuromuscular Junction: Motor End Plate:

Answer 20

• motor nerve axons branch near their
  termini to contact skeletal muscle
  fibres
• myelination halts before the synapse
  in a structure called the motor end
  plate

Question 21

Motor End Plate:

Answer 21

Question 22

NMJ: Label the Diagram:

Answer 22

Question 23

Purpose of Subneural Clefts in NMJs:

Answer 23

subneural clefts increase the surface area for which the neurotransmitter can act

Question 24

NMJ Process:

Answer 24

• there are 300,000 synaptic vesicles
  with acetylcholine
• as with other chemical synapses
  Ca2+ influx initiates exocytosis
• from an action potential 125 vesicles
  of ach are released

Question 25

Acetylcholine receptor at NMJ is composed of

Answer 25

5 homologous subunits (2alpha,beta,gamma,delta)

ach binds to the alpha subunit

the receptor can allow the passage of K+ and Ca2+ but mainly only Na+

Question 26

What in a neuromuscular junction results in local depolarisation?

Answer 26

binding of ach to ach receptors allowing influx of Na+

***enough influx of Na+ results in local depolarisation

the remaining Na+ channels open and an action potential is initiated resulting fiber contraction

Question 27

NMJ action potential summary:

Answer 27

• action potential at axon terminal
• no myelination at motor end plate
• Ca2+ influx initiates exocytosis of ach
• vescicle docks and then fuses to
  membrane
• ach released into synaptic cledt and
  diffuses into subneural clefts which
  provide a larger SA for action and
  hence more binding site
• ach binds to alpha subunits on
  receptor
• influx of Na+ some passage of K+
  and Ca2+
• local depolarisation (end plate
  potential)
• remaining Na+ channels open,
  depolarisation reaches threshold
  initiating action potential
• results in fibre contraction

Question 28

Subneural Clefts:

Answer 28