PS263 – Neural Conduction & Synaptic Transmission
Resting Membrane Potential
Membrane Potential: The difference in electrical charge between the inside
and the outside of the cell.
Recording M.P: Position the tip of one electrode inside the neuron & the tip of
another outside the extracellular fluid.
Microelectrodes: (intracellular) their tips are less than one-thousandth of a
mm in diameter – much to small to be seen with the naked eye.
Resting Potential: The steady membrane potential of about -70mV – with this
charge built up across its membrane, a neuron is said to be polarized.
Ion Basis: The salts in neural tissue separate into + and – charged particles
called ions – resting potential results from the fact that the ratio of negative
to positive charges is greater inside the neuron then outside
Random Motion: (homogenizing) Ions in normal tissue are in constant
random motion, particles in random motion tend to become evenly
distributed because they are more likely to move down their C.G than up.
Electrostatic Pressure: (homogenizing) any accumulation of charges + or – 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.
Sodium Potassium Pumps: The transport of Na+ ions out of neurons & the
transport of them are not independent processes. Performed by energy-
consuming mechanisms in the cell membrane that continually changes 3 Na+
ions inside two K+ ions outside.
Transporters: Mechanisms in the membrane of a cell that actively transport
ions or molecules across the membrane.
Conduction of Postsynaptic Potentials
When neurons fire they release neurotransmitters from their terminal
buttons which diffuse across the synaptic clefts & interact with specialized
receptor molecules on receptive membranes of the next neurons in circuit.
Depolarize: NT’s may decrease the resting membrane potential from -70 to
-67 mV; Excitatory Postsynaptic Potentials (EPSPs): they increase the
likelihood that the neuron will fire. (travel passively)
Hyperpolarize: It increases the resting membrane potential from -70 to -72
mV; Inhibitory Postsynaptic Potentials (IPSPs): decrease the likelihood that
the neuron will fire. (travel passively)
Both potentials are graded responses: the amplitudes are proportional to
the intensity of the signals that elicit them: Weak signals elicit small & vice.
1. Rapid – instantaneous for most purposes.
2. Decremental – IPSPs & EPSPs decrease in amplitude as they travel through
the neuron – most do not travel more than a couple of mm
Postsynaptic Potentials & Action Potentials
Threshold of Excitation: If the sum of depolarizations & hyperpolarizations
reaching the section of the axon adjacent to the axon hillock at any time is
sufficient to depolarize the membrane to a level.
Axon Hillock: Conical structure at the junction between the cell body & the
axon) PS263 – Neural Conduction & Synaptic Transmission
Action Potential: Massive, but momentary (1 millisecond) reversal of the
membrane potential from -70 to about +50mV.
All-or-None Responses: Can either occur to their full extent or not at all.
Integration: Adding or combining a number of individual signals into one
Conduction of Action Potentials
Voltage-Activated Ion Channels: Ion channels that open/close in response to
change in the level of membrane potential.
Absolute Refractory Period: Brief period of about 1 to 2 milliseconds after
the initiation of an action potential during which it is impossible to elicit a
Relative Refractory Period: Period during which it is possible to fire the
neuron again but only applying higher-than-normal levels of stimulation.
Refractory Period is responsible for the fact that A.P.’s normally travel along
axons in only one direction because the portions of an axon over which an A.P.
has just travelled are left momentarily
First, the conduction of A.P along an axon is nondecremental; A.P do not
grow weaker as they travel along the axonal membrane.
Second, A.P are conducted more slowly than postsynaptic potentials – these 2
things occur because conduction of EPSPs and IPSPs are passive, whereas the
axonal conduction of A.P is largely active.
A.P travels passively along axonal membrane to the adjacent voltage-
activated sodium channels, which are yet to open. The arrival opens these
channels allowing Na+ to rush into a neuron and generation full A.P.
Antidromic Conduction: If electrical stimulation of sufficient intensity is
applied to the terminal end of an axon, an A.P will be generated and travel
along the axon back to the cell body.
Orthodromic Conduction: Axonal conduction in the natural direction from
cell body to terminal buttons – triggers exocytosis.
Nodes of Ranvier: Gaps between adjacent myelin segments – ions can pass
through the axonal membrane only here.
Saltatory Conduction: the transmission of action potentials in myelinated
* At what speed are action potentials conducted along an axon? 1) Conduction is
faster in large-diameter axons and faster for myelinated. Mammalian motor
neurons (that synapse on skeletal muscles) are large & myelinated thus some can
conduct at speeds of 100 meters per second where small one conduct at 1 m/sec.
- the maximum velocity of conduction in human motor neurons is about 60
meters per second.
Interneurons: conductions in these are typically passive and decremental.
Hodgkin-Huxley Model in Perspective – 1950s based on the study of squid
motor neurons, large and easily accessible in PNS (these neurons are LARGE)
Due to the simplicity of these neurons it is hard to apply to the mammalian
brain. Properties of c