Overview Course introduction Neural Processing: Basic Issues Neural Communication: Basics Vision,...

postsynapticneuron

science-education.nih.gov

Synapse

axon of presynapticneuron

dendrite ofpostsynapticneuron

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The Membrane

The membrane surrounds the neuron. It is composed of lipid and protein.

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

+

-

+

-

+

-

+

-

+

-

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.

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.

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.

K+ K+

K+Na+

Na+

Na+

+

-

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]

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

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

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

Hierarchical Organization of Motor System

• Primary Motor Cortex and Premotor Areas

Primary motor cortex (M1)

Foot

Hip

Trunk

Arm

Hand

Face

Tongue

Larynx

postsynapticneuron

science-education.nih.gov

Flexor-Crossed ExtensorReflex(Sheridan 1900)

Painful Stimulus

Reflex CircuitsWith Inter-neurons

Gaits of the cat: an informal computational model

Vision and Action

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

Rizzolatti et al. 1998

A New PictureA New Picture

Somato-Centered Bimodal RFs in area F4Somato-Centered Bimodal RFs in area F4

(Fogassi et al. 1996)

The fronto-parietal networks


F5c-PFF5c-PF


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

Murata et al. J Neurophysiol. 78: 2226-2230, 1997

F5 Canonical NeuronsF5 Canonical Neurons

Vision

Overview of the Visual System

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

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

http://www.iit.edu/~npr/DrJennifer/visual/retina.html

Rods and Cones in the Retina

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

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

Overview Course introduction Neural Processing: Basic Issues Neural Communication: Basics Vision,...

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