Maps of the World Within the Brain

A single axon arises from most neurons. The axon is a cylindrical tube of cytoplasm covered by a membrane, the axolemma. A cytoskeleton consisting of neurofilaments and microtubules runs through the axon. The microtubules provide a framework for fast axonal transport (see Axonal Transport section). Specialized molecular motors (kinesin molecules) bind to vesicles containing molecules (eg, neurotransmitters) destined for transport and "walk" via a series of adenosine triphosphate (ATP)-consuming steps along the microtubules The axon is a specialized structure that conducts electrical signals from the initial segment (the proximal part of the axon, near the cell body) to the synaptic terminals. The initial segment has distinctive morphological features; it differs from both cell body and axon. The axolemma of the initial segment contains a high density of sodium channels, which permit the initial segment to act as a trigger zone. In this zone, action potentials are generated so that they can travel along the axon, finally invading the terminal axonal branches and triggering synaptic activity, which impinges on other neurons. The initial segment does not contain Nissl substance (see Fig 2-3). In large neurons, the initial segment arises conspicuously from the axon hillock, a cone-shaped portion of the cell body. Axons range in length from a few microns (in interneurons) to well over a meter (ie, in a lumbar motor neuron that projects from the spinal cord to the muscles of the foot) and in diameter from 0.1 m to more than 20 m. Many axons are covered by myelin. The myelin consists of multiple concentric layers of lipid-rich membrane produced by Schwann cells in the peripheral nervous system (PNS) and by oligodendrocytes (a type of glial cell) in the central nervous system (CNS) (Figs 2-6, 2-7, 2-8, and 2-9). The myelin sheath is divided into segments about 1 mm long by small gaps (1 m long) where myelin is absent; these are the nodes of Ranvier. The smallest axons are unmyelinated. As noted in Chapter 3: Signaling in the Nervous System, myelin functions as an insulator. In general, myelination serves to increase the speed of impulse conduction along the axon.

clinical examination