GRQ #4 - Synaptic Activity Flashcards
Signals are transmitted between neurons at locations called
synapses
what is a synapse?
it is where a neuron meets its target cell
synapses that occur between two neurons are called
neuronal synapses
neuronal synapses can be of 2 types. They are:
electrical and chemical
What is an axodendritic synapse?
a synapse that occurs between an axon and a dendrite
what is an axosomatic synapse?
a synapse that occurs between an axon and a cell body
what is an axoaxonic synapse?
a synapse that occurs between an axon and another axon
What are the two terms used to describe the neurons sending and receiving messages?
Presynaptic and postsynaptic neurons
what is a presynaptic neuron
it is the neuron that is sending the message from its axon terminal
what is a postsynaptic neuron
it is the neuron that is receiving the message from its dendrite, cell body or axon
what is a synaptic transmission
it is the transfer of chemical and electrical signals between neurons at a synapse
___ is the fundamental process for most functions of the nervous system
synaptic transmission
Each neuron has an ___ number of synapses
enormous
An axon generally splits into over __ axon terminals
1,000
Each axon terminal meets up with another
axon, dendrite or cell body
on average, a presynaptic neuron generally forms synapses with about how many postsynaptic neurons?
about 1,000
A post synaptic neuron can receive input from as many as ____ presynaptic neurons
10,000
what is an electrical synapse
they occur between cells that are electrically coupled via gap junctions
how do gap junctions function
gap junctions are between two axolemmas of two neurons that are nearly touching and the junctions contain precisely aligned channels that form pores through which ions and other small substances can travel
Gap junctions allow the __
electric current to flow directly from the axoplasma of one neuron to axoplasma of another neuron
do the axolemmas of the neurons touch at an electrical synapse?
No - they are separated by about 3.5 nanometers (so they are nearly touching)
what are two unique features of electrical synapses
- synaptic transmission is bidirectional
2. synaptic transmission is nearly instantaneous
what do electrical synapses’ unique features allow for
they allow the activity of a group of cells to be synchronized; when stimulated, the cells will produce action potentials in unison
explain synaptic transmission being bidirectional in electrical synapses
it means that either neuron in an electrical synapse may act as the presynaptic or the postsynaptic neuron and thus the electrical current may flow in either direction between the two cells
explain synaptic transmission being nearly instantaneous in electrical synapses
the delay between depolarization of the presynaptic neuron and change in potential of the postsynaptic neuron is less than 0.1 miliseconds which is crazy fast
where are electrical synapses found in the nervous system?
primarily in areas of the brain that are responsible for programmed, automatic behaviors like breathing; also present in developing nervous tissue in the embryo and fetus and are thought to assist in the development of the brain
where are electrical synapses found outside the nervous system?
they can be found in cardiac and visceral smooth muscle where they allow those tissues to engage in coordinated muscle activity
the vast majority of synapses in the nervous system are __. why?
chemical synapses because they are more efficient.
what is a chemical synapse
a type of synapse in which a presynaptic neuron releases neurotransmitters to trigger a change in a postsynaptic neuron
what happens to the electrical current in an electrical synapse?
they eventually become weaker as the current dissipated into the extracellular fluid
do chemical synapses lose strength/weaken?
no - chemical synapses convert an electrical signal into a controlled chemical signal so no strength is lost
where is the chemical signal reconverted back into an electrical signal?
in the postsynaptic neuron
what are the 3 structural differences between a chemical and electrical synapse
- synaptic vesicles
- synaptic cleft
- neurotransmitter receptors
explain the role of the synaptic vesicles
this is how the electrical signal of the action potential is converted into a chemical signal ; every chemical synapse houses synaptic vesicles in the axon terminal of the presynaptic neuron. the vesicles contain chemical messengers called neurotransmitters that transmit signals from the presynaptic to postsynaptic neuron
explain the role of the synaptic cleft
the cells of a chemical synapse are separated by a microscopic gap between the axon terminal of a presynaptic neuron and its target cell called the synaptic cleft. it is filled with extracellular fluid and proteins like enzymes; 20-50 nm (slightly larger than the gap junctions in electrical synapses)
explain the role of neurotransmitter receptors
the postsynaptic neuron must have receptors to which the neurotransmitters released by the presynaptic neuron can bind or it cannot respond to the signal being transmitted; receptors are generally linked directly or indirectly to ion channels
how is the chemical signal converted back into an electrical signal in a chemical synapse
through the ion channels that the neurotransmitter receptors are linked to
what causes chemical synapses to transmit signals more slowly than electrical synapses
the three unique structural differences
what is the synaptic delay
the short delay between the arrival of the action potential at the axon terminal of a presynaptic neuron and the postsynaptic potential of a postsynaptic neuron
how long is the synaptic delay
0.5 millisecond gap between the arrival of the action potential at the axon terminal and the effects on the postsynaptic neuron’s membrane
because chemical synapses are ____ the message can only be sent by the presynaptic neuron
unidirectional
what is allowed by the 3 structural differences of the chemical synapse that is not permitted by the structure of the electrical synapse
the signal can vary in size - this means that if more neurotransmitters are released then the presynaptic neuron has a greater effect on the postsynaptic neuron
the signal in an electrical synapse will always be __
the same size
the effect that the presynaptic neuron triggers can vary with different
neurotransmitters and receptors
what is step 1 (the first thing that takes place) at a neuronal synapse
an action potential in the presynaptic neuron triggers calcium ion channels in the axon terminal to open - an action potential reaches the axon terminal of the presynaptic neuron, which triggers the opening of voltage-gates calcium ion channels in its axolemma
what is step 2 at a neuronal synapse
an influx of calcium ions causes synaptic vesicles to release neurotransmitters into the synaptic cleft - these calcium ions enter the axon terminal causing synaptic vesicles in the area to fuse with the presynaptic membrane and this releases neurotransmitters into the synaptic cleft via exocytosis
what is step 3 at a neuronal synapse
neurotransmitters bind to receptors on the postsynaptic neuron - the neurotransmitters diffuse across the synaptic cleft where they bind to neurotransmitter receptors on the membrane of the postsynaptic neuron
what is step 4 at a neuronal synapse
ion channels open, leading to a local potential and possibly an action potential : the binding of neurotransmitters to receptors generally either opens or closes ligand-gated ion channels in the postsynaptic membrane resulting in a local potential (this local potential may or may not lead to an action potential in the postsynaptic neuron)
