postsynaptic
neuron
science-education.nih.gov
Synapse
axon of presynaptic
neuron
dendrite of
postsynaptic
neuron
bipolar.about.com/library
The Membrane
The membrane surrounds the neuron.
 It is composed of lipid and protein.

The Resting Potential
-
-
-
+
+
-
Resting potential of neuron = -70mV
-
+

+

There is an electrical charge across the membrane.
This is the membrane potential.
The resting potential (when the cell is not firing) is a
70mV difference between the inside and the outside.
+

outside
inside
Artist’s rendition of a typical cell membrane
Ions and the Resting Potential


Ions are electrically-charged molecules e.g. sodium (Na+),
potassium (K+), chloride (Cl-).
The resting potential exists because ions are concentrated on
different sides of the membrane.
Na+ and Cl- outside the cell.
 K+ and organic anions inside the cell.

Na
+
Na
Organic anions (-)
K+
Cl-
+
Na+
Na+
K
Organic anions (-)
+
Cl-
outside
inside
Organic anions (-)
Ions and the Resting Potential


Ions are electrically-charged molecules e.g. sodium (Na+),
potassium (K+), chloride (Cl-).
The resting potential exists because ions are concentrated on
different sides of the membrane.
Na+ and Cl- outside the cell.
 K+ and organic anions inside the cell.

Na
+
Na
Organic anions (-)
K+
Cl-
+
Na+
Na+
K
Organic anions (-)
+
Cl-
outside
inside
Organic anions (-)
Maintaining the Resting
Potential


Na+ ions are actively transported (this uses
energy) to maintain the resting potential.
The sodium-potassium pump (a membrane
protein) exchanges three Na+ ions for two K+
ions.
Na
Na+
+
Na+
outside
K+
K+
inside
Excitatory postsynaptic
potentials (EPSPs)
Opening of ion channels which leads to
depolarization makes an action potential more likely,
hence “excitatory PSPs”: EPSPs.


Inside of post-synaptic cell becomes less negative.
Na+ channels (NB remember the action potential)
Ca2+ . (Also activates structural intracellular changes ->
learning.)
Na+
Ca2+
-

+

outside
inside
Inhibitory postsynaptic
potentials (IPSPs)
Opening of ion channels which leads to
hyperpolarization makes an action potential less
likely, hence “inhibitory PSPs”: IPSPs.


Inside of post-synaptic cell becomes more negative.
K+ (NB remember termination of the action potential)
Cl- (if already depolarized)
Cl-
K+
-

+

outside
inside
Integration of information




PSPs are small. An individual EPSP will not produce
enough depolarization to trigger an action potential.
IPSPs will counteract the effect of EPSPs at the
same neuron.
Summation means the effect of many coincident
IPSPs and EPSPs at one neuron.
If there is sufficient depolarization at the axon
hillock, an action potential will be triggered.
axon hillock
Neuronal firing: the action
potential
The action potential is a rapid
depolarization of the membrane.
 It starts at the axon hillock and passes
quickly along the axon.
 The membrane is quickly repolarized to
allow subsequent firing.

Before Depolarization
Action potentials: Rapid
depolarization



When partial depolarization reaches the activation
threshold, voltage-gated sodium ion channels open.
Sodium ions rush in.
The membrane potential changes from -70mV to +40mV.
Na+
+
-
Na+
Na+
+
Depolarization
Action potentials: Repolarization



Sodium ion channels close and become refractory.
Depolarization triggers opening of voltage-gated
potassium ion channels.
K+ ions rush out of the cell, repolarizing and then
hyperpolarizing the membrane.
Na+
Na+
K
+
Na+
K+
K+
+
-
Repolarization
The Action Potential
The action potential is “all-or-none”.
 It is always the same size.
 Either it is not triggered at all - e.g. too little
depolarization, or the membrane is
“refractory”;
 Or it is triggered completely.

Course of the Action Potential
• The action potential begins with a partial depolarization (e.g. from
firing of another neuron ) [A].
• When the excitation threshold is reached there is a sudden large
depolarization [B].
• This is followed rapidly by repolarization [C] and a brief
hyperpolarization [D].
• There is a refractory period immediately after the action potential
where no depolarization can occur [E]
+40
Membrane
potential 0
(mV)
[C]
[B]
[E]
[A]
[D]
excitation threshold
-70
0
1
2
3
Time (msec)
Action Potential
Local Currents depolarize adjacent channels causing
depolarization and opening of adjacent Na channels
Question: Why doesn’t the action potential travel backward?
Conduction of the action
potential.




Passive conduction will ensure that adjacent
membrane depolarizes, so the action potential
“travels” down the axon.
But transmission by continuous action potentials
is relatively slow and energy-consuming
(Na+/K+ pump).
A faster, more efficient mechanism has evolved:
saltatory conduction.
Myelination provides saltatory conduction.
Myelination



Most mammalian axons are myelinated.
The myelin sheath is provided by oligodendrocytes and
Schwann cells.
Myelin is insulating, preventing passage of ions over
the membrane.
Saltatory Conduction



Myelinated regions of axon are electrically insulated.
Electrical charge moves along the axon rather than across the
membrane.
Action potentials occur only at unmyelinated regions: nodes of
Ranvier.
Myelin sheath
Node of Ranvier
Synaptic transmission



Information is transmitted from the presynaptic
neuron to the postsynaptic cell.
Chemical neurotransmitters cross the
synapse, from the terminal to the dendrite or
soma.
The synapse is very narrow, so transmission is
fast.
Structure of the synapse



An action potential causes neurotransmitter
release from the presynaptic membrane.
Neurotransmitters diffuse across the
synaptic cleft.
They bind to receptors within the
postsynaptic membrane, altering the
membrane potential.
terminal
extracellular fluid
synaptic cleft
presynaptic membrane
postsynaptic membrane
dendritic spine
Neurotransmitter release



Ca2+ causes vesicle membrane to fuse with
presynaptic membrane.
Vesicle contents empty into cleft: exocytosis.
Neurotransmitter diffuses across synaptic
cleft.
Ca2+
Ionotropic receptors (ligand gated)
Synaptic activity at ionotropic receptors
is fast and brief (milliseconds).
 Acetylcholine (Ach) works in this way
at nicotinic receptors.
 Neurotransmitter binding changes the
receptor’s shape to open an ion channel
directly.

ACh
ACh
Ionotropic Receptors
Postsynaptic Ion motion
Requirements at the synapse
For the synapse to work properly, six basic events need to happen:
 Production of the Neurotransmitters


Storage of Neurotransmitters




SV
Release of Neurotransmitters
Binding of Neurotransmitters


Synaptic vesicles (SV)
Lock and key
Generation of a New Action Potential
Removal of Neurotransmitters from the Synapse

reuptake
Motor Control Basics
• Reflex Circuits
– Usually Brain-stem, spinal cord based
– Interneurons control reflex behavior
– Central Pattern Generators
• Cortical Control
Hierarchical Organization of
Motor System
• Primary Motor Cortex and Premotor Areas
Primary motor cortex (M1)
Hip
Trunk
Arm
Hand
Foot
Face
Tongue
Larynx
postsynaptic
neuron
science-education.nih.gov
FlexorCrossed
Extensor
Reflex
(Sheridan
1900)
Reflex
Circuits
With
Inter-neurons
Painful Stimulus
Gaits of the cat: an informal computational model
Vision and Action
Cortical Motor System
Pre-motor cortex
Movement planning/sequencing
• Many projections to M1
• But also many projections directly into
pyramidal tract
• Damage => more complex motor
coordination deficits
• Stimulation => more complex mov’t
• Two distinct somatotopically organized
subregions
• SMA (dorso-medial)
• May be more involved in
internally generated movement
• Lateral pre-motor
• May be more involved in
externally guided movement
Somatotopy of Action Observation
Foot Action
Hand Action
Mouth Action
Buccino et al. Eur J Neurosci 2001
A New Picture
Rizzolatti et al. 1998
Somato-Centered Bimodal RFs in area F4
(Fogassi et al. 1996)
The fronto-parietal networks
Rizzolatti et al. 1998
F5c-PF
Rizzolatti et al. 1998
The F5c-PF circuit
Links premotor area F5c and parietal area PF (or 7b).
Contains mirror neurons.
Mirror neurons discharge when:
Subject (a monkey) performs various types of goalrelated hand actions
and when:
Subject observes another
similar kinds of actions
individual
performing
F5 Canonical Neurons
Murata et al. J Neurophysiol. 78: 2226-2230, 1997
Vision
Overview of the Visual System
Physiology of Color Vision
Two types of light-sensitive receptors
Cones
cone-shaped
less sensitive
operate in high light
color vision
Rods
rod-shaped
highly sensitive
operate at night
gray-scale vision
© Stephen E. Palmer, 2002
The Microscopic View
How They
Fire
• No stimuli:
– both fire at base rate
• Stimuli in center:
– ON-center-OFF-surround
fires rapidly
– OFF-center-ON-surround
doesn’t fire
• Stimuli in surround:
– OFF-center-ON-surround
fires rapidly
– ON-center-OFF-surround
doesn’t fire
• Stimuli in both regions:
– both fire slowly
Rods and Cones in the Retina
http://www.iit.edu/~npr/DrJennifer/visual/retina.html
What Rods and Cones Detect
Notice how they aren’t distributed evenly, and the
rod is more sensitive to shorter wavelengths
•
Center /
Surround
Strong activation in center,
inhibition on surround
• The effect you get using these
center / surround cells is
enhanced edges
top:
the stimuli itself
middle: brightness of the stimuli
bottom: response of the retina
• You’ll see this idea get used in
Regier’s model
http://www-psych.stanford.edu/~lera/psych115s/notes/lecture3/figures1.html
How They
Fire
• No stimuli:
– both fire at base rate
• Stimuli in center:
– ON-center-OFF-surround
fires rapidly
– OFF-center-ON-surround
doesn’t fire
• Stimuli in surround:
– OFF-center-ON-surround
fires rapidly
– ON-center-OFF-surround
doesn’t fire
• Stimuli in both regions:
– both fire slowly