Neurons vary in size and complexity. Motor neurons are usually larger than sensory neurons. Nerve cells with long processes (eg, dorsal root ganglion cells) are larger than those with short processes (Figs 2–2 and 2–3).
Schematic illustration of nerve cell types. A: Central nervous system cells: (1) motor neuron projecting to striated muscle, (2) special sensory neuron, and (3) general sensory neuron from skin. B: Autonomic cells to smooth muscle. Notice how the position of the cell body with respect to the axon varies.
Schematic drawing of a motor neuron. The myelin sheath is produced by oligodendrocytes in the central nervous system and by Schwann cells in the peripheral nervous system. Note the three motor end-plates, which transmit the nerve impulses to striated skeletal muscle fibers. Arrows show the direction of the nerve impulse. (Reproduced, with permission, from Junqueira LC, Carneiro J, Kelley RO: Basic Histology: Text & Atlas. 11th ed. McGraw-Hill, 2005.)
Some neurons project from the cerebral cortex to the lower spinal cord, a distance of 4 ft or more in adults; others have very short processes, reaching, for example, only from cell to cell in the cerebral cortex. These small neurons, with short axons that terminate locally, are called interneurons.
Extending from the nerve cell body are usually a number of processes called the axon and dendrites. Most neurons give rise to a single axon (which branches along its course) and to many dendrites (which also divide and subdivide, like the branches of a tree). The receptive part of the neuron is the dendritic zone (see Dendrites section). The conducting (propagating or transmitting) part is the axon, which may have one or more collateral branches. The downstream end of the axon is called the synaptic terminal, or arborization. The neuron's cell body is called the soma, or perikaryon.
The cell body is the metabolic and genetic center of a neuron (see Fig 2–3). Although its size varies greatly in different neuron types, the cell body makes up only a small part of the neuron's total volume.
The cell body and dendrites constitute the receptive pole of the neuron. Synapses from other cells or glial processes tend to cover the surface of a cell body (Fig 2–4).
Electron micrograph of a nerve cell body (CB) surrounded by nerve processes. The neuronal surface is completely covered by either synaptic endings of other neurons (S) or processes of glial cells. Many other processes around this cell are myelinated axons (M). CB, neuronal cell body; N, nucleus, ×5,000. (Used with permission from Dr. D.M. McDonald.)
Dendrites are branches of neurons that extend from the cell body; they receive incoming synaptic information and thus, together with the cell body, provide the receptive pole of the neuron. Most neurons have many dendrites (see Figs 2–2, 2–3, and 2–5). Because most dendrites are long and thin, they act as resistors, isolating electrical events, such as postsynaptic potentials, from one another (see Chapter 3). The branching pattern of the dendrites can be complex and determines how the neuron integrates synaptic inputs from various sources. Some dendrites give rise to dendritic spines, which are small mushroom-shaped projections that act as fine dendritic branches and receive synaptic inputs (Fig 2–5). Dendritic spines are currently of great interest to researchers. The shape of a spine regulates the strength of the synaptic signal that it receives. A synapse onto the tip of a spine with a thin "neck" will have a smaller influence than a synapse onto a spine with a thick neck. Dendritic spines are dynamic, and their shape can change. Changes in dendritic spine shape can strengthen synaptic connections so as to contribute to learning and memory. Maladaptive changes in spines may contribute to altered function of the nervous system after injury.
Dendrite from pyramidal neuron in the motor cortex. Note the spines on the main dendrite and on its smaller branches. Scale = 10 μm. (Micrograph used with permission from Dr. Andrew Tan, Yale University.)
A single axon or nerve fiber 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 via a series of adenosine triphosphate (ATP)-consuming steps along the microtubules.
The axon 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 μm to more than 20 μm.
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, 2–9, 2–10). The myelin sheath is divided into segments about 1 mm long by small gaps (1 μm long) where myelin is absent; these are the nodes of Ranvier. The smallest axons are unmyelinated. As noted in Chapter 3, myelin functions as an insulator. In general, myelination serves to increase the speed of impulse conduction along the axon.
Diagrammatic view, in three dimensions, of a prototypic neuron. Dendrites (1) radiate from the neuronal cell body, which contains the nucleus (3). The axon arises from the cell body at the initial segment (2). Axodendritic (4) and axosomatic (5) synapses are present. Myelin sheaths (6) are present around some axons.
A: In the peripheral nervous system (PNS), unmyelinated axons are located within grooves on the surface of Schwann cells. These axons are not, however, insulated by a myelin sheath. B: Myelinated PNS fibers are surrounded by a myelin sheath that is formed by a spiral wrapping of the axon by a Schwann cell. Panels 1–4 show four consecutive phases of myelin formation in peripheral nerve fibers. (Reproduced, with permission, from Junqueira LC, Carneiro J, Kelley RO: Basic Histology. 11th ed. McGraw-Hill, 2005.)
Electron micrograph of myelinated (M) and unmyelinated (U) axons of a peripheral nerve. Schwann cells (S) may surround one myelinated or several unmyelinated axons. ×16,000. (Used with permission from Dr. D.M. McDonald.)
Oligodendrocytes form myelin in the central nervous system (CNS). A single oligodendrocyte myelinates an entire family of axons (2–50). There is little oligodendrocyte cytoplasm (Cyt) in the oligodendrocyte processes that spiral around the axon to form myelin, and the myelin sheaths are connected to their parent oligodendrocyte cell body by only thin tongues of cytoplasm. This may account, at least in part, for the paucity of remyelination after damage to the myelin in the CNS. The myelin is periodically interrupted at nodes of Ranvier, where the axon (A) is exposed to the extracellular space (ES). (Redrawn and reproduced with permission from Bunge M, Bunge R, Pappas G: Ultrastructural study of remyelination in an experimental lesion in adult cat spinal cord. J Biophys Biochem Cytol May;10:67–94, 1961.)
Electron micrograph showing oligodendrocyte (OL) in the spinal cord, which has myelinated two axons (A1, A2). ×6,600. The inset shows axon A1 and its myelin sheath at higher magnification. The myelin is a spiral of oligodendrocyte membrane that surrounds the axon. Most of the oligodendrocyte cytoplasm is extruded from the myelin. Because the myelin is compact, it has a high electrical resistance and low capacitance so that it can function as an insulator around the axon. ×16,000.
In addition to conducting action potentials, axons transport materials from the cell body to the synaptic terminals (anterograde transport) and from the synaptic terminals to the cell body (retrograde transport). Because ribosomes are not present in the axon, new protein must be synthesized and moved to the axon. This occurs via several types of axonal transport, which differ in terms of the rate and the material transported. Anterograde transport may be fast (up to 400 mm/d) or slow (about 1 mm/d). Retrograde transport is similar to rapid anterograde transport. Fast transport involves microtubules extending through the cytoplasm of the neuron.
An axon can be injured by being cut or severed, crushed, or compressed. After injury to the axon, the neuronal cell body responds by entering a phase called the axon reaction, or chromatolysis. In general, axons within peripheral nerves can regenerate quickly after they are severed, whereas those within the CNS do not tend to regenerate. The axon reaction and axonal regeneration are further discussed in Chapter 22.
Transmission of information between neurons occurs at synapses. Communication between neurons usually occurs from the axon terminal of the transmitting neuron (presynaptic side) to the receptive region of the receiving neuron (postsynaptic side) (Figs 2–6 and 2–11). This specialized interneuronal complex is a synapse, or synaptic junction. As outlined in Table 2–1, some synapses are located between an axon and a dendrite (axodendritic synapses, which tend to be excitatory), or a thorn, or mushroom-shaped dendritic spine which protrudes from the dendrite (Fig 2–12). Other synapses are located between an axon and a nerve cell body (axosomatic synapses, which tend to be inhibitory). Still other synapses are located between an axon terminal and another axon; these axoaxonic synapses modulate transmitter release by the postsynaptic axon. Synaptic transmission permits information from many presynaptic neurons to converge on a single postsynaptic neuron. Some large cell bodies receive several thousand synapses (see Fig 2–4).
Schematic drawing of a synaptic terminal. Vesicles fuse with the presynaptic membrane and release transmitter molecules into the synaptic cleft so that they can bind to receptors in the postsynaptic membrane.
TABLE 2–1Types of Synapses in the CNS. ||Download (.pdf) TABLE 2–1 Types of Synapses in the CNS.
|Type ||Presynaptic Element ||Postsynaptic Element ||Function |
|Axodendritic ||Axon terminal ||Dendrite ||Usually excitatory |
|Axosomatic ||Axon terminal ||Cell body ||Usually inhibitory |
|Axoaxonic ||Axon terminal ||Axon terminal ||Presynaptic inhibition (modulates transmitter release in postsynaptic axon) |
|Dendrodendritic ||Dendrite ||Dendrite ||Local interactions (may be excitatory or inhibitory) in axonless neurons, eg, in retina |
Axodendritic synapses terminate on dendrities or mushroom-shaped "dendritic spines," and tend to be excitatory. Axosomatic synapses terminate on neuronal cell bodies and tend to be inhibitory. Axoaxonal synapses terminate on an axon, often close to synaptic terminals, and modulate the release of neurotransmitters. (Reproduced, with permission, from Ganong WF: Review of Medical Physiology. 22nd ed. McGraw-Hill, 2005.)
Impulse transmission at most synaptic sites involves the release of a chemical transmitter substance (see Chapter 3); at other sites, current passes directly from cell to cell through specialized junctions called electrical synapses, or gap junctions. Electrical synapses are most common in invertebrate nervous systems, although they are found in a small number of sites in the mammalian CNS. Chemical synapses have several distinctive characteristics: synaptic vesicles on the presynaptic side, a synaptic cleft, and a dense thickening of the cell membrane on both the receiving cell and the presynaptic side (see Fig 2–11). Synaptic vesicles contain neurotransmitters, and each vesicle contains a small packet, or quanta, of transmitter. When the synaptic terminal is depolarized (by an action potential in its parent axon), there is an influx of calcium. This calcium influx leads to phosphorylation of a class of proteins called synapsins. After phosphorylation of synapsins, synaptic vesicles dock at the presynaptic membrane facing the synaptic cleft, fuse with it, and release their transmitter (see Chapter 3).
Synapses are very diverse in their shapes and other properties. Some are inhibitory and some excitatory; in some, the transmitter is acetylcholine; in others, it is a catecholamine, amino acid, or other substance (see Chapter 3). Some synaptic vesicles are large, some small; some have a dense core, whereas others do not. Flat synaptic vesicles appear to contain an inhibitory mediator; dense-core vesicles contain catecholamines.
In addition to calcium-dependent, vesicular neurotransmitter release, there is also a second, nonvesicular mode of neurotransmitter release that is not calcium-dependent. This mode of release depends on transporter molecules, which usually serve to take up transmitter from the synaptic cleft.