Manuel Celedon
OH: Wed. 5-6pm.
Email: mceledon@ucsd.edu
Signaling across chemical synapse
Pre-synaptic site
- The release of neurotransmitter is triggered by the arrival of a nerve impulse (AP).
o Within the pre-synaptic nerve terminal, vesicles containing neurotransmitter sit
“docked” and ready at the synaptic membrane
 The AP produces influx of CA2+ ions thru voltage-gated, calcium selective
ion channels.
 At this point the vesicles fuse with the membrane and release their
contents to the outside.
Postsynaptic site
- receptors on the membrane of the post-synaptic neuron, bind to the NT molecules (in the case
of a Neuromuscular Junction it would be Ach) and respond by opening ion channels in the
post-synaptic cell membrane, causing ions to rush in or out and changing the local transmembrane potential of the cell.
o Excitatory result
 Happens when the cell potential depolarizes (becomes more +)
 Excitatory Post-synaptic potential (EPSP) NT drives Vm to
threshold for AP
o Inhibitory result
 Happens when the cell potential hyperpolarizes (becomes more -)
 Inhibitory Post-synaptic Potential prevents AP’s from being
induced. Vm (membrane potential) does not reach threshold.
- Whether a synapse is excitatory or inhibitory depends on what types of ion channel are open.
(Na+ depolarizes, Cl- hyperpolarizes)
Integration of Synaptic Inputs
- generally, if an excitatory synapse is strong, an AP in the pre-synaptic neuron will trigger
another AP in the post-synaptic cell
o Whereas at a weak synapse the Excitatory post-synaptic Potential (EPSP) will not
reach the threshold for AP initiation.
 Since each neuron typically connects or “synapses” to many others, a single
neuron cell can receive more than one AP fired at the same time or have the
same one fire more than once. There are two types of post-synaptic
potential summation.
 Temporal – same synapse have a bunch or AP’s fired from Presynpt. Site and they reach the post-syn site. Time based.
 Spatial – multiple AP’s fired from more than one pre-synaptic site
upon one neuron cell (post-synaptic site) thus adding up.
Learning and Memory
- Hippocampus is involved in memory formation.
o It is done via a process called long term potentiation (LTP)
 LTP can cause the long-term strengthening of the synapses between two
neurons that are activated simultaneously.
 (Hebbian synapse)
o Glutamate is the NT released into these synapses. It’s an excitatory ionotropic
receptor.
 It binds to several types of receptors in the post-synaptic neuron.
 Receptors – AMPA and NMDA
o AMPA – is paired with an ion channel so that when
glutamate binds to this receptor, this channel lets NA+
ions enter post synaptic neuron.

-
This causes the post-synaptic membrane to
become depolarized and if this depolarization
reaches the critical threshold it can trigger an AP.
(then nerve impulse is then transmitted to the
next neuron)
o NMDA – is also paired with an ion channel, but this
channel admits calcium ions into the post-synaptic cell.
At resting potential, the Ca2+ channel is blocked by
Mg2+, so that even if glutamate binds to NMDA ions
cannot enter.
 For these Mg2+ ions to withdraw from the
channel, the membrane potential must initially be
depolarized (via the AMPA receptor)
The increase [] of Ca2+ in post-syn. Cell sets off several biochemical reactions that make this
synapse more efficient at firing AP’s for extended period.
Mammalian eye
- Anatomy
o Rods are specialized neurons and have:
 Outer segments: contain stacks or discs of membrane, that contain
rhodopsin
 Inner segments: ordinary cell components (ex. Golgi, etc.)
 Synaptic terminal
o Rhodopsin = protein opsin + retinal (absorbs light)
 A photon of light will convert 11-cis retinal to all trans retinal
 This change affects the config. Of opsin, and allows rhodopsin to activate a
G protein called transducin.
o Transducin activates a phosphodiesterase – cleaves a bond between 2 ester
linkages. This leads to the conversion of cGMP to GMP
 The Na Channels are held open by cGMP. This means that you don’t build
up a negative resting potential.
 The cleavage of cGMP from the arrival of light leads to a closing
of the Na Channels, which leads to hyperpolarization, which
inhibits NT release. Light blocks this signaling pathway that is
open while dark.
o “Dark current” (steady state cGMP keeps Na+ channels
open.)
 Rods are for black and white vision
 Cones are for color vision
- Layers of Eye cells
o Ganglion - A ganglion is any structure containing a collection of nerve cell bodies in
the central or peripheral nervous system.
o Bipolar - specialized cells that connect rods and cones to ganglion cells
o Photoreceptors - The light sensitive cells in the eye divided into rods (which see in
dim light and do not distinguish between colors) and cones (which are responsible
for color vision)
 Light goes across ganglion cells, thru bipolar cells, all the way down to hit
the photoreceptors and then it gets absorbed here only to go back up to the
bipolar cells and then the ganglion which sends the signal to be processed
by brain.
o Receptive field – groups of rods and cones associated with 1 ganglion.