The central nervous system (CNS) is responsible for controlling bodily functions, and it is the center for behavioral and intellectual abilities. Neurons within the CNS are organized into highly complex patterns that mediate information through synaptic interactions. Clinicians prescribe CNS drugs to modify the activity of these neurons in order to treat specific disorders or to alter the general level of arousal of the CNS. This chapter presents a simplified introduction to the organization of the CNS and the general drug strategies medical practitioners use to alter activity within the brain and spinal cord. This chapter is not intended to be an extensive review of neuroanatomy and neurophysiology; the references at the end of this chapter include several excellent resources on this topic.1–3 However, this chapter will review certain neuroanatomical and neurophysiological concepts that will help you understand how drugs can affect CNS function. These concepts will likewise be important when we examine the actions of specific CNS drugs in the next several chapters in this book.
The CNS can be grossly divided into the brain and spinal cord (Fig. 5-1). The brain is subdivided according to anatomic or functional criteria. The following is a brief overview of the general organization of the brain and spinal cord, with some indication of where particular CNS drugs tend to exert their effects.
General organization of the CNS.
The largest and most rostral aspect of the brain is the cerebrum (see Fig. 5-1). The cerebrum consists of bilateral hemispheres, with each hemisphere divided anatomically into several lobes (frontal, temporal, parietal, and occipital). The outer cerebrum, or cerebral cortex, is the highest order of conscious function and integration in the CNS. Specific cortical areas are responsible for sensory and motor functions as well as intellectual and cognitive abilities. Other cortical areas are involved in short-term memory and speech. The cortex also operates in a somewhat supervisory capacity regarding lower brain functioning and may influence the control of other activities, such as the autonomic nervous system.
Most CNS therapeutic medications tend to affect cortical function indirectly by first altering the function of lower brain and spinal cord structures. An exception is the group of drugs used to treat epilepsy, which are often targeted directly for hyperexcitable neurons in the cerebral cortex. In addition, drugs that attempt to enhance cognitive function in conditions such as Alzheimer disease (cholinergic stimulants; see Chapter 19) might also exert their primary effects in the cerebrum.
A group of specific areas located deep within the cerebral hemispheres is collectively termed the basal ganglia. Its components include the caudate nucleus, putamen, globus pallidus, lentiform nucleus, and substantia nigra. ...