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C3.1.6—Input through sensory neurons

Input through sensory neurons links named stimuli, coordinating structures and effectors so body systems or plant tissues produce an integrated biological response.

Syllabus
First assessment 2025
Objective
C3.1.6
Level
HL

Sensory neurones carry transduced information to the CNS

Sensory neurones carry transduced information to the CNS.

A receptor detects a stimulus and converts its energy into a change in membrane potential. If threshold is reached, an action potential travels along a sensory neurone to the CNS.

identify stimulus; receptor type; transduction; threshold; direction to CNS.

Salt ions entering taste-receptor channels depolarise the receptor cell, which then excites a sensory neurone carrying the signal to the brain.

A stronger stimulus is not encoded by a larger action potential; it can increase firing frequency or recruit receptors.

Input through sensory neurons

Assessment in practice

1–2 marks
How it is assessed

This objective is assessed through multiple choice, commonly using Identify / Outline.

Command terms

Identify / Outline

What earns marks

Mark schemes reward named structures, correct direction of information flow and explanations that keep the sequence causal.

Watch for

Giving a general body-system answer without the specific pathway step for sensory neuron input pathway.

Representative question

Question 1

[Maximum number: 1]

Between which structures do sensory neurons carry nerve impulses?

A

From receptors to muscles

B

From effectors to the central nervous system (CNS)

C

From the central nervous system (CNS) to receptors

D

From receptors to the central nervous system (CNS)

Body-System Integration

  • Nervous signals, hormones and blood transport integrate organs into coordinated systems; emergent functions arise from their interactions.
  • Sensory neurons carry receptor input to the CNS; motor neurons carry output to effectors. Mixed nerves contain both fibre types.
  • Reflex arcs provide rapid involuntary responses through sensory, relay and motor neurons. The spinal cord also links the brain and peripheral nerves.
  • Cerebral hemispheres support conscious processing; the cerebellum coordinates movement and balance; the medulla adjusts ventilation and heart activity.
  • The hypothalamus links nervous and endocrine control through the pituitary. Pineal melatonin helps time sleep; adrenal epinephrine supports acute stress responses.
  • Baroreceptors and chemoreceptors provide feedback about pressure, CO2, pH and O2.
  • The CNS controls voluntary swallowing and egestion, while the enteric nervous system coordinates gut peristalsis.

Concept essentials

  • Input through sensory neurons depends on linking the stimulus to the coordinating structure or signal.
  • The pathway for input through sensory neurons must keep information flow and response direction clear.
  • Named tissues, hormones, neurons or effectors give input through sensory neurons its biological specificity.
  • Evidence about input through sensory neurons is strongest when structure, mechanism and outcome are connected.