How can the Brain tell a sensation in the index finger apart from one in the middle finger?

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As they’re ultimately both being communicated via the median nerve

In: 5

Index and thumb are on one nerve pinky and ring on another, and middle finger on its own, they may feed into the median nerve, but the signal originates on its own.

Nerves are bundles of cables, but each cable is still separate. In fact the ‘cables’ are actually individual neurons. The longest neurons in your body are bundled into the sciatica nerve, and they go from the base of your spine all of the way to your big toe.

From the spinal cord, the signals go on up to your brain and eventually reach sensory cortex. This is a strip of brain tissue about where a headband sits, and information from the different parts of your body are organized along it in a kind of map.

Your fingers in particular have a lot of brain dedicated to them as they’re so sensitive, and each one has its own separate bit of tissue.

You can see what this sensory body map looks like on your brain here, it’s called a cortical homunculus: https://en.wikipedia.org/wiki/Cortical_homunculus?wprov=sfla1

Who is “the Brian” my brain mistook “the Brain” for?

Knowing how those bundles of cables work, where they originate, and where they end is fascinating and one of the reasons I love being a Neurosurgery Physician Assistant.

When I take a patients history and examine them, I can tell them (most of the time) which nerve is being pinched based on reflexes, muscle strength, and sensation. For instance biceps, and wrist flexor weakness with decreased sensation between the thumb and first finger is most likely a disc herniation compressing the C6 nerve.

If you really want to have your mind boggled, look at a diagram of the brachial plexus and try to follow the nerve paths through it as they leave the neck and travel down the arm.