Synaptic Transmission Flashcards
anterograde and retrograde transport
anterograde = out of cell, retrograde = into cell
molecular motors and examples of cargos
The microtubule motor kinesin mediates fast anterograde transport of mitochondria and vesicles from soma to terminals
Dynein mediates fast retrograde transport of degraded vesicular membranes and absorbed toxins/viruses/growth factors from terminal to soma
electrical versus chemical synapses
In some cases, the electrical response in one cell is transmitted to another cell through electrical synapses. Electrical synapses are extremely fast (little or no delay) and most are BIDIRECTIONAL; they allow direct passive flow of electrotonic current between cells via specialized elements called gap junctions. In the nervous system, electrical synapses are present, but much less common than chemical synapses. DOWNSIDE–They lack the necessary directionality (since they are bidirectional) that chemical synapses have & they are not selective.
Electrical synapses provide speed and synchrony, by allowing direct passive flow of electrotonic current through gap junctions. They are common in heart and smooth muscle. Chemical synapses provide directionality, amplification, and plasticity. They are slower because neurotransmitter must cross the synaptic cleft and activate post-synaptic receptors.
ionotropic and metabotropic receptors
Transmitters signal to the postsynaptic target cell via ionotropic or metabotropic receptors.
Ionotropic receptors contain an ion channel as part of their structure, and transmitter binding triggers a rapid response.
Metabotropic receptors are commonly linked to G-proteins that transduce a slower biochemical signal.
Describe the ionic basis for IPSPs and EPSPs, how they alter synaptic transmission
Postsynaptic potentials (PSPs) are produced as follows: conductance changes due to ion channel openings (and sometimes closings) lead to ionic current flow through the channels that, in turn, lead to changes in the membrane potential.
Excitatory PSPs (EPSPs) increase the probability that an action potential will be triggered.
Inhibitory PSPs (IPSPs) decrease the likelihood that an action potential will be triggered.
Examples of excitatory and inhibitory transmitters that bind to ionotropic and metabotropic receptors
Glutamate is the major excitatory transmitter in the brain (CNS). Acetylcholine is also excitatory. They depolarize the membrane.
GABA is the major inhibitory transmitter in the brain. It hyperpolarizes (IPSP) the membrane by opening up Chloride ion channels.
Steps in synaptic transmission
Steps in synaptic transmission:
(1) transmitter molecules are synthesized & packaged in vesicles
(2) an action potential arrives at the presynaptic terminal
(3) depolarization of terminal opens voltage-gated calcium channels (calcium flows down gradient into cell–calcium is higher in ECF)
(4) increased calcium in terminals triggers vesicle fusion to presynaptic membrane
(5) transmitter diffuses across cleft & binds to postsynaptic receptors
(6) a postsynaptic response occurs
(7) transmitter molecules are cleared/inactivated by enzymatic degradation, uptake, or diffusion (if this does not occur, receptors will become desensitized because they become internalized)
Dendrites
Dendrites are tapered processes arising from the cell body that greatly increase the receptive surface. The membranes contain receptors for chemical transmitters, and voltage-gated ion channels that can amplify the graded synaptic signal.
Cell soma
The cell soma surrounds the nucleus and contains the endoplasmic reticulum, Golgi complex, etc.. It performs many “house-keeping” functions, such as protein synthesis, degradation, and processing.It’s membrane also contains receptors that bind chemical transmitters released by afferent neurons.
Axon
The axon is a single thin process arising from the cell body at the axon hillock. It transmits all-or-none action potentials to the terminals after integrating transmitter-mediated bioelectrical changes received in the dendrites and cell body. Many axons are surrounded by a glial-derived myelin sheath which greatly increases the speed of impulse propagation via the process of saltatory conduction. Cytoplasm of axon = axoplasm.
Presynaptic terminal
Presynaptic terminals are specialized structures that convert electrical signals propagated down the axon (action potentials) into chemical signals (neurotransmitter), released from presynaptic vesicles and transmitted to the target cell at the synapse (the point of contact between the pre- and post-synaptic neuron/target cell). Postsynaptic potentials differ from action potentials; they are small graded changes.
Chemical synapses
Transmission has directionality, move from presynaptic to postsynaptic direction. Signals can be amplified.
Neurotransmitter release
The complex process of neurotransmitter release from the presynaptic terminal is called exocytosis. This calcium-dependent process involves the fusion of proteins on the vesicle membrane called V-SNAREs (synaptobrevin & synaptotagmin) with proteins on the presynaptic target membrane called T-SNAREs (SNAP-25 & syntaxin). This promotes vesicle fusion and release of transmitter into the synaptic cleft. The transmitter then transduces its signal by binding to specific receptors in the postsynaptic target cell. Vesicle membrane that has fused to the presynaptic membrane is recycled through another complex process called endocytosis.
Cable theory
In simple terms, dendrites are long and their membranes are thin and “leaky” to electric current. Thus, before EPSPs can reach the cell soma, a large amount of the potential is lost by leakage through the membrane, a process called decremental conduction. To overcome this & reach the axon hillock & spike an AP, you need to send multiple signals.
Temporal summation
Temporal summation occurs when EPSPs from the same cell arrive in rapid succession.
