L9-Synapses Flashcards
4 types of synapses and their definitions:
- Axon terminal:
- presynaptic terminals at the axon terminal (usually on a motor neuron) - Axon varicosities:
- presynaptic terminals occur along UNMYELINATED axons in specialized compartments. - En passant synapse:
- One neuron joins another but not at the axon terminal (ex: cell body) - Spine synapse:
- Bulge in dendrite that receives an input from a single axon.
axosomatic synapse
axon to cell body
axoaxonic synapse
axon to axon hillock
axodendritic synapse
axon to dendrite
dendrodendritic synapse
dendrite to dendrite
2 mechanisms of synapses
electrical (direct) and chemical (indirect)
Why are AP waveforms transmitted across electrical synapses unperturbed as if the synapse were not present?
Because the cytoplasm of the presynaptic neuron is in DIRECT CONTACT with the postsynaptic neuron via GAP JUNCTIONS.
The resistance of the gap junction membrane is low, permitting nearly full-strength signal to be induced in the postsynaptic cell.
Which is faster? Chemical or electrical synapse?
electrical. Therefore more useful for escape responses in nonvertebrates.
Which is more common? Chemical or electrical synapses?
Chemical
Where are gap junctions present in cardiac muscle?
in the intercalated disks
In an electrical synapse, the cytoplasm of the presynaptic and postsynaptic neurons are in direct contact through __________.
In an electrical synapse, the cytoplasm of the presynaptic and postsynaptic neurons are in direct contact through GAP JUNCTIONS.
Where are electrical synapses located?
- cardiac muscle (intercalated disks)
- smooth muscle
- some nerve networks
T/F: in chemical synapses, the pre- and postsynaptic cells make direct contact
FALSE. There is a large synaptic cleft between the two cells. Too far for an action potential to “jump.”
Why are chemical synapses much slower than electrical?
Because chemical synapses rely on diffusion of the neurotransmitter.
Are chemical synapses unidirectional?
Yass queen
The synaptic knob contains _____ ______, which store ___________ synthesized and packaged by the Golgi apparatus of the presynaptic neuron.
The synaptic knob contains SYNAPTIC VESICLES, which store NEUROTRANSMITTERS synthesized and packaged by the Golgi apparatus of the presynaptic neuron.
Mechanism of a chemical synapse:
- when an AP in the presynaptic neuron has been propagated to the axon terminal, the change in potential triggers the opening of voltage-gated Ca2+ channels in the SYNAPTIC KNOB.
- Because [Ca2+] is much higher in the ECF and the inside of the cell is neg, Ca2+ flows INTO the synaptic knob through the opened channels.
- Ca2+ induces the release of neurotransmitter via exocytosis from synaptic vesicles into synaptic cleft.
- Neurotransmitter diffuses across cleft and binds to receptor sites on sybsynaptic membrane.
- binding triggers opening of specific (CHEMICALLY GATED) ion channels in the subsynaptic membrane, changing the permeability of postsynaptic neuron.
Possible results of altered permeability of the postsynaptic membrane following a chemical synapse:
- Excitatory postsynaptic potential (EPSP)
- Inhibitory post-synaptic potential (IPSP)
- No change in postsynaptic potential
ESPS
Excitatory postsynaptic potential:
A postsynaptic change occurring at an EXCITATORY SYNAPSE (which results in the simultaneous movement of a few K+ ions out of the postsynaptic neuron with a large number of Na+ ions entering).
At resting potential, what favors the movement of K+ and Na+ ions in/out of cell?
At resting potential, both the concentration and electrical gradients for Na+ favor its movement INTO the postsynaptic neuron.
On the other hand, only the concentration gradient favors the movement of K+ ions OUT of the postynaptic cell.
Do EPSPs generate action potentials in the postsynaptic neuron?
Not usually. The net movement of positive ions into the cell (high [Na+] into cell and low [K+] out of cell) causes a SMALL DEPOLARIZATION of the postsynaptic neuron.
This depolarization does bring the cell closer to threshold. Aka, the membrane is MORE EXCITABLE (easier to bring to threshold) than when at rest.
In inhibitory synapses, the binding of a neurotransmitter to ts receptor site increases the permeability of the subsynaptic membrane to what ions by altering their respective channel conformations?
either K+ or CL-
The resulting ion movement in an inhibitory synapse brings about a small ________ of the postynaptic neuron.
The resulting ion movement in an inhibitory synapse brings about a small HYPERPOLARIZATION of the postynaptic neuron.
In the case of increased Pk+, what happens in an inhibitory synapse?
more positive charges leave the cell via K+ efflux, leaving more neg charges behind in cell.
In the case of increased Pcl-, what happens in an inhibitory synapse?
Since [Cl-] is higher OUTSIDE CELL, Cl- travels INTO cell along its concentration gradient, then are driven out by the opposing electrical gradient established by the resting membrane potential.
Small hyperpolarization moves the membrane potential further away from threshold.
What is the small hyperpolarization of the postsynaptic cell called?
IPSP: inhibitory postsynaptic potential.
Response of postsynaptic membrane is dependent on what?
dependent on receptor type, not neurotransmitter.
Termination of response can occur 3 ways:
- Destruction
- Reuptake by presynaptic neuron
- Diffusion away from the synapse
The response to a given neurotransmitter-receptor combination is always ________
constant
Synapses that lead to response mediated by a 2nd messenger are called ____ ____
slow synapses (because responses take longer and last longer)
APs are conducted from the ____ ____ in the ____ to the ____ ____ through large, myelinated MOTOR NEURONS (neuromuscular synapses)
APs are conducted from the CELL BODY in the CNS to the SKELETAL MUSCLE through large, myelinated MOTOR NEURONS. (neuromuscular synapses)
What is the synaptic junction called in neuromuscular synapses?
neuromuscular junction
T/F: an individual muscle cell can have several neuromuscular junctions.
FALSE.
As the axon approaches a muscle, it divides into several terminal branches. Each branch innervates ONLY ONE MUSCLE CELL. Therefore, each muscle cell has only one neuromuscular junction.
A single muscle cell is called a ____ ____
muscle fibre
Neuromuscular chemical messenger?
Acetylcholine
motor neuron axon terminal is called a ___ ____
terminal button
What does the binding of ACh from a motor neuron to the postsynaptic cell cause?
The binding brings about the opening of NONSPECIFIC CATION CHANNELS. These allow a large concentration of Na+ to enter the muscle cell compared to a smaller movement of K+ out.
This results in an end-plate potential. Local current flow occurs between the depolarized end plate and the adjacent membrane. This causes voltage-gated Na+ channels to open. With the subsequently reduced potential to threshold, an AP is initiated, which is propagated throughout the muscle fiber.
ACh is destroyed by _________ following muscle response.
ACh is destroyed by ACETYLCHOLINESTERASE following muscle response.
2 types of inhibitory synapses:
- postsynaptic inhibition
2. presynaptic inhibition
What is the motor end plate?
specialized portion of the muscle cell membrane IMMEDIATELY UNDER THE TERMINAL BUTTON.
Postsynaptic inhibition
- inhibition of POSTSYNAPTIC CELL by PRESYNAPTIC NEURON.
- Action of neurotransmitter drives postsynaptic membrane FARTHER from threshold.
Presynaptic inhibition
- inhibition of PRESYNAPTIC CELL.
- Inhibition of stimulatory neurotransmitter release from presynaptic membrane
What is the mechanism by which inhibitory synapses work? (presynaptic inhibition)
- An excitatory terminal (A) ending on a postsynaptic cell (C) is itself innervated by an inhibitory terminal (B).
- Stimulation of terminal A alone will cause an EPSP in cell C, but SIMULTANEOUS STIMULATION OF TERMINAL B PREVENTS THE RELEASE OF NEUROTRANSMITTERS FROM TERMINAL A.
- Consequently, no EPSP is produced in C, despite terminal A being stimulated.
This mechanisms allows for the inhibition of terminal A without suppressing any other excitatory input to cell C.
Presynaptic facilitation
An innervated presynaptic cell is further activated by an EPSP and is able to thus release MORE neurotransmitters into the synaptic cleft
Temporal summation
summation of multiple graded postsynaptic potentials from the same location.
Spatial summation
summation of graded postsynaptic potentials from more than one location on the cell.
Convergence of input
one cell is influenced by many others
Divergence of output
one cell influences many others
What is substance P an example of?
A neuropeptide
