Neural Conduction and Synaptic Transmission

• Substantia nigra

  • neurons that produce dopamine, which is delivered to the striatum
  • as these neurons die, the amount of dopamine they can deliver to the cells in the striatum goes down
• the striatum helps control movement, and needs dopamine to do so
• dopamine does not readily penetrate the blood-brain barrier
• Dopaminergic transmission has let to the development of an effective treatment: L-dopa, the chemical precursor of dopamine, which penetrates the blood-brain barrier and is converted to dopamine once inside the brain
• Membrane potential - the difference in electrical charge between the inside and the outside of a cell
• To record a neuron's membrane potential, it is necessary to position the tip of one electrode inside the neuron and the tip of another electrode outside the neuron in the ECM
• The tip of the intracellular electrode must be fine enough to pierce the neural membrane without damaging it
• microelectrodes - intracellular electrodes
• the tips of microelectrodes are less than one-thousandth of a millimeter in diameter
• When both electrode tips are in the extracellular fluid, the voltage difference between them is zero
• when the tip of the intracellular electrode is inserted into a neuron that is at rest, a stead potential of about -70mV is recorded
• at rest - not receiving signals from other cells
• The potential inside the resting neuron is about 70mV less than the outside of the neuron
• resting potential - the steady membrane potential of about -70mV
• in the resting state of a membrane, with the −70 mV charge built up across its membrane, a neuron is said to be polarized
• polarized - has a membrane potential that is not zero
• ions - positively or negatively charged particles
• there are many different kinds of ions in neurons
• Sodium ions (Na+) - natrium
• Potassium ions (K+) - kalium
• In resting neurons, there are more Na+ ions outside the cell than inside
• There are more K+ ions inside the cell than outside, in resting neurons
• Ion channels are specialized pores in the neural membrane that maintain the unequal distributions of Na+ and K+ ions
• Each type of ion channel is specialized for the passage of a specific ion
• There are 2 types of pressure on Na+ ions to enter the resiting neurons

  1. electrostatic pressure from the resting membrane potential (bc opposite charges attract, the positively charged Na+ ions are attracted to the -70mV charge inside resting neurons)
  2. Pressure from random motion for Na+ ions to move down their concentration gradient
• Why do Na+ ions under electrostatic pressure and pressure from random movement not come rushing into neurons, thus reducing the resting membrane potential?

  The sodium ion channels in resting neurons are close, thus reducing the flow of Na+ ions into the neuron
• The potassium ion channels are open in resting neurons, but only a few K+ ions exit because the electrostatic pressure that results from the negative resting membrane potential holds them inside
• 3 factors that influence the distribution of Na+ and K+ ions across neural membranes

  1. Ioins in motion move down their concentration gradients, thus Na+ will tend to enter and K+ will tend to exit
  2. The negative internal charge creates pressure for both Na+ and K+ to enter
  3. Na-K pumps transport 3Na+ out for every 2K+ they transport in
• Why does the resting membrane potential stay fixed?

  • studied by Alam Hodgkin and Andrew Huxley
  • bc sodium-potassium pumps
• At the same rate that Na+ ions leaked into resting neurons, other Na+ ions were actively transported out; and at the same rate that K+ ions leaked out of resting neurons, other K+ ions were actively transported in
• Na-K pumps

  • a mechanism in the cell membrane that continually exchanges 3 Na+ ions inside the neuron for 2 K+ ions outside
• transporters - mechanisms int he membrane of a cell that actively transport ions or molecules across the membrane
• disturbances of the membrane potential occur as a result of input from cells that synapse on a neuron
• Postsynaptic potentials (PSPs) - disturbances of the resting membrane potential
• When neurons fire, they release from their terminal buttons chemicals called neurotransmitters, which diffuse across the synaptic clefts and interact with specialized receptor molecules on the receptive membranes of the next neuron in the circuit.
• When neurotransmitter molecules bind to postsynaptic receptors, they may depolarize or hyperpolarize, depending on the neurotransmitter, receptor, and postsynaptic neuron
• Depolarize - decrease the resting membrane potential from -70mV to -67mV *for example)
• Hyperpolarize - increase the resting membrane potential
• Postsynaptic depolarizations are called excitatory postsynaptic potentials (EPSPs) because they increase the likelihood that the neuron will fire
• Postsynaptic hyperpolarizations are called inhibitory postsynaptic potentials (IPSPs) because they decrease the likelihood that the neuron will fire