Topics‎ > ‎WJEC A2 Topics‎ > ‎

Reflex, taxis and kinesis

Subscribe!
I'd like private tutoring!

There are behaviours which are innate and may not require any conscious thought. Escape reflexes, taxes (sg. taxis) and kineses (sg. kinesis) are types of innate behaviours that help organisms avoid danger, find food and respond to environmental stimuli quickly enough to be effective.


Reflex - think of sensory neuron --> interneuron --> motor neuron

The knee-jerk reflex is too famous so it shall be ignored on this occasion (also, what is the point of it anyway? I never recall it saving me from anything, do you?!). Instead we turn to the iris reflex responsible for controlling the amount of light entering your eyes.

(Fun experiment that's also an opportunity for justified procrastination: go in the bathroom and keep covering an eye and uncovering quickly while staring in the mirror to see how quickly the pupil appears to change size!)

...back now.

How does this reflex work? Most reflexes have multiple steps, but the outline goes something like this:

receptor --> sensory neuron --> integration centre --> motor neuron --> effector

In other words: the retina (receptor) at the back of the eyes senses the amount of light present. This information is relayed (sensory neuron) to a centre in the brain (integration centre) which then acts upon it by stimulating the motor neuron to carry out a response command to the effector - the iris muscles which contract.


The above example uses the spinal cord as the integration centre instead of the brain. The escape reflex is a haphazard response to an unknown danger, as seen by insects moving in all directions following an attempt to squash them. It can also happen in humans as exemplified by the reflex to pull away from very hot or cold objects, jump at a loud sound or blink when startled by something.


Taxis and kinesis - think of a maggot jiggling aimlessly



What happens it you expose a maggot (not that you'd ever do such a thing) to an extreme temperature? Will it identify the nearest fire exit and calmly and orderly proceed to the assembly point? Will it pull out its emergency kit and offer first aid to its fellow maggots?

No I'm afraid. It will just jiggle about and move its squishy body utterly and completely aimlessly until it just so - hey presto - happens to get to a better place. It must have worked pretty well apparently! This maggoty aimlessness that characterises their attitude to life is called kinesis.

What if there's a freshly decomposing, nicely rotten carcass lying around, emanating a stream of increasingly concentrated chemicals that give its characteristic smell? That seems like a good dinner for our maggots. But will they find it on time? 

You bet they will. They may not have any sophisticated response to burning flames or death by ice, but they sure have a built-in putrid flesh detector (put-dar?). When they move specifically towards the ever-increasing chemical concentration from a carcass, it's taxis they're up to.

So:

Kinesis - moving randomly hoping for the best (in response to a stressful stimulus)

Taxis - getting in a cab to your destination (ok!!! sorry. Taxis is moving specifically towards/away from a stimulus)


Case study: woodlice

Woodlice are common insects that feed on dead plant matter. They like to live in dark, moist conditions because water conservation is important to them. Their flat bodies and lack of a waterproof cover means that exposure to bright or dry conditions will result in water loss rapidly. Therefore, they respond quickly to changes in temperature.




Their kinesis relating to temperature is called positive orthokinesis because they move faster to get away from a place that is too hot or too cold. The same kinesis response applies to humidity, in terms of speed of movement but also speed of turning direction, called klinokinesis. Turning behaviour is a popular variable that is tested in experiments with woodlice as well as maggots.

A maze is created for them where they begin by being forced to turn in one direction. Having been forced to make a turn, they are then faced with a T junction where they can choose to turn either direction. The expected finding is that they will turn the opposite direction to the one they were forced to turn at the beginning. This prevents them from getting back to their original spot (going around in circles).

Another thing woodlice respond to via kinesis is proximity to other objects including woodlice. They want to stay in groups and sheltered spaces, so unless their body area is touching other things, they will speed up their movement until they get to a tight place, away from predators and the dangers of desiccation. This is called positive thigmokinesis.

Woodlice sense plant litter by picking up the chemical signals given off by bacteria decomposing the plant waste. This enables their taxis to their food.

Ok byeeeeeeee

<< Previous topic: Gene expression and the brain                                                                 Next topic: Learned behaviour >>
Comments