Week 3: Synaptic Transmission & Neurotransmitters Flashcards
What are the 2 primary neurotransmitters in the nervous system?
Glutamate (excitatory) & GABA (inhibitory)
How to locate axon and dendrite under a microscope?
Look for synaptic vesicles
Briefly describe the sequence of chemical events at a synapse. (5 steps)
Hint: synthesise, calcium, cleft, separate, reuptake
1) Neuron synthesizes chemicals that serve as neurotransmitters.
2) Action potential travels down axon. At presynaptic terminal, an action potential enables calcium to enter the cell. Calcium releases neurotransmitters into synaptic cleft.
3) Neurotransmitters diffuse across cleft, attach to receptors, and alter activity of postsynaptic neuron.
4) Neurotransmitter separate from receptors.
5) May either be reabsorbed back into presynaptic neuron (reuptake) or diffuse away.
What are 2 types of receptors at the postsynaptic neuron?
Ligand-gated ion channels (ionotropic receptors)
G-protein-coupled receptors (metabotropic receptors)
Briefly describe how an ionotropic receptor works.
Receptor itself is an ion channel
Opens when a specific neurotransmitter binds to it
Changes configuration and channel opens.
Effect is direct where ions pass through the channel.
Effect is faster but shorter lived (eg. vision & hearing which requires rapid quick changing info)
Briefly describe how a metabotropic receptor works.
opens ion channels indirectly, through the action of intracellular processes
When neurotransmitter binds to this receptor, it evokes biochemical changes in molecules inside the cell, which diffuse and open ion channels in the vicinity.
G-protein, an energy storing molecule, detaches and takes its energy elsewhere.
Exert effect through metabolic pathways (slower but longer effect) (eg. taste, smell, pain – exact timing is not so important)
Do receptors in postsynaptic cells bind to any neurotransmitter?
No, they bind to specific neurotransmitters.
After ion channels of receptors are opened, what happens to postsynaptic cell?
It is depolarised. Ions entering contributes to synaptic input for initial depolarization. Could take many simultaneous active synapses to reach threshold for action potential in postsynaptic cell.
Amphetamines are used to treat ADHD, what do they do?
They inhibit transporters for dopamine, lowers reuptake, prolonging dopamine’s effects. Also increase the release of dopamine into the synapse. Increases the amount of dopamine in the synaptic cleft.
What is spatial summation?
Summing inputs from different dendrites at different locations.
What is temporal summation?
Summing inputs from the same source over time to cross the threshold.
How is spatial summation critical to brain functioning?
Each neuron receives sensory input from many incoming axons. This might produce synchronised responses, exciting a neuron enough to activate it
How do dendrites propagate changes in membrane potential?
Through diffusion of ions. No voltage-gated channels in dendrite, so depolarization just diffuse passively along the membrane.
How does voltage move in dendrites? How is this different from how it moves in axons?
Voltage moves symmetrically in both directions (towards & away from soma) since there are no hyperpolarised segments during refractory period.
Describe the ripple effect in spatial summation.
Some depolarisation occurs at location of synapse.
Ions diffuse to the sides, resulting in depolarisation in locations adjacent to synapse, but of smaller magnitude. A few microseconds later, ion channels in first synapse closes and membrane potential returns to resting state. Effects become smaller and smaller.
Describe how spatial summation can elicit an action potential in postsynaptic neuron.
When the depolarisation from the 1st input reaches the 2nd input, it overlaps with the effect of the 2nd input. They compound and add up evoking a larger depolarisation. Overlap of responses evoke a larger and larger depolarisation, summing the effects from all 4 synapses combined. Depolarization eventually spreads to soma, when it reaches axon hillock, the depolarisation is enough to cross the threshold and elicit an action potential in the postsynaptic neuron
In spatial summation, what happens if we reverse the order of inputs from near to far?
The neuron depolarises much less, and does not evoke an action potential. The summation occurs in the opposite direction away from the soma, so the large depolarisation never reaches the axon hillock.
What does the information theory measure?
It is a measure of the reduction of uncertainty
GABA usually acts as an _____ neurotransmitter.
Glutamate usually acts as an _____ neurotransmitter
inhibitory; excitatory
Note: most of the time, but not all the time. The specific effect of a neurotransmitter depends on the specific ion channels opened by the neurotransmitter and specific ionic balance of the neuron.
What kind of effects do GABA and Glutamate exert on the postsynaptic cell?
Ionotropic effect
What kind of effects do neuromodulators exert on the postsynaptic cell?
metabotropic effect
Influence larger groups of neurons in a more diffused manner and for a longer period of time. They are not reabsorbed so quickly, and affect a larger area.
Where is noredrenalin/norepinephrine produced?
locus coeruleus (project to CNS)
What function is norepinephrine thought to be involved in?
thought to increase arousal and attention
Where is dopamine produced?
Substantia nigra and ventral tegmental area (project to cortical areas and basal ganglia)
What function is dopamine thought to be important for?
important for reward system, motor system, and cognition. Projections are diffuse and long range in an ascending modulatory system (ie. from lower to higher brain regions)
Where is the primary source of serotonin?
dorsal raphe nuclei (project to CNS)
What is serotonin thought to be important for?
mood regulation and sleep
Where is the primary source of acetylcholine?
Basal forebrain and pons (project to CNS)
What is acetylcholine thought to be important for?
learning, memory, reward, arousal etc.
What are neuropeptides?
Chains of amino acid.
Where are neuropeptides released in neurons?
released through dendrites, soma, and axon. mostly from dendrites. Unlike other neurotransmitters released exclusively at axon terminals. Released by repeated depolarisation, not a single action potential.
Which neurons do neuropeptides affect?
affect all nearby neurons that have appropriate receptors through diffusion. diffuse to a wide area and effects last a long time.
What are hormones?
refer to neurotransmitters released into the blood stream and affect every tissue around the body with appropriate receptors. Particularly useful for coordinating long lasting changes in multiple parts of body.
