Neuroanatomy Lecture #17: Synaptic Transmission

Front 1

The two types of synapses btwn neurons

Back 1

Chemical and Electrical

Front 2

The Electrical synapse:
• Type of channel it contains
• The composition of the channel
• fxn of the channel
• an example

Back 2

The Electrical synapse:
• Gap Junction
• Two hemichannels, called Connexons, each having 6 subunits of 4 membrane spanning regions (connexins)
• Fxns to help synchronize the activity of neighboring cells

ex. astrocytes and spatial bufering

Front 3

Name & define 5 types of chemical synapses and how they fxn (excitatory/inhibitory)

Back 3

• Axomatic - a postsynaptic membrane is on the cell body of another neuron; Inhibitory

• Axodendritic - a postsynaptic membrane is on the dendrite of another neuron; Excitatory

• Axoaxonic - the postsynaptic membrane is on the axon of another neuron; Inhibitory

• Symmetric - The pre- and post- synaptic membrane are similar in thickness; Inhibitory

• Asymmetrical - The Post-synaptic membrane is thicker than the pre-synaptic membrane; Excitatory

Front 4

Two types of receptors at the at chemical synapses, how they mediate changes in the post-synaptic cell after ligand binding and examples of each

Back 4

• Ionotropic receptors
- after a ligand binds, they undergo a conformational change to either open/close, thus changing post-synaptic membrane potential

ex. Nicotinic AchR, GABA and NMDA

• Metabotropic receptors
- After a ligand binds, they modify the post-synaptic cell through a G-protein mediated cascade

ex. Muscarinic AChR and NorE receptors

Front 5

There are 3 modes of rapid termination of the action of neurotransmitters at the synaptic cleft. Name them and the types of neurotransmitters predominantly using that termination mode.

Back 5

• Diffusion - Neurotransmitters predominantly use this mode though all neurotransmitters can

• Extracellular enzymatic degradation: Predominantly used by acetylcholine

• Uptake into nerve endings or glia - Very important mode used for catecholamines, glutamate, seratonin, GABA and glycine

Front 6

Ionotropic glutamate receptors:
• The transmitter that binds them
• The 3 most important types
• The types of channels they open
• These ions are involved in influx/efflux
• The net result of ion flux

Back 6

Ionotropic glutamate receptors:
• Glutamate
• NMDA, AMPA, kainate
• cation channels
• Influx: Na+; Efflux: K+
• Net Influx of Na+ leads to a small depolarization ---> EPSP

Front 7

Compare the depolarizing and re-polarizing phases of the EPSP in relation to time and what is happening to the receptors

Back 7

The depolarization phase is fast due to:
- the rapid binding of neurotransmitters to the receptors
- and the opening of channels to allow cations to enter

The repolarization phase is slow and rate controlled due to:
- transmitters unbinding from the receptors,
- cation channels closing
- and positive charge re-distributing itself across the membrane

Front 8

What is an amplitude of an EPSP from an ionotropic receptor dependent on?

And what does this tell us about the type of potentials that can be elicited?

Back 8

The amplitude is dependent on how much glutamate is released by exocytosis in response to a presynaptic impulse

This tells us the depolarizations at the post-synaptic membrane are graded with respect to the [glutamate] applied

Front 9

Describe the ionic basis of Glutamate-induced EPSPs

What causes the depolarization? What is on the post-synaptic membrane that leads to a change in the membrane potential? What is the size of the EPSP dependent on?

Back 9

It is the number of cation channels opened in the post-synaptic membrane that determines the value of depolarization for the membrane. Because the channels will only open when bound with Glutamate, a short pre-synaptic impulse will only release a small amount of glutamate into the synaptic cleft to bind to channels.

Front 10

Definition of the reversal potential

Back 10

It is the value at which there is no net flow of an ion at the synapse (no synaptic current).

Front 11

Describing the ionic basis of Glycine & GABA induced IPSPs:

• What is E(anion) value predominantly dependent on?

• What do - & + driving forces describe?

• Regardless of the -/+ sign of the driving force, what does the potential change of the membrane move towards and ultimately what does it tells us about the actions of Glycine and GABA?

Back 11

Describing the ionic basis of Glycine & GABA induced IPSPs:

• It is dependent on the equilibrium potential of chloride, E(Cl)

• - DF: anions (mainly chloride) will leave the cell
+ DF: anions will enter the cell

• The potential change of the membrane move towards E(Cl) and the actions of Glycine and GABA are inhibitory.

Front 12

The initiation zone:
• Its location in multipolar and sensory neurons
• Two points about its excitation threshold and channels

Back 12

The initiation zone:
• Multipolar neurons - located at the axon hillock
Sensory neurons - located at the sensory endings
1) It is the site where the excitation threshold is lowest
2) and the concentration of v-gated Na+ channels is highest/unit area

Front 13

Considering that EPSPs spread with decrement,

these are two means many EPSPs can be combined to attain excitation threshold and generate an AP. What is the main idea of each

Back 13

Spatial summation:
Due to many neurons synapsing on the post-synaptic cell, an additive effect is created, allowing the sum of the additive currents to yield larger EPSPs than separate EPSPs by themselves

Temporal Summation:
Several EPSPs will reach the post-synaptic neuron sequential, in a timeframe where the individual EPSPs of the 1st potentials have yet to relax back to the resting potential. The series of impulses are additive, creating larger EPSPs.

Front 14

What dictates the firing rate of a neuron?

Back 14

The combined effect of the spatial and temporal summation of EPSPs & IPSPs occurring at many sites in its soma and dendrites

Front 15

Two modes of action for Cocaine

Back 15

1) It blocks Na+ channels from inside the cell (local anesthetic)

2) It blocks the transport roteins responsible for the re-uptake of the following neurotransmitters, thereby prolonging their existence in the synaptic cleft
- Dopamine
- NorAdrenaline
- Seratonin (5-OH tryptamine)

Front 16

Morphine:
• Acts as an agonist for this molecule
• Its receptor and location
• Its competing transmitters (2)
• the outcome of it binding to the receptor

Back 16

Morphine:
• μ-enkephalin
• nocioreceptors located on C-fibers in the dorsal horn (substantia gelatinosa)
• Substance P and Glu (They are excitatory)
• It inhibits the relay of pain information to 2nd order neurons in the spinal cord
• Its competing transmitters
• the outcome of it binding to the receptor

Front 17

Psychiatric, neurological and neurodegenerative diseases are associated with these kinds of activities from these receptors

Back 17

Excessive activation or inactivity of ionotropic and metabotropic receptors