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12.1 Essential ideas

12.1.1 Neural development
12.1.2 The human brain
12.1.3 Perception of stimuli
12.1.4a Innate and learned behaviour 1 (HL)
12.1.4b Innate and learned behaviour 2 (HL)
12.1.5 Neuropharmacology (HL)
12.1.6 Ethology (HL)

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Book: 12.1 Essential ideas
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Date: Sunday, 27 September 2020, 3:41 PM

12.1.1 Neural development

  • During early embryogenesis three germ layers, called ectoderm, mesoderm and endoderm, are formed.
  • Part of the ectoderm develops into the neural tube, and eventually the brain and nervous system.

The neural tube in chordates is formed from infolding and elongation of ectoderm

  • The neural plate is a region of differentiated cells located on the dorsal end of the ectoderm.

Neurulation in a mouseFigure 12.1.1a – Neurulation of a generalised chordate embryo shown in transverse section

  • During the process of neurulation, the neural plate gradually folds inwards forming a neural groove.
  • As the neural groove deepens it elongates bidirectionally from the centre outwards.
  • Eventually the fold is enclosed and separates from the rest of the ectoderm, forming a neural tube.

Neurulation in a mouseFigure 12.1.1b – Neurulation in a mouse (dorsal view, a–c), and morphology of the embryo at the end of neurulation (side view d)

Neurons are initially produced by differentiation in the neural tube

  • At the end of neurulation, the neural tube is closed at both ends.
  • Some of the cells lining the cavity of the tube differentiate to become regions of cell proliferation.
  • These regions undergo repeated mitosis to produce neuronal precursors, called neuroblasts.

Immature neurons migrate to a final location

  • Neuroblasts must travel from their origin to different parts of the developing brain and spinal cord in order to ensure that the appropriate neural connections are made.

Figure 12.1.1c – Neuronal migrationFigure 12.1.1c – Neuronal migration

  • Neuronal migration involves two processes, which may occur simultaneously:
    • Glial locomotion – the entire neuroblast is propelled forward along a scaffold of support cells called glia.
    • Somal translocation – the cytoplasm of the cell extends to the attachment site, then pulls the nucleus and cell body towards the final location.

Axons grow from immature neurons in response to chemical stimuli

  • A neuron with a single axon and many branching dendrites will develop from each neuroblast after its migration.
  • The terminal end of the growing axon is the growth cone.
  • The growth cone responds to chemical stimuli. A proposed mechanism is shown in Figure 12.1.1d.

axon growth coneFigure 12.1.1d – The axon growth cone responds to chemical stimuli

  • Some axons extend beyond the neural tube to reach other parts of the body. A single axon may extend through the entire length of the organism’s body.

Each developing neuron forms multiple synapses

  • Synapses connect neurons to other neurons via chemical neurotransmitters. They can be formed throughout life.
  • During early brain development there is an explosion of brain connectivity as newly formed neurons interact with each other.
  • As a neuron matures, it becomes part of a complex neural network by forming multiple synapses.

Synapses that are not used do not persist

  • During synaptic transmission, metabolic pathways involved in the release of neurotransmitters are re-enforced.
  • This means that specific neural pathways become stronger from use while others become weaker from disuse.
  • Synapses that are not used are eliminated by neural pruning.

neural pruning evidenceFigure 12.1.1e – Synaptic density in humans is evidence for neural pruning

Neural pruning eliminates unused neurons and synapses

  • Neural pruning involves two related processes:
    • Elimination of unused synapses – most synaptic modification occurs after birth.
    • Apoptosis of unused neurons – programmed cell death occurs as the fetus develops. The average human adult has 40% fewer neurons than a newborn.
  • Neural pruning makes existing neural pathways more efficient, and streamlines brain functions.

Nervous system plasticity allows it to change with experience

  • Synaptic re-enforcement is an example of neural plasticity.
  • The number of dendrites, the length of axons and the strength of neural pathways respond to environmental stimuli.
  • For example, learning and practising a new skill influences brain chemistry so that a neural pathway is re-enforced.
  • New neural pathways can be forged throughout life, but neural plasticity is greatest until the age of 6.
  • Neural stem cells are found in some parts of the brain – neurogenesis (the formation of new neurons) also plays a role in nervous system plasticity.

Figure 12.1.1f - Time-lapse of migrating neuronsFigure 12.1.1f – Time-lapse of migrating neurons

Essential idea

Modification of neurons starts in the earliest stages of embryogenesis and continues to the final years of life.

Rita Levi-MontalciniFigure 12.1.1g – Rita Levi-Montalcini (1909–2012) 
Mrs Levi-Montalcini won a Nobel Prize for discovering nerve growth factor, a protein that acts as a positive chemical stimulus during neurogenesis.

Food for thought

  • Differentiation involves genes being switched on or off – remember that DNA and specific proteins, especially enzymes, are involved in every process described on this page.
  • Discuss the saying ‘Use it or lose it’ in relation to neural development.

Concept help

  • In humans, neurulation begins at about week 3 of pregnancy.
  • Neuroblasts are not stem cells – they can only become neurons. However, structural and functional differences can develop depending on where they migrate to.
  • Neural pruning occurs during development and should not be confused with neuroplasticity.

Course link

12.1.2 The human brain

Brain development in a human fetusFigure 12.1.2a – Brain development in a human fetus

  • The human brain develops from expansion of the anterior portion of the embryonic neural tube.
  • Expansion of the neural tube is followed by the development of two distinct hemispheres by inward folding of the outer membrane.
  • The brain continues to fold into lobes and ridges, or gyri, thereby increasing the surface area of cerebral cortex.
  • The posterior portion of the neural tube develops into the spinal cord.

Skill: Identifying parts of the brain from a scan, photograph, or diagram

Print the diagram of the brain from and compare it to Figure 12.1.2b.

sagittal view of the human brainFigure 12.1.2b – Sagittal view of the human brain. Click image to reveal labels.

Different parts of the brain have specific roles:

  • Medulla oblongata
    Coordinates autonomic nervous functions and homeostasis, including the action of smooth and cardiac muscle, e.g. swallowing, breathing, heart rate, digestion.
  • Cerebellum
    Coordinates unconscious actions such as balance, posture, movement.
  • Hypothalamus
    • Acts as a bridge between the nervous and endocrine systems.
    • Coordinates autonomic nervous functions and homeostasis, e.g. body temperature, thirst, sleep patterns.
    • Synthesises hormones and releasing factors and releases them into the pituitary gland.
  • Pituitary gland
    • Synthesises hormones that regulate growth, development and the menstrual cycle.
    • Secretes hormones in response to chemical cues from the hypothalamus


    • Secretes hormones produced by the hypothalamus.
  • Cerebral hemispheres
    Coordinating centre for higher order functions including emotion, memory, learning, reasoning and language.

The cerebral cortex 

  • The cerebral cortex is the outermost layer of the cerebral hemispheres. It is about 2–3mm deep in humans.
  • It contains a high density of neurons and synapses, and is the brain’s centre of conscious thought and action.

Distribution of cortex in the brainFigure 12.1.2c – Distribution of cortex in the brain

  • The human cerebral cortex makes up a larger proportion and is more highly developed in humans than in other animals.
  • The cortex has become enlarged by an increase in total area, with extensive folding to accommodate it within the cranium. 

Skill: Correlation between body and brain size

Figure 12.1.1d - Brain and body sizes of different animalsFigure 12.1.2d – Brain and body sizes of different animals 

  1. State the relationship between brain mass and body mass.
  2. Outline the trend in brain mass to body mass in the hominid species (A. africansus, H. habilis, H. sapiens).
  3. Discuss the hypothesis that fish and reptiles have smaller brains than mammals and birds.
  4. Estimate the brain mass to body mass ratio for humans and elephants.

Download the answers to the above >

 The left and right cerebral hemispheres

  • The left cerebral hemisphere receives input from sensory receptors in the right side of the body and the right side of the visual field in both eyes, and vice versa.
  • The left cerebral hemisphere controls muscle contraction in the right side of the body and vice versa for the left side.

left and right cerebral hemispheres.Figure 12.1.2e – The left and right cerebral hemispheres are ‘cross-wired’ to sensory and motor neurons.

  • The brain looks symmetrical, but there is lateralisation of functions in the hemispheres, meaning that one side is more dominant than the other for certain functions.
  • This explains, for example, why people are more commonly right-handed or left-handed and not ambidextrous.

 Brain metabolism requires large energy inputs

  • Unlike the liver or muscles, the brain stores very little glycogen.
  • It requires a constant supply of oxygen and glucose for cellular respiration.
  • The brain uses up to 20% more oxygen than other tissues in the body. Part of the reason for this is that synapses are continually firing, even when the body is at rest.
  • Much of the energy is used to maintain and re-establish resting potentials after a nervous impulse.

Figure 12.1.f - MRI scanFigure 12.1.f – An MRI (magnetic resonance imaging) scan reveals the structure of the brain.

Key concepts

  • The parts of the brain specialise in different functions.
  • The autonomic nervous system controls involuntary processes using centres in the brainstem.
  • The cerebral hemispheres are responsible for higher order functions.

Concept help

  • The cerebral cortex appears as ‘grey matter’ because it consists of a large density of nuclei and dendrites, while other parts of the brain and spinal cord appear as ‘white matter’ consisting mainly of myelinated axons.

Food for thought

  • Although specific functions can be attributed to different areas of the brain, brain injury shows that some activities are spread in many areas and that neural plasticity allows the brain to re-organise itself following a disturbance such as stroke. See Page 12.2.1.
  • Does studying make you hungry? It’s probably a psychological effect. Researchers are still not sure if thinking hard requires more energy, since the overall metabolic rate of the brain is relatively constant. 

cortical homunculusFigure 12.1.2g - Cortical homunculus

Nature of Science

Using models: The cortical homunculus is a pictorial model showing locations on the somatosensory and motor cortex where information from different parts of the body is processed. The ‘little man’ has distorted features to reflect the relative amount of innervation of the different body parts.

Course link

  • Details on the structure and function of the hypothalamus and pituitary glands can be found in 15.1.5.

12.1.3 Perception of stimuli

  • Living organisms are able to detect changes in the environment.
  • The eye and the ear are two important sensory organs.

Sensory receptors detect changes in the environment

Mechanoreceptors (sensitive to touch, changes in air/liquid pressure, or gravity)

  • Touch receptors in the dermis of skin
  • Sensory hair cells in the cochlea and semi-circular canals (see below)
  • Baroreceptors in the aorta and carotid arteries
  • Stretch receptors in the muscles

Chemoreceptors (sensitive to concentration of chemicals or pH)

  • Taste buds of the tongue
  • Osmoregulatory centre of the hypothalamus
  • Olfactory epithelium of the nose
  • Chemoreceptor cells of the carotid artery

Thermoreceptors (heat sensitive)

  • Temperature centre of the hypothalamus
  • Nerve endings in the dermis of skin

Photoreceptors (light sensitive)

  • Rod and cone cells of the retina (see below)

Rods and cones differ in sensitivity to light intensities and wavelength

Figure 12.1.3a - Absorption spectra for rods and conesFigure 12.1.3a - Absorption spectra for rods and cones





High (respond to lower intensities of light)

Low (respond to higher intensities of light)


Broad range of wavelengths (1 type)

Wavelength ranges corresponding to blue, red and green light (three different types)

Type of vision

Black and white



Skill: Annotate a diagram of the retina

Figure 12.1.3b - Structure of the retinaFigure 12.1.3b - Structure of the retina

  • Download, print and annotate the diagram of the retina. Check your answers by comparing them to Figure 12.1.3b.
  • Be sure to include the:
    • direction that light travels through the retina
    • functions of rods, cones, bipolar cells, ganglion cells.

Bipolar cells send the impulses from rods and cones to ganglion cells

  • Bipolar cells are neurons that synapse with rods and cones at one end and ganglion cells at the other.
  • Rods are connected to bipolar cells in groups, whereas each cone is connected to a single bipolar cell.

Ganglion cells send messages to the brain via the optic nerve

  • The cell bodies of ganglion cells are in the retina, and the axons of ganglion cells extend into the optic nerve.

Skill: Label a diagram of the human eye

Figure 12.1.3c - Structure of the human eye (with eyelid) in cross-sectionFigure 12.1.3c - Structure of the human eye (with eyelid) in cross-section

The information from the right field of vision from both eyes is sent to the left part of the visual cortex and vice versa

  • This is called contralateral processing: each eye receives a flat and slightly different image of the field of view.
  • The visual cortex in each hemisphere superimposes the images from each eye, creating depth perception, or stereoscopic vision.

Figure 12.1.3d - Contralateral processing in the human eyeFigure 12.1.3d - Contralateral processing in the human eye

  • The optic chiasma is the point in the brain where the neurons carrying signals from the opposite field of view switch hemispheres.

Skill: Label a diagram of the human ear

Figure 12.1.3e - Structure of the human earFigure 12.1.3e - Structure of the human ear

Outer ear

• Pinna

• Auditory canal

Middle ear

• Ear drum

• Bones of the middle ear (malleus, incus, stapes)

• Round window

• Oval window

• Eustachian tube

Inner ear (fluid-filled)

• Semi-circular canals

• Cochlea

• Auditory nerve


Structures of the middle ear transmit and amplify sound

  • Sound waves cause air pressure changes.
  • The ear drum vibrates when hit by a sound wave. The membrane extends and pushes against the first of three bones in the middle ear.
  • The first bone hits the second, the second hits the third. The three bones act as levers to amplify the sound.
  • The last bone strikes the oval window. By this time, the sound has been amplified by about twenty times.

Sensory hairs of the cochlea detect sounds at specific wavelengths

  • When struck, the oval window compresses liquid in the cochlea.
  • A portion of the cochlear membrane is made of hair cells that are sensitive to the resulting pressure changes.
  • Different regions of this membrane are stimulated by specific wavelengths.

Impulses caused by sound perception are transmitted to the brain via the auditory nerve

  • Nervous input from the hair cells of the cochlea converge in the auditory nerve.
  • The auditory nerve connects to the auditory cortex in the temporal lobe of the brain.
  • Information from the cochlear hair cells allows the auditory cortex to perceive sound waves as different pitches.

Figure 12.1.3f - Structural detail of the inner ear

Figure 12.1.3f - Structural detail of the inner ear

Hair cells in the semicircular canals detect head movements

  • Hair cells located in fluid-filled semi-circular canals are oriented on three different planes, each perpendicular to the others.
  • Head movements are detected by the relative amounts of stimulation of hair cells in the ampulla of each canal.

Figure 12.1.3g – Sensory receptors are cells or neuronsFigure 12.1.3g – Sensory receptors are cells or neurons

Key questions

  • Name types of sensory receptors.
  • Distinguish between rods and cones.
  • Explain the roles of photoreceptors in vision and mechanoreceptors in hearing.

Figure 12.1.3h – Blind spot test

Try it! 

Cover your right eye. Keep looking at the black dot as you move closer to the screen. The plus sign disappears when you reach a certain distance. You have just found the blind spot in your left eye!

Food for thought

  • The blind spots of the left and right eyes are at slightly different distances. Discuss the significance of this fact.
  • The cochlea is about the size of a pea. The structures in Figure 12.1.3e are not to scale.

12.1.3iFigure 12.1.3i – Keep spinning


Why does your head keep ‘spinning’ after getting off a playground roundabout? Why does the effect worsen with age?

Concept help

  • The Eustachian tube is not involved in hearing. It is connected to the throat and its role is to equalise pressure in the middle ear.
  • Pitch is determined by the frequency of a sound. Wavelength and frequency are related properties of waves.
  • The flexible round window allows fluid to move in the cochlea.

Course link

Balance is maintained by the autonomic nervous system and coordinated by the medulla. Review the parts of the brain in 12.1.2.

12.1.4a Innate and learned behaviour 1 (HL)

  • Behaviours are conscious or unconscious actions that occur in response to external stimuli.

Fig 12.1.4a-a Stimulus - Response

Figure 12.1.4a/a – Stimulus-response pathway

  • Behaviour is coordinated by the central nervous system (CNS).

Innate behaviour is genetically determined and develops independently of the environment

  • Innate behaviour is:
    • Heritable – encoded in DNA and passed from generation to generation
    • Intrinsic – present in animals raised in isolation from others
    • Stereotyped– performed in the same way each time by each individual of a species
    • Inflexible – not modified by development or experience
    • Consummate – fully developed or expressed at first performance
    • Innate behaviours may be expressed at different stages of life. Examples include the suckling reflex in mammals and courtship behaviour in fruitflies. 

Figure 12.1.4a/b – Courtship behaviour in Drosophila meets the criteria for innate behaviour.Figure 12.1.4a/b – Courtship behaviour in Drosophila meets the criteria for innate behaviour.
Innate behaviours are displayed in the same stereotyped sequence or events each time the are performed.

A reflex is an autonomic and involuntary response

  • Reflexes are rapid unconscious responses to external stimuli. They are coordinated by the autonomic nervous system.
  • The pupil reflex and the patellar reflex are two examples. The stimulus-response pathways for these reflexes are summarised in the table below:


Pupil reflex

Patellar reflex (knee jerk)


Bright light shone in the eye

Tendon under the knee struck by object



Touch receptors in skin

CNS coordinator


Spinal cord


Muscles of the iris

Muscles of the quadriceps


Pupils close

Leg extends

A reflex arc is made of neurons that mediate reflexes

A reflex arc is the sequence of neurons that synapse with each other during a reflex response. A typical reflex arc consists of:

  1. Sensory neuron – An external stimulus initiates depolarisation in a sensory neuron. The impulse travels towards the central nervous system and synapses with a relay neuron.
  2. Relay neuron – These are part of the central nervous system. Relay neurons, or interneurons, connect neurons to each other, and are found in grey matter of the brain and spinal cord.
  3. Motor neuron – The relay neuron synapses with a motor neuron. Depolarisation in the motor neuron results in the response at the effector (muscle).

Application: Withdrawal reflex of the hand

  • The withdrawal reflex allows you to remove your hand from a painful stimulus (e.g. hot burner) before you are aware of the pain.
  • This is an important behavioural response that helps to minimise injury.

Skill: Diagram of a reflex arc for the withdrawal reflex

Figure 12.1.4a/c - Withdrawal reflexFigure 12.1.4a/c - Withdrawal reflex

  • Practise drawing the reflex arc shown in Figure 12.1.4a/c.

Learned behaviour develops as a result of experience

  • Learned behaviour is adaptable – it develops over time through experience.
  • There is individual variation in the performance of learned behaviour.
  • Learning involves the formation of new neural pathways – learning alters the phenotype but not the genotype, and therefore is not heritable.

Learning is the acquisition of a new skill or knowledge

  • The capacity for learning differs among species.
  • Social animals, animals with high parental involvement, animals with longer lifespans and more highly developed forebrains, are more likely to learn, especially from each other.


Figure 12.1.4a/d – Innate behaviours
Innate behaviours do not need to be learned and are often necessary for survival.

Essential idea

Behavioural patterns can be inherited or learned.

Key questions

  • Distinguish between innate and learned behaviour.
  • Draw a diagram of a reflex arc for the withdrawal reflex.
  • Outline the processes involved in a reflex response.

Figure 12.1.4a/e – Dolphins using ultrasound
Language skills are learned through experience. Cetaceans, such as dolphins and whales, communicate through learned ultrasound ‘languages’ that vary between social groups.

Concept help

  • External stimuli are perceived by sensors – see 12.1.3. The effector in a stimulus-response pathway is either a muscle or a gland.
  • Sensory neurons transmit impulses to the central nervous system (brain and spinal cord).
  • Motor neurons transmit impulses from the central nervous system.
  • Not all somatic reflexes require an interneuron. For example, the patellar reflex arc consists of a sensory neuron and a motor neuron.


It is easy for us to guess how the behaviour of an animal might influence its chance of survival and reproduction. Is intuition a valid starting point for scientific hypotheses?

12.1.4b Innate and learned behaviour 2 (HL)

Learning involves forming and strengthening associations between regions in the brain.

Reflex conditioning involves forming new associations

  • Reflex conditioning is a form of learning in which behaviour is elicited by a stimulus that previously had no effect.
  • Here is an example. Imagine you eat a certain food and then get sick because of the flu. You may develop a dislike for the food and feel nauseated whenever you smell it, even though the food was not the cause of your illness.
  • Can you identify the unconditioned stimulus, unconditioned response, conditioned stimulus and conditioned response in the example above? 

Fig 12.1.4b-a reflex conditioning

Figure 12.1.4b/a – Reflex conditioning
Reflex conditioning results in new neural associations.

Application: Pavlov’s experiments into reflex conditioning

  • Ivan Pavlov was a Russian behaviourist who studied reflex conditioning in dogs in the 1890s.
  • He found that the salivary reflex could be conditioned by stimuli of varying types and strength, including bells, whistles or flashes of light. 

Figure 12.1.4b/b – Reflex conditioning in Pavlov’s dogsFigure 12.1.4b/b – Reflex conditioning in Pavlov’s dogs

Operant conditioning is learning from trial and error experience

  • Operant, or instrumental, conditioning occurs when an animal learns to associate a behaviour with negative or positive consequences, i.e. by trial and error.
  • When the consequence of a behaviour is positive, the behaviour increases. For example, a rat in a Skinner box learns to get food by pushing a lever.
  • When the consequence of a behaviour is negative, the behaviour decreases. For example, cows learn to avoid the electric fence around their enclosures after being shocked.

Fig 12.1.4b-c operant conditioning

Figure 12.1.4b/c – Operant behaviour
Reward strengthens behaviour and punishment weakens behaviour in operant conditioning.

  • Operant conditioning is the most common form of learning in animals.

Imprinting occurs at a particular life stage and is independent of consequences

  • Imprinting is irreversible phase-sensitive learning that occurs during a critical period in the early stages of life.
  • Filial imprinting of chicks is an example. Goslings and ducklings learn to follow their mothers, who will lead and protect them in the first weeks of life.
  • In the 1930s, Konrad Lorenz discovered that the stimulus for filial imprinting is movement. When hatched in isolation from their mothers, the young birds he studied followed the first moving object they encountered during the critical period of 13–16 hours after hatching.
  • Imprinted hatchlings continue to follow their ‘mothers’ even if it puts their lives in danger.

Memory is the process of encoding, storing or accessing information

  • Encoding is the first step of memory formation.
  • The physiology of memory is not well understood. The current model is that encoding and storage of memory involves the persistent strengthening of synapses. This is an example of neural plasticity. 
  • Research on patients with damage to the hippocampus indicates that the hippocampus (see figure 12.1.4b/d) is necessary for new memory formation.
  • Studies with functional magnetic resonance imaging (fMRI) indicate that many different regions of the cerebral cortex are involved in the storage of long-term memory and that the pre-frontal cortex is active when memories are recalled.

Figure 12.1.4b/d – The hippocampus is necessary for encoding new memoriesFigure 12.1.4b/d – The hippocampus is necessary for encoding new memories 

Fig 12.1.4b/e – Skinner box

Fig 12.1.4b/e – Skinner box
B.F. Skinner (1904–90), an American physiologist and behaviourist, studied operant conditioning in rats in a box with levers and buttons, now known as a Skinner box.


Figure 12.1.4b/f – Principal Skinner
‘Principal Skinner’, a character from the popular television series, The Simpsons, is a namesake of the famous behaviourist.

Key questions

  • Compare reflex conditioning with operant conditioning, using examples.
  • Discuss the roles of innate and learned behaviour in imprinting.

12.1.4b/g Konrad Lorenz

Figure 12.1.4b/g – Konrad Lorenz
Konrad Lorenz (1903–89) being followed by imprinted greylag geese, Anser anser

Figure 12.1.4b/h - Pavlos apparatus

Figure 12.1.4b/h – Pavlos apparatus

Science and Social Responsibility (Aim 8)

Pavlov’s experiments involved presenting dogs with food, then withholding the feeding. The behaviourist approach to reflex conditioning borders on the unethical.

Further reading

The ethics of important studies in the history of behavioural psychology, such as the Stanford Prison experiment, are still being discussed today.

12.1.4b/i smell memoryFigure 12.1.4b/i – Smell memory

Food for thought

The amygdala is important for mediating emotion. The hippocampus, amygdala and olfactory bulbs are located on the temporal lobe. Have you ever noticed that certain smells evoke strong emotional memories?

Course link

Memory is thought to rely on long-term potentiation of synapses. Learn more in 12.1.5.

12.1.5 Neuropharmacology (HL)

Neurotransmitters (NTs) are chemicals released at synapses by the pre-synaptic neuron and acting on the post-synaptic neuron


  • Found in synapses of the parasympathetic nervous system called cholinergic synapses.


  • Found in synapses of the sympathetic nervous system called andrenergic synapses.


  • Associated with the pleasure, motivation, and physical dependence/addiction.


  • An excitatory NT found in many parts of the brain.
  • Its action is inhibited by alcohol.

GABA (gamma-amino butyric acid)

  • An inhibitory NT found in many parts of the brain.
  • It regulates feelings of anxiety.


  • Contributes to mood, appetite, pain, sleep and body temperature control.
  • Low levels are associated with depression.

Excitatory vs inhibitory synapses

Summation is the cellular basis of neural decision-making

  • There are many pre-synaptic neurons connected to a single post-synaptic neuron, as shown in Figure 12.1.5a.
  • Information from one synapse is not enough to trigger an action potential in the post-synaptic neuron.
  • Instead, a nervous impulse is initiated or inhibited depending on the sum of all excitatory and inhibitory neurotransmitters in the post-synaptic neuron.
  • If the sum is greater than the threshold membrane potential, the result is an excitatory post-synaptic potential (EPSP).
  • If the sum is less than the threshold membrane potential, the result is an inhibitory post-synaptic potential (IPSP).

Figure 12.1.5a - Summation of excitatory and inhibitory neurotransmitters in a post-synaptic neuronFigure 12.1.5a - Summation of excitatory and inhibitory neurotransmitters in a post-synaptic neuron

Slow-acting neurotransmitters modulate fast synaptic transmission in the brain

  • When neurotransmitters bind directly to gated ion channels on the post-synaptic membrane, the result is a fast, and short-lived, synaptic transmission in the brain.
  • Sometimes neurotransmitters bind to membrane proteins that subsequently activate second messengers inside the post-synaptic membrane.
  • Second messengers modulate the speed and efficiency of fast synaptic transmission by altering the shape or state of an ion channel. 

Figure 12.1.5b - Slow neurotransmitters affect synaptic transmission through a second messenger.Figure 12.1.5b - Slow neurotransmitters affect synaptic transmission through a second messenger.

The role of slow-acting neurotransmitters in memory and learning

  • Secondary messengers are involved in metabolic cascades that result in the long-term modification of existing proteins or the synthesis of new proteins (by interacting with transcription factors).
  • Experimental evidence demonstrates that synapses are persistently strengthened by high-frequency transmissions. This phenomenon is called long-term potentiation (LTP).

Figure 12.1.5c - Long-term potentiation is a persistent strengthening of synapsesFigure 12.1.5c - Long-term potentiation is a persistent strengthening of synapses.

  • LTP is an example of the structural plasticity of the brain. It has been demonstrated in regions of the brain associated with memory and learning, most notably the hippocampus.

Figure 12.1.5d - Long-term potentiation results in modification of the structures associated with memoryFigure 12.1.5d - Long-term potentiation results in modification of the structures associated with memory 

  • Figure 12.1.5d shows a model for LTP involving the neurotransmitter glutamate and two membrane proteins in the hippocampus.
  • In this model, the increased number of receptors on the post-synaptic neuron explains the increased strength of the synaptic transmission.
  • Many neurobiologists think LTP could be a physiological mechanism for the encoding of memories.

Figure 12.1.5e - Woman with Alzheimer's diseaseFigure 12.1.5e – Woman with Alzheimer's disease
Glutamate is an excitatory neurotransmitter involved in learning and memory. Treatment of Alzheimer’s disease involves drugs that regulate its activity.

Concept help

Fast-acting does not necessarily mean excitatory. GABA is a fast-inhibiting neurotransmitter.

Nature of Science

Assessing risks associated with research: Patient advocates will often press for the speeding up of drug approval processes, encouraging more tolerance of risk.

International mindedness

Attitudes to drugs and the use of drugs differ globally. There are many cultures that use drugs to enhance rituals or religious experiences.


Figure 12.1.5f – Priest wine

12.1.5gFigure 12.1.5g – Rastafari with ganja

Science and social responsibility (Aim 8)

What are the social consequences of psychoactive drug use and abuse … on the user? his or her family? the wider society?

Course link

In 6.2.5 you learned about cholinergic synapses. These are excitatory synapses.

12.1.6 Applications of ethology (HL)

  • Ethology is the study of animal behaviour in natural conditions.
    • Ethology (study of animal behaviour) and ecology (study of the interaction of organisms with the environment) are both important for understanding how natural selection leads to the evolution of behaviour.
  • Natural selection can change the frequency of observed animal behaviour.
    • Behaviour, both innate and learned, affects an animal’s chance of survival and reproductive success.
    • Some behaviours are favoured in different environmental conditions, so the frequency of behaviours changes in response to natural selection. 
  • Behaviour that increases chances of survival and reproduction will become more prevalent in a population.
    • Both innate and learned behaviours become more prevalent if they increase an organism’s chance of survival and reproduction. 
  • Learned behaviour can spread through a population or be lost from it more rapidly than innate behaviour.
    • Many generations are needed for the frequency of innate, or genetic, behaviours to change.
    • A learned behaviour is quickly passed on by social interaction in a single generation, but if one generation doesn’t learn the skill from the previous generation, it may be lost forever.

Application: Synchronised oestrus in female lions – an innate behaviour

  • Lions do not have breeding seasons – instead the females of one pride all ovulate and are ready for mating at the same time.
  • Oestrus can be induced by the introduction of a new dominant male to the pride. Upon arrival, he kills any existing cubs and mates with females to ensure his own cubs are cared for.
  • Synchronised oestrus is an innate response that has survival advantages. For example:
    • Female lions care for each other’s cubs of about the same stage of development, e.g. suckling cubs while others are hunting.
    • Male cubs are about the same age when they are cast off from the pride, so they generally have an equal chance of competing for a new pride. 

Application: Milk-bottle feeding in blue tits – a learned behaviour

  • The Eurasian blue tit Cyanistes caeruleus is unable to digest lactose from milk, but is capable of digesting milk fat – a highly nutritious food source.
  • Between the 1920s and the 1940s, (unhomogenised) milk was widely delivered to doorsteps in glass bottles with cardboard caps in many parts of Europe. Blue tits were observed first in England, then as far away as Sweden, skimming the cream from the top of unclaimed milk bottles.
  • In the 1950s, milk bottles were fitted with aluminium seals, which the blue tits learned to pierce in order to get at the cream. Other birds, such as robins, were also seen at the bottles, but not with the same frequency as blue tits.
  • A possible explanation for the difference in frequency is the family structure and relative sociability of the two species. Blue tits travel in family groups of up to ten, whereas robins are solitary. Blue tits were able to learn from each other, whereas. cream-skimming behaviour would remain isolated to innovative robins.
  • Milk is no longer delivered in glass bottles, and is normally homogenised, so blue tits exposed to the food source do not exploit it. Cream-skimming was a learned behaviour that is now lost.

Application: Breeding strategies of coho salmon affect survival and reproduction

  • Pacific coho salmon Oncorhynchus kisutch migrate to estuaries from the open water annually during the spawning season.
  • Male coho salmon develop into one of two breeding phenotypes – each is associated with a different breeding behaviour: 





Larger body mass
Bright colouration

Less conspicuous colour

Maturation time

18 months

6 months

Dominant breeding strategy

Fight with other males for access to females and eggs

Sneak up on freshly laid eggs without being noticed by females or other males


Large energy investment
More conspicuous to predators

Less able to defend themselves
Success depends on the number of suitable hiding places in the spawning area


  • The breeding success of either strategy is frequency-dependent:
    • When few males develop into jacks, jacks will be more successful at finding suitable hiding places.
    • When many males develop into hooknoses, competition will be greater.

12.1.6aFigure 12.1.6a – Life history of male coho salmon (Ecology 72(4) 1991, p.1181)

Application: Courtship in birds of paradise is an example of mate selection

  • Many species of birds display sexual dimorphism – males have very different morphological features (i.e. bright and colourful plumage) compared to females.
  • In addition to conspicuous plumage, male birds of paradise (about 40 species of the family Paradisaeidae) perform elaborate dances and calls in order to woo mates.
  • The basic moves are inherited genetically, but each male refines his own steps and calls to perform unique courtship behaviours. For example, the male in this video adds a leaf to his perch pivot to ‘add to the effect’: takes you through the courtship display of a male Carola’s parotia (binomial name: Parotia carolae).

  • Females choose males that perform elaborate displays and this choice has driven the evolution of increasingly complex courtship behaviour – this is an example of sexual selection.
  • The reason why females choose elaborate dancers is unclear although suggested hypotheses include the possibility that the more exhibitionist male dancers are stronger than their less elaborate competitors.

12.1.6bFigure 12.1.6b – Niko Tinbergen
Niko Tinbergen (1907–88) and his colleague Konrad Lorenz shared a Nobel Prize in 1973 for their contributions to the field of ethology.

Key questions

Using examples of both innate and learned behaviour, outline how behaviour either increases or decreases the chances of survival and reproduction.

12.1.6cFigure 12.1.6c – A pride of lions
Oestrus synchronisation means all the cubs in a pride of lions are the same age.

12.1.6dFigure 12.1.6d – Blue tit on a milk bottle
The Eurasian blue tit learned to pierce caps and feed on cream. 

12.1.6fFigure 12.1.6e – Homogenised milk
Homogenised cow's milk causes diarrhoea in blue tits.

Concept help

  • Other species of Pacific salmon, including Chinook Oncorhynchus tshawytscha, also have alternative male reproductive phenotypes. Human activity (i.e. fishing) may also affect the frequency and success of jacks vs hooknoses in both species.
  • Distinct layers of cream (fat) and watery milk are visible in unhomogenised milk. The process of homogenisation emulsifies the fats in milk so that they are more evenly distributed.

12.1.6dFigure 12.1.6f – Red salmon
The breeding strategy of male coho salmon affects the chances of survival and reproduction.

12.1.6gFigure 12.1.6g – Bird of paradise
Courtship behaviour is an example of mate selection in birds of paradise in Papua New Guinea.

Course link

Innate behaviours evolve as a result of natural selection. Review the mechanism in 5.1.2.

Further reading

A recent study has suggested that social learning in blue tits is affected by the age of birds and individual innovativeness, where younger birds are more likely to perform a new skill and to learn it from others. See:

Aplin, Lucy M., Sheldon, B. and Morand-Ferran, J. ‘Milk bottles revisited: social learning and individual variation in the blue tit, Cyanistes caeruleus’. Animal Behaviour 85(6), 1225-1232.