Lecture 18: Smooth Muscle Physiology

Question 1

Where can smooth muscle be found?

Answer 1

• Gut
• Respiratory Tract
• Vasculature

Question 2

What are some of the features (physiologically) of smooth muscle?

Answer 2

• Can maintain contractions for long period of time
• Maintain organ shape
• Continuously generate active tension
• Uses relatively little ATP

Question 3

What are the key features of a multi-unit smooth muscle?

Answer 3

• Fibers operate individually
  
  • allows finer control
• Innervated by a single nerve

Question 4

What are some examples of multi-unit smooth muscle?

Answer 4

• Ciliary muscles or eye
• Iris
• Piloerector muscles

Question 5

What are the key features of unitary smooth muscle?

Answer 5

• Syncytial or visceral
• Work together as a unit
• Cell membranes adhere to one another and contain gap junctions

Question 6

Where can you find unitary smooth muscle?

Answer 6

• GI Tract
• Bile Ducts
• Uterus

Question 7

When smooth muscle contracs, what does actin attach to?

Answer 7

Dense Bodies/Adherens Junction

Question 8

What is the latch mechanism?`

Answer 8

• Phosphate is removed from myosin via myosin phosphatase
• Actin and myosin are still latched together
• This complex has LOW affinity for ATP
• Remaining latched: the muscle can continue to generate active tension

Question 9

What are the steps of smooth muscle contraction?

Answer 9

1. Ca²⁺ enters cytosol thorugh Ca²⁺ channels in plasma membrane (much bigger effect than Ca²⁺ released from SR)
2. Ca²⁺ binds to CaM and together they form REVERSIBLE Ca²⁺-CaM complex
3. Ca²⁺-CaM complex activates MLCK (myosin light chain kinase)
4. MLCK phosphorylates myosin
5. Myosin-actin cross bridges can form and contraction can occur

Question 10

What neurotransmitters can control smooth muscle?

What do they do?

Answer 10

• Norepinephrine/Epinephrine: Adrenergic Receptors
  
  • α₁: vasoconstriction
  • β₂: vasodilation
• Acetylcholine: Cholinergic receptors
  
  • Can be excitatory or inhibitory
  • Contraction is direct effect
  • Relaxation is indirect effect
• Angiotensin II, Vasopressin, Endothelin: Contraction
• Adenosine: Relaxation
• Nitric Oxide: Relaxation
• Other: CCK, oxytocin, serotonin, histamine

Question 11

What neurotransmitters contract smooth muscle?

What neurotransmitters relax smooth muscle?

Answer 11

Contraction:

• Norepinephrine/Epinephrine w/ α₁ receptors
• Acteylcholine (direct)
• Angiotensin II
• Vasopressin
• Endothelin

Relaxation:

• Norepinephrine/Epinephrine: β₂
• Acetylcholine (indirect effect)
• Adenosine
• Nitric Oxide

Question 12

What is the NTX of skeletal muscle?

Answer 12

Acetylcholine

Question 13

What environmental ques can cause vasodilation ino smooth muscle?

Answer 13

• Will cause vasodilation:
  
  • Hypoxia
  • Excess CO₂
  • Increased H⁺
  • Increased K⁺
  • Adenosine
  • ANP

Question 14

What type of neurons innervate skeletal muscle?

Answer 14

Alpha motor neurons

Question 15

How is the activity of smooth muscle controlled?

Answer 15

• Spiked potentials: oscillating periods
  
  • more spiked potentials, more strength
  • stimulated by hormones, NT, and stretch
• Plateaus
  
  • stimulated by NT and stretch

Important for fine tuning

Question 16

How can the smooth muscle cell maintain such long contractions?

Answer 16

Latch mechanism

Question 17

What is the transmission specialization of skeletal muscle?

Answer 17

NMJ

Question 18

What is the transmission specialization of smooth muscle?

Answer 18

Varicosities (bumps along the axon that contain neurotransmiiter vesicles)

NO NMJ*

Question 19

What is the NTX receptor on skeletal muscle?

Answer 19

nAChR

Question 20

What is the NTX receptor on smooth muscle?

Answer 20

mAChR or adrenergic

Question 21

Does skeletal muscle have any other NTX receptors?

Answer 21

NO

Question 22

Are there other smooth muscle receptors?

Answer 22

Yes

• Blood bourne (ex: hormones)
• Paracrine (ex: NO-)
• Intrinsic (ex: enteric NS)

Question 23

What are the steps of smooth muscle relaxation?

Answer 23

1. Myosin phosphatase dephosphorylates myosin
2. Ca2+-CAM complex dissociates
3. Ca2+ transported out of the cell (via Na+/Ca2+ exchanger) or pumped back into the SR (via Ca2+/ATPase)
4. CAM hangs out in the smooth muscle cell

Question 24

Why does smooth muscle use ATP at a slower rate than skeletal muscle?

Answer 24

Cross bridge formation is slower in smooth muscle

Question 25

What is active tension? (very general sense)

Answer 25

What the cross bridges do

There is an optimal range of active tension for both smooth and skeletal muscle

Question 26

What is passive tension? (very genrally speaking)

Answer 26

How individual muscle cells are able to stretch their membranes

Question 27

What is important about passive tension?

Answer 27

Limits the muscles ability to continue stretching despite increases in length (this helps prevent injury)

Question 28

What is unique about smooth muscle regarding passive tension?

Answer 28

• Once actin and myosin release maximum passive tension, the smooth muscle cell can reduce its tension back to zero via rearrangement of the dense bodies and cell as a whole
• This enables smooth muscle to maintain contractions for long periods of time

Question 29

Identify the red and blue lines on each graph.

Which type of muscle is represented by each graph?

Answer 29

Left: Skeletal muscle cell

• Blue: active tension
• Red: passive tension

Right: Smooth muscle cell

• Blue: active tension
• Red: passive tension