Development of spinal reflexes in the rat fetus studied in vitro.

Abstract

SUMMARY1. The onset and development of spinal reflex activity was investigated using the isolated spinal cord of the rat fetus. The potential changes generated in motoneurones were recorded extracellularly from L3 ventral roots.2. A spike potential was recorded from the ventral root at embryonic day 13-5 in response to stimulation of the cord surface close to the ventral root. The discharge persisted in Ca2+-free solution but was blocked by tetrodotoxin.3. At embryonic day 14-5, trans-synaptically evoked discharges were detected in motoneurones.4. Stimulation of the dorsal root was first effective in eliciting reflex discharges at embryonic day 15-5. The reflex response then consisted of a prolonged depolarization upon which were superimposed small spikes, and was probably polysynaptic.5. A spike potential, presumably a monosynaptic reflex, was generated at the end of fetal life. This discharge appeared first at embryonic day 17-5 in a primitive form.6. Between embryonic day 16-5 and t7 5, stimulation of the dorsal root of different segments (L1-L6) elicited responses similar to those induced by the corresponding (i.e. L3) dorsal root stimulation. These inter-segmentally induced responses were then reduced in size toward the birth. However, in the presence of strychnine, a train of spike discharges of similar shape to the segmentally induced response was also evoked by stimulation of the dorsal root at L4 or L5. These spikes disappeared during further post-natal development.7. It is concluded that synapses in the segmental polysynaptic pathway become functional in a retrograde sequence with respect to the direction of normal reflex impulse flow. The reflex responses, elicited by stimulation of the dorsal roots of different segments, are suggested to be suppressed first by the development of inhibitory mechanisms and then by neuronal cell death or by elimination of the synapses responsible for generating the inter-segmental reflexes.

Radiation of phasic stretch reflex in biceps brachii to muscles of the arm in man and its restriction during development.

“…In the present study the estimated central delay for the stretch reflex at all ages was similar to, or shorter than, those in adults, with 540 DEVELOPMENT OF PHASIC STRETCH REFLEX the implication that the early component of the phasic stretch reflex is monosynaptic during development. This conclusion receives direct support from developmental studies in animals where the phasic stretch reflex has been shown to have a monosynaptic component in both fetus and newborn (rat: Saito, 1979;Kudo & Yamada 1987;cat: Wilson, 1962;Skoglund, 1960;Eccles & Willis, 1965).…”

“…Excitatory reflex actions from muscle afferents predominate (Skoglund, 1960; Saito, 1979; Myklebust, 1990). The kitten has been shown to have excitatory monosynaptic projections between primary afferents and cc-motoneurones of non-homonymous and non-synergistic muscles, which become greatly reduced in comparison to the adult cat (Eccles, Shealy & Willis, 1963).…”

“…The kitten has been shown to have excitatory monosynaptic projections between primary afferents and cc-motoneurones of non-homonymous and non-synergistic muscles, which become greatly reduced in comparison to the adult cat (Eccles, Shealy & Willis, 1963). Indeed, many excitatory intersegmental projections from primary afferents to motoneurones disappear during development (rat: Saito, 1979). In the human neonate Myklebust, Gottlieb & Agarwal (1986) and Myklebust (1990) observed that a tendon tap to triceps surae evoked excitatory reflex activity, not only in triceps surae, but also in its antagonist, tibialis anterior, and to more distant muscles of the lower limb, quadriceps femoris and the hamstring muscles.…”

Expression of synaptophysin during the prenatal development of the rat spinal cord: Correlation with basic differentiation processes of neurons

“…Moreover, although a direct comparison between two species cannot be made, neuromuscular junctions containing a synaptic vesic1e antigen have been observed at ED 14 in another rodent species, the mouse. '9 In the rat, the motor neurons become excitable around ED 14 28 and electrical as weil as chemical excitability appear one week before birth. 20 The first evidence for functional synaptic transmission within rat intercostal musc1es was reported to occur at ED 15: 6 i.e.…”

The Generation of Neuronal Heterogeneity in a Rat Sensory Ganglion

“…Cutaneous afferents appear in rat proximal hindlimb on embryonic days 14 -15 (E14 -E15) and in skin of distal toes at E16 -E17 (Reynolds et al, 1991;Mirnics and Koerber, 1995). These fibers are functional slightly later, beginning at E17 (Saito, 1979;Kudo and Yamada, 1985;Fitzgerald, 1987;Kucera et al, 1988;Fitzgerald, 1991). Neuropeptides appear around the time of peripheral target functional contacts (E18 -E19) (Kessler and Black, 1981;Senba et al, 1982), suggesting that target interactions could influence sensory neuronal phenotype.…”