Introduction to the Nervous System

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1 Introduction to the Nervous System

The brain seems bewilderingly complex the first few times you look at it. One way to ease the bewilderment is to have an overview of some vocabulary and organizing principles, which the first three chapters of this book attempt to provide. Chapter 1 is a quick introduction to the parts of the nervous system and the cells that make it up, Chapter 2 is an overview of how the parts get arranged that way during development, and Chapter 3 is a closer look at major parts and the wiring principles underlying their interconnections.

The Nervous System Has Central and Peripheral Parts

The nervous system has both central and peripheral parts, roughly corresponding to the parts inside and outside the skull and vertebral column. The peripheral nervous system (PNS) is approximately the same thing as the collection of nerves that reach pretty much every part of the head and body, collecting sensory information and delivering messages to body parts or to PNS neurons. The central nervous system (CNS) is made up of the brain and the spinal cord. The brain in turn is composed of the cerebrum (forebrain), cerebellum, and brainstem (Fig. 1-1, Table 1-1). The cerebrum, by far the largest component, is itself composed of two cerebral hemispheres and the diencephalon (from a Greek word meaning “in-between-brain,” because it’s interposed between the cerebral hemispheres and the brainstem).

Figure 1-1 Major components of the brain (the spinal cord has been cut off at its junction with the brainstem). A, The left side of a brain; anterior is to the left. B, The right half of a hemisected brain; anterior is to the left. C, A coronal section of a cerebrum, in the plane indicated in B. A, Amygdala; H, hypothalamus; L, lenticular nucleus; M, midbrain; Me, medulla; P, pons; T, thalamus.

Table 1-1 Major divisions of the brain

Major Division	Subdivision	Principal Function
Cerebral hemisphere	Cerebral cortex	Perception, cognition, memory, voluntary movement
	Lenticular nucleus	Part of the basal ganglia: movement control
	Caudate nucleus	Part of the basal ganglia: movement control
	Amygdala	Part of the limbic system: drives and emotions
Diencephalon	Thalamus	Relays information to the cerebral cortex
Diencephalon	Hypothalamus	Controls the autonomic nervous system
Brainstem	Midbrain Pons Medulla	Cranial nerve nuclei, long tracts Cranial nerve nuclei, long tracts Cranial nerve nuclei, long tracts
Cerebellum		Coordination of movement

Each cerebral hemisphere has a covering of cerebral cortex and encloses a series of large nuclei. Some of the enclosed nuclei (lenticular and caudate nuclei) are parts of the basal ganglia, which help control movement; another (the amygdala) is part of the limbic system, which deals with drives and emotions. The cerebral cortex is a critical structure for perception, for the initiation of voluntary movement, and for the functions we think of as distinctively human—things like language and reasoning. Corresponding to these several functions, there are cortical areas primarily concerned with sensation, others with movement, and still others with more complex activities. Because of this parceling of functions, it is possible for cortical damage to impair some abilities while leaving others more or less unaffected.

The diencephalon includes the thalamus, a relay station for information on its way to the cerebral cortex, and the hypothalamus, which controls the autonomic nervous system and many aspects of drive-related behavior. The brainstem is subdivided into the midbrain, pons, and medulla. It contains most of the cranial nerve nuclei, as well as long tracts on their way to or from the cerebrum. The cerebellum is interconnected with many other parts of the CNS and, like the basal ganglia, helps control movement.

The Principal Cellular Elements of the Nervous System Are Neurons and Glial Cells

Except for some extrinsic elements such as blood and blood vessels (see Chapter 6) and meninges (see Chapter 4), the whole nervous system is made up of just two general categories of cells: neurons and glial cells (or glia). Each can be divided into a few subcategories, some characteristic of the CNS and others of the PNS (Table 1-2; see THB6 Fig. 1-27, p. 27).

Table 1-2 Major cell types of the nervous system

Location	Major Neurons	Major Glia
CNS	Motor neurons (→ skeletal muscle via PNS) Preganglionic autonomic neurons (→ autonomic ganglia via PNS) Interneurons local interneurons projection neurons	Astrocytes (metabolic support, response to injury) Oligodendrocytes (myelin) Ependymal cells (line ventricles, secrete CSF) Microglia (response to injury)
PNS	Primary sensory neurons (spinal, cranial nerve, and enteric ganglia) Postganglionic autonomic neurons (sympathetic, parasympathetic, and enteric ganglia)	Schwann cells (myelin, satellite cells)

Neurons Come in a Variety of Sizes and Shapes, but All Are Variations on the Same Theme

Although there are lots of variations, a typical neuron (Fig. 1-2) has a collection of tapering dendrites and a single cylindrical axon, all emerging from the cell body. The cell body is the synthetic center of the whole neuron, the dendrites receive most of the inputs (synapses) from other neurons, the axon conducts electrical impulses (action potentials) away from the cell body and toward other neurons, and axon terminals release neurotransmitter onto other neurons. So the anatomical polarization (dendrites → cell body → axon) corresponds to a functional polarization in terms of the direction in which electrical signals move.

Figure 1-2 Principal components of a typical neuron.

Nearly all neurons fall into one of six categories (Fig. 1-3):

1. Sensory neurons, such as those in dorsal root and cranial nerve ganglia, deliver information to the CNS. The cell bodies live in the PNS, but processes extend through both the PNS and CNS.

2. Motor neurons have cell bodies in the CNS and axons that travel through the PNS to reach skeletal muscle.

3. Preganglionic autonomic neurons have cell bodies in the CNS and axons that travel through the PNS to reach autonomic ganglia.

4. Postganglionic autonomic neurons have cell bodies in the PNS (in autonomic ganglia) and axons that travel through the PNS to reach smooth muscle, cardiac muscle, and glands.

5. Local interneurons are entirely contained in the CNS and have short axons that project to nearby CNS areas.

6. Projection neurons are also contained entirely within the CNS, but they have long axons that project in bundles to distant CNS areas.

Figure 1-3 Categories of neurons in the nervous system, as seen in the spinal cord. Some neurons do not fit comfortably into one of these categories (e.g., rods and cones of the retina), but most do. 1, Sensory neurons, in this case a dorsal root ganglion cell (DRG); 2, motor neurons; 3, preganglionic autonomic neurons; 4, postganglionic autonomic neurons, with cell bodies in autonomic ganglia (AG); 5, local interneurons; 6, projection neurons.

It’s a little arbitrary deciding how long an axon has to be before it qualifies as belonging to a projection neuron, but between them local interneurons and projection neurons account for more than 99% of all our neurons.

Neuronal Cell Bodies and Axons Are Largely Segregated Within the Nervous System

The CNS is mostly separated into areas of gray matter, containing neuronal cell bodies and dendrites, and areas of white matter, containing axons. Most synapses are made onto neuronal dendrites and cell bodies, so gray matter contains the sites of neural information processing and white matter is like telephone cables interconnecting these sites.

A specific area of gray matter is most often referred to as a nucleus (e.g., the trigeminal motor nucleus contains motor neurons for jaw muscles); when it forms a surface covering, it may be referred to as a cortex (e.g., cerebral cortex, cerebellar cortex). Ganglion (“swelling”) usually refers to a group of neuronal cell bodies in the peripheral nervous system, but is also used occasionally to refer to masses of CNS gray matter (e.g., basal ganglia). There is also an assortment of other names based on the appearance or location of an area of gray matter (e.g., thalamus, from a Greek word meaning “inner chamber”).

Specific groups of fibers in areas of white matter are most often called tracts, and usually have two-part names that indicate the origin and termination of the fibers. For example, the corticospinal tract consists of axons that emerge from neurons in the cerebral cortex and terminate in the spinal cord. Several other terms are used to refer to structurally prominent areas of white matter; the most common are fasciculus, lemniscus, and peduncle.

Neuronal Organelles Are Distributed in a Pattern That Supports Neuronal Function

Key Concepts

Neuronal cell bodies synthesize macromolecules.

Dendrites receive synaptic inputs.

Axons convey electrical signals over long distances.

Organelles and macromolecules are transported in both directions along axons.

Synaptic contacts mediate information transfer between neurons.

Neurons deal with the same issues as other cells, using the same organelles (Fig. 1-4). However, some of these issues are accentuated because of the specialized structure and function of neurons:

1. Neurons are electrical signaling machines (see Chapters 7 and 8), so they need to control ionic concentration gradients, pumping in the opposite direction ions that enter or leave as part of an electrical signaling process or just leak across the membrane. This requires a lot of energy and a lot of mitochondria.

2. Many neurons are huge in the longitudinal dimension because of their long axons and extensive dendrites, but they have only one cell body and one nucleus. So the cell body is constantly busy synthesizing enzymes, other proteins, and organelles for the rest of the neuron; this too requires a lot of energy and a lot of mitochondria, as well as a lot of endoplasmic reticulum and ribosomes (appearing as Nissl bodies). Diffusion of things like proteins and organelles along an axon would be much too slow, so there are active mechanisms for transporting them. Soluble substances move a few mm/day by slow axonal transport. Membrane-associated components move by fast axonal transport (about a hundred times faster), using microtubules as “railroad tracks.”

3. Neurons, like other cells, are largely bags of water encased by a microscopically thin membrane, so maintaining their elaborate shapes is a trick. To do this, they construct a cytoskeleton from filamentous proteins—microtubules, neurofilaments (neuronal intermediate filaments), and microfilaments (actin).

4. Neurons convey electrical signals to other neurons by dumping small quantities of chemicals (neurotransmitters) onto them. This requires a specialized, very fast secretory process at synapses.