Opposition is a movement of the thumb to face the other fingers of the hand. The movement is an important part of all grasping movements and is possible not only in humans, but also in animals such as primates and birds. Opposition may be impossible with damage to the median nerve involved or with spinal cord lesions of segments C6 to Th1.
What is opposition?
Opposition is a movement of the thumb to face the other fingers of the hand. Grasping movements are part of the everyday life of humans. For these grasping movements, the opposition of the thumb is a crucial element. The opposition or opposability of the thumb refers to its ability to oppose the further fingers. Some animals can realize such a position not only with the thumb, but also with the first toe. The human thumb gets its opposability sometimes by its anatomical position. Compared to the other fingers, the thumb of a human is twisted by 130 degrees. This allows the so-called pincer grip to take place between the thumb and one of the long fingers. In opposition, the thumb performs a movement that opposes the entire palmar side of the palm to the other fingers. In animal species such as primates or even birds, the opposability of foot or claw limbs plays just as important a role as it does for humans, since it is only in this way that they can grasp certain food components.
Function and task
The thumb realizes its oppositional movement by contractions of the opponens pollicis muscle. This muscle is located in the thenar musculature and is innervated motorically by the median nerve. This is a mixed nerve of motor and sensory pathways originating from the brachial plexus. The median nerve originates from the lateral and medial fasciculus of this plexus. Its fibrous portions originate from spinal cord segments C6 to Th1. Its branches of origin run in a distal direction. Near the insertion site of the coracobrachialis muscle, the nerve crosses the brachial artery until it is medial to it. Along the medial ulna, the median nerve runs to the forearm, where it lies between the heads of the pronator teres muscle. Between the flexor digitorum profundus and superficialis muscles, it reaches the wrist by a descent. From there, it extends under the retinaculum flexorum into the palm. In the palm, the median nerve becomes a lateral and medial branch. The median nerve innervates the entire ulnar portion of the flexor digitorum profundus muscle, with one exception, and thus is involved in the musculature of almost all flexors of the forearm. The musculature of the ball of the thumb, i.e., the thenar musculature, is also motor innervated by this nerve. The opposition of the thumb is performed by the opponens pollicis muscle, which is part of the thenar musculature thus innervated. The muscle takes its beginning at the Os trapezium and the ligaments of the palmar carpus. Its short tendon runs obliquely in distal and lateral directions. The opposition of the thumb by these two anatomical structures enables humans to perform grasping movements and the pincer grip. Sometimes the little finger is also said to be opposable. It can move in the direction of the palm through the opponens digiti minimi muscle, which in the broadest sense corresponds to opposition. In the narrower sense, however, only the thumb is capable of complete opposition in human anatomy, and thus of opposition to the rest of the fingers.
Diseases and ailments
The opposability of the thumb has value in clinical practice only when it is limited or even defunct. For example, if the median nerve is damaged, its conductivity may be reduced. Damage to this nerve can be caused by mechanical compression of the nerve pathway, for example, in the context of accidents. On the other hand, any malnutrition or poisoning can also impair the conductivity of the peripheral nerves. In the case of such an impairment, the term neuropathy is used. In most cases, neuropathies are secondary phenomena associated with a primary disease such as diabetes mellitus or chronic dependence on neurotoxic substances. In extreme cases, neuropathy of the median nerve can cause complete paralysis of the motor nerve.If such a paralysis is present, the patient is no longer able to oppose the thumb. Likewise, the thumb can no longer be opposed if the mediating spinal cord segments C6 to Th1 are affected by damage. Lesions of the spinal cord are referred to as damage to the central nervous system. Such damage may be related to neurological diseases or to tumor diseases, spinal cord infarcts or mechanical injuries of the spinal column. Neurological diseases include, for example, the autoimmune disease multiple sclerosis. In this disease, the immune system mistakenly identifies the nervous tissue of the central nervous system as hostile and causes inflammation in it. These inflammations can affect the spinal cord in addition to the brain. Particularly in the spinal cord, they often result in paralysis of certain muscles and thus can also be responsible for paralysis of the thumb. Opposition pain can also occur. They can be due, for example, to inflammation of the structures involved. However, a fracture in the bones near the thumb can also be associated with pain that is noticeable during opposition. More commonly, muscle fiber tears of the opposing thumb muscle cause the pain on opposition.
