Ever wondered how your nervous system processes information faster than...
AQA A Level Biology Topic 6 Notes











Resting Potentials
Your neurones are like tiny electrical circuits that are constantly ready to fire. The cell body contains all the usual organelles plus the machinery for making neurotransmitters. Dendrites receive signals whilst the axon carries impulses along its length, wrapped in a fatty myelin sheath with gaps called nodes of Ranvier.
When a neurone isn't active, it maintains a resting potential of -70mV. This happens because there are more positive ions (Na⁺ and K⁺) outside the cell than inside, making the inside relatively negative.
The sodium-potassium pump works constantly using ATP to maintain this balance. It pumps 3 Na⁺ out for every 2 K⁺ in. Because the membrane is more permeable to K⁺, more potassium diffuses out than sodium diffuses in, keeping that -70mV difference.
Quick Tip: Think of resting potential like a battery that's fully charged and ready to go - your neurones are always primed for action!

Action Potentials
An action potential is what happens when your neurone's voltage jumps above -55mV (the threshold). This triggers a massive change in the membrane's permeability to Na⁺, causing depolarisation up to +40mV.
The process works like dominoes falling. Voltage-gated Na⁺ channels open, sodium rushes in, and the voltage spikes. Then repolarisation occurs as K⁺ channels open and potassium flows out, dropping the voltage back down.
Hyperpolarisation briefly takes the voltage below -70mV before it returns to normal. Once triggered at one point, the action potential spreads along the axon like a Mexican wave, with each section triggering the next.
Key Point: Action potentials are all-or-nothing events - they either happen completely or not at all, always reaching the same peak voltage.

All-or-Nothing Principle
Your neurones follow a strict rule: if the stimulus doesn't reach -55mV, absolutely nothing happens. It's like trying to start a car with a flat battery - you either get enough power or you don't.
Any stimulus that does trigger the threshold will always peak at exactly +40mV. Stronger stimuli don't make bigger action potentials; instead, they increase the frequency of firing. This prevents you from being overwhelmed by every tiny environmental change.
After each action potential, there's a refractory period where the neurone can't fire again. The sodium channels need time to reset, which ensures impulses stay separate and only travel in one direction.
Remember: The refractory period is like a cooldown timer that prevents your nervous system from going into overdrive!

Speed of Conduction
Three main factors determine how fast nerve impulses travel: myelination, axon diameter, and temperature. Understanding these helps explain why some reflexes are lightning-fast whilst others take longer.
Saltatory conduction happens in myelinated axons where action potentials jump between nodes of Ranvier instead of travelling the entire length. This dramatically increases speed since the impulse skips the myelinated sections.
Wider axons conduct faster because there's less ion leakage - it's like having a bigger pipe for water flow. Higher temperatures also speed things up because ions diffuse faster and enzymes work more efficiently, providing more ATP for the sodium-potassium pump.
Fun Fact: Your fastest nerve fibres can conduct at over 100 metres per second - that's faster than most cars on a motorway!

Synapses
Synapses are the gaps between neurones where chemical communication happens. When an action potential reaches the synaptic knob, it triggers a cascade of events that might (or might not) fire the next neurone.
The process involves calcium channels opening, vesicles fusing with the membrane, and neurotransmitters like acetylcholine diffusing across the synaptic cleft. These bind to receptors on the post-synaptic membrane, potentially opening sodium channels.
Summation helps ensure important signals get through. Spatial summation involves multiple neurones working together, whilst temporal summation means one neurone firing repeatedly in quick succession.
Key Insight: Synapses act like biological decision-makers, determining which signals are important enough to pass on to the next neurone.

Summation and Neuromuscular Junctions
Spatial summation works like a group project - multiple neurones pool their neurotransmitter to reach the threshold. Temporal summation is more like rapid-fire texting from one neurone, building up enough signal over time.
Inhibitory synapses do the opposite by making action potentials less likely. They cause Cl⁻ to enter and K⁺ to leave, hyperpolarising the membrane to around -80mV.
Neuromuscular junctions are special synapses between motor neurones and muscles. Unlike regular synapses, they're always excitatory and cause muscle contraction rather than generating new action potentials. They're the final step in turning electrical signals into physical movement.
Clinical Connection: Many muscle relaxants and poisons work by interfering with neuromuscular junctions - that's why understanding them is crucial for medicine!

Pacinian Corpuscles
Pacinian corpuscles are your pressure sensors, found deep in your skin, especially in fingers and feet. They're perfect examples of how mechanical stimuli get converted into electrical signals your brain can understand.
These receptors consist of a single sensory neurone wrapped in layers of connective tissue separated by gel. The neurone membrane contains stretch-mediated sodium channels that are normally too narrow for Na⁺ to pass through.
When pressure is applied, the tissue layers deform, stretching the neurone membrane and widening the sodium channels. This allows Na⁺ to flood in, creating a generator potential that can trigger action potentials if strong enough.
Real-world Application: This is why you can feel the texture of objects through your fingertips - thousands of Pacinian corpuscles are constantly monitoring pressure changes!

Rods and Cone Cells
Your retina contains two types of photoreceptors that work in completely different ways. Rod cells are your low-light specialists, whilst cone cells handle colour vision and fine detail.
Rods contain rhodopsin pigment that breaks down in even dim light. Many rods connect to one sensory neurone (retinal convergence), allowing summation that makes them incredibly sensitive. However, this means poor visual acuity since your brain can't distinguish between individual rod signals.
Cone cells work differently - each connects to its own bipolar cell, giving excellent visual acuity. They contain iodopsin pigments (red, blue, green) that only break down in bright light. Their distribution is uneven, with most cones concentrated in the fovea where light focuses.
Interesting Fact: You have about 120 million rods but only 6 million cones - yet cones give you most of your detailed daytime vision!

Muscles
Skeletal muscles work in antagonistic pairs - when one contracts (the agonist), its partner relaxes (the antagonist). Your biceps and triceps are classic examples, working together to bend and straighten your arm.
Muscle fibres are unique cells with multiple nuclei and a specialised membrane called the sarcolemma. Transverse tubules fold inward to spread electrical impulses throughout the fibre, whilst the sarcoplasmic reticulum stores and releases calcium ions.
Each fibre contains numerous myofibrils - the contractile units made of protein filaments. These are packed with mitochondria because muscle contraction requires massive amounts of ATP.
Remember: Muscles can only pull, never push - that's why you need pairs working in opposition to create movement in both directions!

Myofibrils and Sliding Filament Theory
Myofibrils contain the actual machinery of muscle contraction: thick myosin filaments and thin actin filaments arranged in repeating units called sarcomeres. The alternating pattern creates dark A-bands and light I-bands.
The sliding filament theory explains how muscles contract without the filaments themselves changing length. Instead, myosin and actin slide past each other, making the sarcomeres shorter and the whole muscle contract.
Z-lines mark the ends of each sarcomere, whilst the M-line runs through the middle. During contraction, the Z-lines move closer together as the filaments slide, but the A-bands stay the same width - only the I-bands get shorter.
Key Concept: Think of it like a telescope collapsing - the sections slide together but don't change their individual lengths!
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AQA A Level Biology Topic 6 Notes
Ever wondered how your nervous system processes information faster than you can blink? Understanding neurones, synapses, and muscle contraction is crucial for A-level Biology - and it's actually pretty fascinating once you get the hang of it!

Resting Potentials
Your neurones are like tiny electrical circuits that are constantly ready to fire. The cell body contains all the usual organelles plus the machinery for making neurotransmitters. Dendrites receive signals whilst the axon carries impulses along its length, wrapped in a fatty myelin sheath with gaps called nodes of Ranvier.
When a neurone isn't active, it maintains a resting potential of -70mV. This happens because there are more positive ions (Na⁺ and K⁺) outside the cell than inside, making the inside relatively negative.
The sodium-potassium pump works constantly using ATP to maintain this balance. It pumps 3 Na⁺ out for every 2 K⁺ in. Because the membrane is more permeable to K⁺, more potassium diffuses out than sodium diffuses in, keeping that -70mV difference.
Quick Tip: Think of resting potential like a battery that's fully charged and ready to go - your neurones are always primed for action!

Action Potentials
An action potential is what happens when your neurone's voltage jumps above -55mV (the threshold). This triggers a massive change in the membrane's permeability to Na⁺, causing depolarisation up to +40mV.
The process works like dominoes falling. Voltage-gated Na⁺ channels open, sodium rushes in, and the voltage spikes. Then repolarisation occurs as K⁺ channels open and potassium flows out, dropping the voltage back down.
Hyperpolarisation briefly takes the voltage below -70mV before it returns to normal. Once triggered at one point, the action potential spreads along the axon like a Mexican wave, with each section triggering the next.
Key Point: Action potentials are all-or-nothing events - they either happen completely or not at all, always reaching the same peak voltage.

All-or-Nothing Principle
Your neurones follow a strict rule: if the stimulus doesn't reach -55mV, absolutely nothing happens. It's like trying to start a car with a flat battery - you either get enough power or you don't.
Any stimulus that does trigger the threshold will always peak at exactly +40mV. Stronger stimuli don't make bigger action potentials; instead, they increase the frequency of firing. This prevents you from being overwhelmed by every tiny environmental change.
After each action potential, there's a refractory period where the neurone can't fire again. The sodium channels need time to reset, which ensures impulses stay separate and only travel in one direction.
Remember: The refractory period is like a cooldown timer that prevents your nervous system from going into overdrive!

Speed of Conduction
Three main factors determine how fast nerve impulses travel: myelination, axon diameter, and temperature. Understanding these helps explain why some reflexes are lightning-fast whilst others take longer.
Saltatory conduction happens in myelinated axons where action potentials jump between nodes of Ranvier instead of travelling the entire length. This dramatically increases speed since the impulse skips the myelinated sections.
Wider axons conduct faster because there's less ion leakage - it's like having a bigger pipe for water flow. Higher temperatures also speed things up because ions diffuse faster and enzymes work more efficiently, providing more ATP for the sodium-potassium pump.
Fun Fact: Your fastest nerve fibres can conduct at over 100 metres per second - that's faster than most cars on a motorway!

Synapses
Synapses are the gaps between neurones where chemical communication happens. When an action potential reaches the synaptic knob, it triggers a cascade of events that might (or might not) fire the next neurone.
The process involves calcium channels opening, vesicles fusing with the membrane, and neurotransmitters like acetylcholine diffusing across the synaptic cleft. These bind to receptors on the post-synaptic membrane, potentially opening sodium channels.
Summation helps ensure important signals get through. Spatial summation involves multiple neurones working together, whilst temporal summation means one neurone firing repeatedly in quick succession.
Key Insight: Synapses act like biological decision-makers, determining which signals are important enough to pass on to the next neurone.

Summation and Neuromuscular Junctions
Spatial summation works like a group project - multiple neurones pool their neurotransmitter to reach the threshold. Temporal summation is more like rapid-fire texting from one neurone, building up enough signal over time.
Inhibitory synapses do the opposite by making action potentials less likely. They cause Cl⁻ to enter and K⁺ to leave, hyperpolarising the membrane to around -80mV.
Neuromuscular junctions are special synapses between motor neurones and muscles. Unlike regular synapses, they're always excitatory and cause muscle contraction rather than generating new action potentials. They're the final step in turning electrical signals into physical movement.
Clinical Connection: Many muscle relaxants and poisons work by interfering with neuromuscular junctions - that's why understanding them is crucial for medicine!

Pacinian Corpuscles
Pacinian corpuscles are your pressure sensors, found deep in your skin, especially in fingers and feet. They're perfect examples of how mechanical stimuli get converted into electrical signals your brain can understand.
These receptors consist of a single sensory neurone wrapped in layers of connective tissue separated by gel. The neurone membrane contains stretch-mediated sodium channels that are normally too narrow for Na⁺ to pass through.
When pressure is applied, the tissue layers deform, stretching the neurone membrane and widening the sodium channels. This allows Na⁺ to flood in, creating a generator potential that can trigger action potentials if strong enough.
Real-world Application: This is why you can feel the texture of objects through your fingertips - thousands of Pacinian corpuscles are constantly monitoring pressure changes!

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Your retina contains two types of photoreceptors that work in completely different ways. Rod cells are your low-light specialists, whilst cone cells handle colour vision and fine detail.
Rods contain rhodopsin pigment that breaks down in even dim light. Many rods connect to one sensory neurone (retinal convergence), allowing summation that makes them incredibly sensitive. However, this means poor visual acuity since your brain can't distinguish between individual rod signals.
Cone cells work differently - each connects to its own bipolar cell, giving excellent visual acuity. They contain iodopsin pigments (red, blue, green) that only break down in bright light. Their distribution is uneven, with most cones concentrated in the fovea where light focuses.
Interesting Fact: You have about 120 million rods but only 6 million cones - yet cones give you most of your detailed daytime vision!

Muscles
Skeletal muscles work in antagonistic pairs - when one contracts (the agonist), its partner relaxes (the antagonist). Your biceps and triceps are classic examples, working together to bend and straighten your arm.
Muscle fibres are unique cells with multiple nuclei and a specialised membrane called the sarcolemma. Transverse tubules fold inward to spread electrical impulses throughout the fibre, whilst the sarcoplasmic reticulum stores and releases calcium ions.
Each fibre contains numerous myofibrils - the contractile units made of protein filaments. These are packed with mitochondria because muscle contraction requires massive amounts of ATP.
Remember: Muscles can only pull, never push - that's why you need pairs working in opposition to create movement in both directions!

Myofibrils and Sliding Filament Theory
Myofibrils contain the actual machinery of muscle contraction: thick myosin filaments and thin actin filaments arranged in repeating units called sarcomeres. The alternating pattern creates dark A-bands and light I-bands.
The sliding filament theory explains how muscles contract without the filaments themselves changing length. Instead, myosin and actin slide past each other, making the sarcomeres shorter and the whole muscle contract.
Z-lines mark the ends of each sarcomere, whilst the M-line runs through the middle. During contraction, the Z-lines move closer together as the filaments slide, but the A-bands stay the same width - only the I-bands get shorter.
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