A2 The Human Brain

Question 1

Brain formation

Answer 1

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

The embryonic brain will initially be composed of three primary structures – the forebrain, midbrain and hindbrain

These structures will eventually give rise to the identifiable components of the developed brain

Question 2

Purpose of the brain

Answer 2

The human brain acts as an integration and coordination system for the control of body systems

It processes sensory information received from the body and relays motor responses to effector organ

The human brain is organised into clearly identifiable sections that have specific roles

The major external structures include the cerebral cortex, cerebellum and brainstem
Internal structures include the hypothalamus, pituitary gland and corpus callosum

Question 3

Cerebral cortex

Answer 3

composed of two hemispheres and several lobes:
frontal lobe
parietal lobe
temporal lobe
occipital lobe

Question 4

Frontal lobe

Answer 4

controls motor activity and tasks associated with the dopamine system (memory, attention, etc.)

Question 5

Parietal lobe

Answer 5

is responsible for touch sensation (tactility) as well as spatial navigation (proprioception)

Question 6

Temporal lobe

Answer 6

is involved in auditory processing and language comprehension

Question 7

Occipital lobe

Answer 7

is the visual processing centre of the brain and is responsible for sight perception

Question 8

Cerebellum

Answer 8

The cerebellum appears as a separate structure at the base of the brain, underneath the cerebral hemispheres
It is responsible for coordinating unconscious motor functions – such as balance and movement coordination

Question 9

Brainstem

Answer 9

is the posterior part of the brain that connects to the spinal cord (which relays signals to and from the body)
includes the pons, medulla oblongata (often referred to as the medulla) and the midbrain
The brainstem (via the medulla) controls automatic and involuntary activities (breathing, swallowing, heart rate, etc.)

Question 10

Hypothalmus

Answer 10

The hypothalamus is the region of the brain that functions as the interface with the pituitary gland
As such, the hypothalamus functions to maintain homeostasis via the coordination of the nervous and endocrine systems
The hypothalamus also produces some hormones directly, which are secreted via the posterior pituitary (neurohypophysis)

Question 11

Pituitary gland

Answer 11

The pituitary gland is considered the ‘master’ gland – it produces hormones that regulate other glands and target organs
The anterior lobe is called the adenohypophysis and secretes hormones such as FSH, LH, growth hormone and prolactin
The posterior lobe is called the neurohypophysis and secretes hormones such as ADH and oxytocin

Question 12

Corpus Callosum

Answer 12

The corpus callosum is a bundle of nerve fibres that connects the two cerebral hemispheres
It is the largest white matter structure in the brain, consisting of roughly 250 million axon projections
Damage to the corpus callosum can prevent information exchange between left and right hemispheres (split brain disorders)

Question 13

Brocas area

Answer 13

controls the production of speech
Located within the frontal lobe of the left cerebral hemisphere (not present in the right hemisphere)

Question 14

Sensory cortex

Answer 14

recieves sensory inputs especially touch

Question 15

Wernickes area

Answer 15

controls the understanding of speech

Question 16

Nucleus accumbens

Answer 16

is deep in the frontal cortex
it acts as a pleasure of rewards centre in the brain
It secretes neurotransmitters responsible for feelings of pleasure (dopamine) and satiety (serotonin)
It communicates with other centres involved in the mechanisms of pleasure, such as the ventral tegmental area (VTA)

Question 17

Ventricles

Answer 17

cavities containing cerebrospinal fluid which absorbs shock and delivers nutrients

Question 18

Meninges

Answer 18

membrane covering which protects the brain hemispheres

Question 19

Animal experiments - brain

Answer 19

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
Example: Animal studies using mice and rats have been used to develop drug treatments for diseases such as MS

Question 20

Lesions

Answer 20

Lesions are abnormal areas of brain tissue which can indicate the effect of the loss of a brain area
Lesions can be identified via post-mortem analysis (autopsy) or via scans of the brain (CT scans or MRI)
The effects of lesions can be difficult to identify, as many functions may involve multiple brain areas
Additionally, the brain has the capacity to re-learn certain skills by re-routing instructions to other areas (plasticity)
Example: Split brain patients have been used to identify specific roles of the left and right cerebral hemisphere

Question 21

Autopsy

Answer 21

An autopsy is a post-mortem examination of a corpse via dissection in order to evaluate causes of death
Comparisons can be made between the brains of healthy and diseased corpses to identify affected brain areas
Example: Cadavers who suffered from aphasia (language impairment) in life demonstrate damage to specific areas

Question 22

fMRI

Answer 22

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
Example: fMRI studies have been used to diagnose ADHD and dyslexia, as well as monitor recovery from strokes

Question 23

Visual cortex

Answer 23

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)

Question 24

Evolution and the brain

Answer 24

Through evolution, the human cerebral cortex has been greatly enlarged in comparison to other brain structures

The disproportional enlargement of the cerebral cortex in humans is responsible for our capacity for cognitive thought

The increase in total area is mediated by extensive folding (gyrification) to form wrinkled peaks (gyrus) and troughs (sulcus)

This greatly increases surface area without increasing volume – allowing the brain to fit within the cranium

The extent of gyrification of the cerebral cortex is a reliable indicator of potential cognitive capacity

Primates and humans have a greater degree of folding compared to lower mammals (e.g. rats have a smooth cortex)

Question 25

Human brain vs animals brain

Answer 25

The cerebral cortex is the outer layer of neural tissue found in the cerebrum of humans and other mammals

It is composed of grey matter and is involved in complex actions, such as memory, perception, consciousness and thought

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

The cerebral cortex can be externally classified according to four topographical lobes – frontal, parietal, temporal, occipital

Question 26

Cerebral hemispheres

Answer 26

The left cerebral hemisphere is responsible for processing sensory information from the right side of the body (and vice versa)

Tactile sensation from the left side of the body is processed by the right side of the brain (at the somatosensory cortex)
Objects on the left side of the visual field in both eyes are processed on the right side of the visual cortex

The left cerebral hemisphere is also responsible for processing motor information for the right side of the body (and vice versa)

Muscular contractions are coordinated by the motor cortex (premotor cortex = preparation ; primary motor cortex = execution)

A consequence of this contralateral processing is that damage to one side of the brain affects the other side of the body

For instance, a stroke in the left hemisphere may cause paralysis to the right side of the body

Question 27

Contralateral processing

Answer 27

The processing of information on the opposite side of the body
Tactile information from the left side of the body is transferred to the right side in the spinal cord or brainstem
Visual information from the left visual field is transferred to the right cerebral hemisphere at the optic chiasma

Question 28

Subdivisions of the human nervous system

Answer 28

Firstly, the nervous system can be divided into the central nervous system (brain and spine) and peripheral nervous system
The peripheral nervous system (PNS) can be divided into the sensory (afferent) pathway or the motor (efferent) pathway
The motor pathway can be subdivided according to whether the response is voluntary (somatic) or involuntary (autonomic)

Question 29

Sympathetic nerves

Answer 29

release noradrenaline (adrenergic) to mobilise body systems (‘fight or flight’ responses)

Question 30

Parasympathetic nerves

Answer 30

release acetylcholine (cholinergic) to relax body systems and conserve energy (‘rest and digest’)

Question 31

Pupil reflex

Answer 31

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)

Pupils constrict in bright light (to prevent overstimulation of photoreceptors) and dilate in dim light (to maximise light exposure)

In bright light, parasympathetic nerves trigger circular muscles to contract and cause the pupils to constrict
In dim light, sympathetic nerves trigger radial muscles to contract and cause the pupils to dilate

Question 32

Brain death

Answer 32

Brain death is defined as the permanent absence of measurable activity in both the cerebrum and brainstem

The brainstem is responsible for involuntary autonomic responses and may function alone to maintain homeostasis
Hence, individuals with a non-functioning cerebrum but a functioning brainstem may be kept alive in a vegetative state

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
The Glasgow Coma Scale uses multiple tests to determine the neurological health of someone with suspected brain injury

Question 33

Encephalization

Answer 33

the amount of brain mass relative to an animal’s body mass
here 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

Body mass increases disproportionately to an increase in brain mass as most tasks only require a fixed brain capacity
While there is a correlation between body size and brain size, there is not a correlation between brain size and intelligence

Question 34

Brain metabolism

Answer 34

The human brain consumes ~20% of the body’s energy levels, despite making up only ~2% of the body’s mass

The brain’s rate of energy consumption varies little, regardless of the level of physical exertion by the body

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