A1 Neural Development

Question 1

Outline the process of neurulation.

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Answer 1

• Cells located in the outer germ layer (ectoderm) differentiate to form a neural plate
• The neural plate then bends dorsally, folding inwards to form a groove flanked by a neural crest
• The infolded groove closes off and separates from the neural crest to form the neural tube
• The neural tube will elongate as the embryo develops and form the central nervous system (brain and spinal cord)
• The cells of the neural crest will differentiate to form the components of the peripheral nervous system

Understanding: The neural tube of embryonic chordates is formed by infolding of ectoderm followed by elongation of the tube.

Question 2

Outline how neurons are produced by differentiation in the neural tube.

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Answer 2

The neural plate develops into the neural tube, with continued proliferation of cells by mitosis and differentiation along the pathways leading to the cells becoming functioning neurons.

Understanding: Neurons are initially produced by differentiation in the neural tube.

Question 3

Outline how neurons migrate to a final location.

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Answer 3

The cytoplasm and organelles in it are moved from the trailing end of the neuron to the leading edge by contractile actin filaments.

Understanding: Immature neurons migrate to a final location.

Question 4

Define “axon”.

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Answer 4

An axon is a long narrow outgrowth from the cell body that carries signals to other neurons. Only one axon develops on each neuron, but it may be highly branched.

Understanding: An axon grows from each immature neuron in response to chemical stimuli.

Question 5

Outline how chemical stimuli affects growth in immature neurons.

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Answer 5

Chemical stimuli determine neuron differentiation when the axon grows out from the cell body and also the direction in which it grows in the developing embryo.

Understanding: An axon grows from each immature neuron in response to chemical stimuli.

Question 6

Outline the growth of axons.

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Answer 6

• In some cases they are relatively short and make connections between neurons within the central nervous system, but other neurons develop very long axons which can reach any part of the body.
• As long as the cell body of its neuron remains intact, its axon may be able to regrow if severed or damaged in other ways outside the central nervous system.

Understanding: Some axons extend beyond the neural tube to reach other parts of the body.

Question 7

Define “synapse”.

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Answer 7

A synapse is a junction at which a neuron transmits a signal to another cell (relay neuron or effector)
* Most synapses transmit chemical signals, although electrical synapses also exist

Understanding: A developing neuron forms multiple synapses.

Question 8

Outline the number of synapses a neuron can form.

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Answer 8

In practice most neurons develop multiple synapses and some neurons in the brain develop hundreds, allowing complex patterns of communication.

Understanding: A developing neuron forms multiple synapses.

Question 9

Explain why unused synapses do not persist.

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Answer 9

Synapses often disappear if they are not used.
* When transmission occurs at a synapse, chemical markers are let that cause the synapse to be strengthened.
* Synapses that are inactive do not have these markers so become weaker and are eventually eliminated.

Understanding: Synapses that are not used do not persist.

Question 10

Outline the process of neural pruning.

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Answer 10

• Neurons that are not used destroy themselves by the process of apoptosis.
• The elimination of part of a neuron or the whole cell is known as neural pruning.

Understanding: Neural pruning involves the loss of unused neurons.

Question 11

Define “neuroplasticity”.

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Answer 11

Neuroplasticity describes the capacity for the nervous system to change and rewire its synaptic connections.

Understanding: The plasticity of the nervous system allows it to change with experience.

Question 12

Explain the significance of neural plasticity.

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Answer 12

Plasticity is the basis for forming new memories and also for certain forms of reasoning. It is also very important in repairing damage to the brain and spinal cord.

Understanding: The plasticity of the nervous system allows it to change with experience.

Question 13

Explain how spina bifida occurs.

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Answer 13

Spina bifda is caused by the incomplete closure of the embryonic neural tube.

Application: Incomplete closure of the embryonic neural tube can cause
spina bifida.

Question 14

Outline how strokes occur.

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Answer 14

Most strokes are caused by a blood clot blocking one of the small vessels in the brain, but bleeding from a blood vessel is another cause.

Application: Events such as strokes may promote reorganization of brain
function.

Question 15

Explain how strokes may promote the reorganization of brain function.

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Answer 15

• During a stroke part of the brain is deprived of suffcient oxygen and glucose.
• If cell respiration ceases in neurons, they become irreparably damaged and die.
• Recovery from strokes involves parts of the brain taking on new functions to supplement the damaged areas.

Application: Events such as strokes may promote reorganization of brain

Question 16

Explain how the neural tube expands to form the nervous system

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Answer 16

During embryonic development, the neural tube will enlarge and develop into different components of the nervous system:
* The anterior part of the neural tube will expand to form the brain during cephalisation (development of the head)
* The remainder of the neural tube will develop into the spinal cord
* Cells that comprised the neural crest will differentiate to form most of the peripheral nervous system

Understanding: The anterior part of the neural tube expands to form the brain.

Question 17

Outline the role of the medulla oblongata.

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Answer 17

The medulla oblongata is used in autonomic control of gut muscles, breathing, blood vessels and heart muscle

Understanding: Different parts of the brain have specific roles.

Question 18

Outline the role of the cerebellum.

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Answer 18

The cerebellum coordinates unconscious functions, such as posture, non-voluntary movement and balance.

Understanding: Different parts of the brain have specific roles.

Question 19

Outline the role of the hypothalamus.

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Answer 19

The hypothalamus is the interface between the brain and the pituitary gland, synthesizing the hormones secreted by the posterior pituitary, and releasing factors that regulate the secretion of hormones by the anterior pituitary.

Understanding: Different parts of the brain have specific roles.

Question 20

Outline the role of the pituitary gland.

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Answer 20

The pituitary gland: the posterior lobe stores and releases hormones produced by the hypothalamus and the anterior lobe produces and secretes hormones that regulate many body functions.

Understanding: Different parts of the brain have specific roles.

Question 21

Outline the role of the cerebral hemispheres.

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Answer 21

The cerebral hemispheres act as the integrating centre for high complex functions such as learning, memory and emotions.

Understanding: Different parts of the brain have specific roles.

Question 22

Outline the use of animal experiments to identify the role of different brain parts.

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Answer 22

• Animal experimentation can be used to identify function by stimulating regions with electrodes or removing via lobotomy
• Because such methods are highly invasive and potentially damaging, animal models are frequently used
• Experimentation on animals involves less ethical restrictions than human studies (although ethical standards do exist)
• Animal studies are limited by the differences between animal and human brains, making valid comparisons difficult

Application: Use of animal experiments, autopsy, lesions and fMRI to identify the role of different brain parts

Question 23

Outline the use of lesions to identify the role of different brain parts.

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Answer 23

• Lesions are abnormal areas of brain tissue which can indicate the effect of the loss of a brain area
• Lesions can be identified via autopsy and relating the position of the lesion to observed changes in behaviour and capacities

Application: Use of animal experiments, autopsy, lesions and fMRI to identify the role of different brain parts

Question 24

Outline the use of fMRI to identify the role of different brain parts.

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Answer 24

• Functional magnetic resonance imaging (fMRI) records changes in blood flow within the brain to identify activated areas
• Oxygenated haemoglobin responds differently to a magnetic field than deoxygenated haemoglobin
• These differences in oxygenation can be represented visually and reflect differences in the level of brain activity
• fMRI is non-invasive and can be used to identify multiple brain regions involved in complex, integrated brain activities

Application: Use of animal experiments, autopsy, lesions and fMRI to identify the role of different brain parts

Question 25

Outline the function of the visual cortex of the cerebral hemisphere.

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Answer 25

• Located within the occipital lobe of the cerebrum and receives neural impulses from light-sensitive cells in the eyes
• The visual cortex is the region of the brain responsible for visual perception (sight)

Application: Visual cortex, Broca’s area, nucleus accumbens as areas of the
brain with specific functions.

Question 26

Outline the function of the Broca’s area of the cerebral hemisphere.

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Answer 26

• Located within the frontal lobe of the left cerebral hemisphere (not present in the right hemisphere)
• Is responsible for speech production (if damaged, the individual cannot produce meaningful speech despite intending to)

Application: Visual cortex, Broca’s area, nucleus accumbens as areas of the
brain with specific functions.

Question 27

Outline the function of the Broca’s area of the cerebral hemisphere.

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Answer 27

• The nucleus accumbens is involved in the pleasure reward pathway and is found within each cerebral hemisphere
• It secretes neurotransmitters responsible for feelings of pleasure (dopamine) and satiety (serotonin)

Application: Visual cortex, Broca’s area, nucleus accumbens as areas of the
brain with specific functions.

Question 28

Contrast the two parts of the autonomic nervous system.

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Answer 28

The autonomic nervous system has two parts: sympathetic and parasympathetic. These often have contrary effects on an involuntary process. For example, parasympathetic nerves cause an increase in blood fow to the gut wall during digestion and absorption of food. Sympathetic nerves cause a decrease in blood flow during fasting or when blood is needed elsewhere.

Understanding: The autonomic nervous system controls involuntary processes in the body using centres located mainly in the brain stem.

Question 29

Outline the autonomic response of swallowing.

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Answer 29

• The first phase of swallowing, in which food is passed from the mouth cavity to the pharynx is voluntary and so is controlled by the cerebral cortex.
• The remaining phases in which the food passes from the pharynx to the stomach via the esophagus, are involuntary and are coordinated by the swallowing centre of the medulla oblongata.

Application: Swallowing, breathing and heart rate as examples of activities
coordinated by the medulla.

Question 30

Outline the autonomic response of breathing.

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Answer 30

• There are chemoreceptors in the medulla that monitor blood pH.
• If blood pH falls, indicating an increase in carbon dioxide concentration, breathing becomes deeper and/or more frequent.

Application: Swallowing, breathing and heart rate as examples of activities
coordinated by the medulla.

Question 31

Outline the autonomic response of heart rate.

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Answer 31

• Blood pH and pressure are monitored by receptor cells in blood vessels and in the medulla.
• In response to this information, the cardiovascular centre (in the medulla) can increase or decrease the heart rate by sending signals to the heart’s pacemaker.

Application: Swallowing, breathing and heart rate as examples of activities
coordinated by the medulla.

Question 32

Outline what the pupil reflex is and how it it used to assess brain damage.

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Answer 32

• The pupil reflex is an involuntary response originating at the brainstem and under the control of the autonomic nervous system
• It involves the resizing of the iris to regulate the amount of light that reaches the retina (excess light can damage the retina)
• Brain death can be determined by medical professionals by testing the function of specific autonomic responses
• The pupil reflex is one autonomic test used to assess brain death – brain dead individuals will not exhibit a pupil reflex

Application: Use of the pupil refex to evaluate brain damage.

Question 33

Contrast the cerebral cortex in humans and other animals.

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Answer 33

The cerebral cortex is much more highly developed in humans than other animals and forms a larger proportion of the brain.

Understanding: The cerebral cortex forms a larger proportion of the brain and is more highly developed in humans than other animals.

Question 34

Define the cerebral cortex.

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Answer 34

The cerebral cortex is much more highly developed in humans than other animals and forms a larger proportion of the brain.

Understanding: The cerebral cortex forms a larger proportion of the brain and is more highly developed in humans than other animals.

Question 35

Outline the evolution of the human cerebral cortex.

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Answer 35

• Through evolution, the human cerebral cortex has been greatly enlarged in comparison to other brain structures
• The increase in total area is mediated by extensive folding
• This greatly increases surface area without increasing volume – allowing the brain to fit within the cranium

Understanding: The human cerebral cortex has become enlarged principally by an increase in total area with extensive folding to accommodate it within the cranium.

Question 36

Outline the role of the cerebral hemispheres.

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Answer 36

The cerebrum is organised into two hemispheres that are responsible for higher order functions and complex skills
* These functions include memory, speech, cognitive thought, problem solving, attention and emotions

Understanding: The cerebral hemispheres are responsible for higher order functions.

Question 37

Outline how the left cerebral hemisphere receives sensory input from sensory receptions and controls muscle activity.

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Answer 37

• The left cerebral hemisphere receives sensory 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 for the right hemisphere.
• The left cerebral hemisphere controls muscle contraction in the right side of the body and vice versa for the right hemisphere.

Understanding: The left cerebral hemisphere receives sensory 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 for the right hemisphere.

Question 38

Outline brain metabolism.

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Answer 38

The large amounts of energy required by the brain are used to sustain neurons and their processes
* Energy is needed to maintain a resting potential when neurons are not firing (Na+/K+ pump uses ATP)
* Energy is used to synthesise large numbers of neurotransmitters to facilitate neuronal communication

Understand: Brain metabolism requires large energy inputs.

Question 39

Outline the relationship between brain mass and body mass in animal species.

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Answer 39

There is a positive correlation between body size and brain size in different animals – larger animals have larger brains
* This correlation follows a linear pattern of progression but is not directly proportional

Skill: Analysis of correlations between body size and brain size in different animals

Question 40

List the main types of receptors in humans.

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Answer 40

• Mechanoreceptors respond to mechanical forces and movements.
• Chemoreceptors respond to chemical substances.
• Thermoreceptors respond to heat.
• Photoreceptors respond to light.

Understanding: Receptors detect changes in the environment.

Question 41

Outline the process of olfaction.

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Answer 41

• Olfaction is the sense of smell.
• Olfactory receptor cells are located in the epithelium inside the upper part of the nose.
• These cells have cilia which project into the air in the nose. Their membrane contains odorant receptor molecules, proteins which detect chemicals in the air.
• Only volatile chemicals can be smelled in air within the nose.
• The combination of olfactory receptors activated determines the specific scent perceived by the brain.

Application: Detection of chemicals in the air by the many different olfactory receptors

Question 42

List the main types of photoreceptors in the human retina.

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Answer 42

Two main types of photoreceptor are present in the human retina, rods and cones.

Understanding: Rods and cones are photoreceptors located in the retina.

Question 43

Contrast rods and cones.

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Answer 43

Rod Cells
* better in low light conditions
* all contain the same pigment (rhodopsin) which absorbs a wide range of wavelengths, no colour
* Rod cells cannot differentiate between different colours
* Rod cells are abundant at the periphery of the retina and hence are responsible for peripheral vision
* Rod cells produce poorly resolved images as many rod cells synapse with a single bipolar neuron

Cone Cells
* better in bright light conditions
* three different types of cone cells, therefore differentiate between different colours (red, blue and green)
* Cone cells are abundant at the centre of the retina (within the fovea) and hence are involved in visual focusing
* Cone cells produce well defined images as each cone cell synapses with a single bipolar neuron

Understanding: Rods and cones differ in their sensitivities to light
intensities and wavelengths.

Question 44

Outline the effect of red-green colour blindness.

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Answer 44

• Red-green colour blindness is a genetic disorder whereby an individual fails to discriminate between red and green hues
• There are three different types of cone cells, each of which absorbs different wavelengths (trichromatic: red, green, blue)
• The genes responsible for producing red or green photoreceptors are located on the X chromosome (sex-linked)
• If either of these genes are mutated, red and green wavelengths cannot be distinguished
• As these genes are recessive and located on the X chromosome, red-green colour-blindness is more common in males

Application: Red-green colour-blindness as a variant of normal trichromatic vision.

Question 45

Outline how Bipolar cells send the impulses from rods and cones to ganglion cells.

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Answer 45

• Photoreceptors (rods and cones) convert light stimuli into an electrical nerve impulse (action potential)
• This neural information is relayed to the brain via bipolar cells and ganglion cells
• Bipolar cells transmit the nerve impulses produced by the photoreceptors to ganglion cells
• Ganglion cells transmit nerve impulses to the brain via long axonal fibres that compose the optic nerve
• Signals from ganglion cells may be sent to the visual cortex

Understanding: Bipolar cells send the impulses from rods and cones to
ganglion cells.

Question 46

Outline the function of ganglion cells.

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Answer 46

• Retinal ganglion cells have cell bodies in the retina with dendrites that form synapses with bipolar cells.
• Ganglion cells also have long axons along which impulses pass to the brain.
• The axons of ganglion cells pass across the front of the retina to form a central bundle at the blind spot, because their presence makes a gap in the layer of rods and cones.
• The axons of the ganglion cells pass via the optic nerve to the optic chiasma in the brain.

Understanding: Ganglion cells send messages to the brain via the
optic nerve.

Question 47

Outline the process of visual processing.

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Answer 47

• Contralateral processing is when a stimulus is processed on the opposite side to where it was detected
• Information from the right half of the visual field is detected by the left half of the retina in both eyes and is processed by the left hemisphere (and vice versa for the left half of the visual field)
• Information from each eye may swap at the optic chiasma, so that the right or left visual field is processed together
• The optic nerves that swap sides are moving contralaterally, while those that stay on the same side remain ipsilateral
• Impulses are conducted by the optic nerve to the thalamus, before being transmitted to the visual cortex (occipital lobe)

Understanding: The information from the right feild of vision from both
eyes is sent to the left part of the visual cortex and vice versa.

Question 48

Outline the middle ear and its function.

Answer 48

• The middle ear is separated from the outer ear by the eardrum and the inner ear by the oval window
• It is an air-filled chamber that houses three small bones (collectively called the ossicles)
• The bones of the middle ear are individually called the malleus (hammer), incus (anvil) and stapes (stirrup)
• The malleus is in contact with the eardrum and the stapes contacts the oval window (while the incus connects the two)
• The function of the ossicles is to amplify the sound vibrations by acting like levers to reduce the force distribution

Understanding: Structures in the middle ear transmit and amplify sound.

Question 49

Explain how the sensory hairs of the cochlea detect sounds of specific wavelengths.

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Answer 49

• The cochlea is a fluid-filled spiral tube within the inner ear that converts sound vibrations into nerve impulses
• Displacement of fluid by sound vibrations activates sensory hair cells within the spiral part of the cochlea
• Hair cells are mechanoreceptors
• The cilia on hair cells vary in length and will each resonate to a different frequency of sound (i.e. specific wavelengths)
• When the stereocilia are moved by the cochlear fluid, the hair cell will depolarise to generate a nerve impulse
• The nerve impulse will be transmitted via the auditory nerve to the auditory centres of the brain
• The kinetic movement of the cochlear fluid is dissipated by the vibration of the round window

Understanding: Sensory hairs of the cochlea detect sounds of
specific wavelengths.

Question 50

Outline how impulses caused by sound perception are transmitted to the brain.

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Answer 50

• The vibration of the oval window causes fluid within the cochlea to be displaced – this displacement is detected by hair cells
• Activation of these hair cells generates nerve impulses which are transmitted via the auditory nerve to the brain

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

Question 51

Outline how defective hair cells in the cochlea can be overcome.

A.3

Answer 51

• Cochlear implants may be used to stimulate the auditory centres of the brain in patients with non-functioning hair cells
• Standard hearing aids are ineffective in deaf patients as they amplify sounds but do not bypass defective hearing structures
• The external parts are a microphone to detect sounds, a speech processor that selects the frequencies used in speech and flters out other frequencies, and a transmitter that sends the processed sounds to the internal parts.
• The internal parts are implanted in the mastoid bone behind the ear. They consist of a receiver that picks up sound signals from the transmitter, a stimulator that converts these signals into electrical impulses and an array of electrodes that carry these impulses to the cochlea. The electrodes stimulate the auditory nerve directly and so bypass the non- functional hair cells.

Application: Use of cochlear implants in deaf patients.

Question 52

Outline how movement if detected.

A.3

Answer 52

• The vestibular system is a sensory system in the inner ear that is involved in balance and spatial orientation
• Within the semicircular canals are gelatinous caps called cupula, which are embedded with numerous hair cells
• When the head moves, the fluid in the semicircular canals follows the direction of movement
• This fluid movement exerts pressure on the hair cells embedded in the cupula, triggering nerve impulses
• There are three semicircular canals at 90º angles to one another, allowing head movement to be detected in all three planes
• The brain integrates information from the semicircular canals in each ear in order to identify head position and movement

Understanding: Hair cells in the semicircular canals detect movement of the head.