Ararat

The Upper Limb


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Bones and Joints of the Upper Limb
The upper limb is attached to the axial skeleton at the shoulder joint, which is a ball and socket joint, joining the scapula and the humerus (the only bone of the arm). Moving distally, the next joint is the elbow joint. It is a hinge joint that connects the humerus to the 2 bones of the forearm, the radius and the ulna. These 2 bones are long parallel bones and the radius attaches on the thumb side of your hand. There is a proximal and a distal radioulnar joint that connects the radius and the unla. It is a pivot joint allowing for rotation, pronation and supination of the forearm (twist your arm so your palms are up = supination and then twist so the palm is down = pronation). The wrist joint connects the ulna/radius to the carpals, then the carpals to the metacarpals. The wrist joint is a plane joint. A condyloid joint, named the metacarpal-phalangeal joint, connects the metacarpals to the phalanges. The interphalangeal joint (IP), a hinge joint, connects the phalanges to each other. Finally, a saddle joint joins the metacarpals and carpals at the thumb.





Bones of the upper limb

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-All of the joints of the upper arm are synovial joints which contain lubricating syonvial fluid.




Synovial joint
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-The ends of the bones are covered in articular (hyaline) cartilage, which help absorb compression in the joints.
-Each bone is made up of compact bone and spongy bone. The outside layer is the compact bone and the inside is spongy bone.
-Spongy bone is a honeycomb of small pieces called trabeculae.



Spongy bony tissue
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-Compact bone is made up of long parallel cylinders called the Haversian system.










Compact Bony tissue
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-Nerves and blood vessels pass through the passageways of compact bone and the trabeculae spaces of spongy bone.



Muscles of the Upper Limb
There are 9 muscles that insert (where it attaches on a movable bone) on the humerus, but only 3 of these muscles actually help to move the upper limb. The 3 muscles are the pectoralis major, latissimus dorsi and the deltoids. Of the remaining six muscles, 4 of them make up the rotator cuff, which help reinforce the shoulder joint. The last 2, cross at the shoulder, one of which is the teres major. Of these 9 muscles, the ones that originate (where it attaches to a less movable bone) on the anterior side of the shoulder joint (pectoralis major, anterior deltoids) move the upper limb to flex. The biceps in the arm also flex the upper limb. Muscles that originate posterior to the shoulder joint extend the upper limb. They include the latissimus dorsi and posterior deltoids. Also, the triceps are extensor muscles.

The forearm muscles move the wrist and also move the fingers and thumb. The muscles on the anterior side help flex and pronate, while the posterior muscles are responsible for extension and supination.






Major muscles of upper limb
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-All of these muscles are made of skeletal muscle tissue.
-Skeletal muscle fibers are long and striated.
-Skeletal muscle is considered voluntary muscle because it is under conscious control.
-Skeletal muscle is accountable for body movement.



Skeletal muscle tissue
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Nerves that trigger the muscles to move the skeleton
Motor neurons are found in the ventral horn of the spinal cord. The motor neuron then follows the ventral root out of the ventral horn. Where the ventral root meets the dorsal root, a spinal nerve is formed. A nerve is a parallel bundle of peripheral axons. After the spinal nerve exits the vertebral column through the intervertebral foramen, it splits into ventral and dorsal ramus. The ventral rami form the brachial plexus that innervates the upper limb.











Brachial Plexus
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The axon of each motor neuron divides as it enters the muscle. Each axonal ending forms a neuromuscluar junction with a single muscle fiber. A motor unit is one motor neuron and the muscle cells it synapses with.

An action potential is generated when a neuron is stimulated. The electrical impulse is conducted along the neuron’s axon (which has an insulating myelin sheath formed by Schwann cells) when Na+ influxes and diffuses along the axolemma. As the electrical impulse arrives at the synapse between the nerve and the muscle, calcium channels are opened and Ca2+ enters, which trigger the muscles to contract. Contraction is the sliding of two myofilaments, actin and myosin.








Actin-myosin filaments contracting

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When the muscles contract, they pull on the tendons to move the bones at the joints.

The 4 major nerves from the brachial plexus that innervate the upper limb are the musculo-cutaneous nerve, ulnar nerve, median nerve (all innervate the anterior division) and the radial nerve (which innervates the posterior division).









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