![]() 1 = thumb 2–5 = index, middle, ring and little finger, respectively, W = wrist (with permission from ( 46)). The LED display used to cue different movements (flexion or extension) is shown on the right. An overhead view of a monkey’s finger between two microswitches is shown in detail below. ![]() A pistol-grip device is shown with a monkey’s hand in place, with each digit positioned between two microswitches. ( e) Manipulandum designed to test “individuation” of digit movements in the macaque. Monkeys used a precision grip (opposition of thumb and index finger) to retrieve food pellets from the board, and monkeys were scored for the number of pellets retrieved in 30 s. Food pellets were placed in wells accessed via slots cut in the board, some horizontal, some vertical. ( d) Modified Brinkman board used to assess skilled hand function in monkeys subjected to incomplete spinal lesions and treatment with anti-NoGo antibody ( 21). A plastic cover with two slots above the lever (not shown) restricted access to these two digits and prevented digits 3–5 from contributing these digits were flexed out of the way. ( a–c) Sequences showing a macaque performing a trained precision grip movement which involved displacing two spring-loaded levers, one with the index finger and one with the thumb. Monkeys with prehensile digits but only capable of a side grip between thumb and index finger (pseudo-opposition) (index 5) include the New World squirrel monkey, while the marmoset, the most widely used non-human primate in modern research, has prehensile digits but a non-opposable thumb, and has a dexterity index of 4. An opposable thumb, allowing a somewhat more limited form of precision grip is found in dextrous monkeys such as the Old World macaque (Rhesus monkey) and the New World capuchin (Cebus monkey), and has an index of 6. It is found in humans and in great apes, and is given an index of 7. In the dexterity index, a true precision grip involving pad-to-pad opposition between index finger and thumb is the highest form of dextrous capacity (Figs. The “index of dexterity” is a measure of the degree of functional adaptation of the hand that allows relatively independent use of the digits and the employment of different types of grasp ( 5, 6). There is evidence that these neuroanatomical changes are associated with important behavioural adaptations in the control of the hand. Development of the CST correlates with the improvement in the index of dexterity, particularly in the ability to perform finger-thumb precision grip (with permission from Courtine et al. The primate CST is located mostly in the lateral columns, and a significant proportion of CS fibres (∼10%) descend ipsilaterally (but actually many of these cross and still terminate contralaterally see ( 44)). This is reflected in an increase in the size of the excitatory postsynaptic potential (EPSP) elicited by cortical neurons in hand motoneurons. In non-human primates and humans, direct CS connections with motoneurons have evolved, together with an increase in the size and number of the CS fibres. Most of the CST fibres in rodents travel in the dorsal columns. In rodents, there are no direct connections between CS neurons and the cervical motoneurons which innervate forelimb muscles–brainstem pathways and spinal interneurons relay cortical input to motor neurons. Relationship between the development of the CST and the emergence of fine motor control abilities.
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