History of neuroscience: Camillo Golgi

By the second half of the 19th century, scientists were beginning to develop a better understanding of the overall function of the nervous system. Discoveries like Paul Broca's identification of Broca's area and Fritsch and Hitzig's description of the motor cortex, for example, had led researchers to a deeper appreciation of the functional specialization of different parts of the brain. What was still missing by the 1870s, however, was an awareness of some of the most fundamental information about the basic building blocks of the nervous system: neurons.

In fact, the word neuron (along with the terms axon, dendrite, and synapse) would not be introduced until the 1890s, and at the middle of the 19th century there was some debate about whether the brain was even made up of distinct cells like other tissues in the body. This was because when one viewed brain tissue under a microscope, nerve cells appeared to have many extensions that stretched out to other cell bodies, seeming to make contact with them. In this way, the brain looked as if it consisted of a collection of uninterrupted processes that formed an expansive net of cellular entities. This view of the structure of the brain came to be called reticular theory, as the word reticulum is Latin for "net."

The main obstacle that prevented researchers from being able to describe the true structure of neurons was the lack of a stain that allowed for the clear differentiation of neurons under the microscope. Because cells are not distinctly colored like they often are in textbooks, their clear borders are difficult to discern against a similarly colorless fluid background (even under the microscope). Microscopists must rely on dyes or stains, which selectively color cells or individual components of cells so they stand out and are able to clearly be seen.

Before the 1870s, the most widely used stain in brain science was a substance called carmine, a reddish stain that could be obtained from certain insect species (it is still used today as a coloring agent in a variety of products ranging from cosmetics to yogurt). Carmine was recognized as a staining agent in the 1850s. Its discovery was a major breakthrough, as prior to its widespread use staining was not even a regular practice nor was its utility fully appreciated. The carmine stain, however, still did not allow brain scientists to attain a perfectly clear picture of neurons, instead leading to the view that supported reticular theory as discussed above. 

This was the context when Camillo Golgi made his contributions to the field in the 1870s. Golgi was a thirty-year old physician working at a small, little-known hospital in northern Italy. He did not have the benefit of using the laboratory facilities of a large research institution. Instead, he created a makeshift laboratory in a kitchen of the hospital. The laboratory consisted of not much more than his microscope, which he mostly used in the evening by candlelight.

a pyramidal neuron stained with a golgi stain

It was in that hospital kitchen that Golgi developed a new method of staining that would revolutionize the way people looked at the brain. The stain involved soaking cells in a solution of silver nitrate, and although Golgi was not the first to attempt to stain cells with silver, his method was a tremendous improvement over past efforts.

The silver staining method caused neurons to appear dark against a yellow background (for this Golgi initially called the method the "black reaction"), but the essential feature of the stain for making it useful in visualizing neurons was that it only caused about 3% of the neurons in a tissue sample to be darkened. This was important because if all neurons were stained, the abundance of cells in any sample would cause the whole sample to appear black. Due to its selectivity, the silver stain allowed for the visualization of a selection of neurons in such detail as had never been possible before.

Golgi used his new stain to make a number of important observations about the nervous system. He provided more detailed descriptions of neurons, including the first good descriptions of axon collaterals, or branches that extend off of the main processes of axons. He described two types of neurons in the brain, one that has long a long axon that can stretch from the grey matter of the brain to other parts of the brain or nervous system, and another that has a short axon. These neurons have since been named Golgi I and Golgi II cells, respectively. He detailed ways in which glial cells can be differentiated from neurons and described the structure of the cortex, corpus callosum, and spinal cord. He discovered sensory receptors in muscle that detect muscle tension; these are now known as Golgi tendon organs. And of course he was the first to describe in detail the protein- and lipid-packaging cellular organelle now called the Golgi apparatus. 

Despite all of Golgi's achievements, his contribution of the silver stain may have been the most significant, as it allowed later researchers to appreciate the true structure of neurons for the first time. Golgi's stain (with some refinement) would be used by neuroscientists like Santiago Ramon y Cajal to prove that neurons did not fuse together to form a net, and instead were independent of one another (just like other cells in the body); Cajal's observations on the structure and organization of neurons would soon come to be known as neuron doctrine, and it would supplant reticular theory in the minds of most of the scientific community. Golgi and Cajal would share the 1906 Nobel Prize in Physiology or Medicine for their important contributions to neuroscience.

Although Golgi's stain had helped Cajal to disprove reticular theory, Golgi refused to accept the evidence that suggested neurons were independent of one another. In his Nobel Prize acceptance speech, Golgi railed against the neuron doctrine, shocking much of the audience to whom the evidence in support of it was distinctly clear. Even though he was incredibly stubborn on this point, however, Golgi's obstinance does not overshadow the significant contributions he made to neuroscience and biology as a whole.

Finger S. Minds Behind the Brain. New York, NY: Oxford University Press; 2000.

2-Minute Neuroscience: Amyotrophic Lateral Sclerosis (ALS)

Amyotrophic lateral sclerosis (ALS) is a debilitating neurodegenerative disorder characterized by a progressive loss of motor function. ALS affects upper motor neurons and lower motor neurons. As these motor neurons stop working, muscles also begin to atrophy; this can eventually lead to respiratory failure, which is often the cause of death in ALS patients. The pathophysiology of ALS is not completely understood, but similar to other neurodegenerative diseases like Alzheimer's disease it is characterized by clusters of dysfunctional proteins within neurons. In this video, I discuss ALS symptoms and pathophysiology.

Know your brain: Mammillary bodies

Where are the mammillary bodies?

The mammillary bodies are part of the diencephalon, which is a collection of structures found between the brainstem and cerebrum. The diencephalon includes the hypothalamus, and the mammillary bodies are found on the inferior surface of the hypothalamus (the side of the hypothalamus that is closer to the brainstem). The mammillary bodies are a paired structure, meaning there are two mammillary bodies---one on either side of the midline of the brain. They get their name because they were thought by early anatomists to have a breast-like shape. The mammillary bodies themselves are sometimes each divided into two nuclei, the lateral and medial mammillary nuclei. The medial mammillary nucleus is the much larger of the two, and is often subdivided into several subregions.  

What are the mammillary bodies and what do they do?

The mammillary bodies are best known for their role in memory, although in the last couple of decades the mammillary bodies have started to be recognized as being involved in other functions like maintaining a sense of direction. The role of the mammillary bodies in memory has been acknowledged since the late 1800s, when mammillary body atrophy was observed in Korsakov's syndrome---a disorder characterized by amnesia and usually linked to a thiamine deficiency. Since then a number of findings---anatomical, clinical, and experimental---have supported and expanded upon a mnemonic role for the mammillary bodies.

The mammillary bodies are directly connected to three other brain regions: the hippocampus via the fornix, thalamus (primarily the anterior thalamic nuclei) via the mammillothalamic tract, and the tegmental nuclei of the midbrain via the mammillary peduncle and mammillotegmental tract. Two of the three connections are thought to primarily carry information in one direction: the hippocampal connections carry information from the hippocampus to the mammillary bodies and the thalamic connections carry information from the mammillary bodies to the thalamus (the tegmental connections are reciprocal). 

These connections earned the mammillary bodies the reputation of being relay nuclei that pass information from the hippocampus on to the anterior thalamic nuclei to aid in memory consolidation. This hypothesis is supported by the fact that damage to pathways that connect the mammillary bodies to the hippocampus or thalamus is associated with deficits in consolidating new memories. Others argue, however, that the mammillary bodies act as more than a simple relay, making independent contributions to memory consolidation. Both perspectives emphasize a role for the mammillary bodies in memory but differ as to the specifics of that role.

Further supporting a role for the mammillary bodies in memory, there is evidence from humans that suggests damage to the mammillary bodies is associated with memory deficits. Several cases of brain damage involving the mammillary bodies as well as cases of tumor-related damage to the area of the mammillary bodies suggests that mammillary body damage is linked to anterograde amnesia. Indeed, mammillary body dysfunction has been identified as a major factor in diencephalic amnesia, a type of amnesia that originates in the diencephalon (Korsakoff's syndrome, an amnesia that is seen primarily in long-term alcoholics, is one type of diencephalic amnesia).

Experimental evidence from animal studies also underscores the importance of the mammillary bodies in memory. Studies with rodents and monkeys have found deficits in spatial memory to occur after damage to the mammillary bodies or the mammillothalamic tract. 

In addition to involvement in memory functions, there are cells in the mammillary bodies that are activated only when an animal's head is facing in a particular direction. These cells are thought to be involved in navigation and may act somewhat like a compass in creating a sense of direction.

Vann SD, & Aggleton JP (2004). The mammillary bodies: two memory systems in one? Nature reviews. Neuroscience, 5 (1), 35-44 PMID: 14708002

History of neuroscience: Luigi Galvani

Luigi galvani (1737-1798)

By the start of the 18th century, brain scientists were beginning to develop a better understanding of the complex anatomy of the nervous system. The physiology of the brain, however---or the way in which the brain functions---was still an area dominated by speculation and lacking in experimental evidence. One extremely important but unanswered question at the time involved the physiology of the nerves. Scientists in the beginning of the 18th century still relied on observations made by the ancient Greeks when attempting to explain nerve function, but those insights did not seem to be matching up with recent laboratory discoveries.

The dominant hypothesis regarding nerve function at the beginning of the 1700s centered around the ambiguous concept of animal spirits. The notion of animal spirits is thought to have originated with the ancient Greeks, and was advocated by Galen---whose influence may have helped the doctrine remain dominant for over 1500 years.

The animal spirits hypothesis suggested that the hollow nerves of the body were filled with spirits---invisible, intangible substances that acted in a mysterious manner to cause movement or allow sensation to occur. According to Galen's view, a form of spirits called natural spirits were produced in the liver after the consumption of food. Natural spirits then were sent to the heart, where they were converted to vital spirits. Vital spirits were carried in the carotid arteries to the brain to either the ventricles or to a complex of arteries at the base of the brain that Galen called the rete mirabile, or "wonderful net." In one of these locations, the vital spirits were converted to animal spirits---the highest form of spirits. The animal spirits were then stored in the ventricles until they were needed.

Although the animal spirits hypothesis was still the prevailing hypothesis at the start of the 18th century, investigators were not having success in experimentally verifying the existence of the spirits. This led to the exploration of other hypotheses, like Thomas Willis' idea that the nerves carried fluid that dripped onto muscles to stimulate them. These new hypotheses, however, also did not seem to stand up to experimental scrutiny. But this changed early in the 18th century when some scientists began to suggest that electricity was the enigmatic substance that filled the nerves.

At the time, appreciation for the wonders of electricity was rapidly growing. The first devices that could produce and store electricity, known respectively as friction machines and Leyden jars, appeared in the first half of the 18th century. These contraptions could be used to create dazzling displays, and became a popular novelty at social engagements. It was also soon recognized, however, that electricity had some potential medical applications. It seemed to be particularly effective at stimulating the muscles of paralyzed limbs to contract.

This led some to hypothesize that electricity was the substance that flowed through the nerves. This hypothesis was bolstered when it was verified that the shocks produced by electric fish (e.g. the electric ray) were caused by actual electricity, as it proved that electricity could exist within the confines of an animal's nervous system. It was at this time, when excitement about electricity as a mechanistic component of the nervous system was beginning to grow, that Luigi Galvani would make his seminal contributions to the field.

Galvani was a doctor and professor of anatomy at the University of Bologna in Italy. In the 1770s, he began to explore electricity and its association with the nerves, conducting his experiments in his own home and mostly with frogs as the subjects.

An illustration from Galvani's 1791 publication that shows some of the devices (along with frog preparations) used in his experiments.

In 1791, after 10 years of research into the subject, Galvani published the work that would make him famous, his Commentary on the Effects of Electricity on Muscular Motion. In the treatise, Galvani described a series of experiments that made a strong case for the natural involvement of electricity in the nervous system. First, Galvani discussed an observation that occurred serendipitously. He had placed a dissected frog on a table next to an electrical machine, and when one of his assistants touched the frog's nerves with a metal scalpel at the same time as the electrical machine emitted a spark, the frog's leg muscle contracted, causing a convulsive movement of the limb. 

Galvani further explored the ability of electricity to cause muscle contractions. He found that when a wire was stretched from the electrical machine to the frog's leg, a convulsion was also elicited. He extended the finding to mammals, observing that similar types of contractions could be generated in chickens and sheep.

Galvani then began to investigate the effects of natural sources of electricity, showing that lightning (as conducted by a lightning rod and down a wire) was also capable of producing muscle contractions when it was given a path to a frog's limbs. These experiments were all interesting, but they were not groundbreaking on their own. Other investigators had observed the ability of electricity to elicit muscle movement. But the next experiments Galvani conducted, and the resultant deductions he made, are what really caused his research to stand apart from the rest.

As a way to attach conductors or hang the frogs outside his home for experiments with lightning, Galvani had fastened brass hooks to the frogs' spinal cords. He was inside with a frog that had a brass hook attached to it when he pressed the frog, along with the hook, up against a metal plate. To Galvani's surprise, the frog exhibited the same type of convulsive movements the application of electricity had caused. This suggested the movements were not dependent on some external source of electricity, and led Galvani to make the deduction that "the electricity was inherent in the animal itself."

Galvani went on to hypothesize that this "animal electricity" was produced by the brain and distributed by the nerves to the muscles (the brain as the "source" of electricity would not stand up to scrutiny once researchers began to better understand the electrical properties of neurons). He also postulated that the nerves must be covered with a fatty insulatory material---a hypothesis that preceded the discovery of that insulatory material (i.e. myelin) by over 60 years.

Galvani's findings and deductions were very influential. Other hypotheses about nerve function, like the animal spirits doctrine, began to fall out of favor. Although many questions about the electrical properties of the nervous system remained, at least now investigators had a mechanism for nerve function that could be observed and measured (unlike the elusive animal spirits). Galvani's discoveries would form the foundation of the modern study of nerve function.

Galvani was not able to fully appreciate the significance of his observations or the popularity they engendered. His wife died in the same year he published his findings (1791). He was devastated and never seemed to be the same emotionally. He spent the next seven years defending his conclusions from critics, especially the well-known well-known Alessandro Volta, who incessantly attacked Galvani's work as insufficient to support the deductions he made. Galvani died in 1798, uncertain of how important his discoveries would become and unaware that they would be an essential piece in the foundation modern neuroscience has been built upon.

Finger S. Minds Behind the Brain. New York, NY: Oxford University Press; 2000.

Galvani L. Commentary of the effects of electricity on muscular motion. Foley MG, translator. Norwalk, CT: Burndy Library; 1953.