Nervous System

• Nervous System

  The master controlling and communicating system of the body
• Functions of the Nervous System

  • Sensory input - monitoring stimuli occurring inside and outside the body
  • Integration - interpretation of sensory input
  • Motor output - response to stimuli by activating effector organs

  
• The nervous system has two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS)
• Central Nervous System (CNS)

  • Brain and spinal cord
  • Integration and command center
• Peripheral Nervous System (PNS)

  • Paired spinal and cranial nerves
  • Carries messages to and from the spinal cord and brain
• Functional divisions of the PNS

  • Sensory (afferent) division
  • Motor (efferent) division

  
• Sensory (afferent) division

  • Sensory afferent fibers - carry impulses from skin, skeletal muscles, and joints to the brain
  • Visceral afferent fibers - transmit impulses from visceral organs to the brain

  
• Motor (efferent) division

  Transmits impulses from the CNS to effector organs
• Parts of the motor division

  • Somatic nervous system
  • Autonomic nervous system (ANS)
• Somatic nervous system

  Conscious control of skeletal muscles
• Autonomic nervous system (ANS)

  • Regulates smooth muscle, cardiac muscle, and glands
  • Divisions - sympathetic and parasympathetic
• Principal cell types of the nervous system

  • Neurons
  • Supporting cells
• Neurons

  Excitable cells that transmit electrical signals
• Supporting cells (neuroglia or glial cells)

  • Provide a supportive scaffolding for neurons
  • Segregate and insulate neurons
  • Promote health and growth
• Types of supporting cells

  • Astrocytes
  • Microglia
  • Ependymal cells
  • Oligodendrocytes
  • Schwann cells
  • Satellite cells

  
• Astrocytes

  • Most abundant, versatile, and highly branched glial cells
  • They cling to neurons and their synaptic endings, and cover capillaries
  • Support and brace neurons
  • Anchor neurons to their nutrient supplies
  • Guide migration of young neurons
  • Control the chemical environment

  
• Microglia

  • Small, ovoid cells with spiny processes
  • Phagocytes that monitor the health of neurons
• Ependymal cells

  • Range in shape from squamous to columnar
  • They line the central cavities of the brain and spinal column

  
• Oligodendrocytes

  Branched cells that wrap CNS nerve fibers
• Schwann cells

  Surround fibers of the PNS

  
• Satellite cells

  Surround neuron cell bodies with ganglia

  
• Neurons (Nerve Cells)

  • Structural units of the nervous system
  • Composed of a body, axon, and dendrites
  • Long-lived, amitotic, and have a high metabolic rate
  • Their plasma membrane functions in electrical signaling and cell-to-cell signaling during development
• Nerve Cell Body (Perikaryon or Soma)

  • Contains the nucleus and a nucleolus
  • Is the major biosynthetic center
  • Is the focal point for the outgrowth of neuronal processes
  • Has well-developed Nissl bodies (rough ER)
  • Contains an axon hillock - cone-shaped area from which axons arise

  
• Processes

  • Arm-like extensions from the soma
  • Called tracts in the CNS and nerves in the PNS
  • There are two types: axons and dendrites

  
• Dendrites of Motor Neurons

  • Short, tapering, and diffusely branched processes
  • They are the receptive, or input, regions of the neuron

  
• Axons: Structure

  • Slender processes of uniform diameter arising from the hillock
  • Long axons are called nerve fibers
  • Usually there is only one unbranched axon per neuron
• Axons: Function

  • Generate and transmit action potentials
  • Secrete neurotransmitters from the axonal terminals
• Myelin Sheath

  • Whitish, fatty (protein-lipoid), segmented sheath around most long axons
  • It functions to protect the axon, electrically insulate fibers from one another, and increase the speed of nerve impulse transmission
• Myelin Sheath and Neurilemma: Formation

  • Formed by Schwann cells in the PNS
  • A Schwann cell envelopes an axon in a trough, encloses the axon with its plasma membrane, and has concentric layers of membrane that make up the myelin sheath
  • Neurilemma - remaining nucleus and cytoplasm of a Schwann cell
  • Nodes of Ranvier: Gaps in the myelin sheath between adjacent Schwann cells

  
• Axons of the CNS

  • Both myelinated and unmyelinated fibers are present
  • Myelin sheaths are formed by oligodendrocytes
  • Nodes of Ranvier are widely spaced

  
• Regions of the Brain and Spinal Cord

  • White matter - dense collections of myelinated fibers
  • Gray matter - mostly soma and unmyelinated fibers

  
• Structural classification of neurons

  • Multipolar - three or more processes
  • Bipolar - two processes (axon and dendrite)
  • Unipolar - single, short process

  
• Functional classification of neurons

  • Sensory (afferent) - transmit impulses toward the CNS
  • Motor (efferent) - carry impulses away from the CNS
  • Interneurons (association neurons) - shuttle signals through CNS pathways

  
• Comparison of structural classes of neurons is provided in Tables 11.1.1, 11.1.2, and 11.1.3