Unit 4 Flashcards
Where do synapses form? (CNS and muscle)
CNS: excitatory glutamatergic synapses form on the dendritic spines; inhibitory GABAnergic synapses form on the cell soma
Muscle: they form in the middle of the myofibers
The constituents that make up a synapse are already ____.
are already present in cell waiting to coalesce in the right location.
What does dying the neurotransmitters in a growth cone reveal? And how was this done?
The dye reveals that neurotransmitters are released from the growth cone upon contact with receptor neurons.»_space; Vesicular release must occur before synapse formation.
The neuron was depolarized to generate vesicles which took up dye from the environment during polarization. When the neuron was repolarized by signal transmission, the dye release would be observable.
What does the myoball assay show?
That contact strength increases over time between a neuron and its target cell.
What are “puncta adherentia”?
adhesion junctions; the zone of adhesion between the pre- and postsynaptic cell
Cell adhesion is dependent upon which molecules? ___ and ___.
cadherins and nectins; together, they allow for tight binding at the adhesion junctions.
What does the experiment involving synaptophysin and PSD95 show? (And what are PSD95 and synaptophysin?)
PSD95: a marker for the postsynaptic density
synaptophysin: a marker for presynaptic transport packets
The experiment showed that presynaptic transport packets are recruited to the synapse due to communication between pre and postsynaptic signals. The synapse is stabilized due to the interactions between the postsynaptic density and the presynaptic proteins.
What are neurexin and neuroligin? and what does their interaction lead to?
Neurexin is a presynaptic receptor and neuroligin is a postsynaptic receptor.
Binding of NRXN to NLGN induces presynaptic differentiation.
What does clustering before innervation reveal about postsynaptic differentiation in the muscle? How was this shown?
Acetylcholine receptors (AchR) are present in the post-synaptic cell. Bungarotoxin is used to block these receptors so they cannot communicate. However, PSD clustering still occurs because communication with presynaptic cells isn’t necessary for clustering to occur in the postsynaptic region.
Cell-to-cell contact is important for organizing the synapse, NOT ligand-receptor interactions.
What is bungarotoxin?
Bungarotoxin is a molecule that binds to AchRs and blocks their activation.
Explain the mechanism of postsynaptic AchR clustering determined using Xenopus muscle.
Normally, motor neurons are connected through the basal lamina to target muscle cells with AchRs. If the nerves are cut, the AchRs still cluster to the same location in the basal lamina.
This indicates that there is some extracellular matrix component responsible for clustering near the postsynaptic receptors.
What is the basal lamina?
the protein-rich region in the ECM between neurons and their target cells
What is the component that encourages AchR clustering in the basal lamina?
Agrin
How does Agrin work?
Agrin is a ligand found in the ECM. It binds a co-receptor Lrp4 to MuSK, a receptor in the membrane of the postsynaptic target cell. MuSK provides the downstream scaffold for AchR clustering.
Ach ___ the formation of its receptor when agrin isn’t present.
inhibits
aggregation of AchRs only occurs where agrin is present. this prevents the acetylcholine signal from spreading too far.
___ play a role in CNS receptor clustering
Ephrins
Glutamate receptor clustering depends upon
Ephrin-Eph signaling
Four things that occur after synaptogenesis starts:
Recruitment of receptors to synapse increase
Transcription of new receptors
Recycling time of receptors decreases
Translation of receptors in the dendrites increases
What influences synapse stabilization?
Experience
Two ways to refine synaptic connections:
addition or elimination of synapses during development
modification of synaptic strength - magnitude of postsynaptic response (eg. neurons firing more quickly)
The number of convergent inputs on a target cell ___ through development
decreases
During critical periods, synapses are particularly ___
vulnerable to change and refinement (neural plasticity)
Result of Hubel and Weisel cat experiment - one eyelid closed during development
synapses of the unused eye weakened
greater innervation in the cortex from the eye that was used»_space; monocular cortex neurons
Result and conclusion of Hubel & Weisel - both eyes closed
Normal ocular dominance restored»_space; equal input from both eyes»_space; binocular cortex neurons
Conclusion: Competition Hypothesis - total activity doesn’t matter, only competition between sensory organs
Result of Hubel and Weisel Artificial Strabismus Experiment
Increased competition between the eyes led to increased competition in cortex»_space; more monocular neurons in cortex layer
Fixed environments lead to a ___ of cortical neurons ___
a lack of cortical neurons responding to the deprived stimulus
eg. cat raised with horizontal stripes doesn’t react to vertical movement
Spontaneous retinal activity is ___ for cortical stripe formation
necessary
Coding properties
a neuron’s response to a specific stimuli
eg. a particular Hz tone, line direction, etc
General pathway of signal from sensory system
sensory organ -> thalamus -> cortex
[eg. cochlea > brain stem > inferior colliculus > thalamus > auditory cortex]
Frequency tuning curve experiment
animals were reared in repetitive sound environments. This caused the auditory system to lack the ability to differentiate between different noises and frequencies.
The neuromuscular junction (NMJ): muscles are __ innervated, but they become ___ innervated through development
polyneuronally innervated then they become mononeuronally innervated.
Heterosynaptic depression
an active axon terminal encourages the decrease in receptors of nearby terminals
Synapse refinement in the NMJ
competition between neurons (increased asynchronous firing) leads to synaptic loss.
if both nerves stimulate synchronously, there is no competition and no loss of synapses.
Is synaptic transmission necessary for synapse formation?
No.
General method of synapse formation
make more than needed, then prune
Three classifications of early behavior
Anticipatory, Adaptive, Substrative
Meaning and examples of anticipatory behavior
actions that will be of use later in life
eg. human infants’ visual system is firing well before they are ever exposed to light
Meaning and examples of adaptive behavior
serve specific functions at particular stages of life
eg. chicks using their beaks to push on their eggshells; human baby rooting reflex (instinctively turning towards food source)
Meaning and examples of Substrative Behavior
simple behavior that more complex behaviors can be built upon later
eg. side-to-side movements in fish preparing for swimming; (debatable: crawling > walking)
How do hox genes play a role in nerve circuits?
hox gene location on the spinal cord determines how the muscles in that region fire
eg. hox gene in the wing segment leads to synchronous firing of muscle nerves; hox genes in the leg region lead to alternating firing of muscle nerves.
if the hox genes are swapped, the muscle firing patterns also swap. (done in chicks)
Two events defining sensory input maturation
- absolute sensitivity
- discrimination
What is absolute sensitivity?
the measure of the minimum stimulus level that can be detected. this occurs before discrimination
What is discrimination (sensory input)?
the ability to perceive differences between similar stimuli
Both absolute sensitivity and discrimination ___ during development
increase
(sensitivity increases meaning the minimum stimulus decreases)
Absolute sensitivity in the auditory system reaches adult-like levels by ___ years old
2 years old
Maturation of sound perception (discrimination) takes at least ___ years.
at least 11 years
Visual acuity doesn’t mature until ___ years old
3 years old
