Chapter 5: Communication and Adaptation of Neurons Flashcards
Acetylcholine (ACh)
first neurotransmitter discovered in the PNS and CNS, activates skeletal muscles in the SNS, either excites or inhibits organs in the ANS
Epinephrine (EP)
chemical messenger that acts as a neurotransmitter in the CNS and as a hormone to mobilize the body for flight or fight during times of stress, also known as adrenaline
Norepinephrine (NE)
neurotransmitter that accelerates heart rate in mammals, found in the brain and in the sympathic division of the ANS, also known as noradrenaline
Neurotransmitters
chemical with an excitatory or inhibitory effect when released by a neuron onto a target
Dopamine (DA)
amine neurotransmitter involved in coordinating movement, attention, learning, and reinforcing behaviors
Synaptic Vesicles
membranous compartment that encloses a fixed number (called a quantum) or neurotransmitter molecules
Synaptic Cleft
gap separating the neuronal presynaptic membrane from the postsynaptic membrane
Tripartite Synapse
functional integration and physical proximity of the presynaptic membrane, postsynaptic membrane, and their intimate association with surrounding astrocytes
Chemical Synapse
junction at which messenger molecules are released when stimulated by an action potential
Presynaptic Membrane
axon terminal membrane on the transmitter, or output, side of a synapse
Postsynaptic Membrane
membrane on the transmitter, or input, side of a synapse
Storage Granule
membranous compartment that holds several vesicles containing a neurotransmitter
Anterograde Synaptic Transmission
process that occurs when a neurotransmitter is released from a presynaptic neuron and binds to a receptor on the postsynaptic neuron
What are the five steps of neurotransmission?
- The neurotransmitter is synthesized somewhere inside the neuron
- It is packaged and stored with vesicles at the axon terminal
- It is transported to the presynaptic membrane and released into the cleft in response to an action potential
- It binds to and activates receptors on the postsynaptic membrane
- It is degraded or removed, so it will not continue to interact with a receptor and work indefinitely
Transporter
protein molecule that pumps substances across a membrane
Transmitter-Activated Receptor
protein that has a binding site for a specific neurotransmitter and is embedded in the membrane of a cell
Ionotropic Receptor
embedded membrane protein, acts as a binding site for a neurotransmitter and a pore that regulates ion flow to directly and rapidly change membrane voltage
Metabotropic Receptor
embedded membrane protein with a binding site for a neurotransmitter linked to a G protein, can affect other receptors or act with second messengers to affect other cellular processes, including opening a pore
Autoreceptor
self-receptor in a neuronal membrane, that is, it responds to the same transmitter released by the neuron, part of a negative feedback loop allowing the neuron to adjust its output
Quantum
number of neurotransmitter molecules, equivalent to the content of a single synaptic vesicle, that produces a just-observable change in postsynaptic electric potential
Gap Junction
area of contact between adjacent cells in which connexin proteins in each cell form connecting hemichannels which, when open, allow ions to pass between the two cells, also called an electrical synapse
What are the four criteria for identifying neurotransmitters?
- The transmitter must be synthesized in the neuron or otherwise be present in it
- When neuron is active, the transmitter must be released and produce a response to some target
- The same response must be obtained when the transmitter is experimentally placed on the target
- A mechanism must exist for removing the transmitter from its site of action after its work is done
Reuptake
inactivation of a neurotransmitter when membrane transporter proteins bring the transmitter back into the presynaptic axon terminal for reuse
Small-Molecule Transmitter
quick-acting neurotransmitter synthesized in the axon terminal from products derived from the diet
Rate-Limiting Factor
any chemical in limited supply that restricts the pace at which another chemical can be synthesized
Serotonin (5-HT)
amine neurotransmitter, helps to regulate mood and aggression, appetite and arousal, perception of pain and respiration
Gamma-Aminobutyric Acid (GABA)
amino acid neurotransmitter, typically inhibits neurons
Histamine (H)
neurotransmitter that controls arousal and waking, can cause the constriction of smooth muscles, when activated in allergic reactions, constricts airway and contributes of asthma
Neuropeptide
short, multifunctional amino acid chain (fewer than 100 amino acids)
acts as a neurotransmitter and can act as a hormone, may contribute to learning
Endocannabinoid
class of lipid neurotransmitters, including anandamide and 2-AG
synthesized at the presynaptic membrane to act on receptors at the presynaptic membrane
affects appetite, pain, sleep, mood, memory, anxiety, and the stress response
Nitric Oxide (NO)
gaseous neurotransmitter, acts for example, to dilate blood vessels, aid digestion, and activate cellular metabolism
Carbon Monoxide (CO)
gaseous neurotransmitter, activates cellular respiration
Hydrogen Sulfide
gaseous neurotransmitter, slows cellular metabolism
Zinc
an ion transmitter that is packaged and stored in vesicles and that is then released and interacts with several receptors
G protein
guanylyl nucleotide-binding proteins coupled to a metabotropic receptor, when activated binds to other proteins
Subunit
protein molecule that assembles with other protein molecules
Second Messenger
chemical that initiates a biochemical process when activated by a neurotransmitter (the first messenger)
How were chemical messages discovered?
discoveries about how neurons communicate stem from experiments designed to study what controls an animals heartbeat
heartbeat quickens if you are excited or exercising, if you are resting, it slows
chemical relay excitatory messages to speed up and inhibitory messages to slow down
role of the vagus nerve and neurotransmitter acetylcholine (ACh) in slowing heart rate
How were chemical messages discovered?
discoveries about how neurons communicate stem from experiments designed to study what controls an animals heartbeat
heartbeat quickens if you are excited or exercising, if you are resting, it slows
chemical relay excitatory messages to speed up and inhibitory messages to slow down
role of the vagus nerve and neurotransmitter acetylcholine (ACh) in slowing heart rate
Who was Otto Loewi (1921)?
frog heart experiment
marked the beginning of research into how chemicals carry info from one neuron to another
What did Otto Loewi discover about acetylcholine?
the first neurotransmitter discovered in the PNS and CNS
activates skeletal muscles in the somatic NS
excitatory/inhibitory action dependent upon the ion channel (not the molecule itself)
What did Otto Loewi discover about epinephrine?
chemical messenger that acts as a hormone
mobilize the body for fight or flight during stress
works as a neurotransmitter in the CNS
What did Loewi discover about norepinephrine?
neurotransmitter found in the brain and in the sympathetic division of the autonomic NS
accelerates heart rate in mammals
What is a neurotransmitter?
chemical released by a neuron onto a target (excitatory or inhibitory)
outside the CNS, many of these chemicals circulate in the bloodstream as hormones (have distinct targets, action slower than that of a neurotransmitter)
How are neurotransmitters measured today?
the actual number of transmitters is an open question, with 100 posited as the maximum
the confirmed number is 60, with most of the work being done by 10
whether a chemical is accepted as a neurotransmitter depends on the extent to which it meet certain criteria
What is an electron microscope?
projects a beam of electrons through a very thin slice of tissue
much better resolution than the light microscope
1950s: revealed the structure of a synapse for the first time
allowed researchers to determine that neurotransmitters are packaged into vesicles at the end terminals of axons
What is a chemical synapse?
the junction where messenger molecules (neurotransmitters) are released from one neuron to excite or inhibit the next neuron
most synapses in the mammalian nervous system are chemical
What is the presynaptic membrane?
axon terminal
where the action potential terminates to release the chemical message
What is the postsynaptic membrane?
dendritic spine
the receiving side of the chemical message, where EPSPs or IPSPs are generated
What is the synaptic cleft?
space between
small gap where the chemical travels from presynaptic to postsynaptic membrane
What is the tripartite synapse?
functional integration and physical proximity of the presynaptic membrane, postsynaptic membrane, and their intimate association with surrounding astrocytes
What is the synaptic vesicle?
presynaptic
small membrane-bound spheres that contain one or more neurotransmitters
What is the postsynaptic receptor?
postsynaptic
site to which a neurotransmitter molecule binds
What are the two ways neurotransmitters are derived?
synthesized in the axon terminal (small molecule transmitter)
synthesized in the cell body (peptide transmitters)
How are neurotransmitters synthesized in the axon terminal?
made from building block from food
pumped into cell via transporters
protein molecules embedded in the cell membrane, pumps substances across a membrane
How are neurotransmitters synthesized in the cell body?
according to instructions in the DNA (peptide transmitters)
transported on microtubules to axon terminal
peptide transmitters may also be manufactured within presynaptic terminal by the ribosomes
How are neurotransmitters packaged?
regardless of their origin, neurotransmitters that are packaged into vesicles can be found in three locations at the axon terminal
some vesicles are warehoused in granules
some are attached to microfilaments
others are attached to the presynaptic membrane
How are neurotransmitters released into the synaptic cleft?
synaptic vesicles loaded with neurotransmitters must dock near release sites on the presynaptic membrane
then the vesicles are primed to prepare them to fuse rapidly in response to calcium (Ca) influx
at the terminal, the action potential opens voltage-sensitive calcium channels
Ca2+ enters the terminal and binds to the protein
the complex causes some vesicles to empty their contents into the synapse
What is receptor-site activation?
after release, the neurotransmitter diffuses across the synaptic cleft to activate receptors on the postsynaptic membrane
properties of the receptor determine the effect on the postsynaptic cell
What are transmitter-activated receptors?
protein embedded in the membrane of a cell that has a binding site for a specific neurotransmitter
What effects can a neurotransmitter have on the postsynaptic cell?
depolarize the postsynaptic membrane, causing excitatory action on the postsynaptic neuron (EPSP)
hyperpolarize the postsynaptic membrane, causing inhibitory action on the postsynaptic neuron (IPSP)
initiate other chemical reactions that modulate the excitatory or the inhibitory effect or influence other functions of the receiving neuron
How does diffusion inactivate neurotransmitters?
some of the neurotransmitter simply diffuses away from the synaptic cleft and is no longer available to bind to receptors
How does degradation inactivate neurotransmitters?
enzymes in the synaptic cleft break down the neurotransmitter
How does reuptake inactivate neurotransmitters?
transmitter is brought back into the presynaptic axon terminal by membrane transporters for reuse
by-products of degradation by enzymes also may be taken back into the terminal to be used again
How does astrocyte uptake inactivate neurotransmitter?
nearby astrocytes take up neurotransmitter, can also store transmitters for re-export to the axon terminal
What is flexibility in synaptic function?
if the terminal is very active, the amount of neurotransmitter made and stored there increases
if the terminal is not often used, enzymes within the terminal buttons may breakdown excess transmitter
the by-products are then reused or excreted from the neuron
axon terminals may even send messages to the neuron’s cell body, requesting increased supplies of the neurotransmitter or the molecules with which to make it
What is the variety of synapses?
synapses vary widely
each type is specialized in location, structure, function, and target
variety of connections makes the synapse a versatile chemical delivery system
What are electrical synapses?
gap junction: fused presynaptic and postsynaptic membrane that allows an action potential to pass directly from one neuron to the next
electrical synapses are fast, gap junctions eliminate delays in information flow
What are electrical synapses?
gap junction: fused presynaptic and postsynaptic membrane that allows an action potential to pass directly from one neuron to the next
electrical synapses are fast, gap junctions eliminate delays in information flow
chemical synapses are more flexible (amplify or diminish signal) but slower
What are excitatory synapses?
typically located on dendrites
round vesicles
dense material on membranes
wide cleft
large active zone
What are inhibitory synapses?
typically located on cell body
flat vesicles
sparse material on membranes
narrow cleft
small active zone
What is the relationship between excitatory and inhibitory action within a neuron?
the differing locations of excitatory and inhibitory synapses divide a neuron into two zones: an excitatory dendritic tree and an inhibitory cell body
excitation coming in over the dendrites and spreading past the axon hillock to trigger an action potential at the initial segment
if the message is to be stopped, it is vest stopped by inhibiting the cell body close to the initial segment
so inhibition blocks or cuts excitation from passing through the postsynaptic cell
What are some functions of neurotransmitters?
carry a message from one neuron to another by influencing the voltage on the postsynaptic membrane
change the structure of a synapse
communicate by sending messages in the opposite direction, these retrograde (reverse-direction) messages influences the release or reuptake of transmitters on the presynaptic side
What are small-molecule transmitters?
class of quick acting neurotransmitters
synthesized from dietary nutrients and packaged ready for use in axon terminals
can be quickly replaced at presynaptic terminal
some drugs are designed to emulate the router of small-molecule transmitters
What is acetylcholine (ACh)?
present at junction of neurons and muscles in the CSN
What are some examples of amines?
common biochemical pathway/relatedness
dopamine, norepinephrine, epinephrine, serotonin
What are amino acids?
glutamate: main excitatory
GABA: main inhibitory transmitter (workhorses of the brain)
What are purines?
synthesized as nucleotides, regulate blood flow, sleep, arousal, etc.
What are neuropeptides?
short, multifunctional amino acid chain, acts as a neurotransmitter and can act as a hormone
synthesized through translation of mRNA from instructions in the neuron’s DNA
most are assembled on the neuron’s ribosomes, packaged in a membrane by Golgi bodies, and transported by the microtubules to the axon terminals
act slowly and are not replaced quickly
have no direct effects on postsynaptic membrane voltage
activate receptors that indirectly influence cell structure/function
generally, cannot be taken orally as drugs, as small-molecule transmitters can
What are the function of neuropeptides in the NS?
act as hormones that respond to stress
enable a mother to bond with her infant (oxytocin)
regulate eating and drinking/pleasure and pain
What are lipid transmitters?
can’t be stored in vesicles, created “on demand”
main example: endocannabinoids (endogenous cannabinoids), synthesized at the postsynaptic membrane to act on receptors at the presynaptic membrane, postsynaptic neurons reduces amount of incoming neural signal
affect appetite, pain, sleep, mood, memory, anxiety, stress response
What are endocannabinoids?
investigators hypothesize that endocannabinoids are synthesized on demand after a neuron has depolarized and calcium has entered
acts on presynaptic receptor
reduces the amount of small-molecule transmitter being released, so the postsynaptic neuron reduces the amount if incoming neural signal
CBI receptor is the target of all cannabinoids, endocannabinoids (humans) or phytocannabinoid (plants) or synthetic cannabinoids
found in both glutamate and GABA synapses, cannabinoids act as neuromodulators to inhibit release of glutamate and GABA, cannabinoids thus dampen both neuronal excitation and inhibition
obtained from the hemp plants Cannabis Sativa and Cannabis Indica
used for medical and recreational purposes for thousands of years
What are gaseous transmitters?
nitric oxide, carbon monoxide, hydrogen sulfide
neither stored in synaptic nor released from them
synthesized in cell as needed; easily cross cell membrane
chemical messengers in body, modulate neurotransmitter production
What are ion transmitters?
recent evidence has led researchers to classify zinc as a transmitter
actively transported, packaged into vesicles, usually with another transmitter like glutamate, and released into the synaptic cleft
zinc dysregulation is linked to Alzheimer’s
What are ionotropic receptors?
embedded membrane protein with two parts: a binding site for a neurotransmitter and a pore that regulates ion flow to directly and rapidly change membrane voltage
allows the movement of ions such as Na+, K+, and Ca+ across a membrane
when neurotransmitter attaches to binding site, the pore opens or closes, changing the flow of ions
What are metabotropic receptors?
embedded membrane protein with a binding site for a neurotransmitter but no pore
indirectly produces change in nearby ion channels or in the cell’s metabolic activity
linked to a G protein that can affect other receptors or act with second messengers to affect other cellular processes
What is the amplification cascade?
a single neurotransmitter binding to a metabolic receptor can activate an escalating sequence of events
proteins can be activated or deactivated
Cholinergic Neuron
neuron that uses acetylcholine as its main neurotransmitter, cholinergic applies to any neuron that uses ACh as its main transmitter
Activating System
neural pathways that coordinate brain activity through a single transmitter
its cell bodies lie in a brainstem nucleus
axons are distributed through a wide CNS region
Schizophrenia
behavioral disorder characterized by delusions, hallucinations, disorganized speech, blunted emotions, agitation or immobility, and a host of associated symptoms
Noradrenergic Neuron
a neuron containing norepinephrine
Major Depression
mood disorder characterized by prolonged feelings of worthlessness and guilt, disruption of normal eating habits, insomnia, a general slowing of behavior, and frequent thoughts of suicide
Mania
disordered mental state of extreme excitement
Obsessive-Compulsive Disorder
behavior characterized by compulsively repeated acts (such as hand washing) and repetitive, often unpleasant thoughts (obsessions)
Learning
relatively persistent or even permanent change in behavior that results from experience
Habituation
learned behavior in which the response to a stimulus weakens with repeated presentations
Saccades
small, fast, random eye movements designed to keep photoreceptors exposed to ever-changing visual stimuli to prevent habituation
Sensitization
learned behavior in which the response to a stimulus strengthens with learned presentations
Posttraumatic Stress Disorder (PTSD)
syndrome characterized by psychological arousal associated with recurrent memories and dreams arising from a traumatic event that occurred months or years earlier
What are receptor subtypes?
each neurotransmitter may interact with a number of receptor subtypes specific to that neurotransmitter
each subtype has slightly different properties, which confer different activities
What are neurotransmitter systems and behavior?
a single neuron may use one transmitter at one synapse and a different transmitter at another synapse
different transmitters may coexist in the same terminal or synapse
caution against the assumption of a simple cause-and-effect relationship between a neurotransmitter and a behavior
What is neurotransmission in the somatic nervous system (SNS)?
cholinergic neuron (motor neurons): neuron that uses acetylcholine as its main transmitter, excites skeletal muscles to cause contractions
nicotinic ACh receptor (nAChr): when Ach or nicotine binds to this receptor, its pore opens to permit ion flow, thus depolarizing the muscle fiber
the nicotine receptor pore permits the simultaneous efflux of K+ and influx of Na+
What are the dual activating systems of the autonomic nervous system (ANS)?
the sympathetic division arouses the body for action, producing the fight-or-flight response, heart rate ramps up, digestive functions ramp down
the parasympathetic division calms the body down, producing an essentially opposite rest-and-digest response, digestive functions ramp up, heart rate ramps down
both divisions are controlled by acetylcholine neurons that emanate from the CNS at two levels of the spinal cord
What is the function of cholinergic neurons in the CNS?
cholinergic neurons in the CNS synapse with sympathetic NE neurons to prepare the body’s organs for fight or flight
cholinergic neurons in the CNS synapse with autonomic ACh neurons in the parasympathetic division to prepare the body’s organs to rest and digest
What are excitatory and inhibitory synapses?
during sympathetic arousal, norepinephrine turns up heart rate and turns down digestive functions, NE receptors on the heart are excitatory, whereas NE receptors on the gut are inhibitory
acetylcholine turns down heart rate and turns up digestive functions because its receptors on these organs are reversed, on the heart is inhibitory, on the gut is excitatory
What is enteric nervous system (ENS) autonomy?
ENS can act without input from the CNS
uses al four classes of neurotransmitters, more than 30 transmitters, mainly serotonin and dopamine
sensory ENS neurons detect mechanical and chemical conditions in the gastrointestinal system
What are activating systems?
neuronal pathway that coordinate brain activity through a single neurotransmitter
cell bodies lie in a nucleus in the brainstem, and their axons are distributed through a wide region of the brain
four systems: cholinergic, dopaminergic, noradrenergic, and serotonergic
one system for each small-molecule transmitter
What is the cholinergic system?
normal waking behavior, is thought to function in attention and memory
loss of cholinergic neurons is associated with Alzheimer disease
What is the dopaminergic system?
nigrostriatal pathways and mesolimbic pathways
What are the nigrostriatal pathways in the dopaminergic system?
active in maintaining normal motor behavior (coordination)
loss of DA is related to muscle rigidity and dyskinesia in Parkinson disease
What are the mesolimbic pathways in the dopaminergic system?
dopamine release causes repetition of behaviors
most affected in addiction behaviors (food, drugs, etc.)
related to impulse control
increases in DA activity may be related to schizophrenia
decreases in DA activity may be related to deficits of attention
What is the noradrenergic system?
norepinephrine plays a role in learning by stimulating neurons to change structure
may also facilitate normal development of the brain and organize movements
imbalances associated with depression, mania
decreased NE activity related to ADHD and hyperactivity
What is the serotonergic system?
plays a role in wakefulness and learning
imbalances associated with depression, schizophrenia, OCD, sleep apnea, sudden infant death syndrome (SIDS)
What is the adaptive role of synapses in learning and memory?
learning: relatively permanent change in behavior that results from experience
neuroplasticity: the nervous system’s potential for change, which enhances its ability to adapt, required for learning and memory
hebb synapse: when the axon of cell A is near enough to excite a cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such as A’s efficiency, as one of the cells firing B is increased
cells that fire together, wire together
Who is Eric Kandel?
was awarded the 2000 Nobel Prize in Physiology or Medicine for his descriptions of the synaptic basis of learning using Aplysia
he used enduring changes in simple defensive behaviors to study underlying changes in the nervous system
What is the habituation response?
learning behavior in which a response to a stimulus weakens with repeated presentations of the stimulus
example: gill withdrawal response in the marine snail Aplysia californica
What is the neural basis of habituation?
as habituation develops, the excitatory postsynaptic potentials in the motor neuron become smaller (motor neuron is receiving less neurotransmitter from the sensory neuron across the synapse)
habituation must take place in the axon terminal of the sensory neuron
less activity from a habituated neuron relative to a non-habituated ones (as habituation takes place, calcium influx decreases in response to voltage changes associated with an action potential)
reduced sensitivity of calcium channels and decreased release of neurotransmitter
What is the sensitization response?
learning behavior in which the response to stimulus strengthens with repeated presentations because the stimulus is novel or stronger than normal
What is the neural basis of sensitization?
in response to an action potential on an axon of a sensory neuron, K+ channels are slow to open (K+ ions cannot repolarize the membrane quickly, so action potential lasts longer than normal, prolongs the inflow of calcium and more transmitter is released)
sensitization is the opposite of habituation at the molecular and behavioral levels
in sensitization, more calcium influx results in more transmitter being released
in habituation, less calcium influx results in less neurotransmitter being released
What is learning as a change in synapse number?
neural changes associated with learning must last long enough to account for a relatively permanent change in an organism’s behavior
repeated stimulation produces habituation and sensitization that can persist for months
the number and size of sensory synapses change in well-trained, habituated, and sensitized Aplysia
transcription and translation of nuclear DNA initiate structural changes (formation of new synapses and spines)
second-messenger cAMP molecules plays an important role in carrying instructions regarding structural changes to nuclear DNA
