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A&P
Neurophysiology, day 2 Ch 11
Coding for Stimulus Intensity - once generated, all action potentials are alike.
How can the CNS determine the strength of a stimulus?
- Strong stimuli cause nerve impulses to be generated more often than weak
- Stimulus intensity is coded for by frequency of impulse transmission
-Additionally, the more neurons activated, the greater the perceived intensity
Refractory Periods Absolute - when a section of neuron membrane is generating an action potential, it
is incapable of responding to another stimulus, no matter how strong
- this insures that each action potential is a separate event
Relative - while repolarization is occurring, the axon’s threshold for impulse
generation is elevated
Conduction Velocities of Neurons 1. The wider the axon, the faster it conducts impulses. (less resistance)
2. The presence of a myelin sheath forces the impulse to move from node to node
(about 1 mm)
- Myelin acts as an insulator: the current does not dissipate in adjacent membrane
regions, it is forced to go only from one node to the next
- Saltatory conduction - electrical signal jump only from node to node
- much faster than conduction without myelin
The Synapse:
- a unique junction that mediates the transfer of information from one neuron to the
next or from a neuron to an effector cell
if effector cell = muscle, synapse = neuromuscular junction
if effector cell = gland, synapse = neuroglandular junction
- most occur between the axonal endings of one neuron and the dendrites or cell
bodies of the next, these are axodendritic synapses or axosomatic synapses
-Presynaptic neuron - the neuron conducting impulses towards the synapse;
the information sender
-Postsynaptic neuron - the neuron conducting impulses away from the synapse;
the information recipient
Electrical synapses - bridged junctions that contain protein channels that
interconnect the cytoplasm of adjacent neurons, less common than chemical
- allow current-carrying ions to flow from one cell to the next
Electrical Synapses (cont.)
- found in neurons that need quick transmission- Ex: eye movement, embryonic
nervous tissue
- also found in nonnervous tissues - Ex: heart, smooth muscle
Chemical Synapses - specialized for release and reception of chemical neurotransmitters
- Made of two parts:
1. knoblike axonal terminal of presynaptic neuron, containing synaptic vesicles,
each containing thousands of neurotransmitter molecules
2. receptor region on the membrane of a dendrite or the cell body of the
postsynaptic neuron, which bears neurotransmitter receptors of many types
- neurotransmitters must move across the synaptic cleft, a space 30 to 50 nm wide
Steps of Information Transfer:
1. Calcium gates in the presynaptic axonal terminal open. Ca2+ floods in.
2. Neurotransmitter is released by exocytosis. Ca2+ is quickly removed.
3. Neurotransmitter binds to postsynaptic receptors after diffusing across synapse.
* This is the rate-limiting step of neural transmission (0.3 - 5.0 ms)
4. Ion channels open in the postsynaptic membrane. Neuron may be excited or
inhibited, depending on which neurotransmitter is bound.
Then...
1. Neurotransmitter may be degraded by enzymes on the postsynaptic neuron or in
the synapse, or
2. Removal of neurotransmitter from the synapse by reuptake into the presynaptic
terminal to be stored or destroyed by enzymes, or
3. Diffusion of neurotransmitter away from the synapse, ending its effects.
* There may be more than 50 different neurotransmitters!
Classification of neurotransmitters according to function:
1. Excitatory - cause depolarization -- more likely to make impulse
2. Inhibitory - cause hyperpolarization -- less likely to make impulse
3. Direct - open ion channels, promote rapid responses
4. Indirect - act through second-messenger molecules, promote longer-lasting,
broader responses
* Read ‘A closer look’ and ‘Patterns of Neural processing’. Be sure you
understand reflex arcs.