Welcome to Brain Structure, Imaging, and Development
The human brain is often called the most complex structure in the known universe. In this chapter, we are going to look at the different regions of the brain and how we use modern technology to see inside it without surgery. We will also explore the age-old "nature versus nurture" debate: are we born with our brain's "wiring" already complete, or does our environment shape who we become?
Quick Review: This chapter focuses on the physical structure and development of the brain. For information on how individual nerve cells work, you might want to look at the chapter on Neurones and the action potential.
1. Key Structures of the Brain
The specification requires you to know the location and function of four main areas of the brain. Think of the brain as a high-tech control center where different departments handle different jobs.
A. The Cerebral Hemispheres
This is the large, outer, "wrinkly" part of the brain. It is divided into two halves (hemispheres).
Function: This is where high-level processing happens. It controls voluntary behavior, personality, learning, memory, and language. It is essentially "you."
B. The Hypothalamus
Located just above the brainstem, this is a tiny but mighty area.
Function: It is the boss of homeostasis. It monitors blood temperature and water potential. It also acts as the link between the nervous system and the endocrine (hormone) system by controlling the pituitary gland.
C. The Cerebellum
Found at the back of the brain, underneath the cerebral hemispheres.
Function: It coordinates movement, posture, and balance. If you can walk in a straight line or catch a ball, thank your cerebellum! It doesn’t start the movement, but it makes it smooth and precise.
D. The Medulla Oblongata
This is located in the brainstem, at the very base of the brain.
Function: It controls involuntary (autonomic) functions that keep you alive, such as your heart rate and breathing rate. (You can read more about its role in heart rate in Topic 7: Run for your Life).
Key Takeaway: Memorize these four! A common exam question involves a diagram of the brain where you must label these parts or describe what happens if one is damaged.
2. Brain Imaging Techniques
In the past, scientists could only study the brain after someone had died. Today, we use four main types of scans to see brain structure and brain activity in living patients.
CT Scans (Computed Tomography)
CT scans use X-rays to take many "slices" of the brain, which a computer joins together.
What they show: Mainly brain structure. They are excellent for spotting physical abnormalities like large tumors or bleeding in the brain (strokes).
Limitation: They don't show real-time brain activity.
MRI Scans (Magnetic Resonance Imaging)
These use magnetic fields and radio waves to create high-resolution images.
What they show: Very detailed brain structure. MRI provides much better "soft tissue" detail than CT scans, helping doctors see smaller tumors or damaged areas.
Limitation: Like CT, standard MRI only shows the structure, not what the brain is doing at that moment.
fMRI Scans (functional MRI)
This is the "active" version of an MRI. It works by detecting changes in blood flow and oxygen levels (specifically oxyhaemoglobin) in the brain.
What they show: Brain activity. When a part of the brain is working hard, it needs more oxygen, so blood flow increases to that area. We can see which parts of the brain "light up" when a person performs a task (like talking or moving a hand).
Advantage: High resolution and shows activity in real-time without using radiation.
PET Scans (Positron Emission Tomography)
These use a radioactive tracer (often a form of glucose) injected into the blood.
What they show: Brain activity and metabolism. Active brain cells use more glucose, so the radioactive tracer builds up in the busiest parts of the brain.
Advantage: Very useful for diagnosing diseases like Alzheimer’s where certain brain regions become less active.
Common Mistake: Don't confuse structure with function. CT and MRI are structural. fMRI and PET are functional (they show the brain "in action").
3. The Development of the Brain: Nature vs Nurture
How much of our brain is determined by our genes (nature) and how much by our environment (nurture)? Scientists use several methods to investigate this.
Methods of Investigation
- Animal Experiments: Scientists can raise animals in different environments (e.g., lots of toys vs. none) to see how it affects brain structure.
- Twin Studies: Identical (monozygotic) twins have the same genes. If they are raised in different environments but turn out the same, it suggests "nature." If they are different, it suggests "nurture."
- Cross-cultural Studies: Comparing children from very different cultures to see if they develop the same skills (like depth perception) at the same time.
- New-born Abilities: Testing what babies can do the moment they are born (before they have had much "nurture" time).
- Damaged Brain Areas: Studying adults with brain injuries to see which functions are lost and if the brain can "re-wire" itself.
4. The Critical Period and Vision
One of the most famous examples of nature and nurture working together is the development of vision. There is a critical period in early life when the visual cortex (the part of the brain that "sees") must receive stimuli to develop properly.
The Hubel and Wiesel Experiments
Scientists Hubel and Wiesel performed experiments on monkeys and kittens to understand this. They deprived one eye of light during the early stages of life (the critical period).
Findings:
1. If the eye was covered during the critical period, the animal remained permanently blind in that eye, even after the cover was removed.
2. This wasn't because the eye was broken, but because the visual cortex in the brain hadn't developed the necessary connections (columns of neurones) to process signals from that eye.
3. If an adult animal had its eye covered for the same amount of time, it did not lose its sight. This proves that the "wiring" happens during a specific window of time.
Ethics of Animal Research
The Hubel and Wiesel experiments are controversial. You need to understand the two main standpoints:
1. Pro-research: It is acceptable to use animals if the research leads to medical breakthroughs that save human lives or prevent human suffering (e.g., treating childhood blindness).
2. Anti-research: Animals have a right to not be harmed. Some argue that causing pain to an animal is morally wrong regardless of the potential benefit to humans (this is sometimes called a "utilitarian" vs. "rights-based" debate).
Quick Summary: - Cerebral Hemispheres: Thinking/Language. - Hypothalamus: Homeostasis. - Cerebellum: Balance/Coordination. - Medulla: Heart/Breathing. - Imaging: CT/MRI (Structure), fMRI/PET (Function). - Development: Vision needs environmental input during a critical period to wire the brain correctly.