Chapter 12 (1)

• Nervous system carries out its task in three basic steps

  1. Sense organs receive information about changes in the body and external environment, and transmit coded messages to the brain and spinal cord (CNS: central nervous system)

  2. CNS processes this information, relates it to past experiences, and determines appropriate response

  3. CNS issues commands to muscles and gland cells to carry out such a response

  
• Central nervous system (CNS)


  Brain and spinal cord enclosed by cranium and vertebral column
• Peripheral nervous system (PNS)


  All the nervous system except the brain and spinal cord; composed of nerves and ganglia
• Nerve


  A bundle of nerve fibers (axons) wrapped in fibrous connective tissue
• Ganglion


  A knot-like swelling in a nerve where neuron cell bodies are concentrated
• Peripheral nervous system divisions

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


  Carries signals from receptors to CNS
• Sensory (afferent) division subdivisions

  • Somatic sensory division
  • Visceral sensory division
• Somatic sensory division


  Carries signals from receptors in the skin, muscles, bones, and joints
• Visceral sensory division


  Carries signals from the viscera (heart, lungs, stomach, and urinary bladder)
• Motor (efferent) division


  Carries signals from CNS to effectors (glands and muscles that carry out the body's response)
• Motor (efferent) division subdivisions

  • Somatic motor division
  • Visceral motor division (autonomic nervous system)
• Somatic motor division


  Carries signals to skeletal muscles, producing muscular contraction as well as somatic reflexes—involuntary muscle contractions
• Visceral motor division (autonomic nervous system)


  Carries signals to glands, cardiac and smooth muscle, producing involuntary visceral reflexes
• Autonomic nervous system divisions

  • Sympathetic division
  • Parasympathetic division
• Sympathetic division


  Tends to arouse body for action, accelerating heart beat and respiration, while inhibiting digestive and urinary systems
• Parasympathetic division


  Tends to have calming effect, slowing heart rate and breathing, stimulating digestive and urinary systems
• Universal Properties of Neurons

  • Excitability (irritability): Respond to environmental changes called stimuli
  • Conductivity: Respond to stimuli by producing electrical signals that are quickly conducted to other cells at distant locations
  • Secretion: When an electrical signal reaches the end of nerve fiber, the cell secretes a chemical neurotransmitter that influences the next cell
• Functional Classes of Neurons

  • Sensory (afferent) neurons
  • Interneurons (association neurons)
  • Motor (efferent) neuron

  
• Sensory (afferent) neurons


  Detect stimuli and transmit information about them toward the CNS

  
• Interneurons (association neurons)


  Lie entirely within CNS connecting motor and sensory pathways (about 90% of all neurons), receive signals from many neurons and carry out integrative functions (make decisions on responses)

  
• Motor (efferent) neuron


  Send signals out to muscles and gland cells (the effectors)
• Soma


  Control center of neuron, also called neurosoma or cell body
• Soma


  • Has a single, centrally located nucleus with large nucleolus
  • Cytoplasm contains mitochondria, lysosomes, Golgi complex, inclusions, extensive rough ER and cytoskeleton
  • Inclusions: glycogen, lipid droplets, melanin, and lipofuscin pigment
  • Cytoskeleton has dense mesh of microtubules and neurofibrils (bundles of actin filaments) that compartmentalizes rough ER into dark-staining Nissl bodies
  • No centrioles, no mitosis
• Dendrites


  Branches that come off the soma, primary site for receiving signals from other neurons, the more dendrites the neuron has, the more information it can receive, provide precise pathways for the reception and processing of information

  
• Axon (nerve fiber)


  Originates from a mound on the soma called the axon hillock, cylindrical, relatively unbranched for most of its length, axon collaterals—branches of axon, branch extensively on distal end, specialized for rapid conduction of signals to distant points, only one axon per neuron (some neurons have none), may be enclosed by myelin sheath

  
• Terminal arborization


  Distal end of axon has extensive complex of fine branches
• Synaptic knob (terminal button)


  Little swelling that forms a junction (synapse) with the next cell, contains synaptic vesicles full of neurotransmitter
• Types of Neurons


  • Multipolar neuron
  • Bipolar neuron
  • Unipolar neuron
  • Anaxonic neuron
• Multipolar neuron


  One axon and multiple dendrites, most common – most neurons in CNS

  
• Bipolar neuron


  One axon and one dendrite, olfactory cells, retina, inner ear
• Unipolar neuron


  Single process leading away from soma, sensory cells from skin and organs to spinal cord
• Anaxonic neuron


  Many dendrites but no axon, retina, brain, and adrenal gland
• Neuroglia


  • Protect neurons and help them function
  • Bind neurons together and form framework for nervous tissue
  • In fetus, guide migrating neurons to their destination
  • If mature neuron is not in synaptic contact with another neuron, it is covered by glial cells to prevent neurons from touching each other and give precision to conduction pathways
• Types of Neuroglia in CNS

  • Oligodendrocytes
  • Ependymal cells
  • Microglia
  • Astrocytes

  
• Oligodendrocytes


  Form myelin sheaths in CNS that speed signal conduction, arm-like processes wrap around nerve fibers

  
• Ependymal cells


  Line internal cavities of the brain, secrete and circulate cerebrospinal fluid (CSF), cuboidal epithelium with cilia on apical surface

  
• Microglia


  Wander through CNS looking for debris and damage, develop from white blood cells (monocytes) and become concentrated in areas of damage
• Astrocytes


  • Most abundant glial cell in CNS, covering brain surface and most nonsynaptic regions of neurons in the gray matter
  • Form supportive framework
  • Have extensions (perivascular feet) that contact blood capillaries and stimulate them to form a seal called the blood–brain barrier
  • Convert glucose to lactate and supply this to neurons
  • Secrete nerve growth factors
  • Communicate electrically with neurons
  • Regulate chemical composition of tissue fluid by absorbing excess neurotransmitters and ions
  • Astrocytosis or sclerosis—when neuron is damaged, astrocytes form hardened scar tissue and fill in space

  
• Types of Neuroglia in PNS

  • Schwann cells
  • Satellite cells