Ulnar nerve
In human anatomy, the ulnar nerve is a nerve which runs from the shoulder to the hand, at one part running near the ulna bone. It is the only exposed nerve in the human body (it is unprotected for a few centimeters at the elbow). When someone says "hitting their funny bone" it is the aggravation of this nerve that is being referred to.

# Course
The ulnar nerve comes from the medial cord of the brachial plexus, and runs inferior on the medial/posterior aspect of the humerus down the arm, going behind the medial epicondyle at the elbow. Because of the mild pain and tingling throughout the forearm associated with an inadvertent impact of the nerve at this point, it is usually called the funny bone.  (It may also have to do with its location relative to the humerus, as the name "humerus" is a homonym to the word "humorous").
It enters the anterior (front) side of the forearm and runs alongside the ulna. There it supplies one and a half muscles (flexor carpi ulnaris & medial half of flexor digiti profundus).  It soon joins with the ulnar artery, and the two travel inferiorly together, deep to the flexor carpi ulnaris muscle.
After its journey down the ulna, the ulnar nerve enters the palm of the hand.  Unlike the median nerve which travels below the flexor retinaculum of the hand and through the carpal tunnel, the ulnar nerve and artery pass superficial to the flexor retinaculum via the ulnar canal.

# Branches and innervation

## Muscular
The ulnar nerve and its branches innervate the following muscles in the forearm and hand:
An Articular branch that passes to the elbow joint while the ulnar nerve is passing between the olecranon and medial epicondyle of the femur.
- In the forearm, via the muscular branches of ulnar nerve:
Flexor carpi ulnaris
Flexor digitorum profundus (medial half)
- Flexor carpi ulnaris
- Flexor digitorum profundus (medial half)
- In the hand, via the deep branch of ulnar nerve:
hypothenar muscles
Opponens digiti minimi
Abductor digiti minimi
Flexor digiti minimi brevis
Adductor pollicis
The third and fourth lumbrical muscles
Dorsal interossei
Palmar interossei
- hypothenar muscles
Opponens digiti minimi
Abductor digiti minimi
Flexor digiti minimi brevis
- Opponens digiti minimi
- Abductor digiti minimi
- Flexor digiti minimi brevis
- Adductor pollicis
- The third and fourth lumbrical muscles
- Dorsal interossei
- Palmar interossei
- In the hand, via the superficial branch of ulnar nerve:
Palmaris brevis
- Palmaris brevis

## Cutaneous
The ulnar nerve also provides sensory innervation to the part of the hand corresponding to the fourth and fifth digits:
- Palmar branch of ulnar nerve - anterior
- Dorsal branch of ulnar nerve - posterior

# Ulnar nerve entrapment
The Ulnar nerve can be trapped or pinched in various ways as it proceeds down the arm from the Brachial plexus to the ring and middle fingers.  One common cause is cubital tunnel syndrome, where the tunnel runs the inner outside side of the elbow.  Pinching of the nerve often causes tingling symptoms in the little and ring fingers.  In some cases moderate to severe pain is experienced from pinching this nerve.  Often such pins and needles sensations can be caused by sleeping wrongly on your arm, but sometimes the problems last for days.  In severe cases, surgery is performed.