Orientation to the Structure and Imaging of the Central Nervous System

Central, Peripheral, and Visceromotor Nervous Systems

The human nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS) ( Fig. 1.1 A ). The CNS consists of the brain and spinal cord. Because of their locations in the skull and vertebral column, these structures are the most protected in the body. The PNS is made up of nerves that connect the brain and spinal cord with peripheral structures. These nerves innervate muscle (skeletal, cardiac, smooth) and glandular epithelium and contain a variety of sensory fibers. These sensory fibers enter the spinal cord through the posterior ( dorsal ) root, and motor fibers exit through the anterior ( ventral ) root. The spinal nerve is formed by the joining of posterior (sensory) and anterior (motor) roots and is, consequently, a mixed nerve ( Fig. 1.1 B ). In the case of mixed cranial nerves, the sensory and motor fibers are combined into a single root.

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 and intestines. The visceromotor nervous system is also called the autonomic nervous system because it regulates visceral motor responses normally 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 A ). 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.

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 A ) 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 presynaptic membrane ( Fig. 1.2 A ).

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, such as myasthenia gravis, Lambert-Eaton syndrome, or botulism, that represent a failure of neurotransmitter action at the presynaptic membrane, 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. Fig. 1.2 B 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 quickly extends at the knee without the patient consciously controlling the movement. 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 neural 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 C ). First, impulses from receptors in the muscle stretched by the tendon travel through a reflex arc that causes the leg to extend ( 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 ( Fig. 1.2 C ). As can be seen in Fig. 1.2 C , 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 C ). 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, whereas the posterior column–medial lemniscus system conducts body position and vibratory sense, and the corticospinal system conducts descending information from the cerebral cortex to spinal cord motor neurons.

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.

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) and vestibulocochlear (VIII) nerves; the nucleus of the accessory nerve (XI) is located in cervical levels of the spinal cord. It also contains important relay centers and nuclei that are essential to the regulation of respiration, heart rate, and various visceral functions.