 | a synapse is a junction between nerve cells
 | electrical synapses are the less
common variety. they are bridged junctions that are basically like a gap
junction found in other types of cells |
| chemical synapses are specialized
for release and reception of chemical neurotransmitters. They are
separated by the synaptic cleft and have two parts :
 |
an axonal terminal which has synaptic vesicles containing
neurotransmitters |
|
a receptor region on the postsynaptic neuron which contains
neurotransmitter receptors (of several types) |
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| most synapses occur between the axonal endings of one neuron and the
dendrites or cell bodies of another, and so are referred to as
axodendritic or axosomatic. There are also less common (and
less understood) axoaxonic, dendrodendritic and dendrosomatic
synapses. |
| neurons have anywhere from 1000 to 10000 axonal terminals and are
stimulated by an equal number of neurons. |
| when a nerve impulse reaches the axon terminal, a chain of events
is triggered that culminates in neurotransmitter release and the continuation
of the nerve impulse.
| first, calcium gates open in the
presynaptic axonal terminal (calcium floods into the terminal) |
| the increase in calcium concentration
triggers exocytosis of the neurotransmitters. calcium is then removed via
a membrane pump or taken into the mitochondria. |
| the neurotransmitters cross the
synapse and bind to postsynaptic receptors. |
| the binding causes ion channels
to open in the postsynaptic membrane. this produces a receptor potential.
|
depending on the neurotransmitter, the postsynaptic neuron may be excited
or inhibited |
|
| the neurotransmitter is
"terminated" by one of three methods :
|
degradation by enzymes (ex. acetylcholinesterase degrades acetylcholine) |
| removal by reuptake into the presynaptic terminal, where it
is either stored or degraded (ex. norepinephrine) |
|
diffusion away from the synapse |
|
|
| EPSPs vs. IPSPs
| an excitatory postsynaptic potential
occurs across an excitatory chemical synapse. At this type of synapse,
the neurotransmitter's binding causes a depolarization and a sodium-potassium
channel opens to allow simultaneous diffusion of those ions.
|
the simultaneous flow allows a greater influx of sodium than there is an
outflux of potassium, resulting in a depolarization as steep as 0mV |
|
this extremely strong postsynaptic potential is meant to trigger the AP in
the axon |
|
| an inhibitory postsynaptic potential
is caused by the binding of an inhibitory neurotransmitter at an inhibitory
chemical synapse.
|
most inhibitory neurotransmitters induce hyperpolarization of the membrane
by making the membrane more permeable to potassium ions, chloride ions or
both |
|
this makes it less likely than an AP will reach the axon hillock |
|
|
| a single EPSP cannot induce an AP all by itself. However, if thousands
of excitatory axon terminals are firing on the same postsynaptic membrane,
or if a smaller number are firing impulses very rapidly, the EPSPs can add
together (summate).
| in temporal summation, one
or more presynaptic neurons send impulses very rapidly, so that before the
EPSP on the postsynaptic membrane dissipates, another one occurs, depolarizing
the membrane even further. |
| in spatial summation,
a neuron is stimulated by a large number of terminals at the same time, producing
many EPSPs which produce a whole greater than the sum of the individuals. |
| IPSPs can also summate, inhibiting
a neuron to a greater degree |
|
| synapses used repeatedly or continuously (even for a short period
of time) develop synaptic potentiation, where the presynaptic
neuron's ability to stimulate the postsynaptic neuron is greatly enhanced
due to higher concentrations of calcium ions and the activation of
voltage-regulated receptors called N-methyl-D-aspartate (NMDA) receptors,
which help to depolarize the postsynaptic membrane using calcium ion entry. |
| presynaptic inhibition occurs when release of a excitatory
neurotransmitter is inhibited by another neuron via an axoaxonic synapse.
Less neurotransmitter is released than normal, so smaller EPSPs are formed. |
| neurotransmitters are classified depending on their chemical makeup
|
acetylcholine : the first neurotransmitter to be identified.
It is released at neuromuscular junctions and in the autonomic system and
is usually excitatory. Removed via degradation by acetylcholinesterase (AChE) |
| biogenic amines :
include serotonin, dopamine, norepinephrine (NE), epinephrine and
histamine. They are synthesized from amino acids, are broadly distributed
in the brain, and seem to play a large role in emotional behavior and circadian
rhythms. NE and epinephrine are also used in the ANS. Some mental illnesses
are due to imbalances of one or more of these neurotransmitters and certain
psychoactive drugs may bind to biogenic amine receptors and produce
hallucinations (LSD, for one). |
| amino acids : GABA
(gamma-aminobutyric acid), glycine, glutamate and
aspartate. Only found in the CNS so far. Since they are found in all
cells, proving their role as neurotransmitters is difficult. The ones listed
above are known to definitely be neurotransmitters. There may be others. |
| peptides : substance
P, endorphins, and enkephalins. Include a broad spectrum
of molecules and effects thereof. Substance P mediates pain signals. Endorphins
and enkephalins are natural opiates (euphorics) and reducers of pain perception. |
| novel messengers :
ATP, nitric oxide and carbon monoxide. Not fully understood
yet, but ATP is excitatory and works in the CNS and PNS, and NO is involved
in long term memory and learning. |
|
| neural integration
| nerves are arranged into functional
groups called neuronal pools
|
| circuits are patterns of
synaptic connections within a pool
|
diverging circuits are those where one incoming fiber triggers responses
in many neurons; also called amplifying circuits
|
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converging circuits are those where many fibers converge on one neuron |
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