Neurones and Synapses Flashcards
What is the definition of a neurone
A specialised cell capable of transmitting information in the form of nerve impulses
How do the sensory neurones link
Conduct an impulse from the receptor to the CNS
How do motor neurones link
Conduct an impulse from the CNS, to the effector
How do the connector neurone link
Links between the sensory and motor enurone
How are potential differences caused
By the unequal distribution of positively charged sodium ions on either side of the cell membrane
What is the resting potential
-neurones have a large potential difference- inside is negatively charged with respect to the outside due to the excess of positively charged sodium ions on the outside- occurs in the resting state
-magnitude- -70mV
-can be maintained as the membrane is impermeable to the flow of ions
Describe an action potential
-when a neurone is stimulated, the sodium channels open in the cell surface membrane- allow sodium ions to diffuse in (membrane becomes permeable)
-changes potential difference- less negative inside
-if a critical potential is reached- -55mV (threshold stimulus)- sodium ions surge in- neurone quickly becomes depolarised
-cell reaches a potential difference of +40mV
What are the functions of the refractory period
- Ensures a one way movement of action potentials- repolarisation must happen before another action potential develops
- limits the no. of action potentials that can be fired- ensures each action potential is a discrete entity
What is the law that states that an action potential either occurs if the threshold stimulus is reached, or it doesn’t
All or nothing law
Describe how the speed of the nerve impulse can be altered
- Myelination of the axon- myelin sheath protects the axon/ prevents ion movement/ insulates the axon- local circuits/ depolarisation cannot occur in the parts of the neurone that is myelinated- action potentials can only occur at the nodes of ranvier where the axon is exposed- action potential jumps from one node of ranvier to the next- greatly increases the speed at which the action potential is propagated along the axon- saltatory conduction
- Diameter of the axon- the thicker the axon, the faster the impulse- proportionately less ion leakage in an axon with a larger diameter- if there is too much leakage, it makes it difficult to maintain the potential gradient required to form resting/action potentials
- Temperature- affects the rate at which the ions can diffuse across the membrane
What are synapses
Junctions between the axon of one neurone and the dentrite of another
Explain the transmission of a synapse
- When an impulse arrives at the end of the neurone (synaptic bulb) the membrane becomes permeable to calcium ions, allowing them to diffuse into the synaptic bulb
- Calcium ions cause the synaptic vesicles to move towards the pre- synaptic membrane
- The vesicles fuse with the pre synaptic membrane, releasing the neurotransmitter acetylcholine, by exocytosis into the synaptic cleft.
- Acetylcholine diffuses across the synaptic cleft and binds to the acetylcholine receptors in the post synaptic membrane.
- This causes the opening of sodium ion channels in the membrane of the PSM, as positive ions diffuse in, the membrane becomes depolarised and an excitatory post synaptic potential is generated.
- If sufficient depolarisation occurs, the EPSP will reach the threshold intensity required to produce an action potential in the PSM.
- The enzyme acetylcholinesterase hydrolyses the acetylcholine to choline and ethanoic acid.
- The breakdown products diffuse across the cleft and are reabsorbed in the synaptic cleft. They are resynthesised to acetylcholine whcih is stored in the synpatic vesicles to be reused. ATP is used to recombine choline and ethanoic acid- which is produced in the mitochondria.
Why does the acetylcholine have to be broken down
It’s important that it is broken down and does not continually remain in the receptor- it prevents it continuously generating a new action potential in the post synaptic membrane
Explain the importance of synapses
- Ensure unidirectionality- nerve impulses can only travel in one direction from the pre synaptic neurone to the post neurone, as the neurotransmitter is only made in the pre synaptic neurone and the neurotransmitter receptors are only in the membrane of the post synaptic neurone
- Prevent overstimulation of effectors- too many impulses passing along the same neurone in a short time period will exhaust the supply of the neurotransmitter more quickly than it can be built up- the synapses fatigue
- They provide integration- may involve a number of pre synaptic neurones forming junctions with one post synaptic neurone- synapses provide flexibility
- Synapses enable nerve impulses to pass from one neurone to another- allow nervous communication to continue throughout the body even though the hardware neurones are not continuous.
What are inhibitory synapses
release a neurotransmitter making it more difficult for an EPSP to form in the post synpatic membrane.
-neurotransmitters in inhibitory synapses can lead to an influx of negative ions in the post synaptic membrane- makes the inside of the membrane more negative than normal resting potential (-70mV)- more difficult for excitatory synapses to produce an EPSP that reaches the threshold level needed to product an action potential.
