1. A simple withdrawal reflex
A noxious stimulus applied to the skin (in this example, extreme heat from an iron), produces a burst of action potentials in the sensory neurons. Their axons fire and their terminal buttons, within the spinal cord, release an excitatory transmitter substance. This substance stimulates the interneurons and causes them to fire. The interneurons excite the motor neurons and these neurons cause the muscle to contract causing withdrawal from the source of pain.

2. An inhibited withdrawal reflex
In this example a noxious stimulus is applied to the skin (heat from a casserole dish), but this time you are unable to withdraw from the source of the pain as to do so would cause you to drop the casserole dish.
As before, the pain that you feel increases the activity of excitatory synapses on the motor neurons, which tends to cause the hand to open. However, this excitation is counteracted by inhibition from the brain. Neurons in the brain send information to the spinal cord that prevents the withdrawal reflex from making you drop the dish.

3. Event-related potentials in healthy controls and in patients with Alzheimer’s disease
Event-related potentials (ERPs) have been used to investigate many psychological functions including decision making, language processing and auditory perception.
The absence of an ERP may indicate an impairment in function or a failure to attend to a stimulus.
This slide shows the difference between ERPs evoked by a decision making task in healthy volunteers and in patients with dementia. The ERPs of the patients are characterised by a reduction in the amplitude of the waves (particularly the P300).

4. Brain damage caused by a stroke
A set of CT scans from a patient with a brain lesion caused by a stroke; the lesion can clearly be seen as a white spot in the lower left hand corner of scan 5.
The scans are arranged from near the bottom of the brain (scan 1) to near the top of the brain (scan 6).
CT scans are typically displayed according to the ‘radiological convention’ – this means that the image is essentially reversed, so that while the brain lesion is shown on the left of the image, it is actually in the right hemisphere.
CT images are captured by sending a narrow beam of X-rays through a patient’s head; a computer calculates the amount of radiation that passes through it at various points along each angle. The result is a 2-dimensional image of a ‘slice’ of a person’s head.