Structure of a Neuron

There are several varieties of neurons, as we shall see, but a good starting point for discussing neuronal stucture is a motor neuron of the spinal cord (fig. 12.4). The control center of the neuron is its soma,7 also called the cell body or perikaryon8 (PERR-ih-CARE -ee-on). It has a single, centrally located nucleus with a large nucleolus. The cytoplasm contains mitochondria, lysosomes, a Golgi complex, numerous inclusions, and an extensive rough endoplasmic reticulum and cytoskeleton. The cytoskeleton consists of a dense mesh of microtubules and neurofibrils (bundles of actin filaments) that compartmentalize the rough ER into dark-staining regions called Nissl9 bodies (fig. 12.4c, d). Nissl bodies are unique to neurons and a helpful clue to identifying them in tissue sections with mixed cell types. Mature neurons lack centrioles and apparently undergo no further mitosis after adolescence, but they are unusually long-lived cells, capable of functioning for over a hundred years. Even into old age, however, there are unspecialized stem cells in the CNS that can divide and develop into new neurons (see insight 4.3, p. 143).

The major cytoplasmic inclusions in a neuron are glycogen granules, lipid droplets, melanin, and a golden brown pigment called lipofuscin10 (LIP-oh-FEW-sin)—an end product of lysosomal digestion of worn-out organelles and other products. Lipofuscin collects with age and pushes the nucleus to one side of the cell. Lipofuscin granules are also called "wear-and-tear granules" because they are most abundant in old neurons, but they are apparently harmless.

The soma of a neuron usually gives rise to a few thick processes that branch into a vast number of dendrites11— named for their striking resemblance to the bare branches of a tree in winter. The dendrites are the primary site for receiving signals from other neurons. Some neurons have only one dendrite and some have thousands. The more dendrites a neuron has, the more information it can receive from other cells and incorporate into its decision

Digestion Sensory Neuron
Figure 12.3 Functional Classes of Neurons. Sensory (afferent) neurons carry signals to the central nervous system (CNS); interneurons are contained entirely within the CNS and carry signals from one neuron to another; and motor (efferent) neurons carry signals from the CNS to muscles and glands.

9Franz Nissl (1860-1919), German neuropathologist

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Chapter 12 Nervous Tissue 447

Nervous Tissue Motor Neurone

Figure 1 2.4 A Representative Neuron. The Schwann cells and myelin sheath are explained later in this chapter. (a) A multipolar neuron such as a spinal motor neuron. (b) Detail of myelin sheath. (c) Neurofibrils of the soma. (d) Nissl bodies, stained masses of rough ER separated by bundles of neurofibrils.

Figure 1 2.4 A Representative Neuron. The Schwann cells and myelin sheath are explained later in this chapter. (a) A multipolar neuron such as a spinal motor neuron. (b) Detail of myelin sheath. (c) Neurofibrils of the soma. (d) Nissl bodies, stained masses of rough ER separated by bundles of neurofibrils.

Saladin: Anatomy & I 12. Nervous Tissue I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Variation Neural Structure Image

Figure 12.5 Variation in Neuronal Structure. Top row, left to right: Two multipolar neurons of the brain—a pyramidal cell and Purkinje cell. Second row, left to right: Two bipolar neurons—a bipolar cell of the retina and an olfactory neuron. Third row: A unipolar neuron of the type involved in the senses of touch and pain. Bottom row: An anaxonic neuron (amacrine cell) of the retina.

448 Part Three Integration and Control making. As tangled as the dendrites may seem, they provide exquisitely precise pathways for the reception and processing of neural information.

On one side of the soma is a mound called the axon hillock, from which the axon (nerve fiber) originates. The axon is cylindrical and relatively unbranched for most of its length, although it may give rise to a few branches called axon collaterals along the way, and most axons branch extensively at their distal end. An axon is specialized for rapid conduction of nerve signals to points remote from the soma. Its cytoplasm is called the axoplasm and its membrane the axolemma.12 A neuron never has more than one axon, and some neurons in the retina and brain have none.

Somas range from 5 to 135 ^m in diameter, while axons range from 1 to 20 ^m in diameter and from a few millimeters to more than a meter long. Such dimensions are more impressive when we scale them up to the size of familiar objects. If the soma of a spinal motor neuron were the size of a tennis ball, its dendrites would form a huge bushy mass that could fill a 30-seat classroom from floor to ceiling. Its axon would be up to a mile long but a little narrower than a garden hose. This is quite a point to ponder. The neuron must assemble molecules and organelles in its "tennis ball" soma and deliver them through its "mile-long garden hose" to the end of the axon. How it achieves this remarkable feat is explained shortly.

At the distal end, axons usually have a terminal arborization13—an extensive complex of fine branches. Each branch ends in a synaptic knob (terminal button). As studied in the previous chapter, the synaptic knob is a little swelling that forms a junction (synapse14) with a muscle cell, gland cell, or another neuron. It contains synaptic vesicles full of neurotransmitter.

Not all neurons fit the preceding description. Neurons are classified structurally according to the number of processes extending from the soma (fig. 12.5):

• Multipolar neurons are those, like the preceding, that have one axon and multiple dendrites. This is the most common type of neuron and includes most neurons of the brain and spinal cord.

• Bipolar neurons have one axon and one dendrite. Examples include olfactory cells of the nasal cavity, some neurons of the retina, and sensory neurons of the inner ear.

• Unipolar neurons have only a single process leading away from the soma. They are represented by the neurons that carry sensory signals to the spinal cord. These neurons are also called pseudounipolar because they start out as bipolar neurons in the embryo, but their two processes fuse into one as the neuron

,3arbor = tree

Figure 12.5 Variation in Neuronal Structure. Top row, left to right: Two multipolar neurons of the brain—a pyramidal cell and Purkinje cell. Second row, left to right: Two bipolar neurons—a bipolar cell of the retina and an olfactory neuron. Third row: A unipolar neuron of the type involved in the senses of touch and pain. Bottom row: An anaxonic neuron (amacrine cell) of the retina.

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Chapter 12 Nervous Tissue 449

matures. A short distance away from the soma, the process branches like a T, with a peripheral fiber carrying signals from the source of sensation and a central fiber continuing into the spinal cord. In most other neurons, a dendrite carries signals toward a soma and an axon carries them away. In unipolar neurons, however, there is one long fiber that bypasses the soma and carries nerve signals directly to the spinal cord. The dendrites are the branching receptive endings in the skin or other place of origin, while the rest of the fiber is considered to be the axon (defined in these neurons by the presence of myelin and the ability to generate action potentials—two concepts explained later in this chapter).

• Anaxonic neurons have multiple dendrites but no axon. They communicate through their dendrites and do not produce action potentials. Some anaxonic neurons are found in the brain and retina. In the retina, they help in visual processes such as the perception of contrast.

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