The inputs from sense organs are generally mapped or projectedin an orderly way onto the

86 The inputs from sense organs are generally mapped or projectedin an orderly way onto the

sensory regions in the central nervous system, and the outputs from the motor regions of the central nervous system are mapped in an orderly way onto the muscles. Thus, similar nerve cells in different regions of the vertebrate retina send their axons to synapse with neurons in correspondingly different regions of the optic tectum in the midbrain (Figure 21-110), and similar motor neurons at different locations in the spinal cord send their axons to different muscles.

In principle, the growth cones could be simply channeled to different destinations as a direct consequence of their different starting positions, like drivers on a multilane highway where it is forbidden to change lanes. This possibility was tested in the visual system by a famous experiment in the 1940s. If the optic nerve of a frog is cut, it will regenerate. The retinal axons grow back to the optic tectum, restoring normal vision. If, in addition, the eye is rotated in its socket at the time of cutting of the nerve, so as to put originally ventral retinal cells in the position of dorsal retinal cells, vision is still restored, but with an awkward flaw: the animal behaves as though it sees the world upside down. This is because the misplaced retinal cells make the connections appropriate to their original, not their actual, positions (Figure 21-111). The cells are evidently endowed with positional values, carrying a record of their original position, so that cells on opposite sides of the retina are intrinsically different. As in the cortex of the reeler mouse (see Figure 21-105), it is the intrinsic character, rather than the position, that decides the choice of target site. Such nonequivalence among neurons is referred to as neuronal specificity.

Ax o n s fro m Op p o s it e S id e s o f t h e Re t in a Re s p o n d

D iffe re n t ly t o a Gra d ie n t o f Re p u ls iv e Mo le c u le s in t h e

Te c t u m 87

On reaching the tectum, the retinal axons must choose, according to their individual character, which region of tectum to innervate. Axons from the nasal retina (the side closest to the nose), for example, project to the posterior tectum, and axons from the temporal retina (the side farthest from the nose) project to the anterior tectum. This choice is governed by differences in the intrinsic characters of the cells in different parts of the tectum. Thus the neuronal map depends on

a correspondence between two systems of positional markers, one in the retina and the other in the tectum.

Experiments in vitro with tissues from the chick embryo give some insight into the nature of the tectal markers and the way in which the retinal axons respond to them. Fragments of retina are placed in culture and allowed to send out axons over a substratum that is carpeted with membrane vesicles prepared from tectal cells (Figure 21-112). The carpet is laid out in stripes, with bands of anterior tectal membrane alternating with bands of posterior tectal membrane. Axons from nasal retina, depending on details of the preparation, either show no preference and grow indiscriminately in all of the bands or show a preference, appropriately, for posterior tectal Experiments in vitro with tissues from the chick embryo give some insight into the nature of the tectal markers and the way in which the retinal axons respond to them. Fragments of retina are placed in culture and allowed to send out axons over a substratum that is carpeted with membrane vesicles prepared from tectal cells (Figure 21-112). The carpet is laid out in stripes, with bands of anterior tectal membrane alternating with bands of posterior tectal membrane. Axons from nasal retina, depending on details of the preparation, either show no preference and grow indiscriminately in all of the bands or show a preference, appropriately, for posterior tectal

The peculiar effects of the posterior tectal membrane on the temporal retinal cells have been traced to a specific inhibitory glycoprotein that is distributed in a gradient from posterior to anterior in the tectum. In other parts of the nervous system other surface molecules can be shown to have analogous functions as growth cone repellents. These crude systems of markers are adequate to define the anteroposterior orientation of the map in the frog optic tectum. Other mechanisms of an entirely different sort, however, are required to make the map precise.

D iffu s e P a t t e rn s o f S y n a p t ic Co n n e c t io n s Are S h a rp e n e d b y

Ac t iv it y - d e p e n d e n t S y n a p s e Elim in a t io n 88 , 89

In a normal animal the retinotectal map is initially fuzzy and imprecise. Studies in frogs and fish show that each retinal axon at first branches widely in the tectum and makes a profusion of synapses, distributed over a large area of tectum that overlaps with the territories innervated by other axons. These territories are subsequently trimmed back by elimination of synapses and retraction of axon branches. This refinement of the map through synapse elimination is governed by two competition rules that jointly create spatial order: (1) axons from separate regions of retina, which tend to be excited at different times, compete to dominate the available tectal territory, but (2) axons from neighboring sites in the retina, which tend to be excited at the same time, innervate neighboring territories in the tectum because they collaborate to retain synapses on shared tectal cells (Figure 21-113). The mechanism underlying both these rules depends on electrical activity and signaling at the synapses that are formed. If all action potentials are blocked by a toxin that binds to voltage-gated Na + channels, synapse elimination is inhibited and the map remains fuzzy.

This phenomenon of activity-dependent synapse elimination is encountered in almost every part of the developing vertebrate nervous system. Synapses are first formed in abundance and distributed over a broad target field; then the system of connections is pruned back by competitive processes that depend on electrical activity and synaptic signaling. The elimination of synapses in this way is distinct from the elimination of surplus neurons by cell death, and it occurs after the period of normal neuronal death is over.

The cellular mechanisms of synapse elimination are beginning to be clarified by experiments on the innervation of skeletal muscle in vertebrate embryos, where typically each muscle cell at first receives synapses from several neurons but in the end is left innervated by only one. Co-cultures of motor neurons with muscle cells can be used to analyze the mechanism in vitro. One can identify a muscle cell that is innervated by a single neuron and then directly excite the muscle cell repeatedly with puffs of acetylcholine delivered through a micropipette close to its surface. The synapse made on the muscle cell by the neuron is found to be permanently weakened by this treatment unless the neuron itself is stimulated electrically so that it fires in synchrony with the The cellular mechanisms of synapse elimination are beginning to be clarified by experiments on the innervation of skeletal muscle in vertebrate embryos, where typically each muscle cell at first receives synapses from several neurons but in the end is left innervated by only one. Co-cultures of motor neurons with muscle cells can be used to analyze the mechanism in vitro. One can identify a muscle cell that is innervated by a single neuron and then directly excite the muscle cell repeatedly with puffs of acetylcholine delivered through a micropipette close to its surface. The synapse made on the muscle cell by the neuron is found to be permanently weakened by this treatment unless the neuron itself is stimulated electrically so that it fires in synchrony with the

These and many other findings suggest a simple interpretation of the competition rules for synapse elimination in the retinotectal system. Axons from different parts of the retina fire at different times and so compete. Each time one of them fires, the synapse(s) made by the other on

a shared tectal target cell are weakened, until one of the axons is left in sole command of that cell. Axons from neighboring retinal cells, on the other hand, tend to fire in synchrony with one another: they therefore do not compete but instead maintain synapses on shared tectal cells, creating a precisely ordered map in which neighboring cells of the retina project to neighboring sites in the tectum (see Figure 21-113).

Ex p e rie n c e Mo ld s t h e P a t t e rn o f S y n a p t ic Co n n e c t io n s in

t h e B ra in 89 , 90

The same "firing rule" relating synapse maintenance to neural activity helps to organize our developing brains in the light of experience. In the brain of a mammal axons relaying inputs from the two eyes are brought together in the visual region of the cerebral cortex, where they form two overlapping maps of the external visual field, one as perceived through the right eye, the other as perceived through the left. The organization and development of the cortical projections from the two eyes have been studied in great detail, both by anatomical tracing and by physiological tests in which single cortical cells are monitored to find out what kinds of visual stimulus will excite them. These studies reveal an extraordinary sensitivity to experience early in life: if, during a certain critical period, one eye is kept covered so as to deprive it of visual stimulation, while the other eye is allowed normal stimulation, the deprived eye loses its synaptic connections to the cortex and becomes almost entirely, and irreversibly, blind. In accordance with the firing rule, a competition has occurred in which synapses in the visual cortex made by inactive axons are eliminated while synapses made by active axons are consolidated. In this way cortical territory is allocated to axons that carry information and is not wasted on those that are silent.

But the firing rule also operates in more subtle ways to establish the nerve connections that enable us to see. For example, the ability to see depth - stereo vision - depends on the presence in the visual cortex of cells that receive inputs from both eyes at once, conveying information about the same part of the visual field as seen from two slightly different angles. These binocularly driven cells allow us to compare the view through the right eye with that through the left so as to derive information about the relative distances of objects from us. If, however, the two eyes are prevented during the critical period from ever seeing the same scene at the same time - for example, by covering first one eye and then the other on alternate days or simply as a consequence of a childhood squint - almost no binocularly driven cells are retained in the cortex, and the capacity for stereo perception is irretrievably lost. Evidently, in accordance with the firing rule, the inputs from each eye to a binocularly driven neuron are maintained only if the two inputs But the firing rule also operates in more subtle ways to establish the nerve connections that enable us to see. For example, the ability to see depth - stereo vision - depends on the presence in the visual cortex of cells that receive inputs from both eyes at once, conveying information about the same part of the visual field as seen from two slightly different angles. These binocularly driven cells allow us to compare the view through the right eye with that through the left so as to derive information about the relative distances of objects from us. If, however, the two eyes are prevented during the critical period from ever seeing the same scene at the same time - for example, by covering first one eye and then the other on alternate days or simply as a consequence of a childhood squint - almost no binocularly driven cells are retained in the cortex, and the capacity for stereo perception is irretrievably lost. Evidently, in accordance with the firing rule, the inputs from each eye to a binocularly driven neuron are maintained only if the two inputs

We saw in Chapter 15 that synaptic changes underlying memory in many parts of the brain hinge on the behavior of a particular type of receptor for the neurotransmitter glutamate - the NMDA receptor. Ca 2+ flooding into the postsynaptic cell through the channels opened by this receptor triggers lasting changes in the strengths of the synapses on that cell, just as Ca 2+ entering a muscle cell via acetylcholine-receptor channels during development affects the synapses made on it by motor neurons. The changes that are induced by the NMDA-dependent mechanism in the adult brain obey rules closely akin to the developmental firing rule. In fact, the refinement and remodeling of synaptic connections that we have just described in the developing visual systems of mammals and amphibians can be blocked by an inhibitor of the NMDA receptor. Both memory and the developmental adjustments, therefore, may depend on essentially the same machinery. The molecular basis of this device through which experience molds our brains is one of the central challenges that the nervous system presents to cell biology.