Porth's Essentials of Pathophysiology, 4e

822

Nervous System

U N I T 1 0

Dendrite

Nucleolus

Axon hillock Initial segment

Nucleus

Nissl bodies

Free nerve endings in skin

Oligodendroglial cell myelin

Schwann cell myelin

Axon

CNS PNS

Myelin sheath Nucleus of Schwann cell

Node of Ranvier

Afferent cell body in dorsal root ganglion

Nucleus Nucleolus Nissl bodies

Schwann cell myelin

PNS CNS

FIGURE 34-1. Afferent (A) and efferent neurons (B) , showing the soma or cell body, dendrites, and axon. Arrows indicate the direction for conduction of action potentials. CNS, central nervous system; PNS, peripheral nervous system.

Oligodendroglial cell myelin

Synaptic terminals

Neuromuscular junction

A

B

Muscle fiber

Functionally, the nervous system is divided into two systems: the somatic and autonomic nervous systems. The somatic (Greek soma, “body”) nervous system provides sensory and motor innervation for all parts of the CNS and PNS except viscera, smooth muscle, and glands. The autonomic nervous system (ANS) provides efferent innervation to smooth muscle in blood vessels and visceral structures, the conduction system of the heart, the sweat glands, and the exocrine glands of the gastrointestinal tract. Nervous tissue consists of two principal types of cells—neurons and neuroglia or supporting cells. The neurons are the functional cells of the nervous system. They exhibit membrane excitability and conductivity and secrete neurotransmitters (signal-transmitting chem- icals) and hormones, such as epinephrine and antidi- uretic hormone (ADH). The neuroglial cells, such as the Schwann cells in the PNS and the oligodendrocytes in the CNS, protect the nervous system and provide meta- bolic support for the neurons. Neurons A typical neuron has three distinct parts: a cell body, dendrites, and an axon. The axonal and dendritic pro- cesses form the functional connections, or synapses, with other nerve cells, receptor cells, or effector cells. Afferent, or sensory, neurons of the PNS transmit infor- mation to the CNS (see Fig. 34-1A), whereas efferent, or motor, neurons carry information away from the CNS (see Fig. 34-1B). Interspersed between the afferent and

efferent neurons is a network of interconnecting neurons (also called interneurons or internuncial neurons) that modulate and control the body’s response to changes in the internal and external environments. The cell body of a neuron, also known as the soma, contains a large vesicular nucleus with one or more dis- tinct nucleoli and a well-developed rough endoplasmic reticulum. A neuron’s nucleus has the same deoxyri- bonucleic acid (DNA) and genetic information that is present in other cells of the body, and its nucleolus pro- duces the ribonucleic acid (RNA) associated with pro- tein synthesis. The cytoplasm contains large masses of ribosomes that are prominent in most neurons. These RNA masses, which are involved in protein synthesis, stain as dark Nissl bodies (see Fig. 34-1B). The Nissl bodies and free ribosomes extend into the dendrites, but not into the axon. The area of the cell body, called the axon hillock, is free of large cytoplasmic organelles and serves as a landmark to distinguish between axons and dendrites in microscopic preparations. The dendrites (from the Greek dendron, “tree”) are multiple, branched extensions of the nerve cell body; they conduct information toward the cell body and are the main source of information for the neuron. The den- drites and cell body are studded with synaptic terminals that communicate with axons and dendrites of other neurons (see Fig. 34-1A). Axons are long efferent processes that project from the cell body and carry impulses away from the cell. Most neurons have only one axon; however, axons may exhibit multiple branching that results in many axonal terminals. The cytoplasm of the cell body extends to fill