Membrane Potentials in Smooth Muscle || Spike Potential, Plateau, Slow Wave, Junctional Potential

Summary

This video explores the diverse membrane potentials in smooth muscles, contrasting them with the uniform action potentials of skeletal muscles. It delves into four main types: spike potentials, plateau action potentials, slow wave potentials, and junctional potentials. Spike potentials are akin to those in skeletal muscles and are common in unitary smooth muscles. Plateau potentials, akin to cardiac muscle, enable prolonged contractions in the gastrointestinal tract and other areas. Slow wave potentials, generated internally, oscillate to elicit rhythmic contractions, particularly in the gut. Junctional potentials, found in small multi-unit muscles like the iris, cause local depolarizations leading to contractions without action potentials. The video also discusses the role of calcium channels in smooth muscle action potentials and the unique characteristics of these potentials.

Takeaways

• 💪 Smooth muscles exhibit a greater variety of membrane potentials compared to skeletal muscles, which primarily contract with action potentials.
• 🌟 Smooth muscle action potentials can be spike potentials or plateau potentials, the latter being similar to those in cardiac muscles and allowing for prolonged contractions.
• 🔋 The resting membrane potential in smooth muscles is between -50 to -60 millivolts, setting the baseline for their electrical activity.
• 🚀 Spike potentials in smooth muscles are like those in skeletal muscles, triggered by external factors or internally by the muscle itself.
• 🕒 Plateau potentials have a prolonged depolarized state, lasting from hundreds of milliseconds to a second, and are seen in the gastrointestinal tract, ureter, uterus, and certain vascular smooth muscle cells.
• 🚧 The primary ion channels in smooth muscles are calcium channels, with few sodium channels, leading to a calcium-dominated action potential.
• 🔬 L-type voltage-gated calcium channels in smooth muscles are slow to open and close, contributing to the slower depolarization and delayed repolarization compared to skeletal muscles.
• 🌀 Slow wave potentials are self-generated oscillations in the membrane potential, believed to be due to cyclic changes in ion pumping, and can trigger action potentials if the threshold is reached.
• 🎶 Slow wave potentials, also known as pacemaker waves, do not spread or cause contraction by themselves but can elicit rhythmic contractions in muscles like those in the gut.
• 🔗 Junctional potentials occur in small multi-unit smooth muscle fibers, such as in the iris and piloerector muscles, where neurotransmitters cause local depolarizations without an action potential.

Q & A

What types of membrane potentials do smooth muscles exhibit that are different from skeletal muscles?

Smooth muscles exhibit a variety of membrane potentials unlike skeletal muscles which only contract with action potentials. These include spike potentials, action potentials with plateau, slow wave potentials, and junctional potentials.

What is the resting membrane potential range in smooth muscles?

The resting membrane potential in smooth muscles is between -50 to -60 millivolts.

How do spike potentials in smooth muscles differ from those in skeletal muscles in terms of duration?

Spike potentials in smooth muscles have a duration ranging from 10 to 50 milliseconds, which is generally longer than those in skeletal muscles.

What is the significance of the plateau phase in action potentials with plateau seen in smooth muscles?

The plateau phase in action potentials with plateau allows for a prolonged depolarized state before repolarization, enabling the muscle to contract for an extended period, from hundreds of milliseconds to a second.

In which body structures are action potentials with plateau typically observed?

Action potentials with plateau are typically observed in the gastrointestinal tract, ureter, uterus under certain conditions, and certain vascular smooth muscle cells.

What type of ion channels predominantly contribute to the action potential in smooth muscles?

Smooth muscles mainly have calcium channels, with only a few sodium channels, making the contribution of sodium to the action potential not significant.

How do L-type voltage-gated calcium channels contribute to the action potential in smooth muscles?

L-type voltage-gated calcium channels are slow to open and close, leading to slower depolarization and delayed repolarization, which in turn produces the action potential in smooth muscles.

What is the dual role of calcium in smooth muscle function?

Calcium in smooth muscles has a dual role: it is involved in the stimulation process by producing action potentials and also plays a part in the contractile process itself.

What are slow wave potentials and how do they contribute to muscle contraction?

Slow wave potentials are rhythmic oscillations in the membrane potential generated by the fiber itself without external stimulation. They do not spread or cause contraction directly but can elicit action potentials repetitively, leading to rhythmic contractions.

In what type of smooth muscles are junctional potentials observed and what is their role?

Junctional potentials are observed in multi-unit smooth muscles like those in the iris and piloerector muscles of hair. They cause local depolarizations that are equivalent to end plate potentials in skeletal muscles and lead to muscle contraction without the need for action potential spread.

How do slow wave potentials affect the rhythmic contractions of muscles such as those in the gut?

Slow wave potentials, also known as pacemaker waves, occur at a frequency of several cycles per minute and are responsible for the rhythmic contractions of muscles in the gut by eliciting repetitive action potentials.

Outlines

00:00

💡 Introduction to Smooth Muscle Membrane Potentials

This paragraph introduces the topic of the video, which is the different types of membrane potentials found in smooth muscles, contrasting them with the more uniform action potentials of skeletal muscles. It highlights that smooth muscles exhibit a variety of membrane potentials including action potentials (both spike and plateau types), slow wave potentials, and junctional potentials. The resting membrane potential in smooth muscles is noted to be between -50 to -60 millivolts. The paragraph sets the stage for a detailed exploration of each type of potential, starting with action potentials.

05:03

🚀 Detailed Exploration of Smooth Muscle Membrane Potentials

The second paragraph delves into the specifics of smooth muscle membrane potentials. It begins with a discussion of spike potentials, which are similar to those in skeletal muscles and can be triggered by various external and internal factors. The plateau type of action potential is likened to that in cardiac muscles, characterized by a prolonged depolarized state that facilitates sustained muscle contraction. The paragraph then explains the predominance of calcium channels over sodium channels in smooth muscles, with L-type voltage-gated channels being key to the generation of action potentials. The role of calcium in both stimulation and contraction is emphasized. Moving on, slow wave potentials are described as self-generated oscillations that do not spread or cause contraction but can trigger action potentials if the threshold is reached. Lastly, junctional potentials are discussed as local depolarizations in small multi-unit muscles, such as those in the iris, which can lead to muscle contraction without the need for an action potential. The paragraph concludes with a brief summary of the types of membrane potentials and their functions in smooth muscles.

Keywords

💡Membrane Potentials

Membrane potentials refer to the electrical potential difference across the cell membrane, which is crucial for the functioning of excitable cells like neurons and muscles. In the context of the video, it explains that smooth muscles exhibit a variety of membrane potentials unlike skeletal muscles, which primarily contract with action potentials. The video delves into different types of membrane potentials found in smooth muscles, such as action potentials, slow wave potentials, and junctional potentials.

💡Smooth Muscles

Smooth muscles are a type of muscle tissue found in various parts of the body including the walls of blood vessels, the gastrointestinal tract, and the respiratory tract. They are responsible for involuntary movements. The video script highlights that smooth muscles have more diverse types of membrane potentials compared to skeletal muscles, which is a key point in understanding their unique function and behavior.

💡Action Potentials

Action potentials are electrical signals that travel along the membrane of excitable cells, leading to muscle contraction or nerve impulses. The video script describes two types of action potentials in smooth muscles: spike potentials, which are similar to those in skeletal muscles, and action potentials with a plateau, which allow for prolonged contractions.

💡Resting Membrane Potential

The resting membrane potential is the voltage difference across the cell membrane when the cell is at rest. In smooth muscles, this is typically between -50 to -60 millivolts, as mentioned in the script. This baseline potential is essential as it sets the stage for the generation of action potentials and other membrane potential variations.

💡Spike Potentials

Spike potentials are a type of action potential characterized by a rapid depolarization followed by repolarization. The video script explains that these are seen in most unitary smooth muscles and can be elicited by various external and internal factors, such as neuronal stimulation or stretch.

💡Plateau

A plateau, in the context of action potentials, refers to a prolonged depolarized state before repolarization. The video describes how action potentials with a plateau, seen in certain smooth muscles like those in the gastrointestinal tract, allow for sustained contractions due to this extended phase.

💡Calcium Channels

Calcium channels are proteins in the cell membrane that allow calcium ions to flow into the cell. The video script emphasizes that smooth muscles primarily have calcium channels, which are crucial for generating action potentials. These channels are of the L-type and are slower to open and close, leading to a slower depolarization and repolarization process.

💡Slow Wave Potentials

Slow wave potentials are rhythmic oscillations in the membrane potential that occur spontaneously without external stimulation. The video script explains that these potentials are generated by cyclic changes in ion pumping and can elicit action potentials if the threshold is reached, leading to rhythmic contractions in muscles like those in the gut.

💡Pacemaker Waves

Pacemaker waves are rhythmic electrical oscillations that drive the rhythmic contractions of certain muscles. The video script refers to slow wave potentials as pacemaker waves because they initiate the repetitive sequence of action potentials necessary for rhythmic muscle contractions, particularly in the gut.

💡Junctional Potentials

Junctional potentials are local depolarizations caused by neurotransmitters released near the cell membrane. The video script describes how these potentials are seen in multi-unit smooth muscles, such as those in the iris and piloerector muscles, and how they can cause muscle contraction without the need for an action potential to spread across the muscle fiber.

Highlights

Smooth muscles exhibit more varied membrane potentials compared to skeletal muscles.

Skeletal muscles contract solely with action potentials, unlike smooth muscles.

Smooth muscle membrane potentials include action potentials, slow wave potentials, and junctional potentials.

Resting membrane potential in smooth muscles ranges from -50 to -60 millivolts.

Spike potentials in smooth muscles are similar to those in skeletal muscles and can be triggered by various stimuli.

Action potentials with plateau in smooth muscles are akin to those in cardiac muscles, allowing prolonged contraction.

The plateau phase of action potentials in smooth muscles can last from hundreds of milliseconds to a second.

Smooth muscles primarily utilize calcium channels for action potential generation, with minimal sodium channel involvement.

L-type voltage-gated calcium channels in smooth muscles are slower to open and close, leading to delayed depolarization and repolarization.

Calcium's dual role in smooth muscles includes both stimulation and contraction processes.

Slow wave potentials in smooth muscles are self-generated and do not directly cause muscle contraction.

Slow wave potentials can elicit action potentials if the threshold of approximately -35 millivolts is reached.

Pacemaker waves, or slow waves, are responsible for rhythmic contractions in certain smooth muscles like the gut.

Junctional potentials occur in multi-unit smooth muscles and are akin to end plate potentials in skeletal muscles.

In multi-unit smooth muscles, neurotransmitters cause local depolarizations leading to muscle contraction without action potentials.

Small multi-unit smooth muscle fibers, such as those in the iris and piloerector muscles, utilize junctional potentials for contraction.

A quick summary of the different types of membrane potentials and their roles in smooth muscle function is provided.

Transcripts

In this video, we will talk about types  of membrane potentials, in smooth muscle.  

Let's get started. First of all, smooth  

muscles have more varieties of membrane  potentials, than skeletal muscles.  

The skeletal muscles contract  only with action potentials.  

But the smooth muscles show diverse  types of membrane potentials.  

These potentials include action  potential, which can be spike potential,  

or action potential with plateau(), slow  wave potential, and junctional potential.  

And let's not forget that underneath all these,  

the resting membrane potential in smooth  muscles, is -50 to -60 millivolts.  

Now let's see these types one by one. First, action potentials.  

Here, the spike potentials, are similar to the  action potential that we see in skeletal muscles.  

They are seen in most unitary smooth muscles. Their duration can be from 10 to 50 milliseconds.  

They can be elicited by external factors, such  as neuronal stimulation, hormones, stretch(),  

or even internally by the muscle itself,  as we will see in slow wave potential.  

The next type of action potential,  

the one with plateau, is similar to  action potential in cardiac muscles.  

In this, we have a prolonged depolarized state  before repolarization, called a plateau.  

The duration of such action potential, can be  from hundreds of milliseconds to a second.  

This long plateau, allows prolonged  contraction of the muscle.  

Its seen in gastrointestinal tract,  ureter, uterus under some conditions,  

and certain vascular smooth muscle cells.  

So these were the types of action  potentials, seen in smooth muscles.  

Now let's talk about the  mechanism of these potentials.  

The smooth muscles mainly have calcium channels. There are only a few sodium channels.  

So contribution of sodium to smooth muscle  action potential is not significant.  

The action potential is mainly  produced by calcium channels.  

These channels, are L-type  of voltage-gated channels.  

Such channels are slow to open. So the entry of calcium, and therefore,  

the depolarization, is slower  than that in skeletal muscles.  

Then these channels are slow to close also. So the repolarization is also delayed.  

In addition, the delayed  opening of potassium channels,  

also contributes to delayed repolarization. In some fibers, the repolarization is so  

delayed, that a prolonged plateau is seen. So this is how action potentials are produced.  

The calcium entering during an action potential,  contributes to the contractile process also.  

Thus the calcium in smooth  muscles has a dual role.  

In stimulation(), as well as in contraction. So this was about action potential.  

The next type of membrane potential seen  in smooth muscle, is slow wave potential.  

These are generated by the fiber itself,  without any external stimulation.  

The exact mechanism is not known,  but its believed to be due to  

cyclic changes in the pumping of  positive ions out of the cell.  

Means for some time, positive ions like sodium  or calcium, are pumped rapidly out of the cell.  

Removal of positive ions,  makes the membrane negative.  

Then for some time, their extrusion is reduced.  

So their retention in the fiber,  makes the potential positive.  

This way, the potential keeps oscillating. This is called slow wave potential.  

Such potential changes occur  locally and do not spread.  

Also, they cannot cause muscle contraction.  

However, during the positive phase of the  potential, if the voltage reaches to the  

threshold of about -35 millivolts, the  action potential is generated as usual.  

And as we know, the action potential can spread  along the membrane, and cause muscle contraction.  

In one peak a slow wave, one or  more action potentials may appear.  

This repetitive sequence of action potentials,  causes rhythmic contraction of the muscle.  

So the slow waves, which are responsible for  this rhythm, are also called pacemaker waves.  

They occur in a frequency of  several cycles per minute.  

They are seen in gut smooth muscles,  where they cause rhythmic contractions.  

So this was about the slow waves.  

Now let's talk about the junctional potential. They are seen in multi-unit smooth muscles,  

like those in the iris and  piloerector muscles of hair.  

Fibers in such muscles are very small in  size, to generate an action potential.  

In them, the neurotransmitters are  released close to the cell membrane.  

They cause local depolarizations. This is called junctional potential.  

They are equivalent to end plate  potentials, seen in skeletal muscles.  

These potentials spread  electronically, in nearby areas.  

This much is enough to excite the entire  cell, because of its smaller size.  

Thus the action potential that  spreads in a self-generative  

manner, is not seen in such fibers. So these were all the types of membrane  

potentials, seen in smooth muscles. Now let's have a quick summary.  

Different smooth muscles show different  types of membrane potentials.  

Spike potentials are similar to  those seen in skeletal muscles.  

And they occur in most unitary smooth muscles. Action potential with plateau, allows prolonged  

contraction, in the gastrointestinal  tract, ureter, uterus, etc.  

Slow wave potentials, are rhythmic  oscillations in the membrane potential.  

They do not spread themselves  or cause contraction.  

But they elicit action potentials repetitively,  

which causes rhythmic contraction  of muscles, as seen in the gut.  

Finally, the junctional potential is seen  in small multi-unit smooth muscle fibers,  

like those in the iris and piloerector muscles. In this, local depolarizations cause muscle  

contraction, without action potential. That's it for this video.