neurons and synapses 6.5

Question 1

State the function of the nervous system.

Answer 1

Coordinates the actions of complex organisms via the transmission of electrochemical signals. These signals are transmitted by a specialised network of cells called neurons.

Question 2

Draw the structure of a neuron. Annotate a neuron drawing with the name and function of the following cell parts: dendrites, axon and cell body

Answer 2

see notion

Question 3

Outline the structure and function of myelin.

Answer 3

Structure
Mixture of protein and phospholipids that is produced by glial cells.
Function
Forms an insulating layer around the axon. This increases the speed of electrical transmission via saltatory conduction.

Question 4

State the role of Schwann cells in formation of myelin.

Answer 4

Schwann cells deposit myelin by growing around the nerve fibre. Each time they grow around the nerve fibre a double layer of phospholipid bilayer is deposited.
There may be 20 or more layers when the Schwann cell stops growing.

Question 5

Outline the mechanism and benefit of saltatory conduction.

Answer 5

Along unmyelinated neurons, action potentials propagate sequentially along the axon in a continuous wave of depolarisation.
In myelinated neurons, the action potentials ‘hop’ between the gaps in the myelin sheath called the nodes of Ranvier.
This results in an increase in the speed of electrical conduction by a factor of up to 100-fold.

Question 6

Compare the speed of nerve impulse conduction myelinated and non-myelinated neurons.

Answer 6

Myleinated neurons have a much faster speed than non-myelinated.

Question 7

Define resting potential.

Answer 7

Resting potential is the difference in charge across the membrane when a neuron is not firing.

Question 8

Explain three mechanisms that together create the resting potential in a neuron.

Answer 8

1. plasma membrane of a neuron keeps large negatively charged molecules such as proteins inside the cell.
2. pumps and channels in the membrane regulate the flow of postivetly charged Na+ and K+
   - cell maintains a higher concentration of Na+ outside the neuron membrane and a higher concentration of K+ on the inside.
   - makes outisde of cell positive and inside of cell negative.
3. the sodium potassium pump actively transport more sodium ions out of the cell than potassium ions inside the cell (3:2 ratio).

Question 9

State the voltage of the resting potential.

Answer 9

Approx -70mV.

Question 10

Define action potential, depolarization and repolarization.

Answer 10

An action potential consists of depolarization and repolarization of the neuron (the rapid changes in charge across the membrane that occur when a neuron is firing.)

Depolarisation - a sudden change in membrane potential; usually from a negative to positive charge.

Repolarisaiton - The restoration of a membrane potential following depolarization.

Question 11

Outline the mechanism of neuron depolarization and repolarization

Answer 11

depolarization: occurs when threshold potential is reached in neuron membrane. causes Na+ volatge gated channels to diffuse Na+ down its concentration gradient into the cell. causes the membrane voltage to become positive.
repolarization: after membrane reaches certain postitive voltage level. K+ volatge gated channels diffuse K+ down its concentration gradient to outside of the cell. causes the membrane volatge to be more negatively charged than -70mv.

Question 12

Define nerve impulse.

Answer 12

Nerve impulses are action potentials propagated along the axons of neurons

Question 13

Describe how nerve impulses are propagated along the neuron axon.

Answer 13

Nerve impulses are action potentials that move along the length of an axon as a wave of depolarisation.
Depolarisation occurs when Na+ channels open and cause a change in membrane potential. (positive)
these Na+ diffuse to neighbouring axon region (local current), causing it to reach threshold and triggering an action potential. this pattern repeats.

Question 14

Outline the cause and consequence of the refractory period after depolarization.

Answer 14

Cause: after repolarisation Na+/K+ ATPase will actively transport 3 Na+ back outside and 2 K+ back inside which will re-establish the resting potetnial of -70mv as well as the concentration gradient needed.
consequence: the period where the neuron cannot undergo another action potential. it aslo prevents the backflow from action potential.

Question 15

Describe that cause of and effect of membrane potential reaching the threshold potential.

Answer 15

Threshold potentials are triggered when the combined simulation from dendridites exceeds a minimum level of depolarisation.
If the depolarisation is sufficient to activate the opening of voltage-gated ions in one section of the axon, this triggers into the next section of the axon.

Question 16

State the role of neurotransmitters.

Answer 16

Neurotransmitters are chemical messengers released from neurons and function to transmit signals across the synaptic cleft.
released in response to the depolorisation of the axon terminal of a presynaptic neuron.
bind to receptors on post-synaptic cells and can either trigger or prevent a response.

Question 17

Outline the mechanism of synaptic transmission, including the role of depolarization, calcium ions, diffusion, exocytosis, neurotransmitters, receptors, sodium ions, sodium channels, threshold potential and action potential.

Answer 17

1. pre-synaptic neuron contains vesicles with chemical neurotransmitters and also has Ca+ ions outside of its membrane
2. action potential reaches pre synaptic neuron, causing the Ca+ ions to diffuse inside the membrane, this causes the vesciles to move towards the plasma membrane of the neuron and fuse with it (exocytosis) releasing the neurotransmitter in the synaptic cleft.
3. neurotransmitter diffuses acroos the synaptic cleft and binds with receptors on the post synaptic neuron.
4. this cuases Na+ channels in the post synaptic membrane to open where Na+ ions diffuse into the memebrane, and so if enough Na+ ions diffuse into the post snyaptic neuron a threshold is reached causing the it to depolarize and an action potential is initiated.
5. neurotransmitter is degraded or brocken down by specifc enzymes and released from the receptors causing the Na+ channels to close.
6. the neurotrasnmitter can then diffuse back to the presynaptic neuron where they are re-assembled and then reused.

Question 18

Explain why some synaptic transmissions will not lead to an action potential in a postsynaptic cell.

Answer 18

Sometimes the threshold potential can’t be reached so the action potential can’t propagate. This is because not enough neurotransmitters have reached the receptors at the postsynaptic cell to trigger threshold potential.

Question 19

Outline the secretion and action, acetylcholine.

Answer 19

Secretion: Released at neuromuscular junctions and binds to muscle fibres to trigger muscle contraction

Action: Activates a postsynaptic cell by binding to 1 or 2 classes of specific receptors.
(Acetylcholine must be constantly removed from the synapse as overstimulation can lead to fatal convulsions and paralysis)

Question 20

Outline the mechanism of action of neonicotinoids use as insecticides.

Answer 20

Neonicotinoid pesticides can irreversibly bind to nicotinic acetylcholine receptors to trigger a sustained response.
They can’t be broken down by AChE, which leads to the permanent overstimulation of target cells (in insects).
Consequently, insects die due to the overstimulation of their cells.

Question 21

Define cholinergic synapse.

Answer 21

Cholinergic synapses are synapses that use AChE as their neurotransmitter.

Question 22

Compare the proportion of cholinergic synapses in insects and humans.

Answer 22

More cholinergic synapses in insects than mammals.

Question 23

State why neonicotinoids insecticides are not highly toxic to humans.​

Answer 23

Insects have a different composition of acetylcholine receptors, which neonicotinoids bind to more strongly. Hence they are more toxic to insects.

Question 24

Outline the reabsorption and formation of acetylcholine.

Answer 24

Reabsorption
Acetylcholine is broken down into its two component parts by the AChE enzyme.
AChE is released into the synapse from the presynaptic neuron or embedded on the membrane of the postsynaptic cell.
Liberated choline is returned to the presynaptic neuron where it is coupled with another acetate to reform acetylcholine.

Formation
Created in the axon terminal by combining choline with an acetyl group.
It is stored in vesicles within the axon terminal until released via exocytosis in response to a nerve impulse.