Module 4: Smooth and Cardiac Muscle (6)

What are smooth muscles? What are cardiac muscles?

smooth muscles cells are found in walls of hollow organs and tubes (ex. digestive system, vasculature).

Cardiac muscle cells are only found in the heart.

Smooth muscles lack sarcomeres, and instead have three types of filaments that participate in contraction. What are they?

1. thick myosin filaments that are longer than those in skeletal muscles

2. thin actin filaments that contain tropomyosin but not troponin

3. intermediate filaments that don’t directly support contraction but instead the cytoskeletal framework.

What do smooth muscles have instead of z-lines?

dense bodies!

• positioned throughout the cell as well as on the internal surface of the plasma membrane.

• serve as anchor points for intermediate and contractile filaments.

• thick and thin filaments oriented at angles forming diamond-like patterns

smooth muscle lacks troponin that in blocks cross-bridge formations until calcium is present in skeletal muscles. However, even without troponin, actin and myosin in smooth muscles cannot form cross-bridges at rest. Why is this?

Both skeletal and smooth muscles have a protein called myosin light chain → associated with the myosin head and aids in cross-bridge formation (bigger role in smooth than in skeletal).

• during excitation, calcium enters the smooth muscle cell and binds to calmodulin (calcium binding messenger protein)

• calcium-calmodulin complex binds to and activates myosin light chain kinase.

• Once activated, this kinase phosphorylates the myosin light chain, which allows the myosin cross-bridge to bind to actin.

unlike skeletal muscles, smooth muscles have no t-tubules and they have very little sarcoplasmic reticulum. Thus, calcium (which is needed for contraction) must come from somewhere else. There are two main sources for smooth muscles, what are they?

1. Calcium entry from extracellular fluid

   • voltage-gated dihydropyridine receptors here function as calcium channels.

   • if smooth muscle cell depolarizes enough to open these channels, calcium enters from ECF.

2. Calcium entry from the sarcoplasmic reticulum.

   • once calcium enters the cell, it can activate calmodulin OR it can cause further increase

Why is it so important for muscles to have calcium sources?

calcium is crucial for any type of muscles functions. Body maintains very tight control over calcium in the blood; equilibrium maintained by absorption from intestines, movement of calcium in and out of the bones, kidneys reclamation and excretion of calcium into the urine.

smooth muscle excitation depends on their type. There are single unit and multiunit smooth muscles. How are each type excited?

Single unit

• majority of smooth muscle is single unit.

• muscle fibres are all electrically connected via gap junctions and thus become excited and contract as a single unit or “functional synctium”.

• Found in hollow organs (digestive and reproductive systems, urinary tracts, small blood vessels).

Multiunit

• distinct groups of smooth muscle cells innervated by nerves of the autonomic nervous system to contract.

• stimulation = “neurogenic”

• found in walls of large blood vessels, small airways to lungs, base of hair follicles of the skin, and in the eye.

Single unit smooth muscle is myogenic. What does this mean?

it’s self-excitable and doesn’t require nerve stimulation.

Within a functional synctium, there’s clusters of specialized automatic smooth muscle cells

• automatic meaning they can spontaneously depolarize to generate action potentials that spread throughout the muscle. There are two types of spontaneous depolarization.

There are two types of spontaneous depolarization. What are they? Briefly describe each.

1. Pacemaker potentials

   • membrane gradually depolarizes until it reaches threshold and fires an AP.

   • unlike other excitable tissues, autorhythmic cells have l_f channels (permeable to sodium and potassium) open and depolarize cell until membrane becomes more positive. Here they close and calcium channels open to continue depolarization until it reaches threshold.

2. Slow-wave potentials

   • caused by active transport of calcium across the membrane which causes an oscillating wave of hyper and depolarization.

   • should threshold be reached, AP = generated

   • oscillation amplitude influenced by neural and local factors.

In skeletal muscle, gradation of contraction is under neural control. In single unit smooth muscle what happens?

gap junctions prevent recruitment as all the muscle cells contract together.

What is gradation of contraction in smooth muscles?

occurs by modulating cytosolic calcium concentration. A single excitation stimulus doesn’t cause all cross-bridges to cycle. Continuous excitation of smooth muscle can further increase cytosolic calcium thereby causing more cross-bridges to cycle and give greater contraction.

What is tone?

degree to which single unit smooth muscle is contracted at any given time.

• can be modified by several factors.

What innervates single unit smooth muscles? How does this system innervate the smooth muscles?

innervated by both branches of the autonomic nervous system'

• doesn’t initiate contraction though → the ANS modifies the rate and strength of contraction

Postganglionic autonomic neurons travel across the surface of many smooth muscle cells and release neurotransmitters from specialized areas called varicosities.

• in this manner, a single neuron can affect a large number of smooth muscle cells (diff from motor end plates in skeletal muscles).

At rest, smooth muscle is far below optimal length for tension generation (unlike skeletal muscle) Why is this important?

It’s an important property for the functioning of organs that the smooth muscles are in. As smooth muscles are stretched or distended it can generate stronger contractions to return to its normal length.

The different structure of contractile filaments in smooth muscle allows for far greater stretch than that in skeletal muscle.

Important too because they can be stretched many times their resting length and still develop tension.

Cardiac muscle is found in the hear and shares properties with skeletal and single unit smooth muscles. What’s unique to cardiac muscles?

the length of their action potentials

• last much longer in cardiac muscles

Branching of fibres

What are five similarities between skeletal and cardiac muscles?

1. striated muscles with thick and thin filaments → sarcomeres

2. has troponin and tropomyosin as primary site where calcium activates cross bridge activity.

3. has t-tubules and well defined sarcoplasmic reticulum

4. lots of mitochondria

5. has well defined length-tension relationship

What are three similarities between smooth and cardiac muscles?

1. calcium comes from both extracellular fluid and sarcoplasmic reticulum

2. interconnected by gap junctions to allow the spread of activation

3. innervated by the ANS to modify rate/strength of contraction.