A4: Synapses

what are the two classes of synapses?

electrical and chemical

electrical synapse: size

3.5nm

electrical synapse: synaptic continuity?

yes

electrical synapse: ultrastructural element

gap junction

electrical synapse: agent of transmission

ionic current, electrotonic

electrical synapse: synaptic delay?

no delay due to pores

electrical synapse: direction of transfer

- bidirectional

- pre

electrical synapse: post synaptic event

excitatory, EPSP

chemical synapse: size

20-40nm

chemical synapse: synaptic continuity?

no

chemical synapse: ultrastructural element

pre = active release zones, chemicals are in vesicles

post = receptors, chemical gated channels

chemical synapse: agent of transmission

chemical (neurotransmitter)

chemical synapse: synaptic delay?

0.3 - 5 ms

chemical synapse: direction of transfer

- unidirectional

- pre --> post

chemical synapse: post synaptic event

EPSP or IPSP

what does synaptic continuity mean?

complete separation of intracelluar space

basic structural features of electrical synapse

- gap junction

- connexins on both elements form a pore that allows for current (1.5-2nm)

- current can flow in both directions

location of electrical synapses

- usually in invertebrates escape mechanisms

- rare in mammals (found in the heart)

advantages of electrical synapses

- speed of intracellular communication

- synchronization of different nerve cells

- pure diffusion, no energy required, passive process

disadvantages of electrical synapses

- limited modulation = response is only excitatory

- only EPSP

- simple behaviors

what is the predominant form of synaptic transmission?

chemical

basic structure of chemical synapses?

- EPSP or IPSOS determined by type of ion channel

- arrival of AP makes the vesicles release their NTs into the synaptic cleft

direction of ion transfer in chemical synapses?

always pre --> post

what is an EPSP?

- excitatory post synaptic potential

- DEPOLARIZATION on post-synaptic membrane

what is an IPSP?

- inhibitory post-synaptic potential

- HYPERPOLARIZATION of post-synaptic membrane

location of chemical synapses?

1. Axo-somatic
2. Axo-dendritic
3. Axo-axonic

axodendritic chemical synapses tend to produce....

EPSP

axosomatic chemical synapses tend to produce.....

IPSP, block the signal at the soma

characteristics of axo-axonic chemical synapses?

- axon --> axon communication

- no impact on AP bc it has already propagated down the axon

- can increase or decrease amount of NT released

- modulate Ca permeability

characteristics of axo-somatic chemical synapses?

- axon --> soma

- axon = pre-synaptic element

- soma = post-synaptic element, has electrically gated channels

are most chemical synapses axodendritic or axosomatic?

axodendritic = 8,000

advantages of chemical synapses

- better modulation

- complex behaviors

- can have excitation or inhibition

disadvantages of chemical synapses

- requires energy for chemical processes

- synaptic delay

pre-synaptic neuron is responsible for ...

synthesis, storage, release

post-synaptic neuron is responsible for .....

1. binding on the post synaptic neuron

2. activation of the effector component (opening on channels)

definition of presynaptic events:

1) neurochemical has to be synthesized by _______
2) NT has to be present in the _______
3) NT has to be ___________ with activation
4) if applied exogenously, NT has to stimulate ______
5) a specific mechanism has to exist for NT ________

1) the neuron

2) presynaptic element

3) released

4) nerve activation (from an outside source)

5) removal

how are neurotransmitters stored?

in vesicles

an average presynaptic neuron generally forms synapses with how many postsynaptic neurons?

1000

synaptic vesicles protect the NT from what?

enzymatic degredation

one quanta =

2000

are the contents of each vesicle fixed?

- yes, each have one quanta

- important that they have the same amount of NT

what is the dale principle?

a given neuron will release the SAME NT at each terminal site

what is the co-existence principle?

- some synapses can release multiple NT (more common in ANS)

- if a neuron releases 2 NT at one synapse, it releases same 2 NT at EVERY synapse in accordance with dale principle

how many vesicles are released with the arrival of a single AP?

150 - 300

only a small fraction of synaptic vesicles in the synaptic terminal are positioned in _______.

The remaining represent a _________________

1. active release site

2. reserve

the vesicles in the reserve storage pool, are they free to move about?

no, they are anchored to a network of cytoskeletal filaments

steps in NT release

1) arrival of AP to terminal portion

2) AP causes Ca++ channels to open

3) Ca++ enters neuron through diffusion

4) Ca++ is trigger for vesicles to fuse with plasma membrane

5) NT released by exocytosis into synaptic cleft (cell empties contents into the cell)

what is post tetanic potentiation?

- increase in NT release after a high-frequency train of AP

- high frequency discharge from the nerve, after it just released a bunch of NTs

- electric Ca++ permeability INCREASES

____ + ______ -----> _______ ---> ACh

Acetyl CoA + choline ----> choline acetyl-transferase ----> ACh

ACh is broken down by _____ into _____

enzyme acetylcholinesterase

acetate and choline

1. where are acetylcholinesterase and choline acetyltransferase synthesized?

2. how are they transported?

1. neuronal cell body


2. axonal transport to the presynaptic terminal

where does synthesis of ACh occur?

presynaptic terminal

ACh acts a transmitter at sites in the???

PNS and CNS

what NT is responsible for excitatory transmission at the NMJ?

ACh

what types of neurons utilize ACh as their neurotransmitter?

1. autonomic ganglia
2. post ganglionic parasympathetic neurons
3. parasympathetic neurons

GABA stands for?

gamma-aminobutyric acid

GABA and glycine release leads to _____ on post synaptic targets

IPSP

GABA and glycine ar present in large amounts in the?

gray matter or brain and/or spinal cord

what is the mediator responsible for presynaptic inhibition?

GABA

____ ----> _____ ----> _____ ----> GABA

glucose ----> L-glutamate ----> glutamic acid decarboxylase

Glycine is synthesized from?

serine

what is the most abundant and major excitatory NT in brain and spinal cord?

glutamate

______ is a metabolic precursor for GABA

glutamate

glutamate is the principal mediator of ...

cognition
emotions
sensory information
motor coordination

glutamatergic communication requires the right concentrations of ________________ to be released.

Less results in --->
more can be --->

glutamate

poor communication

neurotoxic and can damage neurons and neural networks

how many types of glutamate receptors have been located?

4

glutamate receptors may provide the basis for?

memory

serotonin is involved in regulation of

1. mood
2. appetite
3. sleep

serotonin is made from?

essential amino acid, TRYTOPHAN

tryptophan deficiency can lead to ...

low serotonin levels which are associated with anxiety and depression

only about ____% of body's serotonin is found in the CNS; the rest is found in the _____?______

1-2%

GI system

serotonin containing neurons are present in the ______?_______

raphe nuclei (part of the reticular formation)

serotonin containing neurons along with NE containing neurons appear to play an important role in determining?

level of arousal

what are the 3 catacholamines?

norepinephrine, epinephrine, dopamine

catecholamines are formed by?

the amino acid, TYROSINE

what produces catecholamines?

1. adrenal medulla

2. post-ganglionic fibers of the SNS (fight or flight)

high levels of circulating catecholamines are associated with ___?___

stress

dopamine neurons generally have what effect?

inhibitory

steps in forming cateachoamines

L-tyrosine --> DOPA --> dopamine --> epinephrine --> norepinephrine

once released into the neurosynaptic cleft, the NTs ______________ towards the post synaptic membrane

diffuse --> lead to an EPSP or IPSP

what determines the sign of a synaptic potential

- TYPE of ion CHANNEL gated by the transmitter in the post synaptic cleft

- NOT by the type of transmitter released from the pre-synaptic neuron

2 major functions of synaptic receptors

1. recognizing of specific transmitters

2. activation of effectors

basic features of chemical transmitter?

1. receptor binding

2. effector function

what are the receptor types?

1. directly gated (ionotropic or transmitter gated)

2. indirectly gated (metabotropic or G-protein coupled receptors)

functions of the directly gated (ionotropic or transmitter gated) receptors?

1. receptor binding and effector function are part of the SAME macromolecule

2. rapid!!! synaptic delay

if a synaptic delay is < .5 ms, you can assume it is what type of receptor?

directly gated (ionotropic or transmitter gated)

functions of the indirectly gated (metabotropic or G-protein coupled) receptors?

1. receptor binding and effector function are 2 SEPARATE molecules

2. slow! synaptic delay > .5 ms

3. leads to a series of chemical processes inside the cell

if a synaptic delay is > . 5ms, you can assume it is what type of receptor?

indirectly gated (metabotropic or G-protein coupled)

direct ACh receptors:

type =
effect =
action =
where? =

1. nicotinic ACh (nAch R)

2. open Na+ and K+ channels

3. EPSP

4. NMJ

indirect ACh receptors:

type =
effect =
action =
where? =

1. muscarinic ACh (mAch R)

2. open of close K+ channels

3. EPSP or EPSP

4. autonomic (PNS)

direct glutamate receptors:

effect =
action =

1. open Na+ channels

2. EPSP

indirect glutamate receptors:

effect =
action =

1. ?

2. EPSP

indirect dopamine receptors:

effect =
action =
where? =

indirect
1. opens K+
2. IPSP

OR

indirect
1. close Ca++
2. inhibition
3, axoaxonic synapses

direct GABA receptors:

type =
effect =
action =

1. GABA a R

2. Open Cl- channels

3. IPSP

indirect GABA receptors:

type =
effect =
action =

1. GABA b R

2. close Ca++

3. inhibition

once released, how do you turn off the action of the neurotransmitter?

1. diffusion
2. enzymatic degredation
3. active reuptake

NT removal: diffusion

small fraction of NT will just diffuse away

NT removal: enzymatic degredation

- enzyme breaks it down

- primary method for getting rid of ACh

- ex: ACh broken down by acetylcholinesterase to choline and acetate

NT removal: active reuptake

- predominant method, except for ACh

- NT actively reabsorbed by presynaptic element (required energy)

what NT are removed by active reuptake?

GABA
glycine
glutamate
seratonin