AHL: Neurobiology
IB Biology Higher LevelΒ· 7 min read
1. Neuron Structure and Functionβ β β βββ± 15 min
Neuron
Specialized excitable cells that transmit electrical and chemical signals throughout the nervous system, adapted for rapid long-distance communication
Example:
Motor neurons transmit signals from the central nervous system (CNS) to effector muscles
Neurons are classified by their function into three main types, each with a distinct structure that matches its role in the nervous system:
Sensory neurons: Transmit signals from sensory receptors towards the CNS, typically unipolar in structure
Relay neurons (interneurons): Connect neurons within the CNS; 99% of human neurons are interneurons
Motor neurons: Transmit signals from the CNS to effector muscles/glands, multipolar with many dendrites
A student observes a neuron with one long axon extending from the spinal cord to the calf muscle, and many branching dendrites attached to the cell body. Identify the neuron type and explain how its structure suits its function.
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Step 1: Match the location and structure to neuron classification
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This neuron connects the CNS (spinal cord) to a muscle effector, with multiple dendrites at the cell body
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Step 2: Identify the neuron type
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This is a motor neuron
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Step 3: Explain structure-function matching
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Multiple dendrites allow the motor neuron to receive and integrate input from many different interneurons in the spinal cord before generating a response. The long axon extends the full distance from the spinal cord to the calf muscle, enabling long-distance transmission of the signal to the effector.
Exam tip:
IB exam questions almost always expect you to link neuron structure to its function, not just list structural features.
2. Resting and Action Potentialsβ β β β ββ± 20 min
Resting Potential
The steady electrical potential difference across a neuron's membrane when it is not transmitting a signal, typically around mV, maintained by active ion transport.
Resting potential is maintained by the sodium-potassium (Na+/K+) ATPase pump, which moves 3 Na+ ions out of the cell for every 2 K+ ions moved in. This creates a concentration gradient with higher Na+ outside the cell and higher K+ inside. When a stimulus depolarizes the membrane to the threshold potential (~ mV), voltage-gated Na+ channels open, triggering an action potential.
Explain how ion movement generates the depolarization (rising) phase of an action potential.
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Start with the resting state, where the membrane potential is:
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A stimulus depolarizes the membrane until it reaches threshold potential. Voltage-gated sodium channels respond to this voltage change by opening their activation gates.
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Sodium ions flow passively down their concentration gradient into the axon, since [Na+] is much higher outside the cell than inside.
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This influx of positive charge rapidly makes the membrane potential more positive, reaching approximately mV at the peak of the action potential, completing depolarization.
3. Propagation of Action Potentialsβ β β β ββ± 20 min
Action potentials propagate along the axon because the influx of Na+ during depolarization creates a local current that diffuses along the inside of the axon. This current depolarizes the adjacent region of membrane to threshold, triggering a new action potential. Two main factors increase conduction speed: increased axon diameter and myelination.
Explain why a thick myelinated axon conducts action potentials faster than a thin unmyelinated axon.
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First, explain the effect of myelination:
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Myelin is an insulating layer produced by glial cells that prevents ion flow across the axon membrane, except at small unmyelinated gaps called nodes of Ranvier. Action potentials are only regenerated at these nodes, so the signal jumps between nodes (called saltatory conduction) instead of propagating along every section of membrane, which drastically increases speed.
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Next, explain the effect of axon diameter:
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A larger axon diameter reduces internal resistance to the diffusion of ions along the axon. Local currents from depolarization spread further and faster, so the next region of membrane reaches threshold more quickly, increasing overall conduction speed.
4. Chemical Synaptic Transmissionβ β β β ββ± 20 min
Synapse
A specialized junction between a presynaptic neuron and a postsynaptic cell (neuron or effector) that allows signal transmission.
Most synapses in the nervous system are chemical. When an action potential arrives at the presynaptic axon terminal, it triggers a sequence of events that leads to release of a neurotransmitter, which then alters the activity of the postsynaptic cell.
Explain why acetylcholinesterase activity is required for normal function of cholinergic synapses.
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After acetylcholine (ACh) is released into the synaptic cleft and binds to postsynaptic receptors to trigger a response, the enzyme acetylcholinesterase breaks ACh down into acetate and choline.
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Choline is then transported back into the presynaptic terminal to be resynthesized into new ACh for future signals.
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If acetylcholinesterase did not break down ACh, ACh would remain bound to postsynaptic receptors, causing continuous stimulation of the postsynaptic cell. This leads to unregulated repeated action potentials and uncontrolled muscle contraction.
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Breakdown of ACh resets the synapse, allowing it to respond to new signals and enabling precise control of neural communication.
5. Common Pitfalls
Wrong move:
Claiming the sodium-potassium pump causes depolarization during an action potential
Why:
The pump only maintains resting potential; depolarization comes from passive Na+ flow through voltage-gated channels
Correct move:
State that voltage-gated Na+ channels open at threshold to allow Na+ influx for depolarization, while the Na+/K+ pump maintains resting potential
Wrong move:
Saying action potentials jump along the entire myelinated axon in saltatory conduction
Why:
Action potentials are only regenerated at nodes of Ranvier; depolarization spreads passively between nodes
Correct move:
Explain that saltatory conduction involves action potentials forming only at unmyelinated nodes, which speeds up conduction
Wrong move:
Omitting calcium from descriptions of synaptic transmission
Why:
Calcium entry into the presynaptic terminal is the direct trigger for neurotransmitter exocytosis
Correct move:
Always include the step: action potential opens voltage-gated Ca2+ channels, Ca2+ enters, triggering exocytosis of neurotransmitter vesicles
Wrong move:
Claiming all synapses are excitatory
Why:
Synapses can be excitatory or inhibitory depending on the neurotransmitter and receptor
Correct move:
Note that excitatory synapses depolarize the postsynaptic membrane, while inhibitory synapses hyperpolarize it to reduce action potential generation
Wrong move:
Stating Na+ concentration is higher inside a resting neuron
Why:
The sodium-potassium pump moves Na+ out of the cell, reversing this gradient
Correct move:
Remember the gradient: Na+ higher outside the cell, K+ higher inside the cell at rest
6. Quick Reference Cheatsheet
Feature | Key Exam Details |
|---|---|
Resting Neuron | Membrane potential: ~ mV, Na+ higher out, K+ higher in, maintained by Na+/K+ pump |
Action Potential Phases | Depolarization: Na+ channels open, Na+ influx; Repolarization: K+ channels open, K+ efflux; Refractory period resets the membrane |
Conduction Speed Factors | Faster with thicker axon diameter and myelination; saltatory conduction in myelinated axons |
Synaptic Transmission Steps | AP arrives β Ca2+ enters β neurotransmitter exocytosis β binds postsynaptic receptors β response β neurotransmitter degraded/reuptake |
Neuron Types | Sensory: Receptor β CNS; Relay: Connect neurons in CNS; Motor: CNS β effector |
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 Β· 2
Action potential propagation factors
- 2023 Β· 1
Synaptic transmission steps
- 2024 Β· 2
Neuron type function comparison
What's Next
This sub-topic lays the core foundation for all higher-level neurobiology topics in IB Biology HL, including the organization of the nervous system, brain structure and function, and the mechanism of drug and toxin action on neural signaling. These concepts are frequently tested in extended response questions, so mastering action potentials and synaptic transmission is critical for high marks. Neurobiology also connects closely to other themes in IB Biology, linking membrane transport, cell signaling, and organism-level homeostasis to explain how the nervous system enables interaction with the environment. Explore the following related topics to build on this foundation: