Lecture 5: synaptic transmission Flashcards
RECAP - what is a synapse?
- 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
What is an electrical synapse?
- 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 **
Where in the body do we find gap junctions?
- myocardial cells
- hepatocytes
- intestinal smooth muscle
- neurons in CNS
Describe the **structure **of gap junctions
- 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
What is a chemical synapses between neurons?
- 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
what is the difference in direction between chemical synpases vs electrical synapses
- chemical synapses occur in 1 direction
- electrical synapses occur in both directions
what are ionotropic receptors vs metabotropic receptors?
- 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
Describe the first few steps of a chemical synapse
- 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
Describe the next few steps of a chemical synapse
- 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
How is the neurotransmitter removed from the postsynaptic cell?
- NT is removed by either **glial cell **uptake (eg astrocytes) or enzymatic degradation
What are the 2 kinds of chemical synapses?
- 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
What is an** EPSP**?
- 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
What is the most well known excitatory neurotransmitter?
glutamate - it increases membrane permeability to Na+ and K+
What is the membrane time constant?
- the time it takes the membrane potential to move 63% of way towards final value
what is the membrane length constant?
- 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
What is the ‘summation’ of graded potentials ie EPSP?
- 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**
what are the** 2 forms** of summation of graded potentials?
- temporal summation
- spatial summation
What is spatial summation?
- 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
what is temporal summation?
- 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
What is an IPSP?
- 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
What is the effect of the time constant on summation?
- 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
what is the effect of the length constant on summation?
- 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