postsynaptic neuron scienceeducation nih gov Synapse axon of

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postsynaptic neuron science-education. nih. gov

postsynaptic neuron science-education. nih. gov

Synapse axon of presynaptic neuron dendrite of postsynaptic neuron bipolar. about. com/library

Synapse axon of presynaptic neuron dendrite of postsynaptic neuron bipolar. about. com/library

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

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

The Resting Potential - - - + + - Resting potential of neuron =

The Resting Potential - - - + + - Resting potential of neuron = -70 m. V outside - + n There is an electrical charge across the membrane. This is the membrane potential. The resting potential (when the cell is not firing) is a 70 m. V difference between the inside and the outside. + n inside

Artist’s rendition of a typical cell membrane

Artist’s rendition of a typical cell membrane

Ions and the Resting Potential n n Ions are electrically-charged molecules e. g. sodium

Ions and the Resting Potential n n 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+ K Organic anions (-) + Cl- outside inside Organic anions (-)

Ions and the Resting Potential n n Ions are electrically-charged molecules e. g. sodium

Ions and the Resting Potential n n 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+ K Organic anions (-) + Cl- outside inside Organic anions (-)

Maintaining the Resting Potential n n Na+ ions are actively transported (this uses energy)

Maintaining the Resting Potential n n 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+ outside K+ K+ inside

Excitatory postsynaptic potentials (EPSPs) Opening of ion channels which leads to depolarization makes an

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) ¨ Ca 2+. (Also activates structural intracellular changes -> learning. ) ¨ Ca 2+ outside - Na+ + n inside

Inhibitory postsynaptic potentials (IPSPs) Opening of ion channels which leads to hyperpolarization makes an

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+ outside - Cl- + n inside

Integration of information n n PSPs are small. An individual EPSP will not produce

Integration of information n n 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

Neuronal firing: the action potential The action potential is a rapid depolarization of the membrane. n It starts at the axon hillock and passes quickly along the axon. n The membrane is quickly repolarized to allow subsequent firing. n

Before Depolarization

Before Depolarization

Action potentials: Rapid depolarization n When partial depolarization reaches the activation threshold, voltage-gated sodium

Action potentials: Rapid depolarization n When partial depolarization reaches the activation threshold, voltage-gated sodium ion channels open. Sodium ions rush in. The membrane potential changes from -70 m. V to +40 m. V. + - - Na+ Na+ +

Depolarization

Depolarization

Action potentials: Repolarization n Sodium ion channels close and become refractory. Depolarization triggers opening

Action potentials: Repolarization n 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+ K Na+ + K+ K+ + -

Repolarization

Repolarization

The Action Potential The action potential is “all-or-none”. n It is always the same

The Action Potential The action potential is “all-or-none”. n It is always the same size. n Either it is not triggered at all - e. g. too little depolarization, or the membrane is “refractory”; n Or it is triggered completely. n

Course of the Action Potential • The action potential begins with a partial depolarization

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 (m. V) [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

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. n n Passive conduction will ensure that adjacent membrane

Conduction of the action potential. n n 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 n Most mammalian axons are myelinated. The myelin sheath is provided by oligodendrocytes

Myelination n 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 n Myelinated regions of axon are electrically insulated. Electrical charge moves along

Saltatory Conduction n 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 n Information is transmitted from the presynaptic neuron to the postsynaptic cell.

Synaptic transmission n 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 n n n An action potential causes neurotransmitter release from

Structure of the synapse n n n 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 n n n Ca 2+ causes vesicle membrane to fuse with presynaptic

Neurotransmitter release n n n Ca 2+ causes vesicle membrane to fuse with presynaptic membrane. Vesicle contents empty into cleft: exocytosis. Neurotransmitter diffuses across synaptic cleft. Ca 2+

Ionotropic receptors (ligand gated) Synaptic activity at ionotropic receptors is fast and brief (milliseconds).

Ionotropic receptors (ligand gated) Synaptic activity at ionotropic receptors is fast and brief (milliseconds). n Acetylcholine (Ach) works in this way at nicotinic receptors. n Neurotransmitter binding changes the receptor’s shape to open an ion channel directly. n ACh

Ionotropic Receptors

Ionotropic Receptors

Postsynaptic Ion motion

Postsynaptic Ion motion

Requirements at the synapse For the synapse to work properly, six basic events need

Requirements at the synapse For the synapse to work properly, six basic events need to happen: n Production of the Neurotransmitters ¨ n Storage of Neurotransmitters ¨ n n n SV Release of Neurotransmitters Binding of Neurotransmitters ¨ n 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

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

Hierarchical Organization of Motor System • Primary Motor Cortex and Premotor Areas

Primary motor cortex (M 1) Hip Trunk Arm Hand Foot Face Tongue Larynx

Primary motor cortex (M 1) Hip Trunk Arm Hand Foot Face Tongue Larynx

postsynaptic neuron science-education. nih. gov

postsynaptic neuron science-education. nih. gov

Flexor. Crossed Extensor Reflex (Sheridan 1900) Reflex Circuits With Inter-neurons Painful Stimulus

Flexor. Crossed Extensor Reflex (Sheridan 1900) Reflex Circuits With Inter-neurons Painful Stimulus

Gaits of the cat: an informal computational model

Gaits of the cat: an informal computational model

Vision and Action

Vision and Action

Cortical Motor System Pre-motor cortex Movement planning/sequencing • Many projections to M 1 •

Cortical Motor System Pre-motor cortex Movement planning/sequencing • Many projections to M 1 • 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

Somatotopy of Action Observation Foot Action Hand Action Mouth Action Buccino et al. Eur J Neurosci 2001

A New Picture Rizzolatti et al. 1998

A New Picture Rizzolatti et al. 1998

Somato-Centered Bimodal RFs in area F 4 (Fogassi et al. 1996)

Somato-Centered Bimodal RFs in area F 4 (Fogassi et al. 1996)

The fronto-parietal networks Rizzolatti et al. 1998

The fronto-parietal networks Rizzolatti et al. 1998

F 5 c-PF Rizzolatti et al. 1998

F 5 c-PF Rizzolatti et al. 1998

The F 5 c-PF circuit Links premotor area F 5 c and parietal area

The F 5 c-PF circuit Links premotor area F 5 c and parietal area PF (or 7 b). Contains mirror neurons. Mirror neurons discharge when: Subject (a monkey) performs various types of goalrelated hand actions and when: Subject observes another individual performing similar kinds of actions

F 5 Canonical Neurons Murata et al. J Neurophysiol. 78: 2226 -2230, 1997

F 5 Canonical Neurons Murata et al. J Neurophysiol. 78: 2226 -2230, 1997

Vision

Vision

Overview of the Visual System

Overview of the Visual System

Physiology of Color Vision Two types of light-sensitive receptors Cones cone-shaped less sensitive operate

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

The Microscopic View

How They Fire • No stimuli: – both fire at base rate • Stimuli

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/Dr. Jennifer/visual/retina. html

Rods and Cones in the Retina http: //www. iit. edu/~npr/Dr. Jennifer/visual/retina. html

What Rods and Cones Detect Notice how they aren’t distributed evenly, and the rod

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

• 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 http: //www-psych. stanford. edu/~lera/psych 115 s/notes/lecture 3/figures 1. html Regier’s model

How They Fire • No stimuli: – both fire at base rate • Stimuli

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