Synaptic Transmissions Flashcards
1
Q
What is a refractory period?
A
- during an impulse, the portion of the axon actively conducting the action potential is not able to respond to another threshold stimulus of normal strength.
- refractory period limits number of action potentials generated per second
2
Q
What is the absolute refractory period?
A
- time when threshold stimulus cannot ternate another action potential
- voltage-gated Na+ channels are briefly unresponsive
3
Q
What is the relative refractory period?
A
- time when only high-intensity stimulus can generate another action potential
- re-polarization is not complete and membrane is re-establishing resting potential
4
Q
What causes the variation in the speed of impulse conduction?
A
- myelination of an axon
5
Q
what does myelin consist of?
A
- is rich in lipids and prevents water and water-soluble substances (such as ions) from crossing membrane; acts as electrical insulator
- ions can cross membrane only thru gaps in myelin sheath, “Nodes of Ranvier”
6
Q
What is saltatory conduction?
A
- myelinated axons transmit impulses thru saltatory conduction, in which action potentials “jump” from node to node down the axon
- saltatory conduction is much faster than impulse conduction in unmyelinated axons
7
Q
how does axon diameter affect conduction speed?
A
- thick axons transmit faster than thin axons
8
Q
What are synapses?
A
- connections where neurons communicate
- they are able to communicate chemical directions to one another
9
Q
What is the presynaptic neuron?
A
- referring to the neuron releasing neurotransmitter at a synapse on the “sending side of a synapse
10
Q
What is the postsynaptic neuron?
A
- referring to the neuron or other type of cell on the “receiving” side of a synapse
- could also be a muscle or glandular cell
11
Q
What is synaptic transmission?
A
- the mechanism by which the impulse in the presynaptic neuron signals the postsynaptic cell
- as a result of synaptic transmission, the presynaptic neuron stimulates or inhibits a postsynaptic cell
- is a one-way process carried out by neurotransmitters
12
Q
how does an impulse continue from one neuron to another?
A
- it has to cross the synaptic cleft
13
Q
What happens when an impulse reaches the synaptic knob at the end of an axon?
A
- the synaptic vesicles release a neurotransmitter that diffuses across the synaptic cleft
14
Q
What is the structure of a synaptic knob?
A
- is the transmissive segment of a neuron
- the synaptic knob contains vesicles filled with neurotransmitter
- upon stimulation, they are released into the synaptic cleft and bind with NT-specific ligand-gated channels on the postsynaptic membrane
15
Q
What happens when action potential reaches the synaptic knob of the presynaptic axon?
A
- the presynaptic axon contains large numbers of NT-containing synaptic vesicles
- When the AP reaches this region, folate-gated calcium channels open and result in an influx of calcium
- this triggers the release of the NTs by exocytosis
16
Q
What happens when neurotransmitters (NTs) diffuse across the synaptic cleft?
A
- the NTs diffuse across the cleft and then bind to NT-specific receptors present on the postsynaptic membrane
- this binding triggers the opening of ligand-gated ion channels
- this causes either depolarization or hyperpolatization of the postsynaptic membrane
17
Q
What happens to NTs that are left within the synaptic cleft?
A
- they are either broken dow or taken back up into the synaptic knob
- enzymes within the extracellular fluid break down the NTs to be taken back up to the synaptic knob
18
Q
What drives neural communication between cells?
A
- is driven by changes in membrane permeability to ions
- neurons receive input from multiple different types of stimuli
- the summation of graded potentials and the unidirectional propagation of APs are due primarily to changes in Na+ and K+ membrane permeability
- all of this ultimately results in the release of NTs
19
Q
Describe synaptic transmission
A
- is the process by which an impulse in a presynaptic neuron transfers info to a postsynaptic cell
- involves a chemical NT, released by the presynaptic cell that binds to receptors on the postsynaptic cell
- this binding causes chemically-gated ion channels to open in the postsynaptic cell, creating a synaptic potential in that cell
20
Q
what are the effects of NTs?
A
- they vary
- some open to ion channels and others close ion channels
21
Q
What are synaptic potentials?
A
- local potentials resulting from changes in chemically gated ion channels
22
Q
What are excitatory neurotransmitters?
A
- increase permeability to Na+ ions
- bring membrane closer to threshold; increase likelihood of generating impulses
23
Q
What are inhibitory neurotransmitters?
A
- move membrane farther from threshold
- decrease likelihood of generating impulses
24
Q
What is an excitatory postsynaptic potential? (EPSPs)
A
- membrane change in which neurotransmitter opens Na+ channels
- de-polarizes membrane of postsynaptic neuron as Na+ enters axon
- action potential in postsynaptic neuron becomes MORE likely
25
What is inhibitory postsynaptic potential? (IPSPs)
- membrane change in which neurotransmitter opens K+ channels
- hyper-polarizes membrane of postsynaptic neuron as K+ leaves axon
- action potential of post-synaptic neurons becomes LESS likely
26
What is summation?
- a process where EPSPs and IPSPs are added together
| - summation of all inputs usually occurs at the trigger zone (axon hillock)
27
What does net excitatory effect lead to?
- greater probability of an action potential
28
What does net inhibitory effect lead to?
- effect does not generate action potentials
29
What does acetylcholine do?
- stimulates skeletal muscle contraction
- used in CNS to control skeletal muscle actions
- used in PNS to stimulate skeletal muscle contraction at neuromuscular junctions; may excite or inhibit at autonomic nervous system synapses
30
What types of proteins can neurotransmitters be?
- monoamines
- amino acids
- peptides
31
Where are NTs produced?
- in rough endoplasmic reticulum
| - or cytoplasm
32
How are NTs released when it reaches the synaptic knob of the axon?
- by exocytosis
33
What are the events that lead to neurotransmitter release?
- action potential passes along an axon and over the surface of its synaptic knob
- synaptic knob membrane becomes more permeable to calcium ions, and they diffuse inward
- in the presence of calcium ions, synaptic vesicles fuse to synaptic knob membrane
- synaptic vesicles release their NTs by exocytosis into the synaptic cleft
- synaptic vesicle membrane becomes part of the cell membrane
- the added membrane provides material for endocytotic vesicles
34
What is vesicle trafficking?
- process of membrane recycling
- synaptic vesicle becomes part of cell membrane as it releases NT
- endocytosis returns membrane to cytoplasm; forms new vesicles
35
What are neuronal pools?
- groups of interneurons that make synaptic connections with each other and are located completely with the CNS
- interneurons work together to perform a common function
- each pool receives input from other neurons
- each pool generates output to other neurons
- pools may affect other pools or peripheral effectors
36
What is convergence?
- one neuron receives input from several neurons
- incoming impulses often represent info from different types of sensory receptors
- allows nerve system to collect, process, and respond to info
- makes it possible for a neuron to sum impulses from different sources
37
What is divergence?
- one neuron sends impulses to several neurons via branching of its axon
- can amplify an impulse
- impulse from a single neuron in CNS may activate several motor units in a skeletal muscle
- impulse from a sensory receptor may reach different regions of the CNS for processing
38
What happens during the absolute refractory period?
- the axon's voltage-gated sodium channels are temporarily not responsive at all, and the axon cannot be stimulated
39
What does the refractory period ensure?
- that an action potential is conducted in only one direction
- down the axon because the area upstream from where the action potential has just occurred is still in the refractory period from the previous action potential
40
What are the events leading to impulse conduction?
- nerve cell maintains resting membrane potential by diffusion of Na+ and K+ down their concentration gradients as the cell pumps them up the gradients
- neurons receive stimulation causing local potential changes which many sum to reach threshold
- if threshold is reached, sodium channels in the trigger zone of the axon open
- sodium ions diffuse inwards, depolarizing the membrane
- potassium channels in the membrane open
- potassium ions diffuse outward, re-polarizing the membrane
- the resulting action potential causes an electric current that stimulates adjacent portions of the membrane
- the action potential propagates along the length of the axon
