o Action potentials propagate down the neuron. Myelination speeds up that process and
refractory ensures that this propagation occurs in a single direction.
o The signal has now reached the presynaptic terminals and now needs to communicate
with the next neuron (cell-cell communication).
o The pre and postsynaptic cells are physically separated by a gap known as a synapse (the
space itself is known as the synaptic cleft).
o Electrical signals cannot jump physical gaps and so the electrical signal (which is in the
form of a depolarization – charge “flipping”) needs to be converted to a chemical signal.
o Chemical signals are in the form of neurotransmitters which are released by the
presynaptic neuron and diffuse across the synaptic cleft and reach the plasma membrane
of the postsynaptic cell.
o Now the voltage-gated calcium (Ca ) channels become involved (as they are not
involved in the actual action potential). They are present only in the presynaptic
o The wave of depolarization travels down the length of the neuron. The membrane of the
presynaptic terminal becomes depolarized and become positive on the inside of the cell.
o This flip of charge is the key to activate (opens) the voltage gated calcium channels.
o Calcium is much more concentrated on the outside than the inside and so will flood into
the presynaptic neuron through those channels.
o Calcium is important as it is involved in the movement of the vesicles containing
o Vesicles of neurotransmitters are “just sitting around” at a resting state. Calcium causes
them to move to the end (plasma membrane) of the presynaptic terminals. This is known
as calcium-dependent trafficking. o The vesicles fuse with the plasma membrane and release their contents into the synaptic
o Theydiffuse to the postsynaptic cell where there are ligand-gated channels on the plasma
membrane of the postsynaptic cell. In which case the ligands are the neurotransmitters.
o There are two important neurotransmitters for the situation studied. They aren’t the only
o One neuron will only produce one neurotransmitter.
o Acetylcholine (Ach) is one type of neurotransmitter, if a neuron produces Ach then it will
not produce any other neurotransmitter.
o There are ligand-gated sodiumchannels on the plasma membrane of the postsynaptic cell
(like the ones found on a nociceptor), however, they are only specific to acetylcholine.
o Onlyacetylcholinewill open them.When Achis boundto the channel, it opens and sodium
is free to move into the cell.
o This causes small depolarizations in the postsynaptic cell as there is generally not a lot of
sodium entering the cell (subthreshold stimulus). It would then return to resting state.
o If the acetylcholine release occurs a short
time after the first release this can build
onto the depolarizations.
o Collectively these depolarizations caused
by acetylcholine are known as EPSPs
(excitatory postsynaptic potential).
Depolarizations makes the neuron closer
to threshold so is referred to as excitatory. o A second important neurotransmitter is GABA. On the postsynaptic cell are ligand-gated
chlorine (Cl ) channels which opens when the ligand GABA is bound to the channel.
o That allows chloride to move down its concentration gradient (into the cell). This makes
the cell more negative – referred to as a hyperpolarization – this decreases the membrane
o Afterthestimulusisgone –thecellreturnstoitsrestingmembranepotential –andsimilarly
this stimulus can be built upon.
o These hyperpolarizations created by the influx of chlorine ions are deemed IPSPs
(inhibitory postsynaptic potential). Makes the cell’s potential further from threshold.
o Any one cell is receiving stimuli from hundreds of presynaptic neurons. A single
presynaptic neuron is never strong enough to stimulate the postsynaptic cell enough to
o Theirsignals can add up. Someofthese presynapticneurons will beinhibitory (releasing
GABA) and others will be excitatory (releasing acetylcholine).
o When these signals are adding/taken in at the same time – this is referred to as summation
– which refersto theaddition of all thenextchargedifferences inducedbythepresynaptic
neurons (all the hyper and depolarizations from GABA and Ach respectively).
o If the summation is enough for the membrane potential to reach threshold, an action
potential will result.
o One form of summation is known as temporal summation – which is repeated signaling
o For example – inhibitory signal → delay(returns to resting due to delay). Excitatorysignal
→ delay (returns to resting).
o Gradual “up and down,” if there are enough positive signals (EPSPs) to outweigh the
negative signals (IPSPs) – an action potential will be reached. Adding over time.
o Analogy – your mom telling to clean up your room over and over. Might not do it the first
time, over time if she tells you constantly you get tired of her nagging and do it – temporal
summation. o With spatial summation, it involves many neurons stimulating a postsynaptic cell at the
same time. For example – 10 EPSPs and 2 IPSPs at the same time (the EPSPs outweigh
and cause a large increase in membrane potential).
o Firingoccursat different parts ofthe target cell.The net effects of excitatoryandinhibitory
stimuli will determine what occurs at the postsynaptic cell with spacial summation.
o With respect to the reflex arch, ligand-gated sodium channels at the nociceptors are
involved in the detection of a stimulus. This generates an action potential that propagates
down the sensory neuron.
o This leads to the release of neurotransmitters at the presynaptic terminals of the sensory
neuron that stimulate the next postsynaptic cell (an interneuron in the spinal cord in the
case for a reflex arch).
o At the interneuron, this presynaptic stimulation is always excitatory (Ach). Sensory
neurons are always excitatory.
o The same events as above will again occur in the interneuron. The interneuron synapses to
a motor neuron and it also always induces an excitatory signal.
o This occurs again within the motor neuron and the motor neurons are also all excitatory
as they synapse to a target organ (muscle).
o Similar events occur at the target organ; however, it does not generate an action potential,
it will instead contract. o Presynaptic terminals of the motor neuron synapse with the muscle cells – referred to as a
neuromuscular synapse (the muscle cells contain bundles of myofibrils). The myofibrils
are wrapped in the blue sarcoplasmic reticulum.
o Acetylcholineis released into thesynaptic cleft at themusclecell.Attheplasmamembrane
of the muscle cells are ligand-gated sodium channels. Ach binds → sodium rushes into
the muscle cells → the muscle cells are depolarized (same as with postsynaptic neurons).
o The plasma membrane of the muscle cells form deep invaginations known as T-tubules
which make physical contact with the sarcoplasmic reticulum around the myofibrils
inside the muscle cell.
o The sarcoplasmic reticulum is the endoplasmic reticulum of a muscle fibre. Its main role
in muscle contraction is the storage of calcium ions. There is a much larger concentration
of calcium ions in the sarcoplasmic reticulum than in the cytosol of the muscle cell.
o There is also a greater concentration of calcium ions in the outside interstitial fluid than
in the cytosol.
o Calcium is vital in the