Comunidades de aprendizaje: aprendizaje dialógico y medidas de éxito

1.2.3.1 Anatomical development

circuitry in the human has not been extensively studied because standard anatomical tracers must be actively transported, 'which requires living tissue. However, this problem has been partly solved by the introduction o f intensely fluorescent lipid-soluble tracers, which enable the tracing o f connections over short distances (Konstantinidou et a l 1995).

At 7.5 weeks PCA, primary afferents are seen at the lateral end o f the dorsal funiculus o f the spinal cord (Okado & Kojima 1984), and by 8 weeks PCA, they are seen in the gray matter o f the spinal cord (Okado & Kojima 1984, Konstantinidou e t a l 1995). Connections between primary afferents and dorsal hom intemeurons are also made at this time (Okado & Kojima 1984). Some axons, considered to be spindle afferents, have already reached the motor pools at 8 weeks PCA, having traversed the length o f the gray matter in bundles (Konstantinidou et a l 1995). A s developm ent proceeds, these axons project to the ventral hom and send branches to the intermediate zone aswell as to the motor pools (Konstantinidou et a l 1995). Between 11 and 19 weeks PCA, afferent fibres which project to the ventral hom form synaptic connections with both the somata and proximal dendrites o f motoneurons (Okado & Kojima 1984, Konstantinidou

et a l 1995). Concurrently with the development o f afferents projecting to the ventral hom, other groups o f axons penetrate the gray matter all along the mediolateral extent o f the dorsal hom, descending to lamina IV and subsequently tum ing upwards to terminate in laminae III and IV (Konstantinidou et a l 1995).

There have been many more studies conducted on the developm ent o f the primary afferent projection to the spinal cord in the rat and other species (Thor e t a l 1982, Smith 1983, Fitzgerald 1985, 1987b, 1988, Mendelson & Frank 1991, Fitzgerald et a l 1994,

Seebach & Ziskind-Conhaim 1994, M im ics & Koerber 1995b). In the rat, primary afferents penetrate the gray matter o f the lumbar spinal cord at E l 5 at which time they are confined to the segments o f entry (Ziskind-Conhaim 1990, M im ics & Koerber 1995b). The growth o f fibres into the gray matter o f adjacent segments begins one day later at E l 6, and this pattem o f one day's delay is continued for each successive segment (M im ics & Koerber 1995b). Monosynaptic connections between primary afferents and lumbar motoneurons are established at E17-18 (Kudo & Yamada 1987, Ziskind- Conhaim 1990). At E l 9, putative small-diameter axons grow into the substantia gelatinosa and display the same rostrocaudal delay (M im ics & Koerber 1995b). Both the C and A ô classes o f small-diameter afferents produce terminals in the substantia gelatinosa just prior to birth (Fitzgerald 1987b), although C-fibres do not form mature synaptic connections there until the end o f the first postnatal week (Fitzgerald 1985,

1988).

The neurochemical development o f C fibre terminations in the rat spinal cord has been investigated using substance P (SP) and fluoride-resistant acid phosphatase (FRAP) as specific C-fibre markers (Fitzgerald & Gibson 1984, Pignatelli et al 1989). These studies have shown that Substance P is present in the spinal cord from birth, and FRAP from within 12 hours o f birth (Fitzgerald & Gibson 1984, Pignatelli et a l 1989). The adult neurochemical appearance o f C-fibre terminals in the dorsal hom is established between postnatal days 6 and 8 (Fitzgerald & Gibson 1984, Pignatelli et a l 1989).

Studies in the rat using a variety o f different methods, including horseradish peroxidase conjugated wheat-germ agglutinin (WGA-HRP), which is transported preferentially by C fibres (Fitzgerald & Swett 1984), horseradish peroxidase conjugated to choleragenoid

(B-HRP) which preferentially labels A fibres (Fitzgerald et a l 1994), and carbocyanine dyes (M im ics & Koerber 1995b) have found that the ingrowth o f primary afferent fibres into the lumbar spinal cord is specific and occupies the somatotopic area appropriate for the adult. The organisation o f the presynaptic neuropil in laminae III and IV does not change significantly during development (M im ics & Koerber 1995b), and the somatotopic arrangement o f A-fibre afferent terminals in the dorsal hom is also established early in development, although at first, A-fibre terminals project throughout laminae I-V, including in lamina II (substantia gelatinosa, Fitzgerald et a l 1994). This pattem o f widespread termination is observed until the end o f the third postnatal week, after which time the A-fibre terminal field becom es restricted to its normal sites in laminae III and IV (Fitzgerald et al 1994). Thor and coworkers (1982), in a study o f pudendal nerve afferent (PNA) projections to the sacral spinal cord in the cat, similarly found that,’ in the neonatal cat, the terminal field o f these afferents is substantially expanded compared to the adult. This is especially tm e on on the side o f the cord contralateral to the labelled fibres, where there is substantial labelling in laminae I, V and VI, areas which are normally only sparsely labelled in the adult. This phenomenon is also evident ipsilaterally, with increased labelling in lamina I on the lateral border o f the dorsal hom, extending into laminae V and VI (Thor et a l 1982).

In a study o f muscle afferent fibre projections to the rat thoracic spinal cord. Smith (1983) found that the projection o f primary afferents in the dorsal and ventral rami to the ipsilateral and contralateral dorsal hom s was appropriate from the very outset. However, in a study o f monosynaptic muscle afferent projections to motoneurons in the rat lumbar spinal cord, Seebach & Ziskind-Conhaim (1994) found that at E l 8-21, 30% o f motoneurons were innervated by primary afferents o f antagonistic m uscles. These

functionally inappropriate synapses persisted at birth, but their percentage w as significantly reduced within 3-5 days after birth (Seebach & Ziskind-Conhaim 1994).

Finally, in an investigation o f the development o f monosynaptic connections in the lumbosacral cord o f the chick embryo, M endelson & Frank (1991) found that connections are formed between afferents and motoneurons o f synergistic but not antagonistic or functionally unrelated m uscles. Furthermore, as functional activity between afferents and motoneurons had been blocked with c?-tubocurarine (Dtc), the specificity o f these connections is not dependant on normal patterns o f neuronal activity or motoneuronal cell death, both o f which are blocked when Dtc is administered (M endelson & Frank 1991). Conversely, in another study examining the same projections in the chick embryo, and also using neuromuscular blocking agents, a widespread pattem o f excitatory connections between afferents and motoneurons o f 6

thigh m uscles was found which were not organised in synergist-antagonist patterns (Lee & O'Donovan 1991).

1.2.3.2 Phvsiological development

In the human, it is obviously difficult to study the developm ent o f physiological activity in the central terminals o f different classes o f primary afferent fibres. However, the onset o f reflex activity has been studied in the human (Humphrey & Hooker 1959, Humphrey 1964), and w ill be reviewed in more detail in an ensuing section (see section

1.3).

In the rat, the onset o f functional central primary afferent connections has been studied largely in vitro in isolated spinal cord preparations (Saito 1979, Kudo & Yamada 1987).

It appears that, in the rat as in the human (Okado & Kojima 1984), connections are first formed between primary afferent fibres and intemeurons, so stimulation o f the L4 dorsal root evokes what is probably a polysynaptic reflex in the ventral root beginning at E l 5.5 (Saito 1979, Kudo & Yamada 1987). Long-latency excitatory postsynaptic potentials (EPSPs) are first recorded from motoneurons in response to dorsal root stimulation at E l6 (Ziskind-Conhaim 1990). Primitive monosynaptic reflexes are observed first at E l 7.5 (Saito 1979), and their latency is markedly shortened between then and E l 8.5 (Kudo & Yamada 1987). A more detailed account o f reflex developm ent in the rat is given in an ensuing section (see section 1.3). In the fetal sheep, functional connections between central primary afferent terminals and dorsal horn cells are evident after m id­ gestation at 92 days (Rees et al 1991).

In the postnatal period, Ap fibres evoke spikes o f electrical activity in both the superficial and deep laminae o f the dorsal horn from day 0 (PC, Fitzgerald 1988). However, although an afferent volley attributable to small diameter fibres (Aô and C) can be detected from P2, it cannot evoke spikes in dorsal horn cells until 1 w eek later at P9 (Fitzgerald 1985, 1988). Nevertheless, before that, this small-diameter afferent volley can. produce long-lasting sub-threshold depolarisations in the dorsal horn (Fitzgerald 1985, Nussbaumer er al 1989), and sensitization to subsequent stimuli (Fitzgerald 1987b).

The neurochemical maturity o f C-fibre afferent connections in the spinal cord o f the neonatal rat has been studied using flexion reflex responses to mustard oil, which is a specific C-fibre irritant (Fitzgerald & Gibson 1984). The authors found that mustard oil could not elicit flexion reflex responses until P I0-11, and neurogenic oedema, a C-fibre-

mediated inflammatory reaction, did not occur until PI 1.