Transcript Ch 2 lec 3
Chapter 2 Structure and functions of cells of the nervous system Cells of the Nervous System Supporting Cells Glia (glial cells) - Supporting cells that “glue” the nervous system together; 3 most important types are: Astrocytes Oligodendrocytes Microglia Summary: Things to think about Membrane potentials Lipid bilayer Ion types (cations and anions contributing) Distribution of ions across the membrane Membrane proteins Channels Pumps/transporters: Passive vs active movement of ions Action potentials Threshold Temporal explanation of ion movement across the membrane. An Action Potential Temporal and sequential importance of ion transfer across the membrane. Dependent on voltage-gated (dependent) channels Figure 2.21 Factors Influencing Conduction Velocity Saltatory conduction High density of Na+ V-D at Nodes of Ranvier 2 advantages of Saltatory Conduction Economical Much less Na+ enters cell (only at nodes of Ranvier) mush less has to be pumped out. Speed Conduction of APs is faster in myelinated axons because the transmission between the nodes is very fast. Communication Between Neurons Some Simple Vocab Details of a Synapse Figure 2.28 The Synapse Synaptic transmissiontransmission of signal from one cell to another Neurotransmitter Postsynaptic potentials Excitatory Inhibitory Scanning electron micrograph (real) shows the synapses between nerve fibres (purple) and a nerve cell (yellow). Magnified 10,000 times. NOVA Release of Neurotransmitters Small, clear Large, dense core False colour electron micrograph Vesicles After synthesis, NTs are stored in vesicles (lipids). Varying numbers of vesicles at the button Terminal button could contain both large and small sized vesicles Scanning electron micrographnerve ending (broken) with vesicles Small vesicles (neurotransmistters) Synthesized in the terminal button and packaged in synaptic vesicles Large dense core (typically neuropeptides) Assembled in the cell body, packaged in vesicles, and then transported to the axon terminal. Vesicle and Release Proteins Vesicle Transporters: Get substances into vesicles Each vesicle: 1000s NT molecules Trafficking Proteins: Docking Release Recycle Vesicle Pools Very few vesicles are docked (<1%) Most in the reserve pool (85-90%) Recycling pool (10-15%) Neurotransmitter Release Exocytosis The arrival of an AP at the terminal opens dependent Ca2+ channels The entry of Ca2+ causes vesicles to fuse with the terminal membrane and release their contents Release of Neurotransmitters Figure 2.31 Release of Neurotransmitters Figure 2.31 Docked 1. Synaptic vesicle migrates to presynaptic membrane. Release of Neurotransmitters Figure 2.31 2. Vesicle fuses with presynaptice membrane. Release of Neurotransmitters Figure 2.31 3. Neurotransmitter is released into the synaptic cleft. Vesicles After Release Recycling of vesicle material (<1sec) 1. Kiss and Run (leave) Release most NT, reseals and moves into cytoplasm to be refilled Merge and Recycle 2. Vesicle fuses completely with the membrane Bulk Endocytosis 3. Large pieces of the membrane fold in to reform vesicles Figure 2.33 Activation of Receptors Pos I. Postsynaptic Receptors Ligand – a molecule that binds to another A NT is a ligand of its receptor I. Postsynaptic receptors • • Released NT molecules produce signals in postsynaptic neurons by binding to receptors Receptors are specific for a given NT 1) Ionotropic Receptors Receptor that contains a binding site for a neurotransmitter and an ion channel that opens when a molecule of the neurotransmitter attaches to the binding site. Figure 2.34 Ionotropic Receptors NT binds and an associated ion channel opens or closes, causing a PSP Excitatory e.g. Nicotinic (N1) receptors (depolarizes) If Na+ channels are opened, for example, an EPSP occurs If K+ or Cl- channels are opened, for example, an IPSP occurs Inhibitory e.g. BZP receptors (hyperpolarizes) 1) Short cut 2) Second messenger Figure 2.35 2) Metabotropic Receptors Slower variety (short cut faster than second messenger system) • Actions are reliant on activation of G-proteins located in the internal membrane of the postsynaptic cell • 2 basic varieties: 1) short cut 2) second messenger • Figure 2.36 Ionic Movement During Postsynaptic Potentials Figure 2.37 1) REUPTAKE Mediated by transporter molecules on neurons and glia After it is taken up it may be degraded or recycled in vesicles 2) ENZYMATIC DEGRADATION Removal at the cleft E.g. Cholinergic synapses (ACh) Neuromuscular junction ED can occur in the synapse or in the cytoplasm Used to recycle: ACh -> choline by ACh-esterase (AChE) 3) DIFFUSION Away from the synapse Glia cells Transporters for uptake II. Autoreceptors Sensitive to neurotransmitter released by presynaptic terminal Act as safety valve to reduce release when levels are high in synaptic cleft (autoregulation) Excitatory Post-Synaptic Potential • • Transmitter causes the receptor sites to open gated ion channels that permit Na+ into the cell (depolarizing event) Known as an EPSP Inhibitory Post-Synaptic Potential Transmitter causes the receptor sites to open gated ion channels that permit K+ out of the cell or Cl- into the cell (hyperpolarizing event) Known as an IPSP 1. 2. Spatial Summation Temporal Summation INTEGRATION of Input Signals + SPATIAL SUMMATION 1. Summation of EPSPs + Two distinct synaptic inputs onto postsynaptic cell • Same time • EPSP + EPSP = larger EPSP • Cell is depolarized - SPATIAL SUMMATION 2. Summation of IPSPS - • • Two independent inhibitory inputs Postsynaptic cell hyperpolarized SPATIAL SUMMATION 3. Summation of EPSP and IPSP + EPSP (depolarizing) and IPSP (hyperpolarizing) input Not net change in membrane potential TEMPORAL Summation Single synapse initiating a sequence of membrane events Presynaptic Inhibition • Axoaxonic- decreases NT released • Presynaptic facilitation can occur also (increasing NT released)