Nervous system 1 (CH11)

• Functions of the Nervous System

  • Maintaining homeostasis. Regulate and coordinate physiology.
  • Receiving sensory input. Monitor internal and external stimuli.
  • Integrating information. Brain and spinal cord process sensory input and initiate responses.
  • Controlling muscles and glands.
  • Establishing and maintaining mental activity. Consciousness, thinking, memory, emotion.
• Nervous system translates your environment
• Central nervous system (CNS)


  Brain and spinal cord
• Peripheral nervous system (PNS)


  Sensory receptors and nerves
• Divisions of the PNS

  • Sensory receptors
  • Sensory (afferent)
  • Motor (efferent)
• Somatic nervous system


  From CNS to skeletal muscles. Voluntary; single neuron system.
• Autonomic nervous system (ANS)


  From CNS to smooth muscle, cardiac muscle and certain glands. Subconscious or involuntary control. Two neuron system: first from CNS to ganglion; second from ganglion to effector.
• Divisions of the ANS


  • Sympathetic
  • Parasympathetic
  • Enteric
• Neurons


  Electrically excitable cells of the nervous system
• Major parts of a neuron

  • Neuron cell body or soma
  • Dendrites
  • Axons
• Functional classification of neurons

  • Sensory or afferent
  • Motor or efferent
  • Interneurons or association neurons
• Structural classification of neurons

  • Multipolar
  • Bipolar
  • Pseudo-unipolar
  • Anaxonic
• Astrocytes


  • Regulate what substances reach the CNS from the blood (blood-brain barrier)
  • Produce chemicals that promote tight junctions to form blood-brain barrier
  • Regulate extracellular brain fluid composition
• Ependymal Cells


  Line brain ventricles and spinal cord central canal
• Choroid plexus


  Secrete cerebrospinal fluid
• Microglia


  Specialized macrophages. Respond to inflammation, phagocytize necrode tissue, microorganisms, and foreign substances that invade the CNS
• Oligodendrocytes


  Form myelin sheaths around axons
• Schwann cells or neurolemmocytes

  Wrap around portion of only one axon to form myelin sheath
• Satellite cells


  Surround neuron cell bodies in sensory ganglia, provide support and nutrients
• Myelinated axons


  • Myelin protects and insulates axons from one another, speeds transmission, functions in repair of axons
  • Not continuous
  • Nodes of Ranvier
  • Completion of development of myelin sheaths at 1 yr
  • Degeneration of myelin sheaths occurs in multiple sclerosis and some cases of diabetes mellitus
• Unmyelinated axons


  Rest in invaginations of Schwann cells or oligodendrocytes. Not wrapped around the axon; gray matter
• Voltage-gated ion channels


  Open or close in response to small voltage changes across the cell membrane
• Resting membrane potential


  Unequal distribution of charge exists between the immediate inside and immediate outside of the plasma membrane: -70 to -90 mV
• Establishing the resting membrane potential

  1. Membrane more permeable to K+ due to many leak channels, and K+ diffuses from inside to outside the cell

  2. Positive charges accumulate outside the membrane

  3. Negatively charged proteins cannot diffuse with K+, so as K+ diffuses out of cell, inside of membrane becomes more negative

  4. Na+, Cl, and Ca2+ do not have a great affect on resting potential since there are very few leakage channels for these ions

  5. Maintained by the Na+/K+ pump
• Depolarization


  Inside of cell becomes more positive; for example, from -70mV to -55mV
• Voltage-gated ion channels and the action potential

  1. Resting membrane potential

  2. Depolarization

  3. Repolarization

  4. End of repolarization and afterpotential

  5. Resting membrane potential
• Propagation of action potentials

  • Threshold graded current at trigger zone causes action potential
  • Continuous conduction: Action potential in one site causes action potential at the next location
  • Cannot go backwards because initial action potential site is depolarized yielding one-way conduction of impulse
  • Myelinated axons employ saltatory conduction
• Speed of conduction


  • Faster in myelinated than in non-myelinated
  • In myelinated axons, lipids act as insulation forcing ionic currents to jump from node to node
  • In myelinated, speed is affected by thickness of myelin sheath
  • Diameter of axons: large-diameter conduct more rapidly than small-diameter. Large have greater surface area and more voltage-gated Na+ channels
• Nerve fiber types


  • Type A: large-diameter, myelinated. Motor neurons supplying skeletal and most sensory neurons.
  • Type B: medium-diameter, lightly myelinated. Part of ANS.
  • Type C: small-diameter, unmyelinated. Part of ANS.
• Electrical synapses


  Cells connected by gap junctions that allow graded current to flow between adjacent cells
• Chemical synapses


  Components: Presynaptic terminal, Synaptic cleft, Postsynaptic membrane
• Neurotransmitter removal


  1. ACh: acetylcholinesterase splits ACh into acetic acid and choline. Choline recycled within presynaptic neuron.

  2. Norepinephrine: recycled within presynaptic neuron or diffuses away from synapse. Enzyme monoamine oxidase (MAO). Absorbed into circulation, broken down in liver.
• Spatial and temporal summation


  Many postsynaptic potentials combine in summation at the trigger zone. If threshold is reached, and action potential is triggered.