The reflex arc

A reflex is a rapid response to a stimulus.

The withdrawal of a hand when a finger touches a hot object is an example of a reflex. Describe the pathway of a nerve impulse in the reflex arc involved in this response.

The photograph shows an African land snail.

(Source: © MYN / JORIS VAN ALPHEN / NATURE PICTURE LIBRARY / SCIENCE PHOTO LIBRARY) When an African land snail is touched on its eyestalk, the snail goes back into its shell. The snail then remains in its shell for a period of time. A student investigates the effect that repeatedly touching the eyestalk has on the time the snail remains in its shell. This is the student’s method.

• gently touch the eyestalk of a snail with a cottonwool bud so that the snail goes back into its shell

• measure the time the snail remains in its shell

• after the snail has come back out of its shell, touch the eyestalk again so that the snail goes back into its shell again

• measure the time the snail remains in its shell

• repeat the stimulus six more times

The student’s results are shown in the table.

Plot a graph to show the effect of stimulus number on the time the snail remains in its shell. Join your points with ruled, straight lines.

Describe the effect of increasing stimulus number on the time the snail remains in its shell.

Suggest a benefit to the snail of the difference in response to the repeated stimulus.

The diagram shows a human reflex arc with some structures labelled. It is the withdrawal reflex, which allows a person to remove their hand from a hot object quickly.

Which structure is the sensory receptor?

Which structure is the cell body of the relay neurone?

Which structure is the motor neurone?

Which structure is the effector?

A nerve impulse travels at 50 metres per second.

Calculate the time taken for an impulse to travel 180 cm.

The length of the nerve pathway in the withdrawal reflex arc is 180 cm. The time a person takes to remove their hand away from a hot object is called the response time. Explain why the actual response time is different from the time taken for a nerve impulse to travel 180 cm.

The diagram shows a human reflex response to touching a hot object. Some of the structures in the reflex arc are labelled.

Give the name of this reflex response.

Draw an arrow on the diagram to show the direction of the nerve impulse at point X.

Which structure detects the stimulus?

Which structure is the motor neurone?

A neurone is 1.10 m in length. The speed of the nerve impulse in this neurone is 120 metres per second. Calculate the time, in seconds, for the impulse to pass along the neurone. Give your answer in standard form.

Describe how the impulse passes from neurone to neurone in the reflex arc.

Pain has a survival function. If a person damages their ankle, they sense pain.

Explain how sensing pain benefits an organism.

Some medicines are used to reduce pain. These medicines work by preventing communication between the injured ankle and the brain. Explain which components of the nervous system may be affected by these medicines.

Reflexes are responses that protect the body and involve the central nervous system (CNS).

Give the name of one part of the CNS.

If an animal steps on a sharp object, a reflex arc occurs so that it picks up its foot to prevent further damage.

Describe how named neurones bring about a withdrawal reflex to protect the animal from damage.

The photograph shows a breed of dog called a border collie.

Some border collies have a genetic condition called sensory neuropathy.

Border collies with sensory neuropathy may be injured as their reflexes do not work well.

Sensory neuropathy is caused by a recessive allele, n. The dominant allele for not having sensory neuropathy is N.

The diagram shows a family pedigree for some border collies.

Sex determination in dogs is controlled in the same way as in humans.

Which combination of sex chromosomes is present in the body cells of a male dog?

How many individuals in this family have the genotype Nn?

Determine the probability that the next offspring produced by individuals P and Q is a male with sensory neuropathy.

Include a genetic diagram in your answer.

Dog breeders use selective breeding to try to remove harmful alleles from dog breeds.

Explain how selective breeding could be used to remove the allele for sensory neuropathy from a population of border collies.

The diagram shows the structure of a human eye.

Which of these is the structure labelled X?

When looking at a close object, which row of the table shows the state of the ciliary muscles and suspensory ligaments?

Explain how structure Y controls the light entering the eye when someone walks into a dark room.

Multiple sclerosis is a condition that can slow down the speed at which electrical impulses travel along neurones.

The time taken for the blink reflex to occur can be used to help diagnose if someone has multiple sclerosis.

The blink reflex causes the eyelid to close.

Air is blown on to the eye and the time taken for the eyelid to close is recorded.

The diagram shows the reflex pathway.

The speed the impulse moves along the reflex arc consisting of all three neurones in a person without multiple sclerosis is 77 metres per second.

The time taken for the blink reflex to occur in a person with multiple sclerosis is 0.0050 s.

The total length of the neurones in the reflex arc for the person with multiple sclerosis is 25 cm.

Calculate the difference between the speed of impulse for the person with multiple sclerosis and for the person without multiple sclerosis, in metres per second.

The speed of an impulse along the axon of the motor neurone for someone without multiple sclerosis is 120 metres per second.

Suggest why the speed of the impulse calculated along all three neurones is less than the speed of the impulse along only the motor neurone.

The diagram shows a food web from a woodland ecosystem.

Which organism in the food web is the producer?

Which organism acts as both a secondary consumer and a tertiary consumer in the food web?

The amount of energy transferred changes as you move along a food chain. The data comes from an ecosystem containing producers, primary consumers and secondary consumers.

The light energy reaching the producers is 7.1 × 10⁶ kJ per m² per year. Explain why the plants cannot absorb all of this energy.

The table shows that energy is transferred between producer and primary consumer and between primary consumer and secondary consumer. A student states that the energy transfer between producer and primary consumer is the most efficient. Determine whether the student’s statement is correct.

Woodlice feed on dead and decaying plant material in the soil. The photographs show how a woodlouse can curl up into a ball. This behaviour is an example of a reflex response.

(Source: © StellaNature Alamy Stock Photo)

State what is meant by a reflex response.

Give a reason why this reflex response benefits the woodlouse.

Describe how this reflex response could have evolved by natural selection.

Humans can control their internal environment.

State the term used to describe the control of an organism’s internal environment.

Coordination uses hormones and nerves.

Some responses are simple reflex arcs.

Describe the structure and functioning of the withdrawal reflex of a finger from a hot object.

Humans use their skin to regulate their body temperature. The diagram shows a section through the skin with two structures labelled A and B.

Changes take place in the skin when a person moves from a warm environment to a cold environment.

Explain the changes that take place in structure A as a person enters a cold environment.

Explain the changes that take place in structure B as the person enters a cold environment.

Modern humans have less body hair than their ancestors.

Explain how this evolutionary change was brought about by natural selection.

Neurones transmit nerve impulses.

The diagram shows a neurone.

This neurone is involved in the withdrawal reflex of a finger from a hot object.

The neurone transmits electrical impulses from the central nervous system to the effector.

Draw a circle on the diagram to show the part of the neurone where the impulses are transferred to the effector.

What is the name of this type of neurone?

Explain the advantage of a withdrawal reflex when a finger touches a hot object.

Some neurones do not have a myelin sheath.

A scientist investigates the impulse speed for neurones with different diameters. She does this for neurones with a myelin sheath and neurones without a myelin sheath.

The graph shows the scientist’s results.

Which conclusion is supported by the graph?

A neurone with a myelin sheath has a diameter of 1.0 µm.

Use the graph to determine the speed of an impulse in this neurone.

speed =_........ m per s

The neurone is 90 cm long.

Calculate the time taken for an impulse to travel along this neurone.

Give your answer in standard form.

The diagram shows three types of neurone.

Give the names of these neurones.

Explain the role of these neurones in the withdrawal reflex of a finger from a hot object.

A teacher uses this method to estimate the speed of a nerve impulse.

● students stand in a circle and hold hands

● student A in the circle starts a timer and at the same time squeezes the hand of student B on his left

● when student B feels his hand being squeezed, he immediately squeezes the hand of the student on his left

● this process continues around the circle of students until student A feels his hand being squeezed and he stops the timer

Explain what other measurements the teacher would need to make in order to calculate the speed of a nerve impulse.

Describe whether the teacher’s method is likely to give an accurate estimate of the speed of a nerve impulse.