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Chapter 48: Neurons, Synapses, and Signaling (pg1047-1063)
1. Communication of neurons is categorized in 2 types of signals: long distance and short distance.
- Long distance: transmission electrical signals from neuron to neuron over a long distance throughout the body.
- Short distance: the transmission of chemical signals from neuron to neuron between nearby cells.
- Signals are interpreted by a small cluster of neurons called ganglia or a larger group of neurons organized into a
brain.
2. 3 main functions:
-- Sensory input: the sensory neurons transmit either external information from the sensors(eyes, nose, ears) or
internal information ( blood pressure, muscle tension) to the brain and ganglia.
-- Integration: uses interneurons, located in the brain and the spinal cord, to receive the signals from sensory
neurons, analyze them then send out the proper response to the motor neurons .
-- Motor output: when a signal is sent from the brain or ganglia via nerves which are bundles of neurons extended
throughout the body. The motor neurons will then carry out the transmitted signal.
3. Parts of neurons.
--cell body: contains the neuron’s nucleus, organelles and extensions.
-- dendrites: branched extensions that receive signals from other neurons.
-- axons: extensions that transmit signals to other neurons. Usually they’re much longer than dendrites.
-- axon hillock: region where the axon is connected to the cell body.
-- Synapse: is the region/ junction where chemical signals are transmitted between 2 neurons.
-- The synaptic terminal: located at the very tip of the axon allows the axon to release chemical signals aka
neurontransmitters.
- The transmitting cells is called presynaptic and the receiving cell is the postsynaptic cell.
- Glia: the spinal cord of neurons, are vertebrates that nourish neurons, protect axons, or regulate extracellular fluids.
4. Membrane potential results from the difference in electrical charge across the plasma membrane.
Resting potential is membrane potential of a neuron that isn’t transmitting any signal. It usually ranges between -60
and -80mV(millivolts).
5. A neuron‘s membrane potential and resting potential are heavily dependent on the concentration of
K+(Potassium) and Na+(Sodium), both positively charged. Across a neuron’s plasma membrane, the
concentration of K+ is relatively higher in the inside the cell compared to outside. The opposite is true for
Na+(Sodium). To maintain a neuron’s resting potential which ranges around -60 to -80 mV, these ions diffuses in
and out of the cell through specific ion channels that are selectively permeable meaning K+ can only diffuse across
the membrane through a K+ channel proteins and a Na+ ions only diffuse through Na+ channel..
6. Equilibrium potential is an ion’s specific voltage(electrical charge) when the membrane is at equilibrium. The
equilibrium potential of K+(EK+ ) ranges about -90mV where as the equilibrium potential of Na+ (E Na+) ranges
about +62 mV. The sodium-potassium pump is used so each ions can reach and maintain its own equilibrium
potential . As a result of a larger quantity of K+ ions are in the cell, K+ ion channels will open to allow K+ ions to
diffuse out of the cell, thus making the inside of the cell more negative. Compared to K+ ion channels, fewer Na+
channels are opened to allow the Na+ ions to diffuse inside the cell. Because of more K+ ions are diffusing outside
of the cell compared to Na+ ions diffusing in, the neuron’s resting potential ranges around -60mV to -80 mV, closer
to the ENa+ than EK+ .
7. - Gated ion channels- When a neuron is inactive, un-gated channels are used to reach the neuron’s resting
potential. On the other hand, when a neuron is active, the presence of a stimuli will trigger the gated ion channels to
open or close thus the membrane’s permeability to some ions and its membrane potential are altered. If a stimuli
triggers the opening of gated K+ channel, K+ ions will diffuse out of the cell thus making the inside of the cell more
negative. This outflow of positive ions or inflow of negative ions that make the cell more negative is known as
hyperpolarization. On the other hand, depolarization occurs when a stimuli triggers the opening of Na+ ion
channels or ion channels that would turn the inside of the cell more positive. The diffusion of Na+ ions from outside
to inside the cell will turn the cell’s overall charge away from its membrane potential and become more positive.
Since the change of the membrane potential heavily depends on the strength of the stimuli, both hyperpolarization
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and depolarization are considered as graded potentials.
- Voltage-gated ion channels- Most gated ion channels are voltage-gated channels; that is, they open or close to a
change in the membrane potential. If depolarization opens Na+ voltage-gated ion channels then the inflow of Na+
ions will start a chain reaction that leads to the opening of more Na+ gated ions channels which in turn increase the
depolarization of the cell.
8. Action potentials are the nerve impulses or signals that carry information along an axon. It results from a
massive change in membrane potential. As mentioned in number 7, if a stimulus causes a rapid opening of the
voltage-gated sodium channels, then the cell will be depolarized because of the inflow of the Na+ ions. If this
depolarization increases the membrane voltage to a certain value, called the threshold, action potentials will occur.
9. The course of action potentials may involve both sodium and potassium voltage-gated channels in a series of
stages.
-Stage 1: resting potential- At resting potential, voltage-gated sodium channels and most of voltage-gated
potassium channels are closed while some un-gated potassium channels are open.
-Stage 2: threshold- When a stimulus depolarizes the cell, like a chain reaction, more voltage-gated sodium
channels will open and allow even more Na+ to enter and depolarize the cell. The depolarization from inflow of
Na+ allow the cell to reach the threshold which triggers an action potential.
-Stage 3: rising phase- Once threshold is reached, most sodium channels will be opened while potassium channels
are closed which result in further depolarization of the cell, bringing the membrane potential closer to E Na+ .
-Stage 4: falling phase- Since the cell favors its membrane potential to be closer to EK+ than ENa+ , the voltage-gated
sodium channels will be inactivated while the voltage-gated potassium channels open to quickly bring the cell’s
membrane potential closer to EK+.
-Stage 5: undershoot- the voltage-gated sodium channels will be closed while some potassium channels will be
opened to allow the cell to return to its resting potential.
10. During undershoot, the stage when the cell returns to its resting potential, the presence of a second stimulus will
not trigger any reaction from the cell. This period between the first and second action potential is known as the
refractory period.
11. Action potential moves in one direction only, that is, away from the axon hillock and toward the synaptic
terminals, where signals will be transmitted to another neuron. Think of it as a relay and the action potential is
the baton. Let’s name the region where the action potential is initiated(usually at the axon hillock) region A. The
inflow of Na+ during the rising phase of region A creates an electrical current that causes depolarization of the
neighboring region, region B. The depolarization of region B will soon be large enough for it to reach the threshold,
causing the original potential action to be reinitiated in there, similar to the passing of the baton. By then, region B
will be its rising phase and it will cause depolarization of region C and the same process happens in following
neighboring regions until the potential action reaches the synaptic terminal. While region B is at its rising phase,
region A will be at its falling and undershoot stage and allow the membrane to return to its resting potential.
12. There are several factors that affect the speed at which the action potentials are conducted. One factor that
affect the speed of the conduction of an axon is its wideness. The wider the axon is, the less resistance it will have
and the action potentials travel more rapidly through the axon. Second factor that speeds up the conduction of the
action potential are the myelin sheaths which are layers of membrane that wraps around the axon. Since
membranes are mostly lipids, the sheaths are poor conductor of electricity which allow it provide an electrical
insulation for the axon. In the peripheral nervous system (PNS) these sheaths are called oligodendrocytes. In the
central nervous system (CNS) they’re known as Schwann cells. The exposed axon gaps in between the
oligodendrocytes and the Schwann cells where depolarization takes place are known as nodes of Ranvier. Action
potentials are spread inside the axon but the depolarization only takes place from node to node.
13. Electrical synapses are direct transmission of electrical signals from one neuron to the next using gap
junctions. Most synapses, however, are chemical synapses that require the release of neurotransmitter from the
presynaptic membrane which are from the cells that do the transmitting of chemical signals,
to the postsynaptic membrane which are from cells that receive the signals. The presynaptic neurons synthesize and
pack the neurotransmitter into synaptic vesicles. When the action potential finally reaches the synaptic terminal,
depolarization causes voltage-gated Ca2+ channels to open and the diffusion of Ca 2+ causes the synaptic vesicle to
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fuse with the terminal membrane which results in the release of neurotransmitters to the postsynaptic cells.
14. The postsynaptic cells contain ligand-gated ion channels that allow the binding of transmitted
neurotransmitters. The binding of neurotransmitters may cause the opening of certain ion channels that will change
the postsynaptic membrane potential. If the neurotransmitters cause the K+ and Na+ ion channels to open and
depolarization takes place then they’re called excitatory postsynaptic potentials(EPSPs) because the inside of the
cell will be more positive which will bring the membrane potential closer toward threshold. If the neurotransmitters
happen to open K+ or CL- ion channels then they are inhibitory postsynaptic potentials (IPSPs) because they
move the membrane potential further away from threshold by making it more negative on the inside.
15. Major neurotransmitters include acetylcholine, biogenic amines, amino acids, neuropathies, and gases.
Each neurotransmitter has dozens of distinguished receptors that can have different effects on the postsynaptic cell.
16. Acetylcholine is the most common neurotransmitter in invertebrate and vertebrates alike. It has both excitatory
and inhibitory effects depending on which receptor it binds on, but most often are the receptors of ligand-gated
channels in muscle cells. It is most often released in the gap between the neuron and a muscle cell where it
stimulates muscle cells to contract. Acetylcholine is inactivated by an enzyme named acetylcholinesterase that
hydrolyzes the neurotransmitter in the synaptic cleft.
- Biogenic Amines- neurotransmitters that come from amino acids. Serotonin comes from tryptophan and
catecholamines come from tyrosine. Dopamine, one catecholamine amino acid acts as a neurotransmitter while
others such as epinephrine and norepinephrine act as neurotransmitters and hormones. Biogenic amines are
involved in mood regulation. Dopamine and serotonin affect sleep, mood, attention, and learning. They also have
an important role in treating disorders related to the brain such as depression and Parkinson’s disease, when the
brain lacks dopamine. Among these, norepinephrine is the most prominent because of its role of generating EPSPs
in the autonomomic nervous system.
17. Along with epinephrine and nor epinephrine, there are 3 other amino acids, gamma-amino butyric(GABA),
glutamate, and glycine that also act as neurotransmitters. In the brain, GABA as an inhibitory effect, glutamate as
an excitatory while glycine is inhibitory in parts of the central nervous system outside the brain.
- Neuropeptides are short chains of amino acids that serve as neurotransmitters through a signal transduction
pathway. The neuropeptide substance P is an excitatory neurotransmitter that in involved in our perception of pain
while another neuropeptide, endorphin, which is released during physical or emotion stress, decreases both
emotional and physical pain perceptions.
18. Gases- both nitric oxide(NO) and carbon monoxide(CO) act as local regulators in the cell.
- Nitric Oxide is a dissolved gas is not stored in synaptic vesicles but is made on demand and after it acts upon its
target cell, it is immediately broken down. In many of its target cells, NO triggers an enzyme that relaxes smooth
muscle cells or stimulates an enzyme to synthesize a second messenger.
-Carbon Monoxide- large consumption of carbon monoxide can be deadly but the body produces a small amount as
neurotransmitters. It affects the release of hypothalamic hormones and acts as an inhibitory neurotransmitter in
intestinal smooth muscle cells.
Key Vocabularies
 Sensory receptors: an organ or structure within the cell that responds to a specific stimuli from the organism’s
internal or extern anal environment.
 Central nervous system (CNS) includes the brain and longitudinal nerve cord.
 Neurons are nerve cells that transfer information within the body.
 cell body: contains the neuron’s nucleus, organelles and extensions.
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dendrites: branched extensions that receive signals from other neurons.
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axons: extensions that transmit signals to other neurons. Usually they’re much longer than dendrites.
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axon hillock: region where the axon is connected to the cell body.
 Myelin sheath are produced by two types of glia in the CNS and PNS that wraps around the axon to create a
create an electrical insulator membrane around the axon that speeds up the conduction of the action potential.
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Synapse: is the region/ junction where chemical signals are transmitted between 2 neurons.
The synaptic terminal: located at the very tip of the axon allows the axon to release chemical signals aka
neurontransmitters.
Presynaptic cells are cells that transmit chemical signals via neurotransmitters.
postsynaptic cells are the cells that receive the signal through receptors if neurotransmitters.
Glia: the spinal cord of neurons, are vertebrates that nourish neurons, protect axons, or regulate extra cellular
fluids.
Sensory neurons transmit information from external and internal stimuli to the brain or ganglia to be analyzed.
Motor neurons transmit signals sent from the brain to cells for muscle or gland activity.
Interneurons a nerve cell within the CNS that forms synapses with sensory and/or motor neurons and integrates
sensory input and motor output.
Ganglia a cluster of nerve cell bodies in the CNS.
Oligodendrocytes are a type of glial cell that forms insulating myelin sheaths around the axons of neurons in the
CNS.
Schwann cells are a type of glial cell that forms insulating myelin sheaths around the axons of neuurons in the
PNS.
Membrane potential is the difference in electrical charge across a cell’s plasma membrane due to the differential
distribution of ions.
Gated ion channels are protein channels which its opening and closing are triggered by the binding of a specific
ion.
Voltage-gated ion channel is a specialized ion channel that opens or closes in response to changes in the
membrane potential.
hyperpolarization: a change in a cell’s membrane potential such that the inside of the membrane becomes more
negative relative to the outside, Hyperpolarization reduces the chance that a neuron will transmit a nerve
impulse.
Depolarization: a change in the cell’s membrane potential such that the inside of the membrane is made less
negative relative to the outside.
Threshold potential is the potential that an excitable cell membrane must reach before an action potential can be
initiated.
Action potential is the rapid change in the membrane potential of an excitable cell, caused by stimulustriggered, selective opening and closing of voltage-sensitive gates in sodium and potassium ion channels.
Refractory period is the time period between the first and second action potential when the cell uses the time to
return to its resting potential. During this time, a second stimulus will result in no action.
Salutatory conduction: rapid transmission of a nerve impulse along an axon, resulting from the action potential
“jumping” from one node of Ranvier to another, skipping the myelin-sheathed regions of the membrane.
Synaptic cleft is the region between the synapse terminal of the presynaptic cell and the receptor of the
postsynaptic cell. In this region, the neurotransmitters are transmitted.
Synaptic vesicle is a membranous sac containing neurotransmitter molecules at the tip of an axon.
Neurotransmitter is a molecule that is released from the synaptic terminal of a neuron at a chemical synapse,
diffuses across the synaptic cleft, and binds to the postsynaptic cell, triggering a response.
Presynaptic membrane: the transmitting cell at a synapse. The neurotransmitters are released when the vesicle
fuses with the membrane.
Postsynaptic membrane: the target cell at a synapse, contains receptors of the 0neurotransmitters.
Excitatory postsynaptic potential (EPSP) is an electrical change(depolarization)in the membrane of a
postsynaptic cell caused by the binding of an excitatory neurotransmitter from a presynaptic cell to a
postsynaptic receptor.
Inhibitory postsynaptic (IPSP) is an electrical change(hyperpolarization) in the membrane of a postsynaptic
neuron caused by the binding of an inhibitory neurotransmitter from a presynaptic cell to a postsynaptic
receptor. It makes it more difficult for the postsynaptic cell to generate an action potential.
Acetylcholine is a neurotransmitter that has both excitatory and inhibitory effects depending on which receptor
it binds on, but most often are the receptors of ligand-gated channels in muscle cells. It is most often released in
the gap between the neuron and a muscle cell where it stimulates muscle cells to contract. Acetylcholine is
inactivated by an enzyme named acetylcholinesterase that hydrolyzes the neurotransmitter in the synaptic cleft.
Biogenic amines are neurotransmitters derived from amino acids.
Epinephrine is an amino acid that acts as a neurotransmitter and a hormone.
norepinephrine is the most prominent biogenic amine because of its role of generating EPSPs in the autonomic
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nervous system.
Dopamine is a neurotransmitter that is a catecholamine, like epinephrine and norepinephrine.
Serotonin is a neurotransmitter synthesized from the amino acid tryptophan, that functions in the CNS.
Gamma amino butyric acid (GABA) is an amino acid that functions as a CNS neurotransmitter in the CNS of
vertebrates.
Glycine is an amino acid that acts as an inhibitory synapses in part of the CNS outside the brain.
Glutamate is an amino acid that functions as a neurotransmitter in the CNS.
Neuropeptides: a relatively short chain of amino acids that serve as a neurotransmitter.
Substance P is neuropeptide that is a key excitatory neurotransmitter that mediates the perception of pain.
Endorphin is any of several hormones produced in the brain and anterior pituitary that inhibits pain perceptions.