Chapter 1

Welcome to the World of Neurobiology

IN THIS CHAPTER

Getting to know how neurons evolved

Seeing how the nervous system is organized

Meeting neurons and glial cells

Taking a tour around the brain

What makes you you? Your brain, most people would answer. Then what is it about your brain that makes you you? The brain is made of neurons and glial cells. Worms and bees have brains with neurons and glia. So do dogs and chimpanzees. What about the brain distinguishes these animals from each other, and for that matter, one human from another? Is it more neurons, different neurons, special neural circuits? And what happens when these neurons, glia, and circuits are disrupted or dysfunctional?

Neurobiologists like the two of us want to answer these questions. Thousands of us at universities, research institutes, and pharmaceutical companies all over the world are working on these questions. We have many hypotheses and data sets. This book, in a way, is a progress report on the efforts.

Virtually all neurobiologists believe that intelligence comes from nervous systems that are broadly programmed by genes and fine-tuned by experience. Generally, the human genetic program creates a brain with more cortical neurons than other animals have, allowing for richer experience to produce a unique kind of cognition and intelligence.

Here we introduce you to the basics of neurobiology and give you a jumping-off point to use this book in understanding the human nervous system.

Starting with Neurobiology - Just the Basics

Neurobiology is the study of neurons, glia, and the nervous system. It involves many fields of study including physiology, anatomy, biochemistry, molecular biology, cognitive and behavioral psychology, and artificial intelligence. The basic goals of neurobiology are to describe how the nervous system operates in terms of what neurons and glial cells do, how they're built, how they work, and what happens when their functions are disrupted.

Neurons are the detecting and signaling cells of the nervous system that use electrical and chemical signaling to communicate. Glial cells are the supporting cells that provide metabolic, structural, and functional support.

EVOLVING THE NERVOUS SYSTEM

Many of the DNA sequences, proteins, and reactions that exist in neurons and glia are similar to those in single-celled organisms. This apparent conservation of genes and biochemistry is an important argument for life having a common origin.

Cells have membranes that separate their insides from the external environment. Receptors embedded in the membrane enable cells to respond to external signals. Receptor responses include biochemical cascades inside the cell, and, in neurons particularly, electrical activity.

Animals are multicellular eukaryotic organisms that evolved around 600 million years ago. Early animals evolved different types of cells that are specialized to do things like secrete hormones or digestive enzymes and undergo contraction. As animals increased cells and got bigger, their sensor cells became separated from other cells and movement required coordination.

Neurons evolved between about 600 to 550 million years ago, most likely from epithelial cells. This probably occurred when animals needed to send sensory information over longer distances and control contraction of muscles for movement. Neurons developed as electrical signaling cells that formed nerve nets in cnidarian and/or ctenophore evolution.

Later, the evolution of special sense organs and nerve ganglia occurred several times in bilaterans (the clade that gave rise to crustaceans, insects, and vertebrates). There's still much debate over the origin of neurons and nervous systems in the tree of life.

Neurons evolved extensions called axons and dendrites that allow them to communicate rapidly, specifically, and over long distances. Action potentials are electrical signals transmitted along the axon. The axon forms connections called synapses with its targets (the majority on dendrites) where chemical communication occurs and typically produces electrical responses. The following sections discuss when and how neurons most likely evolved during animal evolution.

Arriving at a nervous system

Electrical signaling in the form of action potentials and cell-to-cell communication evolved in single-celled organisms before multicellularity evolved. Some nonmetazoan single-celled organisms express genes for secreted molecules and synaptic-related proteins. Evolving neurons adapted functions that single cells use to interact with the environment and other cells.

The next step was the extension of an axon, from one cell to distant cells where a specific signaling substance, called a neurotransmitter, is released. Now, instead of a multicellular signaling soup, there are circuits. Electrical and chemical signaling allowed for rapid communication across the distances from one end of an animal to another for sensing the surrounding environment and performing coordinated movements.

Coordinating responses in simple circuits

Nervous systems are complex and hard to study. The human brain is estimated to contain about 86 billion neurons and about the same number of glial cells. An average neuron receives a thousand or more synapses, meaning there are more than a hundred trillion synapses in the human brain. Neurobiologists don't really know yet how single neurons work and don't know, and can't count, all of the connections between neurons.

Studying invertebrates

People often wonder why scientists study the nervous systems of flies, worms, squids, and slugs. The reason is they have advantages in that the cells are fewer, bigger, and/or more amenable to genetic manipulation. Alan Hodgkin and Andrew Huxley won the Nobel Prize for deducing the ionic basis of the action potential in the squid giant axon. Eric Kandel won the Nobel Prize for identifying memory mechanisms in the sea slug Aplysia.

Many invertebrates such as worms and insects have several hundred to a few thousand neurons. This vastly simplifies the problem of working out a complete neural circuit, including which neurotransmitters are used by which neurons, what responses are produced and how activity is integrated.

Recent progress has been made in making model systems from mammals, studying intact brains and behavior, and using brain slices, neural tissue cultures, and brain organoids and that can be investigated using molecular biology, microscopy, and electrical techniques.

Introducing Neurons and Glia

When neurobiologists talk about the nervous system, we often use the term neural. Neural refers to any part of the nervous system that involves neurons or glia. Part 1 of this book gives you more detailed information about neurons and glia. Here we describe how neurons and glia are functionally organized in the nervous system.

Beginning with functional anatomy

The nervous system has two main components that are connected and communicate with each other:

• Central nervous system (CNS): The CNS is composed of the brain and spinal cord.
• Peripheral nervous system (PNS): The PNS includes the spinal nerves, cranial nerves, ganglia, and the enteric nervous system.

Neurons in the PNS are responsible for detecting and relaying sensory signals in the external world (somatic sensory system) and in the internal body (visceral sensory system). The PNS then relays that information to the CNS.

The CNS receives sensory information and uses it for perception and homeostasis. These are crucial for the other main functions of the CNS: to control the body's movements and produce survival responses and behaviors. To help control and regulate this output, the mammalian CNS also provides emotion and cognitive functions.

The CNS sends output signals from motor neurons that extend axons in the nerves (the PNS) to your muscles, organs, and glands that control their contraction, relaxation, or secretion. The output in the PNS has two main divisions:

• The somatic system controls skeletal muscles to control our movements.
• The autonomic system controls organs and glands to regulate internal functions.

Establishing the cells

Both neurons and glial cells have a cell body where the nucleus and cellular organelles are located. Cellular organelles include mitochondria, the endoplasmic reticulum, Golgi complex, and lysosomes. The nucleus contains the chromosomes and is where RNA is produced. Organelles carry out biochemical activities including energy metabolism and protein synthesis. Figure 1-1 gives you a snapshot of typical neurons and glial cells that we discuss in the following sections. We describe the biochemistry and cell biology of neurons and glia in Chapter 2.

Neurons and glia are derived from parts of ectoderm that form the neural tube and neural crest. Prenatal development involves hard-wired genetic programs. Postnatal development involves the sculpting of synaptic connections and depends on experience and activity. Chapter 3 focuses on the development of the nervous system.

Specializing the functions

Neurons, also referred to as nerve cells or neuronal cells, are the main signaling cells...