Chapter 13 Synapses Flashcards
What is a synapse?
- Specialized site of contact between a neuron and another neuron or effector.
- cells that respond to neural stimulation
What is a synapse’s function?
- Synaptic transmission: The rapid and transient transmission of a signal across the synaptic cleft. (communication)
- Synaptic plasticity: The ability to change the functional properties of synapses. (malleable;ability to change)
The types of synpases are?
- electrical- direct
- chemical- a space has to be crossed (synaptic cleft)
Electrical Synapse
- These are less common (vertebrate retina, some sensory receptors)
- The neurons are connected by a specialized protein structure called a gap junction which allows direct flow of cytoplasm (including ions) between cells
- bidirectional
Electrical Synapse: gap junction
Stimulus only goes in one direction…? idk
a —> b yes
b —> a no
Chemical Synapse
- not directly connected and no sharing of cytosol
- synaptic vesicles hold neurotransmitters
Chemical Synapse: Neurotransmitter Release
Chemical Synapse: Neurotransmitter Release contin
5 criteria to be a neurotransmitters
- Must be in presynaptic terminal
- Including all components for synthesis
- Must be released when stimulated
- In sufficient amounts for a response
- Addition to extracellular fluid produces a response similar to neural stimulation
- A removal mechanism should exist
- The effects of drugs must be similar to and consistent with their effect on experimental application of the candidate neurotransmitters
Neurotransmitter Classification system 1
- Ionotropic: fast, direct
- Metabotropic: slow, indirect
Neurotransmitter Classification system 2
- Small molecule
- Neuroactive peptides
Neurotransmitter Classification system 3
- Most CNS synapses use amino acid NTs
- Fast EPSPs from glutamate
- Fast ISPs: GABA or glycine
- Biogenic amines
- Ach, NE, DA, 5-HT
- Peptides
- Co-released with other NTs
Signal reception
provides a means for the cell to detect a signal
Signal transduction
a mechanism of modifying intracellular activity in response to a signal
Four classes of receptors
- Ligand-gated channels
- G protein-coupled receptors
- Enzyme/enzyme-linked receptors
- Intracellular receptors
*Evolution has caused multiple forms and alleles of the receptors
Cell Signaling: Ligand-Gated Channels
- A membrane-bound protein
- Serves as a receptor and channel
- Primarily used for neurotransmission
Ionotropic Chemical Synapses and the Neuromuscular Junction
Ionotropic Chemical Synapses and the Neuromuscular Junction
Cell Signaling: G protein-coupled Receptors
- A membrane-bound protein
- Serves as a receptor and activates a G protein
- No chemical passes through the membrane
- Used for hormones, neurotransmitters and sensory neurons
Cell Signaling: Enzyme/Enzyme-linked Receptors
- A membrane-bound protein
- Are either enzymes, or are directly linked to enzymes
- No chemical passes through the membrane
- Used for hormones such as ANP
- Increases intracellular cGMP
Cell Signaling: Intracellular Receptors
- Not membrane-bound
- Bind to hydrophobic molecules that can cross the cell membrane
- Steroids, thyroid hormones, retinoic acid, vitamin D, nitric oxide
- Interact with DNA
Endplate Potentials
- If you measure the electrical potential at a large distance from the motor nerve, you can only see an action potential in the muscle.
- However, when measured near the motor neuron you can see that a small depolarization precedes the action potential. This is called the endplate potential.
- The endplate potential is propagated electronically as its amplitude decreases as the distance from the motor neuron increases.
Postsynaptic Inhibition and Excitation
- Excitatory postsynaptic potentials (EPSCs) increase the probability of AP firing while inhibitory postsynaptic currents decrease the probability of AP firing.
- Excitatory currents are usually carried by Na or Ca, while inhibitory currents are carried by K or Cl.
Termination of Neurotransmitter Transmission
Synaptic desensitization: the postsynaptic membrane fails to respond to the neurotransmitter, even if it is present at high concentrations.
Synaptic integration
Multiple neurons interact with each other
Synaptic integration continued
- Each neuron receives input from several other neurons (dendrites).
- The integrative area is the cell body (soma).
- At the axon hillock, there is a “spike initiation zone.”
- The spike initiation zone has the lowest threshold potential in the whole neuron; it also has a high density of voltage-gated Na+ channels.
Spatial Summation
Here we have three axons from three different neurons (a,b,c) converging upon another neuron from which we record a voltage tracing.
When postsynaptic potentials are produced from more than one axon simultaneously, their amplitudes are additive (#3).
This is called “spatial summation”
Spatial Summation
Here a neuron receives both an excitatory input (producing an EPSP) and an inhibitory input (producing an IPSP).
If these occur at the same time, they will also summate (i.e. they “cancel” each other out if they are of the same amplitude).
Temporal summation
When the same input neuron fires repeatedly, its postsynaptic potentials also summate since the PSP lasts longer than the actual postsynaptic current.
Synaptic plasticity
- Neuronal plasticity is the modification of neuronal function as a result of experience.
- Much of this plasticity is thought to reside in synaptic function.
- Any learned behavior is dependent upon this process.
- Synaptic plasticity can occur presynaptically or postsynaptically.
- It can be homosynaptic (i.e. at a single terminal) or heterosynaptic (changes induced by modulation from a nearby terminal).
- It can also be short-term or long-term.
- (memory)
Synaptic Facilitation
- An increase in amplitude of PSPs in response to successive presynaptic impulses
- The amount of NT is altered
- A decrease is synaptic antifacilitation
The response of the 2nd AP is greater than the sum of the 1st and 2nd
In this example, a frog neuromuscular junction is stimulated twice in rapid succession
As expected, we see some temporal summation
However, the 2nd postsynaptic potential is even larger than one would expect from temporal summation alone
This is an example of short-term, homosynaptic facilitation, which is thought to be related to an increased availability of free Ca+ at the presynaptic terminal for neurotransmitter release
Habituation
- Decrease in the intensity of a reflex response when the stimulus is presented repeatedly
- learning to ignore
- ex of learning
- aplysia
Sensitization
- Prolonged enhancement of a reflex response to a stimulus, which results from the presentation of a second stimulus that is novel or noxious
- learning to pay attention again
- ex of learning
- aplysia
Habituation & Sensitization in Aplysia
- Mechanical stimulation of the siphon or mantle shelf results in gill withdrawal
- After repeated stimulation, the response decreases (habituation)
- After a shock to the head, the response is large (sensitization)
Habituation & Sensitization in Aplysia Continued
- EPSPs closely match the time-course of gill withdrawal
- Habituation results from a decrease in NT quanta
- Antifacilitation
- Sensitization uses presynaptic facilitation
Presynaptic Inhibition
Not only can the postsynaptic potential be inhibitory, but presynaptic inhibition is also possible.
In this case, the axonal terminal of an inhibitory neuron synapses with another neuron’s axonal terminal (in this case a motor neuron—just proximal to the neuromuscular junction).
(a tracing of an EPSP, a tracing of an IPSP)
an inhibitory stimulus after the EPSP has no effect
the same inhibitory stimulus occurring before the EPSP (as in presynaptic inhibition) nullifies the EPSP
*Spatial and Temporal Summation are POSTSYNAPTIC events
Long-term Potentiation
- Long-lasting changes have been studied in the CA1 region of the hippocampus
- LTP is a long lasting enhancement of synaptic transmission following intense stimulation
- Any synapse active while the postsynaptic cell is strongly depolarized will be potentiated, for a period that can last up to weeks in an intact animal
