2.11B — The closed-skull problem and CT

Higher-tier Biology (2.11B): directly examining living brain tissue would require passing through the skull. Opening the skull and disturbing the tissue risks bleeding, infection and damage to regions with important functions. Medical imaging gathers evidence from inside the living head while the skull remains closed.

CT means computed tomography. It produces structural evidence in a sequence:

1. A narrow beam of X-rays passes through the head from many angles as the X-ray source rotates.
2. Detectors measure the X-rays that emerge from the head.
3. A computer combines the measurements to construct cross-sectional images called slices. Successive slices can be combined into a three-dimensional structural image.
4. The images can reveal the position of physical abnormalities such as damaged tissue, bleeding or a tumour.

CT does not directly record the activity of neurones. It can still contribute to an investigation of function by linking structure to outcome. Suppose a CT scan locates damage in one part of a cerebral hemisphere and the patient has lost a particular voluntary ability. The association is evidence that the damaged area contributes to that function. Evidence from more patients with a similar link makes the conclusion stronger, but does not show that the area works alone.

2.11B — PET maps activity-related uptake

Higher-tier Biology (2.11B).

PET means positron emission tomography. It uses a radioactive tracer to produce an activity-related map rather than only a picture of physical structure.

1. A small amount of radioactive tracer is introduced into the bloodstream. In many brain PET scans, the tracer behaves like glucose.
2. Blood carries the tracer to the brain. Regions with greater metabolic activity take up more of the glucose-like tracer during the measurement period.
3. As the tracer decays it emits positrons. Positron-electron interactions produce gamma rays that detectors outside the skull can detect.
4. A computer uses the detections to map the distribution of tracer uptake in the brain.
5. Researchers compare uptake patterns during a task with a resting or other comparison condition. A region with greater task-associated uptake is likely to contribute to that function.

PET does not show a thought or an electrical impulse directly. It shows tracer uptake related to tissue activity, so its patterns must be interpreted. A highlighted region may contribute to a task without being the only region involved.

Both methods therefore overcome the main access problem, but they answer different questions. CT is most directly structural; PET is most directly functional.

2.12B — Why spinal injuries are difficult to repair

Higher-tier Biology (2.12B): the brain and spinal cord form the central nervous system. Their protection by the skull and vertebral column makes damaged tissue difficult to reach, and an operation can itself injure nearby nervous tissue. There is also a deeper problem: the tissue contains highly specialised neurones arranged in precise communication pathways.

Most mature central nervous system neurones are not readily replaced by cell division, and damaged central axons have very limited spontaneous regrowth. Even if an axon regrows some distance, restoring function requires it to reconnect with suitable target cells. Recovery can occur through surviving pathways and changes in how networks are used, but major destroyed pathways cannot yet be rebuilt reliably.

A severe spinal cord injury shows the consequences:

1. Compression or cutting damages neurones and axons at the injury site.
2. Impulses can no longer travel normally between the brain and parts of the body below that site.
3. Movement, sensation or control of body functions below the injury may be reduced or lost.
4. Treatment can stabilise the spine, relieve pressure and prevent additional damage. Rehabilitation can strengthen useful movement and make better use of surviving pathways.
5. These treatments do not necessarily replace destroyed neurones or reconnect all severed axons, so recovery may be incomplete.

Preventing further injury and repairing the original spinal pathways are therefore different outcomes. A successful operation can stop the damage worsening without restoring tissue that has already been destroyed.

2.12B — Brain tumour treatment trade-offs

Higher-tier Biology (2.12B).

A brain tumour is a mass of abnormal cells formed by uncontrolled cell division. Treatment aims to remove or destroy tumour cells while preserving nearby healthy brain tissue and its functions. That need for selectivity creates several limitations.

The relevant limitation depends on the tumour's type, size and position. A scan may locate a tumour accurately enough to plan treatment, but locating abnormal tissue is not the same as being able to remove or destroy it without harming nearby healthy tissue.