Excitation and Contraction of Smooth Muscle DSA

Question 1

Two types of smooth muscle =

Answer 1

1. multi-unit smooth muscle

2. unitary (single-unit) smooth muscle

Question 2

Multi-Unit Smooth Muscle: 4 generalized traits

Answer 2

1. composed of discrete, separate smooth muscle fibers
2. each fiber innervated independently of the others
3. look like skeletal muscle on the outside
4. covered in basement like membrane

Question 3

Important characteristic of multi smooth muscle fibers regarding innervation

Answer 3

they can each contract independently of the others. control is exerted mainly by nerve signals

Question 4

a major share of nervous control of unitary smooth muscle is by

Answer 4

non-nervous means

Question 5

examples of multi unit smooth muscle

Answer 5

piloerector muscles, ciliary muscle of the eye, iris muscle of the eye

Question 6

Unitary smooth muscle is also called

Answer 6

syncytial smooth muscle or visceral smooth muscle

Question 7

define “unitary”

Answer 7

doesnt mean a single muscle fiber, but a single mass of hundreds to thousands of fibers that act together

Question 8

unitary smooth muscle is arranged in

Answer 8

sheets or bundles, and their cell membranes are connected so force is transmitted over all of them

Question 9

what specific complex does skeletal muscle possess that smooth muscle does not have?

Answer 9

troponin complexes.

Question 10

what visible structural quality immediately distinguishes smooth muscle from skeletal muscle?

Answer 10

it doesnt have striations found in the actin/myosin arrangement of skeletal muscle

actin filaments attach to DENSE bodies instead

Question 11

Dense bodies

Answer 11

actin fibers in smooth muscle attach to “Dense bodies” which themselves attach to membrane, while others are dispersed inside the cell

Question 12

ratio of actin to myosin in smooth muscles

Answer 12

5-10 times as many actin filaments as myosin filaments

Question 13

dense bodies = _____ of smooth muscles

Answer 13

Z discs

Question 14

troponin is to skeletal muscles as _____ is to smooth muscle

Answer 14

calmodulin

Question 15

sequence of events leading to smooth muscle contraction

Answer 15

1) Ca2+ increases intracellularly
2) calcium binds to calmodulin
3) calmodulin-calcium complex then joins and activates myosin light chain kinase (MLCK) a phosphorylating enzyme
4. a myosin light chain called the Regulatory Chain becomes phosphorylated in response to myosin kinase. Head of the regulatory chain now has ability to repetitively bind with actin filaments and proceed through entire cycling process of intermittent “pulls”

Question 16

where does the majority of the Ca2+ in smooth muscle come from?

Answer 16

comes from the extracellular matrix, only a small amount comes from the sarcoplasmic reticulum

Question 17

the time required for calcium diffusion into smooth muscle is called the

Answer 17

latent period
happens before contractions begins
latent period is 50 x as great for smooth muscle as for skeletal muscle

Question 18

How does smooth muscle cause relaxation?

Answer 18

A calcium pump removes Ca2+ from intracellular fluid and pumps it into the sarcoplasmic reticulum and the ECM.

important: this is ATP dependent, is slow acting compared to the one in skeletal muscle, and therefore lasts for seconds rather than hundreths to tenths of a second, as occurs for skeletal muscle

Question 19

when Ca falls below a certain level, what happens in the cell

Answer 19

1. Ca is released from calmodulin
2. myosin phosphatase removes phosphate from myosin light chain –> causes detachment of myosin head from actin
   3.

Question 20

Myosin Phosphatase

Answer 20

important in cessation of contraction

Question 21

sequence leading to smooth muscle relaxation

Answer 21

1. calcium is pumped out of cytosol by ATP-Ca pump (pumps Ca into SR and ECM)
2. Ca falls to critical level, Calmodulin releases Ca
3. Myosin phosphatase removes Pi from regulatory light chain
4. cycling stops, contraction ceases

Question 22

what two big factors determine time of relaxtion?

Answer 22

calcium concentration in the muscle and amount of myosin phosphatase