Essentials of Human Physiology for Pharmacy

46 Essentials of Human Physiology for Pharmacysensory input and experiences adds further complexity and distinctivenessto neurological responses and behavior. The nervous system is divided intotwo anatomically distinct regions:• Central nervous system• Peripheral nervous systemThe central nervous system (CNS) consists of the brain and spinal cord.The peripheral nervous system (PNS) consists of 12 pairs of cranial nerves thatarise from the brainstem and 31 pairs of spinal nerves arising from the spinalcord. These peripheral nerves carry information between the CNS and thetissues of the body. The PNS consists of two divisions:• Afferent division• Efferent divisionThe afferent division carries sensory information toward the CNS and theefferent division carries motor information away from the CNS toward theeffector tissues (muscles and glands). The efferent division is further dividedinto two components: (1) the somatic nervous system, which consists ofmotor neurons that innervate skeletal muscle; and (2) the autonomic nervoussystem that innervates cardiac muscle, smooth muscle, and glands.6.2 Classes of neuronsThe human nervous system has three functional classes of neurons:• Afferent neurons• Efferent neurons• InterneuronsAfferent neurons lie predominantly in the PNS (see Figure 6.1). Each hasa sensory receptor activated by a particular type of stimulus, a cell bodylocated adjacent to the spinal cord, and an axon. The peripheral axon extendsfrom the receptor to the cell body and the central axon continues from thecell body into the spinal cord. Efferent neurons also lie predominantly in thePNS. In this case, the cell bodies are found in the CNS in the spinal cord orbrainstem and the axons extend out into the periphery of the body wherethey innervate the effector tissues. By way of convergence, the centrallylocated cell bodies may receive inputs from several different regions of thebrain that will influence their activity.The third class of neurons includes the interneurons, which lie entirelywithin the CNS. Because the human brain and spinal cord contain well over100 billion neurons, interneurons account for approximately 99% of all theneurons in the body taken together. Interneurons lie between afferent and