6.5 - neurons and synapses

Question 1

what are neurons?

Answer 1

• nerve cells that transmit electrical information around the body
• electrical information is often referred to as electrical impulses or nerve impulses

Question 2

what is the structure of a neuron?

Answer 2

• 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

Question 3

what is the structure of the myelin sheath?

Answer 3

• Myelin has multiple layers of phospholipid bilayers from Schwann cells

Question 4

what is nerve fibre?

Answer 4

• 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

Question 5

what are Schwann cells and Oligodendrocytes?

Answer 5

• 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.

Question 6

what is the node of Ranvier and what does it do?

Answer 6

• 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

Question 7

how does the structure of a neuron affect nerve impulses?

Answer 7

• 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)

Question 8

how is neuron transmission impacted?

Answer 8

• 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.

Question 9

what is neurons charge?

Answer 9

• 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

Question 10

what is the membrane potential?

Answer 10

• concentration of Na+ higher outside cell and K+ concentration higher inside the neuron
• unequal distribution makes concentration gradient of charge

Question 11

what is resting potential?

Answer 11

• 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

Question 12

what is an action potential?

Answer 12

• 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

Question 13

what is Depolarisation in action potentials?

Answer 13

• 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

Question 14

what is Repolarisation in action potentials?

Answer 14

• 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

Question 15

what is refractory period in action potentials?

Answer 15

• 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

Question 16

what is a nerve impulse?

Answer 16

• 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

Question 17

what are local currents?

Answer 17

• 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

Question 18

threshold potential in depolarisation?

Answer 18

• 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.

Question 19

what do electrodes do?

Answer 19

• 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).

Question 20

what are the 4 stages of a typical action potential?

Answer 20

• 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

Question 21

what is a synapse and synapsis cleft?

Answer 21

• 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

Question 22

what is the role of neurotransmitters?

Answer 22

• 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

Question 23

1 - 5 of synaptic transfer?

Answer 23

1. Nerve impulse propagated to the presynaptic neuron terminal.
2. Depolarisation of the presynaptic membrane causes voltage-gated Ca2+ channels to open and Ca2+ diffuses into the presynaptic neuron.
3. 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.
4. Neurotransmitters are released by exocytosis into the synaptic junction where it passively diffuses across the synaptic cleft towards the postsynaptic membrane.
5. Neurotransmitters complementary bind to receptors on adjacent neuron (postsynaptic neuron) and causes the Na+ ion channels to open.

Question 24

6 - 9 of synaptic transfer?

Answer 24

6. Na+ ions diffuse down the concentration gradient into the postsynaptic neuron and causes the postsynaptic membrane to depolarise and reach the threshold potential.
7. An action potential is triggered in the postsynaptic neuron and it is transmitted in the postsynaptic neuron.
8. 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.
9. 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.

Question 25

what is Acetycholine?

Answer 25

• 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

Question 26

how is Acetycholine made?

Answer 26

• produced in the presynaptic neuron by combining choline (absorbed from the diet) with an acetyl group (produced from aerobic respiration)

Question 27

what does Acetycholine do?

Answer 27

• 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

Question 28

what are Neonicotinoids?

Answer 28

• synthetic compounds that have a similar structure to nicotine

Question 29

what do Neonicotinoids do?

Answer 29

• 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

Question 30

what are the consequences of Neonicotinoids?

Answer 30

• 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.

Question 31

what is threshold potential?

Answer 31

• 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).

Question 32

what happens when theres not enough neurotransmitters to cause threshold potential?

Answer 32

• 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.