Cardiac Muscle/Smooth Muscle

Question 1

What are muscle spindles?

Answer 1

• small tapered structures located in midst of myofibers
• responsive to mechanical pressure
• send information re: movement and position to CNS
• facilitates degree of contraction and number of motorneurons involved
• proprioception

Question 2

How do muscle spindles respond to stretch?

Answer 2

by reflexive contraction

Question 3

Spindle stretch opens a…

Answer 3

mechanically sensitive channel

Question 4

What are golgi tendon organs?

Answer 4

• located in the muscle tendon (attachment point to bone)
• its role is to prevent damage to muscle through excessive stretching
• XS stimulation produces an inhibitory reflex of the same muscle

Question 5

________ and _______ modulate our skeletal muscle system via reflexes.

Answer 5

Muscle spindles and GTOs (they monitor muscle stretch and tension, respectively)

Question 6

Describe vertebrate cardiac muscle.

Answer 6

• striated
• branch-like
• uninucleated
• intercalated disks
  
  • occur at z-line
  • fascia adherens (actin binding)
  • macula adherens (desmosome, join cells)
  • gap junctions (electrical continuity)
    
    • = synctium

Question 7

Hearts are innervated by ____, but this serves for…

Answer 7

ANS, modulatory function only

Question 8

________ cells initiate myogenic contractions in the heart.

Answer 8

Sinoatrial

Question 9

Do sinoatrial cells have a stable RMP?

Answer 9

No.

• Starts at ~60mV
• they have cation channels (funny channel) activated by hyperpolarization
• Na+ enters and initiates AP+
• Leakage through the Na⁺ channel (If) depolarizaes the cell until it gets to ~40mV
• Depolarization is mediated by VGCC not Na channels
• Connected by gap junctions to other cells in the heart

Question 10

Where is the AV node located?

Answer 10

In the right atrium

Question 11

What serves as the conduit between the atrium and ventricle?

Answer 11

AV node (+fibres)

Question 12

Why does the AV node help difficult conduction from atria to ventricles?

Answer 12

Allows temporally separate contractions for each.

Question 13

What structure carries electrical impulses from the AV node into the ventricles?

Answer 13

The bundle of his

Question 14

Describe the AV node action potential.

Answer 14

• There is a delay in the conduction of the AP from the SA node to the AV node
  
  • enables the ventricles to fill with blood before contraction
• AV node cells will function as pacemaker cells themselves, just at a slower rate.

Question 15

What is the heart contraction sequence?

Answer 15

1. contraction initiated in SA
2. L and R atria contract
3. AV node conducts depolarization from atria to heart apex
4. ventricular contraction starts at apex and moves superiorly

Question 16

RMP in cardiac myocytes is dominated by…

Answer 16

K⁺

Question 17

Why is the cardiac AP much longer in duration than a neuronal skeletal muscle AP?

Answer 17

Because of a long duration Ca²⁺ current.

Question 18

What does an electrocardioram (ECG) allow us to do?

Answer 18

Record electrical activity through electrodes placed on the skin. The waves of depolarization and hyperpolarization experienced by the atrie and ventricels can be recorded with extracellular electrodes.

Question 19

What are the 5 major deflections of voltage on the ECG?

Answer 19

1. P - atrial depolarization
2. QRS complex - ventricular depolarizatio
3. T - ventricualr repolarization

Question 20

Are VGCC and RyR coupled in cardiac cells?

Answer 20

No. RyR opening is mediated by Ca²⁺influx

Question 21

What are the key differences of smooth muscle from skeletal muscle?

Answer 21

• no sarcomeres (no striations)
  
  • thick and thin filaments are scattered in cells
  • attached to cell membrane at adhesion plaques
• no t0tubules and minimal SR
• often connected by gap junctions
  
  • typically function as a single unit (synctium)
• contracts in all dimensions
• different mechanism of EC coupling
  
  • spontaneous contractions
  • stimulated by exogenous substances (hormones, NT etc)
  • pacemaker contractions

Question 22

At rest, what protein is bound to acin and blocks myosin binding in smooth muscle?

Answer 22

Calesmon (smooth musle DOES NOT have troponin)

Question 23

Stimulation of smooth muscle cell increases intracelluar Ca²⁺…

Answer 23

• Ca²⁺ binds to calmodulin (CaM)
  
  • CaM binds to caldesmon and removes it from actin
    
    • cross-bridges form and contraction ocurs
  • CaM also causes phosphorylation of myosin
    
    • increase in myosin ATPase activity

Question 24

Epiniephrine mediates ________ of smooth muscle contraction.

Answer 24

inhibition

Question 25

Describe smooth muscle relaxation.

Answer 25

• Decrease [Ca]
  
  • buffering transporters
• MLC-phosphatase
• hyperpolarize smooth muscle cell
• alter actin/myosin dynamics