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.
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Integrates incoming signals
and generates outgoing
signal to 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?
Action potentials propagate by positive feedback.
Speed is critical: (1) large diameter and (2) myelination
Action potentials jump down 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
1. As charge spreads down
an axon, myelination (via
Schwann cells) prevents
ions from leaking out across
the plasma membrane.
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.
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
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
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 ?
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.
From stimulus to action potential: ionotropic example
From stimulus to action potential: metabotropic example
cAMP activates many channels
1 active receptor ~10 GTP conversions each GTP powers ~10 cAMP about 1:100
Vision: also metabotropic
5’ cGMP changes many ion channels
1 active receptor ~500 transducin activations -> each one converts 103 GMPs
Characteristics of the stimulus: intensity, frequency, location, modality
Ways the nervous system encodes these: spike rate, tuning curves, receptive fields, labeled line
(brightness, concentration, loudness, pressure, temperature)
Different neurons respond best to different frequencies
Threshold (dB SPL)
Shape of curve = selectivity for frequency
These receptive fields form spatiotopic maps of the world on the sensory organ… and these maps usually translate to areas of cortex as well
Specific sensory cells with specific receptors project to specific parts of the thamalus…which project to specific parts of cortex. LABELED LINE.