TBR action potentials along smooth muscle fibers

[Meredith92]

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I'm a bit confused on this question....

(problem 2 on passage 1 in chapter 3 bio)
The conduction velocity of action potentials along smooth muscle fiber is low because activation of:
A Potassium channels is slow
B sodium channels is slow
C calcium channels is slow
D Magnesium channels is slow


"C is correct, calcium channels is slow. As in cardiac muscle, an inward calcium current is an important component of the action potential in smooth muscle. We are told that the conduction velocity is low along smooth muscle fiber. It is logical to assume that the conduction velocity of action potentials along GI smooth muscle fibers is slow because activation of the calcium channels is slow. This question was just designed to make one think about theof calcium in the conduction velocity of an action potential of smooth muscle. The correct choice is C."


1. So my question is about the difference between muscle contraction and action potentials. For action potentials down a muscle fiber, would slow-opening sodium channels cause a slow velocity of an action potential? I know that calcium is needed for contraction, but the question is action about the action potential itself. Are they referring to the calcium that is needed for acetylcholine release the NMJ?

2. Also separate question, but the muscle cell is the myofibril right? and a bunch of muscle cells/ myofibrils make up the muscle fiber

Thanks for your help!

 

[Leonardo Noto]

I'm a bit confused on this question....

(problem 2 on passage 1 in chapter 3 bio)
The conduction velocity of action potentials along smooth muscle fiber is low because activation of:
A Potassium channels is slow
B sodium channels is slow
C calcium channels is slow
D Magnesium channels is slow


"C is correct, calcium channels is slow. As in cardiac muscle, an inward calcium current is an important component of the action potential in smooth muscle. We are told that the conduction velocity is low along smooth muscle fiber. It is logical to assume that the conduction velocity of action potentials along GI smooth muscle fibers is slow because activation of the calcium channels is slow. This question was just designed to make one think about theof calcium in the conduction velocity of an action potential of smooth muscle. The correct choice is C."


1. So my question is about the difference between muscle contraction and action potentials. For action potentials down a muscle fiber, would slow-opening sodium channels cause a slow velocity of an action potential? I know that calcium is needed for contraction, but the question is action about the action potential itself. Are they referring to the calcium that is needed for acetylcholine release the NMJ?

2. Also separate question, but the muscle cell is the myofibril right? and a bunch of muscle cells/ myofibrils make up the muscle fiber

Thanks for your help!

Well, I'm a physician, not a physiologist, but this question has been unanswered for several days so I'll at least answer the second part and partially answer the first part.

Let's start with #2 first: Muscle cell = muscle fiber = myocyte. Myofibrils are contractile units inside STRIATED muscle fibers/cells/myocytes. Striated muscle = skeletal muscle and cardiac muscle, both of which are arranged in a regularly repeating pattern of myofilaments [myofilaments = thin filaments (mostly actin) and thick filaments (mostly myosin) --> arranged into regularly repeating units called sarcomeres --> which make up myofibrils --> myocytes/muscle fibers/cells are "tubes" that are chok full of myofibrils]. There is a good cartoon on Wiki of myofibrils inside a muscle fiber, http://en.wikipedia.org/wiki/Myofibril. Smooth muscle has a more haphazard arrangement than striated muscle, not surprising since it is the regularly repeating sarcomere structure of striated muscle that makes it appear striated (striped) under the microscope. Here's a good website (Yale) with a brief explanation of the arrangement of smooth muscle about halfway down the page, http://medcell.med.yale.edu/lectures/muscle.php

As for the first question, I don't have my copy of Guyton's Physio in front of me right now so I'm unfortunately not going to be able to answer your question. However, I will say that you should remember that the biochemical cascade that leads to muscle contraction is quite different in skeletal muscle, smooth muscle, and cardiac muscle--that's something that a lot of people fail to realize; and yes, it is enough to make you pull your hair out! For the MCAT just remember the basics for each type and skip the minutiae because it isn't high yield enought to perseverate over!

FYI: The most high yield muscle-related material for the MCAT is the function of the sarcomere in skeletal muscle.

Dr. Leonardo Noto
www.leonardonoto.com

 

[sp808]

Cardiac and smooth muscle have two phases of inward current: a fast depolarization carried by sodium followed by a slower, but longer depolarization carried by calcium. This is extracellular calcium, whereas the calcium you refer to is from the sarcoplasmic reticulum. A slower voltage-gated sodium channel would slow the action potential hypothetically, but the question is asking for what is true, not what could be true.

 

[Meredith92]

Cardiac and smooth muscle have two phases of inward current: a fast depolarization carried by sodium followed by a slower, but longer depolarization carried by calcium. This is extracellular calcium, whereas the calcium you refer to is from the sarcoplasmic reticulum. A slower voltage-gated sodium channel would slow the action potential hypothetically, but the question is asking for what is true, not what could be true.

Thanks that makes a lot of sense!

 

[sunshine02]

Cardiac and smooth muscle have two phases of inward current: a fast depolarization carried by sodium followed by a slower, but longer depolarization carried by calcium. This is extracellular calcium, whereas the calcium you refer to is from the sarcoplasmic reticulum. A slower voltage-gated sodium channel would slow the action potential hypothetically, but the question is asking for what is true, not what could be true.

I know this is over a year-old, but I'm having a similar question.
I read that smooth muscle cells do not have voltage-gated sodium channels. Therefore, in cardiac muscle there is a fast depolarization carried by sodium followed by the slower calcium depolarization, but in smooth muscle depolarization is all caused by calcium???

Thanks for clearing this up!