PSYC271 Chapter 4: Neural Conduction and Synaptic Transmission
“The Lizard”: A case of Parkinson’s Disease
Parkinson’s disease caused by lack of dopamine production.
Prescribing dopamine does nothing because dopamine doesn’t readily penetrate the
bloodbrain barrier. Solution: Ldopa, a chemical precursor to dopamine that gets
converted to dopamine after passing through the bloodbrain barrier
4.1 Resting Membrane Potential
Membrane Potential: the difference in electrical charge between the inside and outside
of a cell
Recording Membrane Potential:
Microelectrodes: extremely fine recording electrodes, which are used for
One electrode inside body, other on the outside. Measures volt both sides.
Resting Membrane Potential:
Resting Potential: the steady membrane potential of a neuron at rest, usually
When both electrodes outside cell, voltage usually 0.
With a 70mV charge built up across its membrane, a neuron is said to be
Ionic Basis of the Resting Potential:
Ions: positively or negatively charged particles
Greater ratio of negative charges to positive charges inside the cell, than outside
it. This unequal distribution of charges and homogenization occurs due to:
Random Motion ▯ions in neural tissue are always in random motion.
Particles tend to become evenly distributed because are more likely to
move down concentration gradients (from areas of high concentration to
areas of low concentration)
Electrostatic Pressure ▯like charges repel each other and opposite
charges attract. Left with areas of opposite charges.
4 ions most important in resting potential: Sodium (Na+), Potassium (K+),
Chloride (Cl), various negatively charged ions
More Na+ and Cl ions outside neuron than inside. More K+ ions inside.
Proteins synthesized inside cell are negatively charged and tend to stay inside
Neural membrane partly responsible for this distribution of ions:
Differentiated Permeability (passiveno energy used) ▯In resting
neurons, K+ and Cl pass easily, Na+ with difficulty, and negative ions not
Ion Channels: pores in neural membranes through which specific ions
pass. Specialized to particular ions.
Hodgkin & Huxley found that the 70mV of electrostatic pressure keeps
out the negatively charged ions (keeps them from travelling to areas of
lower concentration). Alternatively, 90mV would be necessary to let K+
ions out of the neuron, but only 70mV available. For Na+, 120mV would
be necessary to push ion inside the neuron. Passive factors constantly drive K+ ions out of the neuron and Na+ ions
in. Therefore, K+ ions must be actively pumped in and Na+ ions must be
actively pumped out to maintain resting equilibrium.
Active mechanisms in the cell membrane counteract the influx of Na+
ions by pumping Na+ ions out as rapidly as they pass in. Counteract efflux
of K+ ions by pumping K+ ions as rapidly as they pass out.
SodiumPotassium Pump: active transport mechanisms that pump Na+ ions
out of neurons and K+ ions in. Found in cell membrane and constantly exchanges
three Na+ ions inside the neuron for two K+ ions outside.
4.2 Generation and Conduction of Postsynaptic Potentials
When neurotransmitter attaches to postsynaptic receptor, can have one of 2 effects
depending on structure of both neurotransmitter and receptor:
1. Depolarize: decrease the resting membrane potential
Excitatory Postsynaptic Potentials (EPSPs): postsynaptic depolarizations,
which increase the likelihood that an action potential will be generated
2. Hyperpolarize: increase the resting membrane potential
Inhibitory Postsynaptic Potentials (IPSPs): postsynaptic hyperpolarizations,
which decrease the likelihood that an action potential will be generated
Graded Responses: responses whose magnitude is indicative of the magnitude of the
stimuli that induce them. (ex. Weak signals elicit small postsynaptic potentials, strong
signals elicit large postsynaptic potentials). This is true for both EPSPs & IPSPs
Important: whether EPSPs or IPSPs, both cause reactions the instant they hit the
Transmission of EPSPs and IPSPs is decremental ▯decrease in amplitude as they travel
through the neuron. Most do not make it far into the axon
4.3 Integration of Postsynaptic Potentials and Generation of Action Potentials
Most postsynaptic potentials created at a single synapse have little to no effect on the
firing of the postsynaptic neuron (doesn’t get it excited enough).
Multiple neurotransmitters can be released into a synapse and attach to multiple
posysynaptic receptors. If the sum of EPSPs + IPSPs that reach the first part of the axon
is low enough voltage to excite the neuron (65mV), an action potential will be generated
and the message will be passed on to the next neuron.
Threshold of Excitation: the level of depolarization necessary to generate an action
potential, usually about 65mV
Action Potential (AP): a massive momentary reversal of a neuron’s membrane
potential from about 70mV to about +50mV. Allornone response.
AllorNone Response: nongraded responses. Either occur to their full extent or not at
Integration: adding or combining a number of individual signals into one overall signal
(ex. Sum of EPSPs and IPSPs before generating action potential)
Neurons integrate signals in 2 ways:
Spatial Summation: the integration of signals that occur at different sites on the
neuron’s membrane. Three possible outcomes:
1. Two simultaneous EPSPs sum to produce greater EPSP 2. Two simultaneous IPSPs sum to produce greater IPSP (more ve)
3. Simultaneous IPSP and EPSP cancel each other out
Temporal Summation: the integration of neural signals that occur at different
times at the same single synapse.
If particular synapse is activated and then activated again before the
original postsynaptic potential has completely dissipated, the effect of the
second stimulus will be added to the postsynaptic potential produced by
Integration of signals is always occurring because a single neuron has thousands of
synapses and dendrites.
Which dendrite or synapse the signal hits has no effect on its being more/less
influential. Whether or not dendrite or synapse is located closer to axon – doesn’t matter.
Neuron firing similar to gun firing. Graded response as neuron is stimulated (becomes
less polarized until the threshold of excitation reach) much like a gun’s trigger is slowly
pulled back. The actual firing is an allornone event, firing the AP/bullet either happens
or doesn’t, the speed or amplitude of firing can’t be influenced.
4.4 Conduction of Action Potentials
Ionic Basis of Action Potentials:
VoltageActivated Ion Channels: ion channels that open/close in response to
changes in the level of the membrane potential
Membrane potential for resting neuron remains relatively the same.
However when membrane potential gets reduced to the threshold of excitation,
sodium channels in the axon membrane open and allow Na+ ions to rush in.
Membrane potential suddenly increase from 70 to +50mV, triggering K+
channels to open. K+ rush out of the cell.
‘Rising phase’ ▯after 1 millisecond of opening, Na+ channels close.
Repolarization ▯Potassium channels gradually close. Because of this, too many
K+ escape, leaving the cell hyperpolarized. Eventually, original conditions are
Note: this all happens so quickly that only ions closet to the membrane (both
inside the cell and out) are involved in this exchange. Since the majority of ions
inside and outside the cell are unaffected, restoring original conditions isn’t very
Absolute Refractory Period: a brief period after the initiation of an action
potential during which it is impossible to elicit another action potential in the
Relative Refractory Period: a period after the absolute refractory period during
which a higherthannormal amount of stimulation is necessary to make a neuron
If neuron is subject to high level continual stimulation, it will fire after a relative
refractory period. If lower level, must wait for absolute refractory period to be
Axonal Conduction of Action Potentials: Action potentials are nondecremental & are conducted more slowly than
postsynaptic potentials (because an active process)
Antidromic Conduction: Axonal (action potential) conduction in the
backwards direction – from terminal buttons to cell body.
Orthodromic Conduction: Axonal (action potential) conduction in the natural
direction – from cell body to terminal buttons.
Conduction in Myelinated Axons:
In myelinated axons, ions can only pass through the nodes of Ranvier
(unmyelinated segments). This means that the signal passes through a node,
reaches a myelinated area, and must create another action potential to get through
sodium pumps. This is done, and signal travels to next node. Same thing happens
when it reaches the next myelinated area. Continues until terminal buttons are
When dealing with myelinated axons, action potentials are conducted passively
and decrementally. By the time the signal travels from the node to the next myelin