nervous system Flashcards
central nervous system (CNS)
The central nervous system (CNS) consists of the brain and spinal cord, which occupy the dorsal body cavity and act as the integrating and command centers of the nervous system. They interpret incoming sensory information and issue instructions based on past experience and current conditions.
The peripheral
pĕ-rif′er-al) nervous system (PNS) includes all parts of the nervous system outside the CNS. It consists mainly of the nerves that extend from the spinal cord and brain. Spinal nerves carry impulses to and from the spinal cord. Cranial (kra′ne-al) nerves carry impulses to and from the brain. These nerves serve as communication lines. They link all parts of the body by carrying impulses from the sensory receptors to the CNS and from the CNS to the appropriate glands or muscles.
The sensory division, or afferent (af′er-ent; literally “to go toward”) division
consists of nerves (composed of many individual nerve fibers) that convey impulses to the central nervous system from sensory receptors located in various parts of the body. The sensory division keeps the CNS constantly informed of events going on both inside and outside the body. Sensory fibers delivering impulses from the skin, skeletal muscles, and joints are called somatic (soma = body) sensory (afferent) fibers, whereas those transmitting impulses from the visceral organs are called visceral sensory (afferent) fibers.
The motor division, or efferent (ef′er-rent) division (think: efferent exits the CNS)
(think: efferent exits the CNS), carries impulses from the CNS to effector organs, the muscles and glands. These impulses activate muscles and glands; that is, they effect (bring about or cause) a motor response.
The motor division in turn has two subdivisions
somatic an automatic
sonatic and automatic
The somatic (so-mat′ik) nervous system allows us to consciously (voluntarily), control our skeletal muscles. Hence, we often refer to this subdivision as the voluntary nervous system. However, not all skeletal muscle activity controlled by this motor division is voluntary. Skeletal muscle reflexes, such as the stretch reflex (described later in the chapter), are involuntary.
The autonomic (aw″to-nom′ik) nervous system (ANS) regulates events that are involuntary (no conscious control), such as the activity of smooth muscle, cardiac muscle, and glands. This subdivision, commonly called the involuntary nervous system, itself has two parts, the sympathetic and parasympathetic, which typically bring about opposite effects. What one stimulates, the other inhibits. We will describe these later.
Supporting cells in the CNS are “lumped together” as neuroglia (nu-rog′le-ah), literally, “nerve glue,” also called glial cells or glia. Neuroglia include many types of cells that support, insulate, and protect the delicate neurons (Figure 7.3). In addition, each of the different types of neuroglia has special functions. CNS neuroglia include the following:
Astrocytes: abundant star-shaped cells that account for nearly half of neural tissue (Figure 7.3a). Their numerous projections have swollen ends that cling to neurons, bracing them and anchoring them to their nutrient supply lines, the blood capillaries. Astrocytes form a living barrier between capillaries and neurons, help determine capillary permeability, and play a role in making exchanges between the two. In this way, they help protect the neurons from harmful substances that might be in the blood. Astrocytes also help control the chemical environment in the brain by “mopping up” leaked potassium ions, which are involved in generating a nerve impulse, and recapturing chemicals released for communication purposes.
Microglia (mi-kro′-gle-ah): spiderlike phagocytes that monitor the health of nearby neurons and dispose of debris, such as dead brain cells and bacteria (Figure 7.3b).
Ependymal (ĕ-pen′dĭ-mal) cells: neuroglia that line the central cavities of the brain and the spinal cord (Figure 7.3c). They participate in the production of cerebrospinal fluid (CSF) and the beating of their cilia helps to circulate the cerebrospinal fluid that fills those cavities and forms a protective watery cushion around the CNS.
Oligodendrocytes (ol″ĭ-go-den′dro-sĭtz): neuroglia that wrap their flat extensions (processes) tightly around CNS nerve fibers, producing fatty insulating coverings called myelin sheaths (Figure 7.3d).
Supporting cells in the PNS come in two major varieties
Schwann cells and satellite cells (Figure 7.3e). Schwann cells form the myelin sheaths around nerve fibers in the PNS. Satellite cells act as protective, cushioning cells for peripheral neuron cell bodies.
Did You Get It?
Neurons, also called nerve cells
are highly specialized to transmit messages (nerve impulses) from one part of the body to another. Although neurons differ structurally from one another, they have many common features (Figure 7.4). All have a cell body, which contains the nucleus and one or more slender processes extending from the cell body.
The cell body
is the metabolic center of the neuron. Its transparent nucleus contains a large nucleolus. The cytoplasm surrounding the nucleus contains the usual organelles, except that it lacks centrioles (which confirms the amitotic nature of most neurons). The rough ER, called Nissl (nis′l) bodies, and neurofibrils (intermediate filaments that are important in maintaining cell shape) are particularly abundant in the cell body.
Processes
The armlike processes, or fibers, vary in length from microscopic to over 3 feet long, reaching from the lumbar region of the spine to the great toe. Neuron processes that convey incoming messages (electrical signals) toward the cell body are dendrites (den′drītz), whereas those that generate nerve impulses and typically conduct them away from the cell body are axons (ak′sonz). Neurons may have hundreds of branching dendrites (dendr = tree), depending on the structural type. However, each neuron has only one axon, which arises from a conelike region of the cell body called the axon hillock.
axon terminals
An occasional axon gives off a collateral branch along its length, but all axons branch profusely at their terminal end, forming hundreds to thousands of axon terminals. These terminals contain hundreds of tiny vesicles, or membranous sacs, that contain chemicals called neurotransmitters (review our discussion of events at the neuromuscular junction in Chapter 6). As we said, axons transmit nerve impulses away from the cell body. When these impulses reach the axon terminals, they stimulate the release of neurotransmitters into the extracellular space between neurons, or between a neuron and its target cell.
synaptic (sĭ-nap′tik) cleft.
Each axon terminal is separated from the next neuron or its target by a tiny gap called the synaptic (sĭ-nap′tik) cleft. Such a functional junction, where an impulse is transmitted from one neuron to another, is called a synapse (syn = to clasp or join). Although they are close, neurons never actually touch other neurons or target cells. You will learn more about synapses and the events that occur there a bit later.
Myelin Sheaths
Most long nerve fibers are covered with a whitish, fatty material called myelin (mi′ĕ-lin), which has a waxy appearance. Myelin protects and insulates the fibers and increases the speed of nerve impulse transmission. Compare myelin sheaths to the many layers of insulation that cover the wires in an electrical cord; the layers keep the electricity flowing along the desired path just as myelin does for nerve fibers. Axons outside the CNS are myelinated by Schwann cells, as previously noted. Many of these cells wrap themselves around the axon in a jelly-roll fashion (Figure 7.5). Initially, the membrane coil is loose, but the Schwann cell cytoplasm is gradually squeezed from between the membrane layers. When the wrapping process is done, a tight coil of wrapped membranes, the myelin sheath, encloses the axon. Most of the Schwann cell cytoplasm ends up just beneath the outermost part of its plasma membrane. This part of the Schwann cell, external to the myelin sheath, is called the neurilemma (nu″rĭ-lem′mah, “neuron husk”). Because the myelin sheath is formed by many individual Schwann cells, it has gaps, or indentations, called nodes of Ranvier (rahn-vĕr), at regular intervals (see Figure 7.4).
Clusters of neuron cell bodies and collections of nerve fibers are named differently in the CNS and in the PNS.
For the most part, cell bodies are found in the CNS in clusters called nuclei. This well-protected location within the bony skull or vertebral column is essential to the well-being of the nervous system—remember that neurons do not routinely undergo cell division after birth. The cell body carries out most of the metabolic functions of a neuron, so if it is damaged, the cell dies and is not replaced. Small collections of cell bodies called ganglia (gang′le-ah; ganglion, singular) are found in a few sites outside the CNS in the PNS.
Bundles of nerve fibers (neuron processes) running through the CNS are called tracts, whereas in the PNS they are called nerves.
The terms white matter and gray matter refer respectively to myelinated versus unmyelinated regions of the CNS. As a general rule, the white matter consists of dense collections of myelinated fibers (tracts), and gray matter contains mostly unmyelinated fibers and cell bodies.
