Nervous system
 
  1. Neuron structure
    1. cell body, dendrites, one or more axons
      1. dendrites highly branched
      2. number of dendrites varies among neurons
      3. many brain cells lack axons
      4. Central nervous system (CNS): brain & spinal cord
      5. Peripheral nervous system (PNS): all parts of nervous system outside of CNS
    2. Neuron function
      1. information transmitted along neuron as electrical impulse
        1. speed related to diameter of axon
        2. speed in vertebrates increased by mylination (see below)
      2. sensory cells receive information
        1. internal or external environments monitored
        2. sensory cells may or may not be nerve cells
          1. CNS integrates information from many sensory cells
          2. CNS stimulates a motor neuron
            1. signals go to effector cells
          3. reflexes go to spinal cord but not brain
  2. Membrane potential
    1. Cells are inherently electrical
      1. ions carry an electrical charge
      2. membrane potential: different concentrations of positive and negative ions across a membrane
        1. ions on either side of the membrane have potential energy
      3. ions move across membranes in response to both electrical and concentration gradients
        1. electrochemical gradient
      4. the flow is an electric current
    2. Resting potential
      1. neuron membranes at rest have voltage: resting potential
        1. primary positive charged ion inside is potassium
        2. extra cellular fluid has high concentrations of sodium and chloride ions
      2. Resting potential due to electrochemical gradient of cell at rest
        1. potassium ion concentration inside cell is a balance of electrical and chemical gradients
        2. resting potential typically -70 mV because some sodium also moves into cell
          1. if the situation was unchecked, there would be a steady efflux of potassium and influx of sodium
          2. sodium potassium ATPase maintains the gradients
    3. Changes in membrane potential produce nerve impulses
      1. all cells have membrane potential
      2. only certain cells are excitable
        1. capable of generating large changes in membrane potential
      3. Changes in membrane potential are made possible by special ion channels
        1. gated ion channels
          1. chemically-gated ion channels: open or close in response to chemical stimulus
          2. voltage-gated ion channels: open or close in response to change in membrane potential
        2. A change in membrane potential is a localized event at a particular point of the membrane
          1. some stimuli produce hyperpolarization (opening potassium channel)
          2. some produce depolarization (opening sodium channel)
        3. action potential: a rapid depolarization of the membrane
        4. threshold potential
          1. level of depolarization causing an action potential
          2. typically about 15-20 mV more positive than resting (about -50 to -55 mV in an axon)
          3. hyperpolarization does not produce action potentials
          4. sodium and potassium voltage-gated channels have staggered response
        5. refractory period
          1. sodium channels not responsive to stimulus
      4. Nerve impulses propagate themselves along an axon
        1. the action potential does not actually travel but is repeatedly regenerated
        2. sodium entering the cell creates an electrical change that depolarizes neighboring membrane
        3. depolarization exceeds threshold: action potential "downstream"
        4. depolarization "backwards" does not produce action potential
      5. mylination increases speed of propagation along axon
        1. saltatory conduction
        2. action potential appears to "jump" between unmylinated regions (nodes of Ranvier)
  3. Signal communication between cells
    1. synapses are unique cell junctions controlling communcation between neuron and another cell
      1. found between neurons and: other neurons, sensory cells, muscle cells, gland cells
    2. synapses between neurons conduct signals from axon to dendrite or cell body of next neuron
      1. Electrical synapse: local ion currents of an action potential move from one cell to the next
        1. very rapid, no signal degradation
      2. Chemical synapse: much more common in vertebrates, most invertebrates
        1. synaptic cleft separates presynaptic and postsynaptic cells
          1. postsynaptic membrane may be dendrite or cell body
        2. synaptic vesicles in axon tip contain neurotransmitters
        3. presynaptic neuron releases neurotransmitter into synapse when action potential arrives
        4. neurotransmitter molecules diffuse to postsynaptic membrane
        5. specialized receptor proteins on postsynaptic membrane
          1. chemically gated ion channels
        6. binding of neurotransmitter to receptors alters membrane potential of postsynaptic cell
          1. depolarization or hyperpolarization
  4. neural integration occurs at the cellular level
    1. a single neuron receives information for many neighbors via thousands of synapses
      1. synapses may be excitatory or inhibitory
        1. excitatory synapse: neurotransmitter causes depolarization
        2. inhibitory synapse: neurotransmitter causes hyperpolarization
      2. both are graded potentials in response to number of molecules binding to receptors
    2. summation: additive affect of several synaptic terminals on same postsynaptic cell is cumulative
    3. if potential in axon hillock reaches threshold, action potential initiated
Study Questions

1.  Will diffusion across a semipermiable membrane with no active transport result in equality of charge as well as of chemical concentrations?

2.  Which of the following ions are actively transported,  which are passively channeled, and which do not move across a nerve cell membrane?  sodium (Na+), potassium (K+), chloride (Cl-), anionic macromolecules.

3.  Make sure you understand how the electrochemical status of the nerve cell membrane is maintained.

4.  A patch-clamp is a devise that allows membrane physiologists to isolate individual channels and pumps in a cell membrane.  In the comparisons described in the following experimental protocol, determine which variables are manipulated, controlled for, measured, or not manipulated, measured or controlled for.

A cell physiologist isolates a series  neurons from the spine of a zebra fish (an organism recently developed as a new model system for development and genetics).  She succeeds in keeping three motor neurons and three sensory neurons alive.  In each neuron, she uses a patch clamp to isolate a potassium channel and monitors the flux of potassium across the membrane following stimulus of an action potential.  The proceedure allows her to test the response of the neurons to different stimuli, investigating which neurotransmitters initiate action potentials in each cell.  She repeats this for three to five channels at different positions in each surviving neuron.  

Variables:  type of neuron, type of channel, neurotransmitter, genotype of cell

Comparison:  compare channels within the same neuron as they do or do not show response to a neurotransmitter

Comparison:  compare channels among the motor neurons as they do or do not show response to a neurotransmitter

Comparison:  compare channels between motor and sensory neurons as they do or do not show response to a neurotransmitter

5.  How is the response of the postsynaptic cell different if it is a nerve cell or a skeletal muscle cell?
A.  the nerve cell shows signal summation and the skeletal muscle does not
B.  the nerve cell may have depolarization or hyperpolarization in response, the muscle cell always depolarizes
C.  the nerve cell will always depolarize, the muscle cell will always hyperpolarize
D.  the nerve cell will respond to various neurotransmitters, the muscle cell only responds to acetylcholine
E.  the nerve cell has voltage-gated channels in the postsynaptic membrane, the muscle cell has ligand-gated (chemically-gated) channels in the postsynaptic membrane

6.  What are the potential advantages and disadvantages of the chemically-mediated synapse compared to the electrical transmission of information?  

7.  Explain in your own words the circumstances under which the axon hillock will or will not initiate an action potential following transmission of a signal by two presynaptic cells, one inhibitory and one excitatory.  Your answer should include the significance of repeated transmission, timing between repeats, and the physical distance of the synapsis from the axon hillock.

8.  There are neurodegenerative diseases in which the Swann cells do not form a mylin sheath.  How might this affect the function of the motor neuron?

See also:

Content review #1-5, Concept review #1, 2, 4;  Applying ideas #2, 3

Answers