Lecture 5: synaptic transmission Flashcards
1
Q
RECAP - what is a synapse?
A
- a synapse is a structure that allows a neuron to pass an electrical or chemical signal to another neuron
- they are essential for the transmission of nerve impulses from one neuron to another
2
Q
What is an electrical synapse?
A
- an electrical synapse is a mechanical & electrically conductive link between 2 neurons that is formed at a narrow gap between pre and postsyanptic neurons called a gap junction
- these gap junction permit the flow of ions from one cell to another
- there is no synaptic delay
- there is** conduction in both directions **
3
Q
Where in the body do we find gap junctions?
A
- myocardial cells
- hepatocytes
- intestinal smooth muscle
- neurons in CNS
4
Q
Describe the **structure **of gap junctions
A
- they are plaque like structures
- they have 6 subunits surrounding a central channel
- the hexagonal array of the subunits (proteins) is called a connexon which consists also of connexins
5
Q
What is a chemical synapses between neurons?
A
- chemical synapses involve transmission of a nerve impulse via chemicals called neurotransmitters
- these are transported from the axon terminal of the presynaptic cell to the postsynaptic cell and they bind to postsynaptic receptors
6
Q
what is the difference in direction between chemical synpases vs electrical synapses
A
- chemical synapses occur in 1 direction
- electrical synapses occur in both directions
7
Q
what are ionotropic receptors vs metabotropic receptors?
A
- ionotropic - ligand gated receptors - ie they open in response to the binding of a neurotransmitter to allow the passage of ions
- metabotropic - NT receptors that act via a **secondary messanger system **eg cAMP
8
Q
Describe the first few steps of a chemical synapse
A
- the neurotransmitter is formed and stored in the axon terminal of presynaptic cell
- an AP arrives at the axon terminal and causes depolarisation, this causes opening of Ca2+ channels
- an influx of calcium occurs which causes the synaptic vesciles (with NT) to fuse with membrane
9
Q
Describe the next few steps of a chemical synapse
A
- the NT is released across the synaptic cleft and binds to receptors on the postsynaptic membrane
- this causes either the opening/closing of the postsynaptic channels
- postsynaptic current causes either excitatory or inhibitory postsynaptic potentials that change the excitability of the cell
10
Q
How is the neurotransmitter removed from the postsynaptic cell?
A
- NT is removed by either **glial cell **uptake (eg astrocytes) or enzymatic degradation
11
Q
What are the 2 kinds of chemical synapses?
A
- excitatory - the postsynaptic response is a depolarization, bringing membrane closer to threshold
- inhibitory -postsynaptic response is hyperpolarisation
- they are differentiated by the effects of the NT on the postsynaptic cell - type of ion channel influenced by binding of NT
12
Q
What is an** EPSP**?
A
- an** excitatory post synaptic potential **
- it is a depolarising graded potential (ie it is NOT AN ALL OR NONE RESPONSE) (graded potential - changes in potential that vary in size)
- involves NT called glutamate that makes membrane more permeable to Na+ and K+
- the depolarised response is caused by the flow of positively charged ions into the postsynaptic cell due to the opening of ligand gated ion channels
- it only functions to bring the membrane closer of postsynptic cell to threshold
13
Q
What is the most well known excitatory neurotransmitter?
A
glutamate - it increases membrane permeability to Na+ and K+
14
Q
What is the membrane time constant?
A
- the time it takes the membrane potential to move 63% of way towards final value
15
Q
what is the membrane length constant?
A
- a mathematical constant used to measure the distance that a **graded electrical potential will travel **along a neurite via passive electrical conduction
- the greater the value of the length constant, the farther the graded potential will travel
16
Q
What is the ‘summation’ of graded potentials ie EPSP?
A
- since an individual EPSP has very little effect on the postsynaptic membrane potential, its not enough to reach threshold level
- in order to reach threshold, several EPSP’s must occur either one after another or several at the same time - this is refered to as** summation**
17
Q
what are the** 2 forms** of summation of graded potentials?
A
- temporal summation
- spatial summation
18
Q
What is spatial summation?
A
- this occurs when several EPSPs or IPSPs arrive at the postsynaptic cell at the same time but from different presynaptic neurons
- summation of EPSP’s can allow the AP to occur, or summation of IPSP’s can prevent the cell from generating an AP
19
Q
what is temporal summation?
A
- it is the additive effect of EPSP’s or IPSP’s from only a single presynaptic neuron on the membrane of the postsynaptic cell
- they synapses occur one after another
20
Q
What is an IPSP?
A
- an** inhibitory post synaptic potential**
- this graded potential causes a hyperpolarised response to transmitter release
- involves the NT called GABA that makes the membrane more permeable to Cl-
- inhibits an AP in the postsynaptic cell
21
Q
What is the effect of the time constant on summation?
A
- neurons with a larger membrane
time constant have a greater capacity for temporal summation than the neurons with shorter membrane time constant - As a result of a larger time constant, the greater the likely hood of the 2 consecutive potentaials summate to reach threshold
22
Q
what is the effect of the length constant on summation?
A
- because the depolarisation produced by a presynaptic neuron is almost never sufficient enough to reach threshold, the inputs from many presynaptic neurons acting on different sites of the neuron are added together
* neurons with a larger length constant (ie electrical signal doesnt decay as much with distance) are more likely to be brought to threshold by inputs acting at different sites
