Psych 255 Unit 5

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50 Terms

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Neurotransmitters

chemical released by a neuron onto a target that has either an excitatory or inhibitory effect (or other more complex effects)

outside the CNS, many of these chemicals circulate in the bloodstream as hormones

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Presynaptic Membrane

- output side of a synapse (sends out neurotransmitters)

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Postsynaptic Membrane

- input side of a synapse (receives neurotransmitters)

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Synaptic Vesicle

a membrane structure that contains a neurotransmitter

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Synaptic Cleft/Synaptic Gap

- Gap that separates the presynaptic membrane from the postsynaptic membrane
- where neurotransmitters are released when stimulated by an action potential

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Steps of Neurotransmission

1. Synthesis
2. Packaging & storage
3. release of NT
4. receptor activation
5. deactivation of NT

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Steps 1 & 2: Synthesis, Packaging, and Storage of NT

- synthesized in axon terminal with chemical precursors from food pumped into the cell via transporter proteins
- synthesized in the soma using the DNA code and transported on microtubules

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Step 3: Release of the NT- EXOCYTOSIS

-at terminal, the AP opens voltagesensitive calcium (Ca++) channels
- incoming Ca++ binds to calmodulin, forming a complex -complex causes some vesicles to empty or get ready

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Step 4: Activation of Receptor Site pt.1

-after release, the NT diffuses across the synapse and activates postsynaptic receptors
- These transmitter activated receptors are proteins embedded in the membrane that have a binding site for a specific NT

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Step 4: Activation of Receptor Site pt.2

- depolarize the postsynaptic membrane causing an EPSP (e.g., open Na+ ion channels)
Or
- hyperpolarize the postsynaptic membrane causing an IPSP (e.g., open K+ or Cl- ion channels)
Or
- initiate other chemical reactions that modulate either the excitatory or inhibitory effect, or influence the receiving neuron in other ways

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Step 4: Activation of Receptor Site pt.3

• Autoreceptors - "Self-receptor" in a neural membrane that responds to the transmitter that the neuron releases (on the presynaptic membrane)

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Step 5: Deactivation of NT

- diffuse away from synaptic gap degraded by enzymes in gap
- reuptake into the presynaptic neuron for subsequent re-use
-taken up by nearby glial cells

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Type I Synapse

- excitatory
- typically on dendrites
- large active zone
- wide cleft
- round vesicles
- dense material on membranes

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Type II Synapse

- inhibitory
- typically on cell body
- small active zone narrow cleft
- flat vesicles
- sparse material on membranes

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Types of Neurotransmitters

• ~ 100+ different kinds
• Some are inhibitory at one location and excitatory at another
• >1 neurotransmitter may be active at one synapse
• No simple one-to-one relationship between a single neurotransmitter and a single behaviour

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Identifying Neurotransmitters

4 Criteria
• must be synthesized and/or packaged in the neuron • the chemical must produce a response in a target cell when released
• The same response must be obtained when the chemical is experimentally placed on the target
• must be a mechanism for removal after the chemical's work is done

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Varieties of Neurotransmitters

Small-Molecule transmitters
• Peptide transmitters
• Lipid transmitters
• Transmitter gases
• Ion transmitters

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Small Molecule transmitters

- quick acting neurotransmitters
- synthesized from precursor chemicals found in the diet - Packaged in the axon terminal

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Peptide transmitters

- Peptide transmitters are chains of amino acids synthesized from mRNA based on DNA code
- synthesized in soma and shipped
- act slower & replaced slower than small molecule neurotransmitters
- often act as hormones

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Lipid transmitters

- not stored in vesicles Endocannabinoids
- synthesized and released from postsynaptic membrane
- Binds to receptor on presynaptic membrane, which decreases NT release (GABA, glutamate) from the presynaptic membrane
- "retrograde transmitter"

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Receptors (2 types)

- Ionotropic
Direct
fast

- Metabotropic
Indirect
slow

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Direct: Ionotropic Receptors

- embedded membrane protein with a binding site for a NT and pore (similar to a gated channel)
- regulates ion flow to directly & rapidly change the membrane voltage

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Indirect: Metabotropic Receptors

- Causes a metabolic reaction that is slower and longer lasting
- Receptors have binding site for a neurotransmitter but no pore/channel
- Receptors are coupled with G-proteins
- G-protein releases a subunit that may affect ion channels

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Noradrenergic System

- projections from locus coeruleus
- Related to attention and arousal
- decreases related to depression
- increases related to mania

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Serotonergic System

- projections from raphe nuclei
- Related to arousal
- Increases related to schizophrenia
- decreases related to depression

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Cholinergic System

- projects from midbrain & basal forebrain nuclei
- Related to arousal
- role in memory
- decrease in ACh related to Alzheimer's

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Mesolimbic Dopaminergic System

- projects from ventral tegmental area
- role in pleasure and reward
- system mediating drug addiction
- related to schizophrenia

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Nigrostriatal Dopaminergic System

- projects from substantia nigra
- role in normal motor behavior
- decrease in DA related to Parkinson's disease

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tripartite synapse

Synapse between a presynaptic neuron, a postsynaptic neuron, and an astrocyte.

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chemical synapse

junction at which messenger molecules are released when stimulated by an action potential

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presynaptic membrane

the specialized membrane of the axon terminal of the neuron that transmits information by releasing neurotransmitter

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postsynaptic membrane

the cell membrane opposite the terminal button in a synapse; the membrane of the cell that receives the message

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anterograde synaptic transmission

process that occurs when a neurotransmitter is released from a presynaptic neuron and binds to a receptor on the postsynaptic neuron

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Transporters

proteins that move neurotransmitter molecules from the synapse across the cell membrane and back into the axon terminal as part of the reuptake process

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ionotropic receptors

Embedded membrane protein that acts as (1) a binding site for a neurotransmitter and (2) a pore that regulates ion flow to directly and rapidly change membrane voltage.

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metabotropic receptors

Embedded membrane protein, with a binding site for a neurotransmitter but no pore, linked to a G protein that can affect other receptors or act with second messengers to affect other cellular processes.

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autoreceptors

"Self-receptor" on the presynaptic membrane that responds to the transmitter that the neuron releases

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quantum (pl. quanta)

number of neurotransmitter molecules, equivalent to the content of a single synaptic vesicle, that produces a just-observable change in postsynaptic electric potential

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gap junctions

Area of contact between adjacent cells in which connexin proteins in each cell form connecting hemichannels; when open, the hemichannels allow ions to pass between the two cells; also called an electrical synapse

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small molecules transmitter

Quick-acting neurotransmitter synthesized in the axon terminal from products derived from the diet.

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histamine (H)

Neurotransmitter that controls arousal and waking; can cause the constriction of smooth muscles and so, when activated in allergic reactions, contributes to asthma, a constriction of the airways.

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Serotonin (5-HT)

amine neurotransmitter; helps to regulate mood and aggression, appetite and arousal, perception of pain, and respiration

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glutmate (GLU)

Amino acid neurotransmitter; typically opens the Na+ and Ca2+ channels and therefore excites neurons.

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Gamma-aminobutyric acid (GABA)

amino acid neurotransmitter; typically inhibits neurons

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Neuropeptides

short chains of amino acids

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Endocannabinoids

natural, marijuana-like substances produced by the body

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Cholinergic neuron

neuron that uses acetylcholine as its main neurotransmitter

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Activating systems in the CNS

neural pathways that coordinate brain activity through a single neurotransmitter

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noradrenergic neurons

neurons that release norepinephrine

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Neuromodulators

Neurotransmitters that alter the firing and synaptic characteristics of other neurons, typically over an extended period