E1 Stimulus and Response


E.1.1  Define the terms stimulus, response and reflex in the context of animal behaviour

Stimulus:  A change in the environment (external or internal) that is detected by a receptor

Response:  A change in an organism (an action) resulting from a stimulus

Reflex:  A type of response that is rapid and involuntary (unconscious)

E.1.2  Explain the role of receptors, sensory neurons, relay neurons, motor neurons, synapses and effectors in the response of animals to stimuli

Receptors transform stimuli into electrical nerve impulses

Sensory neurons relay the nerve impulse to the central nervous system (via the dorsal root of the spinal cord)

Relay neurons (also called interneurons or connector neurons) transmit nerve impulses within the CNS

Motor neurons receive nerve impulses from the CNS (via the ventral root) and relays the signal to an effector

Effectors (muscles or glands) produce a response to the stimulus

Synapses are the junctions between two nerve cells and transmission of a signal across these junctions require chemical neurotransmitters

The Stimulus-Response Pathway

stimulus response

E.1.3  Draw and label a diagram of a reflex arc for a pain withdrawal reflex

reflex arc

E.1.4  Explain how animal responses can be affected by natural selection, using two examples

Natural selection describes the process by which the frequency of an inheritable characteristic changes as a result of external (environmental) agents

These beneficial inheritable traits (adaptations) can include instinctive behaviours such as migrating, foraging, hunting, communication, etc.

Example:  Blackcap Migration Patterns

  • The blackcap (Sylvia atricapilla) exhibits behavioural variation in its migration patterns from its summer breeding grounds in Germany
  • Historically, most blackcaps migrated south to Spain (warmer climate in winter) with a minority migrating west to the UK (closer, but cooler)
  • With a rise in global temperatures, more blackcaps are now migrating west to the UK (more favourable survival prospects increase allele frequency)

Example:  House Sparrow Feeding Behaviours

  • The house sparrow (Passer domesticus) will chirp and gape as a fledgling in order to be fed by its parents
  • In a nest of chicks, those which chirp louder and gape more obviously are more likely to be fed more and survive longer
  • Hence the alleles responsible for chirping and gaping are passed to offspring increasing prevalence of the behaviour