Nucleus Dendrites Collect electrical signals Cell body Integrates incoming signals and generates outgoing signal to axon Axon Passes electrical signals to dendrites of another cell or to an effector cell
The membrane potential drives the responsiveness to stimulation. How are signals conducted along the length of a neuron? Na+ 440 mM OUTSIDE K+ 20 mM INSIDE -70mV Na+ 50 mM 40 0 -40 -80 K+ 400 mM
1.Depolarization phase 2.Repolarization phase Threshold potential Resting potential 3.Hyperpolarization phase Figure 45.6
Action potentials propagate by positive feedback. Speed is critical: (1) large diameter and (2) myelination
Action potentials jump down axon. Axon Nodes of Ranvier Schwann cells (glia) wrap around axon, forming myelin sheath Schwann cell membrane wrapped around axon WHY ACTION POTENTIALS JUMP DOWN MYELINATED AXONS Schwann cell 1. As charge spreads down an axon, myelination (via Schwann cells) prevents ions from leaking out across the plasma membrane. Node of Ranvier 2. Charge spreads unimpeded until it reaches an unmyelinated section of the axon, called the node of Ranvier, which is packed with Na+ channels. 3. In this way, electrical signals continue to jump down the axon much faster than they can move down an unmyelinated cell.
Sample problem. The distance from your toe to your spinal column is about 1m. If your sensory axon is 5 um in diameter, how much time elapses before your CNS receives the signal? How much time would elapse if your nerve was not myelinated?
What you should understand How the generation of an action potential represents an example of positive feedback. How voltage gated channels generate and keep brief the action potential. The flows of major ions during resting, depolarization, repolarization, and hyperpolarization. How myelination leads to rapid propagation velocities.
Synapses: Calcium mediates synaptic vesicle fusion with SNARE, SNAP AND SYNAPTOTAGMIN
Neurotransmitters lead to either Excitatory or Inhibitory Postsynaptic Potentials: EPSPs and IPSPs 40 0 -40 -80 -120
presynaptic membrane Neurotransmitter Enzyme recycler Receptor Myasthenia gravis Acetylcholine (Ach) binds to receptors Positive ions flow in – depolarizing postsynaptic cell Acetylcholinesterase breaks Ach into acetate + choline These are transported back into cell Very fast (~25,000/sec)!
Summation: EPSPs and IPSPs from multiple inputs sum at postsynaptic cells 40 0 -40 -80 -120
Temporal summation A neuron in your spinal column receives input from a sensor in your leg. Under resting conditions, that sensor sends a signal every 10 seconds. Under extreme stretch of your leg, it sends signals every second. Why would our spinal nerve only respond to the more frequent stimulus ? 40 0 -40 -80 -120
Worksheet Neurotransmitter Enzyme recycler Receptor
What you should understand The roles of neurotransmitters, postsynaptic receptor molecules and enzyme recycling components of synapses. Summation of IPSPs and EPSPs by postsynaptic cells (temporal and spatial) The consequences of up- and down-regulation of postsynaptic receptor molecules.
Sensory systems • Stimuli are transduced into changes in membrane potential by ionotropic and metabotropic mechanisms • Four characteristics of the stimulus are encoded • Intensity: spike rate • Frequency: tuning curves • Location: receptive fields • Modality: labeled line • Sensory systems are diverse and adapted for their specific tasks…and amazing!
40 0 -40 -80 From stimulus to action potential: ionotropic example
From stimulus to action potential: metabotropic example G protein Na+ Adenylate cyclase ATP cAMP GTP GDP receptor cAMP activates many channels Amplification: 1 active receptor ~10 GTP conversions each GTP powers ~10 cAMP about 1:100
Vision: also metabotropic transducin Phosphodiesterase cGMP 5’cGMP Rhodopsin GTP GDP Disk membrane 5’ cGMP changes many ion channels Amplification: 1 active receptor ~500 transducin activations -> each one converts 103 GMPs
How does a single sensory neuron encode stimuli? Characteristics of the stimulus: intensity, frequency, location, modality 00000001100010000000001110000100000001000000000 Ways the nervous system encodes these: spike rate, tuning curves, receptive fields, labeled line
Stimulus intensity is encoded by spike rate Spike rate Intensity (brightness, concentration, loudness, pressure, temperature)
Different neurons respond best to different frequencies Louder Threshold (dB SPL) Quieter Frequency Shape of curve = selectivity for frequency
Receptive fields: area of space in which the presence of a stimulus will alter the firing of a sensory neuron These receptive fields form spatiotopic maps of the world on the sensory organ… and these maps usually translate to areas of cortex as well
If all neurons communicate using action potentials, how can we keep the modalities apart? Specific sensory cells with specific receptors project to specific parts of the thamalus…which project to specific parts of cortex. LABELED LINE.