Dendritic Territories Research Articles

Postnatal development of cat hind limb motoneurons. I: Changes in length, branching structure, and spatial distribution of dendrites of cat triceps surae motoneurons.

The postnatal development of length, branching structure, and spatial distribution of dendrites of triceps surae motoneurons, intracellularly stained with horseradish peroxidase, was studied from birth up to 44-46 days of postnatal (d.p.n.) age in kittens and compared with corresponding data from adult cats. The number of dendrites of a triceps surae motoneuron was about 12, and the arborization of each dendrite generated an average of 12-15 terminal branches. There was no net change in the number of dendrites of a neuron or in the degree of branching of the dendrites despite the occurrence of both a transient remodeling of the dendritic branching structure and changes of the spatial distribution of the dendritic branches during postnatal development. The perisomatic territory in the transverse plane occupied by the dendritic branches of a motoneuron increased in parallel with the overall growth of the spinal cord. Thus, the relative size of the dendritic territory in this plane was kept almost constant, whereas dendritic branches projecting in the rostrocaudal direction grew much faster than the spinal cord and also became more numerous. At birth the rostro-caudal dendritic span of individual motoneurons bridged 1:6 to 1:5 of the L7 spinal cord segment length; this figure was 1:3 at 22-24 d.p.n. Hence, in this direction, the growing dendritic branches invaded novel dendritic territories. The change in dendritic branch length from birth to 6 weeks of age corresponded to an average growth rate of 2 to 4 microns per dendritic branch and day, which implies that the total increase in length of the dendrites of a neuron could amount to 1 mm/day. The increase in branch length did not occur in a uniform or random manner; instead, it followed a spatiotemporal pattern with three phases: From birth to 22-24 d.p.n., growth was particularly prominent in greater than or equal to 3rd order preterminal and 2nd through 6th order terminal branches. From 22-24 to 44-46 d.p.n., a large increase in branch length confined to terminal branches of greater than or equal to 3rd branch orders was observed. As indicated by topological analysis, this length increase was probably due in part to a resorption of peripheral dendritic branches during this stage of development. From 44-46 d.p.n. to maturity, the increase of dendritic branch length was restricted to preterminal branches of low (less than or equal to 4th) branch order.(ABSTRACT TRUNCATED AT 400 WORDS)

Postnatal development of cat hind limb motoneurons. II: In vivo morphology of dendritic growth cones and the maturation of dendrite morphology.

The maturation of dendrite morphology was studied by light and electron microscopy in cat spinal alpha-motoneurons intracellularly labeled with horseradish peroxidase. Alpha-motoneurons supplying the triceps surae (TS) and the intrinsic foot sole (SP) muscles were investigated in kittens from birth to 44-46 days of postnatal (d.p.n.) age. At birth, a large number of dendritic branches displayed growth cones, filopodia, and fusiform processes. The growth cones were of lamellipodial and filopodial types, but intermediate forms also occurred. The growth cones shared several morphological features with the neuritic growth cones studied in vitro. It was suggested that the occurrence of different types of growth cones--even in the same dendrite--may reflect their transformation from one type to the other and the level of growth activity could be inferred from the number and form of the growth cones. About 50-70% of the terminal branches in the dendrites of newborn kittens possessed growth cones, filopodia, and/or fusiform processes. The corresponding figure for preterminal branches was 20-30%, with a clear decrease in incidence when approaching the soma. During the period under study, most of these growth-associated processes disappeared from the dendrites so that at 44-46 d.p.n. of age only about 10% of the terminal and less than 1% of the preterminal branches had growth-associated processes. Analysis of the three-dimensional distribution of dendritic branches with such processes disclosed that they were relatively more frequent in the medial, rostral, and caudal dendritic territories. It was concluded that the pattern of distribution and disappearance of growth cones, filopodia, and fusiform processes coincided with postnatal longitudinal dendritic growth and the development of the adult dendritic territories described in a preceding paper (Ulfhake et al., '88). Dendritic growth, with respect to length and caliber, also occurred in the absence of growth cones and filopodia. It is suggested that the important role of these processes may be to act as a steering device in establishing the adult distribution and synaptology of the dendrites. Comparison of TS and SP alpha-motoneuron dendrite morphology at birth and at 22-24 d.p.n. age showed that the SP neurons lagged in the maturation process. Light and electron microscopic observations indicated that postnatally direct contacts might exist between dendrites and fine blood vessels in the neuropil without any interposing glial sheath. The number of such suspected contacts diminished during the period under study, indicating that the glial ensheathment of the blood vessel takes place, in part, postnatally.

The morphology of dorsal column postsynaptic spinomedullary neurons in the cat.

Dorsal column postsynaptic ( DCPS ) spinomedullary neurons from the cat's lumbosacral enlargement were identified by antidromic stimulation of the cervical dorsal columns and stained intracellularly with horseradish peroxidase. The cell bodies were located in laminae III-IV. Their dendritic arbors were elongated rostrocaudally but narrow mediolaterally. On the average, the arbors were X 5 longer than they were wide. Most of the neurons had nearly all of their dendrites in laminae III-IV and some of the neurons had, in addition, a considerable amount of dendritic surface area in lamina V. Only one neuron had more than a very small amount of dendritic surface area dorsal to lamina III. Seven of the neurons had unmyelinated axon collaterals that arborized extensively and issued varicosity-bearing terminal branches in laminae III-V, both within and beneath their dendritic territories. All of the neurons were excited by myelinated, low-threshold mechanoreceptors. Since the rostrocaudally elongated and mediolaterally narrow dendritic arbors of DCPS neurons are in register with the laminae III-IV terminal distributions of myelinated, low-threshold mechanoreceptors, it is probable that this excitation arises from a monosynaptic and topographically discrete innervation. About one-half of the DCPS neurons were also excited by noxious stimuli. It is probable that this excitation is accomplished by a polysynaptic pathway since DCPS dendritic arbors and nociceptor terminal distributions are largely or completely separate.

The superior vestibular nucleus: an intracellular HRP study in the cat. II. Non-vestibulo-ocular neurons.

Superior vestibular neurons were penetrated with horseradish peroxidase (HRP)-loaded glass microelectrodes in anesthetized cats and identified electrophysiologically following electrical stimulation of the vestibular nerves and oculomotor complex. Neurons that were not antidromically activated from the oculomotor complex were stained by intracellular injection of horseradish peroxidase. Three types of neurons are identified according to their initial axonal trajectories into the cerebellum, the dorsal pontine reticular formation, or the brachium conjunctivum. Ipsilateral vestibular nerve input to all neurons is primarily monosynaptic and excitatory, whereas the contralateral is inhibitory. The neurons are located in the periphery of the superior vestibular nucleus. Soma diameters range from 20.5 micrometers to 44 micrometers. Most neurons exhibit globular and ovoid cell bodies. The dendritic arbors are intermediate between iso- and allodendritic branching patterns. The few spines and dendritic appendages present are distributed mainly distally on the dendrites. Soma size does not correlate with axon diameter, number of dendrites, or dendritic territories.

Spinal cord layer I neurons with axon collaterals that generate local arbors

The morphology and location of physiologically characterized neurons in layer I of the spinal cord dorsal horn were revealed by the intracellular deposition of horseradish peroxidase (HRP). This material showed that the axons of some layer I neurons issue varicosity-bearing collaterals that generate arbors that overlap the neurons' dendritic territories in layer I.

Physiology and morphology of substantia gelatinosa neurons intracellularly stained with horseradish peroxidase.

Neurons in Rexed's layer II were physiologically characterized with natural and electrical stimuli applied to their cutaneous receptive fields. The neurons were then intracellularly stained with horseradish peroxidase. Three general patterns of physiological responses were found. Nociceptive specific neurons did not respond to gentle mechanical stimulation. Most responded exclusively to tissue-damaging stimuli. Some also responded to moderately heavy pressure, but these responded to noxious stimuli with an increased discharge frequency. Wide dynamic range neurons responded to both gentle mechanical stimulation and to tissue-damaging stimulation. Low-threshold mechanoreceptive neurons responded only to gentle mechanical stimulation. Some of the low-threshold mechanoreceptive neurons were innervated by primary afferents with unmyelinated axons. Excepting those low-threshold mechanoreceptive neurons with input from unmyelinated afferents, the patterns of primary afferents innervation of layer II neurons were similar to the patterns of innervation that have been found for neurons in layers I and IV-V. All but 2 of the 22 neurons that we found were recognized as being of two general morphological types. Stalked cells had their perikarya situated along the superficial border of layer II. Most of their dendrites traveled ventrally while spreading out rostrocaudally. This gave their dendritic arbors a fan-like shape. Stalked cell axons arborized largely in layer I. Islet cell perikarya were found throughout layer II. Most of their dendrites traveled rostrocaudally. Their dendritic arbors were shaped like cylinders with their long axes parallel to the long axis of the spinal cord. Islet cell axons arborized in the immediate vicinity of their dendritic territories, within layer II. Stalked cells and those islet cells whose dendritic arbors were largely contained within the superficial one-third of layer II (layer IIa) were either nociceptive specific or wide dynamic range neurons. The islet cells whose dendritic arbors were largely within the deeper two-thirds of layer II (layer IIb) were all low-threshold mechanoreceptive neurons. These observations suggest that layers IIa and IIb have different functional roles and that stalked cells and islet cells are separate and distinct components of the neural circuitry of the superficial dorsal horn.

The Terminal Fields of Dorsal Root Fibers in the Lumbosacral Spinal Cord of the Cat, and the Dendritic Organization of the Motor Nuclei

Publisher Summary This chapter describes the pattern of degenerating dorsal root fibers, visualized by means of the Nauta-Laidlaw technique, in the spinal gray matter of the lumbosacral segments of the cat. The pattern, especially of the terminal fields, is related to the cytoarchitectonic laminae determined with the Nissl stain, and to data on synaptic areas revealed by the use of the Golgi method. These anatomical findings have been compared in detail with electrophysiological data on the areas of termination of various muscles and cutaneous nerve fibers, and the distribution of interneurons activated by specific types of peripheral stimulation. The chapter also describes the pattern of dendrites of various motor and interneuron nuclei of the lumbosacral segments by use of the Golgi-Cox technique. The relative independence and overlap of the dendritic territories of the motor nuclei are described in the chapter, and a possible relationship between these and the synaptic linkages formed by the large, sensory fibers of muscle nerves is indicated.