Grey matter · Grey Matter

Grey matter is the tissue of the brain and spinal cord where the computing happens: it holds the neurons' cell bodies, their dendrites, the local stretches of their axons and the synapses between them, together with glial cells and a dense bed of capillaries.

Grey matter. Grey matter is the tissue of the brain and spinal cord where the computing happens: it holds the neurons' cell bodies, their dendrites, the local stretches of their axons and the synapses between them, together with glial cells and a dense bed of capillaries.

It looks grey (pinkish in a living brain, because of the blood) since it carries little myelin, the fatty insulation that makes white matter pale. In the brain it lies outside, as the cerebral cortex and the cerebellar cortex, and in islands deep inside, the nuclei such as the thalamus, the basal ganglia and the nuclei of the brainstem. In the spinal cord the layout is inverted: grey matter forms an H or butterfly in the centre, wrapped by white matter.

Grey matter is where the brain's energy goes. Synapses and the pumps that restore ion gradients after each signal are concentrated here, so it is the tissue that suffers first when blood flow fails.

Its thickness can be measured on MRI. Cortical thickness changes with development, ageing and disease, which makes it a common measure in imaging studies.

Its neurons are the subject of the neuron card, and the white matter card explains the wiring between them.

Questions: What is the difference between grey matter and white matter? Grey matter is where neurons have their cell bodies, dendrites and synapses, so it is where signals are combined and decisions about firing are made. White matter is made of the axons that carry those signals between distant regions, most of them wrapped in myelin, a fatty sheath that gives the tissue its pale colour and speeds conduction. In the brain the grey matter is outside (the cortex) and in deep nuclei, with white matter between; in the spinal cord the layout is reversed, grey in the centre and white around it. Which parts of the neuron make up grey matter, and why does it use most of the brain's energy? Grey matter holds the neuron's cell bodies, its branching dendrites with their synapses, and the local unmyelinated stretches of axons, packed among glial cells and capillaries. Without much myelin it lacks the pale fat of white matter and looks grey in a preserved brain. Synaptic transmission and the ion pumping that follows every signal happen mostly here, so grey matter uses several times more energy per gram than white matter and is the first to suffer when blood flow falls. Why is the spinal cord's grey matter inside, the reverse of the brain's cortex, and what does it do there? In the cord the grey matter forms a central H, with sensory neurons in the back horns and motor neurons in the front horns, and the white matter around it is made of the long tracts that run between the body and the brain. The layout keeps the local circuits compact, where sensory fibres enter and motor fibres leave at every segment, while the through traffic runs on the outside. Those local circuits are enough for reflexes: in the knee jerk a stretch receptor's fibre excites the motor neuron of the same muscle directly in the front horn, and the leg kicks with no part played by the brain.