Nervous System

• Nervous system functions


  • Receiving sensory input
  • Integrating information
  • Controlling muscles and glands
  • Maintaining homeostasis
  • Establishing and maintaining mental activity
• Main divisions of the nervous system

  • Central nervous system (CNS)
  • Peripheral nervous system (PNS)
  • Sensory division
  • Motor division
  • Somatic nervous system
  • Autonomic nervous system
  • Enteric nervous system
• Neurons


  • Receive stimuli, conduct action potentials, and transmit signals to other neurons or effector organs
• Glial cells


  • Supportive cells of the CNS and PNS, do not conduct action potentials, carry out different functions that enhance neuron function and maintain normal conditions within nervous tissue
• Structural types of neurons

  • Multipolar neurons
  • Bipolar neurons
  • Pseudo-unipolar neurons
• Types of glial cells

  • Astrocytes
  • Ependymal cells
  • Microglial cells
  • Oligodendrocytes
  • Schwann cells
• Myelin sheath


  Specialized layers that wrap around the axons of some neurons, formed by oligodendrocytes in the CNS and Schwann cells in the PNS, an excellent insulator that prevents almost all ion movement across the cell membrane
• Unmyelinated neurons


  Axons that lack the myelin sheaths, rest in indentations of the oligodendrocytes in the CNS and the Schwann cells in the PNS
• Gray matter


  Consists of groups of neuron cell bodies and their dendrites, where there is very little myelin
• White matter


  Consists of bundles of parallel axons with their myelin sheaths, which are whitish in color
• Resting membrane potential


  A small, but measurable, voltage that exists across the cell membrane when the cell is at rest, due to differences in concentrations of ions across the membrane
• Nerve cell communication


  Resting membrane potential can change in response to a stimuli, changes in membrane potential are called action potentials
• Gated membrane channels


  Closed until opened by specific signals, opening and closing changes the permeability of the membrane to ions and can therefore change the membrane potential
• Action potential


  Electrical signals conducted along the cell membrane, caused by the opening of voltage-gated Na+ and K+ channels, resulting in depolarization, repolarization, and hyperpolarization
• Continuous conduction


  Action potentials in unmyelinated axons travel along the entire membrane
• Saltatory conduction


  Action potentials on myelinated axons occur in a jumping pattern at the nodes of Ranvier
• Axon conduction speed


  Varies based on the diameter of axon fibers, medium-diameter lightly myelinated axons conduct at 3-15 m/s, large-diameter heavily myelinated axons conduct at 15-120 m/s
• Synapse


  Junction where the axon of one neuron interacts with another neuron, with a presynaptic terminal, synaptic cleft, and postsynaptic membrane, neurotransmitters are released from the presynaptic terminal and bind to receptors on the postsynaptic membrane, causing stimulation or inhibition of an action potential
• Neurotransmitters


  • Acetylcholine
  • Norepinephrine
• Membrane receptors


  Cause chemically gated channels for Na+, K+, or Cl− to open or close in the postsynaptic membrane
• Neurotransmitter type and channel response

  Depend on the type of neurotransmitter in the presynaptic terminal and the type of receptors on the postsynaptic membrane
• Response


  May be either stimulation or inhibition of an action potential in the postsynaptic cell
• Response when Na+ channels open

  Postsynaptic cell becomes depolarized, and an action potential will result if threshold is reached
• Response when K+ or Cl− channels open

  Inside of the postsynaptic cell tends to become more negative, or hyperpolarized, and an action potential is inhibited from occurring
• Neurotransmitters


  • Acetylcholine
  • Norepinephrine
• Neurotransmitters do not normally remain in the synaptic cleft indefinitely, thus their effects are short duration</b>
• Neurotransmitter reduction


  Rapidly broken down by enzymes within the synaptic cleft or are transported back into the presynaptic terminal
• Acetylcholinesterase


  Enzyme that breaks down acetylcholine
• Norepinephrine


  Either actively transported back into the presynaptic terminal or broken down by enzymes
• Converging pathway


  • Simple pathway in which two or more neurons synapse with the same postsynaptic neuron
  • Allows information transmitted in more than one neuronal pathway to converge into a single pathway
• Diverging pathway


  • Simple pathway in which an axon from one neuron divides and synapses with more than one other postsynaptic neuron
  • Allows information transmitted in one neuronal pathway to diverge into two or more pathways
• Summation


  1. Many presynaptic action potentials are needed

  2. Allows integration of multiple subthreshold local potentials

  3. Can bring the membrane potential to threshold and trigger an action potential
• Spatial summation


  Local potentials originate from different locations on the postsynaptic neuron—for example, from converging pathways
• Temporal summation


  Local potentials overlap in time, from a single input that fires rapidly
• Central nervous system (CNS)


  Brain and spinal cord
• Peripheral nervous system (PNS)


  All the nerves and ganglia outside the brain and spinal cord
• Spinal cord


  • Extends from the foramen magnum to the 2nd lumbar vertebra
  • Protected by vertebral column
  • Spinal nerves allow movement
  • If damaged paralysis can occur
• Gray matter


  Center of spinal cord, looks like letter H or a butterfly
• White matter


  Outer layer of spinal cord, contains myelinated fibers
• Columns in white matter

  • Dorsal, ventral, and lateral
  • Contain ascending and descending tracts