Two Flashcards
(16 cards)
What are the two types of synapses?
Electrical and Chemical
What are electrical synapses also called? What do they consist of? How do they work? What is the postsynaptic response like? How quickly do they move? Which direction do they go? What do electrical synapses produce?
Gap junctions.
The membranes are very close together and contain connexons which consist of six subunits called connexins. Connexons can be open or closed. Open, they allow through most small molecules and ions.
The postsynaptic response is the same polarity but smaller
Fast (
What are chemical synapses? How do they differ from electrical synapses? What are the 3 surfaces involved?
They are highly specialized contact zones between neurons and effectors
They contain a wide cleft (20-40nm)
It is slower (0.5 ms delay) and unidirectional
The postsynaptic response can differ in time and polarity from the presynaptic response
They have signal amplification
Presynaptic surface, synaptic cleft, postsynaptic surface
What does the presynaptic element consist of? How do these things look morphologically? What do flattened and round vesicles contain?
They contain synaptic vesicles that contain NTs. These can appear empty or have an electron dense core. They can be flat (inhibitory) or round (excitatory)
Presynaptic density: Dense material located near PM where vesicles dock (active zones)
Mitochondria-Synaptic activity requires energy
What does the postsynaptic element consist of?
Postsynaptic density-Various molecules that anchor NT receptors
what does the synaptic cleft consist of?
EC fluid
What are 4 ways that chemical synapses are classified? Give and explain examples.
Location on the receptive neuron: Axodendritic, axosomatic, axoaxonic
Function: EPSP, IPSP
NT released
Ultrastructure: Type I (asymmetrical, excitatory): wide cleft, large active zone, round vesicles, extensive postsynaptic density, and dense material in cleft. Type II (Symettrical, inhibitory): small syn. cleft, smaller active zone, flattened vesicles, equal densities, lack dense material in cleft.
What are the two general steps of chemical syn. transmission?
Transmitting step and Receptive step
What are 7 transmitting steps?
The transmitting step consists of the following steps: 1) synaptic vesicles are created and filled with neurotransmitter; 2) an action potential invades the presynaptic terminal; 3) depolarization of axon terminal opens voltage-gated channels 4) Ca2+ rushes into the terminal. The increased intracellular Ca2+ triggers fusion of vesicles with the cell membrane. This process involves cytoplasmic proteins that hold each vesicle, moves them to the active zone, and docks them with the plasma membrane; 5) release of neurotransmitter into synaptic cleft. Neurotransmitters are released in discrete packets or quanta. A fusion pore forms, allowing escape of neurotransmitter via exocytosis; 6) neurotransmitter molecules diffuse across the synaptic cleft, binds to postsynaptic receptors causing activation of ion channels in postsynaptic membrane; 7) neurotransmitters are inactivated by degradation of neurotransmitters by enzymes located in the synaptic cleft or reuptake of neurotransmitter into the presynaptic neuron or
diffusion of neurotransmitters into extracellular environment and taken up by glia.
Compare and contrast the direct and indirect method of the receptive step?
Direct method – ionotropic /ligand-gated receptors—>Fast signal transduction. Usually one transmitter molecule opens a pore to many ions.
Indirect method – metabotropic receptors—>SLOW signal transduction. Usually, one transmitter molecules activates many g-protein molecules which activates many effector enzymes which catalyze many 2nd messenger molecules which open pores to many ions.
What are 2 types post-synaptic potential? How does each work?
1) the excitatory postsynaptic potentials (EPSPs) are produced by the binding of neurotransmitter molecules to receptors that result in the opening of Na+ and K+ channels allowing Na+ to rush into cell and K+ to rush out producingdepolarization. These occur at Type I synapses.
2) the inhibitory postsynaptic potentials (IPSPs) are caused by localized increase in membrane permeability to Cl- or to K+ leading to Cl- inflow or K+ outflow producinghyperpolarization. These occur at Type II synapses.
What are 5 mechanisms of agonists?
Agonist actions include those that
1) serve as a neurotransmitter precursor,
2) stimulate the release of a neurotransmitter,
3) stimulate postsynaptic receptors,
4) block presynaptic reuptake or
5) inactivate enzymes that breakdown neurotransmitters following their release.
What are 5 mechanisms of antagonists?
1) prevent storage of neurotransmitter in synaptic vesicles,
2) inhibit the release of neurotransmitter,
3) block postsynaptic receptors,
4) inactivate synthetic enzymes,
5) stimulates presynaptic reuptake.
What must happen for an action potential to occur? Where does it begin? What are the two types of summation?
Within a single neuron the synaptic current propagates passively as a graded potential. If the threshold potential is met an action potential will be initiated at the axon hillock. The integration of graded potentials can be additive or subtractive, referred to as summation.
There are two types of synaptic summation. In spatial summation, two excitatory inputs arrive at a postsynaptic neuron simultaneously. Together, they produce a greater depolarization. Same applies for inhibitory inputs. In temporal summation, two
excitatory inputs arrive at a postsynaptic neuron in rapid succession. The resulting postsynaptic depolarizations overlap in time and are added in a stepwise fashion. Same applies for inhibitory inputs.
What are convergence and divergence? How does this differ at the neuromuscular junction?
In the CNS many axon terminals synapse onto one neuron. The postsynaptic cell then integrates the entire input into one output signal, the action potential. Many presynaptic neurons converge on any single postsynaptic neuron, referred to as convergence. The average neuron receives 10,000 synapses.
Conversely, axons of most presynaptic neurons divide into many branches that diverge to end on many postsynaptic neurons, referred to as divergence. The average neuron sends out 1000 synapses.
At NM junction, one axon terminal makes contact onto one muscle fiber. One AP in the axon terminal will almost always elicit one AP in the muscle.
What is the neuromuscular junction? What does it consist of? What is another name for it? Where are ACh receptors located?
Specialized synaptic connection in the PNS
Motor end plate
The neuromuscular junction consists of an axon terminal (of a motor neuron) and a muscle cell. A typical neuromuscular junction contains:
1) a presynaptic element (an axon terminal) which contains numerous mitochondria and synaptic vesicles that contain the neurotransmitter acetylcholine (Ach),
2) a synaptic cleft which contains a basal lamina and acetylcholinesterase (AChE), the enzyme that breaks down acetylcholine, and
3) a postsynaptic element (muscle cell) whose postsynaptic membrane is increased by foldings of the membrane adjacent to the nerve terminal enabling the neuromuscular junction to fully utilize the ACh released. Ach receptors are present in small quantities over most of the muscle membrane surface but are concentrated heavily at the tips of the each fold.
