Chapter 4 Flashcards
The ______ potential is the difference in electrical charge between the inside and the
outside of a cell.
membrane
Although the size of the extracellular electrode is not critical, it is paramount that the
tip of the intracellular electrode be fine enough to
pierce the neural membrane without severely damaging
it. The intracellular electrodes are called _______;
their tips are less than one-thousandth of a millimeter in
diameter much too small to be seen by the naked eye.
microelectrodes
However, when
the tip of the intracellular electrode is inserted into a neuron, a steady potential of about ___ ______ is
recorded.
70 millivolts (mV)
This steady membrane potential of about 70 mV
is called the neuron s ____ _____. In its resting state,
with the 70 mV charge built up across its membrane, a
neuron is said to be polarized.
resting potential
Like all salts in solution, the salts in neural tissue separate into positively and
negatively charged particles called __________. The resting potential results from the fact that the ratio of negative to
positive charges is greater inside the neuron than outside.
ions
The first of the two homogenizing factors is _____
motion. The ions in neural tissue are in constant random
motion, and particles in random motion tend to become
evenly distributed because they are more likely to move
down their _____ ______ than up them; that is,
they are more likely to move from areas of high concentration to areas of low concentration than vice versa.
random, concentration gradients
The
second factor that promotes the even distribution of ions
is ______ ______. Any accumulation of charges, positive or negative, in one area tends to be dispersed by the
repulsion among the like charges in the vicinity and the
attraction of opposite charges concentrated elsewhere.
electrostatic pressure
The situation turned out to be different for the K
*
ions.
Hodgkin and Huxley calculated that __ mV of electrostatic pressure would be required to keep intracellular K
*
ions from moving down their concentration gradient and
leaving the neuron some 20 mV more than the actual
resting potential.
90 mV
It was subsequently discovered that the transport of
Na
*
ions out of neurons and the transport of K
*
ions into
them are not independent processes. Such ion transport is
performed by energy-consuming mechanisms in the cell
membrane that continually exchange three Na
*
ions inside the neuron for two K
*
ions outside. These transporters are commonly referred to as ___ ____ ____
sodium potassium
pumps.
Since the discovery of sodium potassium pumps, several other classes of ______ (mechanisms in the
membrane of a cell that actively transport ions or molecules across the membrane) have been discovered (e.g.,
Tzingounis & Wadiche, 2007). You will encounter more of
them later in this chapter
transporters
When neurotransmitter
molecules bind to postsynaptic receptors, they typically
have one of two effects, depending on the structure of
both the neurotransmitter and the receptor in question.
They may _______ the receptive membrane (decrease
the resting membrane potential, from 70 to 67 mV,
for example) or they may ______ it (increase the
resting membrane potential, from 70 to 72 mV, for
example).
depolarize, hyperpolarize
Postsynaptic depolarizations are called ___ ___ ____ (EPSPs) because, as you will
soon learn, they increase the likelihood that the neuron
will fire.
excitatory
postsynaptic potentials
Postsynaptic hyperpolarizations are called
____ ____ ____ (IPSPs) because they
decrease the likelihood that the neuron will fire.
inhibitory postsynaptic potentials
Both
EPSPs and IPSPs are ____ _____. This means that
the amplitudes of EPSPs and IPSPs are proportional to
the intensity of the signals that elicit them: Weak signals
elicit small postsynaptic potentials, and strong signals
elicit large ones.
graded responses
The receptive areas of
most neurons are covered with thousands of synapses,
and whether or not a neuron fires is determined by the
net effect of their activity. More specifically, whether or
not a neuron fires depends on the balance between the
excitatory and inhibitory signals reaching its axon. Until
recently, it was believed that action potentials were generated at the ____ _____, but they are
actually generated in the adjacent section of the axon
axon hillock (the conical structure at the junction between the cell body and the axon)
If the sum of the depolarizations
and hyperpolarizations reaching the section of the axon
adjacent to the axon hillock at any time is sufficient to
depolarize the membrane to a level referred to as its
____ of ____ usually about __ mV an action potential is generated near the axon hillock.
threshold of excitation, 65 mv
The
____ ____ is a massive but momentary lasting for 1 millisecond reversal of the membrane potential from about 70 to about *50 mV.
action potential (AP)
Unlike postsynaptic
potentials, action potentials are not graded responses;
their magnitude is not related in any way to the intensity
of the stimuli that elicit them. To the contrary, they are
___ ___ ___ _____; that is, they either occur to their
full extent or do not occur at all. See Figure 4.3 for an illustration of an EPSP, an IPSP, and an AP.
all-or-none responses
In effect, each multipolar neuron adds together all the
graded excitatory and inhibitory postsynaptic potentials
reaching its axon and decides to fire or not to fire on the
basis of their sum. Adding or combining a number of
individual signals into one overall signal is called
_______. Neurons integrate incoming signals in two
ways: over space and over time.
integration
Figure 4.4 shows the three possible combinations of
____ ______. It shows how local EPSPs that are produced simultaneously on different parts of the receptive
membrane sum to form a greater EPSP, how simultaneous
IPSPs sum to form a greater IPSP, and how simultaneous
EPSPs and IPSPs sum to cancel each other out.
spatial summation
Figure 4.5 on page 82 illustrates ____ ______. It shows how postsynaptic potentials produced in
rapid succession at the same synapse sum to form a
greater signal. The reason that stimulations of a neuron
can add together over time is that the postsynaptic potentials they produce often outlast them. Thus, if a particular synapse is activated and then activated again
before the original postsynaptic potential has completely
dissipated, the effect of the second stimulus will be superimposed on the lingering postsynaptic potential produced by the first.
temporal summation
- Roberto Garcia d Orta referred to himself as a great
lizard frozen in a dark, cold, strange world. He suffered from ______. - Tremor-at-rest is a symptom of ______.
- Microelectrodes are required to record a neuron s
resting ______. - The ______ is about 70 mV.
- In its resting state, a neuron is said to be ______.
- Two factors promote the even distribution of ions
across neural membranes: ______ and electrostatic
pressure. - In the resting state, there is a greater concentration of
Na
*
ions ______ the neural membrane than ______
the neural membrane. - Natrium is Latin for ______.
- Ions pass through neural membranes via specialized
pores called ______. - From their calculations, Hodgkin and Huxley inferred
the existence of ______ in neural membranes. - Neurotransmitters typically have one of two effects on
postsynaptic neurons: They either depolarize them or
______ them. - Postsynaptic depolarizations are commonly referred to
in their abbreviated form: ______. - Action potentials are generated near, but not at, the
______. - An action potential is elicited when the depolarization
of the neuron reaches the ______. - Unlike postsynaptic potentials, which are graded, action potentials are ______ responses.
- Neurons integrate postsynaptic potentials in two ways:
through spatial summation and through ______
summation.
can Your Brain answers: (1) Parkinson s disease, (2) Parkinson s disease,
(3) potential, (4) resting potential, (5) polarized, (6) random motion,
(7) outside, inside, (8) sodium, (9) ion channels, (10) sodium potassium
pumps, (11) hyperpolarize, (12) EPSPs, (13) axon hillock, (14) threshold of
excitation, (15) all-or-none, (16) temporal
How are action potentials produced, and how are they conducted along the axon? The answer to both questions is basically the same: through the action of ____ ____ ____ ____ ion channels that open or close in response
to changes in the level of the membrane potential
voltage-activated
ion channels
There is a brief period of about 1 to 2 milliseconds after
the initiation of an action potential during which it is impossible to elicit a second one. This period is called the
____ ____ ____. The ____ _____ ____ is followed by the ____ ____ ____ the
period during which it is possible to fire the neuron again,
but only by applying higher-than-normal levels of stimulation. The end of the ____ ____ ____ is the
point at which the amount of stimulation necessary to
fire a neuron returns to baseline.
absolute refractory period, relative refractory period
If electrical stimulation of sufficient intensity is
applied to the terminal end of an axon, an action potential will be generated and will travel along the axon
back to the cell body; this is called ___ ____
antidromic conduction.
Axonal conduction in the natural direction
from cell body to terminal
buttons is called ___ ____
orthodromic
conduction.
In myelinated axons, ions can pass through the axonal membrane only at the \_\_\_ \_\_\_ \_\_\_ the gaps between adjacent myelin segments.
nodes of Ranvier
. There is, of
course, a slight delay at each node of Ranvier while the action potential is actively generated, but conduction is still
much _____ in myelinated axons than in unmyelinated
axons, in which passive conduction plays a less prominent
role
faster
The transmission of action
potentials in myelinated axons is called ____ _____ (saltare means to skip or jump ). Given the important role of myelin in neural conduction, it is hardly
surprising that the neurodegenerative diseases (diseases
that damage the nervous system) that attack myelin have
devastating effects on neural activity and behavior
saltatory conduction
Conduction is faster in large-diameter
axons, and as you have just learned it is faster in those
that are myelinated. Mammalian motor neurons (neurons
that synapse on skeletal muscles) are large and myelinated; thus, some can conduct at speeds of ___ meters per secound. In contrast, small,
unmyelinated axons conduct action potentials at about
1 meter per second.
100 meters per
second (about 224 miles per hour)
Many cerebral neurons fire continually even when
they receive no input
Action potentials of all motor neurons are the same,
but action potentials of different classes of cerebral
neurons vary greatly in
duration, amplitude, and frequency
The dendrites of some cerebral neurons can actively
conduct
action potentials
Clearly, cerebral neurons are far more complex than
motor neurons, which have traditionally been the focus of
________ research, and thus, results of studies of
motor neurons should be applied to the brain with caution.
neurophysiological
ome communication among neurons occurs across
synapses such as the one illustrated in Figure 4.8. Neurotransmitter molecules are released from buttons into
synaptic clefts, where they induce EPSPs or IPSPs in other
neurons by binding to receptors on their postsynaptic
membranes. The synapses featured in Figure 4.8 are
axodendritic synapses synapses of axon terminal buttons
on ______. Notice that many axodendritic synapses terminate on ______ spines (nodules of various shapes that are
located on the surfaces of many ______) see Figure 3.31
on page 73. Also common are axosomatic synapses
synapses of axon terminal buttons on somas (cell bodies).
dendritic, dendrites
The advantage of presynaptic facilitation
and inhibition (compared to EPSPs and IPSPs, which
you have already learned about) is that they can
they can selectively influence one particular
synapse rather than the entire
presynaptic neuron.
The synapses depicted in Figure 4.9 are \_\_\_\_ \_\_\_\_\_ synapses at which the site of neurotransmitter release and the site of neurotransmitter reception are in close proximity. This is a common arrangement, but there are also many nondirected synapses in the mammalian nervous system.
directed synapses
\_\_\_\_\_ synapses are synapses at which the site of release is at some distance from the site of reception. One type of \_\_\_\_\_\_\_ synapse is depicted in Figure 4.10
Nondirected
There are two basic categories
of neurotransmitter molecules: small and large. The
small neurotransmitters are
of several types; large neurotransmitters are all ______ . ______ are short
amino acid chains comprising
between 3 and 36 amino acids;
in effect, they are short proteins.
neuropeptides
Small-molecule neurotransmitters are typically synthesized
in the cytoplasm of the terminal
button and packaged in ______ by the button’s _______
synaptic
vesicles
Golgi
complex
It was once believed that each neuron synthesizes
and releases only one neurotransmitter, but it has been
clear for some time that many neurons contain two
neurotransmitters a situation that is generally referred
to as _______.
coexistence
________ is the process of neurotransmitter release is
illustrated in Figure 4.11 (see Schweizer & Ryan, 2006).
When a neuron is at rest, synaptic vesicles that contain
small-molecule neurotransmitters tend to congregate
near sections of the presynaptic membrane that are particularly rich in voltage-activated calcium channels (see
Rizzoli & Betz, 2004, 2005). When stimulated by action
potentials, these channels open, and Ca
2
ions enter the
button. The entry of the Ca
2
ions causes synaptic vesicles to fuse with the presynaptic membrane and empty
their contents into the synaptic cleft (see Collin, Marty, &
Llano, 2005; Schneggenburger & Neher, 2005).
Exocytosis
Once released, neurotransmitter molecules produce signals in postsynaptic neurons
by binding to ______ in the
postsynaptic membrane. Each _____ is a protein that
contains binding sites for only particular neurotransmitters;
thus, a neurotransmitter can influence only those cells that
have receptors for it.
receptors
Any molecule that binds to another is
referred to as its _____, and a neurotransmitter is thus said to be a ______ of its receptor.
ligand
The different types of receptors to which a particular neurotransmitter
can bind are called the receptor _____
for that neurotransmitter.
subtypes
Ionotropic receptors are those receptors
that are associated with ligand-activated ion
channels; ______ receptors are those
receptors that are associated with signal proteins and _ proteins (guanosine-triphosphate sensitive proteins)
metabotropic, G
The subunit may move along the inside
surface of the membrane and bind to a nearby ion channel, thereby inducing an EPSP or IPSP; or it may trigger
the synthesis of a chemical called a ____ _____
second messenger
______ are metabotropic
receptors that have two unconventional characteristics:
They bind to their neuron s own neurotransmitter molecules; and they are located on the presynaptic, rather
than the postsynaptic, membrane.
Autoreceptors
These two message-terminating mechanisms are
____ by transporters and ____ ____
reuptake
enzymatic degradation
In contrast, other neurotransmitters are degraded
(broken apart) in the synapse by the action of
_____ proteins that stimulate or inhibit biochemical reactions without being affected by them. For example, acetylcholine, one of the few neurotransmitters for
which enzymatic degradation is the main mechanism of
synaptic deactivation, is broken down by the enzyme
_______.
enzymes, acetylcholinesterase
Interest in gap junctions has recently
been rekindled. ____ _____ are narrow spaces between
adjacent neurons that are bridged by fine tubular channels, called connexins, that contain cytoplasm. Consequently, the cytoplasm of the two neurons is continuous,
allowing electrical signals and small molecules to pass
from one neuron to the next
Gap junctions
The neurotransmitters in the vast majority of fast-acting,
directed synapses in the central nervous system are amino
acids the molecular building blocks of proteins. The four
most widely studied amino acid neurotransmitters are
glutamate, aspartate, glycine, and gamma-aminobutyric
acid (GABA)
Glutamate is
the most prevalent excitatory neurotransmitter in the
mammalian central nervous system. GABA is the most
prevalent inhibitory neurotransmitter
Monoamines are another class of small-molecule neurotransmitters. Each is synthesized from a single amino
acid hence the name monoamine (one amine).
____ ____ are slightly larger than
amino acid neurotransmitters, and their effects tend to be
more diffuse
Monoamine neurotransmitters
There are four monoamine neurotransmitters:
dopamine, epinephrine, norepinephrine, and serotonin.
There are four monoamine neurotransmitters:
dopamine, epinephrine, norepinephrine, and serotonin.
They are subdivided into two groups,
catecholamines and
indolamines, on the basis of their structures.
One class of
unconventional neurotransmitters, the soluble-gas
neurotransmitters, includes nitric oxide and
carbon
monoxide
_________ are neurotransmitters that are adrenergic. There are two reasons for this naming. One is
that epinephrine and norepinephrine used to be called
adrenaline and noradrenaline, respectively, by many scientists, until a drug company registered Adrenalin as a
brand name. The other reason will become apparent to
you if you try to say norepinephrinergic.
Endocannabinoids
_______ is the neurotransmitter at neuromuscular
junctions, at many of the synapses in the autonomic nervous system, and at synapses in several parts of the central
nervous system.
Acetylcholine
So far, two endocannabinoids have been discovered (Van Sickle et al., 2005). The
most widely studied is _____
anandamide
It is usual to loosely group ____ _____
into five categories. Three of these categories acknowledge that neuropeptides often function in multiple capacities, not just as neurotransmitters: One category
(____ _____) contains neuropeptides that were
first identified as hormones released by the pituitary
neuropeptide transmitters, pituitary peptides
a
second category (_____ peptides) contains neuropeptides that were first identified as hormones released
by the hypothalamus; and a third category (____ ___
peptides) contains neuropeptides that were first discovered in the gut. The fourth category (opioid peptides) contains
neuropeptides that are similar in
structure to the active ingredients
of opium, and the fifth
(miscellaneous peptides) is a
catch-all category that contains
all of the neuropeptide transmitters that do not fit into one of the
other four categories.
hypothalamic, brain gut
Drugs have two fundamentally different kinds of effects on synaptic transmission: They facilitate it or they
inhibit it. Drugs that facilitate the effects of a particular
neurotransmitter are said to be _____ of that neurotransmitter. Drugs that inhibit
the effects of a particular neurotransmitter are said to be its
______.
agonists, antagonists
For example, some agonists of a particular neurotransmitter
bind to postsynaptic receptors and activate them, whereas
some antagonistic drugs, called ____ _____, bind to
postsynaptic receptors without activating them and, in so
doing, block the access of the usual neurotransmitter.
receptor blockers
______,
which is the main active ingredient of belladonna, is a receptor blocker that exerts its antagonist effect by binding
to muscarinic receptors, thereby blocking the effects of
acetylcholine on them.
Atropine
_____, a neurotoxin released by a bacterium often found in spoiled food, is
another nicotinic antagonist, but its
mechanism of action is different: It
blocks the release of acetylcholine at neuromuscular junctions and is thus a deadly poison. However,
injected in minute doses at specific sites, it has applications
Botox (short for Botulinium toxin)
Several families of ___ (occurring naturally
within the body) opioids have been discovered. First discovered were the ____ (meaning in the head ). Another major family of endogenous opioids are the
_____ (a contraction of endogenous morphine ). All
endogenous opioid neurotransmitters are neuropeptides,
and their receptors are metabotropic.
endogenous, enkephalins, endorphins
