Nervous Coordination

Question 1

Describe a neurones cell membrane at rest

Answer 1

A neurone’s cell membrane is polarised at rest

In a neurone’s resting state (when it is not being stimulated), the outside of the membrane is positively charged compared to the inside

This is because there are more positive ions outside the cell than inside

So the membrane is polarised - there is a difference in charge (called a potential difference or voltage) across it

Question 2

Define the resting potential

Answer 2

The resting potential - is the voltage (potential difference /difference in charge) across the (cell) membrane of a neurone when it is at rest

Question 3

What is the resting potential of a neurone

Answer 3

The resting potential of a neurone is about -70mV (millivolts)

Question 4

Describe how the resting potential is created and maintained

Answer 4

The resting potential of a neurone is created and maintained by sodium-potassium pumps and potassium ion channels in a neurone’s membrane

The sodium potassium pumps use active transport to move three sodium ions (Na+) out of the neurone for every two potassium ions (K+) moved in. ATP is needed to do this

Potassium ion channel - these channels allow facilitated diffusion of potassium ions out of the neurone, down their conc. gradient

The sodium-potassium pumps move sodium ions out of the neurone, but the membrane isn’t permeable to sodium ions, so they cant diffuse back in.
This creates a sodium ion electrochemical gradient (a concentration gradient of ions) because there are more positive sodium ions outside the cell than inside

The sodium-potassium pumps also move potassium ions in to the neurone, but the membrane is permeable to potassium ions so they diffuse back out through potassium ion channels

This makes the outside of the cell positively charged compared to the inside

https://s3.eu-west-2.amazonaws.com/elements.cognitoedu.org/36ee2615-cd3a-4ebb-8555-f22cb8c8c480/resting-potential-neurone-diagram.png

Question 5

What are the sodium-potassium pump, potassium ion channel and sodium ion channel examples of

Answer 5

Na-K pump, K+ channel and Na+ channel are all types of transport protein

Question 6

Explain what happens when a neurone is stimulated

Describe action potential

Answer 6

Neurone cell membranes become depolarised when they are stimulated

a stimulus triggers other ion channels, called sodium ion channels, to open. If the stimulus is big enough, it will trigger a rapid change in potential difference. Sequence of events is known as an action potential.

1) Stimulus - this excites the neurone ell membrane, causes sodium ion channels to open
The membrane becomes more permeable to sodium, so sodium ions diffuse into the neurone down the sodium ion electrochemical gradient.
This makes the inside of the neurone less negative

2) Depolarisation - if the potential difference reaches the threshold (around -55mV), more sodium ion channels open. More sodium ions diffuse rapidly into the neurone.

3) Repolarisation - at a potential difference of around +30mV, the sodium ion channels close and potassium ion channels open.
The membrane is more permeable to potassium so K+ ions diffuse out of the neurone down the potassium ion conc. (electrochemical) gradient.
This starts to get the membrane back to its resting potential.

4) Hyperpolarisation - potassium ion channels are slow to close so there is a slight overshoot where too many potassium ions diffuse out of the neurone. The potential difference becomes more negative than the resting potential (less than -70mV)

5) Resting potential - the ion channels are reset. The sodium-potassium pump returns the membrane to its resting potential and maintains it until the membrane is excited by another stimulus

https://s3.eu-west-2.amazonaws.com/elements.cognitoedu.org/a1b3f698-4a58-4d89-824c-9be6e31b135a/action-potential-graph.png

time in ms (1000ms in 1s)

Question 7

What is the refractory period

Answer 7

After an action potential, the neurone cell membrane cant be excited again straight away
This is because the ion channels are reovering and they cant be made open (sodium ion channels closed during repolarisation and potassium ion channels are closed during hyperpolarisation). This period of recovery is called the refractory period

Refractory period - Various ion pumps and channels work together to restore the membrane back to the resting potential.

Question 8

How does the action potential move along the neurone

Answer 8

The action potential moves along the neurone as a wave of depolarisation

When an ation potential occurs, some sodium ions that enter the neurone diffuse sideways

This causes sodium ion channels in the next region of the neurone to open and sodium ions diffuse into that part

This causes a wave of depolarisation to travel along the neurone

The wave moves away from the parts of the membrane in the refractory period, because these parts can’t fire (produce) an action potential

This ensures that the wave moves in one direction, preventing the backward flow of the nerve impulse.

Once triggered, an action potential self-propagates through local currents along the axon without any decrease in size.

https://s3.eu-west-2.amazonaws.com/elements.cognitoedu.org/702944b2-b880-4dd0-b5ba-3a5cfcd1f7f0/wave-of-depolarisation-action-potential.png

Question 9

Purpose of the refractory period

Answer 9

Refractory period produces discrete impulses

during the refractory period, ion channels are recovering and cant be opened

so the refractory period acts as a time delay between one action potential and the next

this means that:
action potentials do not overlap, but pass along discrete (separate impulses)

there is a limit to the frequency at which the nerve impulses can be transmitted

action potentials are unidirectional (they only travel in one direction)

Question 10

What is the all-or-nothing principle

Answer 10

Action potentials have an All-or-Nothing nature

Once the threshold is reached, an action potential will always fire (occur) with the same change in voltage, no matter how big the stimulus is

If the threshold isnt reached, an action potential wont fire (occur).
This is the all-or-nothing nature of action potentials

A bigger stimuls wont cause a bigger action potential, but it will cause them to fire more frequently

Question 11

Describe the structure of a myelinated motor neurone

Answer 11

Cell body
Dendrites (extensions of the cell body that connect with other neurones)
Axon
Myelin sheath - made up of a schwann cell
node of ranvier
axon terminal
effector

direction of impulse - from the cell body to the effector

https://s3.eu-west-2.amazonaws.com/elements.cognitoedu.org/2310f41d-b39e-4905-868b-810d37dfe59a/neurone-structure-illustration.png

https://s3.eu-west-2.amazonaws.com/elements.cognitoedu.org/0b1be98f-f1e3-427d-b6f5-b8c921d83571/myelinated-neurone-diagram.png

Cell body - This part contains the nucleus and cytoplasmic organelles, such as mitochondria and the endoplasmic reticulum, which are crucial in the production of neurotransmitters.
Dendrons - These are short branches extending from the cell body, further dividing into highly branched dendrites to receive nerve impulses from many other neurones and transmit them towards the cell body.
Axon - A singular, long nerve fibre responsible for carrying impulses away from the cell body to other neurones or effectors

Schwann cells have several functions:

Their membranes form the myelin sheath.
They remove debris via phagocytosis.
They aid regeneration.

The myelin sheath surrounds parts of the axon, acting as an insulator that prevents the passage of ions into or out of the axon at the regions it covers.

Question 12

What are the three factors that affect the speed of conduction of action potential

Answer 12

Myelination
Axon diamter
Temperature

Question 13

Describe and explain how myelination affects speed of conduction of action potential

Answer 13

Some neurones are myelinated - have myelin sheat
Myelin sheath is an electrical insulator
in peripheral nervous system - the sheath is made up of a type of cell called a Schwann cell

Between the Schwann cells are tiny patches of bare membrane called the nodes of ranvier
Sodium ion channels are concentrated at the nodes

In a myelinated neurone, depolarisation only happens at the nodes of Ranvier (where sodium ions can get through the membrane)

The neurones cytoplasm conducts enough electrical charge to depolairse the next node

(so the imulse ‘jumps’ from node to node) (, in a myelinated neurone, sodium ions that enter the axon at one node of Ranvier diffuse sideways through the cytoplasm (axoplasm) to the next node)

This is called saltatory conduction and it is really fast

In a non-myelinated neurone, the impulse travels as a wave along the whole length of the axon membrane (so you get depolarisation along the whole length of the membrane)

This is slower than saltatory conduction (although it is still quite quick)

This process involves action potentials ‘jumping’ between nodes of Ranvier, which is faster than continuous depolarisation.

Question 14

Describe and explain how axon diameter affects speed of conduction of action potential

Answer 14

Action potentials are conducted quicer along axons with bigger diamters because there is less resistance to the flow of ions than in the cytoplasm of a small axon

With less resistance, depolarisation reaches other parts of the neurone cell membrane quicker

Question 15

Describe and explain how temperature affects speed of conduction of action potential

Answer 15

The speed of conduction increases as the temp. increases too
because ions diffuse faster
this speed only increases up to around 40degreesC though - after that the proteins begin to denature (ion channels + Na-K pump) and the speed decreases

Question 16

What is a synapse

Answer 16

A synapse is the junction between a neurone and another neurone or between a neurone and an effector cell (e.g. a muscle or gland cell)

When an action potential arrives at the end of a neurone, the information has to be passed on to the next cell - this could be another neurone, muscle cell or gland cell

Question 17

What is a synaptic cleft

Answer 17

The tiny gap between the cells at a synapse is called the synaptic cleft

Question 18

What is the presynaptic neurone and synaptic knob

Answer 18

The neurone before the synapse - has a swelling called a synaptic knob
Synaptic knob contains vesicles filled with chemicals called neurotransmitters

Question 19

What is on the surface of postsynaptic membrane

Answer 19

Receptors (to the neurotransmitters) is on the postsynaptic membrane

Question 20

draw a synpase

Answer 20

https://s3.eu-west-2.amazonaws.com/elements.cognitoedu.org/22b8efa7-a9b7-4482-82d7-bb21336f7abd/synapse-structure-diagram.png

https://cdn.savemyexams.com/cdn-cgi/image/f=auto,width=1920/https://cdn.savemyexams.com/uploads/2020/01/A-synapse.png

Question 21

Explain what ocurs when an action potential reaches the end of a neurone

Answer 21

When an action potential reaches the end of a neurone, it causes neurotransmitters to be released into the synaptic cleft
they diffuse across to the postsynaptic membrane (the one after the synapse) and bind to specific receptors

when neurotransmitter bind to receptors, they might trigger an action potential (in a neurone), cause muscle contraction (in a cel), or cause a hormone to be secreted from a gland cell

Question 22

Explain the reason for the receptors only being on the postsynaptic membrane

Answer 22

since the receptors are only on the postsynaptic membranes, synapses make sure impulses are unidirectional - the impulse can only travel in one direction

Question 23

Explain why neurotransmitters are removed from the cleft

Answer 23

Neurotransmitters are removed from the cleft so the response doesnt keep happening

they are taken back into the presynaptic neurone or they are broken down by enzymes (and the products are taken into the neurone

Question 23

Examples of neurotransmitters

Answer 23

examples of different neurotransmitters: acetylcholine and noradrenaline

Question 24

What is a cholinergic synapse

Answer 24

Synapses that use acetylcholine are called cholinergic synapses Cholinergic synapses are specific types of synapses that use acetylcholine (ACh) as their neurotransmitter.

Question 25

Explain how a nerve impulse is transmitted across a cholinergic synapse

Answer 25

ACh transmits the nerve impulse across a cholinergic synapse an action potential arrives at the synaptic knob of the presynaptic neurone the action potential stimulates voltage gated calcium ion channels in the presynaptic neurone to open (voltage-gated ion channels open at a certain voltage) calcium ions diffuse into the synaptic knob (they are pumped out afterwards by active transport) the influx of calcium ions into the synaptic knob causes the synaptic vesicles to move to the presynaptic membrane. they then fuse with the presynaptic membrane the vesicles release the neurotransmitter acetylcholine (ACh) into the synaptic cleft - this is called exocytosis ACh diffuses across the synaptic cleft and binds to specific cholinergic receptors on the postsynaptic membrane this causes sodium ions channels in the postsynaptic neurone to open the influx of sodium ions into the postsynaptic membrane causes depolarisation. an action potential on the postsynaptic membrane is generated if the threshold is reached ACh is removed from the synaptic cleft so the response doesnt keep happening. it is broken down by an enzyme called acetylcholinesterase (AChE) and the products are reabsorbed by the presynaptic neurone and used to make more ACh

Question 26

what do inhibitory neurotransmitters do

Answer 26

Inhibitory neurotransmitters hyperpolarise the postsynaptic membrane (make the potential difference more negative), preventing it from firing an action potential (prevent action potential). E.g. acetylcholine is an inhibitory neurotransmitter at cholinergic synapses in the heart. When it binds to receptors here, it can cause potassium ion channels to open on the postsynaptic membrane, hyperpolarising it.

Question 26

what do excitatory neurotransmitters do

Answer 26

Excitatory neurotransmitters depolarise the postsynaptic membrane, making it fire (trigger) an action potential, if the threshold is reached e.g. acetylcholine is an excittory neurotransmitter at cholinergic synapses in the CNS - it binds to cholinergic receptors to cause an action potential in the postsynaptic membrane - and at neuromuscular junctions

Question 26

types of neurotransmitters

Answer 26

Neurotransmitters can be excitatory, inhibitory or both

Question 27

what happens if a stimulus is weak, in terms of amount of neurotransmitter that is released

Answer 27

If a stimulus is weak, only a small amount of neurotransmitter will be released from a neurone into the synaptic cleft. Weak Stimulus: Produces a lower frequency of action potentials or smaller graded potentials, which leads to a smaller amount of neurotransmitter being released. This might not be enough to excite the postsynaptic membrane to the threshold level and stimulate an action potential.

Question 27

What is summation

Answer 27

Summation is the effect of neurotransmitter released from many neurones (or one neurone that is stimuated a lot in a short period of time) is added together

Question 28

Types of sumamtion

Answer 28

Spatial and Temporal summation

Question 29

Describe spatial summation

Answer 29

Where many neurones connect to one neurone the small amount of neurotransmitter released from each of these neurones can be enough altogether to reach the threshold in the postsynaptic neurone and trigger an action potential if some neurones release an inhibitory transmitter then the total effect of all the neurotransmitters might be no action potential many neurones release neurotransmitters = action potential (+) , (+), (+) -> action potential more inhibitor neurotransmitters are released (-) than excitatory neurotransmitters (+) = no action potential (-) , (-), (+) -> no action potential

Question 30

describe temporal summation

Answer 30

one presynaptic neurone connected to one post synaptic neurone temporal summation is where two or more nerve impulses arrive in quick succession from the same presynaptic neurone. this makes an action potential more likely because more neurotransmitter is released into the synaptic cleft high frequency of weak impulses (from same neurone) = action potential

Question 30

Advantages of both types of summation

Answer 30

Both types of summation mean synapses accurately process information, finely tuning the response

Question 31

What is a neuromuscular junction

Answer 31

A neuromuscular junction is a synapse between a motor neurone and a muscle cell

Question 32

Which type of neurotransmitter do neuromuscular junctions use and what do they do

Answer 32

Neuromuscular junctions use the neurotransmitter acetylcholine (ACh), which binds to cholinergic receptors called nicotinic cholinergic receptors

Question 32

How do neuromuscular junctions work

Answer 32

Neuromuscular junctions work in basically the same way as the cholinergic synapse but there are a few differences The post synaptic membrane has lots of folds that form clefts these clefts store the enzyme that break down ACh (acetylcholinesterase -AChE) The postsynaptic membrane has more receptors than other synapses ACh is always excitatory at a neuromuscular junction. So when a motor neurone fires an action potential, it normally triggers a response in a muscle cell. This isnt always the case for a synapse between two neurones (in this case the response could be inhibitory) https://s3.eu-west-2.amazonaws.com/elements.cognitoedu.org/e58424df-e016-4762-9595-dbd534ccc1fb/neuromuscular-junction-muscle-contraction.png

Question 32

describe the different ways drugs can affect the action of neurotransmitters at synapses

Answer 32

Some drugs affect synaptic transmission ways drugs can affect synaptic transmission: 1) some drugs are the same shape as neurotransmitters so they mimic their action at receptors (these drugs are called agonists) This means more receptors are activated e.g. nicotine mimics ACh so binds to nicotinic cholinergic receptors in the brain drug binds to receptor -> action potential (or if an inhibitory transmitter - no action potential) 2) Some drugs block receptors so they cant be activated by neurotransmiters (these drugs are called antagonists). This means that fewer receptors (if any) can be activated curare blocks the effects of ACh by blocking nicotininc cholinergic receptors at neuromuscular junctions so muscle cells cant be stimulated. results in muscles being paralysed drug prevents neurotransmiter binding to receptor -> so no action potential/resposne 3) some drugs inhibit the enzyme that breaks down neurotransmitters (they stop it from working) this means there are more neurotransmitters in the synaptic cleft to bind to receptors and they are there fore longer e.g. nerve gases stop ACh from being broken down in the synaptic cleft. leads to loss of muscle control enzyme inhibitor (competitive or non-competitive) , repeated and prolong binding -> action potential 4) Some drugs stimulate the release of neurotransmitter from the presynaptic neurone so more receptors are activated 5) some drugs inhibit the release of neurotransmitters from the presynaptic neurone so fewer receptors are activated e.g. alcohol