LATERAL GENICULATE NUCLEUS (LGN)
9.6.1 Anatomy
To begin with, I am mentioning the presence of well-defined laminar structure. Cells of the geniculate are grouped into three major layers, with the optic tract fibers from the contralateral eye projecting exclusively to the dorsal and ventral layers.
while the ipsilateral fibers project to the middle layer (Garey & Powell, 1968; Guillery, 1966; Hayhow, 1958; Laties & Sprague, 1966; Stone & Hansen, 1966). An additional sublamina of the ventral layer which also receives an ipsilateral projection was described by Guillery (1970). Neurons of the LGN are classified into 4 types based on peculiar morphological characteristics. These are classes 1, 2, 3, and 4. Class 1 neurons have large cell bodies and thick, straight dendrites punctuated by occasional spiney appendages. Class 2 neurons have relatively smaller somata and thin, arching dendrites that terminate in a cluster spheroid appendage. Class 3 neurons have smallest cell bodies and very thin, wavy dendrites along which appear complex arrays of ovoid specializations (Garey & Powell,1967). Class 4 cells are small in size and possess dendrites resembling class 1 cells (Stone & Dreher, 1973). Based in distribution, class 1 and 2 cells are found in layers A, AI and, while class 4 cells are confined mainly to the C1 and C2 laminae of the LGN. Class 1 and 2 cells are exclusively relay cells (not interneurons), this ascertained through the technique of intracellular staining (Ogawa, Takimori, & Takahashi, 1978). There exist some morphological correlations between earlier mentioned X and Y cells with class 1, 2, 3 and 4 cells. Class 1 cells are morphologically associated with Y cells, class 3 with X type, while class 2 structural traits exhibited by some X cells, others for Y (Garey & Blakemore, 1977a, 1977b; Gilbert & Kelly, 1975; LeVay & Ferster, 1977).
LeVay and Ferster (1977) suggested that class 1 neurons are Y cells, class 2 are X, and class 3 are simply interneurons. Afferent input to the cells of LGN arrive from numerous sources, with axons of retinal ganglion cells accounting for major excitatory input. These axons divide into several branches prior to terminating on geniculate cells (Guillery, 1966), suggesting that each individual optic fiber innervates several cells of the LGN. The projection of the fibers onto geniculate cells is retinotopically organized in a very precise manner (Stone & Hansen, 1966). The cells of the cat LGN also receive modified topographical projections from the visual cortex (Beresford, 1961; Guillery, 1966; Hollaander, 1970; Kalil & Chase, 1970; Szentagothai, Harmori, & Tombol, 1966; Updyke, 1975), as well as a less organized projection from the superior colliculus (Altman, 1962). There is proposal that the LGN receives afferents from other nonvisual structures of the brain, this is because LGN activity can be modulated by stimulation of the midbrain reticular formation (Burke & Cole, 1978; Suzuki & Taira, 1961).9.6.2 Physiology
Here, we are to elaborate how the LGN components bring about visualization possibilities. To begin with, geniculate neurons in the cat possess receptive fields which are arranged concentrically, with both ON-center and OFF- center types (Hubel & Wiesel, 1961). Receptive field centers of LGN cells have same diameters as the ganglion cells and show field sizes increase with retinal eccentricity (Hoffman, Stone, & Sherman, 1972; Hubel & Wiesel, 1961).
Besides rare exceptions (Daniels, Norman, & Pettigrew, 1977), LGN fields show virtually no specificity for orientation or direction of movement, which is seen in the retina too (Kozak, Rodieck, & Bishop, 1965). Note that there exist some peculiar differences between ganglion cell and geniculate fields and are not to be used interchangeably. For instance, at LGN level, the surround mechanism exerts more of inhibitory influence on the center, in comparison to surrounds at the retinal level, thus enhancing the sensitivity of geniculate neurons to luminance gradients (Hammond, 1973; Hubel & Wiesel, 1961).
Also, the surround portion of the geniculate field show some levels of responsiveness, even at scotopic levels very much below those at which the effects of ganglion cell surrounds disappear (Hammond, 1972; Maffei & Fiorentini, 1972). LGN contains some cells which can be influenced via either eye, such that the cell is excited by stimulation of one eye and inhibited through the other. This is practically impossible for the retina (Sanderson, Bishop, & Darian-Smith, 1971). Another feature to be noted is binocular inhibitory interactions, which bring about absolute segregation of left- and right-eye affer- ents to the different laminae. It is evident that the receptive fields of LGN cells include a third component not seen in retinal fields, such as annular region, surrounding the prominent surround portion, which is of the same polarity as the field center (That is ON or OFF) (Hammond, 1973). Under normal circumstances, stimulation of outer surround reduces the effectiveness of the inner surround, thus disin- hibiting the center (Maffei & Fiorentini, 1972). This third field component, called the suppressive field, based on the inhibitory nature, is a feature peculiar to the LGN (Cleland et al., 1971).Most LGN neurons are innervated by few optic fibers and in some cases by only a single fiber (Cleland et al., 1971; Hammond, 1973; Hoffmann, Stone, & Sherman, 1972; Singer & Creutzfeldt, 1970). Geniculate cells with ON-center fields receive excitatory input only from ON-center ganglion cells, and OFF-center geniculate cells are excited just by OFF-center ganglion cells (Cleland et al., 1971). This is in the order of LGN synaptic organization (Singer & Creutzfeldt, 1970). Secondly, the X/V grouping described for retinal cells is also preserved at the LGN is the sense that X-type retinal cells innervate only X-type geniculate neurons and Y-retinal cells only Y-geniculate neurons (Cleland et al., 1971; Hoffmann et al., 1972). X-type geniculate neurons have slower conducting axons than the V-type neurons, as seen in the retina.
A group of W type cells, which are strictly seen in the ventral portion of layer C (Cleland et al., 1976), while X and Y cells, in contrast, consist almost exclusively of the population of neurons in laminae A and Al (Wilson, Rowe, & Stone, 1976). These X cells in the LGN are more strongly subject to intrage- niculate inhibition than are Y cells (Fukada & Stone, 1976). Bullier and Norton (1977) notice some cells of the LGN that exhibit some properties of both X and Y cells. The response properties of geniculate cells and other neurons in the visual pathways is strictly time-dependent (Stevens & Gerstein, 1976a, 1976b).9.6.3 Lateral Geniculate Nucleus Projections
Axons of geniculate neurons project to four visual areas of the cortex, named areas 17, 18, 19, and the lateral wall of the suprasylvian gyrus in cats (Garey & Powell, 1967; Niimi & Sprague, 1970). Larger geniculate axons with faster conducting abilities project more heavily to area
18, while smaller axons with slower conduction terminate predominantly in areas 17 and 19 (Garey & Powell, 1967; Hollaander & Vanegas, 1977). In general, fast conducting LGN cells are Y type while the slower conducting cells are X type. This mode of projections to visual cortex suggests that areas 17 and 18 may perform different visual functions in the cat, with termination of radiation fibers from the LGN upon several cortical areas within the geniculocorti- cal pathways occurring in parallel, not series.
9.7