In this drawing from rat, Cajal shows signals from the interstitial nuclei (c,B) coming from the root of the oculomotor nerve (A), and being sent through the thalamus.  But experimental evidence in the past 25 years has revealed that these nuclei actually receive signals from the vestibular nuclei and send signals to the oculomotor nucleus through the posterior commissure (none of which appear in the plane of this illustration), so that information flows in the opposite direction than Cajal’s supposition.  

Moving the eye from its resting central position requires sustained movement of the extraocular muscles, but neurons that control various aspects of eye position encode only movement velocity. Consequently, signals must be integrated prior to reaching the oculomotor nucleus, a computation that occurs as they travel through the interstitial nuclei of Cajal.

The trapezoid body, located in the brainstem, is part of the auditory pathway where nerve fibers from the cochlea on one side of the brain cross over on their way to the superior olivary nuclei to the other side, which functions in multiple aspects of hearing.  This crossing over is believed to help with localization of sound.  

Although modern-day diagrams generally show the nerve fibers originating on only one side of the brain for clarity, sections of the trapezoid body examined under a microscope display nerve fibers running in every direction.  Despite this, Cajal was able to clearly see the crossing of the nerve fibers from one side of the brain to the other and to correctly infer that signals ran from the ciliated cells in the Organ of Corti in the cochlea (B) up and across through the trapezoid body (E) to the superior olivary nucleus (J) on the opposite side of the brain, and from there up through the inferior colliculus (G) and towards the cerebral cortex.  

In addition, Cajal depicts some auditory fibers originating in the superior olivary nucleus as sending signals to the 7th cranial nerve (VII), otherwise known as the facial nerve, as well as to the 11th cranial nerve (XI), which controls muscles in the neck and shoulder.  He hypothesized that the connections of these nerves to the auditory system would thus explain the reflex displayed when one hears a sound and immediately turns one’s head and neck in the direction of the source.

The olfactory bulb transmits odorant information from nose to brain through six layers of neurons: the olfactory nerve layer (not shown), the glomerular layer (A), the external plexiform layer (B), the mitral cell layer (C), the internal plexiform layer (D), and the granule cell layer (E), which is deepest in towards the brain.  Cajal definitively delineated the above six layers in 1890, building on earlier work by Schwalbe, and noted that scent information was not passed from single neuron to single neuron, as in other sensory systems, but was sent from the group of neurons comprising an olfactory fiber to multiple cells in the olfactory bulb.  

Cajal was also the first to observe that scent signals are not necessarily transmitted in a serial manner; the mitral cells project into the glomerular layer and are in contact with some ends of the olfactory fibers, and their output is sent to structures deeper within the olfactory cortex.  While he identified several types of cells present in the granule cell layer on a morphological and spatial basis, the function of these cells was not elucidated until recently: the provision of inhibitory feedback for mitral cells.

Interstitial Cells of Cajal

Intestinal villi are small projections that extend into the lumen of the small intestine to increase the surface area of the intestinal wall, thereby increasing absorption of nutrients.  As the small intestine is made of smooth muscle, one might expect that it is innervated to enable consistent peristalsis.  

Cajal’s studies of the innervation of the small intestine revealed several types of cells that responded to the Golgi staining method he usually used.  Cajal believed that the stained cells were neurons, but his contemporaries such as Kolliker and Dogiel maintained that the stained cells in the intestinal villi were fibroblasts, as Golgi’s staining method was known to sometimes highlight connective tissue as well as nervous tissue.  

This controversy continued until the mid-1990s, when it was determined through the use of marker genes that these cells are generally of mesodermal origin, although some may originate from neural tissue.  Such cells are now identified through ultrastructural characteristics and are known definitively to serve as pacemakers for smooth muscle contraction.

The fovea centralis, a small avascular depression at the center of the inner retinal surface filled with densely packed cone cells, is responsible for collecting visual information from the focus of visual gaze to form high-resolution images.  Signals from the fovea constitute half of all input to the visual cortex.  

In this drawing of the foveal pit (F) and perifoveal area by Cajal, the midget bipolar and ganglion cells (so called due to their small size) are highlighted.  In the image, each cone cell (A) contacts a single midget bipolar cell (B), which in turn makes synapses with a single midget ganglion cell (C), which then transmits information through the optic nerve to the visual cortex.  Cajal correctly noted that as distance from the foveal pit increases, the number of cone cells providing input to a single ganglion cell (C2) increases, a process known as convergence.  The lack of convergence at the foveal pit is what allows for maximum visual acuity.  

What Cajal did not realize was that, in order to maintain such visual acuity, each cone cell in the fovea is actually connected to two midget bipolar cells and midget ganglion cells (each pair of which exists in mutually exclusive planes of focus in microscope preparations) — one pair will signal only when the center of the cone’s photoreceptive area is stimulated, and the other pair signals only when the off-center area of the cone’s photoreceptive area is stimulated.  In addition, information transmitted from the fovea to the visual cortex also contributes to color vision.  

The Calyx of Held, first described by Hans Held in 1893, is one of the largest synapses found in the mammalian brain.  As part of the auditory system, each Calyx is part of the axon of a globular bushy cell in the anteroventral cochlear nucleus, which forms a synapse with a principal cell in the medial nucleus of the trapezoid body.  These synapses are integral to detection and localization of high frequency sounds.  

At a time when the existence of synapses between nerve cells were not yet accepted as fact, Cajal’s drawings of the mammalian auditory system revealed a sophisticated understanding of the relationship between the Calyx of Held and the neuronal cell body it envelops.  Rather than portraying[WC([1] them as a single, connected entity, Cajal’s coloring of this drawing indicates that he knew each part belonged to a separate cell, and that information would need to travel between them.  

Because of the large size of a Calyx of Held and its relative accessibility, it has been a popular model system for neurobiological research.  In particular, its amenability to patch-clamping has made it a favorite for systematic studies of presynaptic mechanisms, which can be involved in neurological disorders such as Parkinson’s disease.

The neocortex, the complex strata of neurons in the mammalian cerebrum, comprises the ridges and grooves seen on the top layer of any 3-D depiction of the human brain. It is involved in processes such as sensory perception, language, and conscious thought.  

In the 1890s, Cajal and another scientist, Gustaf Retzius, independently identified bipolar neurons with horizontal nuclei in the developing neocortex of different types of mammals.  Shown in layer d of the drawing above, these cells – now known as Cajal-Retzius cells – are not only important for the transmission of information through the neocortex, but they also play a role in brain development.  They secrete a protein called Reelin, which is a critical component of a signaling pathway for neuronal migration, ensuring that new cortical cells migrating along the long radial glia (a) and into the upper layers of the neocortex will stop when they reach the correct position.  

Diseases associated with a lack of Reelin expression include schizophrenia, bipolar disorder, and autism.  Although Cajal was not aware of the existence of Reelin, or the developmental importance of the Cajal-Retzius cells, he accurately captured many of the major features of neocortical development using only his microscope.

The olfactory system comprises three major parts: the olfactory epithelium, which contains olfactory sensory neurons in direct contact with the environment; the olfactory bulb, which integrates input from the olfactory sensory neurons; and the olfactory cortex, which processes input from the olfactory bulb is processed and transmitts information to other structures in the brain such as the thalamus and hypothalamus.  

By observing the positions of the axons and dendrites in relation to each other, Cajal was able to infer the direction of the flow of information in the olfactory system (represented above by arrows) from the periphery of the body to deeper brain structures, nicely illustrating his Law of Dynamic Polarization.  Cajal also noted that some information was transmitted back from the cortex to the bulb; he named this bidirectional information flow “centrifugal input”.  

The drawing above shows several olfactory sensory neurons (A) forming synapses with a single glomerulus cell in the olfactory bulb (B); in fact, olfactory sensory neurons expressing the same odorant receptor do converge on a pair of glomeruli cells, which integrate the multiple inputs before sending the signal onwards.  The olfactory sensory neurons are not necessarily spatially close, but Cajal seems to have found a lucky grouping in this drawing, illustrating again his talent for capturing structures with future significance.

Insect eyes are compound eyes: each eye is made of many small unit eyes (ommatidia), as opposed to a single large unit eye as in vertebrates.  Cajal’s drawing above details the many ommatidia present in a single fly eye, as well as the neural circuitry relaying visual signals in towards the brain.  In the lamina complex, labeled C, we can see the axons of the photoreceptor cells crossing over each other, suggesting neural superposition.  

Neural superposition, discovered by Valentino Braitenberg and by Kuno Kirschfeld in 1967, is a phenomenon where photoreceptors from different ommatidia that receive the same signals converge upon the same synapses in the brain, providing vision that is both highly sensitive and high resolution.  Cajal was unaware of the phenomenon, but his drawings suggest that he recognized the organization of structures which make it possible.  

While visibly different from the vertebrate visual system, the insect visual system shares with it certain similarities in neural circuits: both systems have intermediary neurons that receive information from photoreceptor cells and direct it onwards towards the “neural switchboard” or neuropil.

Once thought to be mere “filler” for the space around neurons, astrocytes are star-shaped glial cells found throughout the brain and spinal cord that are now known to perform many important functions, such as regulating the transmission of ions and glucose between blood vessels and the brain.  

When Cajal drew these protoplasmic astrocytes, the prevailing theory was that astrocytes only provided structural support for neurons.  He rejected this idea and instead hypothesized that all the astroglia in the brain formed a sort of gland, which would release substances that affect brain function; it is now known that astroglia do indeed release substances that affect neuronal signaling, including glutamate, GABA, and ATP.  

Having observed the close association of astrocytes with blood vessels and neurons, as well as the fact that all astrocytes appeared to have a prominent appendage, or “foot,” Cajal further hypothesized that astrocytes used this “foot” to stimulate blood vessel dilation.  Astrocyte “feet” do indeed regulate blood vessel diameter, albeit via the release of signaling molecules rather than through physical manipulation, as Cajal imagined.  Astrocyte-evoked changes in blood flow, and thus in oxygenation, constitute the signal that is measured by fMRI, a tool used to image brain activity.

Cajal inferred the flow of information between neurons from their structure and relative position.  His ‘Law of Dynamic Polarization’ posits that each neuron is polarized: it has dendrites, through which signals are received, and an axon through which signals are transmitted to the dendrites of the next cells in the pathway.  Thus, simply by observing the morphology and location of neurons in a tissue, he was able to discern the direction of signal transmission.

While this was relatively straightforward in the retina, which receives outside stimuli arrive from a particular direction and must be carried inward, Cajal’s real genius was revealed in the deduction of information flow in a tissue such as the hippocampus, where the sites of input and output were not immediately obvious.  

The hippocampus, named for its resemblance to a seahorse (genus Hippocampus), comprises the Horns of Ammon (Cornu Ammonis, or CA regions), the dentate gyrus, and the subiculum.  It receives signals from various parts of the brain via the entorhinal cortex, and signals flow through the hippocampus in the path shown by Cajal above (dentate gyrus  CA3  CA1); its output travels through the fornix to the anterior thalamic nuclei and other destinations.  Prior to Cajal’s observations, the fornix was thought to be a source of input to the hippocampus.

Cajal observed that motor neuron axons originated at the end of the neuron facing away from the brain.  Although he did not know the nature of the signal being sent, Cajal hypothesized that the command to move began in the motor cortex of the brain and traveled down the spinal cord to the muscles involved.  

In the same way, he noticed that the axons of sensory neurons originated at the end of the neuron that was closer to the brain and inferred that sensory input travels up the spinal cord to be processed by the brain.  

These observations were instrumental in the formulation of his ‘Law of Dynamic Polarization,’ which was extended further by Sir Charles Sherrington, winner of the 1932 Nobel Prize in Physiology or Medicine, who showed that spinal cord reflex circuits involved “reciprocal innervation” of opposing muscles.

In the 1960s, Sir John Eccles used electro-physiological analyses to reveal elegant feedforward circuitry.  Incoming mossy fibers deliver sensory signals from pre-cerebellar nuclei and the spinal cord to granule cells (g), the most numerous neuron type in the brain.  Granule cell axons bifurcate to form the densely arrayed parallel fibers, which contact the (relatively) giant Purkinje cells (a). 

Meanwhile, climbing fibers transmit signals from the inferior olivary nucleus to Purkinje cells.  Each parallel fiber makes a single weak synapse onto a Purkinje cell, whereas the parallel fiber connection is very strong, comprising hundreds of distinct synaptic contacts.

Purkinje cells transmit inhibitory signals to the deep nuclei, outside the cerebellar cortex, which will relay information to the cerebral cortex.  The cerebellum coordinates and fine-tunes movements, although recent research has revealed roles for the cerebellum in spatial cognition and language.

Cajal’s careful studies of retinal sections from a variety of animals led him to conclude that retinal structure is remarkably similar between species: all vertebrates have rod and cone photoreceptors, as well as bipolar cells, horizontal cells, amacrine cells and ganglion cells, in roughly the same arrangement.  

The vertebrate retina develops as an outgrowth of the brain, and signals received from stimulation by light travel through the retinal network and then into the brain via the ganglion cell axons in the optic nerve.  The clear signaling pathway through the retina, from the photoreceptors through the ganglion cells, was one of Cajal’s initial inspirations for his ‘dynamic polarization’ theory.  

Further studies of the retina led to the Nobel Prize in Medicine in 1967 for Granit, Hartline, and Wald, for their respective discoveries of the electrophysiological properties of retinal neurons, lateral inhibition among neurons, and the mechanism of rhodopsin function.

Neuron theory was advanced through tireless promotion of Cajal’s stained brain sections, in which distinct neuron boundaries were clearly visible.  Cajal was able to detect the delicate structure of neurons by applying Golgi’s silver nitrate staining method to samples from embryological or perinatal tissue, in which the neurons were unmyelinated and thus more easily susceptible to staining.  

This section of the cerebral cortex from a human infant is an excellent example of the success of his technique.  Though their processes overlap, the cells are clearly distinct from one another.  By noting the placement of the axons and dendrites, Cajal was also able to postulate the direction in which information flows through this tissue—from the deeper layers of the cortex up toward the surface.  

The precentral gyrus is now known to be part of the primary motor cortex, which coordinates with several other parts of the brain to plan and execute muscle movements.