Reflex arcs and the forms of response – Nervous Coordination (Senior 6 Biology) | Ganzaa

Nervous Coordination

Lessons

Senior 6 · Biology

Nervous Coordination

5 lessons

Lesson 5 of 5

Reflex arcs and the forms of response

30 min

Faster than thought

A child reaching for a cup brushes the hot side of a charcoal stove (imbabura). The hand jerks away before the child has even felt the burn; the pain and the cry come a moment later. The movement was a reflex: it was organised in the spinal cord, without waiting for the brain to decide. This lesson puts together everything in the unit, receptors, neurones, impulses and synapses, into the reflex arc, and then compares reflexes with the other ways animals respond, from the simple turning of a woodlouse to behaviour that is learned.

Definition

Reflex action: a rapid, automatic, involuntary response to a stimulus, which is the same every time the stimulus occurs.

Reflex arc: the pathway of neurones along which impulses travel in a reflex: receptor → sensory neurone → (relay neurone) → motor neurone → effector.

Innate behaviour: behaviour that is inherited, present without learning and the same in all members of a species. Learned behaviour: behaviour that changes as a result of experience.

Kinesis: a non-directional response in which the speed of movement or the rate of turning changes with the intensity of a stimulus. Taxis: a directional movement of a whole organism towards (positive) or away from (negative) a stimulus.

The spinal reflex arc

Heat stimulates receptors (free nerve endings) in the skin; generator potentials reach threshold.
Impulses travel along the sensory neurone, whose cell body is in the dorsal root ganglion, into the grey matter of the spinal cord.
The sensory neurone synapses with a relay neurone, which synapses with a motor neurone.
Impulses travel along the motor neurone, out through the ventral root, to the effector, the arm muscle, which contracts and pulls the hand away.
Other relay neurones carry impulses up the spinal cord to the brain, so the person becomes aware of the pain, but only after the response has begun.

Feature of a reflex	Why it is useful
Fast: few neurones and few synapses (often only two or three)	Reduces injury, such as a burn or a fall
Involuntary: does not depend on conscious decision	The brain is free for other tasks; it works even in a newborn baby
Stereotyped: the same response every time	Reliable protection, such as blinking when something touches the eye

The knee-jerk reflex is even simpler: the sensory neurone from stretch receptors in the thigh muscle synapses directly with the motor neurone, so there is only one synapse and no relay neurone (a monosynaptic reflex). Reflexes coordinated in the brain, such as blinking and the pupil reflex, are cranial reflexes.

Worked example 1: measuring reaction time with a ruler

A student catches a dropped ruler after it has fallen 0.20 m. The distance fallen from rest is $d = \frac{1}{2} g t^{2}$ , with $g = 9.8\,\mathrm{m\,s^{-2}}$ . Calculate her reaction time, and explain why it is longer than the time for a withdrawal reflex.

Rearrange: $t = \sqrt{\frac{2d}{g}} = \sqrt{\frac{2 \times 0.20}{9.8}} = \sqrt{0.0408} = 0.20\,\mathrm{s}$ .
Catching the ruler is a voluntary response: impulses from the eyes must reach the visual cortex, the brain must decide, and impulses must travel down to the motor neurones of the hand. This uses many more neurones and synapses than a spinal reflex arc, and each synapse adds about 0.5 ms of delay, as well as the time for processing in the brain.

Forms of response in animals

Form of response	Description	Example
Innate (inherited)
Reflex	Rapid, automatic response of one part of the body	Withdrawing the hand from heat; blinking; the knee jerk
Kinesis	Non-directional: the animal moves faster, or turns more often, when conditions are unfavourable, so it spends longer in favourable places	Woodlice move fast and turn often in dry air, and slow down in damp places
Taxis	Directional movement of the whole animal towards or away from a stimulus	Euglena swims towards light (positive phototaxis); blowfly maggots crawl away from light (negative phototaxis)
Instinct	A complex, inherited sequence of behaviour	A weaver bird building its first nest without being taught
Learned (changed by experience)
Habituation	Gradually ceasing to respond to a repeated, harmless stimulus	Birds no longer flying from a scarecrow that never moves
Classical conditioning	A new stimulus becomes linked with an existing reflex (a conditioned reflex)	Ivan Pavlov's dogs salivated at a sound that had been paired with food
Operant (trial-and-error) conditioning	An action is repeated more often when it is rewarded and less often when it is punished	A rat learns to press a lever for food
Imprinting	A young animal learns, in a short sensitive period after hatching, to follow the first moving object it sees	Konrad Lorenz's young geese followed him
Insight learning	Solving a new problem by thought, using past experience, without trial and error	A chimpanzee stacking boxes to reach fruit hung out of reach

	Reflex action	Voluntary action
Controlled by	Spinal cord or lower brain	Cerebrum
Conscious decision	None	Yes
Speed	Very fast	Slower
Response	Always the same	Can vary
Learned?	No, inborn (except conditioned reflexes)	Often learned

One of the dogs that Ivan Pavlov used in his experiments, preserved in the Pavlov Museum in Ryazan, Russia. The tube and small container at its muzzle collected saliva, so that the amount produced could be measured. Photo: Rklawton, CC BY-SA 3.0, via Wikimedia Commons

Worked example 2: woodlice in a choice chamber

Twenty woodlice were placed in the centre of a choice chamber with dry air on one side and damp air on the other. Illustrative results: after 10 minutes, 16 were on the damp side. Observations showed that woodlice on the dry side moved faster and turned more often than those on the damp side. (a) What percentage chose the damp side? (b) Is this a taxis or a kinesis? (c) Suggest one improvement to the method.

(a) $\frac{16}{20} \times 100 = 80\%$ .
(b) A kinesis. The woodlice did not move directly towards the damp side; they moved faster and turned more in dry air, so by chance they spent more time where it was damp and slowed down there.
(c) Any one: keep light even over both sides, so that light is not a second variable; repeat with fresh woodlice and take a mean; swap the sides of the chamber between trials. Afterwards return the woodlice to a damp place unharmed.

Why it matters: woodlice lose water easily, so a response that keeps them in damp places protects them from drying out.

Warning

Common mistakes

"The brain controls a spinal reflex." The spinal cord coordinates it; the brain is told afterwards.
"Every reflex arc has a relay neurone." The knee jerk has none.
"Kinesis and taxis are the same." A taxis has a direction relative to the stimulus; a kinesis changes only speed or turning rate.
"A conditioned reflex is innate." The reflex is innate, but its link to the new stimulus is learned.

Launchpad Learning: "The Reflex Arc | A-level Biology". Watch for the route of impulses through the spinal cord and the roles of the three neurones.

✦

Practice: reflexes and forms of response

List, in order, the five parts of a reflex arc.
Where is the cell body of (a) a sensory neurone, (b) a motor neurone, in a spinal reflex?
Explain why a withdrawal reflex is faster than a voluntary movement.
A dropped ruler falls 0.15 m before it is caught. Using $t = \sqrt{\frac{2d}{g}}$ and g = 9.8 m s⁻², calculate the reaction time.
Distinguish between a taxis and a kinesis, with one example of each.
Explain why the knee-jerk reflex is described as monosynaptic.
Explain how Pavlov's experiment shows classical conditioning.
A boy carrying a hot dish he knows he must not drop does not let go, although it is painful. Suggest how the nervous system overrides the withdrawal reflex.

Show answersHide answers

Receptor, sensory neurone, relay neurone, motor neurone, effector.
(a) In the dorsal root ganglion, just outside the spinal cord. (b) In the grey matter of the spinal cord.
Fewer neurones and synapses are involved, the pathway is short, and no time is spent on conscious decision in the brain.
$t = \sqrt{\frac{2 \times 0.15}{9.8}} = \sqrt{0.0306} = 0.17\,\mathrm{s}$ .
A taxis is a directional movement towards or away from a stimulus (Euglena swimming towards light). A kinesis is non-directional: speed or turning rate changes with stimulus intensity (woodlice moving faster in dry air).
The sensory neurone synapses directly with the motor neurone, so there is only one synapse and no relay neurone.
Food (which makes dogs salivate by reflex) was repeatedly paired with a sound. Later the sound alone made the dogs salivate: a new stimulus had become linked with the existing reflex through learning.
Neurones descending from the brain form inhibitory synapses with the motor neurones of the reflex. They hyperpolarise the motor neurones, so the excitation from the relay neurones no longer reaches threshold and the muscles do not contract.

Key points

Reflex arc: receptor → sensory neurone (cell body in the dorsal root ganglion) → relay neurone → motor neurone → effector. The knee jerk is monosynaptic.
Reflexes are fast, involuntary, stereotyped and protective; the brain is informed afterwards and can inhibit them.
Innate responses include reflexes, kineses, taxes and instincts; learned responses include habituation, classical and operant conditioning, imprinting and insight.
Reaction time can be measured by a ruler drop: $t = \sqrt{\frac{2d}{g}}$ .

1Faster than thought

2The spinal reflex arc

3Forms of response in animals

Faster than thought

The spinal reflex arc

Forms of response in animals