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The Nervous System: Introduction to Cells and Systems


LEARNING OBJECTIVES

Upon completion of this chapter, the student should be able to answer the following questions:

  • 1

    What are the major cell types of the central and peripheral nervous systems?

  • 2

    What are the major components of a neuron, and what are their functional roles?

  • 3

    What are the functional roles of the major glial cell types?

  • 4

    What are the main divisions of the central nervous system?

  • 5

    How and where is the cerebrospinal fluid formed, and how does it circulate and exit the ventricular system?

  • 6

    How is axon transport related to the response of the axon to transection?

The nervous system is a communication and control network that allows an organism to interact rapidly and adaptively with its environment, where environment includes both the external environment (exteroceptive; the world outside the body) and the internal environment (interoceptive; the components and cavities of the body). To carry out its function the nervous system takes in sensory information from a variety of sources using specialized sensors (receptors), integrates this information with previously obtained information stored as memories and with the intrinsic goals and drives of the organism that have been embedded in its nervous system through evolution, decides on a course of action, and then issues commands to the effector organs (muscles and glands) to execute the chosen behavioral response.

Moreover, almost all behavioral responses require the coordination of many body parts. For example, even a simple reaching movement of the arm may require coactivation of axial muscles and possibly muscles in the lower extremity to maintain posture and balance, which themselves may be monitored by up to three different sensory systems (vision, vestibular, and proprioceptive) whose information has to be integrated. Furthermore, movements can alter the internal environment and thus can require compensatory changes in heart and breathing rates, blood vessel diameters, and other internal processes. All these variables are monitored and controlled by various specialized subsystems of the nervous system, all of which must work together for the organism to perform movements and more generally to survive. The succeeding chapters will describe these major subsystems individually; however, it should be remembered that in reality their activity is integrated to generate normal behavior.

To begin, it is useful to divide the nervous system into central and peripheral parts. The central nervous system (CNS) consists of the brain and spinal cord. The peripheral nervous system (PNS) consists of nerves and ganglia (small groups of neurons) that innervate all parts of the body and provide an interface between the environment and the CNS. The transition between the CNS and PNS occurs on the dorsal and ventral rootlets near to where they emerge from the spinal cord and on the cranial nerve fibers near to where they arise from the brain.

Cellular Components of the Nervous System

The nervous system is made up of cells, connective tissue, and blood vessels. The major cell types are neurons (nerve cells) and glia (neuroglia = “nerve glue”). In its most general form a neuron’s function can be defined as generation of signals (to be sent to other neurons or effector cells [e.g., muscle cells]) based on an integration of its own electrical properties with electrochemical signals from other neurons. The points where specific neuron-to-neuron communication occurs are known as synapses, and the process of synaptic transmission is critical to neuronal function (see Chapter 6 ). Neuroglia, or just glia, traditionally have been characterized as supportive cells that sustain neurons both metabolically and physically, isolate individual neurons from each other, and help maintain the internal milieu of the nervous system; however, it is now known that they also have important roles in shaping the flow of activity through the nervous system.

Neurons

The typical neuron consists of three main cellular compartments: a cell body (also referred to as a perikaryon or soma ), a variable number of processes that extend from the soma called dendrites and an axon ( Fig. 4.1 ). A tremendous number of morphological variants of this basic template exist, including cases where dendrites or an axon may be absent ( Fig. 4.2 ). These variations do not occur randomly but rather relate to the distinct functional properties of each neuronal class. Indeed, neurons with similar morphologies often characterize specific regions of the CNS and reflect the distinct neuronal processing performed in each CNS region.

Fig. 4.1, Schematic diagram of an idealized neuron and its major components and connections. A, Afferent input from axons of other cells terminates in synapses on the dendrites and cell body. The initial segment of the axon attaches at the axon hillock. This axon is myelinated, as indicated by the blue structures that encapsulate segments of the axon. The axon terminates on two postsynaptic neurons by forming synaptic terminals. B, Nodes of Ranvier are the gaps between the myelin segments where the axon membrane is exposed to the extracellular space. C, Higher-magnification view of synapse.

Fig. 4.2, A, Purkinje cell. B, Pyramidal cell. C, Golgi cell. D, Granule cell. E, Inferior olive cells. F, Bipolar cells.

The cell body is the main genetic and metabolic center of the neuron. Correspondingly it contains the nucleus and nucleolus of the cell and also possesses a well-developed biosynthetic apparatus for manufacturing membrane constituents, synthetic enzymes, and other chemical substances needed for the specialized functions of nerve cells. The neuronal biosynthetic apparatus includes Nissl bodies, which are stacks of rough endoplasmic reticulum, and a prominent Golgi apparatus. The soma also contains numerous mitochondria and cytoskeletal elements, including neurofilaments and microtubules.

The cell body is also a region in which the neuron receives synaptic input (i.e., electrical and chemical signals from other neurons). Although quantitatively the synaptic input to the soma is usually much less than that to dendrites, it often differs qualitatively from dendritic inputs, and by virtue of the closeness of the soma to the axon, inputs to the soma can override those to the dendrites (see Chapter 6 ).

Dendrites are tapering and branching extensions of the soma and are the main direct recipients of signals from other neurons. They can be thought of as a way to expand and specialize the surface area of a neuron, and indeed, they may account for more than 90% of the surface area available for synaptic contact (soma plus dendrites). Dendrites can be divided into primary dendrites (those that extend directly from the soma) and higher-order dendrites (daughter branches extending from a more proximal branch, in which proximal refers to closeness to the soma). The main cytoplasmic organelles in dendrites are microtubules, neurofilaments, and smooth endoplasmic reticulum; the primary dendrites can also contain Nissl bodies and parts of the Golgi apparatus.

A neuron’s set of dendrites is termed its dendritic tree. Dendritic trees differ tremendously between different types of neurons in terms of the size, number, and spatial organization of the dendrites. A dendritic tree can consist of just a few unbranched dendrites or of many highly ramified dendrites. Individual dendrites can be longer than 1 mm or only 10 to 20 µm in length. Another major morphological variation is whether or not a dendrite has spines, which are small mushroom- or lollipop-shaped protrusions from the main dendrite. Spines are sites specialized for synaptic contact (usually but not always) from excitatory inputs. The shape and size of the dendritic tree, as well as the population and distribution of channels in the dendritic membrane, are all important determinants of how the synaptic input will affect the neuron (see Chapter 6 ).

The axon is an extension of the cell that conveys the output of the cell to other neurons or, in the case of a motor neuron, to muscle cells as well. In general, each neuron has only one axon, and it is usually of uniform diameter. The length and diameter of axons vary with the neuronal type. Some axons do not extend much beyond the length of the dendrites, whereas others may be a meter or more long. Axons may have orthogonal branches en passant, but they often end in a spray of branches called a terminal arborization (represented by the four terminal branches and their synaptic terminals in Fig. 4.1A ). The size, shape, and organization of the terminal arborization determine which other cells it will contact. The first part of the axon is known as the initial segment and arises from the soma (or sometimes from a proximal dendrite) in a specialized region called the axon hillock. The axon differs from the soma and proximal dendrites in that it lacks rough endoplasmic reticulum, free ribosomes, and a Golgi apparatus. The initial segment is usually the site where action potentials (spikes) that are propagated down the axon are initiated (see Chapter 5 ). An axon may terminate in a synapse and/or it may make synapses along its length. Synapses will be described in detail in Chapter 6 .

Neurons are special because of their ability to control and respond to electricity. Moreover, the response and control mechanisms of each part of a neuron are distinct from those in other parts. This intraneuronal specialization is a consequence of the particular morphology and the ion channel composition of each part of the neuron. For example, dendrites have ligand-gated ion channels that allow neurons to respond to chemicals released by other neurons, and their characteristic branching pattern allows for integration of multiple input signals. In contrast the axon typically has a long length and high concentration of voltage-gated channels that allow it to convey electrical signals (action potentials) rapidly over long distances without alteration.

Axonal Transport

Because the soma is the metabolic engine of the neuron, substances needed to support axonal and synaptic function are synthesized there. These substances must be distributed to replenish secreted or inactivated materials along the axon and especially to the presynaptic terminals. Most axons are too long to allow efficient movement of substances from the soma to the synaptic endings by simple diffusion. Thus special axonal transport mechanisms have evolved to accomplish this task ( Fig. 4.3 ). A consequence of this metabolic dependency is that axons degenerate when disconnected from the cell body, a fact that has been used by scientists tracing out neuronal pathways; they would cut an axonal pathway and then determine where the degenerating axons distal to the cut projected to.

Fig. 4.3, Axonal transport. Schematic of neuron and enlargement of axonal transport mechanism. Axonal transport depends on movement of material along transport filaments such as microtubules. Transported components attach to transport filaments by means of cross-bridges. Different objects are transported anterogradely (from cell body to axon terminal) and others retrogradely (toward the cell body). The direction of transport—retrograde and anterograde—is determined by specific proteins such as dynein and kinesin, respectively.

Several types of axonal transport exist. Membrane-bound organelles and mitochondria are transported relatively rapidly by fast axonal transport. Substances that are dissolved in cytoplasm (e.g., proteins) are moved by slow axonal transport. In mammals, fast axonal transport proceeds as rapidly as 400 mm/day, whereas slow axonal transport occurs at about 1 mm/day. Synaptic vesicles, which travel by fast axonal transport, can travel from the soma of a motor neuron in the spinal cord to a neuromuscular junction in a person’s foot in about 2.5 days. In comparison the movement of some soluble proteins over the same distance can take nearly 3 years.

Axonal transport requires metabolic energy and involves calcium ions. Microtubules provide a system of guidewires along which membrane-bound organelles move (see Fig. 4.3 ). Organelles attach to microtubules through a linkage similar to that between the thick and thin filaments of skeletal muscle fibers. Ca ++ triggers movement of the organelles along the microtubules. Special microtubule-associated motor proteins called kinesin and dynein are required for axonal transport.

Axonal transport occurs in both directions. Transport from the soma toward the axonal terminals is called anterograde axonal transport. This process involves kinesin, and it allows replenishment of synaptic vesicles and enzymes responsible for the synthesis of neurotransmitters in synaptic terminals. Transport in the opposite direction, which is driven by dynein, is called retrograde axonal transport. This process returns recycled synaptic vesicle membrane to the soma for lysosomal degradation.

IN THE CLINIC

Certain viruses and toxins can be conveyed by axonal transport along peripheral nerves. For example, herpes zoster, the virus of chickenpox, invades dorsal root ganglion cells. The virus may be harbored by these neurons for many years. However, eventually the virus may become active because of a change in immune status. The virus may then be transported along the sensory axons to the skin, causing shingles, a very painful disease. Another example is the axonal transport of tetanus toxin. Clostridium tetani bacteria may grow in a dirty wound, and if the person had not been vaccinated against tetanus toxin, the toxin can be transported retrogradely in the axons of motor neurons. The toxin can escape into the extracellular space of the spinal cord ventral horn and block the synaptic receptors for inhibitory amino acids. This process can result in tetanic convulsions.

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