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Topic 8: Grey matter›The structure and function of the brain (8.8, 8.9)

The structure and function of the brain (8.8, 8.9)

An overview of the structure and function of the brain (8.8, 8.9) from Edexcel A level Biology A
4 min

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The brain is the coordination centre of the nervous system, enabling us to regulate our internal environment and to respond to our external environment.

The brain is comprised of a left and a right cerebral hemisphere, sitting over a collection of structures linking the brain to the body.

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Each hemisphere contains four distinct lobes linked by white matter. Each lobe is associated with different functions.

Diagram titled 'The Four Lobes in Each Cerebral Hemisphere' depicting a side view of a human brain with labeled lobes. The frontal lobe is colored pink and labeled 'Frontal lobe - thinking, movement.' The parietal lobe is green and labeled 'Parietal lobe - sensation, spatial navigation.' The occipital lobe is purple and labeled 'Occipital lobe (visual cortex) - seeing.' The temporal lobe is yellow and labeled 'Temporal lobe - listening.' The cerebellum is shown in brown and labeled 'Cerebellum.'
  • The frontal lobe is linked with higher brain functions such as thinking, planning and forming associations. It also contains the motor cortex, linking directly to the spinal cord to send signals to muscles.
  • The parietal lobe is primarily involved in spatial navigation and orientation, as well as certain forms of recognition and memory.
  • The temporal lobe is concerned with the processing of auditory stimuli (sound). It also has links to memory.
  • The occipital lobe (visual cortex) enables the processing of visual stimuli, and enables shape recognition and depth perspective.
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The outer layer of each cerebral hemisphere is called the cortex, also known as the grey matter, and is composed mostly of nerve cell bodies, synapses and dendrites.

The more central regions of each hemisphere are the white matter, formed from nerve axons. White matter links the different parts within each hemisphere. The corpus callosum is the band of white matter that connects the left and right hemispheres of the brain.

Diagram titled 'Grey and White Matter' showing a cross-section of the human brain. The grey matter is labeled on the outer regions of the brain, indicated by a line with the text 'Grey matter on the outside of the brain.' White matter is labeled internally, with a line pointing to it and the text 'White matter forming connections between parts of the brain.' The corpus callosum is highlighted and labeled with a line and the text 'Corpus callosum, mass of white matter connecting the left and right hemispheres.'
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Below the corpus callosum, a series of structures provides the connection between body and brain.

Illustration of the human brain labeled 'THE BRAIN-BODY CONNECTION'. Key parts are labeled: 'Corpus callosum: white matter connecting the hemispheres', 'Thalamus: routing sensory information', 'Hippocampus: long term memory', 'Hypothalamus: thermoregulation and instruction of the pituitary gland', 'Midbrain', 'Pons', 'Medulla oblongata: controlling unconscious processes', 'Cerebellum: balance and motor functions', and 'Spinal cord: brain-body connection'.
  • The thalamus routes sensory information to the appropriate part of the brain.
  • The hypothalamus controls thermoregulation and the secretion of some hormones.
  • The hippocampus records long-term memory.
  • The cerebellum coordinates balance and motor functions.
  • The medulla oblongata connects the spinal cord to the brain and regulates processes we don’t consciously control, e.g., heart rate and blood pressure.
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MRIs, fMRIs, PET and CT imaging are all methods used in medical diagnosis and to investigate the structure and function of the brain.

A table comparing four imaging techniques with their key details. The columns are labeled 'Imaging technique' and 'Key details.' Row 1: 'MRI - magnetic resonance imaging' with details 'Frozen moment picture of a single layer of the brain at a time. High resolution, measuring the response of water in soft tissue to a magnetic field.' Row 2: 'fMRI - functional magnetic resonance imaging' with details 'Multiple images in a short time interval showing brain activity. Uses radio waves to track the increased presence of oxyhaemoglobin required for brain activity.' Row 3: 'PET - positron emission tomography' with details 'Multiple images in a short time interval showing brain activity. Uses radioactive tracer injections into the blood with a higher blood supply, linking to higher brain activity.' Row 4: 'CT - computerised axial tomography' with details 'Frozen moment picture of a single layer of the brain at a time. Low-resolution imaging of soft tissue using harmful X-rays.'
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CT and MRI are static techniques and are more useful for analysing soft-tissue structure than for brain activity.

MRI is a higher-resolution technique and is safer for the patient as it works by tracking the energy of water molecules responding to a magnetic field rather than ionising X-rays.

CT scans are faster, and the equipment is cheaper, so they are used when a quick, indicative answer is required. If X-rays are used only rarely, the potential for harm is lower.

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Brain activity requires energy; active areas of the brain receive an increased blood supply.

fMRI and PET can monitor the blood flow within the brain. They are used to generate multiple images in a short period of time, enabling monitoring of brain activity in response to different stimuli.

fMRI uses radio waves alongside the magnetic field. The soft tissue is mapped by MRI, and the radio waves provide information about blood flow to the image: where the level of oxyhaemoglobin is higher, more radio waves are reflected, and the image appears darker. This indicates increased blood supply and brain activity.

PET uses isotopes with short half-lives incorporated into biological molecules, such as glucose, to form radiotracers. When the isotope decays, a positron is emitted, which then produces gamma rays on collision with an electron. These gamma rays are detected in the PET scan. More gamma rays are detected in areas with increased blood supply due to brain activity.

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PET radiotracers can be created from molecules designed to bind to certain receptors in the body. The radiotracer level will build up where the target substance is found, making it visible in the image.

The buildup of -amyloid between synapses in the brain can prevent brain signals from travelling effectively. This buildup is linked to the progression of Alzheimer’s syndrome.

PET scans with an amyloid radiotracer are used to detect -amyloid in the brain. This allows for early indicators of Alzheimer’s to be captured and a useful metric for research into preventing the progression of the disease.

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