Title: .Biology Semester 2 / Nerves
Tags: nerves number definition

Q: Define neurone
A: Specialised cells that allow communication between different parts of the body in the form of nerve impulses.

Q: State two important characteristics of neurones
A: Excitable - can detect and respond to stimuli
Conductive - can transmit nerve impulses - action potentials

3 types of neurones
Sensory neurones: transmit impulses from receptors to the central nervous system
Relay neurones: found inside the central nervous system and transmit impulses to and from other neurones
Motor neurones: transmit nerve impulses from the central nervous system to an effector

Skin receptors

Myelin sheath

Node of Ranvier

Dendron - towards cell body

Cell body

Synaptic endings

Motor neurone

Dendrite

Nucleus

Schwann cell

Axon terminal

Q: Electrical signals are brief changes in the [...] across the [...] of neurones.
A: distribution of electrical charges; plasma membrane

Resting potential is the imbalance of electrical charge that exists between the interior of electrically excitable neurones and their surroundings, around -65mV

Q: When the neurone is at resting potential, it is said to be [...]
A: polarised

Sodium potassium pumps
Pumps 3 sodium ions out of the cell, 2 potassium ions in
Active process as it is against concentration gradient: requires ATP
Leads to more positive interstitial space and more negative cytoplasm
Ions diffuse back, more potassium diffuses than sodium
Creates potential difference of -65mV
Extra: Outside +++
Inside - - -

Action potential sequence
1. Membrane is polarised with a resting potential of -65mV
2. Synaptic inputs cause voltage gated sodium ion channels to open and cell depolarises
3. If depolarisation reaches -50mV threshold, channels allow many sodium ions to rush into cell, reaching +40mV. Cell is depolarised.
4. Sodium ion channels close and potassium ion channels open
5. Potassium ions diffuse out of cell and it becomes repolarised; reaches more than -65mV; hyperpolarisation
6. Potassium channel closes and the sodium potassium pumps restore the resting potential of -65mV

+40mV

-50mV

-65mV

Lower than 65mV

mV

Time (ms)

Action potential

Threshold

Resting potential

Hyperpolarisation

Failed initiation

An action potential at a particular point triggers the production of another action potential ahead of it, via a local circuit
This is where the depolarisation of one region triggers the opening of voltage gated sodium ion channels in the neighbouring region, creating a new action potential

Q: An action potential can only be transmitted in [...] direction
Due to the [...]: time needed for a region of the axon to [...], temporarily incapable of generating another
A: one; refractory period; recover from its own action potential

Transmission of action potentials
1. Temporary depolarisation of the axon membrane due to action potential creates a local circuit - between depolarised region and resting region ahead of it
2. Causes resting region to depolarise, opening voltage gated sodium ion channels, allowing sodium ions to rush into the cell
3. Potassium ions then rush out of the cell; repolarisation

Direction of impulse transmission
Action potentials are transmitted in one direction
Due to refractory period: region behind action potential takes time to recover; temporarily incapable of generating a new one

The speed of conduction of action potentials along a nerve depends on:
1. Axon diameter
2. Myelination of neurone
3. Number of synapses

Myelin Sheath structure
Schwann cells made of lipids and protein wrap around the axon
Gaps between them are called nodes of Ranvier

Myelin sheath function
Insulates axons and increases speed of transmission of impulses
Sodium and potassium ions cannot diffuse through myelin sheath; can only diffuse through axon membrane at nodes of Ranvier
Therefore action potentials jump from one node to another, called saltatory conduction
Increases speed of transmission by up to 50 times

Q: The gap between neurones is [...] in length
A: 20nm

Q: A synapse is [...]
A: the junction between two neurones

Q: The gap between two neurones is known as the synaptic [...], and is [...] in length
A: cleft; 20nm

Q: An action potential cannot cross the [...] so chemicals called [...] are used
A: synaptic cleft; neurotransmitters
Extra: E.g. acetylcholine

Q: Synapses that use [...] as the [...] are called [...]
A: acetylcholine; neurotransmitter; cholinergic synapses

The presynaptic neurone is the neurone which transmits the impulse before the synaptic cleft
The post synaptic neurone is the neurone which receives that impulse after the synaptic cleft

Neurotransmitters are made by the presynaptic neurone and stored in synaptic vesicles at the end of the axon in the synaptic knob

The plasma membrane of the post synaptic neurone has membrane receptors with specific, complementary binding sites for neurotransmitters

The all or nothing law
The principle that the strength by which a nerve or muscle fibre responds to a stimulus is independent of the strength of the stimulus.
If that stimulus exceeds the threshold potential, the nerve or muscle fibre will give a complete response; otherwise, there is no response.

5 roles of synapses
1. Summation: amplification of low level signals
2. Action: increase the possible range of actions in response to stimuli
3. Direction: ensure one way transmission of impulses
4. Acclimatisation: synapse becomes fatigued with repeated use; repetitive stimuli filtered out
5. Memory and learning: plasticity of synapses allows selective strengthening of paths

How cholinergic synapses work
1. Action potential arrives at presynaptic membrane. Stimulates voltage gated calcium ion channels to open; calcium ions enter the presynaptic neurone

How cholenergic synapses work
1. An action potential arrives at the presynaptic membrane, stimulates voltage gated calcium ion channels in presynaptic membrane to open and calcium ions enter
2. Calcium ions stimulate synaptic vesicles containing acetylcholine to fuse with presynaptic membrane, releasing it into synaptic cleft by exocytosis

How cholinergic synapses work
1. An action potential arrives at the presynaptic membrane, stimulates voltage gated calcium ion channels in presynaptic membrane to open and calcium ions enter
2. Calcium ions stimulate synaptic vesicles containing acetylcholine to move towards and fuse with the presynaptic membrane, releasing acetylcholine into synaptic cleft via exocytosis
3. Acetylcholine diffuses across synaptic cleft and binds to specific, complimentary receptor sites on postsynaptic membrane

How cholenergic synapses work
1. An action potential arrives at the presynaptic membrane, stimulates voltage gated calcium ion channels in presynaptic membrane to open and calcium ions enter
2. Calcium ions stimulate synaptic vesicles containing acetylcholine to move towards and fuse with the presynaptic membrane, releasing acetylcholine into synaptic cleft via exocytosis
3. Acetylcholine diffuses across synaptic cleft and binds to specific complementary receptor sites in post synaptic membrane
4. Sodium ion channels open in post synaptic neurone; sodium ions enter and depolarise region. May trigger action potential if reaches threshold

How cholenergic synapses work
1. An action potential arrives at the presynaptic membrane, stimulates voltage gated calcium ion channels in presynaptic membrane to open and calcium ions enter
2. Calcium ions stimulate synaptic vesicles containing acetylcholine to move towards and fuse with the presynaptic membrane, releasing acetylcholine into synaptic cleft via exocytosis
3. Acetylcholine diffuses across synaptic cleft and binds to specific complementary receptor sites in post synaptic membrane
4. Sodium channels in post synaptic neurone open; sodium ions enter which may initiate action potential if threshold is reached
5. Acetylcholinesterase enzyme hydrolyses acetylcholine to choline and ethanoic acid in the synaptic cleft
Products diffuse back into presynaptic neurone where acetylcholine is resynthesised using ATP from mitochondria and stored in synaptic vesicles for future use I.e.: recoupled and repackaged

Acetylcholine ---- Acetylcholinesterase -----> choline + ethanoic acid

Q: Effects of chemicals at synapses
[...] - similar in shape to acetylcholine so fits in Ach receptors; initiates action potentials; large dose can be fatal
[...] - prevents release of Acetylcholine
[...] - inhibits acetylcholinesterase so Ach not broken down so continuous action potentials on postsynaptic membrane
A: Nicotine; Botulinum toxin; Organophosphorus insecticides

Effects of chemicals at synapses
Nicotine - similar in shape to acetylcholine so fits in Ach receptors; initiates action potentials; large dose can be fatal
Botulinum toxin - prevents release of Acetylcholine
Organophosphorus insecticides - inhibits acetylcholinesterase so Ach not broken down so continuous action potentials on postsynaptic membrane

Q: Effects of chemicals at synapses
Nicotine - similar in shape to acetylcholine so fits in Ach receptors; initiates action potentials; large dose can be fatal
[...] toxin - prevents release of [...] causing [...]
Organophosphorus insecticides - inhibits acetylcholinesterase so Ach not broken down so continuous action potentials on postsynaptic membrane
A: Botulinum; Acetylcholine; flaccid paralysis

Effects of chemicals at synapses
Nicotine - similar in shape to acetylcholine so fits in Ach receptors; initiates action potentials; large dose can be fatal
Botulinum toxin - prevents release of Acetylcholine
Organophosphorus insecticides - inhibits acetylcholinesterase so Ach not broken down so continuous action potentials on postsynaptic membrane - like Sarin❤️

Q: An [...] is a [...] or a [...]
A: effector; muscle; gland

Q: The two communication systems are [...] and [...]
A: nervous; endocrine

Stimuli are defined as detectable changes in environment, either cellular or external

The need for communication systems in multicellular organisms:
Coordinate activities of different systems
Homeostasis
Response to external stimuli

Q: The cell membrane of neurones is also known as the [...] membrane or the [...]
A: axon; neurolemma

Q: Ions [...] pass through the phospholipid bilayer directly
A: cannot
Extra: They use transport proteins

Q: [...] can be understood as a [...]
A: Potential difference; charge separation

Q: Potential difference, or [...], is measured in [...] by a [...]
A: pd; volts; voltmeter

Differences between motor and sensory neurones

1. Position of cell body:
Sensory on the side, motor at one end


2. Location of cell body in nervous system:
Sensory PNS, motor CNS


3. Length of axon
Sensory shorter, motor longer (axon, away)


4. Length of dendron
Sensory longer, motor absent; shorter dendrites


5. Communication direction
Sensory from receptor to CNS, motor from CNS to effector

Q: At resting potential, the cytoplasm is more negative [...], but not [...]
A: with respect to the outside; negative per se

Q: An [...] is a measurement device used to produce the [...]
A: oscilloscope; classic action potential trace

The role of the dendrites is to transmit stimuli towards the cell body in motor and relay neurones

Q: Name two organelles that would be abundant in the cell body of neurones, and what functions they serve
A: Mitochondria: provide ATP for the active transport of sodium potassium pumps
Rough ER: synthesis of transport proteins

Q: Where does the cell body of a motor neurone lie?
A: At one end of the cell, within the central nervous system

Title: Role of synapses
Q: [...]
Action: increases range of possible actions in response to stimuli
...
...
...
A: Summation: amplification of low level signals
Q: Summation: amplification of low level signals
[...]
Direction: ensures one way transmission of impulses
...
...
A: Action: increases range of possible actions in response to stimuli
Q: ...
Action: increases range of possible actions in response to stimuli
[...]
Acclimatisation: synapse becomes fatigued and inactive with repeated use
...
A: Direction: ensures one way transmission of impulses
Q: ...
...
Direction: ensures one way transmission of impulses
[...]
Memory and learning: plasticity of synapses allows selective strengthening of paths
A: Acclimatisation: synapse becomes fatigued and inactive with repeated use
Q: ...
...
...
Acclimatisation: synapse becomes fatigued and inactive with repeated use
[...]
A: Memory and learning: plasticity of synapses allows selective strengthening of paths
Summation: amplification of low level signals
Action: increases range of possible actions in response to stimuli
Direction: ensures one way transmission of impulses
Acclimatisation: synapse becomes fatigued and inactive with repeated use
Memory and learning: plasticity of synapses allows selective strengthening of paths
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Q: Name Q
A: Myelin

Myelin inhibits the diffusion of ions through the axon membrane, so depolarisation is only possible at the nodes of ranvier, where no myelin is present
Therefore local circuits are lengthened and action potentials are transmitted by saltatory conduction from one node to the next, increasing the speed of the impulse transmission

Title: How to structure: questions about the classic action potential trace
Q: [...]
The movement of ions
...
A: Activity of ion channels or pumps
Q: Activity of ion channels or pumps
[...]
How this effects the potential difference of the membrane
A: The movement of ions
Q: ...
The movement of ions
[...]
A: How this effects the potential difference of the membrane
Q: [...]
[...]
[...]
A: Activity of ion channels or pumps; The movement of ions; How this effects the potential difference of the membrane
Other fields (not shown on cards):
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Q: Explain why temperature effects the speed of nerve transmission
A: • Higher temperature increases kinetic energy of ions, increases rate of diffusion of ions
• Temperatures over 40⁰c may denature transport proteins, changing their structure that will cause them to lose their function: creating action potentials by transporting ions over the bilayer, reducing rate of or inhibiting nerve impulse transmission

Remarks: Speed measured in meters per second

Draw a table outlining the differences between myelinated and non-myelinated neurones

Q: Explain how resting potential is maintained in a neuron
A: Active transport of 3 sodium ions out and 2 potassium ions in by Na/K pumps
Neurolemma is more permeable to potassium ions than sodium ions; potassium ions diffuse out

Q: Explain how skin pressure changes effect depolarisation of neurones in the region
A: Pressure stretches neurolemma
Sodium ion channels open, allow influx of sodium ions
The more pressure, the more channels open, the more depolarised the region

Q: Explain why there are slower responses to stimuli in the nerves of patients with damaged myelin sheath (like in MS)
A: - Less saltatory conduction
- Depolarisation must occur over a larger area, takes longer

Q: Name the enzyme that breaks down ATP
A: ATPase

Q: Why do fast twitch muscle fibres require more ATPase?
A: More ATPase means quicker hydrolysis of ATP and quicker recovery