The median nerve arises from the brachial plexus, which exits the spine between the 6th cervical and 1st thoracic vertebrae (C6 – Th1). The nerve is classified as part of the peripheral nervous system and motor and sensory enervates part of the muscles of the forearm and hand, including fingers.
What is the median nerve?
Graphic representation of the anatomy of the hand, carpal tunnel, median nerve and carpal ligament. The median nerve (middle arm nerve) is classified as part of the peripheral nervous system (PNS) and performs a dual function. Its efferent fibers send sensory (sensitive) impulses to the brain, while the afferent fibers transmit motor signals from the spinal cord or brain to the muscles involved. The fact that the nerve belongs to the peripheral nervous system means that, unlike the nerves of the central nervous system (CNS), it is not subject to the blood–brain barrier. The excitation conductors of the median arm nerve, consisting of bundled nerve fibers, are – as with all peripheral nerves – encased in 3 layers of connective tissue for protection, supply and to impart a certain tensile strength. It branches in its course from the axillary sinus to the underside of the palm to provide impulses for contraction or relaxation to all the muscles for which it is responsible, and to feed back sensory “data” that are important for autonomic control.
Anatomy and structure
The median nerve arises from the lateral nerve cord of the brachial plexus. It is one of the three major arm nerves and, originating from the nerve root in the axillary sinus (median fork), runs up the upper arm to the elbow. It remains unbranched until it reaches the elbow. For the sensory and motor supply of certain forearm muscles, which mainly provide for flexion and inward rotation of the forearm, the nerve branches into three branches. In the forearm, it runs – protected by two muscles – between the ulnar nerve and the radial nerve. At the carpus, the nerve travels under the broad tendon ligament (retinaculum flexorum) through the carpal tunnel into the hollow palm, where it branches again to energize certain finger muscles. The nerve is encased by three layers of connective tissue. The endoneurium, the innermost layer, consists of loose connective tissue that contains a multitude of blood and lymph vessels to supply the nerve fibers. The overlying perineurium contains tight connective tissue capable of bundling and separating many nerve fibers. Externally, the median nerve is encased by the epineurium, which may contain firm collagenous fibers as well as fatty tissue that provides some cushioning and displacement.
Function and Tasks
As a mixed nerve of the peripheral nervous system, the median nerve has two main functions. It must motorically energize certain muscles of the forearm, the ball of the thumb, and the palm. The corresponding muscles respond to voluntary signals to contract or relax, which are transmitted from the median nerve to the muscles. Specifically, the median nerve provides motor enervation to almost all forearm flexors responsible for flexion and rotation of the forearm. Similarly, it supplies several muscles on the palmar side of the hand, which mainly enable movements of the thumb, and two other muscles in the metacarpus. The second main function is the sensitive enervation of the skin over the ball of the thumb and over part of the palm (palmar surface). The sensitive nerves report physical conditions such as pressure, vibration, and temperature, as well as pain. These involuntary messages, which must be distinguished from feedback from sensory senses such as hearing, seeing, tasting, smelling, etc., trigger involuntary reactions – roughly in the manner of self-controlling regulatory circuits. When an upward temperature deviation is reported, the skin pores are instructed to open further to allow them to cool the effect of evaporative cooling back to the set point.
Diseases and ailments
Total loss of the median nerve due to paralysis, lesion, or severe compression has serious effects on the motor function of the thumb and index and middle fingers, depending on the location of the disorder. When the affected person tries to clench the hand into a fist, the index and middle fingers remain extended. The symptom is also colloquially referred to as swear hand.Likewise, the mobility of the wrist and forearm is restricted if the disturbance or interruption of the nerve is localized above the elbow. The restricted or totally failed sensory innervation is manifested by limitations or omission of pain and touch sensations, as well as by a lack of involuntary feedback, which serves to control the regulatory circuit. A relatively common condition, manifested by the above symptoms among others, is carpal tunnel syndrome. The median nerve is compressed or completely pinched off in the carpal tunnel area below the palmar tendon plate of the hand. The causes of obstruction of the nerve are very diverse and may include injury, hematoma, or edema, which in turn may be caused by other diseases. If the nerve blockage persists for a long period of time, muscle loss is the result. Carpal tunnel syndrome can be treated conservatively or surgically.