Physiology Wk 3

Question 1

What is a synapse

Answer 1

A junction between two neurons

the connection between two excitable neurons

Question 2

What are the two types of synapses

Answer 2

Electrical synapses and chemical synapses

Question 3

What is an electrical synapse

Answer 3

Two neurons are connected by gap junctions which allow charge carrying ions to flow directly between two cells in either direction. At an electrical synapse, an AP in one neuron leads to an AP in the connected neuron

Question 4

What is an example of an electrical synapse

Answer 4

ES interconnect a cluster of neurosecretory neurons in the brain that all secrete the same neurohormone, GnRH, which is the head of an endocrine chain of command that governs reproductive function/

Question 5

What are chemical synapses

Answer 5

Majority of synapses in the Human NS are chemical synapses at which the chemical messenger transmits information one way across a space separating the two neurons

Question 6

What does a chemical synapse typically involve

Answer 6

Junction between an axon terminal of one neuron known as the presynaptic neuron and dendrites or a cell body known as the post synaptic neuron

Question 7

Name the features of the chemical synapse

Answer 7

Presynaptic neuron- conducts its AP toward the synapse ends in a slight swelling, the SYNAPTIC KNOB

The synaptic knob contains synaptic vesicles which store a specific chemical messenger neurotransmitter that has been synthesised and packaged by the presynaptic neurone

The post synaptic neurone- whose APs are propagated away from the synapse

The space between the pre and post synaptic neurones are called the synaptic cleft

Question 8

Explain the process and function of a single synapse

Answer 8

Action potential reaches the axon terminal of the presynaptic neurone

Ca2+ enters the presynaptic knob

Neurotransmitter is released by exocytosis into synaptic cleft

Neurotransmitter binds to receptors that are an integral part of chemically gated channels on subsynaptic membrane of postsynaptic neurone

Binding of NT to receptor channel opens that specific channel

Question 9

Explain how a neurotransmitter carries signal across a synapse

Answer 9

Local change in potential triggers the opening of voltage gated ca channels in the synaptic knob

Because the ca is more highly conc in the ECF this ion flows into the synaptic knob through the open channels

Ca2+ promotes the release of NT from some vesicles in the synaptic cleft

The release is done by exocytosis. The released NT diffuses across the synaptic cleft and binds with protein receptors on the subsynaptic membrane

Binding of the NT on the receptor channels causes the channels to open changing the ion permeability and thus potential of postsynaptic membrane

Question 10

what are excitatory synapses

Answer 10

the receptor channels to which the NT binds are non specific cation channels that permit the passage of Na+ and K+

activation of one excitatory synapse cannot depolarise the postsynaptic neuron enough to bring it to the threshold. too few channels are involved at a single synaptic site to permit adequate ion flow to reduce the potential to threshold. this small depolarisation does bring the postsynaptic membrane closer to threshold increasing the likelihood that threshold will be reached and an AP will occur as the membrane is more excitable

a change in the post synaptic potential is called the excitatory postsynaptic potential

Question 11

what are inhibitory synapses

Answer 11

binding of the NT with its receptor channels increases the permeability of the subsynaptic membrane wither K+ or Cl- depending on the synapses

a small hyperpolarisation moves the membrane potential even farther away from the threshold reducing the likelihood that the postsynaptic neuron will reach threshold and undergo an AP and the membrane is less excitable that when its at RP

this is called inhibitory post synaptic potential

EPSPs and IPSPs are produced by opening of chemically gated channels

Question 12

what receptors does Ach bind to

Answer 12

muscalinic
nicotinic

Question 13

look at goodnotes book NTs

Question 14

how are neurotransmitters removed from the synaptic cleft

Answer 14

It may diffuse away from the synaptic cleft, be inactivated by spe- cific enzymes within the subsynaptic membrane, or be actively taken back up into the axon terminal by transport mechanisms in the presynaptic membrane. Once the neurotransmitter is taken back up, it can be stored and released another time (recycled) in response to a subsequent action potential or destroyed by enzymes within the synaptic knob. The method used depends on the particular synapse.

Question 15

what are EPSPs and IPSPs

Answer 15

graded potentials

The postsynaptic neuron can be brought to threshold by either temporal summation or spatial summation.

Question 16

what is GPSP

Answer 16

The total potential in the postsynaptic neuron, the grand postsynaptic potential (GPSP), is a composite of all EPSPs and IPSPs occurring around the same time.

Question 17

what is temporal summation

Answer 17

The summing of several EPSPs occurring very close together in time because of successive firing of a single presynaptic neuron is known as temporal summation. In reality, up to 50 EPSPs might be needed to bring the postsynaptic membrane to threshold. Each action potential in a single presynaptic neuron triggers the emptying of a certain number of synaptic vesicles. The amount of neurotransmitter released and the resultant magnitude of the change in postsynaptic potential are thus directly related to the frequency of presynaptic action potential

intense stimulation by one presynaptic neuron, EPSPS spread from one synapse to trigger zone,
postsynaptic neurone fires

Question 18

explain the determination of the grand post synaptic potential by the sum of activity in the presynaptic inputs

Answer 18

if an excitatory presynaptic input (ex1) is stimulated a second time after the first EPSP in the postsynaptic cell has died off, a second EPSP of the same magnitude will occur

if the EX1is stimulated a second time before the first EPSP has died off, the second EPSP will add onto, or sum with, the first EPSP resulting in temporal summation, which may bring the postsynaptic cell to threshold

the postsynaptic cell may also be brought to threshold by spatial summation of EPSPs that are initiated by simultaneous activation of two or more excitatory presynaptic inputs

Simultaneous activation of an excitatory andinhibitory presynaptic input does not change the postsyantpic potential because the resultant EPSP and IPSP cancel each other out

Question 19

what is spatial summation

Answer 19

the summation of EPSPs originating simultaneously from several presynaptic inputs

Simultaneous stimulation by several presynaptic neurones
EPSPs spread from several synapses to trigger zone

post-synaptic neuron fires

Question 20

cancellation of concurrent EPSPS and IPSPs

Answer 20

if an excitatory and an inhibitory input are activated, the concurrent EPSP and IPSP more or less cancel each other out- depends on their respective magnitudes

Question 21

importance of the postsynaptic integration

Answer 21

the magnitude of the GPSP depends on the sum of activity in all presynaptic inputs and in turn determines whether or not the postsynaptic neuron will undergo an AP to pass information in to the cells in which the nerve terminates

Question 22

what is convergence

Answer 22

A given neuron may have many other neurons synapsing on it.

Question 23

what is divergence

Answer 23

refers to the branching of axon terminals so that a single cell synapses with and influences many other cells.

Question 24

differences between chemical and electrical synapses

Answer 24

chemical
unidirectional
uses NTs
slow impulse flow

Electrical
uses channel proteins
bidirectional
gap junction channels
conexins (6 monomers)
conexons unstoppable flow of current