topic 6.5 - neurones and synapses Flashcards
Difference between nervous and endocrine system
endocrine system consists of glands that release hormones; nervous system consists of nerve cells called neurone
What is the function of neurons?
to transmit electrical impulses
draw a diagram of a standard neuron
dendrite
short branched nerve fibres, for example those used to transmit impulses between neurone in one part of the brain or spinal cord
axons
very elongated nerve fibres, for example those that transmit impulses from the fingers to the spinal cord
how are nerve fibres adapted?
they are myelinated, which allows for saltatory conduction
saltatory conduction
in myelinated nerve fibres the nerve impulse can jump from one node of ranvier to the next
describe myelin
- consists of many layers of phospholipid bilayer
- deposited by Schwann cells, which grow round and round the nerve fibre
node of ranvier
gap between the myelin deposited by adjacent Schwann cells.
why is saltatory conduction faster than continuous transmission?
action potential to be conducted much faster and prevents the loss of the electrical signal through the cell membrane.
define resting potential
the potential difference or voltage across the membrane of a neuron that is not transmitting a signal
why is there a resting potential?
due to the imbalance of positive and negative charges across the membrane
describe resting potential and how it is maintained
- sodium-potassium pumps transfer sodium and potassium ions across the membrane. Na+ ions pumped out and K+ ions pumped in. The number of ions pumped is unequal - when 3 Na+ ions are pumped out, only two K+ ions are pumped in, creating concentration gradients for both ions
- the membrane is about 50x more permeable to K+ ions than Na+ ions, so K+ ions leak back across the membrane faster than Na+ ions. As a result, the Na+ concentration gradient across the membrane is steeper than the K+ gradient, creating a charge imbalance
- there are proteins inside the nerve fibre that are negatively charged (organic anions) which increase the charge imbalance
define an action potential
a rapid change in membrane potential, consisting of:
- depolarisation - a change from negative to positive
- repolarisation - a change back from positive to negative
value of resting potential
-70mV
describe depolarisation
due to the opening of sodium channels in the membrane, allowing Na+ ions to diffuse into the neuron down the concentration gradient. The entry of Na+ ions reverses the charge imbalance across the membrane, so the inside is positive relative to the outside. This raises the membrane potential to a positive value of about +30mV
describe repolarisation
happens rapidly after depolarisation and is due to the closing of the sodium channel and the opening of potassium channels in the membrane. This allows K+ ions to diffuse out of the neuron, down their concentration gradient, which makes the inside of the cell negative again relative to the outside. The potassium channels remain open until the membrane has fallen to a potential close to -70mV
why does it take a few milliseconds for the neuron to be able to transmit another nerve impulse?
the diffusion of potassium depolarises the neuron, but does not restore the resting potential as the concentration gradients of sodium and potassium ions have not yet been re-established.
define a nerve impulse
action potentials propagated along the axons of neutrons
why does the propagation of the action potential happen?
because the sodium ion movements that depolarise one part of the neutron trigger depolarisation in the neighbouring part of the neuron
what is the function of the refractive period after a depolarisation?
prevents propagation of an action potential backwards along an axon
describe how local currents, or propagation of the action potential, come about
- depolarisation of part of the axon reduces the concentration of sodium ions outside the axon and increases it inside
- depolarised part of the axon therefore has different sodium ion concentrations to the neighbouring part of the axon that has not yet depolarised
- inside the axon sodium ions diffuse along inside the axon to the neighbouring part that is still polarised
- outside the axon the sodium ions diffuse from the polarised part back to the part that has just depolarised
state the effect of local currents
reduce the concentration gradient in the part of the neuron that has not yet depolarised. this makes the membrane potential rise from the resting potential of -70mV to about -50mV. sodium channels in the axon membrane are voltage-gated and open when a membrane potential of -50mV is reached (known as threshold potential). this causes depolarisation
draw a diagram for local current
pg 323
draw and describe an action potential peak
pg 324
define a synapse
a junction between neurons and between neurone and receptor or effector cells
when pre-synaptic neurons are depolarised, they release
a neurotransmitter into the synapse
describe synaptic transmission
- a nerve impulse is propagated along the pre-synaptic neuron until it reaches the end of the neuron and the pre-synaptic membrane
- depolarization of the pre-synaptic membrane causes calcium ions (Ca2+) to diffuse through channels in the membrane into the neuron
- influx of calcium causes vesicles containing neurotransmitter to move to the pre-synaptic membrane and fuse with it
- neurotransmitter is released into the synaptic cleft by exocytosis
- the neurotransmitter diffuses across the synaptic cleft and binds to receptors on the post-synaptic membrane
- the binding of the neurotransmitter to the receptors causes adjacent sodium ion channels to open
- sodium ions diffuse down their concentration gradient into the post-synaptic neuron, causing the post-synaptic membrane to reach the threshold potential
- an action potential is triggered in the post-synaptic membrane and is propagated on along the neuron
- the neurotransmitter is rapidly broken down and removed from the synaptic cleft
function of acetylcholine
used as the neurotransmitter in many synapses
how is acetylcholine produced
in the presynaptic neuron by combining choline, absorbed from the diet, with an acetyl group produced during aerobic respiration
describe the use of acetylcholine as a neurotransmitter
- loaded into vesicles and then released into the synaptic cleft during synaptic transmission
- receptors for acetylcholine in the post-synaptic membrane have a binding site to which acetylcholine will bind
- acetylcholine only remains bound to receptor for a short time, during which only one action potential is initiated in the post-synaptic neuron
- enzyme acetylcholinesterase is present in the cleft and rapidly breaks down acetylcholine down into choline and acetate
- choline reabsorbed into pre-synaptic neuron, where it is converted back into active neurotransmitter by recombining it with an acetyl group
what do neonicotinoids do?
insecticides:
- they bind to the acetylcholine receptor in cholinergic synapses in the CNS of insects
- acetylcholinesterase does not break them down, so binding is irreversible
- receptors blocked so acetylcholine is unable to bind and synaptic transmission is prevented
- results in insect paralysis and death
give an advantage and disadvantage of neonicotinoid use
+ not highly toxic to humans and other mammals: greater proportion of synapses in CNS are cholinergic in insects than in mammals and neonicotinoids bind less strongly to acetylcholine receptors in mammals than insects
- effects on honeybees and other beneficial insects
a nerve impulse is only initiated if…
the threshold potential is reached
nerve impulses follow an —- why?
all or nothing principle; only at the threshold potential do voltage-gated sodium channels start to open, causing depolarisation
what determines whether a nerve impulse travels to the neighbouring neuron? explain
the amount of neurotransmitter secreted following depolarisation of the pre-synaptic membrane may not be enough to cause the threshold potential to be reached in the post-synaptic membrane, which then does not depolarise.
what happens when the post-synaptic membrane does not depolarise?
sodium ions that have entered the post-synaptic neuron are pumped out by sodium-potassium pumps and the post-synaptic membrane returns to the resting potential
what mechanism can be used to process information from different sources in the body to help in decision-making?
- a typical post-synaptic neuron in the brain or spinal cord has synapses with many pre-synaptic neurone
- it may be necessary for several of these to release neurotransmitter at the same time for the threshold potential to be reached and nerve impulse to be initiated
