Chapter 4 11/30/2013 4:56:00 PM
Membrane potential is the difference in electrical charge between the
inside and outside of a cell.
Recording Membrane Potential
Must position the tip of one electrode inside the neuron and the tip of
another electrode outside the neuron in the extracellular fluid.
Intracellular electrode must be thin and precise: Microelectrode
Resting Membrane Potential
If both electrodes are in the ECF: Potential= 0mV
If electrode is in the intracellular neuron:Potential= -70mV
KNOWN as Resting Potential of a Neuron
Ionic Basis of the Resting Potential
Salts in neural tissues separate into positive and negative ions
Potential is a result of an IMBALANCE between +ve / -ve ions.
This occurs because…
1) Random Motion: ions in neural tissues are always in constant motion
2) Concentration Gradient: particles in random motion tend to become
evenly distributed because they move DOWN concentration gradients.
3) Electrostatic Pressure: accumulation of charges in one area tends to be
dispersed by the repulsion of like charges and attraction of opposite
Sodium (Greater OUTSIDE neuron)
Chlorine (Greater OUTSIDE neuron)
Potassium (Greater INSIDE neuron)
Negatively Charged Proteins (Tends to not leave the inside)
Unequal distribution of ions occurs from two properties
1)Passive: No energy needed 2)Active: Energy is required
K+/Cl- pass readily
Na+ pass but with difficulty
Proteins do not pass at all
Na/K Pump allows for homeostasis of the neuron along with action
3 Na OUT
2 K IN
Generation & Conduction of Post-Synaptic Potentials
when a neuron fires, they release chemicals from the terminal bouton
these cross synaptic cleft and interact with specialized receptors
When these molecules bind they either
DEPOLARIZE (decrease membrane potential)
HYPERPOLARIZE (increase membrane potential)
Post-synaptic depolarizations are called excitatory post-synaptic
potentials (EPSPs). INCREASE likelihood of firing
Post-synaptic hyperpolarizations are called inhibitory post-synaptic
potentials (IPSPs). DECREASE likelihood of firing.
Both graded responses because the amplitudes of EPSPs/IPSPs are
proportional to their intensities
WEAK SIGNALSSMALL POTENTIAL
STRONG SIGNALSLARGE POTENTIAL
action potentials are generated ADJACENT to the axon hillock If the SUM of the depolarizations and hyperpolarizations reaching this
section at any time is sufficient to DEPOLARIZE the membrane.
Threshold of excitation -65mV
this action potential is STRONG but MOMENTARY
all or none response
They occur to their Full Extent or Not at All
Adding all Potentials is known as integration
o EPSPs produced simultaneously= GREATER EPSPs
o IPSPs produced simultaneously= GREATER IPSPs
o EPSPs + IPSPs simultaneously= CANCELLED OUT
o Shows how post-synaptic potentials produced in rapid
succession at the same synapse sum to form a greater signal.
o Ex. One fires, before it dies off it fires again. Makes a stronger
Conduction of Action Potentials
Voltage-activated ion channels ope/close in the response to changes in the
level of the membrane potential.
There is a brief period about 1-2 milliseconds after the initiation of an
action potential during which it is impossible to elicit a second one
ABSOLUTE Refractory Period
This is followed by the RELATIVE Refractory Period
The period during which it is possible to fire a neuron again, but
only by firing with a HIGHER stimulation then the one previous.
1) Responsible for the fact that action potentials normally travel along axons
in only ONE DIRECTION. CANNOT reverse direction, allowing a refractory
2) Refractory period is responsible for the fact that the rate of neural firing is
related to the intensity of the stimulation. If a neuron is subjected to a high level of continual stimulation, it
fires and then fires again after the absolute period.
If a neuron is subjected to a level just sufficient enough to fire, it
won’t fire again until the absolute AND relative refractory periods
The conduction of action potentials along an axon is NONdecremental;
Aps do not grow weaker as they travel along the axonal membrane they are
Action potentials are conducted more slowly than post-synaptic potentials
This is due to the fact that EPSPs/IPSPs are PASSIVE and action
potentials are ACTIVE.
Antidromic Conduction: If electrical stimulation of sufficient intensity is
applied to the terminal end of the axon, an AP will be generated and will
travel along the axon back to the cell body.
Orthodromic Conduction: Axonal conduction in the NATURAL direction –
Cell BodyTerminal Boutons
Conduction in Myelinated Axons
axons of many neurons are insulated from ECF by fatty tissues = MYELIN
In myelinated axons, ions pass through the axonal membrane only at
NODES OF RANVIER.
The transmission of action potentials in myelinated axons is called
SALTATORY CONDUCTION. (Jumping)
LARGER myelinated axons conduct at faster speeds.
SMALLER myelinated axons conduct at slower speeds.
Internuerons do not have axons, conduction in these is typically passive
Hodgkin-Huxley Model Many cerebral neurons fire continuously even when they receive no input.
Axons of some cerebral neurons can actively conduct both graded and
Many cerebral neurons have NO axons and do not display action
The dendrites of some cerebral neurons can actively conduct action
CEREBRAL NEURONS= MORE COMPLEX THAN MOTOR NEURONS Chapter 4 11/30/2013 4:56:00 PM
Structure of Synapses
Axodendritic synapses of axon terminal boutons to dendrites. Many
axodendritic synapses terminate on dendritic spines.
Axosomatic Synapses of axon terminal boutons on somas (cell bodies).
Dendrodendritic Capable of transmission in either direction.
Axoaxonic can mediate presynaptic facilitation and inhibition.
Directed Synapses: synapses at which the site of neurotransmitter release
and the site of neurotransmitters reception is in CLOSE PROXIMITY
NON-Directed Synapses: are synapses at which the site of release is at
some distance from the site of reception.
In this type of arrangement, neurotransmitter molecules are
released from a series of varicosities along the axon and its
branches and thus are widely dispersed to surrounding targets.
Because of their appearance, they are reffered to as string-of-
Synthesis, Packaging and Transport of Neurotransmitters
Small Molecules: synthesized in cytoplasm, packaged in synaptic vesicles by
Large Molecules: (Neuropeptides) Short amino acid chains (~3)
Many neurotransmitters can produce 2 different effects on different
receptors. This is known as coexistence.
Exocytosis: the process of neurotransmitter s release.