Chapter 2: COMMUNICATION BETWEEN NEURONS Flashcards
What does the lock and key theory state?
Neurotransmitter molecules fit the binding sites of receptors like a key fits in a lock. Neurotransmitter binding conveys the neural message to the postsynaptic cell.
What are the two sides of the synapses?
- presynaptic cell (axon terminal of a neuron)
- postsynaptic cell (dendrite or soma)
- they are separated by a synaptic cleft.
Under what form is the synaptic transmission? (electrical or chemical)
- presynaptic: electrical to chemical (electrical impulse travel down the axon, then neurotransmitter release in extracellular fluid at the synaptic cleft)
- postsynaptic: electrical
What does synaptic transmission dysfunction lead to ?
It leads to mental disorders
What are the general characteristics of synaptic transmission?
- mostly chemical (what happens between presynaptic and postsynaptic)
- synaptic cleft: filled with a matrix of fibrous extracellular protein
- presynaptic: usually axon terminal
- postsynaptic: contains neurotransmitter receptors.
What different synaptic arrangements are there and what are they based upon?
- axodendritic, axospinuous, axosomatic, axoaxonic, dendrodendritic
- based upon which part of neurons is postsynaptic to the axon terminal
What are the different steps of chemical synaptic transmission?
- Neurotransmitter synthesis
- Load neurotransmitter into synaptic vesicles
- Vesicles fuse to presynaptic terminal
- Neurotransmitter spills into synaptic cleft
- Binds to postsynaptic receptors
- Biochemical/electrical response elicited in postsynaptic cell
- Removal of neurotransmitter from synaptic cleft
What does the presynaptic terminal contain?
- small spheres: synaptic vesicles, 50nm, store NT.
- larger vesicles: secretory granules, contains soluble protein, also called dense-core vesicles.
What is the site of NT release called?
-active zone: synaptic vesicles clustered in cytoplasm adjacent to active zone
How are NT released by vesicles?
-Released when synaptic vesicles fuse with membrane and break open, spilling contents into the synaptic cleft
How is NT release triggered?
- Arrival of AP
- Depolarization of terminal membrane causes voltage gated calcium channels to open
- Elevation in Ca+ triggers to be released
How is NT released via exocytosis?
- Basically what was explained earlier when vesicles fuse with membrane.
- Process in which intracellular vesicle moves to plasma membrane and fuse at the active zone.
How does exocytosis exactly happen?
- Rapid: 0.2 msec of Ca+ influx
- Ca+ enter at active zone (where voltage gates are located)
- synaptic vesicles ready and waiting
- calcium modifies snare proteins to initiate exocytosis
What is docking?
A molecular modeling technique that is used to predict how a protein (enzyme) interacts with small molecules (ligands).
What happens once binding occurs?
- the postsynaptic receptors open neurotransmitter-dependent ion channels (also called ligand-gated ion channels), which allow the passage of specific ions in and out of the cell
- NT open ion channels by at least 2 methods, direct and indirect.
What is the direct method in which NT open ion channels?
-ionotropic receptor: When a NT-dependent ion channel is equipped with it’s own binding site, a molecule of the appropriate NT attaches to it, and the ion channel opens.
What is the indirect method of opening ion channels?
- Metabotropic receptors are located close to G-proteins.
1. Molecule of NT binds to metabotropic receptor
2. Receptor activates G-protein
3. G-protein activates enzyme that stimulates the production of chemical second-messenger.
4. Second-messengers (molecules) travel through cytoplasm, attach themselves to ion channel and cause them to open.
What are the characteristics of the direct and indirect methods?
- Direct: transmitter-gated, fast, Amines and AAs.
- Indirect: G-protein-coupled, slower long-lasting and diverse actions, n type of NT can bind.
What determines the nature of the postsynaptic potential at a particular synapse? (EPSP or IPSP)
-determined by the particular type of ion channel the NT open.
What are the 4 major types of NT-dependent ion channels found in the post-synaptic membrane?
- sodium (Na+)
- potassium (K+)
- chloride (Cl-)
- Calcium (Ca2+)
What is an EPSP? (Excitatory Postsynaptic Potential)
-Excitatory depolarization of the postsynaptic membrane of a synapse caused by the liberation of a neurotransmitter by the terminal button.
What is an example of EPSP?
- NT-dependent Sodium channel is the most important source of EPSP.
- When sodium channels open, diffusion and electrostatic push Na+ in.
- Causes depolarization
What is IPSP? (Inhibitory Postsynapatic Potential)
An inhibitory hyperpolarization of the postsynaptic membrane of a synapse caused by the liberation of a NT by the terminal button.
What is an example of IPSP?
- Sodium-potassium transporters maintain a small surplus of K+ inside the cell.
- If K+ channels open, these positive ions will leave the cell, so inside of cell will become more negative: hyperpolarization.
What happens when inhibitory nt open chloride channels?
- If the membrane is at resting potential: nothing happens because diffusion and electrostatic balance the chloride ion.
- If membrane has already been depolarized by excitatory synapses located nearby, opening chloride channels will allow Cl- to enter. So these negative ions will bring resting potential back. Helps neutralize EPSP
What happens when Ca+ channels open?
- Same as sodium: depolarizes the membrane, producing EPSP.
- But also does more like trigger migration of vesicles in presynaptic and release of NT, and activating special enzymes in postsynaptic.
How are postsynaptic potentials terminated with reuptake?
- extremely rapid removal of a TN from synaptic cleft by terminal button
- After Nt relaesed into synapses, presynaptic uses special transporter molecules to return NT into presynaptic.
How are postsynaptic potentials terminated by enzymatic deactivation?
- accomplished by an enzyme that destroys the molecules of the NT
- Ex: acetylcholinesterase (AChE) deactivates acetylcholine.
- This prevents desensitization.
What are called the interaction of the effects of excitatory and inhibitory synapses?
- neural integration
- more excitatory=more rate of firing=depolarization
- more inhibitory=less rate of firing=hyperpolarization=bring membrane potential away from threshold
- IPSP cancel effects of EPSP
What is EPSP summation and what are the two types?
- addition of different EPSPs to produce depolarization
- spatial summation: adding EPSPs generated spontaneously at different synapses in same dendrite.
- temporal summation: adding EPSPs generated at the same synapse in rapid succession
What are autoreceptors?
-Some neurons possess receptors (autoreceptors) that respond to the nt that they themselves release.
What do autoreceptors do when stimulated by a molecule of the nt?
- They regulate internal processes, like the synthesis and release of the nt.
- Mostly inhibitory activity
- Decreases the rate of synthesis and release of nt.
What do axoaxonic synapses do?
- They alter the amount of nt released by postsynaptic axon.
- They can produce presynaptic inhibition or presynaptic facilitation.
What are neuromodulators?
- chemicals released by neurons that travel farther and are dispersed more widely than are nt.
- Most are peptides (chains of amino acids)
- secreted and larger amounts and travel longer distances.
What do neuromodulators affect ?
General behavior states such as vigilance, fearfulness, and sensitivity to pain.
What are hormones and what do they affect?
- secreted by cells of endocrine glands, or cells located in various organs.
- cells that secrete hormones release them into the extracellular fluid.
- hormones distributed to the body through the bloodstream
What are target cells?
- cells that contain receptors for a particular hormone
- many neurons are target cells and hormones can affect their behavior.
