Chapter 2 Flashcards
1906, Charles Scott Sherrington physiologically demon-
strated
that communication between one neuron and the next
differs from communication along a single axon. He inferred a
specialized gap between neurons and introduced the term
synapse to describe it.
reflexes
—automatic muscular responses
to stimuli.
Reflex arc: In a leg flexion reflex
a sensory neuron excites a
second neuron, which in turn excites a
motor neuron, which excites a muscle,
as in Figure 3.1 (p. 40) The circuit from sen-
sory neuron to muscle response is called
a reflex arc.
Sherrington observed several properties of reflexes sug-
gesting special processes at the junctions between neurons: (3)
(a) Reflexes are slower than conduction along an axon.
(b) Several weak stimuli presented at slightly different times
or locations produce a stronger reflex than a single stimulus
does. (c) When one set of muscles becomes excited, a different
set becomes relaxed. Let’s consider each of these points and
their implications.
What evidence led Sherrington to conclude that transmis-
sion at a synapse is different from transmission along an
axon?
Sherrington found that the velocity of conduction
through a reflex arc was significantly slower than the
velocity of an action potential along an axon. There-
fore, some delay must occur at the junction between
one neuron and the next.
temporal summation
repeated stimuli within a brief time
have a cumulative effect. He referred to this phenomenon as
temporal summation (summation over time). A light pinch
of the dog’s foot did not evoke a reflex, but a few rapidly re-
peated pinches did. Sherrington surmised that a single pinch
did not reach the threshold of excitation for the next neuron.
presynaptic neuron
The neuron that delivers transmission is the presynaptic neuron.
postsynaptic neuron
The neuron that receives transmission is the postsynaptic neuron.
excitatory postsynaptic potential (EPSP)
A graded depolarization is
known as an excitatory postsynaptic potential (EPSP).
Partial depolarization is a graded potential.
Unlike action potentials, which are always depolarizations.
It results from a flow of sodium ions into the neuron. If an EPSP does not cause the cell to reach its threshold, the depolarization decays quickly.
spatial summation
that is, summation over space. Synap-
tic inputs from separate locations combine their effects on a neuron.
What is the difference between temporal summation and
spatial summation?
Temporal summation is the combined effect of
quickly repeated stimulation at a single synapse.
Spatial summation is the combined effect of several
nearly simultaneous stimulations at several synapses
onto one neuron.
inhibitory postsynaptic potential (IPSP)
That is, it increases the negative charge within the
cell, moving it further from the threshold and decreasing the
probability of an action potential (point 5 in Figure 3.3, p. 54).
This temporary hyperpolarization of a membrane—called
an inhibitory postsynaptic potential, or IPSP—resembles
an EPSP. An IPSP occurs when synaptic input selectively
opens the gates for potassium ions to leave the cell (carrying
a positive charge with them) or for chloride ions to enter the
cell (carrying a negative charge).
What was Sherrington’s evidence for inhibition in the nervous system?
Sherrington found that a reflex that stimulates a
flexor muscle prevents contraction of the extensor
muscles of the same limb. He therefore inferred that
an axon sending an excitatory message for the flexor
muscle also sent an inhibitory message for the extensor muscle.
What ion gates in the membrane open during an EPSP?
What gates open during an IPSP?
During an EPSP, sodium gates open. During
an IPSP, potassium or chloride gates open.
Can an inhibitory message flow along an axon?
No. Only
action potentials propagate along an axon. Inhibitory
messages—IPSPs—decay over time and distance.
spontaneous firing rate
a periodic
production of action potentials even without synaptic input.
In such cases, the EPSPs increase the frequency of action po-
tentials above the spontaneous rate, whereas IPSPs decrease
it.
What was Loewi’s evidence that neurotransmission de-
pends on the release of chemicals?
When Loewi stimulated a nerve that increased or
decreased a frog’s heart rate, he could withdraw some
fluid from the area around the heart, transfer it to
another frog’s heart, and thereby increase or decrease
its rate also.
Every year, research-
ers discover more and more details about synapses, their
structure, and how those structures relate to function. Here
are the major events (6):
- The neuron synthesizes chemicals that serve as
neurotransmitters. It synthesizes the smaller
neurotransmitters in the axon terminals and synthesizes
neuropeptides in the cell body.
2. Action potentials travel down the axon. At the
presynaptic terminal, an action potential enables
calcium to enter the cell. Calcium releases
neurotransmitters from the terminals and into the
synaptic cleft, the space between the presynaptic and
postsynaptic neurons.
3. The released molecules diffuse across the cleft, attach to
receptors, and alter the activity of the postsynaptic neuron.
4. The neurotransmitter molecules separate from their
receptors.
5. The neurotransmitter molecules may be taken back into the
presynaptic neuron for recycling or they may diffuse away.
6. Some postsynaptic cells send reverse messages to control
the further release of neurotransmitter by presynaptic cells.
Figure 3.13 (p. 60) summarizes these steps.
neurotransmitters
At a synapse, a neuron releases chemicals that affect another
neuron. Those chemicals are known as neurotransmitters.
major categories of neurotransmitters:
"amino acids" acids containing an amine group (NH 2 ) "monoamines" chemicals formed by a change in certain amino acids "acetylcholine" (a one-member “family”) a chemical similar to an amino acid, except that it includes an N(CH 3 ) 3 group instead of an NH 2 "neuropeptides" chains of amino acids "purines" a category of chemicals including adenosine and several of its derivatives "gases" nitric oxide and possibly others
The oddest transmitter is “nitric ox-
ide” (chemical formula NO), a gas re-
leased by many small local neurons. (Do
not confuse nitric oxide, NO, with ni-
trous oxide, N(small2)O, sometimes known as
“laughing gas.”) Nitric oxide is poison-
ous in large quantities and difficult to
make in a laboratory. Yet, many neurons
contain an enzyme that enables them to
make it efficiently. One special function
of nitric oxide relates to blood flow:
When a brain area becomes highly ac-
tive, blood flow to that area increases.
How does the blood “know” which
brain area has become more active? The
message comes from nitric oxide. Many neurons release nitric oxide when they are stimulated. In addi-
tion to influencing other neurons, nitric oxide dilates the nearby
blood vessels, thereby increasing blood flow to that brain area
What does a highly active brain area do to increase its
blood supply?
In a highly active brain area, many stimulated neurons
release nitric oxide, which dilates the blood vessels in
the area and thereby increases blood flow to the area.
catecholamines
compounds known as catecholamines,
because they contain a catechol group and an amine group ( epinephrine, norepinephrine, and dopamine)
tryptophan
The amino acid tryptophan, the precursor to serotonin, crosses the blood–brain barrier by a special transport system that it shares with other large amino acids. The amount of tryptophan in the diet controls the amount of serotonin in the brain
how to increase tryptophan entry in the brain
eat foods
richer in tryptophan, such as soy, and fall after something low in
tryptophan, such as maize (American corn). However, trypto-
phan has to compete with other, more abundant large amino
acids, such as phenylalanine, that share the same transport sys-
tem. One way to increase tryptophan entry to the brain is to
decrease consumption of phenylalanine. Another is to eat car-
bohydrates. Carbohydrates increase the release of the hormone
insulin, which takes several competing amino acids out of the
bloodstream and into body cells, thus decreasing the competi-
tion against tryptophan
vesicles
Most neurotransmitters are synthesized in the presynaptic
terminal, near the point of release. The presynaptic terminal
stores high concentrations of neurotransmitter molecules in
vesicles, tiny nearly spherical packets (Figure 3.15, p. 62). (Nitric
oxide is an exception to this rule. Neurons release nitric oxide
as soon as they form it instead of storing it.)
, MAO (mono-amine oxidase)
It is possible for a neuron to accumulate excess levels of a neurotransmitter. Neurons that release serotonin, dopamine, or norepinephrine contain an enzyme, MAO (mono-amine oxidase), that breaks down these transmitters into inactive chemicals.
how does an action potential lead to the release of a neurotransmitter (exocytosis)?
At the end of an axon, the action potential itself does not
release the neurotransmitter. Rather, the depolarization
opens voltage-dependent calcium gates in the presynaptic terminal. Within 1 or 2 milliseconds (ms) after calcium en-
ters the presynaptic terminal, it causes exocytosis—release
of neurotransmitter in bursts from the presynaptic neuron
into the synaptic cleft that separates one neuron from an-
other. An action potential often fails to release any trans-
mitter, and even when it does, the amount varies
For many years, investigators believed that each neuron released just one neurotransmitter, but later researchers found that
many, perhaps most, neurons release a combination of two or more transmitters.
Why does a neuron release a combination of transmitters instead of just one?
The combination makes the neuron’s message
more complex, such as brief excitation followed by slight but prolonged inhibition
Motor neurons in the spinal chord release what transmitters?
different transmitters from different branches of its axon.
Motor neurons in the spinal cord have one branch to the muscles, where they release acetylcholine, and another branch to other spinal cord neurons, where they release both acetylcholine and glutamate
Although a neuron releases only a limited number of neu-
rotransmitters, it may receive and respond to
many neu-
rotransmitters at different synapses. For example, at various
locations on its membrane, it might have receptors for gluta-
mate, serotonin, acetylcholine, and others.
When the action potential reaches the presynaptic ter-
minal, which ion must enter the presynaptic terminal to
evoke release of the neurotransmitter?
Calcium.
ionotropic effects (receptor, neurotransmittter)
brief on/off effect: when a neurotransmitter binds to the receptor it opens its central channel to let ions pass (transmitter gated/ ligand gated channels)
well suited to conveying visual information, auditory information, and anything else that needs to be updated as quickly as possible.
