Physiology Wk 3 Flashcards
What is a synapse
A junction between two neurons
the connection between two excitable neurons
What are the two types of synapses
Electrical synapses and chemical synapses
What is an electrical synapse
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
What is an example of an electrical synapse
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/
What are chemical synapses
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
What does a chemical synapse typically involve
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
Name the features of the chemical synapse
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
Explain the process and function of a single synapse
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
Explain how a neurotransmitter carries signal across a synapse
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
what are excitatory synapses
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
what are inhibitory synapses
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
what receptors does Ach bind to
muscalinic
nicotinic
look at goodnotes book NTs
how are neurotransmitters removed from the synaptic cleft
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.
what are EPSPs and IPSPs
graded potentials
The postsynaptic neuron can be brought to threshold by either temporal summation or spatial summation.
what is GPSP
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.
what is temporal summation
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
explain the determination of the grand post synaptic potential by the sum of activity in the presynaptic inputs
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
what is spatial summation
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
cancellation of concurrent EPSPS and IPSPs
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
importance of the postsynaptic integration
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
what is convergence
A given neuron may have many other neurons synapsing on it.
what is divergence
refers to the branching of axon terminals so that a single cell synapses with and influences many other cells.
differences between chemical and electrical synapses
chemical
unidirectional
uses NTs
slow impulse flow
Electrical
uses channel proteins
bidirectional
gap junction channels
conexins (6 monomers)
conexons unstoppable flow of current
