PY 105 Chapter Notes - Chapter 2: Central Nervous System, Inhibitory Postsynaptic Potential, Neuromuscular Junction
Refractory Period
• Passage of one action potential makes the neuron nonresponsive to membrane depolarization
and unable to transmit another action potential, or refractory
• Two phases
o Absolute refractory period: a neuron will not fire another action potential no matter how
strong a membrane depolarization is induced
• Voltage-gated sodium channels have been inactivated
• They will not open again until the membrane potential reaches the resting potential
and Na+ channels return to their closed state
o Relative refractory period: a neuron can be induced to transmit an action potential, but the
depolarization required is greater than normal because the membrane is hyperpolarized
Synaptic Transmission
• A synapse is a junction between the axon terminus of a neuron and the dendrites, soma or axon of
a second neuron
• Two types of synapses
a. Electrical - occur when the cytoplasms of two cells are joined by gap junctions. An action
potential will spread directly from one cell to the other
i. Smooth muscle and cardiac muscle
b. Chemical - found at the end of axons where they meet their target cell; an action potential is
converted into a chemical signal
i. Nervous system
• Steps involved in transmission across a chemical synapse
1. Action potential reaches end of an axon, the synaptic knob
2. Depolarization of presynaptic membrane opens voltage-gated calcium channels
3. Calcium influx into presynaptic cell causes exocytosis of neurotransmitter stored in secretory
vesicles
4. Neurotransmitter molecules diffuse across narrow synaptic cleft
5. Neurotransmitter binds to receptor proteins in postsynaptic membrane. These receptors are
ligand-gated ion channels
6. The opening of thee ion channels in postsynaptic cell alters membrane polarization
7. If depolarization of post synaptic cell reaches threshold of voltage-gated sodium channels, action
potential is initiated
8. Neurotransmitter in synaptic cleft is degraded to terminate the signal
Example: the neuromuscular junction between neurons and skeletal muscle
The neurotransmitter released is acetylcholine (Ach)
• When an action potential reaches the synapse, Ach is released into the synaptic cleft
• The acetylcholine binds to the Ach receptors on the surface of the post synaptic cell membrane
• When Ach binds, the receptor opens its associated sodium channel, allowing sodium to flow down
a gradient into the cell, depolarizing the postsynaptic cell membrane
• Acetylcholine is degraded by the enzyme acetylcholinesterase
There are other neurotransmitters and receptors
• Gamma-aminobutyric acid (GABA)
• Serotonin
• Dopamine
• Norepinephrine
Excitatory - when a neurotransmitter opens a channel that depolarizes the postsynaptic membrane
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Document Summary
An action potential will spread directly from one cell to the other. Smooth muscle and cardiac muscle: chemical - found at the end of axons where they meet their target cell; an action potential is converted into a chemical signal. These receptors are ligand-gated ion channels: the opening of thee ion channels in postsynaptic cell alters membrane polarization. If depolarization of post synaptic cell reaches threshold of voltage-gated sodium channels, action potential is initiated: neurotransmitter in synaptic cleft is degraded to terminate the signal. Example: the neuromuscular junction between neurons and skeletal muscle. There are other neurotransmitters and receptors: gamma-aminobutyric acid (gaba) Excitatory - when a neurotransmitter opens a channel that depolarizes the postsynaptic membrane. Inhibitory - neurotransmitters that make the postsynaptic membrane potential more negative than the resting potential, or hyperpolarized. Sensory neurons, which carry information toward cns, are called afferent neurons.
