The Synapse and Neurotransmitters Flashcards
What is a synapse?
The synapse is a junction where 2 neurons come close enough to send chemical signals from one cell to the other.
- The neurons are not connected but separated by a space → synaptic cleft
What is the pre-synaptic neuron? (2)
- Pre-synaptic neuron transmits the signal whereas post-synaptic neuron receives the signal.
- Pre-synaptic neurons have chemical messengers called neurotransmitters enclosed in vesicles.
What happens when pre-synaptic neurons are excited?
When pre-synaptic neurons are excited by an action potential, this causes the vesicles to fuse with the pre-synaptic cell membrane and release neurotransmitters into the synaptic cleft via exocytosis.
Post-synaptic neurons have neurotransmitter receptors, to which these neurotransmitters bind, causing an action in the post-synaptic neuron.
After this, neurotransmitters are removed from the synaptic cleft via: (3)
- Diffusion
- Reuptake: the neurotransmitters return back to the pre-synaptic neuron and are retaken into the vesicles for reuse
- Enzymatic breakdown: neurotransmitters are broken down and then the broken fragments return to the pre-synaptic neuron to stimulate production of neurotransmitters
Electrical activity (the action potential) (via the activation of voltage-gated calcium channels) in the _____ neuron is converted into the release of a chemical called a _______ that binds to receptors in the _____ cell and initiates an AP or secondary pathway.
presynaptic
neurotransmitter
postsynaptic
What does the neurotransmitter initiate?
The neurotransmitter may initiate an electrical response or a secondary messenger pathway that may either excite or inhibit the postsynaptic neuron. In short, a chemical neurotransmitter is released by a presynaptic neuron into the synaptic cleft.
Neurotransmitters and the synapse: (5)
- Neurotransmitters are stored in synaptic vesicles at the ends of axons
- AP reaches synaptic terminal → depolarises the terminal pre-synaptic
membrane opening voltage-gated Ca2+ channels - Calcium triggers exocytosis of neurotransmitters (held in membrane-bound vesicles) into the synapse
- Neurotransmitter diffuses across synaptic cleft → effect either direct or indirect
- Many receptors for neurotransmitters are ligand-gated channels
Synaptic Transmission: (3)
- Action potential arrives; triggers entry of Ca2+ into axon bulb of
presynaptic neuron (voltage-gated Ca2+ channels) - In response to Ca2+, synaptic vesicles fuse with the membrane, release neurotransmitters (synthesized by presynaptic neurons)
- Neurotransmitters reach the postsynaptic membrane by diffusion, chemically/ligand-gated ion channels open when a neurotransmitter binds, causing a change in postsynaptic cell potential (graded potentials)
Ligand-gated ion channels open when bound with the chemical stimulus (neurotransmitter) allowing for: (2)
- Influx in Na+ and K+ ions
This could lead to the formation of graded-potentials
Graded potentials can either be _____ (depolarization) or (hyperpolarization) ______.
excitatory
inhibitory
- Changes in membrane potential that ____ the probability of an action potential are excitatory postsynaptic potentials (EPSPs).
- Changes in membrane potential that _____ the probability of an action potential are inhibitory postsynaptic potentials (IPSPs).
increase
decrease
EPSPs and IPSPs are summed at the___ ____ and may result in an action potential.
axon hillock
Synaptic integration: EPSPs and IPSPs are graded potentials that vary in magnitude but may act together to produce an action potential.
Summation can occur in two ways:
- Temporal summation – repeated EPSPs occur before previous EPSPs decay, summation brings neuron to firing level
- Spatial summation – closely spaced synaptic terminals release neurotransmitters simultaneously on same postsynaptic neuron.
- When action potentials arrive from the same axon close together (temporal) in time or from different axons that have synapses close to one another (spatial), the postsynaptic potentials are _____.
- If excitatory postsynaptic potentials depolarize the axon hillock past a point called the _____, enough ___ channels open to trigger an action potential.
additive
threshold
Na+
What happens to neurotransmitters after transmission of signal? (4)
Removed from cleft
1. Taken back up by presynaptic neurons and repackaged into membrane-bound vesicles
2. Destroyed by enzymes in the synaptic cleft
3. Diffuse away from the synaptic cleft into general circulation where later destroyed
4. Glia take up neurotransmitters and actively use as metabolic fuel
What are neurotransmitters?
Endogenous chemicals that enable neurotransmission i.e., a type of chemical messenger which transmits signals across a chemical synapse, such as a neuromuscular junction, from one neuron to another “target” neuron, muscle cell, or gland cell.
