6.5 - neurons and synapses Flashcards
1
Q
what are neurons?
A
- nerve cells that transmit electrical information around the body
- electrical information is often referred to as electrical impulses or nerve impulses
2
Q
what is the structure of a neuron?
A
- has a cytoplasm, a nucleus and a long nerve fibre
- have dendrites that transmit impulses from the cell body along the axon nerve fibre to the
effectors e.g. muscle or glands - Nerve impulses can be
transmitted long distances
3
Q
what is the structure of the myelin sheath?
A
- Myelin has multiple layers of phospholipid bilayers from Schwann cells
4
Q
what is nerve fibre?
A
- thin, approximately 1mm in diameter, long with a plasma membrane around a narrow region of cytoplasm
- can also have an additional coating along the length of the fibre called myelin
5
Q
what are Schwann cells and Oligodendrocytes?
A
- Schwann cells are specialised cells in the peripheral nervous system
- they deposit phospholipid bilayers to the myelin sheath
- Oligodendrocytes are similar to
Schwann cells and provide insulation to the central nervous system fibres.
6
Q
what is the node of Ranvier and what does it do?
A
- Between the myelin deposits are gaps called the node of Ranvier.
- nerve impulses travel down the axon, it jumps from one node of Ranvier to the next as it is not myelinated
7
Q
how does the structure of a neuron affect nerve impulses?
A
- because of the node of Ranviers salutatory conduction occurs and increases the speed of the transmission
- neurons that are coated with myelin to transmit nerve impulses rapidly (up to 100ms-1) compared to unmyelinated neurons (1ms-1)
8
Q
how is neuron transmission impacted?
A
- immune system can attack the myelin sheath and Schwann cells causing inflammation and scarring.
- this prevents effective nerve transmission.
- This results in demyelinating diseases including multiple sclerosis and neuromyelitis optica.
9
Q
what is neurons charge?
A
- generate and transmit electrical signals by pumping + ions (sodium and potassium ions) across mem
- pumped by Na-K pumps, 3 Na ions pumped out and 2 K ions pumped in
10
Q
what is the membrane potential?
A
- concentration of Na+ higher outside cell and K+ concentration higher inside the neuron
- unequal distribution makes concentration gradient of charge
11
Q
what is resting potential?
A
- nerve cell not transmitting an impulse has a potential difference across its plasma membrane
- membrane is more permeable to K+ ions so they move across more
- concentration gradient for Na+ is greater than the K+ gradient, creating a charge imbalance where the outside is more positive than the inside of the cell
- resting potential is created by
the Na-K pump using ATP - value of the resting potential is -70mV
12
Q
what is an action potential?
A
- a rapid change in the membrane potential of a neuron when an electrical impulse is passed along it
- two phases: depolarisation is a change from negative to positive and
repolarisation is a change back from positive to negative
13
Q
what is Depolarisation in action potentials?
A
- due to the voltage-gated sodium channels opening for more Na+ ions to passively diffuse into the neuron down its concentration gradient
- Na+ entry reverses charge imbalance across membrane, so it becomes more positive than the outside, increases the membrane potential to about +30 mV
14
Q
what is Repolarisation in action potentials?
A
- after depolarisation when sodium channels close and voltage-gated potassium channels in membrane open
- K ions passively diffuse out of the neuron, so inside of the cell is
negative compared to the outside - k channels remain open until the membrane potential falls below -70mV
15
Q
what is refractory period in action potentials?
A
- occurs after a nerve impulse and before the neuron can fire another impulse
- ion distribution across the membrane is being restored by the sodium-potassium pump
- membrane potential is restored to -70 mV, which is the resting potential
16
Q
what is a nerve impulse?
A
- an action potential that starts at one end of a neuron and it is spread along the axon to the next neuron
- occurs as the sodium ion movements that cause depolarisation in one part of the neuron, and will trigger depolarisation in the next part of the neuron
- travel in one direction from
dendrite to axon terminal of a neuron as it is initiated from one neuron before being passed to the next - a refractive period after depolarisation, and this prevents the propagation of an action potential in the backwards direction of the axon
17
Q
what are local currents?
A
- The movement of Na+ ions into neuron propagates the action potential along the axon, and so reduces the Na+ concentration outside the neuron and increases the concentration inside.
- The part of the axon that has depolarised will have a different Na+ concentration compared to the neighbouring sections of the axon that is not yet depolarised.
- Na+ ions diffuse between these regions both inside and outside the axon.
- The higher Na+ ions inside the axon will diffuse to neighbouring parts of the axon along the inside
- Outside the axon, the concentration gradient is in the opposite direction, so Na+ ions diffuse from the polarise section back to the part that has just be depolarised
18
Q
threshold potential in depolarisation?
A
- During depolarisation, the inside of the neuron becomes less negative as the membrane moves from -70mV towards 0mV.
- The neuron must reach the threshold potential of -50mV to trigger an active response.
- Voltage-gated sodium channels open when the membrane potential reaches the threshold potential -50mV.
- Once the threshold potential is reached, an action potential is produced.
19
Q
what do electrodes do?
A
- Electrodes can be placed on either side of the membrane to measure the membrane potentials.
- The potentials can be displayed using an oscilloscope that produces a graph with time on the x-axis (milliseconds,ms) and the membrane potential on the y-axis (millivolts, mV).
20
Q
what are the 4 stages of a typical action potential?
A
- Resting potential: Before the action potential occurs and the neuron is at rest (~70mV).
- Depolarisation: An increasing spike corresponds to Na+ ions influx causing depolarisation of the neuron to +30mV).
- Repolarisation: A falling spike due to K+ ions efflux that causes repolarisation (~-80mV).
- Refractory period: The oscilloscope trace returns to the resting potential by the Na-K ATP pump action.
- An action potential will only occur when depolarisation exceeds –50mV
21
Q
what is a synapse and synapsis cleft?
A
- the junction between two adjacent neurons
- between sensory receptor cells and neurons, brain to spinal cord synapses, and between muscles/glands/effectors and neurons
- plasma membranes of the neurons are separated by a narrow fluid-filled gap called the synaptic cleft that’s about 20nm wide
22
Q
what is the role of neurotransmitters?
A
- Electrical impulses can only be propagated within the plasma membrane.
- cuz of gap, electrical signals cannot pass directly across the synapse.
- The electrical message is converted to a chemical message
- released from the presynaptic neuron towards the receptors
on the postsynaptic neuron
23
Q
1 - 5 of synaptic transfer?
A
- Nerve impulse propagated to the presynaptic neuron terminal.
- Depolarisation of the presynaptic membrane causes voltage-gated Ca2+ channels to open and Ca2+ diffuses into the presynaptic neuron.
- This influx of Ca2+ causes the vesicles containing neurotransmitters in the presynaptic cleft to move towards the presynaptic membrane and the membranes begin to fuse.
- Neurotransmitters are released by exocytosis into the synaptic junction where it passively diffuses across the synaptic cleft towards the postsynaptic membrane.
- Neurotransmitters complementary bind to receptors on adjacent neuron (postsynaptic neuron) and causes the Na+ ion channels to open.
24
Q
6 - 9 of synaptic transfer?
A
- Na+ ions diffuse down the concentration gradient into the postsynaptic neuron and causes the postsynaptic membrane to depolarise and reach the threshold potential.
- An action potential is triggered in the postsynaptic neuron and it is transmitted in the postsynaptic neuron.
- The neurotransmitters in the synaptic cleft are rapidly broken down by enzymes and removed. It is reabsorbed back into vesicles of the presynaptic neuron to prevent continuous synaptic transmission.
- Ca2+ ions are actively pumped back into the synaptic cleft.
- There is a large number of
mitochondria present in the presynaptic neuron to provide sufficient ATP.
25
what is Acetycholine?
- a neurotransmitter in many synapses in animals, and in particular
between neurons and muscle fibres
- stored in vesicles near the presynaptic terminal of the neuron ready for synaptic
transmission
26
how is Acetycholine made?
- produced in the presynaptic neuron by combining choline (absorbed from the diet) with an acetyl group (produced from aerobic respiration)
27
what does Acetycholine do?
- There are ACh receptors on the postsynaptic membrane that bind specifically to ACh
- ACh binds for a short amount of time, allowing for one action potential to be generated in the postsynaptic neuron
- metabolic enzyme called acetylcholinesterase (AChE) will rapidly break down ACh into choline and acetate in the synaptic cleft
- choline is reabsorbed into the presynaptic neuron so that it can be converted into ACh again
28
what are Neonicotinoids?
- synthetic compounds that have a similar structure to nicotine
29
what do Neonicotinoids do?
- useful in pesticides against insects as it binds to ACh receptor in cholinergic synapses in the central nervous system of insects
- AChE cannot break down neonicotinoids and so the binding of neonicotinoids to ACh receptors is irreversible
- This blocks the ACh from binding to the ACh receptors, and thus, it is unable to bind and synaptic transmission is prevented
30
what are the consequences of Neonicotinoids?
- in insects is paralysis and then death, thus neonicotinoids are
highly effective insecticides
- neonicotinoids are not highly toxic to humans and other mammals. so widely used in agriculture
- One variant of neonicotinoid is imidacloprid, which is most used
- controversial as they can hurt bees.
31
what is threshold potential?
- Action potential follow an “all-or-nothing principle”.
- action potential only initiated when threshold potential, ~-50mV, reached.
- because the voltage-gated sodium channels remain closed until the threshold potential is reached.
- Once the threshold potential is reached, there will always be full depolarisation (positive feedback effect on more Na+ channels opening).
32
what happens when theres not enough neurotransmitters to cause threshold potential?
- this results in the postsynaptic neuron to not depolarise.
- typical that a postsynaptic neuron has synapses with many presynaptic neurons.
- allows neurotransmitters from several presynaptic neurons to be released, so enough neurotransmitters are exocytosed to cause depolarisation and a nerve impulse in the postsynaptic neuron.
