The nervous system consists of organs composed primarily of nervous tissue supported and protected by connective tissues.
There are two types of cells that compose nervous tissue: neurons and neuroglia.
Neurons, or nerve cells, are the structural and functional units of the nervous system. They are delicate cells that are specialized to generate and transmit nerve impulses. Neurons may vary in size and shape but they have many common features.
The cell body is the portion of a neuron that contains the large, spherical nucleus. The cell body also contains the usual cytoplasmic organelles. Two types of neuronal processes extend from the cell body: dendrites and axons. A neuron may have many dendrites but it has only one axon.
Dendrites are usually short, highly- branched, tapering processes that receive impulses (electrochemical signals) from other neurons and sensory receptors. Dendrites carry impulses toward the cell body or axon.
An axon, or nerve fiber, is a long, thin process of a neuron. It may have one or more side branches, called axon collaterals. It also forms a number of short, fine branches, the terminal arborization, at its distal tip. The slightly enlarged tips of the terminal arborization are the terminal boutons, which form junctions (synapses) with other neurons, muscles, adipose tissue, or glands. An axon carries nerve impulses away from the cell body or dendrites.
Some axons are enclosed in an insulating myelin sheath formed by special neuroglia. Such axons are referred to as myelinated axons. The myelin sheath increases the speed of nerve impulse transmission. The tiny spaces between adjacent myelin-forming cells, where the axon is exposed, are known as myelin sheath gaps (or nodes of Ranvier). Axons lacking a myelin sheath are referred to as unmyelinated axons and have a much slower speed of nerve impulse transmission.
Types of Neurons
Neurons may be classified according to their anatomy or their function. Structurally, there are three basic types of neurons: multipolar, bipolar, and unipolar neurons (figure 8.4).
Multipolar neurons have several dendrites and a single axon extending from the cell body. Most of the neurons whose cell bodies are located in the brain and spinal cord are multipolar neurons.
Bipolar neurons have only two processes: a dendrite and an axon extending from opposite ends of the cell body. Bipolar neurons occur in the sensory portions of the eyes, ears, and nose.
Unipolar neurons have a single process extending from the cell body. This process quickly divides into two branches extending in opposite directions, with both branches functioning as a single axon. One end of the axon ends in a terminal arborization, while the other ends in dendrites. Unipolar neurons carry nerve impulses from sensory receptors to the CNS. Clusters of cell bodies of unipolar neurons often form ganglia (singular, ganglion), which are located in the PNS.
Functionally, there are three basic types of neurons: sensory neurons, interneurons, and motor neurons.
Sensory neurons carry nerve impulses from the peripheral parts of the body to the CNS. Their dendrites are associated with sensory receptors or are specialized to detect changes directly. Nerve impulses are carried over an axon within cranial or spinal nerves to the CNS. Cell bodies of sensory neurons are located external to the CNS in ganglia. Structurally, most sensory neurons are unipolar neurons, although bipolar neurons are found in special sense organs.
Interneurons are located entirely within the CNS and synapse with other neurons. They are responsible for the processing and interpretation of nerve impulses by the CNS. Interneurons receive nerve impulses from sensory neurons and transmit them from place to place within the CNS. They also activate motor neurons, which results in a stimulation of effectors. Interneurons are multipolar neurons.
Motor neurons carry nerve impulses from the CNS to effectors to produce an action. Their cell bodies and dendrites are located within the CNS, while their axons are located in cranial and spinal nerves. Motor neurons are multipolar neurons (table 8.1).
The neuroglia (nu-rog’-le-ah) provide support and protection for neurons. One type of neuroglia-Schwann cells-occurs in the PNS. Four types of neuroglia occur in the CNS, where they are even more numerous than neurons (figures 8.5 and 8.6).
Schwann cells form the myelin sheath around PNS myelinated axons. They wrap tightly around an axon many times so that the nucleus and most of the cytoplasm become squeezed into the superificial layer. The deep layers, formed by layers of plasma membrane, constitute the myelin sheath. The most superficial layer forms the neurilemma, which is essential for axon regeneration after injury.
Oligodendrocytes (ol-i-go-den’-dro-sitz) form the myelin sheath of myelinated axons within the CNS but they do not form a neurilemma. Lack of a neurilemma is one factor that contributes to the inability of axons within the brain and spinal cord to regenerate after injury.
Astrocytes (as ‘-tro-sitz) are the primary supporting cells for neurons in the CNS. They stimulate the growth of neurons and influence synaptic transmission. Astrocytes also join with the epithelium of blood vessels to form the blood-brain barrier, which protects neurons by tightly regulating the exchange of materials between the blood and neurons.
Microglial cells are scattered throughout the CNS, where they keep the tissues clean by engulfing and digesting cellular debris and pathogens.