nervous system

• the central nervous system receives input via sensory fibers and issues commands via motor fibers. The sensory and motor fibers together form the nerves that constitute the peripheral nervous system.
• Structural classification of nervous system

  Includes all nervous system organs
• Structural classification subdivisions


  • Central nervous system
  • Peripheral nervous system
• Central nervous system (CNS)

  Consists of the brain and spinal cord, occupies the dorsal body cavity, acts as the integrating and command centers of the nervous system
• Central nervous system functions

  • Interpret incoming sensory information
  • Issue instructions based on past experience and current conditions
• Peripheral nervous system (PNS)


  Includes all parts of the nervous system outside the CNS, consists mainly of the nerves that extend from the spinal cord and brain
• Nerves in the PNS

  • Spinal nerves carry impulses to and from the spinal cord
  • Cranial nerves carry impulses to and from the brain
• Nerves in the PNS


  Serve as communication lines, link all parts of the body by carrying impulses from the sensory receptors to the CNS and from the CNS to the appropriate glands or muscles
• Functional classification scheme


  Concerned only with PNS structures
• Functional classification scheme


  • Divides PNS structures into two principal subdivisions
• Sensory division (afferent division)


  Nerves that convey impulses to the central nervous system from sensory receptors located in various parts of the body
• Somatic sensory (afferent) fibers

  Sensory fibers delivering impulses from the skin, skeletal muscles, and joints
• Visceral sensory (afferent) fibers


  Sensory fibers transmitting impulses from the visceral organs
• Motor division (efferent division)


  Carries impulses from the CNS to effector organs, the muscles and glands
• Effector organs


  Muscles and glands
• Motor division (efferent division)


  Activates muscles and glands, effecting a motor response
• The somatic nervous system allows us to consciously (voluntarily), control our skeletal muscles. Hence, we often refer tothis subdivision as the voluntary nervous system. However, not all skeletal muscle activity controlled by this motor division is voluntary. Skeletal muscle reflexes, such as the stretch reflex (described later in the chapter), are involuntary.
• The autonomic nervous system (ANS) regulates events that are involuntary (no conscious control), such as the activity of smooth muscle, cardiac muscle, and glands. This subdivision, commonly called the involuntary nervous system, itself has two parts, the sympathetic and parasympathetic, which typically bring about opposite effects. What one stimulates, the other inhibits. We will describe these later.
• Supporting cells in the CNS are “lumped together” as neuroglia (nu-rog9le-ah), literally, “nerve glue,” also called glial cells or glia. Neuroglia include many types of cells that support, insulate, and protect the delicate neurons . In addition, each of the different types of neuroglia has special functions. CNS neuroglia include the following:
• Astrocytes


  Abundant star-shaped cells that account for nearly half of neural tissue
• Astrocytes


  • Their numerous projections have swollen ends that cling to neurons, bracing them and anchoring them to their nutrient supply lines, the blood capillaries
  • They form a living barrier between capillaries and neurons
  • They help determine capillary permeability
  • They play a role in making exchanges between capillaries and neurons
  • They help protect the neurons from harmful substances that might be in the blood
  • They help control the chemical environment in the brain by "mopping up" leaked potassium ions and recapturing chemicals released for communication purposes
• Microglia (mi-kro9-gle-ah): spiderlike phagocytes that monitor the health of nearby neurons and dispose of debris, such as dead brain cells and bacteria
• Ependymal (˘e-pen9d˘ı-mal) cells: neuroglia that line the central cavities of the brain and the spinal cord (Figure 7.3c). They participate in the production of cerebrospinal fluid (CSF) and the beating of their cilia helps to circulate the cerebrospinal fluid that fills those cavities and forms a protective watery cushion around the CNS.
• Oligodendrocytes: neuroglia that wrap their flat extensions (processes) tightly around CNS nerve fibers, producing
• Supporting cells in the PNS come in two major varieties—Schwann cells and satellite cells. Schwann cells form the myelin sheaths around nerve fibers in the PNS. Satellite cells act as protective, cushioning cells for peripheral neuron cell bodies
• Neurons


  Highly specialized to transmit messages (nerve impulses) from one part of the body to another
• Neurons


  • Have a cell body
  • Have one or more slender processes extending from the cell body
• Cell body


  The metabolic center of the neuron
• The cell body contains a transparent nucleus with a large nucleolus
• The cytoplasm surrounding the nucleus contains the usual organelles, except that it lacks centrioles
• The rough ER, called Nissl bodies, and neurofibrils (intermediate filaments that are important in maintaining cell shape) are particularly abundant in the cell body
• Processes


  The armlike processes, or fibers, vary in length from microscopic to over 3 feet long, reaching from the lumbar region of the spine to the great toe
• Dendrites


  Neuron processes that convey incoming messages (electrical signals) toward the cell body
• Axons


  Neuron processes that generate nerve impulses and typically conduct them away from the cell body
• Neurons may have hundreds of branching dendrites, depending on the structural type
• Each neuron has only one axon, which arises from a conelike region of the cell body called the axon hillock
• Axon


  • Gives off a collateral branch along its length
  • Branches profusely at their terminal end, forming hundreds to thousands of axon terminals
• Axon terminals


  Contain hundreds of tiny vesicles, or membranous sacs, that contain chemicals called neurotransmitters
• Axon transmits nerve impulses


  1. Impulses reach the axon terminals

  2. Stimulate the release of neurotransmitters into the extracellular space between neurons, or between a neuron and its target cell
• Synaptic cleft


  Tiny gap between the axon terminal and the next neuron or its target