ABSTRACT
The central nervous system (CNS) consist of roughly 100 billion neurons, and about an order of magnitude more neuroglia. Neurons (nerve cells) and neuroglia (glial cells) are the two types of neural tissue comprising the CNS. Neurons are excitable cells that have the ability to transmit electrical signals without loss, allowing them to communicate with other neurons and cells by chemical and electrical signals at synapses. An average neuron has several thousand synapses (links) with other neurons. A neuron typically consists of a cell body, dendrites, and axon. Generally, the cell body contains organelles similar to other cells. The dendrites are typically short and highly branched processes that receive (via synapses) signals from other nerve cells. Axons are usually long and cylindrical, and typically only branch (forming collaterals) toward the end of their length. Neurons are classi‹ed into several structural categories, the main ones being bipolar, unipolar, and multipolar. Bipolar neurons, located in special sense organs such as the eye, are small with very distinct axon and dendritic processes. Unipolar neurons, where essentially the dendrites and axon are continuous (the cell body being off to one side), are the predominant type of neurons in the peripheral nervous system (PNS). Some unipolar neurons may have axons a meter or so long (such as sensory nerves from toes), as do some axons of multipolar neurons, speci‹cally motor neurons (which control muscles in the toes). On the other hand, some neurons have axons as short as 100 µm. Most of the neurons in the
CONTENTS
15.1 Introduction ........................................................................................................................ 371 15.2 Development of the Central Nervous System ............................................................... 374 15.3 The Spinal Cord (or Medulla Spinalis) ........................................................................... 375 15.4 The Brainstem .................................................................................................................... 378
15.4.1 Medulla (or Medulla Oblongata) ......................................................................... 378 15.4.2 Pons (or Pons Varolii) ............................................................................................380 15.4.3 Midbrain (or Mesencephalon) .............................................................................. 381
15.5 Cerebellum .......................................................................................................................... 382 15.6 Diencephalon ......................................................................................................................384 15.7 Telencephalon .....................................................................................................................386 15.8 Supporting Structures of the Brain ................................................................................. 389 Short Answer/Multiple Choice Questions ............................................................................. 392 Answers to Questions .................................................................................................................402 Bibliography ................................................................................................................................405
