The nervous system is divided anatomically into the central nervous system (CNS) andperipheral nervous system (PNS). The CNS consists of the brain and spinal cord, whereas thePNS includes cranial nerves that arise from the brain and spinal nerves that arise from the spinalcord.The PNS connects the entire body to the integrating centers of the brain and spinal cord. Theconnection between the CNS and the body through the PNS provides the fast, immediate controlover homeostasis by transmittingaction potentialsor nerve impulses.Figure 10.1: Organization of the Nervous systemFun Brain Facts:The adult brain has ~100 billion neurons and weighs about 1.5 kg (3−3.5 pounds)The adult brain receives 15% of the total blood flow in the body per minuteScientists have discovered thatneurogenesis(the formation of new brain cells fromneural stem cells) can occur in adult brains – this process is helping us understand andtreat neurodegenerative diseases such as Alzheimer’s diseaseThe CNS is composed of thebrain and spinal cord. The CNS receives inputfromsensoryneurons and directs activity ofmotorneurons that innervate muscles andglands.
Figure 10.2: CNS structureCNS OverviewIn simple terms, sensory peripheral nerves in the body send information to the brain,which integrates the information and generates a response. This response can take theform of motor neuron responses or higher brain functions including emotions, memory,learning and perceptions.The brain is connected caudally to thespinal cord, a large bundle of axonal nervefibers that connects the spinal nerves with the brain.Ascending tractsof nervesconvey sensory information from the periphery to the brain whereasdescendingtractsof nerves send motor nerve impulses from the brain down the spinal cord. The
spinal cord also performs some information integration independent of the brain in theform ofreflex arcs.Brain developmentBrain development is a complicated, fascinating process that we will only touch uponbriefly in this class. Around day 14 after conception, the embryo has split into threedistinct "germ layers" - ectoderm, endoderm, and mesoderm - which eventually will giverise to all tissues of the body. Around 20 days after conception, theectoderm germlayerforms a groove that will become theneural tube, a single hollow channel that willeventually develop into the brain and spinal cord.By week 4 after conception, the brain begins to organizeinto three distinct swellings inthe neural tube:Prosencephalon (forebrain)Mesencephalon (midbrain)Rhombencephalon (hindbrain)By week 5, these large swellings further differentiate intofive regions: thetelencephalon and diencephalon from the prosencephalon, the mesencephalon, and themetencephalon and myelencephalon from the rhombencephalon.