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postsynaptic neuron

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.

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postsynaptic neuron

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  1. postsynaptic neuron science-education.nih.gov

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

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

  4. 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. outside + + + + + - inside - - - - Resting potential of neuron = -70mV

  5. Artist’s rendition of a typical cell membrane

  6. 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. Cl- Na+ Cl- Na+ Na+ Na+ outside inside Organic anions (-) K+ Organic anions (-) Organic anions (-) K+

  7. 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. Cl- Na+ Cl- Na+ Na+ Na+ outside inside Organic anions (-) K+ Organic anions (-) Organic anions (-) K+

  8. 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 inside K+ K+

  9. 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.) Ca2+ Na+ + outside inside -

  10. 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- + outside inside - K+

  11. 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

  12. 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.

  13. Before Depolarization

  14. Na+ + - - + Na+ Na+ 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.

  15. Depolarization

  16. Na+ K+ + K+ - Na+ Na+ K+ 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.

  17. Repolarization

  18. 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.

  19. +40 Membrane potential (mV) [C] [B] 0 [A] [D] excitation threshold -70 Time (msec) 0 1 2 3 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] [E]

  20. Action Potential Local Currents depolarize adjacent channels causing depolarization and opening of adjacent Na channels Question: Why doesn’t the action potential travel backward?

  21. 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.

  22. 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.

  23. 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

  24. 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.

  25. 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 dendritic spine postsynaptic membrane

  26. Neurotransmitter release • Ca2+ causes vesicle membrane to fuse with presynaptic membrane. • Vesicle contents empty into cleft: exocytosis. • Neurotransmitter diffuses across synaptic cleft. Ca2+

  27. 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

  28. Ionotropic Receptors

  29. Postsynaptic Ion motion

  30. 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

  31. Motor Control Basics • Reflex Circuits • Usually Brain-stem, spinal cord based • Interneurons control reflex behavior • Central Pattern Generators • Cortical Control

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

  33. Primary motor cortex (M1) Hip Trunk Arm Hand Foot Face Tongue Larynx

  34. postsynaptic neuron science-education.nih.gov

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

  36. Gaits of the cat: an informal computational model

  37. Vision and Action

  38. 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

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

  40. A New Picture Rizzolatti et al. 1998

  41. Somato-Centered Bimodal RFs in area F4 (Fogassi et al. 1996)

  42. The fronto-parietal networks Rizzolatti et al. 1998

  43. F5c-PF Rizzolatti et al. 1998

  44. 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

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