Figure 1-1. A, General relationships of central (CNS), peripheral (PNS), and visceromotor nervous systems. Visceromotor regions of CNS and PNS are shown in red. B, A representation of the thoracic spinal cord in a clinical orientation showing the relationships of efferent (outgoing, motor) and afferent (incoming, sensory) fibers to spinal nerves and roots. Motor fibers are visceral efferent (visceromotor; red) and somatic efferent (green); sensory fibers are somatic afferent (blue) and visceral afferent (black).
The visceromotor nervous system (also called visceral motor) is a functional division of the nervous system that has parts in both the CNS and the PNS (Fig. 1-1). It is made up of neurons that innervate smooth muscle, cardiac muscle, or glandular epithelium or combinations of these tissues. These individual visceral tissues, when combined, make up visceral organs such as the stomach. The visceromotor nervous system is also called the autonomic nervous system because it regulates motor responses outside the realm of conscious control.
Neurons
At the histologic level, the nervous system is composed of neurons and glial cells. As the basic structural and functional units of the nervous system, neurons are specialized to receive information, to transmit electrical impulses, and to influence other neurons or effector tissues. In many areas of the nervous system, neurons are structurally modified to serve particular functions. At this point, we consider the neuron only as a general concept (see Chapter 2).
A neuron consists of a cell body (perikaryon or soma) and the processes that emanate from the cell body (Fig. 1-2). Collectively, neuronal cell bodies constitute the gray matter of the CNS. Named and usually function-specific clusters of cell bodies in the CNS are called nuclei (singular, nucleus). Typically, dendrites are those processes that ramify in the vicinity of the cell body, whereas a single, longer process called the axon carries impulses to a more remote destination. The white matter of the CNS consists of bundles of axons that are wrapped in a sheath of insulating lipoprotein called myelin.
Figure 1-2. A representative neuron and synapse, a simple (monosynaptic) reflex, and a pathway.
In general, there is a direct relationship between (1) the diameter of the axon, (2) the thickness of the myelin sheath, (3) the distance between the nodes of the myelin sheath (nodes of Ranvier), and (4) the conduction velocity of the nerve fiber. Axons with a large diameter have thick myelin sheaths with longer internodal distances and therefore exhibit faster conduction velocities. Likewise, axons with a thin diameter that have thin myelin sheaths with shorter internodal distances have slower conduction velocities. The axon terminates at specialized structures called synapses or, if they innervate muscles, motor end plates (neuromuscular junctions), which function much like synapses.
The generalized synapse (Fig. 1-2) is the most common type seen in the CNS and is sometimes called an electrochemical synapse. It consists of a presynaptic element, which is part of an axon; a gap called the synaptic cleft; and the postsynaptic region of the innervated neuron or effector structure. Communication across this synapse is accomplished as follows. An electrical impulse (the action potential) causes the release of a neuroactive substance (a neurotransmitter, neuromodulator, or neuromediator) from the presynaptic element into the synaptic cleft. This substance is stored in synaptic vesicles in the presynaptic element and is released into the synaptic space by the fusion of these vesicles with the cell membrane (Fig. 1-2).
The neurotransmitter diffuses rapidly across the synaptic space and binds to receptor sites on the postsynaptic membrane. On the basis of the action of the neurotransmitter at receptor sites, the postsynaptic neuron may be excited (lead to generation of an action potential) or inhibited (prevent generation of an action potential). Neurotransmitter residues in the synaptic cleft are rapidly inactivated by other chemicals found in this space. In this brief example, we see that (1) the neuron is structurally specialized to receive and propagate electrical signals, (2) this propagation is accomplished by a combination of electrical and chemical events, and (3) the transmission of signals across the synapse is in one direction (unidirectional), that is, from the presynaptic neuron to the postsynaptic neuron. There are a number of neurologic disorders that represent a failure of neurotransmitter action at the synapse or at the receptors on the postsynaptic membrane.
Reflexes and Pathways
The function of the nervous system is based on the interactions between neurons. Figure 1-2 illustrates one of the simplest types of neuronal circuits, a reflex arc composed of only two neurons. This is called a monosynaptic reflex arc because only one synapse is involved. In this example, the peripheral end of a sensory fiber responds to a particular type of input. The resulting action potential is conducted by the sensory fiber into the spinal cord, where it influences a motor neuron. The axon of the motor neuron conducts a signal from the spinal cord to the appropriate skeletal muscle, which responds by contracting. This is an example of a muscle stretch reflex, which is actually one of the more commonly tested reflexes in clinical medicine. Reflexes are involuntary responses to a particular bit of sensory input. For example, the physician taps on the patellar tendon and the leg jerks; the patient does not think about it—the motor response just happens. The lack of a reflex (areflexia), an obviously weakened reflex (hyporeflexia), or an excessively active reflex (hyperreflexia) is usually indicative of a neurologic disorder.
By building on these summaries of the neuron and of the basic reflex arc, we shall briefly consider what neuronal elements constitute a pathway. If the patient bumps his or her knee and not only hits the patellar tendon but also damages the skin over the tendon, two things happen (Fig. 1-2). First, impulses from receptors in the tendon travel through a reflex arc that causes the leg to jerk (knee jerk, or patellar reflex). The synapse for this reflex arc is located in the lumbosacral spinal cord. Second, impulses from pain receptors in the damaged skin are transmitted in the lumbosacral cord to a second set of neurons that convey them via ascending axons to the forebrain. As can be seen in Figure 1-2, these axons cross the midline of the spinal cord and form an ascending tract on the contralateral side. In the forebrain, these signals are passed to a third group of neurons that distribute them to a region of the cerebral cortex specialized to interpret them as pain from the knee.
This three-neuron chain constitutes a pathway, a series of neurons designed to carry a specific type of information from one site to another (Fig. 1-2). Some pathways carry information to a level of conscious perception (we not only recognize pain but know that it is coming from the knee), and others convey information that does not reach the conscious level. It is common to refer to all the neurons comprising a pathway and conducting a specific type of information as a system. For example, the anterolateral system conducts pain and thermal information, the posterior column–medial lemniscus system conducts vibratory and position sense, and the corticospinal system conducts information from the cerebral cortex to the spinal cord.
REGIONS OF THE CENTRAL NERVOUS SYSTEM
Spinal Cord
The spinal cord is located inside the vertebral canal and is rostrally continuous with the medulla oblongata of the brain (Fig. 1-3). An essential link between the PNS and the brain, it conveys sensory information originating from the body wall, extremities, and gut and distributes motor impulses to these areas. Impulses enter and leave the spinal cord through the 31 pairs of spinal nerves (Fig. 1-1; see also Fig. 9-2). The spinal cord contains sensory fibers and motor neurons involved in reflex activity and ascending and descending pathways or tracts that link spinal centers with other parts of the CNS. Ascending pathways convey sensory information to higher centers, whereas descending pathways influence neurons in the spinal cord or brainstem.
Figure 1-3. The basic divisions of the central nervous system.
Medulla Oblongata
At the level of the foramen magnum, the spinal cord is continuous with the most caudal part of the brain, the medulla oblongata, commonly called the medulla (Fig. 1-3). The medulla consists of (1) neurons that perform functions associated with the medulla and (2) ascending (generally sensory) and descending (generally motor) tracts that pass through the medulla on their way from or to the spinal cord. Some of the neuronal cell bodies of the medulla are organized into nuclei associated with specific cranial nerves. The medulla contains the nuclei for the glossopharyngeal (cranial nerve IX), vagus (X), and hypoglossal (XII) nerves as well as portions of the nuclei for the trigeminal (V), vestibulocochlear (VIII), and spinal accessory (XI) nerves. It also contains important relay centers and nuclei that are essential to the regulation of respiration, heart rate, and various visceral functions.
Pons and Cerebellum
The pons and cerebellum originate embryologically from the same segment of the developing neural tube. However, in the adult, the pons forms part of the brainstem (the other parts being the midbrain and medulla) and the cerebellum is a suprasegmental structure because it is located posterior (dorsal) to the brainstem (Fig. 1-3).
Like the medulla, the pons contains many neuronal cell bodies, some of which are organized into cranial nerve nuclei, and it is traversed by ascending and descending tracts. The pons contains the nuclei of the abducens (VI) and facial (VII) nerves and portions of the nuclei for the trigeminal (V) and vestibulocochlear (VIII) nerves. The anterior (ventral) part of the pons contains large populations of neurons (pontine nuclei) that form a relay station between the cerebral cortex and cerebellum and descending motor fibers that travel to all spinal levels.
The cerebellum is connected with diverse regions of the CNS and is considered part of the motor system. It serves to coordinate the activity of individual muscle groups to produce smooth, purposeful, synergistic movements.
Midbrain
Rostrally, the pons is continuous with the midbrain (Fig. 1-3). This part of the brain is, quite literally, the link between the brainstem and the forebrain. Ascending or descending pathways to or from the forebrain must traverse the midbrain. The nuclei for the oculomotor (III) and trochlear (IV) cranial nerves as well as part of the trigeminal (V) complex are found in the midbrain. Other midbrain centers are concerned with visual and auditory reflex pathways, motor function, transmission of pain, and visceral functions.
Thalamus
The forebrain consists of the cerebral hemispheres, large groups of neurons that comprise the basal nuclei, and the thalamus (Fig. 1-3). We shall see later that the thalamus actually consists of several regions—for example, the hypothalamus, subthalamus, epithalamus, and dorsal thalamus. The thalamus is also commonly called the diencephalon, a term that reflects its embryologic origin.
The thalamus is rostral to the midbrain and almost completely surrounded by elements of the cerebral hemisphere. Individual parts of the thalamus can be seen in detail only when the brain is cut in coronal or axial planes.
With the exception of olfaction, all sensory information that eventually reaches the cerebral cortex must pass through the thalamus. One function of the thalamus, therefore, is to receive sensory information of many sorts and to distribute it to the specific regions in the cerebral cortex that are specialized to decode it. Other areas of the thalamus receive input from pathways conveying information on, for example, position sense or the tension in a tendon or muscle. This input is relayed to areas of the cerebral cortex that function to generate smooth, purposeful movements.
Although it is small, the hypothalamus functions in sexual behavior, feeding, hormonal output of the pituitary gland, body temperature regulation, and a wide range of visceromotor functions. Through descending connections, the hypothalamus influences visceral centers in the brainstem and spinal cord.
Cerebral Hemispheres
The largest and most obvious parts of the human brain are the two cerebral hemispheres. Each hemisphere is composed of three major subdivisions. First, the cerebral cortex is a layer of neuronal cell bodies about 0.5 cm thick that covers the entire surface of the hemisphere. This layer of cells is thrown into elevations called gyri (singular, gyrus) separated by creases called sulci (singular, sulcus).
The second major part of the hemisphere is the subcortical white matter, which is made up of myelinated axons that carry information to or from the cerebral cortex. The largest and most organized part of the white matter is the internal capsule. This bundle contains fibers passing to and from the cerebral cortex, such as corticospinal and thalamocortical fibers.
The third major component of the hemisphere is a prominent group of neuronal cell bodies collectively called the basal nuclei. These prominent forebrain centers are involved in motor function. Parkinson disease, a neurologic disorder associated with the basal nuclei, is characterized by a profound impairment of movement.
The gyri and sulci that make up the cerebral cortex are named, and many are associated with particular functions. Some gyri receive sensory input from thalamic relay nuclei, whereas descending fibers from these gyri may influence centers in the brainstem or spinal cord. The cerebral cortex also has association areas that are essential for analysis and cognitive thought.
FUNCTIONAL SYSTEMS AND REGIONS
A functional system is a set of neurons linked together to convey a particular block of information or to accomplish a particular task. In this respect, systems and pathways, in some cases, may be similar, and their meanings may frequently overlap.
Anatomic parts of the CNS, such as the medulla and pons, are commonly called regions
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