Keywords
central nervous system, peripheral nervous system, neurons, glial cells, gray and white matter, demyelinating diseases
Chapter Outline
The Nervous System Has Central and Peripheral Parts, 1
The Principal Cellular Elements of the Nervous System Are Neurons and Glial Cells, 1
Neurons Come in a Variety of Sizes and Shapes, but All Are Variations on the Same Theme, 1
Neuronal Cell Bodies and Axons Are Largely Segregated Within the Nervous System, 3
Neuronal Organelles Are Distributed in a Pattern That Supports Neuronal Function, 4
Schwann Cells Are the Principal PNS Glial Cells, 4
CNS Glial Cells Include Oligodendrocytes, Astrocytes, Ependymal Cells, and Microglial Cells, 5
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 become arranged during development, and Chapter 3 is a closer look at major parts and the wiring principles underlying their interconnections. Throughout the text you will have brief introductions to pathology and pharmacology.
The Nervous System Has Central and Peripheral Parts
The nervous system has central parts that are mainly found within the skull and vertebral column and peripheral parts that are mainly found outside the skull and vertebrae. The peripheral nervous system (PNS) is composed of an assortment of nerves that reach pretty much every part of the head and body, collecting sensory information and delivering messages to body parts or to peripheral neurons found in the gastrointestinal tract (i.e., enteric nervous system). 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 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).
| Major Division | Subdivision | Principal Function |
|---|---|---|
| Cerebral hemisphere | Cerebral cortex | Perception, cognition, memory, voluntary movement |
| Lenticular nucleus (globus pallidus and putamen) | Part of the basal ganglia: initiation of movement and thought | |
| Caudate nucleus | Part of the basal ganglia: initiation of thought and movement | |
| Amygdala | Part of the limbic system: drives and emotions | |
| Diencephalon | Thalamus | Relays information to the cerebral cortex |
| 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/adjust 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 ).
| Location | Major Neurons | Major Glia |
|---|---|---|
| CNS | Motor neurons (→ skeletal muscle via PNS) Preganglionic autonomic neurons (→ autonomic ganglia via PNS) Interneurons
| 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. The cell body and dendrites can be seen using a Nissl stain (identifies the rough endoplasmic reticulum [RER]) but does not stain the axon, since they do not contain RER.
Nearly all neurons fall into one of seven 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.
Enteric neurons have cell bodies within the walls of the gastrointestinal tract and axons that travel throughout the smooth muscle layers. The enteric nervous system renders many connections with the autonomic nervous system.
- 6.
Local interneurons are entirely contained in the CNS and have short axons that project to nearby CNS areas.
- 7.
Projection neurons are also contained entirely within the CNS, but they have long axons that project in bundles to distant CNS areas.
