Know which cell types are considered excitable and why. - ANSWER - Excitable cells are
*capable of developing action potentials* because they *express voltage-gated cation channels*
in specific membrane domains.
1.) *Cardiac muscle - myogenic*
- Striated. Functional syncytium (each muscle cell does not have to directly be innervated by a
neuron)
- May or may not maintain resting Vm
- *Signal comes from muscle itself not external*
2.) *Smooth muscle* - contraction via *neurogenic or myogenic or non-neural signals*
(hormones, stretch, pH, temp)
- each unit must be stimulated on its own.
- Voltage gated Ca2+ channels
3.) *Skeletal muscle - neurogenic* (more information later in notes)
Know the different membrane domains and the types of channels present in each of the
different types of excitable cells. - ANSWER - 1.) *Cardiac muscle* - myogenic muscle$
- *Intercalated disks* contain:
A) *Gap junctions - allow direct electrical coupling of myocytes. Rapid conduction of AP.*$$$
B) *Desmosomes*
2.) *Smooth muscle*
- *Multi-unit* - *NO gap junctions*, *neurogenic* , not responsive to stretch, no muscle AP
, - *Single-unit* - *Gap junctions present*, myogenic , responsive to stretch, muscle AP
Cardiac contractile cells - ANSWER - C) *Contractile cells*
- Maintains resting Vm
- Relatively similar to skeletal muscle but some differences: (1) *no motor end-plate* BECAUSE
*signals are brought from other cells* not neurons (2) slow voltage-gated Ca2+ channel keeps
membrane potential depolarized for a longer time creating a *plateau phase*
- *Ca2+ DHP receptor IS A FUNCTIONAL CA2+ CHANNEL HERE*
- *CALCIUM INDUCED CALCIUM RELEASED* - Vm changes -> DHP L-type opens -> Ca2+
comes in through DHP -> Ryanodine receptor allows SR calcium to be released into intracellular
-> contraction -> SERCA pumps CA2+ back in
- Variable Ca2+ released
3.) *Skeletal muscle*- *neurogenic muscle$$$* that is induced by neurotransmitter binding
- Muscle plasma membrane = sarcolemma
- *T-tubules* (part of sarcolemma) CONTAINS *chloride channels* to main neg resting Vm and
*Voltage-sensitive DHP Ca2+ Channel*
- *Two domains in SR (sarcoplasmic reticulum)*
a.) Longitudinal elements - *SERCA pump*
b.) *Terminal cisternae* - in close proximity to t-tubules, contains *Ryanodine receptors* (RyR)
which are calcium release channels
NB:
- *Major role of SR is intracellular CA2+ storage$$$*
- *the main way to tell
- DHP channels are voltage-sensitive proteins with conformational changes that are of more
important than the channel function itself
- difference between multi and single unit smooth muscles is via presence of gap junctions $$$$
$$*
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