Functional Organization of Nervous Tissue

• Nervous system


  Made up of the brain, spinal cord, nerves and sensory receptors. Responsible for sensory perceptions, mental activities, stimulating muscle movements and secretion of many glands.
• Central nervous system (CNS)


  Brain and spinal cord
• Peripheral nervous system (PNS)


  Everything that comes off the brain and spinal cord, including cranial and peripheral nerves
• Brain and spinal cord


  Continuous with each other at the foramen magnum
• Subdivisions of peripheral nervous system

  • Sensory/afferent
  • Motor/efferent
• Sensory/afferent


  Conducts sensations from the periphery up to the brain
• Motor/efferent


  Conducts signals from the brain down to the periphery
• Divisions of peripheral nervous system

  • Somatic
  • Autonomic
• Somatic nervous system


  Conscious control over skeletal muscles
• Autonomic nervous system


  Automatic control, no conscious control
• Components of autonomic nervous system


  • Sympathetic
  • Parasympathetic
• Sympathetic nervous system


  Fight-or-flight response, increases heart rate, blood pressure, pupil dilation, airway dilation, slows digestion
• Parasympathetic nervous system


  Rest and digest response, decreases heart rate, blood pressure, pupil constriction, airway constriction, increases digestion
• Enteric nervous system


  Nervous system of the gut, responds to sympathetic and parasympathetic actions
• Neuron


  Nerve cell that receives stimuli and transmits action potentials
• Functional neuron classifications


  • Sensory/afferent
  • Motor/efferent
  • Interneurons/association
• Structural neuron classifications


  • Multipolar
  • Bipolar
  • Unipolar
• Neuroglia


  Support and protective cells in the nervous system, including astrocytes, ependymal cells, microglia, oligodendrocytes, Schwann cells, and satellite cells
• Myelin sheath


  Insulates and protects myelinated axons, formed by oligodendrocytes in the CNS and Schwann cells in the PNS
• Nodes of Ranvier


  Gaps in the myelin sheath of myelinated axons
• Unmyelinated axons are encased in Schwann cell cytoplasm without a wrapped myelin coating
• Action potential


  Electrical signals cells produce to transfer information from one part of the body to another
• Membrane potential


  Electrical property resulting from ionic concentration differences across the plasma membrane and its permeability
• Ion channels


  • Non-gated/leak channels are always open and responsible for permeability, specific to one type of ion
  • Gated channels open or close in response to ligands or voltage changes
• Resting membrane potential


  1. Charged molecules and ions inside and outside the cell are nearly equal

  2. Potassium concentration higher inside, sodium higher outside

  3. Little movement of potassium ions across membrane
• Action potential generation


  1. Stimulus causes membrane depolarization

  2. Voltage-gated sodium channels open, sodium rushes in

  3. Membrane becomes more positive inside

  4. Voltage-gated potassium channels open, potassium rushes out

  5. Membrane repolarizes, slightly more negative than resting
• Local potentials


  Graded changes in membrane potential caused by ligand binding, mechanical stimulation, temperature changes, or spontaneous permeability changes
• Action potential


  Series of permeability changes, depolarization followed by repolarization, all-or-none response
• Refractory period


  Decreased sensitivity to further stimulation after an action potential, includes absolute and relative refractory periods
• Action potential propagation


  1. Propagates in one direction along the axon

  2. Depolarization causes adjacent membrane to depolarize, triggering new action potential

  3. Recently depolarized area is refractory and cannot generate new action potential
• Saltatory conduction


  • Action potential propagates by jumping between nodes of Ranvier in myelinated axons, faster than continuous propagation
• Synapse


  Junction between two cells where action potentials propagate from one cell to another
• Electrical synapse


  • Uses gap junctions to allow current flow between adjacent cells, found in cardiac and smooth muscle
• Chemical synapse


  • Presynaptic terminal releases neurotransmitters into synaptic cleft, which bind to receptors on postsynaptic membrane
  • Neurotransmitters are removed by enzymes or reuptake
• Neuronal pathways and circuits

  • Convergent (many to fewer neurons)
  • Divergent (fewer to many neurons)
  • Oscillating (circular pathways allowing repeated firing)
• Organization of neurons in the central nervous system allows for complex cognition and memory through neuronal pathways and circuits