Page 65 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

!
concentration will not yet drop to the resting threshold. This type of temporal summation
value, and residual Ca 2+ will accumulate. As a therefore increases the excitability of the post-
result, the more recent rise in [Ca ] i builds on synaptic neuron (! C).
2+
the former one. [Ca ] i rises to a higher level Inhibitory transmitters include substances
2+
after the second stimulus than after the first, as glycine, GABA (γ-aminobutyric acid), and
and also releases more transmitters. Hence, acetylcholine (at M2 and M3 receptors;
the first stimulus facilitates the response to the ! p. 82). They increase the conductance, g, of
+
second stimulus. Muscle strength increases at the subsynaptic membrane only to K (e.g., the
–
high stimulus frequencies for similar reasons metabotropic GABA B receptor.) or Cl (e.g., the
Nerve and Muscle, Physical Work and glutamate (Glu). They are often released creases in g K occur when E m approaches E K
(! p. 67 A).
ionotropic glycine and GABA A receptors; ! F).
Among the many substances that act as ex-
The membrane usually becomes hyper-
polarized in the process (ca. 4 mV max.). In-
citatory transmitters are acetylcholine (ACh)
together with co-transmitters which modulate
(! p. 44). However, the main effect of this in-
the transmission of a stimulus (e.g., ACh to-
hibitory postsynaptic potential IPSP (! D) is not
gether with substance P, VIP or galanin; Glu
hyperpolarization–which works counter to
EPSP-related depolarization (the IPSP is some-
with substance P or enkephalin). If the trans-
mitter’s receptor is an ion channel itself
times even slightly depolarizing). Instead, the
IPSP-related increase in membrane conduct-
(ionotropic receptor or ligand-gated ion chan-
apse (! p. 82), the channels open more often
the EPSP (high g K or g Cl levels). Since both E K
and allow a larger number of cations to enter
and E Cl are close to the resting potential
2 nel; ! A6 and F), e.g., at the N-cholinergic syn- ance short circuits the electrotonic currents of
(! p. 44), stabilization occurs, that is, the EPSP
+
(Na , sometimes Ca ) and leave the cell (K ).
+
2+
Other, so-called metabotropic receptors in- is cancelled out by the high K and Cl short-
+
–
fluence the channel via G proteins that control circuit currents. As a result, EPSP-related
channels themselves or by means of “second depolarization is reduced and stimulation of
messengers” (! A7 and F). Because of the high postsynaptic neurons is inhibited (! D).
electrochemical Na + gradient (! p. 32), the Termination of synaptic transmission (! E)
+
number of incoming Na ions is much larger can occur due to inactivation of the cation
than the number of exiting K ions. Ca 2+ can channels due to a conformational change in
+
also enter the cell, e.g., at the glutamate-NMDA the channel similar to the one that occurs
receptor (! F). The net influx of cations leads during an action potential (! p. 46). This very
to depolarization: excitatory postsynaptic rapid process called desensitization also func-
potential (EPSP) (maximum of ca. 20 mV; ! B). tions in the presence of a transmitter. Other
The EPSP begins approx. 0.5 ms after the ar- terminating pathways include the rapid enzy-
rival of an action potential at the presynaptic matic decay of the transmitter (e.g., acetylcho-
terminal. This synaptic delay (latency) is line) while still in the synaptic cleft, the re-up-
caused by the relatively slow release and diffu- take of the transmitter (e.g., noradrenaline)
sion of the transmitter. into the presynaptic terminal or uptake into
A single EPSP normally is not able to extraneuronal cells (e.g., in glial cells of the
generate a postsynaptic (axonal) action poten- CNS), endocytotic internalization of the recep-
tial (AP A), but requires the triggering of a large tor (! p. 28), and binding of the transmitter to
number of local depolarizations in the den- a receptor on the presynaptic membrane (au-
drites. Their depolarizations are transmitted toceptor). In the latter case, a rise in g K and a
electrotonically across the soma (! p. 48) and drop in g Ca can occur, thus inhibiting transmit-
summed on the axon hillock (spatial summa- ter release, e.g., of GABA via GABA B receptors or
tion; ! B). Should the individual stimuli arrive of noradrenaline via α 2-adrenoceptors (! F
at different times (within approx. 50 ms of and p. 86).
each other), the prior depolarization will not
have dissipated before the next one arrives,
52 and summation will make it easier to reach
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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