Neurons, Signals, Synapses Chapter 37

Dendrites receive signals from other neurons I. Neuron Structure Dendrites receive signals from other neurons Cell body contains nucleus and organelles Axon transmits signals to other neurons or muscles Dendrites Cell body Figure 48.5 Structural diversity of neurons. Axon Fig. 37.5 Motor neuron

Figure 37.2 Presynaptic neuron Stimulus Cell body Axon Synaptic terminals Synaptic terminals Synapse Figure 48.4 Neuron structure and organization. Postsynaptic neuron Neurotransmitter

Synapse – junction of a nerve cell with another cell Relative terms depending on cell location in network: Pre-synaptic cell – cell sending signal across synapse Post-synaptic cell – cell receiving signal

Cyan = neuron cell bodies Red = support cells (glia) Figure 37.3 Real neurons Green = dendrites Cyan = neuron cell bodies Red = support cells (glia) 80 m Cell bodies of neurons Figure 48.6 Glia in the mammalian brain.

II. Resting potential Bioflix: How Neurons Work Intro + Resting Potential

II. Resting potential Resting potential = charge difference across a cell membrane of ALL Body Cells Develops because of Na+ & K+ movement Ion movement through 3 membrane proteins: Na+-K+ pump Passive K+ channels Passive Na+ channels

Figure 37.6 Key Na K Sodium- potassium pump OUTSIDE OF CELL Potassium channel Sodium channel Figure 48.7 The basis of the membrane potential. INSIDE OF CELL

Description of ion movement Na+-K+ pump: pumps 3 Na+ out & 2 K+ in to the cell Result: [Na+] = 15 mM inside 150 mM outside [K+] = 140 mM inside 5 mM outside

Passive K+ channel: always open K+ leaves cell Cl- and Protein- are stuck inside cell Result: (-) charge now greater inside cell (-) charge attracts (+) charge on outside, electrical gradient limits K+ efflux (outflow)

III. Action potential Bioflix: Action Potential

III. Action Potential Depends on voltage-gated Na+ & K+ channels Passive channels that are closed at rest Open in response to a change in voltage across cell membrane Like an electrically operated gate

Description of ion movement Stimulus changes membrane potential (voltage) If change is large enough that voltage exceeds a threshold, voltage-gated Na+ channels open Na+ flows into cell Change in potential CLOSES voltage-gated Na+ channels and OPENS voltage-gated K+ channels K+ leaves cell

Na+-K+ pump re-establishes gradients of Na+ and K+ Action potential spreads because Na+ diffuses along the inside of the cell membrane, changing voltage & opening the next batch of voltage- gated Na+ channels

Figure 37.11 Key Na K 4 Falling phase of the action potential 3 Rising phase of the action potential 50 Action potential 3 Membrane potential (mV) Threshold 4 2 50 1 1 5 2 Depolarization Resting potential 100 Figure 48.11 The role of voltage-gated ion channels in the generation of an action potential. Time OUTSIDE OF CELL Sodium channel Potassium channel INSIDE OF CELL Inactivation loop 1 Resting state 5 Undershoot

IV. Synapse Bioflix: How Synapses Work

A real synapse Postsynaptic neuron Synaptic terminals of pre-synaptic neurons 5 m Figure 48.16 Synaptic terminals on the cell body of a postsynaptic neuron (colorized SEM). Figure 37.16

Presynaptic cell Synaptic cleft Figure 37.15 Presynaptic cell Postsynaptic cell Axon Synaptic vesicle containing neurotransmitter 1 Postsynaptic membrane Synaptic cleft Presynaptic membrane 3 Figure 48.15 A chemical synapse. K Ca2 2 Voltage-gated Ca2 channel Ligand-gated ion channels 4 Na

At chemical synapse Electrical signal crosses the synaptic cleft = gap between neurons How? A.p. reaches end of neuron Voltage changes in the neuron membrane Voltage-gated Ca2+ channels open Ca2+ binds to vesicles containing neurotransmitter Vesicles fuse with neuron membrane

(continued) Neurotransmitter is released by exocytosis into the synaptic cleft Neurotransmitter binds to receptors on next neuron Receptors are ligand-gated Na+ channels = Na+ channels that open when the right molecule (ligand) attaches Neurotransmitter detaches from receptors and is degraded by enzymes to stop signal

What happens next? The postsynaptic cell responds if the postsymatic potential reaches threshold Postsynaptic cells could be : Nerve cells Muscle cells Glandular cells