NERVOUS

• Functions of the Nervous System

  • Sensory input—gathering information
  • Integration
  • Motor output
• Sensory input


  Sensory receptors monitor changes, called stimuli, occurring inside and outside the body
• Integration


  Nervous system processes and interprets sensory input and decides whether action is needed
• Motor output


  A response, or effect, activates muscles or glands
• Nervous system classifications

  • Structural classification
  • Functional classification
• Central nervous system (CNS)

  • Integration; command center
  • Interprets incoming sensory information
  • Issues outgoing instructions
• Peripheral nervous system (PNS)


  • Serve as communication lines among sensory organs, the brain and spinal cord, and glands or muscles
• Sensory (afferent) division

  • Somatic sensory (afferent) fibers
  • Visceral sensory (afferent) fibers
• Motor (efferent) division

  • Somatic nervous system (voluntary)
  • Autonomic nervous system (involuntary)
• Neuroglia

  • Support
  • Insulate
  • Protect neurons
• Nervous tissue cell types

  • Supporting cells (neuroglia, glial cells, glia)
  • Neurons
• Astrocytes


  • Brace and anchor neurons to blood capillaries
  • Determine permeability and exchanges between blood capillaries and neurons
  • Protect neurons from harmful substances in blood
  • Control the chemical environment of the brain
• Microglia


  • Monitor health of nearby neurons
  • Dispose of debris
• Ependymal cells


  • Line cavities of the brain and spinal cord
  • Cilia assist with circulation of cerebrospinal fluid
• Oligodendrocytes


  • Wrap around nerve fibers in the central nervous system
  • Produce myelin sheaths
• Schwann cells


  • Form myelin sheath around nerve fibers in the PNS
• Satellite cells


  • Protect and cushion neuron cell bodies
• Neuron


  Nerve cell specialized to transmit messages (nerve impulses)
• Cell body


  • Nucleus and metabolic center of the cell
• Dendrites


  • Conduct impulses toward the cell body
• Axons


  • Conduct impulses away from the cell body
  • End in axon terminals, which contain vesicles with neurotransmitters
• Synaptic cleft


  Gap between axon terminals and the next neuron
• Synapse


  Functional junction between nerves where a nerve impulse is transmitted
• Myelin


  • White, fatty material covering axons
  • Protects and insulates fibers
  • Speeds nerve impulse transmission
• Myelin sheaths in PNS

  • Schwann cells wrap axons in a jelly roll–like fashion to form the myelin sheath
  • Neurilemma is part of the Schwann cell external to the myelin sheath
  • Nodes of Ranvier are gaps in myelin sheath along the axon
• Myelin sheaths in CNS

  • Oligodendrocytes produce myelin sheaths around axons
  • Lack a neurilemma
• Nervous tissue terminology

  • Nuclei
  • Ganglia
  • Tracts
  • Nerves
  • White matter
  • Gray matter
• Functional classification of neurons

  • Sensory (afferent) neurons
  • Motor (efferent) neurons
  • Interneurons (association neurons)
• Structural classification of neurons

  • Multipolar neurons
  • Bipolar neurons
  • Unipolar neurons
• Irritability


  • Ability to respond to a stimulus and convert it to a nerve impulse
• Conductivity


  • Ability to transmit the impulse to other neurons, muscles, or glands
• Resting neuron's membrane

  Fewer positive ions are inside the neuron's plasma membrane than outside
• Action potential initiation and generation

  1. Stimulus changes the permeability of the neuron's membrane to sodium ions

  2. Sodium channels open, and sodium (Na+) diffuses into the neuron

  3. Inward rush of sodium ions changes the polarity at that site and is called depolarization

  4. If the stimulus is strong enough and sodium influx great enough, local depolarization activates the neuron to conduct an action potential (nerve impulse)
• Propagation of the action potential

  • All-or-none response means the nerve impulse either is propagated or is not
  • Fibers with myelin sheaths conduct nerve impulses more quickly
• Repolarization


  1. Membrane permeability changes again—becoming impermeable to sodium ions and permeable to potassium ions

  2. Potassium ions rapidly diffuse out of the neuron, repolarizing the membrane

  3. Sodium-potassium pump restores the original configuration
• Transmission of the signal at synapses

  1. Action potential reaches the axon terminal, opening calcium channels

  2. Calcium causes the vesicles containing the neurotransmitter chemical to fuse with the axonal membrane

  3. Neurotransmitter is released into the synaptic cleft

  4. Neurotransmitter diffuses across the synaptic cleft and binds to receptors on the next neuron

  5. If enough neurotransmitter is released, a graded potential will be generated, eventually causing an action potential in the next neuron

  6. Neurotransmitter is quickly removed from the synapse either by reuptake or by enzymatic activity
• Reflex


  Rapid, predictable, and involuntary response to stimuli
• Types of reflexes

  • Somatic reflexes
  • Autonomic reflexes
• Somatic reflexes


  • Reflexes that stimulate the skeletal muscles
  • Involuntary, although skeletal muscle is normally under voluntary control
• Autonomic reflexes


  • Regulate the activity of smooth muscles, the heart, and glands