CHAPTER 6
Synaptic & Junctional Transmission 121SLOW POSTSYNAPTIC POTENTIALS
In addition to the EPSPs and IPSPs described previously, slow
EPSPs and IPSPs have been described in autonomic ganglia,
cardiac and smooth muscle, and cortical neurons. These
postsynaptic potentials have a latency of 100 to 500 ms and last
several seconds. The slow EPSPs are generally due to decreases
in K
- conductance, and the slow IPSPs are due to increases in
K
conductance.
GENERATION OF THE ACTION
POTENTIAL IN THE
POSTSYNAPTIC NEURON
The constant interplay of excitatory and inhibitory activity on
the postsynaptic neuron produces a fluctuating membrane
potential that is the algebraic sum of the hyperpolarizing and
depolarizing activity. The soma of the neuron thus acts as a
sort of integrator. When the 10 to 15 mV of depolarization
sufficient to reach the firing level is attained, a propagated
spike results. However, the discharge of the neuron is slightly
more complicated than this. In motor neurons, the portion of
the cell with the lowest threshold for the production of a full-
fledged action potential is the
initial segment,
the portion of
the axon at and just beyond the axon hillock. This unmyelinat-
ed segment is depolarized or hyperpolarized electrotonically
by the current sinks and sources under the excitatory and in-
hibitory synaptic knobs. It is the first part of the neuron to fire,
and its discharge is propagated in two directions: down the
axon and back into the soma. Retrograde firing of the soma in
this fashion probably has value in wiping the slate clean for
subsequent renewal of the interplay of excitatory and inhibito-
ry activity on the cell.
FUNCTION OF THE DENDRITES
For many years, the standard view has been that dendrites are
simply the sites of current sources or sinks that electrotonical-
ly change the membrane potential at the initial segment; that
is, they are merely extensions of the soma that expand the area
available for integration. When the dendritic tree of a neuron
is extensive and has multiple presynaptic knobs ending on it,
there is room for a great interplay of inhibitory and excitatory
activity.
It is now well established that dendrites contribute to neural
function in more complex ways. Action potentials can be
recorded in dendrites. In many instances, these are initiated in
the initial segment and conducted in a retrograde fashion, but
propagated action potentials are initiated in some dendrites.
Further research has demonstrated the malleability of den-
dritic spines. Dendritic spines appear, change, and even disap-
pear over a time scale of minutes and hours, not days and
months. Also, although protein synthesis occurs mainly in the
soma with its nucleus, strands of mRNA migrate into the den-
drites. There, each can become associated with a single ribo-
some in a dendritic spine and produce proteins, which alters
the effects of input from individual synapses on the spine.
Changes in dendritic spines have been implicated in motiva-
tion, learning, and long-term memory.ELECTRICAL TRANSMISSION
At synaptic junctions where transmission is electrical, the im-
pulse reaching the presynaptic terminal generates an EPSP in
the postsynaptic cell that, because of the low-resistance bridge
between the two, has a much shorter latency than the EPSP at
a synapse where transmission is chemical. In conjoint syn-
apses, both a short-latency response and a longer-latency,
chemically mediated postsynaptic response take place.INHIBITION & FACILITATION
AT SYNAPSES
DIRECT & INDIRECT INHIBITION
Inhibition in the CNS can be postsynaptic or presynaptic.
Postsynaptic inhibition
during the course of an IPSP is called
direct inhibition
because it is not a consequence of previous
discharges of the postsynaptic neuron. There are various
forms of
indirect inhibition,
which is inhibition due to the ef-
fects of previous postsynaptic neuron discharge. For example,
the postsynaptic cell can be refractory to excitation because it
has just fired and is in its refractory period. During after-
hyperpolarization it is also less excitable. In spinal neurons, es-
pecially after repeated firing, this after-hyperpolarization may
be large and prolonged.POSTSYNAPTIC INHIBITION IN THE
SPINAL CORDVarious pathways in the nervous system are known to mediate
postsynaptic inhibition, and one illustrative example is pre-
sented here. Afferent fibers from the muscle spindles (stretch
receptors) in skeletal muscle project directly to the spinal mo-
tor neurons of the motor units supplying the same muscle
(Figure 6–6). Impulses in this afferent fiber cause EPSPs and,
with summation, propagated responses in the postsynaptic
motor neurons. At the same time, IPSPs are produced in mo-
tor neurons supplying the antagonistic muscles which have an
inhibitory interneuron interposed between the afferent fiber
and the motor neuron. Therefore, activity in the afferent fibers
from the muscle spindles excites the motor neurons supplying
the muscle from which the impulses come, and inhibits those
supplying its antagonists
(reciprocal innervation).