PSYC 2410 DE S12 Textbook Notes Chapter 4

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19 Aug 2012
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Chapter 4: Neural Conduction and Synaptic Transmission
The Lizard: A Case of Parkinson's Disease
Like many patients with Parkinson's Disease, Roberto looked much older than he actually was
Used to be an active, thriving business man, but now is like granite and shuffles around when he
moves
The first symptom had been tremors at rest - tremors that become worst when the muscle is relaxed
Other symptoms include rigid muscles, a marked poverty of spontaneous movements, difficulty
starting to move and slowness in executing voluntary movements once they have been initiated.
Term "reptililan stare" is often used to describe the characteristic face of Parkinson's disease - lack
of blinking, widely open eyes, gazing out, motionless face
Caused by a small group of neurons called the substantia nigra unaccountably dying.
oThese neurons produce dopamine, which helps control voluntary movement.
oWithout dopamine, the brain begins to degrade in certain functions.
Although low dopamine levels are the cause of Parkinson's Disease, administering dopamine
directly is not a solution because dopamine does not readily cross the blood-brain barrier.
A knowledge of dopaminergetic transmitters has led to the development of an effective treatment.
oL-dopa is a precursor to dopamine which does readily cross the blood brain barrier
oL-dopa is converted to dopamine once within the brain
4.1: Resting Membrane Potential
Membrane Potential: The difference in electrical charge between the inside and the outside of the
cell.
Recording the Membrane Potential
oIn order to record the membrane potential of a cell, the tip of one electrode must be positioned
in the space outside the cell and the tip of another electrode must be placed inside a cell.
oThe electrode which penetrates the cell membrane must be fine enough to pierce it without
causing severe damage.
oMicroelectrodes: Fine-tipped electrodes whose tips are less than one-thousandth of a
millimeter in diameter. Microelectrodes are too small to be seen by the naked eye.
Resting Membrane Potential
oThe difference in potential in the extracellular fluid is 0
oThe difference in potential between the extracellular fluid and the internal environment of a
neuron is -70 mV. (The resting neuron has about 70mV less electric potential energy than the
extracellular fluid outside the cell)
oResting Potential: The steady membrane potential of a neuron at rest, usually about -70mV.
oWhen a neuron has a charge built up across its membrane in this manner, it is said to be
polarized.
Ionic Basis of the Resting Potential
oIons: Positively or negatively charged particles. An ion is formed when a particle gains or loses
electrons and hence alters their electric charge. A positive ion has lost electrons, while a
negative ion has gained electrons.
oSalts in solution naturally separate into positively and negatively charged ions.
oHow charge is distributed evenly across the membrane can be explained by 2 reasons:
Random Motion: Ions in neural tissue are in constant motion and particles in random motion
tend to become evenly distributed because they tend to move from areas of dense
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concentration to areas of low concentration.
Electrostatic Pressure: Any accumulation of charges in one area tends to be dispersed by the
repulsion among like charges in the vicinity and the attraction of opposite charges
concentrated elsewhere.
oDespite homogenization, no single class of ions is distributed equally on the two sides of the
membrane.
oFour kinds of ions contribute significantly to the resting potential of the neuron:
Sodium Ions (Na+)
Concentration is greater outside a resting neuron than inside
Symbol developed from latin name for Sodium, Natrium
Na+ ions tend to be driven into the neurons by both the high concentration of Na+ ions
outside the neuron and the negative internal resting potential of -70 mV.
The membrane of the neuron is resistant to the passive diffusion of Na+
Sodium-potassium pumps are able to maintain a high external concentration of Na+ by
pumping them out at the same rate they flow in.
Potassium Ions (K+)
Concentration is greater inside a resting neuron than outside.
Symbol developed from latin name for Potassium, Kalium
K+ ions tend to move out of the neuron because of their high internal concentration.
oThis tendency is partially offset since the inside of the neuron has a negative
potential, which attracts positively charged potassium ions.
The membrane of the neuron offers little passive resistance to the flow of K+ ions from
the inside to the outside, so they flow out of the cell body at a fairly substantial rate.
To maintain the high concentration of K+ ions, the sodium-potassium pumps in the cell
membrane pump K+ ions into the neuron at the same rate as they move out.
Chlorine Ions (Cl-)
Concentration is greater outside a resting neuron than inside
Passage of Cl- is not inhibitted by the neural membrane.
Cl- ions are actively forced out of neuron because of its negative charge of -70mV.
As Cl- ions accumulate in the area directly outside the cell, they begin to move down
their concetration gradient back into the neuron.
The neuron is in equilibrium when the electrostatic pressure for Cl- ions to move out of
the neuron is equal to the tendency for them to move down the gradient back into the
cell.
oThe equilibrium point occurs at -70mV.
Various negatively charged protein ions
Most negatively charged protein ions are synthesized in the neuron and remain there.
oTwo properties are responsible for the uneven distribution of ions across the membrane
One property is passive and does not require energy, the other process is active and does
require energy.
Passive Property: In resting neurons, K+ and Cl- ions pass readily through the neural
membrane, whereas Na+ ions have trouble passing through it and negatively charged
proteins do not cross the resting membrane at all.
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Ion Channels: Specialized pores on the surface of the neural membrane which allow
the passage of a particular ion.
Active Property: K+ ions are continuously being drawn out of resting neurons, so the
neuron has to actively pump K+ ions back in at the same rate. Similarly, Na+ ions are
continuously being pulled into the resting neuron, so the neuron has to actively pump Na+
out of the cell at the same rate.
Energy-consuming mechanisms in the cell's membrane actively exchange Na+ ions from
within the cell with K+ ions from outside of cell.
Sodium Potassium Pumps: Specialized mechanisms on the cell membrane which
transport Na+ ions out of the cell and K+ ions into the cell.
Transporters: Mechanisms in the membrane of a cell which actively transport ions or
molecules across the membrane.
4.2: Generation and Conduction of Postsynaptic Potentials
When neurons fire they release their terminal button chemicals, called neurotransmitters.
Neurotransmitters spread out across the synaptic cleft and bind with specialized receptor molecules.
Receptor molecules are located on the receptive membranes of the next neuron in the circuit (ie. not
the sending neuron)
When a neurotransmitter binds to a receptor molecule it can generally have one of two effects:
oDepolarization: Occurs when a neurotransmitter binds to a receptor molecule, with the effect
of decreasing the resting membrane potential. For example, from -70 mV to -67 mV.
Excitatatory Postsynaptic Potentials (EPSPs): Postsynaptic depolarizations.
Depolarization makes the neuron more likely to fire.
Travel passively from their sites of generation at synapses like electricity through a
cable.
oHyperpolarization: Occurs when a neurotransmitter binds to a receptor molecule with the
effect of increasing the resting membrane potential. For example, from -70 mV to -72 mV.
Inhibitory Postsynaptic Potentials (IPSPs): Postsynaptic hyperpolarizations.
Hyperpolarization reduces the liklihood that a neuron will fire.
Travel passively from their sites of generation at synapses like electricity through a
cable.
Graded Response: The amplitudes of both EPSPs and IPPs are proportional to the intensity of the
signals that elicit them.
oWeak signals elicit small postsynaptic potentials.
oStrong signals elicit large postsynaptic potentials.
oOpposite to all-or-none responses.
oGraded responses can vary depending on the strength of the signal, where as all-or-none
responses remain constant.
Transmission of postsnaptic potentials happens so quickly, it can be considered instantaneous for
most practical purposes.
It is important not to confuse duration with speed of transmission.
oA neuron can fire continuously for a long duration but will still have a fast transmission speed.
oThe duration of firing varies widely from neuron to neuron, the transmission speed, by contrast,
does not. The transmission speed remains much more constant and nearly instantaneous.
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