By the end of this lesson
- Explain receptor stimulation and impulse generation.
- Trace impulses into and out of the CNS.
- Relate neurone adaptations to rapid transmission.
BIG QUESTION
How can a stimulus at the skin trigger muscle contraction in a fraction of a second?
Fast electrical signalling along specialised neurones links detection to coordinated output.
Receptors transduce stimulus energy
A receptor responds when a stimulus exceeds an effective threshold and changes it into an electrical signal. This conversion is called transductionConversion of a stimulus, such as light or pressure, into electrical impulses by a receptor..
Different receptors are specialised for light, pressure, chemicals, temperature or other stimuli.
Impulses travel through ordered pathways
Sensory neurones carry impulses from receptors into the CNS. Relay neurones transmit within the CNS, and motor neurones carry impulses out to effectors.
At a skeletal muscle, impulses cause contraction and therefore movement.
Neurone structure supports speed
Long axons transmit signals over distance; branched endings connect with other cells. A myelin sheathA fatty insulating layer around some neurones that makes impulses travel faster. electrically insulates many axons and enables faster saltatory conduction between gaps.
Synapses introduce a short delay, so pathways with fewer synapses can respond more rapidly.
Stages of a rapid response
Keep electrical and chemical stages distinct.
| Stage | Signal/event | Outcome |
|---|---|---|
| Receptor | Stimulus transduced into electrical activity. | Sensory impulse begins. |
| Sensory neurone | Electrical impulse toward CNS. | Information arrives centrally. |
| CNS/synapse | Chemical transmission then new impulse. | Pathway is integrated. |
| Motor neurone | Electrical impulse away from CNS. | Effector is activated. |
| Effector | Muscle contraction or gland secretion. | Observable response. |
Language that earns precision
- electrical impulse
- Rapid propagated change in membrane potential along a neurone.
- transduction
- Conversion of stimulus energy into a biological signal.
- axon
- Long neurone process conducting impulses away from the cell body.
- myelin sheath
- Insulating covering that increases conduction speed along many axons.
Common misconceptions
Claim: Electricity flows through nerves exactly like current in a metal wire.
Correction: Impulses are propagated changes in ion movement across cell membranes.
Claim: Stronger stimuli make each impulse travel faster.
Correction: They usually increase impulse frequency/recruitment rather than greatly changing conduction speed.
Claim: All neurones are myelinated.
Correction: Myelination varies between neurone types and fibres.
Match the answer to the command word
Use into the CNS for sensory and out of the CNS for motor.
Link myelin to insulation and faster transmission.
Recall, apply, then justify
Explain how stimulation of a skin receptor produces a rapid muscle response.
Hint
Follow the signal into and out of the CNS.
Suggested marking points
- The receptor detects/transduces the stimulus.
- An electrical impulse travels along a sensory neurone.
- The impulse enters the CNS.
- Relay neurones/synapses transmit the signal.
- A motor neurone carries an impulse out of the CNS.
- The muscle contracts as the effector.
Explain two neurone adaptations for rapid communication.
Hint
Long axon and myelin.
Suggested marking points
- A long axon carries impulses over distance with few cell-to-cell junctions.
- This reduces the number of synaptic delays.
- Myelin electrically insulates the axon.
- Saltatory conduction between gaps increases transmission speed.
Explain why increasing stimulus intensity can strengthen a response without making individual impulses larger.
Planning hint
Use frequency and recruitment.
What a strong design includes
- An action potential is an all-or-nothing event.
- Individual impulses have approximately fixed amplitude.
- A stronger stimulus can increase impulse frequency.
- It can also recruit more receptors/neurones.
- The CNS receives a stronger coded input.
- More motor units or stronger effector activity can result.