Neuronal communication — OCR A-Level Biology
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Neuronal communication explained
Neuronal communication covers 4 specification points, including (a) the roles of mammalian sensory receptors… and (b) the structure and functions of sensory,….
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(a) the roles of mammalian sensory receptors in converting different types of stimuli into nerve impulses
Sensory receptors are specialised cells or nerve endings that detect a specific stimulus and convert its energy into a nerve impulse, a process called transduction. Each receptor type responds to one form of energy: rods and cones in the retina detect light, hair cells in the cochlea detect sound, thermoreceptors detect temperature change, mechanoreceptors in skin detect pressure, and chemoreceptors detect chemicals. The stimulus alters the membrane permeability of the receptor cell, so ions move across the membrane and a generator potential builds. If the generator potential reaches the threshold, an action potential is generated and travels along a sensory neurone to the central nervous system. Pacinian corpuscles in skin illustrate this: pressure deforms the layered capsule, stretch-sensitive sodium channels open, Na⁺ enters and depolarises the membrane.
(b) the structure and functions of sensory, relay and motor neurones
Neurones share a basic structure: a cell body containing the nucleus, dendrites that receive impulses, and an axon that carries impulses away, often wrapped in a myelin sheath formed by Schwann cells with nodes of Ranvier between them. Sensory neurones carry impulses from receptors to the central nervous system; they have a cell body in a dorsal root ganglion and a long dendron. Relay (intermediate) neurones lie within the central nervous system and connect sensory to motor neurones, often with many short dendrites and a short axon. Motor neurones carry impulses from the central nervous system to effectors such as muscles or glands; they have a large cell body in the spinal cord and a long myelinated axon. Myelination allows saltatory conduction, speeding transmission.
(c) the generation and transmission of nerve impulses in mammals
In mammals, a resting neurone keeps its inside negative relative to the outside (about −70 mV) because the sodium–potassium pump moves 3 Na⁺ out for every 2 K⁺ in, and the membrane is far more permeable to K⁺ than to Na⁺, so K⁺ leaks out. A stimulus above threshold opens voltage-gated Na⁺ channels: Na⁺ rushes in, the membrane depolarises to about +40 mV, then Na⁺ channels close and K⁺ channels open, repolarising and briefly hyperpolarising the membrane before the pump restores the resting potential. This local depolarisation triggers the next patch of membrane, so the action potential travels as a wave. In myelinated axons the impulse jumps between nodes of Ranvier (saltatory conduction), which is faster. The all-or-nothing law means a stronger stimulus gives more frequent impulses, not bigger ones, and the refractory period enforces one-way travel.
(d) the structure and roles of synapses in neurotransmission.
A synapse is the junction between a presynaptic neurone and a postsynaptic cell, separated by the synaptic cleft (about 20 nm). The presynaptic knob contains vesicles of neurotransmitter, mitochondria and voltage-gated Ca²⁺ channels. When an action potential arrives, Ca²⁺ channels open and Ca²⁺ enters; vesicles fuse with the presynaptic membrane and release neurotransmitter by exocytosis into the cleft. The transmitter diffuses across and binds receptors on the postsynaptic membrane, opening ion channels. This may depolarise the postsynaptic membrane (excitatory, EPSP) or hyperpolarise it (inhibitory, IPSP). Synapses ensure one-way transmission, allow summation of signals, and let one neurone connect to many cells. Acetylcholinesterase breaks down acetylcholine to stop the signal.
Your focus
- Describe how a named mammalian sensory receptor converts its stimulus into a generator potential.
- Explain the role of threshold in generating a nerve impulse.
- Explain how stimulus intensity is represented by impulse frequency.
Show all 12 objectives
- Describe the structure of sensory, relay and motor neurones.
- Relate the structure of each neurone type to its function and direction of impulse transmission.
- Explain how the myelin sheath and nodes of Ranvier affect the speed of impulse conduction.
- Describe how the resting potential is established and maintained.
- Explain the sequence of ion channel events during an action potential.
- Explain how myelination and the refractory period affect transmission.
- Describe the structure of a synapse and the synaptic cleft.
- Explain the sequence of events in neurotransmission across a synapse.
- Explain the roles of synapses, including summation and one-way transmission.
Neuronal communication flashcards
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Sensory receptor
Neuronal communication exam tips
Marking Points
- Sensory receptors are specialised cells or nerve endings that detect a specific type of stimulus.
- Transduction is the conversion of stimulus energy into a generator potential and then into nerve impulses.
- Each receptor type is sensitive to one form of energy, for example rods and cones detect light and thermoreceptors detect temperature.
- The stimulus changes membrane permeability so ions such as Na⁺ move across the membrane, producing a generator potential.
- A generator potential that reaches threshold triggers an action potential in a sensory neurone.
- A larger stimulus produces a larger generator potential and a higher frequency of impulses, encoding intensity.
- Pacinian corpuscles detect pressure: deformation of the capsule opens stretch-mediated sodium channels and depolarises the membrane.
- All neurones have a cell body with a nucleus, dendrites that receive impulses and an axon that transmits impulses away.
- Sensory neurones carry impulses from receptors to the central nervous system and typically have a cell body in a dorsal root ganglion with a long dendron.
- Relay neurones are found within the central nervous system and connect sensory neurones to motor neurones, often with many short dendrites and a short axon.
- Motor neurones carry impulses from the central nervous system to effectors such as muscles or glands, with a cell body in the spinal cord and a long axon.
- The myelin sheath, formed by Schwann cells with nodes of Ranvier between them, enables saltatory conduction and faster transmission.
- Resting potential is maintained by the Na⁺/K⁺ pump moving 3 Na⁺ out and 2 K⁺ in, plus greater K⁺ permeability, giving an inside-negative membrane.
- Depolarisation results from voltage-gated Na⁺ channels opening and Na⁺ diffusing in, reversing the membrane polarity.
- Repolarisation follows when Na⁺ channels close and K⁺ channels open, allowing K⁺ to leave the axon.
- The action potential is all-or-nothing; stimulus strength is coded by impulse frequency, not amplitude.
- Myelination speeds transmission by saltatory conduction between nodes of Ranvier.
- The refractory period prevents the impulse reversing direction along the axon.
- The synaptic cleft separates the presynaptic and postsynaptic membranes and is the gap across which neurotransmitter diffuses.
- An arriving action potential opens voltage-gated Ca²⁺ channels, and Ca²⁺ entry triggers vesicle fusion and exocytosis of neurotransmitter.
- Neurotransmitter binds specific receptors on the postsynaptic membrane, opening ion channels.
- Excitatory synapses depolarise (EPSP); inhibitory synapses hyperpolarise (IPSP) the postsynaptic membrane.
- Synapses ensure unidirectional transmission and allow summation of impulses.
- Acetylcholinesterase hydrolyses acetylcholine, ending the signal and preventing continuous stimulation.
Examiner Tips
- 💡Link each named receptor to the specific stimulus energy it detects rather than listing receptors alone.
- 💡Use the term generator potential and state the threshold requirement explicitly.
- 💡Explain intensity coding through frequency of impulses, not through bigger impulses.
- 💡Match each neurone type to its direction of impulse travel and its typical cell-body position before selecting an option.
- 💡Use the terms dendron, axon, myelin sheath, Schwann cell and node of Ranvier precisely, as distractors often swap them.
- 💡Link structure to function, for example long myelinated axons to rapid conduction, to eliminate plausible but incorrect options.
- 💡Learn the sequence resting → depolarisation → repolarisation → hyperpolarisation → restoration as an ordered chain.
- 💡Quote the 3 Na⁺ out : 2 K⁺ in ratio and link it to the inside-negative resting potential.
- 💡When asked why conduction is faster, name saltatory conduction and nodes of Ranvier explicitly.
- 💡Write the transmission sequence in order: impulse arrives → Ca²⁺ in → vesicles fuse → exocytosis → diffusion → receptor binding → ion channels open.
- 💡Distinguish EPSP (depolarisation) from IPSP (hyperpolarisation) when explaining summation.
- 💡Name acetylcholinesterase and state its role in terminating the signal.
Common Mistakes
- Thinking any stimulus directly creates an action potential: correct this by explaining that a generator potential must first reach threshold.
- Assuming one receptor detects all stimulus types: correct this by stating each receptor is specific to one form of energy.
- Believing impulse size increases with stimulus strength: correct this by explaining that impulse frequency, not amplitude, encodes intensity.
- Confusing the generator potential with the action potential: correct this by distinguishing the graded receptor potential from the all-or-nothing impulse.
- Reversing the direction of sensory and motor neurones: the correction is that sensory neurones carry impulses towards the central nervous system and motor neurones carry impulses away from it to effectors.
- Placing relay neurones outside the central nervous system: the correction is that relay neurones are located within the central nervous system.
- Confusing the dendron of a sensory neurone with the axon: the correction is that the sensory neurone has a long dendron carrying impulses towards its cell body.
- Ignoring the role of Schwann cells and nodes of Ranvier: the correction is that these structures produce the myelin sheath and allow saltatory conduction.
- Thinking a bigger stimulus makes a taller action potential: correction — the response is all-or-nothing, so intensity is encoded by frequency of impulses.
- Believing the pump fires continuously during the spike: correction — the pump mainly restores ion gradients after the impulse; the spike depends on channel opening.
- Confusing depolarisation with repolarisation: correction — depolarisation is Na⁺ entering (membrane becoming positive inside); repolarisation is K⁺ leaving.
- Assuming myelin carries the current itself: correction — the impulse jumps between nodes of Ranvier; the myelin insulates.
- Saying the electrical impulse crosses the cleft: correction — a chemical neurotransmitter diffuses across; the electrical signal is regenerated postsynaptically.
- Thinking Ca²⁺ enters the postsynaptic cell: correction — Ca²⁺ enters the presynaptic knob to trigger vesicle release.
- Ignoring inhibition: correction — inhibitory synapses hyperpolarise the postsynaptic membrane, raising the threshold.
- Forgetting to remove transmitter: correction — acetylcholinesterase breaks down acetylcholine so the signal stops.