15.5 - Structure And Function Of Synapses Flashcards
What is the function of synapses in the nervous system?
Synapses transmit information, but not impulses, from one neurone to another by means of chemicals known as neurotransmitters.
What is the synaptic cleft, and how wide is it?
The synaptic cleft is the small gap between neurones, which is 20-30 nm wide.
What is the name of the neurone that releases the neurotransmitter?
The neurone that releases the neurotransmitter is called the presynaptic neurone.
What is the synaptic knob, and where is it located?
The synaptic knob is the swollen portion at the end of the axon of the presynaptic neurone.
What organelles are abundant in the synaptic knob, and why?
The synaptic knob contains many mitochondria and large amounts of endoplasmic reticulum. These are required for the manufacture of neurotransmitters, which takes place in the axon.
Where are neurotransmitters stored within the presynaptic neurone?
Neurotransmitters are stored in synaptic vesicles within the presynaptic neurone.
What happens to the neurotransmitter after it is released from the synaptic vesicles?
After being released from the synaptic vesicles, the neurotransmitter diffuses across the synaptic cleft to the postsynaptic neurone.
What feature does the membrane of the postsynaptic neurone possess to interact with neurotransmitters?
The membrane of the postsynaptic neurone possesses specific receptor proteins to receive the neurotransmitter.
What is unidirectionality in synapses?
Unidirectionality refers to the property of synapses that allows them to pass information in only one direction—from the presynaptic neurone to the postsynaptic neurone. This ensures that synapses act like valves.
Why do synapses act like valves?
Synapses act like valves because they allow information to be transmitted in only one direction, ensuring controlled and directional communication between neurones.
What is summation in the context of synapses?
Summation is a process by which low-frequency action potentials that release insufficient concentrations of neurotransmitter can still trigger a new action potential in the postsynaptic neurone through a rapid build-up of neurotransmitter.
What are the two methods of summation in synapses?
1) Spatial summation – where multiple presynaptic neurones release enough neurotransmitter together to exceed the threshold of the postsynaptic neurone and trigger a new action potential.
2) Temporal summation – where a single presynaptic neurone releases neurotransmitter many times over a very short period, and the accumulated concentration exceeds the threshold of the postsynaptic neurone.
Describe spatial summation.
Spatial summation occurs when multiple presynaptic neurones release neurotransmitter simultaneously. The combined release exceeds the threshold value of the postsynaptic neurone, thereby triggering a new action potential
Describe temporal summation.
- Temporal summation occurs when a single presynaptic neurone releases neurotransmitter multiple times over a very short period.
- If the concentration of neurotransmitter builds up and exceeds the threshold value of the postsynaptic neurone, it triggers a new action potential.
What are inhibitory synapses?
Inhibitory synapses are synapses that make it less likely that a new action potential will be created on the postsynaptic neurone.
How do inhibitory synapses function in general?
- Inhibitory synapses function by releasing neurotransmitters that cause changes in ion movements across the postsynaptic membrane, resulting in hyperpolarisation and making the membrane potential more negative.
- This reduces the likelihood of a new action potential.
What happens when the presynaptic neurone releases its neurotransmitter at an inhibitory synapse?
The neurotransmitter binds to chloride ion protein channels on the postsynaptic neurone, causing them to open.
What effect does the opening of chloride ion protein channels have?
When the chloride ion protein channels open, chloride ions (Cl⁻) move into the postsynaptic neurone by facilitated diffusion.
How does the neurotransmitter affect potassium ion channels in the postsynaptic neurone?
The neurotransmitter causes the opening of nearby potassium (K⁺) protein channels.
What happens when potassium ion protein channels open at an inhibitory synapse?
Potassium ions (K⁺) move out of the postsynaptic neurone into the synapse.
What is the combined effect of chloride ions entering and potassium ions leaving the postsynaptic neurone?
The combined effect is that the inside of the postsynaptic membrane becomes more negative, and the outside becomes more positive.
What is hyperpolarisation, and how does it relate to inhibitory synapses?
- Hyperpolarisation is the increase in membrane potential, making it more negative than the usual resting potential.
- In inhibitory synapses, hyperpolarisation occurs due to the influx of negatively charged chloride ions and the efflux of positively charged potassium ions.
What is the membrane potential during hyperpolarisation, and how does it compare to the resting potential?
During hyperpolarisation, the membrane potential increases to as much as -80 mV, compared to the usual resting potential of -65 mV.
Why does hyperpolarisation reduce the likelihood of a new action potential?
- Hyperpolarisation makes the membrane potential more negative, so a larger influx of sodium ions (Na⁺) is required to reach the threshold for triggering a new action potential.
- This reduces the likelihood of an action potential being created.
