BIOL1030 Chap.13 Animal Hormones, Neurons and Behaviour
Animal Hormones, Neurons and Behaviour
Animal Hormones, Neurons and Behaviour covers the Week 9 lectures on how animals take in information and turn it into a coordinated response. It contrasts fast nervous control with slower, longer-lasting hormones, explains why adrenaline acts differently in liver, muscle vessels and gut vessels, and shows how thyroid hormone must be activated inside tissues.
The neuron section covers graded potentials, summation at the axon hillock, all-or-none action potentials and why tetrodotoxin paralyses. Vision links the two halves, from photoreceptors to lateral inhibition. The chapter ends with behaviour: proximate and ultimate explanations, character release under relaxed predation, and why common garden experiments are needed before calling a difference genetic.
What this chapter covers
- 01
Nervous versus endocrine responses to danger
- 02
Adrenergic receptor types and their effects
- 03
T4 to T3 conversion and negative feedback
- 04
EPSPs, IPSPs and summation at the axon hillock
- 05
Voltage-gated sodium channels and tetrodotoxin
- 06
Rods, cones and lateral inhibition
- 07
Proximate and ultimate causes of behaviour
Worked example · free
Decide which synaptic inputs make a neuron fire
- 1One EPSP: minus 70 plus 8 gives minus 62 mV, below threshold, so there is no action potential.
- 1One EPSP with one IPSP: minus 70 plus 8 minus 5 gives minus 67 mV, still below threshold.
- 1Two EPSPs: minus 70 plus 16 gives minus 54 mV, which passes the minus 55 mV threshold.
- 1Voltage-gated sodium channels open and a full-sized action potential fires, the same size whatever the input above threshold.
Key terms
- Axon hillock
- The region where a neuron's graded potentials combine and an action potential is initiated if threshold is reached.
- Adrenergic receptor
- A receptor that binds adrenaline or noradrenaline; its alpha or beta type decides the cell's response.
- Lateral inhibition
- Retinal processing in which active cells suppress neighbours, sharpening contrast and edges.
- Ultimate explanation
- An account of why a behaviour evolved, in terms of its effect on survival or reproduction.
- Common garden experiment
- A study that rears individuals from different populations under the same conditions to test for inherited differences.
Animal Hormones, Neurons and Behaviour FAQ
Why does adrenaline speed up the heart but constrict blood vessels to the gut?
The hormone is the same everywhere, but tissues carry different receptors and signalling machinery. Beta receptors in liver and muscle vessels mobilise glucose and dilate vessels, while alpha receptors in gut vessels activate a different pathway that contracts smooth muscle, sending blood to the muscles needed for immediate action.
What is the difference between a graded potential and an action potential?
A graded potential varies in size with the stimulus and fades with distance, and several can add together. An action potential begins only when the combined potential at the axon hillock reaches threshold, and it is all-or-none: once triggered, its size does not depend on how strong the stimulus was.
Why can a population difference in behaviour not be assumed to be genetic?
Animals observed in their own habitats differ in experience as well as genes, so plasticity can produce the same contrast. Only a common garden design, rearing both populations under identical conditions, ideally over a couple of generations, shows whether the difference is inherited.
Why does tetrodotoxin cause paralysis?
Tetrodotoxin blocks voltage-gated sodium channels. Without sodium rushing in at threshold, neurons cannot produce the rapid depolarisation of an action potential or pass one along the axon, so motor neurons stop activating muscles reliably. The result is paralysis, which also explains why toxin-resistant sodium channels evolve in predators of toxic prey.
Exam move
Practise summation with your own numbers until adding EPSPs and IPSPs against a threshold is automatic, then explain in words why the spike is all-or-none. For hormones, draw a three-column table of tissue, receptor and effect for adrenaline, and rehearse the thyroid case where blood levels look normal but tissues are starved of T3.
For behaviour, take any example and write one proximate and one ultimate sentence, then propose the common garden test. Check each answer against the week's model answers for the receptor names and measurements they expect. Revise vision last, as a chain from light to behaviour.
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