Neural Conduction and Synaptic Transmission (76- 100)
Resting Membrane Potential
- (The Lizard, a Case of Parkinson’s Disease) Roberto Garcia d’Orta his symptoms from Parkinsons changed
him from an active, vigorous business man to a lizard. He had tremors, rigid muscles, a marked poverty of
spontaneous movements, difficulty in starting to move, and slowness in executing voluntary movements once
they have been initiated. Reptile stare describes the characteristic lack of blinking and the widely opened eyes
gazing out of a motionless face, a set of features that seems mre reptilian than human. In his brain a small group
of nerve cells called the substantia nigra were dying, therefore the amount of dopamine delivered to the striatum
- Although dopamine levels are low in Parkinson’s disease, dopamine is not an effective treatment because it
does not readily penetrate the blood-brain barrier. L-dopa the chemical precursor of dopamine, which readily
penetrates the blood-brain barrier and is converted to dopamine once inside the brain, is an effective treatment.
Mr. d’Orta was prescribed this and his symptoms diminished.
- The membrane potential is 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 extracellular fluid. The intracellular electrodes are called
microelectrodes. When both tips are in the extracellular fluid, the boltage difference between them is zero,
however when the tip of the intracellular electrode is inserted into a neuron, a steady potential of about -70mV
is recorded, this is known as the neuron’s resting potential. In its resting state with the -70mV charge built up
across its membrane, a neuron is said to be polarized.
- The salts in neural tissue separate into positively and negatively charged particles called ions. The resting
potential results from the ratio of negative to positive charges being greater inside the neuron than outside. The
ions in neural tissue are in constant random motion, and particles in random motion tend to become evenly
distributed because they are more likely to move down their concentration gradients than up them. Any
accumulation of charges in one area tends to be dispersed by the repulsion among the like charges in the
vicinity and the attraction of opposite charges concentrated elsewhere. The four kinds of ions that contribute to
resting potential are Na
, and various negatively charged protein ions.
The concentrations of Na
ions are greater outside a resting neuron than inside, whereas K
ions are more concentrated on the inside.
The negatively charged protein ions are synthesized inside the neuron and, for the most part, stay there.