Membrane potential

• Resting membrane potential


  • 70 mV, steady state where a neuron is polarized
• How neurons work


  1. Malfunction

  2. Recording
• Recording neuron's membrane potential

  1. Position intracellular electrode inside neuron

  2. Position extracellular electrode outside neuron

  3. Measure voltage difference
• Microelectrodes


  Intracellular electrodes
• When both electrode tips are in extracellular fluid, the voltage difference is 0
• When the intracellular electrode tip is inserted into a neuron, the resting potential is -70 millivolts
• The potential inside the resting neuron is about 70 mV less than outside the neuron
• Ions


  Salts in neural tissue that separate into positively and negatively charged particles
• Ions in neural tissue

  • Sodium
  • Potassium
• In resting neurons, there are more sodium ions outside the cell than inside
• In resting neurons, there are more potassium ions inside than outside
• Ion channels


  Pores in neural membranes through which ions can pass
• Electrostatic pressure


  Causes sodium ions to enter resting neurons due to opposite charges attracting
• Random motion


  Causes sodium ions to move down their concentration gradient from high to low concentration
• The sodium ion channels in resting neurons are closed, greatly reducing the flow of Na+ ions into the neuron
• In resting neurons, the potassium channels are open but only a few K+ ions exit because they are largely held inside by the negative resting membrane potential
• Sodium-potassium pumps


  Transporters that continually exchange 3 Na+ ions inside the neuron for 2 K+ ions outside
• Neurotransmitters


  Released from terminal buttons of neurons, diffuse across synaptic clefts, and interact with receptor molecules
• Depolarization


  Decreasing the resting membrane potential, increasing the likelihood of the neuron firing (excitatory postsynaptic potentials)
• Hyperpolarization


  Increasing the resting membrane potential, decreasing the likelihood of the neuron firing (inhibitory postsynaptic potentials)
• Postsynaptic potentials have two important characteristics: they are rapid and decremental (decrease in amplitude as they travel through the neuron)
• How postsynaptic potentials are generated

  1. Summate over space

  2. Summate over time
• Axon hillock


  Specialized region where the cell body meets the axon, where electrical signals are gathered before being transmitted down the axon
• Action potentials are all-or-none responses, their magnitude is not related to the intensity of the stimuli that elicit them
• Integration


  Adding or combining a number of individual signals into one overall signal
• Spatial summation


  How local EPSPs and IPSPs sum to form greater signals
• Temporal summation


  How postsynaptic potentials produced in rapid succession sum to form a greater signal
• How action potentials are produced and conducted

  1. Through the action of voltage-activated ion channels

  2. Absolute refractory period

  3. Relative refractory period
• Action potentials can only travel along axons in one direction due to the refractory period
• The refractory period is responsible for the rate of neural firing, related to the intensity of stimulation
• EPSP and IPSP


  Crucial in determining whether a neuron will fire an action potential
• Myelinated axons


  Ions can only pass through the axonal membrane at the nodes of Ranvier
• How action potentials are transmitted in myelinated axons


  Generated at first node, then passively conducted to next node where another action potential is elicited
• Myelination increases the speed of axonal conduction
• Saltatory conduction


  The transmission of action potentials in myelinated axons
• Action potential conduction speed depends on axon diameter and myelination
• Many neurons in the mammalian brain either do not have axons or have very short ones, and do not display action potentials, with conduction being passive and decremental
• Properties of cerebral neurons not shared by motor neurons


  • Fire continually even without input
  • Can actively conduct both graded signals and action potentials
  • Action potentials vary greatly in duration, amplitude, and frequency
  • Many do not display action potentials
  • Dendrites can actively conduct action potentials
• Axodendritic synapses


  Synapses where the axon terminal buttons contact the dendrites
• Axosomatic synapses


  Synapses where the axon terminal buttons contact the cell bodies