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Chapter 48  Neurons, Synapses, and Signaling
Student Guided Notes
Overview: Lines of Communication

Neurons are nerve cells that ____________________________________________________________.

Communication by neurons is based on two distinct types of signals: long-distance _________________
signals and short-distance __________________________ signals.

If an organism does NOT have an integration center, it would not be able to interpret stimuli.
Concept 48.1 Neuron structure and organization reflect function in information transfer
Networks of neurons with intricate connections form nervous systems.

The neuron is the structural and functional unit of the nervous system.

Most of a neuron’s organelles, including its _____________________, are located in the
_________________________.

Two types of extensions arise from the cell body: numerous dendrites and a single axon.
o Dendrites (many) are _____________________________________________________________
_______________________________________________________________________________.
o The _______________ (single) is a _______________________________________ that sends
signals to neurons or effector cells.
o Axons are covered in myelin sheaths (insulated layer)  these are necessary because regular
cell membranes are made of lipids which are poor conductors of electrical currents, so the
myelin works better
o Each branched end of an axon transmits information to another cell at a junction called a
_________________________.
o At most synapses, information is passed from the transmitting neuron (the presynaptic cell)
to the receiving/target cell (the postsynaptic cell) by means of chemical messengers called
___________________________________.

Glia are supporting cells that _______________________________, insulate the _________________
___________________, and regulate the __________________________________________________.
o Some glia form the blood brain barrier  restricts passage of most substances into the brain
which controls the chemical environment of the CNS. Formed by TIGHT JUNCTIONS.
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o __________________________________ (in CNS) and ___________________________ (in
PNS) are glia that form _______________________ sheaths around the axons of neurons
(LOOK ON PAGE 1070 FOR THIS BULLET)
Information Processing Overview

The CNS (____________________________________________) includes the brain and spinal cord

The PNS (____________________________________________) includes neurons that bring information
into and out of the CNS; communicates motor and sensory signals between the CNS and the rest of the body

Cerebrospinal Fluid  made in the brain by filtering the blood; fills the space in the brain and spinal cord

There are three stages in the processing of information by nervous systems:
o Sensory input  sensory receptors take info from inside the body and the outside world and
convey it to integration centers
o Integration  carried out in the CNS – input is interpreted and sends a signal so body can
respond appropriately
o Motor output  signal is conducted from integration center to effector cells (ex. muscles or
glands) that carry out the response
Concept 48.2 Ion pumps and ion channels establish the resting potential of a neuron

Nerve signals are changes in voltage across the membrane due to movement of ions.

Because ions are unequally distributed between the interior of cells and the fluid that surrounds them,
the inside of a cell is ________________________________________ relative to the outside.
o _________________________________________________________________________________
____________________________________________________ is called the membrane potential.

The membrane potential of a neuron that is NOT transmitting signals is called the _________________.
The resting potential depends on ionic gradients that exist across the plasma membrane.

Concentration gradients of __________________ ions (____) and _______________ ions (______)
across the plasma membrane of a neuron play critical roles in the formation of the resting potential.

In mammalian neurons, the concentration of K+ is highest _____________________ the cell, while the
concentration of Na+ is highest ______________________.

These gradients are maintained by ___________________________________ in the plasma membrane.
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o The pumps use the energy of ATP hydrolysis to ____________________________________ 3Na+
_________________________ and 2K+ ____________________________ (so the inside is MORE
NEGATIVE and the outside is MORE POSITIVE.)
o Gated ion channels are specialized proteins that span the membrane, and allow ions to diffuse back
and forth across the membrane according to their respective gradients.
o Chemically-gated ion channels  respond to a chemical stimulus (ex. neurotransmitter)
o Voltage-gated ion channels  respond to a change in membrane potential
o Allows only ONE kind of ion to pass through
o Any resulting net movement of positive or negative charge generates a _____________________
______________________ or ___________________________ across the membrane.
o In other words: the action of the ion channels serves to “defeat” the action of the sodium
potassium pump (because the ions move with their concentration gradients, not against)
Concept 48.3 Action potentials are the signals conducted by axons
SEE FIGURES 48.10 p. 1067 and 48.11 p. 1068 – KNOW THESE!!!!!!
Gated ion channels are responsible for generating the signals of the nervous system.

Changes in membrane potential occur because neurons have gated ion channels, which open or close
____________________________________________________________.
o Opening K+ channels increases ______________________________________________________
______________________, increasing the net diffusion of K+ _____________________ the neuron.
o This INCREASE in the membrane potential, is called _____________________________________,
and makes the inside of the cell _______________________________________.
o SO  Open K+ channel; K+ flows out and causes the inside of the cell to become more
negative
o Opening Na+ channels increases the membrane’s permeability to Na+, increasing the net diffusion of
Na+ into the neuron
o This __________________________ in the membrane potential is called
___________________________, and makes the inside of the cell MORE POSITIVE.
o SO  Open a Na+ channel; Na+ flows in and causes the inside of the cell to become more
positive
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
These changes in membrane potential are called graded potentials because the _________________
________________________________________—either hyperpolarization or depolarization—varies
with the __________________________________________ (larger stimulus will open more channels).
Changes in membrane voltage accompany an action potential.

If a depolarization shifts the membrane potential sufficiently, the result is _______________________
______________________________________________________________ called an action potential.
o Unlike graded potentials, actions potentials are “all or nothing” depolarizations.
o (NOTE: hyperpolarizations do not cause action potentials!)
o Action potentials can therefore spread along axons, making them well suited for _______________
________________________________________________________________________________.

Action potentials arise because some __________________________________________________ are
voltage-gated ion channels, opening or closing ____________________________________________
______________________________________________________________.

Action potentials occur whenever a ____________________________ increases the _______________
______________________ to a particular value, called the ___________________________. The
threshold is usually 15-20 mV more positive than the resting potential.
The generation of action potentials.
THE BASICS:

Action potentials are “all or none”, NOT “graded” events

The neuron is at RESTING STATE

DEPOLARIZATION causes an action potential to start

When the THRESHOLD is reached, it causes an ACTION POTENTIAL
DEPOLARIZATION causes
an ACTION POTENTIAL.
In RESTING neurons, the
membrane potential is more
NEGATIVE than the
threshold potential.
(the nerve impulse)

After the neuron “fires”, REPOLARIZATION occurs

After a short UNDERSHOOT, the RESTING POTENTIAL is restored and the neuron may generate
another impulse
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THE MECHANISM OF ACTION POTENTIALS:
(Read along with Figure 48.11 p. 1068 referenced earlier)
1) _______________________________  the gated Na+ and K+ gates are _________________ (so
only the sodium potassium pump and ungated channels are moving ions to maintain resting potential)
2) ___________________________  triggered by an action potential which signals the opening of ____
gates, and sodium flows INTO the cell, making the inside more POSITIVE. A THRESHOLD is then
reached and the ___________________________________ can be triggered.
3) Rising phase of the action potential  depolarization continues with sodium gates open and
__________________________ gates closed until ACTION POTENTIAL occurs
4) Repolarization (falling phase of the action potential) _______ gates start to close and ________
gates open, allowing potassium to LEAVE the cell, making the inside more _______________________
5) _____________________________  sodium channels close but some potassium gates are still open
causing a lag in the time to restore _____________________________________.
a. During this brief refractory period, the neuron is unable to depolarize.
***It is the number of action potentials per second, not their amplitude, that codes for a stimulus intensity
in the nervous system***
Axon diameter and myelination affect conduction speed.

One factor that affects the speed at which action potentials are conducted along an axon is the diameter
of the axon: The larger the axon’s diameter, the faster the conduction.

Saltatory conduction  action potentials that jump from __________________________________
Concept 48.4 Neurons communicate with other cells at synapses
SEE FIGURE 48.16 p. 1072

In MOST cases, action potentials are not transmitted from neurons to other cells.
o However, information is transmitted at the synapse.

Synapses  unique cell junctions that control communication between a neuron and another cell; two
types:
o _____________________________ synapses  contain gap junctions that allow electrical
current to flow directly from cell to cell
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o Chemical synapses  involve the ________________________________________________
_____________________________________________________________________________;
most neurons are this type
o The presynaptic neuron synthesizes the neurotransmitter and packages it in membrane-bound
_____________________________________, which are stored in the neuron’s synaptic terminals.
THE MECHANISM OF CHEMICAL TRANSMISSION ACROSS A SYNAPSE:
(read along with Figure 48.16 p. 1072 referenced earlier)
1) An action potential DEPOLARIZES the PRESYNAPTIC membrane
2) The depolarization opens voltage gated channels, allowing ___________ to enter the cell (neuron).
3) Calcium causes __________________________________ to fuse with the pre-synaptic membrane
4) Neurotransmitters are released into the _____________________________
5) Neurotransmitters bind to the ________________________________ membrane
6) Ion channels open, allowing Na+ and K+ ions to enter the postsynaptic cell
***A series of events converts the electrical signal of the action potential arriving at the synaptic
terminal into a chemical signal that travels across the synapse, where it is converted back into an
electrical signal in the postsynaptic cell. (electrical signal  chemical signal  electrical signal)
Structure of a Chemical Synapse

Neurotransmitters (intracellular messengers) are held in the tip of the presynaptic axon.

Action potential releases these neurotransmitter molecules into the synapse (the action potential
depolarizes the membrane)

The postsynaptic membrane has special receptors for neurotransmitters
o Neurotransmitter binds  opens ion channels (chemically gated!) – postsynaptic membrane is either
hyperpolarized or depolarized (depending on receptor)
o Neurotransmitter is removed quickly (enzyme degradation)  therefore the effect is brief and
precise
o NOTE: nerve impulses can only transmit ONE way
Neural integration occurs at the cellular level.

At some synapses, the _______________________________________ ion channel is permeable to both
Na+ and K+. (Na+ IN and K+ OUT)
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o When the channel opens, the depolarization brings the membrane potential toward threshold, and it
is called an ____________________________________________________________ (_________).
o This DEPOLARIZES the plasma membrane (inside of the cell becomes more positive)

At other synapses, the ligand-gated ion channel is selectively permeable to only K+ or Cl-. (K+ OUT
and Cl- IN)
o When the channel opens, the hyperpolarization moves the membrane potential away from threshold,
and produces an __________________________________________________________ (_______).
o This HYPERPOLARIZES the membrane (the inside of the cell becomes more negative)
** BOTH EPSP AND IPSP ARE GRADED POTENTIALS

Summation  Several synaptic terminals working simultaneously on the same postsynaptic cell and
have an ADDITIVE effect
Temporal summation  Two ________________ produced in rapid succession at the _________
____________________
Spatial summation  Two __________________ produced nearly simultaneously by _________
____________________ on the same postsynaptic neuron
The same neurotransmitter can produce different effects on different types of cells.
o Same neurotransmitter can produce different effects on different types of cells (depends on the
receptors).
SEE FIGURE 48.2 p. 1075 for major neurotransmitters

Acetylcholine (most common) is vital for nervous system functions that include __________________
________________________, ____________________________________, and __________________.
o It can be either excitatory or inhibitory

Amino acid neurotransmitters are active in the vertebrate CNS and PNS.
o Glutamate is the most common __________________________________________.

The amino acid gamma-aminobutyric acid (GABA) is the neurotransmitter at most _______________
______________________________________________________________________.
o Binding of GABA to receptors in postsynaptic cells increases membrane permeability to ______,
resulting in an ______________ (no action potential).

Biogenic amines are synthesized from _________________________________.
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o _________________________________, made from ___________________, is an excitatory
neurotransmitter in the ______________________________ nervous system.
o Outside the nervous system, norepinephrine functions as a _____________________, as does the
related biogenic amine _____________________________.

The biogenic amines ________________________, made from tyrosine, and ____________________,
made from ___________________________, are released at many sites in the brain and affect
_________________, ______-__________, ______________________, and ____________________.
o Depression can be treated with drugs that increase the brain concentrations of biogenic amines:
Prozac enhances the effect of serotonin by inhibiting its reuptake after release.

Neuropeptides, _____________________________________________________, serve as
neurotransmitters that operate via metabotropic receptors.
o The neuropeptide substance P is an excitatory neurotransmitter that mediates ________ perception.

Neuropeptide endorphins function as _______________________________________, decreasing
__________________________________________.
o Endorphins are produced in the brain during times of _____________________________________.

Some neurons in vertebrates release dissolved gases like nitric oxide (NO) that act as local regulators.

Unlike other neurotransmitters, NO is not stored in ________________________________________
but is instead __________________________________________.

NO diffuses into ________________________________________________, produces a change, and is
_________________________________________ within a few seconds.
o In its target cells, NO works like many hormones, stimulating an enzyme _____________________
________________________________________________________________________________.

Vertebrates produce small amounts of CO as a neurotransmitter.

__________________________________________ (gas) is generated in certain populations of neurons
in the brain and PNS.
o In the brain, CO regulates the _______________________________________________________.
o In the PNS, it acts as an __________________________________________________ that
hyperpolarizes intestinal smooth muscle cells.
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