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Nervous Structure of the Arm

This article is intended to serve as a general introduction to the nerve layout in the human arm and hand. It is not concerned with bone or muscle layout, but simply with the areas affected by each nerve pathway, and data type.

The hope is it will be of some use in reconstructing sensory data, as opposed to medical data, which frequently goes into too great an anatomical detail to be of extensive help in recreating sensory stimulation, without considerable data wading.

The primary purpose is to provide sensory data maps, and explanations with which to aid virtual and artificial arm deployment, with a direct description of target sites and interface points.

Overview of the nerves


Public Domain Image from the 20th U.S. edition of Gray's Anatomy of the Human Body, originally published in 1918

The arm is divided between many different nerve fibres. Each sendingo ut tendrils into flesh over defined areas of the arm, as illustrated in the image above. It is colour-coded, such that areas in the same colouration, are gaining sensory and movement data from the same nerve pathway.

Contrary to most medical texts, and perhaps reflecting the differing purpose of this resource, we will not start at the shoulder and flow down, but rather at the fingertips and work up. After all, it is the sensory and motor data we are interested in, not large sections of anatomy.

The Hand

The hand is divided into sections of the median, ulnar, and radial nerves.


Modified from an original Public Domain Image from the 20th U.S. edition of Gray's Anatomy of the Human Body, originally published in 1918

Ulnar nerve in Hand

The ulnar nerve enters the hand at the back. It extends tendrils into the little and ring fingers, it is the only nerve for both motor and sensory functions for the little finger, and for the entire line of the hand flowing back from these fingers to the wrist. The radial nerve does not have as high a sensory bandwidth as the other nerves. When using it, you will find it impossible to move either the ring or little fingers of the same hand, without also moving the other, to a lesser extent but in the same way.In essence, this is because the same nerve branch goes to both, only splitting at hte base of the fingers.

Radial Nerve in Hand

The radial nerve also enters the hand at the back. Unlike the ulnar, it stays at the back. Tendrils from it cover sensory stimulation and muscle comntrol to the flat of the hand, and the back of part of the ring finger, then middle finger, index finger and thumb, back only. This nerve does not extend upwards to the nerve clusters of the sensitive fingertips, and does not provide sensation behind the nail.

Median nerve in Hand

The median nerve is the third and final nerve to enter the hand. It enters at the front of the hand, unlike the others. The median nerve is mostly a sensory channel, with very little motor control. As such, it essentially has additional sensory bandwidth. This branches out to cover the palm of the hand, except for the portion under the ring and middle fingers, which is controlled by the ulner nerve. The median nerve flows up the sensitive fronts of the fingers and thumb, covering all fingertip areas, and overlapping onto the back of the fingers, for the nail area only.

Hand Recap

Only three nerves reach the hand: radial and ulner nerve pass over the back of the wrist, ulner curving round towards the front as it travels. Both bring motor signals as well as ferryinhg sensory data back.

Median nerve enters via the middle of the front of the wrist, branching out swiftly, almost no motor data, enabling high sensor fidelity.

Forearm

The forearm being defined as the section from the elbow to the wrist, inclusive.


Modified from an original Public Domain Image from the 20th U.S. edition of Gray's Anatomy of the Human Body, originally published in 1918

The forearm, perhaps surprisingly, seems at first glance at a medical journal, more complex than the hand, in terms of sheer numbers of separate nerve branches.

Fortunately, that is not actually the case, instead it is more 'as one road flows into another, the name changes'.

Forearm Deep Branch / Dorsal Branch (or ulnar nerve)

What you might take as two nerves at least initially, are in fact the ulnar nerve once more. Rising out of the back of the hand, the ulnar gains tributaries from surrounding muscle and skin tissue down the outside of the arm facing out from the body. Swelling and thickening, the nerve collects signal data and delivers motor data to/from the outside of the wrist. This is known as the deep branch. It is so named because it is a spur at which the ulnar excessively branches, if you are coming down the arm, or collects a great number of tributaries, if you are going up as we are.

Higher up, beyond the wrist and into the forearm proper, the ulnar nerve is known as the dorsal branch. Again, many tributory nerves join it, reaching in from the sensory nerve endings on the outside surface of the forearm. Labelled on the diagram at the top of this document as the 'Medial branchial', this section of tissue is predominantly the underside of the forearm, following down in a line from the fingers controlled by the ulnar in the hand itself. The dorsal branch is responsible for sensory data and motor control on the inside of the elbow only. It has nothing to do with the outside.

Forearm Lateral Antibrachial Cutaneous (or radial nerve)

The lateral antibrachial cutaneous nerve, is basically a surface nerve that marks the passage of the radial back up the arm. Collecting tributaries as it goes, this nerve is primarilly responsible for sensory information on the interior surface of the forearm, running down the same side as the thumb of the hand. It likewise to the ulnar, takes control of half of the motor control for the wrist, and all sensory data down that side. It does not collect any information from the top of the forearm, where major muscle groups are located. That, is left for another nerve.

Forearm Median Antibrachial Cutaneous (or median nerve)

The third nerve in the hand, is also the last nerve in the forearm. It does however, go through several name changes along the way. Each name change corresponding with an additional influx of tributaries.

This nerve runs down the middle ofthe arm, as two branches. One, ulnar branch, digs deep into the middle of the arm, and collects sensory filaments from the middle of the back. It provides all sensory data for the back of the forearm.

The second branch of the median, the volar branch, is a little different. It is a surface nerve, and sends filaments into the far outside ofthe arm, where the ulnar nerve is. These filaments co-ordinate with the ulnar filaments present there, with some data interchange taking place.

Forearm Recap

As with the hand, there are only three nerve channels of consequence for both sensory and motor data: ulnar, radial and median. Each gets substantually thicker as it flows up the arm, gaining thousands of filament nerves and smaller branches. However, each keeps mostly to descrete areas of the arm, and is responsible for muscle control only in that area. Likewise, the sensory channels are fairly narrow, and save for the volar median, they do not interact.

Upper Arm

Everything from the shoulder, to the elbow inclusive, constitutes the upper arm.

*Note: Shoulder area torso nerves are not considered as part of the arm.


Modified from an original Public Domain Image from the 20th U.S. edition of Gray's Anatomy of the Human Body, originally published in 1918

Upper arm Dorsal Branch (or ulnar nerve)

The dorsal branch of the ulnar nerve continues up past the elbow, and slowly wraps round. No longer on the outside of the arm, it dips inside, below the skin. This nerve passes under the elbow, creating the funny bone. If struck, this is the nerve that creates the funny bone sensation.

Filaments are next seen interacting with the sensory system on the inside of the arm above the elbow, flowing into the armpit, before disappearing into the torso. There, it enters the spine at the seventh and eighth cervican nerve bunches splitting from the spinal cord, at the base of the neck, as well as the first thorasic nerve, splitting from the spinal cord at the top of the torso.

Upper arm Posterior cord (or radial nerve)

The posterior cord is another new name given to the radial nerve, as it too largely disappears from view. The posteriod cord runs up the muscle on the outside of the upper arm, to about midway. It then drops under the surface, and joins the dorsal branch in emerging in the triangle of bone at the armpit. From there, it connects to the fifth, sixth, seventh, and eighth cervican nerve bunches splitting from the spinal cord, at the base of the neck, as well as the first thorasic nerve, splitting from the spinal cord at the top of the torso.

Upper arm Superior lateral cutaneous nerve (or radial nerve)

A second section of the radial nerve. As the main section, the posterior cord, entersthe body, this branches off, and flows to the skin covering the top of the upper arm, just below the shoulder. Avoiding any joints, this section of nerve deals solely with sensory data in a band that completely circumnavigates the skin - coloured blue, in the diagram at the top of this resource.

Upper arm Medial cord (median nerve)

The medial cord rises to the top of the arm, passing through the armpit with the other nerves, and joining with the eighth cervican nerve bunch splitting from the spinal cord, at the base of the neck, as well as the first thorasic nerve, splitting from the spinal cord at the top of the torso.

Upper arm Recap

The upper arm is where the arrangement changes slightly. Still the same three nerve sheaths controlling every aspect, with branches drifting arcoss the arm. The shoulder itself is controlled by the chest nerves more than anything else. However, all arm nerves go through the triangular mesh of muscle and bone under the armpit, and all connect to the spinal cord at the same nerve branchings:

Fifth, sixth, seventh, and eighth cervican nerve bunches splitting from the spinal cord, at the base of the neck, as well as the first thorasic nerve, splitting from the spinal cord at the top of the torso.

Closing Statement

This is simply an introductory resource. It has skipped over much detail, and has - not by chance - ignored medical terminology and for the most part, specific structures in the arm's anatomy.

The purpose ofthis resource is not to go into depth, but rather to provide a non-anatomy-based starting point into how the nerves of the arm interact, and control our sensation. Motor control is mentioned, but is not seen as important within the scope of this, except to describe how specific nerves divide up a joint.

It is hoped that this and other resources, at varying levels of detail and depth, will be of aid to the target audience. That is to say, those working to replace natural organs with artificial, or virtual.

References

Gray's Anatomy: The Anatomical Basis of Medicine and Surgery, 39th edition (2004), 1600 pages, Churchill-Livingstone, ISBN 0-443-07168-3

Gray's Anatomy: The Anatomical Basis of Clinical Practice, 20th edition (for image use, see 39th edition for text)

Imagery of the anatomy of the hand, University of Michegan

Anatomy Tables - Nerves, University of Michegan

Further Reading

PreventDisease - Musculoskeletal Atlas

The Spinal Cord - VWN

Staff Comments

 


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