HHF Part2 - Evolutionary Development and Anatomy of the Hand

Part 2 Evolutionary Development and Anatomy of the Hand

Evolutionary Development of the Hand

08/05/2024

What is the difference between human hands and hands of other primates?

Opposable thumb. The thumb of humans can touch the tip of the index finger. Many primates cannot do this. This is mainly due to the length ratio of the thumb and other fingers like the index finger. This opposition capability is crucial for exploring small objects. 

Other differences are also related to a thumb. Mass. Distance betwen thumbs and other fingers. 

This movement of opposition is essential for effective handling and exploration of small objects and greatly facilitated primate tool use and construction. Not all primates have an opposable thumb;

The tip pinch between the pads of the thumb and index finger that is used to pick up very small objects appears to be a unique human characteristic and is possible because of the human ability to hyperextend the distal phalanx of the index finger passively.

 

As the lengths of the index finger and thumb become more similar, the opposability index increases. In Homo sapiens, this ratio is 60, in the baboon 57–58 (Papio in figure 2.2), but in the orangutan (Pongo in figure 2.2) only 40 (Napier, 1993; Schultz, 1956). The baboon has a high ratio because its fingers are short relative to the metacarpals, presumably as part of its adaptation to terrestrial digitigrade locomotion.

In addition, human hands are distinctive in the relative mass of the thumb musculature, which makes up about 39% of the weight of the intrinsic muscles within the hand, as compared to 33% in gibbons and 24% in chimpanzees and orangutans (Tuttle, 1969).

 

Is the opposable thumb also related to the used of tools and celebral evolution?

No.

Studies of contemporary stone tool-making behavior in both human(New Guinea stone tool makers) and nonhuman primates reveal the patterns of hand use required to make different types of tools. They indicate that a number of precision grips are used in tool manufacture, but that a fine precision grip between the tips of the thumb and index finger is not essential (Marzke, 1997).

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Anatomy of the Hand

0/8/06/2024

전문 의학 용어가 너무 많이 나오는 부분은 그렇구나 하면서 그냥 읽음...

 

What are abduction and adduction movements? What are flexion and extension movements and how are they different from abduction and adduction?

flexion - folding, extension - unfolding, abduction - spreading, adduction - unspreading

What are saddle joints and hinge joints? What is the DoF of movements of the human hand, including wrist?

21. 9 from 9 interphalangeal hinge joints + 2*5 from MP (meta-carprophalangeal) joints + 2 from wrist joints.

I think abduction/adduction movements make a convex shape and flexiono/extension movements make a concave shape, so 5 MP joints are called saddle joints.

The five meta-carpophalangeal joints(biaxial or condyloid type) are universal or saddle joints capable of both flexion/extension and abduction/adduction movements (movements away from and toward the midline of the hand), whereas the nine interphalangeal joints in the digits are hinge joints capable of only flexion and extension. The three bones in the fingers are known as the proximal, middle, and distal phalanges, and each finger has three joints, the metacarpophalangeal (MP), the proximal interphalangeal (PIP), and the distal interphalangeal (DIP) joints.

Although there is considerable axial rotation in addition to flexion/extension and abduction/adduction movements, this is constrained and so it is not considered a true third degree of freedom (Cooney, Lucca, Chao, & Linscheid, 1981). In total, the human hand, including the wrist, has 21 degrees of freedom of movement.

Characteristics of a Saddle Joint:

1. Biaxial Movement: Saddle joints permit movement in two planes—flexion/extension and abduction/adduction. This allows for movements such as bending and straightening, as well as spreading and bringing together.
2. Stability and Mobility: The saddle shape provides a good balance between stability and mobility, allowing for more movement than a hinge joint but less than a ball-and-socket joint.

Which muscles control hand?

External hand muscles in a forearm. Intrisic hand muscles in a hand.

Most of the muscles that control movements of the hand are in the forearm and are known as the extrinsic hand muscles (see figure 2.5). These long flexor and extensor muscles of the wrist and fingers take their origin from the bones in the arm, and as they approach the wrist the muscle bellies are replaced by tendons (Kapandji, 1970).

Muscles that take their origin and insertion within the hand are known as the intrinsic hand muscles. There are four groups of these, as shown in figure 2.6. The three thenar muscles (i.e., the abductor pollicis brevis, opponens pollicis, and flexor pollicis brevis) cover the thumb metacarpaland are involved in pronating the thumb, that is, raising the thumb up straight to form a 90° angle with the palm, and in opposing the thumb to the fingertips. The three muscles of the hypothenar eminence (abductor digiti minimi, flexor digiti minimi, and opponens digiti minimi) abduct (move the finger away from the midline of the hand) and flex the little finger. The four lumbrical muscles flex the MP joints of the fingers and extend the interphalangeal joints together with the four dorsal and three palmar interosseus muscles, which also adduct and abduct the fingers. The lumbrical muscles are unique in the human body in that they originate from tendons and not from bone.

Four main groups of Intrinsic hand muscles

What are flexor and extensor muscles?

Bending muscles / Strecthing muscles

How are muscles, ligaments, tendons different?

Muscles

• Function: Muscles are responsible for producing movement by contracting and relaxing. They generate force and enable various bodily movements.

Ligaments

• Function: Ligaments connect bones to other bones, providing stability and support to joints. They help prevent excessive movement that could lead to injury.

Tendons

• Function: Tendons connect muscles to bones, transmitting the force generated by muscle contractions to the bones, causing movement.

How is dorsal skin different from volar skin?

More stretchable due to thinnr epidermis and dermis.

This allows the dorsal skin to stretch considerably with finger movements, and so as the middle finger moves from extension to full flexion, the skin can be lengthened by up to 30 mm. As discussed in chapter 3, it is this stretching of the dorsal skin during finger movement that provides an important input to cutaneous mechanoreceptors.

Why is fingertips important?

Gateway of information. Sensitive too small forces. So important for tactile perception and control of grasping.

The fingertip is essential for haptic exploration of the external world and is the point of contact in the hand for many motor activities, such as grasping and typing. Any force that is applied at the fingertips is transmitted to the muscles, tendons, and other soft tissues in the hand and arm. There has therefore been interest in determining how the mechanics of the fingertip contribute to tactile perception and the control of grasping.

They also indicate why the fingertip functions effectively as a tactile sensor at low forces (less than 1 N) in that small changes in force result in a considerable enhancement in tactile mechanoreceptor activity, due to the increase in contact area.

음. nerve, skin 파트 읽으면서 약간 "이런 거 까지 알아야하나. Kinematics에만 관심있는데.."라고 생각하는데 이 문단이 뼈 때림.

However, it is not only the structural elements of the hand that have contributed to its versatility, but also the soft tissues within the hand and the properties of the glabrous and hairy skin that covers the palmar and dorsal surfaces, respectively. The sensory mechanoreceptors within the skin are uniquely positioned to provide the central nervous system with information about finger movements and the material and geometric properties of objects held within the hand. These receptors are the subject of the next chapter.