6 - The nervous system

Summary

Introduction

Marsupials come in many shapes and sizes (Strahan 1995) and so do their brains. Some of the extinct forms such as Diprotodon were as large as small elephants while others such as the living planigales are among the smallest mammals. The smallest marsupials though are still over twice the weight of the smallest eutherians – a bat and a shrew both of which as adults weigh 2 g. The nervous systems of marsupials all have the same basic mammalian pattern (Johnson 1977). Large or small, that most complex of all computers, the mammalian central nervous system, is wired up to the rest of the body via the cranial, peripheral and autonomic nerves.

The central nervous system (CNS) is composed of the brain and spinal cord. The peripheral nervous system (PNS) carries information into and out of the brain or spinal cord. The PNS includes some of the cranial (head) nerves, the spinal nerves and the various ganglia in which the cell bodies of many of these peripheral nerves are located.

Cells of the nervous system

The nervous system is composed of two major types of cells, the neurons or nerve cells and the glial cells. Neurons are made up of a nerve cell body and its processes. In the CNS the neurons have short branching processes or dendrites which carry bioelectrical impulses to the cell body, and long processes called axons which carry bioelectrical impulses from the cell body. There are about 10,000 different types of neurons.