6B - Nervous Coordination Flashcards
Describe the polarisation of a neurone at rest.
The membrane is polarised.
For a neurone at rest, which part of the membrane is more positive: inside or outside?
Outside
At rest, are the charge across a neurone membrane the same?
No, there are more positive charges outside compare to inside the neurone.
Define resting potential.
The potential difference across a neurone membrane when it is at rest.
What is the value of the resting potential for a neurone?
About -70mV.
What causes the resting potential across a neurone membrane?
- Na⁺-K⁺ pumps move 3 sodium ions out of the cell for every 2 potassium into the cell
- Na⁺ ions can’t move back in, but K⁺ ions can move back out of the neurone using potassium ion channels
- There is more positive charge outside of the neurone compared to inside it, which causes there to be a resting potential
How does a sodium-potassium pump work?
Pumps 3 Na⁺ ions out of the cell for every 2 K⁺ ions that go into the neurone.
Do sodium-potassium pumps require energy?
Yes
What sort of transport is involved in potassium ion channels?
Facilitated diffusion
Which way do Na⁺-K⁺ pumps move sodium and potassium?
- Sodium -> Out of the cell
* Potassium -> Into the cell
What role do potassium ion channels have in a neurone membrane?
They allow potassium to move out of the cell.
What are the 3 types of transport protein involved in neurone membranes?
- Na⁺-K⁺ pump
- K⁺ channel
- Na⁺ channel
Describe the polarisation of a neurone when stimulated.
Depolarised
What is an action potential?
When a stimulus triggers sodium ion channels to open, causing a rapid change in potential difference.
What are the stages of an action potential?
1) At rest
2) Stimulus
3) Depolarisation
4) Repolarisation
5) Hyperpolarisation
6) Resting potential
Describe an action potential (including membrane potentials).
1) At rest:
• The membrane is polarised at a constant -70mV
• Na⁺ and K⁺ channels are closed
2) Stimulus:
• The neurone cell membrane is excited, causing Na⁺ channels to open
• Sodium ions diffuse into the neurone
• This causes the potential difference to become less negative
3) Depolarisation:
• If the potential difference reaches the threshold (-55mV), more Na⁺ channels open
• More sodium ions diffuse into the neurone
• The potential difference becomes rapidly more positive
4) Repolarisation:
• At about +30mV, Na⁺ channels close, while K⁺ channels open
• Potassium ions can diffuse out of the neurone
• The potential difference becomes more negative
5) Hyperpolarisation:
• K⁺ channels are slow to close, so there’s some “overshoot” when too many potassium ions diffuse out of the neurone
• Potential difference becomes slightly more negative than resting potential (-90mV)
6) Resting potential
• Ion channels are reset and the Na⁺-K⁺ pump returns the potential difference to the resting potential, then maintains it
What is the order of the events and channel openings in an action potential?
- Stimulus
- Na⁺ channels open
- Depolarisation
- Na⁺ channels close and K⁺ channels open
- Repolarisation
- Hyperpolarisation and K⁺ channels close
- Resting potential
What is the usual threshold voltage in an action potential?
-55mV
What is the usual peak voltage in an action potential?
+30mV
What is the usual hyperpolarisation voltage in an action potential?
-90mV
Give all of the important voltages in an action potential.
- Resting potential = -70mV
- Threshold potential = -55mV
- Peak voltage = +30mV
- Hyperpolarisation = -90mV
Remember to practise drawing out the shape of an action potential.
See diagram pg 146 of revision guide.
What is the refractory period?
The period after an action potential, during which the neurone cell membrane can’t be excited again.
What causes the refractory period?
The ion channels are recovering and can’t be made to open.
Where on the graph of an action potential is the refractory period?
From the peak voltage to the start of the resting potential.
Remember to practise writing out the order of an action potential.
Pg 146 of revision guide
To what order is the potential difference across a cell membrane?
mV
Describe how an action potential moves along a neurone.
1) When an action potential happens, some of the sodium ions that enter the neurone diffuse sideways.
2) This causes sodium ion channels in the next region to open, so sodium ions diffuse into that part.
3) This causes a wave of depolarisation to travel along the membrane.
4) The wave moves away from the parts of the membrane in the refractory period, because these can’t fire an action potential.
Why does the wave of repolarisation not move the wrong way along a neurone (i.e. back along where it has just travelled)?
The part that is has just covered is in its refractory period, so it cannot trigger an action potential.
What is the term for the movement of an action potential along a neurone?
Wave of depolarisation
What is the purpose of the refractory period?
- Action potentials remain as discrete impulses
- Limit to the frequency at which nerve impulses can be transmitted
- Action potentials are unidirectional
Does the strength of a stimulus have an effect on the peak voltage of the action potential across a membrane?
No, once the threshold is reached, an action potential will always fire with the same change in voltage.
What happens if the threshold voltage in an action potential is not reached?
The action potential will not fire.
How does the strength of a stimulus affect the action potential difference?
- It has NO effect on the peak voltage reached
* But a bigger signal causes action potentials to fire more frequently
Remember to practise drawing the graphs for an action potential by a small and large stimulus.
See diagram pg 147 of revision guide
What 3 factors affect the speed of conduction of action potentials?
1) Myelination
2) Axon diameter
3) Temperature
What is myelination of a neurone?
When the neurone has a myelin sheath, which is an electrical insulator.
What is a Schwann cell?
The type of cell that myelin sheaths are made of.
Where are Schwann cells found?
On the neurones of the peripheral nervous system.
What are the spaces between Schwann cells called?
Nodes of Ranvier
What are nodes of Ranvier?
Small patches of bare membrane between Schwann cells where Na⁺ channels are concentrated.
Describe the structure of a myelinated motor neurone.
- Cell body with nucleus
- Dendrites spread out on one side of the cell body
- Axon is on the other side of the cell body
- Schwann cells are along the axon, with small gaps between them (nodes of Ranvier)
- Axon ends in axon terminal (which joins to the effector)
Remember to practise drawing out the structure of a myelinated motor neurone.
Pg 148 of revision guide
On a myelinated motor neurone, what do the dendrites do?
Connect with other neurones to receive the impulse.
On a myelinated motor neurone, what does the axon terminal do?
Connects to the effector.
How does myelination affect the speed of conduction of an action potential and why?
Speeds it up, because:
• Along the axon are Schwann cells that are insulators
• Depolarisation can only happen at the nodes of Ranvier between them, which have many sodium channels
• The cytoplasm conducts enough electrical charge to depolarise the next node, so the impulse jumps between them by saltatory conduction
• This is faster than normal conduction
What is the term for the impulse jumping between nodes of Ranvier in a myelinated neurone?
Saltatory conduction
What allows the impulse to jump between nodes of Ranvier in a myelinated neurone?
The neurone’s cytoplasm conducts enough charge to depolarise the next node.
Does wide axon diameter speed up or slow down conduction along a neurone?
Speeds up
How does wide axon diameter affect the speed of conduction along a neurone and why?
Speeds it up, because:
• There is less resistance to the flow of ions in the cytoplasm
• So depolarisation reaches other parts of the neurone quicker, transmitting the impulse quicker
How does temperature affect the speed of conduction along a neurone and why?
Speeds it up (until about 40°C), because:
• Ions diffuse faster
• BUT above 40°C, the proteins begin to denature
Why does the speed of conduction along a neurone decrease above 40°C?
The proteins involved begin to denature.
What is a synapse?
A junction between:
• A neurone and another neurone
OR
• A neurone and an effector call
What is the gap between the cells in a synapse called?
Synaptic cleft
What is the neurone before a synapse called?
Presynaptic membrane
What is the swelling on the presynaptic neurone called?
Presynaptic knob
What is found in the presynaptic membrane?
Synaptic vesicles filled with neurotransmitters.
What is on the surface of the postsynaptic membrane?
Receptors
Give two examples of neurotransmitters.
- Acetylcholine
* Noradrenaline
Describe briefly how a synapse works in general.
1) Action potential reaches end of neurone
2) This causes neurotransmitters to be released into the synaptic cleft, which diffuse across it
3) These then bind to specific receptors on the postsynaptic membrane
4) This triggers an action potential, muscle contraction or hormone secretion
5) The neurotransmitter is removed from the cleft to stop the response happening over and over
What type of synapse do you need to know about?
Cholinergic synapse (one that uses acetylcholine as a neurotransmitter)
Describe how a cholinergic synapse works.
1) Action potential arrives at synaptic knob of presynaptic neurone
2) This stimulates voltage-gated calcium ion channels to open
3) Ca²⁺ ions diffuse into the synaptic knob
4) Influx of Ca²⁺ ions into the synaptic knob causes the synaptic vesicles to move to the presynaptic membrane, where they then fuse with it
5) The vesicles release acetylcholine into the synaptic cleft
6) Acetylcholine diffuses across the synaptic cleft and binds to the cholinergic receptors on the postsynaptic membrane
7) This causes Na⁺ channels in be postsynaptic membrane to open
8) Influx of Na⁺ ions causes depolarisation, which triggers an action potential if the threshold is reached
9) Acetylcholine is removed by acetylcholinesterase and the products are re-absorbed by the presynaptic neurone
What are voltage-caged ion channels?
Ion channels that only open at a certain voltage.
What is the technical term for vesicles releasing acetylcholine into the synaptic cleft?
Exocytosis
What is the name of the enzyme that breaks down acetylcholine?
Acetylcholinesterase
What is the shorthand for acetylcholine?
ACh
What is the shorthand for acetylcholinesterase?
AChE
Why are synapses unidirectional?
They only have receptors on the postsynaptic membrane.
What are the different types of neurotransmitter?
- Excitatory
* Inhibitory
What are excitatory neurotransmitters?
Those that depolarise the postsynaptic membrane, making it fire an action potential if the threshold is reached.
What are inhibitory neurotransmitters?
Those that hyperpolarise the postsynaptic membrane and prevent it from firing an action potential.
Are neurotransmitters either excitatory or inhibitory?
No, a single neurotransmitter can be both, depending on the location where they are.
Give an example of where acetylcholine is excitatory.
- CNS
* Neuromuscular junctions
Give an example of where acetylcholine is inhibitory.
• Heart
Describe how an inhibitory synapse works.
- Pre-synaptic membrane releases neurotransmitter that binds to the Cl⁻ channels on the postsynaptic membrane
- This causes the Cl⁻ channels to open, so Cl⁻ moves into the postsynaptic membrane by facilitated diffusion
- The binding of the neurotransmitter also causes the opening of nearby K⁺ channels, so K⁺ moves out of the postsynaptic membrane and into the synapse
- The combined effect of the Cl⁻ moving in and K⁺ moving out hyperpolarises the membrane, making it harder to trigger an action potential
What is summation?
Where the effect of neurotransmitter released from many neurones (or one neurone stimulated rapidly) is added together.
What are the two types of summation?
- Spatial summation
* Temporal summation
What is spatial summation?
- When many neurones connect to one neurone
- A small amount of neurotransmitter released from each can be enough together to reach the threshold and trigger an action potential
Does spatial summation only involve excitatory neurotransmitters?
No, it can also involve inhibitory neurotransmitters, which can override the effect of the excitatory neurotransmitters.
What is temporal summation?
- When two of more nerve impulses arrive in quick succession from the same presynaptic membrane
- A small amount of neurotransmitter released each time can quickly build up to reach the threshold and trigger an action potential
What is the advantage of summation?
Synapses can accurately process information, finely tuning the response.
What is a neuromuscular junction?
A synapse between a neurone and muscle.
What are the two types of synapse you need to be able to compare?
- Cholinergic synapse
* Neuromuscular junction
What neurotransmitter and receptors do neuromuscular junctions use?
- Neurotransmitter: Acetylcholine
* Receptor: Nicotinic cholinergic receptors
Compare a cholinergic synapse and a neuromuscular junction.
They work in basically the same way, except in the neuromuscular junction:
• Postsynaptic membrane has lots of folds called clefts -> These store acetylcholinesterase
• Postsynaptic membrane has more receptors
• Acetylcholine is always excitatory
Remember to practise drawing out the stricture of a neuromuscular junction.
See diagram pg 150 of revision guide
What are the folds in the postsynaptic membrane of a neuromuscular junction called?
Clefts
What are the clefts in the postsynaptic membrane of a neuromuscular junction for?
They store acetylcholinesterase (an enzyme that breaks down acetylcholine).
What are some of the types of drug that can affect a synapse?
- Mimic action of neurotransmitter
- Block receptors
- Inhibit enzymes that break down neurotransmitter
- Stimulate release of neurotransmitter
- Inhibit release of neurotransmitter
Predict the effect of this drug:
Molecule with same shape as the neurotransmitter.
Binds to receptors and activate them.
Predict the effect of this drug:
Molecule with similar shape to the enzyme that breaks down the neurotransmitter.
Binds to the enzyme, which allows it to slowly accumulate.
Predict the effect of this drug:
Molecule with complementary shape to the neurotransmitter.
Breaks down neurotransmitters.
Remember to practise practise drawing out and explaining the effect of various drugs that affect synapses.
Pg 151 of revision guide
What is skeletal muscle?
The type of muscle used to move.
What are some other names for skeletal muscle?
- Striated
- Striped
- Voluntary
What attached muscles to bones?
Tendons
What holds bones together?
Ligaments
How many muscles work in a joint movement?
Two - one relaxes and one contracts.
What does the bone at a joint work as?
A lever, giving the muscles something to pull against.
Why do muscles work in pairs?
They can only pull, not push.
What is the term for two muscles that work together to move a bone?
Antagonistic pairs
What is each of the muscles in an antagonistic pair called?
- Contracting muscle -> Agonist
* Relaxing muscle -> Antagonist
During a bicep curl upwards, which muscle is the agonist and which is the antagonist?
- Agonist -> Biceps
* Antagonist -> Triceps
In the nervous system, what do muscles act as?
Effectors
Describe the structure of skeletal muscle.
- Muscle is made up of large bundles of long cells, called muscles fibres
- The cell membrane surrounding each muscle fibre cell is called the sarcolemma
- Parts of the sarcolemma fold inwards into the sarcoplasm (cytoplasm of a muscle cell). These are called transverse (T) tubules and they help spread electrical impulses throughout the sarcoplasm.
- A network of internal membranes called the sarcoplasmic reticulum runs through the sarcoplasm. It stores and releases Ca²⁺ ions for muscle contraction.
- Muscle fibres have lots of nuclei and lots of mitochondria.
- Muscle fibres also contain lots of long, cylindrical organelles called myofibrils.
(See diagram pg 152 of revision guide)
What is the sarcolemma?
The cell membrane that surrounds a muscle fibre cell.
What is the sarcoplasm?
The cytoplasm of a muscle fibre cell.
What are transverse (T) tubules?
Bits of sarcolemma that fold inwards across a muscle fibre.
What is the purpose of transverse (T) tubules?
They help spread electrical impulses throughout the sarcoplasm so they reach all parts of the muscle fibre.
What is the sarcoplasmic reticulum?
A network of internal membranes that runs through the sarcoplasmic reticulum.
What is the purpose of the sarcoplasmic reticulum?
It stores and released Ca²⁺ ions that are needed for muscle contraction.
What important organelles does a muscle cell contain?
- Mitochondria
- Multiple nuclei
- Sarcoplasmic reticulum
- Myofibrils
What is the term for muscle cells having many nuclei?
Multinucleate
What are myofibrils?
Long, cylindrical organelles in muscle cells that are specialised for contraction.
What molecule are myofibrils made of?
Protein
Describe the general structure of a myofibril.
Made up of myofilaments:
• Thick myofilaments -> Made of myosin
• Thin myofilaments -> Made of actin
What are the thick myofilaments in a myofibril made of?
Myosin
What are the thin myofilaments in a myofibril made of?
Actin
What type of molecule are myosin and actin?
Proteins
How does a myofibril look like under a microscope?
Alternating light and dark bands
What do the dark bands in a myofibril contain?
Thick myosin filaments and some overlapping parts of thin actin filaments
What do the light bands in a myofibril contain?
Thin actin filaments ONLY
What is the name for the dark bands in a myofibril?
A-bands
What is the name for the light bands in a myofibril?
I-bands
How can you remember the names of the light and dark bands in a myofibril?
- L(I)GHT -> I-bands
* D(A)RK -> A-bands
What are the short units in a myofibril called?
Sarcomere
What marks each end of a sarcomere?
Z-line
What marks the middle of each sarcomere?
M-line
How do you remember what the M-line is?
M-line, middle, myosin
What is the H-zone?
The part of the sarcomere that contains only myosin filaments.
Describe the parts of a sarcomere.
- Z-line at each end of the sarcomere -> Actin is attached to this (A to Z)
- M-line is in the middle of sarcomere -> It is in the middle of myosin
- I-band is the area with just actin
- A-band is the area with myosin and myosin overlapping with actin
- H-zone is the area with just myosin
(See diagram pg 153 of revision guide)
Remember to practise drawing out the structure of a sarcomere.
Pg 153 of revision guide
What is the theory that explains how muscle contraction works?
Sliding filament theory
What is the sliding filament theory?
The way in which myosin and actin filaments slide over one another to make the sarcomeres contract.
Describe what happens to A-bands when sarcomeres contract.
Stay the same length
Describe what happens to I-bands when sarcomeres contract.
Get shorter
Describe what happens to H-zone when sarcomeres contract.
Get shorter
Describe what happens to A-bands, I-bands and H-zones when sarcomeres contract.
- A-bands -> Stay the same length
- I-bands -> Get shorter
- H-zones -> Get shorter
Describe the structure of a myosin filament.
- Have globular heads that are hinged, so they can move back and forth
- Each head has a binding site for actin and a binding site for ATP
Describe the structure of an actin filament.
- Have binding sites for myosin heads
* These are called actin-myosin bonding sites
What is tropomyosin and what does it do?
- A protein found between actin filaments
* It helps myofilaments move past each other
In a muscle’s resting state, what does tropomyosin do?
- Tropomyosin blocks the actin-myosin binding site
- So the myofilaments can’t slide past each other because the myosin heads can’t bind to the actin-myosin binding site on the actin filaments
Describe how a muscle contraction is triggered.
1) Action potential from a motor neurone depolarises the sarcolemma. This depolarisation spreads down the T-tubules to the sarcoplasmic reticulum.
2) This causes the sarcoplasmic reticulum to release stored Ca²⁺ ions into the sarcoplasm.
3) Ca²⁺ ions bind to a protein attached to tropomyosin, causing the tropomyosin to change shape and pulling it out of the binding site, so it is revealed.
4) The myosin head can now bind to the binding site (forming an actin-myosin cross bridge).
5) Ca²⁺ ions also activate ATP hydrolyse, which hydrolyses ATP to provide energy for the myosin head to bend, which pulls the actin along in a rowing-like motion.
6) Another ATP molecule is hydrolysed to break the actin-myosin cross bridge.
7) The myosin head moves back and reattaches to another binding site.
8) This process is repeated multiple times as long as Ca²⁺ ions are present.
Do Ca²⁺ ions bind to tropomyosin?
No, they bind to a protein in tropomyosin, which changes its shape.
What is the name for a myosin head joined to actin?
Actin-myosin cross bridge
What is ATP used for in muscle contraction?
- Moving the myosin head during contraction
* Breaking the actin-myosin cross bridge
Describe how muscle contraction is stopped.
- When the muscle stops being stimulated, Ca²⁺ ions leave their binding sites and are actively transported back into the sarcoplasmic reticulum
- This causes the tropomyosin to move back to blocking the actin-myosin binding sites
- The actin filaments slide back to their relaxed position, which lengthens the sarcomere
How do Ca²⁺ ions leave the sarcoplasm once contraction is finished?
Active transport into the sarcoplasmic reticulum
Remember to practise writing and drawing out the process of muscle contraction.
Pg 154 of revision guide
What are the 3 ways of generating ATP for muscle contraction?
1) Aerobic respiration
2) Anaerobic respiration
3) ATP-Phosphocreatine (PCr) system
Describe briefly how ATP is produced in aerobic respiration.
Oxidative phosphorylation in the cells’ mitochondria.
Describe briefly how ATP is produced in anaerobic respiration.
Glycolysis, which has an end product of pyruvate (which is converted back to lactate and can cause muscle fatigue).
Describe how ATP is produced by ATP-Phosphocreatine system.
• A phosphate group is taken from PCr and used to phosphorylase ADP
• ADP + PCr -> ATP + Cr
(Cr = Creatine)
What are the advantages and disadvantages of using an ATP-Phosphocreatine system to generate ATP?
ADV. • Very fast • Anaerobic and alactic (so lactate is not formed) DIS. • PCr runs out very fast
What is the equation for an ATP-Phosphocreatine system generating ATP?
ADP + PCr -> ATP + Cr
Cr = Creatine
When is each method of generating ATP used?
- Aerobic respiration -> Long period of low-intensity exercise
- Anaerobic respiration -> Short periods of hard exercise
- ATP-Phosphocreatine system -> Very short bursts of vigorous exercise (e.g. a tennis serve)
What happens to creatine produced by an ATP-Phosphocreatine system?
- Some of it is broken down into creatinine
* This is removed through the kidneys
What sort of people are likely to have high levels of creatinine?
Those who:
• Exercise regularly
• Have a high muscle mass
What might very high creatinine levels suggest?
Kidney damage (since it is removed through them)
What are the two types of muscle fibres?
- Slow twitch
* Fast twitch
Compare the contraction speed of slow twitch and fast twitch muscle fibres.
- Slow twitch -> Slow contractions
* Fast twitch -> Fast contractions
Compare which muscles will have a high proportion of slow twitch and fast twitch muscle fibres.
- Slow twitch -> Muscles used for posture
* Fast twitch -> Muscles used for fast movement
Compare what slow twitch and fast twitch muscle fibres are good for.
- Slow twitch -> Endurance activities without getting tired
* Fast twitch -> Short bursts of speed and power
Compare the respiration type and rate of slow twitch and fast twitch muscle fibres.
- Slow twitch -> Slow energy release + Aerobic respiration
* Fast twitch -> Fast energy release + Anaerobic respiration
Compare the mitochondria and blood vessels in slow twitch and fast twitch muscle fibres.
- Slow twitch -> Many mitochondria and blood vessels
* Fast twitch -> Few mitochondria or blood vessels
Compare and explain the colour of slow twitch and fast twitch muscle fibres.
- Slow twitch -> Reddish, because they’re rich in myoglobin (protein that stores oxygen)
- Fast twitch -> Whitish, because they don’t have much myoglobin (protein that stores oxygen)
Remember to practise drawing out a table comparing slow twitch and fast twitch muscles.
Pg 155 of revision guide
