week 2

• Synapse

  Specialized junction where neurons communicate by transmitting chemicals

  
• Chemicals are the main way neurons communicate
• Properties of Synapses

  • Reflexes are slower than conduction along an axon
  • Several weak stimuli produce a stronger reflex than one stimulus alone
  • When one set of muscles becomes excited, a different set becomes relaxed
• Sequence of events at a synapse

  1. Synthesis of neurotransmitters

  2. Stimulation of receptors

  3. Disposition of transmitter molecules

  
• Excitatory postsynaptic potential (EPSP)

  Graded depolarization resulting from a flow of sodium ions into the neuron
• Inhibitory postsynaptic potential (IPSP)

  Temporary hyperpolarization of a membrane, occurring when synaptic input selectively opens the gates for potassium ions to leave the cell or chloride ions to enter
• Excitation reaches the dendrite before inhibition

  Results in brief excitation of the dendrite
• Excitatory message reaches neuron 3 faster than inhibitory message

  Results in a burst of excitation (EPSP) in neuron 3, which quickly slows or stops
• Certain combinations of synapses summate with one another more strongly than others do
• The strength of a synapse can vary from one time to another
• Most neurons have a spontaneous firing rate, a periodic production of action potentials even without synaptic input
• EPSPs increase the frequency of action potentials above the spontaneous rate
• Cell X

  • Responds to "A or B"
  • Responds to "A and B"
• Axon from cell A or cell B stimulates cell X

  1. With 11 unit

  2. If threshold of cell X is 11, then cell X responds to "A or B"

  3. If threshold of cell X is 12, then cell X responds to "A and B"

  
• Researchers have discovered complexities that Sherrington did not anticipate
• Complexities discovered

  • Some synapses produce fast, brief effects
  • Others produce slow, long-lasting effects
  • Effect of two synapses at the same time can be more than double the effect of either one, or less than double
  • Certain combinations of synapses summate with one another more strongly than others do
  • The strength of a synapse can vary from one time to another
• The nervous system is indeed complex
• Spontaneous firing rate

  Periodic production of action potentials even without synaptic input
• EPSPs increase the frequency of action potentials above the spontaneous rate

  IPSPs decrease the frequency of action potentials below the spontaneous rate
• Transmission along an axon merely sends information from one place to another
• Synapses determine whether to send the message
• The EPSPs and IPSPs reaching a neuron at a given moment compete with one another, and the net result is a complicated, not exactly algebraic summation of their effects

  
• The summation of EPSPs and IPSPs can be regarded as a decision because it determines whether or not the postsynaptic cell fires an action potential
• Complex behaviors depend on the contributions from a huge network of neurons
• The synapse is the point of communication between two neurons
• Transmission through a reflex arc is slower than transmission through an equivalent length of axon, so Sherrington concluded that some process at the synapses delays transmission

  
• Graded potentials

  EPSPs and IPSPs
• Temporal summation

  Summation of graded potentials from stimuli at different times
• Spatial summation


  Summation of potentials from different locations
• Inhibition is more than just the absence of excitation. It is an active brake that suppresses excitation
• For effective functioning of the nervous system, inhibition is just as important as excitation
• EPSP

  Excitatory graded potential (depolarization)
• IPSP

  Inhibitory graded potential (hyperpolarization)
• An EPSP occurs when gates open to allow sodium to enter the neuron's membrane
• An IPSP occurs when gates open to allow potassium to leave or chloride to enter
• The EPSPs on a neuron compete with the IPSPs; the balance between the two increases or decreases the neuron's frequency of action potentials
• The great majority of synapses rely on chemical processes, which are much faster and more versatile than Sherrington or anyone else of his era would have guessed
• Applying the hormone adrenaline directly to the surface of the heart, the stomach, or the pupils produces the same effects as those of the sympathetic nervous system
• Elliott therefore suggested that the sympathetic nerves stimulate muscles by releasing adrenaline or a similar chemical
• Loewi's experiment demonstrated that nerves send messages by releasing chemicals