Download Understanding Electrical and Chemical Synapses' Role in Neurotransmission and more Quizzes Philosophy of psychiatry in PDF only on Docsity!
The rapid transfer of information between
cells in the nervous system
(neurotransmission) is accomplished by what?
the nervous system through synapses. TERM 2
Two primary divisions of synapses
DEFINITION 2 electrical and chemical. TERM 3
Types of electrical synapses
DEFINITION 3 gap junctions, , connexions, electrotonic synapses, ephaptic connections TERM 4
what are Electrical synapses?
DEFINITION 4 specialized modifications of the membranes between two adjacent cells that allow a direct transfer of small molecules TERM 5
electrical synapses are most common in what
kind of cells?
DEFINITION 5 glial cellscan also occur between neurons
most common type of electrical synapse
gap junctions TERM 7
what are gap
junctions
DEFINITION 7 are small regions closely (2-4 nm) apposed cell membranes containing channels (gap junction channels). TERM 8
the channels are permeable to what?
DEFINITION 8 to ions (Na, K, Ca) and small metabolites up to 1,000 MW (e.g. ATP) TERM 9
gap junctions can create what kinds of
regions of cells?
DEFINITION 9 cells of similar electrical potential or other metabolic characteristics (syncytium). TERM 10
Typical gap junctions occur between
what?
DEFINITION 10 astrocytes, oligodendrocytes, Schwann cells in the PNS, cells of the meninges, ependymal cells lining the ventricles, and between cell types.
Charcot-Marie-Tooth disease
genetic disorder that appears related to mutations in the gene for gap junctions in Schwann cells.This results in demyelination and slowing of conduction in peripheral axons, with subsequent muscle weakness, atrophy and sensory loss. TERM 17
what are chemical synapses?
DEFINITION 17 characterized by the use of a chemical substance (neurotransmitter) to convey electrical activity and sometimes other information between neurons. TERM 18
Neurotransmitter synapses require what?
DEFINITION 18 an electrical signal (action potential) to be converted to a chemical signal (neurotransmitter release) and then back to an electrical signal (post synaptic potential). This process of change from one signal type to another is transduction TERM 19
comparison of speed in chemical vs electrical
synapses?
DEFINITION 19 While chemical neurotransmission is somewhat slower (1- ms) than electrical, less reliable, and more subject to toxins, it also allows a much more complex system of information processing. TERM 20
Chemical synapses have what three primary
elements?
DEFINITION 20
- presynaptic element (bouton)2. synaptic cleft3. postsynaptic element
how are chemical synapses named?
according to the source of the pre to postsynaptic element (e.g. axodendritic, axosomatic, dendrodendritic) TERM 22
sequence of neurotransmission in the
presynaptic bouton
DEFINITION 22 presynaptic bouton synthesis -- secretory vesicle synthesis and loading with transmitter and transport to synaptic terminal (synthesis and packaging) depolarization of the terminal by an action potential opening of calcium channels release of NT into synaptic cleft TERM 23
sequence of neurotransmission in the
postsynaptic membrane
DEFINITION 23
- Neurotransmitter diffuses across the cleft and binds to and activates of a postsynaptic cell receptor (changes shape)6. Transduction of the signal by the postsynaptic cell.7. Pre and Post synaptic inactivation of neurotransmitter that can include reuptake or degradation of the neurotransmitter (termination). TERM 24
From a synthesis point of view,
neurotransmitters are broadly divided into
which two categories?
DEFINITION 24 according to the size of the molecule used (large and small neurotransmitters). Neurons may release one or both types as part of their function TERM 25
examples of small
NT's
DEFINITION 25 Small neurotransmitters include acetylcholine, amino acids, purines and biogenic amines
describe Diffusion and Receptor
Binding
After release into the synaptic cleft, neurotransmitters diffuse across the synaptic cleft and contact receptors on the post synaptic membrane. Note that this distance is very small ( nanometers).Receptors and their reaction to the neurotransmitter will define the nature of the transduction in the postsynaptic membrane. TERM 32
Receptor Binding and Activation of the
Postsynaptic Cell
DEFINITION 32 A typical transduction response, would be the opening of ion channels such as sodium to produce a depolarization of the postsynaptic membrane. However, there are may other types of receptor responses. TERM 33
Neurotransmitter Inactivation
DEFINITION 33 Once the neurotransmitter has bound to the receptor, it must be inactivated in some manner to prepare for the next signal.The binding of neurotransmitter to the postsynaptic receptors is reversible, and the transmitter can be released to diffuse away. It is then rapidly removed from the synaptic cleft by two primary mechanisms: Enzymatic inactivation and reuptake TERM 34
NT reuptake
DEFINITION 34 Most neurotransmitters or their inactivated products are directly taken up by transporter proteins on the presynaptic membrane, and on adjacent neuroglial cells. Some can be taken up by a process of endocytosis (also called pinocytosis) in which vesicles containing the contents of the synaptic cleft are formed from the presynaptic membrane TERM 35
vesicle recycling
DEFINITION 35 Vesicle membranes are recycled. The the synaptic vesicle membrane is reincorporated into new synaptic vesicles. To a lesser extent, vesicles can also be recycled by detaching after fusion (kiss & run). For small molecule transmitters, the vesicles are then reloaded within the terminal and reused.
Neurotransmitter Inactivation- Enzyme
induced breakdown
Enzymatic Inactivation- the molecule is destroyed or modified by membrane bound enzymes around the synaptic cleft. These enzymes may be located on the postsynaptic membrane, adjacent astrocytes, or on the presynaptic membrane. These enzymes typically cleave the transmitter into smaller, inactive but recycleable parts. TERM 37
types of receptor activation
DEFINITION 37 Ligated channels- open or close transmembrane pores or channelsG-protein coupled and other second messenger receptorsTransmembrane receptors with modifiable enzymatic activityLigand -dependent regulators of nuclear transcriptionSequestration of intracellular ions (calcium in particular). TERM 38
example of neurotransmitters which may be
released directly from axons through diffusion
and into the extracellular space
(neuromodulation).
DEFINITION 38 Catecholamines and neuropeptides TERM 39
Peripherally, autonomic axons (usually
sympathetic) use this type of release often in
relation to what?
DEFINITION 39 smooth muscle of blood vessels TERM 40
Neurotransmitter receptor agonists
DEFINITION 40 are agents which mimic or enhance the normal action of a neurotransmitter either through action on cell receptors or other mechanisms
how do ionotropic synapses work?
Ionotropic synapses are relatively easily activated (neurotransmitter released) and their binding to ionotropic receptors (aka ligated ion channels) in the postsynaptic membrane opens an ion channel to depolarize or hyperpolarize the postsynaptic membrane. The signal is rapid (1 millisecond scale) and allows rapid responses. TERM 47
how do metabtropic synapses work?
DEFINITION 47 Metabotropic synapses (metabotropic, second messenger, slow synapses) have receptors that activate an enzymatic process to produce a second messenger molecule that acts usually through a different mechanism to produce signal transduction. This may or may not include the opening of ion channels and depolarization of the cell. It is also possible for the receptor to both open an ion channel and generate a second messenger. TERM 48
About 75% of metabotropic receptors act
through what?
DEFINITION 48 a guansoine nucleotide-binding protein (G-protein).The neurotransmitter binding to the receptor allows coupling of the receptor to a G protein. TERM 49
G protein in combo with gaunosine nucleotide
forms what?
DEFINITION 49 a complex with an enzyme system that generates a second messenger. The G protein can also act as a second messenger itself in function such as the opening of ion channels. This process can generate either stimulatory or inhibitory signals. TERM 50
The most common second messenger
systems are:
DEFINITION 50 Adenylyl cyclase-produces cAMP as a second messenger (guanlyate cyclase is also relatively common)-acts for protein phosphoylationPhospholipase C-produces phosphoinositol and diacylglycerol as second messengers-releases calcium storesPhospholipase A- initiates an arachidonic acid cascade.All of these systems can amplify a postsynaptic response by enzymatic means depending upon the biochemical pathways in the cell
Activities Produced by Second Messengers
While the second messenger transduction system in postsynaptic cells can produce any of the postsynaptic effects of other receptors, the transduction is typically more difficult to stimulate and much slower, but has the advantage of amplification and the ability to stimulate many events simultaneously. TERM 52
Other Forms of Synaptic Communication and
Regulation
DEFINITION 52 presynaptic receptor mediated autoregulation (autoreceptors and heteroreceptors) and retrograde transmission.These effects can be long lasting, seconds to days. TERM 53
Presynaptic Autoregulation
DEFINITION 53 there are receptors on the presynaptic membrane for the same neurotransmitter(s) that it releases or other substances that are released from the vesicle at the same time. They may be ionotropic or metabotropic. TERM 54
These receptors can modulate what?
DEFINITION 54 neurotransmitter synthesis, release, reuptake and package and in general the efficiency of information transfer. In most cases this autoregulation is inhibitory.Presynaptic receptors and/or their bound transmitter may be retrogradely transported as well to produce metabolic effects as well TERM 55
example of presynaptic autoregulation
DEFINITION 55 The autonomic regulation of the sinoatrial node of the heart by synapses from the autonomic nervous systemIncreased activity of the sympathetic innervation of pacemaker cells will produce a negative feedback to autoreceptors on the presynaptic terminal of the sympathetic innervation (norepinephrine) and heteroreceptors of the acetylcholine containing vagus nerve synapses.
gamma-aminobutyric acid
(GABA)-
the primary inhibitory neurotransmitter in the brain (the converse of glutamate). Much of the signaling in the brain is inhibitory. GABA is found in high amounts in many areas.GABA is synthesized from glutamate by glutamic acid decarboxylase. The recycling mechanisms are similar to glutamate. TERM 62
Glycine
DEFINITION 62 inhibitory neurotransmitter primarily in the brainstem and spinal cord.It is primarily associated with mechanisms of recurrent inhibition (in the spinal cord Renshaw cells). It is part of a feedback inhibitory mechanism common to motor neuron pools to limit activity.It is recycled through an active transporter. TERM 63
Acetylcholine
DEFINITION 63 important neurotransmitter in both the central and peripheral nervous systems with both excitatory and inhibitory properties with both ionotropic and metabotropic receptors. It is involved in diverse functions from memory to muscle contraction.After release, acetylcholine is broken down by acetylcholinesterase to choline and acetic acid. Both components are transported into the terminal and recycled. TERM 64
Catcholamines
DEFINITION 64 both excitatory and inhibitory. They are synthesized from tyrosine in successive steps to create the neurotransmitters dopamine, norepinephrine, and epinephrine.most enzymes along this pathway are modifiable by pharmacologic or toxic agents TERM 65
Catecholamines participate in diverse
processes including what?
DEFINITION 65 motor function (dopamine- Parkinson disease), rewarding behavior/addiction and sleep/waking cycles in addition to autonomic sympathetic effects.
The major enzymes involved in inactivation
and recycling of catecholamines?
catechol-O-methyl transferase (COMT) on the post synaptic membrane and monoamine oxidase in the presynaptic terminal cytoplasm.These enzymes are the site of action of many drugs TERM 67
serotonin
DEFINITION 67 excitatory and inhibitory- synthesized from tryptophandiverse functions including sleep/wake cycles, emotional behavioral states and is a prominent component of platelets (vasoconstriction)it is associated with the action of may hallucinogens (LSD, mescaline, ecstasy). TERM 68
what breaks down seratonin?
DEFINITION 68 it is broken down by monoamine oxidase and or transported back into the presynaptic terminal.Many antidepressants act through inhibiting monoamine oxidases (MAOI) or inhibiting the reuptake of serotonin (SSRIs). TERM 69
histamine
DEFINITION 69 excitatory- is synthesized from histidine by histidine decarboxylase. It is also a component of mast cells and basophils (vasodilation) and involved in gastric secretion. Centrally, it is associated with sleep/wake cycles an the maintenance of a conscious state. TERM 70
what breaks down histamine
DEFINITION 70 It is broken down by histamine N methyltransferase and diamine oxidase. Antihistamines are generally receptor blockers.
