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The Resting Potential
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.
inside
outside
Resting potential of neuron = -70mV
+
-
+
-
+
-
+
-
+
-
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.
inside
outsideNa+Cl-Na+
K+
Cl-
K+
Organic anions (-)
Na+Na+
Organic anions (-)
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.
inside
outsideNa+Cl-Na+
K+
Cl-
K+
Organic anions (-)
Na+Na+
Organic anions (-)
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.
inside
outside
Na+
Na+
K+K+
Na+
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.)
inside
outsideNa+ Ca2+
+
-
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)
K+
Cl- +
- inside
outside
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.
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+
-
+
+
-
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.
K+ K+
K+Na+
Na+
Na+
+
-
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]
Membrane potential (mV)
[A]
[B] [C]
[D] excitation threshold
Time (msec)-70
+40
0
0 1 2 3
[E]
Action Potential
Local Currents depolarize adjacent channels causingdepolarization and opening of adjacent Na channelsQuestion: 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.
Node of RanvierMyelin sheath
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.
terminal
dendritic spine
synaptic cleftpresynaptic membrane
postsynaptic membrane
extracellular fluid
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.
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
Requirements at the synapse
For the synapse to work properly, six basic events need to happen: Production of the Neurotransmitters
Synaptic vesicles (SV) Storage of Neurotransmitters
SV Release of Neurotransmitters Binding of Neurotransmitters
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
Cortical Motor System
Pre-motor cortexMovement 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 ObservationSomatotopy of Action Observation
Foot ActionFoot Action
Hand ActionHand Action
Mouth ActionMouth Action
Buccino et al. Eur J Neurosci 2001
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 goal-related hand actions
and when:
Subject observes another individual performing similar kinds of actions
Physiology of Color Vision
© Stephen E. Palmer, 2002
Cones cone-shaped less sensitive operate in high light color vision
Rods rod-shaped highly sensitive operate at night gray-scale vision
Two types of light-sensitive receptors
cone
rod
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
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 itselfmiddle: brightness of the
stimulibottom: 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