The nervous system is an integrating system which acts rapidly by transmitting signals as electrical impulses over often considerable distances to coordinate bodily activities. The brain and spinal cord make up the central nervous system (CNS); incoming information travels in ascending (sensory) tracts that link the spinal cord to the brain and outgoing information passes down descending (motor) tracts linking the brain to the spinal cord. The CNS integrates responses to incoming information and sends the information to effector tissues (usually striated or smooth muscles or glands). Incoming and outgoing information is carried to and from the periphery to the CNS via 12 pairs of cranial nerves connected to the brain and 31 pairs of spinal nerves connected to the spinal cord; they constitute the peripheral nervous system (PNS). Sensory (afferent) information from the external environment is obtained through the organs of special sense in the eyes, ears, nose and tongue, and skin and mucosa lining bodily cavities: we are aware of these stimuli. Information from internal sources is equally important and vital for maintaining homeostasis, but we are usually Neurons are the basic cellular units of the nervous system. As the principal function of the nervous system is conduction of electrical signals over considerable distances, neurons are highly specialized for this f unction. Neurons have: • A specific shape with long cellular extensions; • Highly specialized membranes to control ionic movements to allow electrical activity to spread along the cellular extensions; • A very specialized internal transport system to distribute cellular metabolites along the processes. The general shape of neurons is shown in Figure 3.1. Note first of all, the relatively large cell body near the top of the picture; this contains the nucleus and the intracellular organelles necessary for synthetic functions so is similar to any other cell. What make neurons special are the long processes that emanate from the cell body. Dendrites are short multiple processes that branch extensively from and transmit impulses towards the cell body. Compare the dendrites in Figure 3.1 with the other process, the axon, which transmits impulses away from the cell body.