Simple Reflexes

Rapid, innate and stereotyped animal reflex responses

  1. Nervous coordination of reflexes
  2. The involuntary reflex response
  3. The hatchling Xenopus laevis tadpole
  4. Rapid reflex responses in a hatchling tadpole
  5. The Xenopus laevis tadpole nervous system
  6. Summation

1. Nervous coordination of reflexes

In this section, we are going to consider how nervous systems allow animals to coordinate involuntary actions and react to stimuli. 

Reaction to certain stimuli may mean the difference between life or death (or serious injury!) and therefore need to be fast.

For example, many animals have involuntary escape reflexes that allow them to escape quickly from predators or other types of dangerous stimuli.

These responses are normally rapid, innate reflex responses involving a short nerve pathway, allowing quick movement, usually away from the stimulus.

For example, when a tadpole is touched on its side, a reflex pathway is triggered and the animal flexes and swims away as seen in the video the right.

Before we consider how a reflex response is generated, first let’s recap on how a nervous system is organised: 

A nervous system contains two regions:

The central nervous system (CNS), including the brain and spinal cord.

The peripheral nervous system (PNS), including the sensory and motor systems.

Sensory information is processed by the PNS and passed to the CNS where it is processed and sent back to the PNS in the form of motor information, for some kind of response.

This motor response can be voluntary (involving decision making in the brain) or involuntary (involving no decision making and only the spinal cord).

As no conscious thought is required by the brain, involuntary responses (also called automatic or reflex responses) involve short nerve pathways and as a result, the time from stimulus to motor response is short.

These responses are often innate (do not require learning) and active from birth. They are therefore responses often key for survival.

Hatchling Xenopus tadpole

tiny hatchling tadpole needs to stay safe and swimming away from predators is crucial for its survival.

Therefore its developing nervous system has key pathways in place that allow rapid, innate behaviours. These include a touch reflex and a struggling reflex.

Click here for more info on the hatchling Xenopus tadpole. (hatchling story pop-up coming soon!)

2. The involuntary reflex response

Before we focus on the tiny tadpole case study, let’s first get to grips with the general background of reflex responses. 

reflex response is an involuntary reaction to a stimulus that results a response from an effector (usually a muscle or gland).

stimulus is detected by a receptor, which triggers an action potential (AP) in a sensory neuron in the PNS.

This AP is carried to the spinal cord in the CNS by the sensory neuron along its axon, where it synapses with an interneuron(or relay neuron).

An action potential is generated in the interneuron, which in turn synapses with a motor neuron that has its cell body in the spinal cord of the CNS.

An action potential is carried from the CNS to the PNS along the axon of the motor neuron, which then triggers an effector to respond.

If the effector is a muscle, it may contract due to a release of neurotransmitter at the neuromuscular junction. If the effector is a gland, the gland may secrete something.

Examples often given in exam text books include the retraction of an arm when a hot stimulus is touched or the knee-jerk reflex.

However, with 86 billion nerve cells, the human brain is so complicated that even with decades of research by many dedicated scientists, we still do not understand the nerve circuits allowing us to carry out even the simplest of reflexes.

picture of complex nerve pathway coming soon!

3. The hatchling Xenopus laevis tadpole

One way to get around this difficulty in studying complex nerve pathways is to study simpler animals or indeed young animals that have not fully developed their nervous systems. 

We can then begin to understand how specific neurons work together to produce a rapid, innate response, key to survival.

Adult and hatchling African clawed toad

At the University of Bristol, we study the African clawed toad (Latin name: Xenopus laevis)In fact, we focus on tiny tadpoles that have just hatched out of their eggs. Only a few days old, these hatchling Xenopus laevis tadpoles provide a perfect animal model to study simple nervous systems.

Click here to read more about the hatchling Xenopus tadpole. POP-Up coming soon!

4. Rapid reflex responses in a hatchling tadpole

In this section we look more closely at a specific reflex pathway example. The concept is the same as you will find in text books, but with more specific examples. This section is meant to be an aid to understanding, not something to rote learn, so sit back and enjoy! 

Hatchling tadpole

Xenopus laevis hatchling tadpoles of a few days old, have a simple nervous system and are able to react to stimuli they are likely to encounter in their watery environment.

Being small, numerous and nutritious, tadpoles are often the prey of a number of different animals and because of this, they need to be able to react fast to avoid predators.

Examples of tadpole predators: 

Film of tadpole predators coming soon!

One rapid reflex response that the Xenopus hatchling has evolved is the flexion manoeuvre, so called because the body of the tadpole flexes when it bends away from the stimulation of the skin on its head or body.

The tadpole may then swim away in the opposite direction to the stimulus, which is an innate response to escape predators (see short film above).

In a reflex experiment in the lab, if the tadpole is touched very gently on one side of the body (at the little arrow), it bends away.

To the left is a series of stills from a high speed video (150 fps), showing a tadpole bending away from a stimulus on its body (represented by arrow).

The flexion of the hatchling represents a unique reflex example in neurobiology, as it is one where we know all of the neurons involved in the reflex arc and how the behaviour is generated.

To the right is a diagram of the front end of a hatchling Xenopus laevis, including the nervous system, eye and muscles.

5. The Xenopus laevis tadpole nervous system

In this section we will look more closely at the hatchling Xenopus laevis nervous system and how behaviour, in response to a simple touch to the skin is generated. 

The Xenopus hatchling tadpole has a very simple brain and spinal cord (0.1 mm in diameter), with swimming muscles running down each side of the body.

If the tadpole is preserved and a slice is taken through the body and stained blue, we can see the skin, spinal cord and muscles on right and left sides.

Below is a diagram of the front end of a hatchling Xenopus laevis (left) and a cross section as seen under a microscope (right):

Here you can see the circular spinal cord in the middle, surrounded by the muscles and then the skin on the outside.

The sensory receptors that respond to touch in the tadpole are actually sensory neurons

These sensory cells innervate the skin, which means their dendrites are close to the skin surface and are activated when the skin is touched.

To the right is a diagram of the spinal cord of the tadpole. Note the dendrite of the sensory neuron (yellow) is feeding information into the spinal cord in the CNS from the skin.

When the sensory cells are activated by the touch stimulus they fire an action potential, which is carried along their axons to interneurons (called sensory pathway neurons, in red),which in turn synapse with motor neurons (green).

An action potential is then generated in the motor neurons, causing muscles on the opposite side to stimulation to contract. The tadpole will then pull its body away from the stimulus.

Left is the high speed outline of the tadpole flexion. Notice how the stimulus on the left side causes the muscles to contract on the right side, pulling the body away from the stimulus, before returning to the original straight position. 

6. Summation

In this section we are looking even more closely into the flexion pathway in the hatchling Xenopus tadpole and the process of summation. 

Summation is where input from a number of neurons causes a small amount of excitation (a small depolarisation) in another neuron and this excitation adds up to reach the threshold, triggering an action potential.

For a recap on action potentials and threshold voltages click here(Pop-up box recapping action pots coming soon!)

Text books often imply that neurons synapse in a linear system, i.e. one sensory neuron, one interneuron and one motor neuron, and that the excitation (depolarisation) triggered in the post-synaptic neuron comes from one neuron only.

In fact, when one sensory neuron is stimulated, its axons may indeed run along the spinal cord and produce strong synaptic excitation of many interneurons.

This is the case in the Xenopus laevis hatchling flexion pathway.

In response to stimulation from a single sensory neuron, each of many sensory pathway neurons/interneurons fires an action potential and their axons cross to the other side of the spinal cord and make synaptic connections with motor neuronson that side.

However, each of these sensory pathway neurons/interneurons will only produce a weak excitation (a small amount of depolarisation) in the motor neuron and many will sum together to reach a threshold voltage.

If that threshold is reached, an action potential is generated in the motor neuron. This is called sensory summation (i.e. the summing together of excitation/depolarisation from a number of neurons all synapsing with one motor neuron).

The final part of the story is the response: When the motor neurons fire an action potential, the excitation or depolarisation travels down its axon and causes a neurotransmitter to be released at a neuromuscular junction. The muscles on the opposite side of the body to where the skin was stimulated will then contract, pulling the tadpole away from the stimulus.

Below is a diagram showing how a single sensory neuron (yellow) stimulates many pathway neurons (orange), which produce excitation in a single motor neuron (green). 

For further information on how this process works click here. (link to further info coming soon!)

The Digestive System


The digestive system is modified to account for the change of the herbivorous diet of the tadpole to the carnivorous diet of the frog.

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The Skin


The skin adapts for the change from a purely aquatic lifestyle to an amphibious lifestyle.

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The Reproductive System


The urogenital system develops to allow for reproduction in adulthood.

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The Skeleton


The skeletal structure develops to accommodate the change from tail swimming to using legs to move around. The skull also needs to be remodelled for a frog's change in vision.

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The Nervous System


A tadpole sees from eyes that are positioned on opposite sides of the head. During metamorphosis, the optical nerves develop to accommodate a frog's binocular vision, where the eyes are positioned at the front of the head.

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1. Mating and Laying Spawn


Male and female frogs go to ponds in the winter. They mate in the spring, and the female lays big clumps of eggs.

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2. Frog Spawn


Frogs eggs are called frogspawn. Each round black egg is about 1 mm wide and is surrounded by a blob of jelly. Other animals produce spawn as well, which you can look at here.

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3. Maturing Frog Spawn


After a few days, the eggs begin to grow into tiny tadpoles inside the jelly.

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4. Hatchlings


Then the tadpoles hatch! They are about 5 mm long and they can’t swim (yet). They can bend their body from side to side using special muscles along their trunks and tails.

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5. Young Tadpoles


When their tail is big enough, they swim off into the pond to start to feed. At first they have gills (the pale protrusions from the head region in the left photo) so they can breath underwater like fish. Young tadpoles feed by grazing the surface of pond weeds and also eating tiny floating plants called algae.

Click here to play a tadpole feeding game called Taddypole!

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6. Maturing Tadpoles


Later they develop lungs and can swim up to the surface of the water to breath. The gills are absorbed back into their bodies and eyes develop. Older tadpoles are then able to feed on small animals like young insects.

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7. Mature Tadpoles

The next stage in development is to grow back legs. Tadpoles during this stage need to eat meat in order to get the proper nutrients to grow.

If you are looking after tadpoles, be careful as they can eat each other if you don't give them meat to eat! Click here to learn more about how to look after tadpoles as pets.

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8. Froglet

Finally, tadpoles grow front legs and their tail shrinks until it almost disappears. This is when they climb out of the pond and start living on dry land. Small frogs are commonly called froglets.

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9. Adult Frog

The little froglets will stay by the pond and slowly grow over the summer, eating small insects and worms. They will hibernate just like other adult frogs in damp spots near ponds from autumn until the next spring.

After four years, the new frogs will become adults and will be ready to mate and begin the cycle again.

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Copy - 9. Adult Frog

The little froglets will stay by the pond and slowly grow over the summer, eating small insects and worms. They will hibernate just like other adult frogs in damp spots near ponds from autumn until the next spring.

After four years, the new frogs will become adults and will be ready to mate and begin the cycle again.

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Desert Habitats

Desert habitats are the driest habitats in the world. Most people only think of very hot habitats as being deserts, but cold habitats can be deserts as well! Animals and plants that live in deserts have the ability to survive on very little water and animals can control their body temperatures so they stay at the right level.

 Some examples of plants and animals that live in deserts are cacti, the desert tortoise and the artic fox.

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Coral Reef Habitats

Coral reefs are found in warm tropical oceans all around the world. Coral reefs can be found in both shallow and deep water and take hundreds of thousands of years to grow! They provide food and shelter to many fish and other animals, making them habitats that are home to so many different types of life.

Some examples of plants and animals that live in coral reefs are the sea star, sea grass, the octopus and clown fish.

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