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Worksheet 16.1
Chapter 16: Neurobiology and behaviour – fifteen
summary facts
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A stimulus is a change in the environment (internal or external) that is detected by a receptor and
elicits a response. A reflex is a rapid, unconscious response.
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A coyote responds to a pain stimulus when his leg is caught in a trap. Pressure receptors generate a
nerve impulse in a sensory neurone which carries the impulse to the spinal cord. In the spinal cord,
a relay neurone connects with the motor neurone. The motor neurone carries the impulse to the leg
muscle (effector) which contracts as the coyote tries to withdraw his leg from the trap.
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Animal behaviour is a complicated series of responses to the environment in which the animal
lives. Some whole populations of animals have changed their responses to the environment so
drastically that new species have been formed.
The study of European blackcaps and Sockeye salmon show that natural selection can cause
population change.
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Sensory receptors include:
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mechanoreceptors – which are sensitive to pressure (hair cells of the cochlea)
chemoreceptors – which respond to chemical substances (taste buds)
thermoreceptors – which respond to change in temperature (nerve endings in skin)
photoreceptors – which respond to light energy (rods and cones in the eye).
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Sound is received by the ear in the following manner. The outer ear catches the sound waves and
carries them down the auditory canal to the eardrum (tympanic membrane), which moves back and
forth slightly. The bones of the middle ear (malleus, incus and stapes) receive the vibrations and
multiply them 20 times. The stapes strikes the oval window causing it to vibrate. This vibration is
passed to the fluid of the cochlea. The hair cells in the cochlea respond to the changes in pressure
and send a message to the sensory neurones of the auditory nerve, which carries the message to the
brain.
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Innate behaviour develops independently of environmental context. Two types of innate behaviour
in invertebrates are taxis and kinesis. Planaria moving toward food is an example of chemotaxis.
An example of kinesis is the observation that woodlice move about less in optimum (humid)
conditions and more in unfavourable or (dry) conditions.
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Learned behaviour develops as a result of experience. Learning can improve chances of survival.
Imprinting in ducklings ensures that they stay close to their mother. Newly hatched male songbirds
have species-specific song which is inherited (crude template) but they can also learn to improve
the song they have inherited. First, the males memorize the song of the adults nearby to modify
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their template during the first 100 days of life. Next is the motor phase when they practise singing
the song they have heard from the adults. When the male is sexually mature, this song will help
him find a mate.
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Classical conditioning can be used to modify a reflex response as shown in Pavlov’s experiment
with dogs. Salivation in dogs is a reflex response to food. The UCS (unconditioned stimulus) of
food elicits the UCR (unconditioned reflex) of salivation. Pavlov rang a bell (neutral stimulus, NS)
just before the dog tasted food. After training, Pavlov could ring the bell (conditioned stimulus,
CS) and the dog would salivate (conditioned reflex, CR) in response to the bell alone. The dog had
learned to respond to the neutral stimulus.
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Presynaptic neurones can either excite or inhibit postsynaptic transmission. Neurones are on the
receiving end of many excitatory and inhibitory stimuli. Decision-making in the CNS occurs when
the sum of one type of stimuli, is greater than the sum of the other type of stimuli. For example, if
the sum of excitatory stimuli is greater, the axon will fire.
10 Excitatory neurotransmitters diffuse into the synapse and increase the permeability of the
postsynaptic membrane to the sodium ion (Na+). The influx of sodium depolarizes the neurone in a
wave from one adjacent area to another resulting in an action potential. This is how an impulse is
carried along a nerve. Inhibitory neurotransmitters have the opposite effect by causing K+ to move
out of the postsynaptic membrane.
11 Psychoactive drugs affect mood and behaviour. Three excitatory drugs are nicotine, cocaine and
amphetamines. Three inhibitory drugs are benzodiazepine, alcohol and THC. Excitatory drugs can
increase postsynaptic transmission. For example, cocaine excites the postsynaptic neurones
causing them to produce dopamine in the synapses of the brain. Dopamine stimulates the pleasure
centre in the brain. This results in euphoria, talkativeness and, sometimes, violent behaviour. The
inhibitory drug THC binds to receptors in the cerebellum and lessens coordination. It produces a
mellow and relaxed mood. Endorphins are molecules the body produces which are also inhibitory.
They bind to pain receptors in the brain cells and block transmission of pain impulses.
12 Causes of addiction could be genetic (as shown in experiments with identical twins) or social (as
in family addiction or peer pressure to use drugs). Another cause of addiction is the fact that when
taking drugs continuously, the dopamine receptors that are constantly stimulated become less and
less sensitive to dopamine. Thus, more and more of the drug is needed to just maintain a normal
sense of well-being.
13 The sympathetic nervous system is concerned with ‘fight or flight’. It increases the heart rate,
opens the iris of the eye to dilate the pupil and inhibits the flow of blood to the gut. The
parasympathetic system is just the opposite. It is concerned with relaxation. It decreases the heart
rate, constricts the iris of the eye and increases the flow of blood to the gut.
14 The pupil reflex is an observation of brain activity. When trying to determine ‘brain death’, one
test is the pupil reflex. The optic nerve receives a message from the retina and connects with the
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pretectal nucleus of the brain stem. From here, a message is sent to the Edinger–Westphal nucleus
whose axons run along the oculomotor nerves back to the eye. These nerves synapse at the ciliary
ganglion. The axons of the ciliary ganglion stimulate the circular muscles of the iris so that it
contracts. This closes the pupil. In the dark, the opposite occurs and the circular muscles relax.
This opens the pupil.
15 Many examples of animal behaviour must be understood: social organization of honey bees and
chimpanzees, altruistic behaviour of Belding’s ground squirrels and naked mole rats, foraging
behaviour of small-mouth bass and bluegill sunfish, mate selection in peacocks and the biological
clock of the North American flying squirrel.
© Pearson Education Ltd 2010. For more information about the Pearson Baccalaureate series please visit
www.pearsonbacc.com
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