Synaptic Neurotransmission

Question 1

Electrical transmission

Answer 1

1. Gap junctions allow direct electrical coupling between neurons.
2. Generally rare in neurons.
3. Common in cardiac and smooth muscle.

Question 2

Chemical transmission

sequence of events

Answer 2

1. AP into presynaptic nerve terminal.
2. Activation of voltage gated calcium channels (VGCCs) and Ca++ influx (large gradient 2mM0.00001mM (20,000 :1).
3. Synaptic vesicles fuse with presynaptic membrane and release their content into the synaptic cleft.
4. The neurotransmitter diffuses to postsynaptic membrane.
5. Activation of neurotransmitter-gated ion channels.
6. Ions travel through channels> may cause depol. or hyperpolarisation.
7. Transmitter is removed after very short period. Three mechanisms:
   • Broken down by enzymes.
   • Diffuse away and broken down elsewhere.
   • Re uptake into presynaptic terminal or glial cells.

Question 3

Fast synaptic transmission:

Answer 3

Mediated by ligand-gated ion channels.

• Fast excitatory NTs> Ach and glutamate {both activate channels equally permeable to Na+ and K+}

Question 4

• Excitatory synapse

Answer 4

• cell depolarises
• “excitatory post-synaptic potential” (EPSP) -
• initiates AP
• acetyl choline

Question 5

• Inhibitory synapse

Answer 5

• inhibitory post-synaptic potential” (IPSP)
• cell hyperpolarises
• either it prevents cell from reaching AP threshold or stops AP from propagating
  gaba
• if cell goes negative on inside, threshold remains steady so resting membrane potential decreases

Question 6

Synaptic transmission: chemical synapses

Answer 6

• One transmitter can act on different receptor/channel combos
• Receptor/channel combinations determine transmitter effects
  AP opens voltage- gated Ca++ channels
  • Ca++enters & triggers exocytosis of transmitter from vesicles
  • Transmitter inactivated by one or more of these:
• enzymes in synapse - reuptake by releasing neuron
• diffusion from synapse

Question 7

Synaptic transmission: chemical synapses
ACETYLCHOLINE (ACh)
Nicotinic receptor

Answer 7

“Nicotinic” receptor

• e.g. skeletal muscle cells - directly gated channel
• opened by ACh binding
• permeable to Na+ and K+ - main effect is Na+ influx - fast EPSP results

Question 8

Synaptic transmission: chemical synapses
ACETYLCHOLINE (ACh)
“Muscarinic” receptor

Answer 8

e. g. smooth muscle cells
- indirectly gated channel (causes slow contraction by decreasing permeability to potassium )
- closed by ACh binding
- permeable decreased to K+
- slow EPSP results

Question 9

Synaptic transmission: chemical synapses

Type #1: directly-gated ion channels

Answer 9

Receptor is part of the ion channel

Allows fast post-synaptic potentials (10-100 ms duration)

Question 10

Type #2: indirectly-gated ion channels

Answer 10

receptor separate from ion channel
Allows slow post-synaptic potentials (100 ms - 2 s dura
* Change in intracellular second messenger

Question 11

NEUROTRANSMITTERS

ADRENALINE

Answer 11

some CNS neurons

• all autonomic pre-ganglionic neurons
• all parasympathetic post-ganglionic neurons

Question 12

NEUROTRANSMITTERS

Noradrenaline:

Answer 12

• some CNS neurons

- all sympathetic post-ganglionic neurons - adrenal medulla secretory cells

Question 13

NEUROTRANSMITTERS

Adrenaline

Answer 13

• some CNS neurons

- adrenal medulla secretory cells

Question 14

NEUROTRANSMITTERS

Serotonin

Answer 14

• some CNS neurons

Question 15

NEUROTRANSMITTERS

Gamma-amino-butyric acid (GABA):

Answer 15

• many CNS neurons (generally inhibitory)

Question 16

NEUROTRANSMITTERS

Glutamate

Answer 16

• many CNS neurons (generally EXITATORY)