Chapter 5 - Synaptic Communication Flashcards
electrical current (ions) flowing from one cell to another - no NT release
electrical synapses
An organized collections of protein channels in cell membranes that allows ions and small molecules to pass between adjacent cells. The protein channels that make up _______ consist of two connexons.
One connexon resides in the membrane of one cell. It aligns and joins the connexon of the neighboring cell, forming a continuous aqueous pathway by which ions and small molecules can freely pass (passively) from one cell to the other.
Each connexon consist of six subunits called connexins.
Connexon - the channel, the functional unit
Connexin protein - subunits making up the channels (6 sub units)
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Neurites of two cells connected by a gap junction. An enlargement showing gap junction channels, which bridge the cytoplasm of the two cells. Ions and small molecules cal pass in both directions through these channels. Six sonnecin subunits comprise one connexon, two connexons comprise one gap junction channel, and many gap junction channels comprise one gap junction
gap junction
the most numerous and diverse neuroglial cells in the CNS
astrocytes
Why electrical synapses?
1) speed (fast)
2) high threshold to fire (Ohms law) but, they fire synchronously
3) metabolic signals: components that can pass through non-selective channels.
chemical synapses
can mediate excititory or inhibitory signalling
more complex - plasticity
amplifies signals
axon to dendrite
axondendritic
axon to cell body
axosomatic
axon to axon
axoaxonic
dendrite to dendrite
dendrodendritic
asymmetrical synapses, excititory
gray’s type I
symmetrical synapses, inhibioty
Gray’s Type II
is a synapse between a motor neuron and skeletal muscle. (outside the CNS)
neuromuscular junction
What are the seven basic steps of neurochemical release?
1) NT synthesis
2) load NT into synaptic vesicles
3) vesicles fuse to presynatpic terminal (membrane)
4. NT release into synaptic cleft
5) binds to postsynaptic receptors
6) biochemical/electrical response elicited in postsynaptic cell
7) removal of NT from synaptic cleft
Classes of Neurotransmitters
1) Amino acids ex: glycine
2) Biogenic Amines ex: DA
3) Peptides ex: Dynorphin
a single neuron always produces the same transmitter at every one of its synapses.
Dale’s law
process by which vesicles release their contents
Exocytosis
types of endocytosis
kiss and run
merge and recycle
bulk endocytosis
release most NT, reseals and moves into cytoplasm to be refilled.
short amount of time.
release most NT
recycled quickly
Kiss and Run (leave)
vesicle fuses completely with the membrane
Merge and Recycle
Large pieces of the membrane fold in to reform vesicles
bulk endocytosis
A receptor protein that forms part of a ligand-gated ion channel, so that binding of ligand (e.g. a hormone or neurotransmitter) to the receptor causes opening of the channel, permitting ions to flow through it.
- Ionotropic receptors are not opened (or closed) all the time. They are generally closed until another small molecule (called a ligand — In our case, a neurotransmitter) binds to the receptor.
As soon as the ligand binds to the receptor, the receptor changes conformation (the protein that makes up the channel changes shape), and as they do so they create a small opening that is big enough for ions to travel through.
Therefore, ionotropic receptors are “ligand-gated transmembrane ion channels”.
Ionotropic Receptor
do not have a “channel” that opens or closes. Instead, they are linked to another small chemical called a “G-protein.”
As soon as a ligand binds the metabotropic receptor, the receptor “activates” the G-Protein (it basically changes the G-Protein). Once activated, the G-protein itself goes on and activates another molecule. This new molecule is called a “secondary messenger.”
Metabotropic (G-coupled) receptors
A neurotransmitter receptor located in the presynaptic terminal of the same neuron that produces the neurotransmitter. Autoreceptors have a higher affinity for the neurotransmitter than does the postsynaptic receptor, and thus have an autoregulatory function.
- Presynatpic receptors that are sensitive to the neurotransmitter released by the presynaptic terminal
Typically G-protein-coupled receptors that stimulate second messenger formation
Common effect is inhibition of NT release and in some cases, NT synthesis. - safety valve to reduce release when [NT] is synaptic cleft gets too high.
Autoreceptor
transient postsynaptic membrane depolarization by presynaptic release of neurotransmitter
Excitatory postsynaptic potential (ESPS)
Transient hyperpolarization of postsynaptic membrane potential caused by presynaptic release of NT
Inhibitory postsynaptic potential (ISPS)
mechanisms of NT removal
1) diffusion
2) enzymatic degradation
3) reuptake
a type of passive transport, therefore, it is a net movement of molecules in and out of the cell across the cell membrane along a concentration gradient.
diffusion
a specific enzyme changes the structure of the neurotransmitter so it is not recognized by the receptor. For example, acetylcholinesterase is the enzyme that breaks acetylcholine into choline and acetate.
enzymatic degradation
the whole neurotransmitter molecule is taken back into the axon terminal that released it. This is a common way the action of norepinephrine, dopamine and serotonin is stopped…these neurotransmitters are removed from the synaptic cleft so they cannot bind to receptors.
reuptake
increase or facilitate activity of the NT
agonist
decrease or inhibit the activity of a NT
antagonist
inhibits of neurotransmitter receptors
receptor antagonists
bind and blocks nicotinic receptors, the ionotropic receptors at the neuromuscular junction
causes paralysis
curare
binds and blocks muscarinic receptors
many of these metabotropic receptors are in the brain
high doses disrupt memory
Atropine
mimic actions of naturally occuring neurotransmitters at receptor
receptor agonist
process by which multiple synaptic potentials combine within one postsynaptic neuron
synaptic integration
elementary units of synaptic release
synaptic vesicles
an indivisible unit
when NT are released, they are released in multiples based on this unit
Quantum
- each quantum thought to = ~ 500 molecules from each vesicle
mini
minature postsynaptic potential
The size of the postsynaptic response to this spontaneously released NT can be measured electrophysiologically. The tiny response is called a ______.
Each ____ is generated by the transmitter contents of one vesicle.
The amplitude of the postsynaptic EPSP evoked by a presynaptic action potential is simply an integer muktiple of the mini amplitude
- the results of stimulation of 2000 nitonic Ach channels
minature postsynaptic potential (mini)
a method of comparing amplitudes of miniature and evoked postsynaptic potentials, can be used to determine number of vesicles that release during neurotransmission
Quantal analysis
is a way of achieving an action potential in a neuron with input from multiple presynaptic cells. It is the algebraic summation of potentials from different areas of input, usually on the dendrites.
spatial summation
Two (or more) distinct synaptic inputs onto the postsynaptic cell
same time
cell is depolarized
Spatial summation: Summation of EPSP
Two (or more) independent inhibitory inputs
postsynaptic cell hyperpolarization
Spatial summation: Summation of IPSPS
EPSP (depolarizing) and IPSP (hyperpolarizing) input
not net change in membrane potential
stronger input wins
Spatial summation: Summation of EPSP and IPSP
single synapse initiating a sequence of membrane events.
Temporal summation
the process of information transfer at a synapse
synaptic transmission
special proteins that span narrow gap between two cells in a gap junction
connexin
six connexins combine to form a challen called a
connexon
two connexons (one from each cell) combine to form a
gap junction channel
because electrical current (in the form of ions) can pass through these channels, cells connected by gap junctions are said to be
electrically coupled
receptors known as ______ are membrane-spanning proteins consisting of four or five subunits that come together to form a pore between them. In the absence of NT, the pore is usually closed. When NT binds to specific sites on extracellular region of the channel, it induces a conformation change - just a slight twist of the subunits - which within microseconds causes the pore to open.
transmitter gated ion channels
a transient postsynaptic membrane depolarization caused by the presynaptic release of neurotransmitter
excititory postsynaptic potential (EPSP)
bring the membrane away from thresholf for generating action potentials
inhibitory
a transient hyperpolarization of the postsynaptic membrane cause by presynatpic release of neurotransmitter
inhibitory postsynaptic potential (IPSP)
G-protein coupled receptors
1) neurotransmitter molecules bind to receptor proteins embedded in the postsynaptic membrane
2) The receptor proteins activate small proteins called G-proteins that are free to move along the intracellular face of the postsynaptic membrane
3) the activated G-proteins activate “effector” proteins
because G-protein-coupled receptors can trigger widespread metabolic effects, they are often refered to as ____
metabotropic repceptors
despite continuous presence of NT, the NT-gated channels close
desensitization
study of nervous system tissue
neuropharmacology
inhibit the normal function of specific proteins involved in synaptic transmission
inhibitors
inhibitors of neurotransmitter receptors
bind to the receptors and block (antagonize) the normal action of the NT
receptor antagonists
mimic the actions of naturally occuring NT
receptor agonistst
indivisible unit that reflects the number of transmitter molecules in a single synaptic vesicle and the number of postsynaptic receptors available at the synapse
quantum
several stimuli control the opening/closing of ion channels
1) ligand gated
2) phosphorylation gated
3) voltage-gated
4) stretch (mechanical)
ionotropic: direct gating
receptor is part of the channel protein
5 sub-units
4 membrane spanning a-helices
- also called receptor channels or ligand gated channels
steps of NT action on GPCRs
- bind receptor proteins
- activate small proteins (g-protein)
- activate “effector” proteins
- directly gate channel
- activate second messenger systems
metabotropic: indirect gating
simplified steps:
- activated GTP-binding proteins
- second messenger cascade
effects: may be to open/close phosphorlyation gated channel - typical receptor has single subunit
- 7 a helices
- binding regions is within the plane (pore) of the membrane
ionotropic
fast actions
relatively transient responses (msec)
found: systems with rapid responses (reflex)
metabotropic
- slower synaptic actions (sec-mins)
alter excitability of neurons and strength of synaptic connections
crucial for learning
