Excitation and Contraction of Smooth Muscle DSA Flashcards
Two types of smooth muscle =
- multi-unit smooth muscle
2. unitary (single-unit) smooth muscle
Multi-Unit Smooth Muscle: 4 generalized traits
- composed of discrete, separate smooth muscle fibers
- each fiber innervated independently of the others
- look like skeletal muscle on the outside
- covered in basement like membrane
Important characteristic of multi smooth muscle fibers regarding innervation
they can each contract independently of the others. control is exerted mainly by nerve signals
a major share of nervous control of unitary smooth muscle is by
non-nervous means
examples of multi unit smooth muscle
piloerector muscles, ciliary muscle of the eye, iris muscle of the eye
Unitary smooth muscle is also called
syncytial smooth muscle or visceral smooth muscle
define “unitary”
doesnt mean a single muscle fiber, but a single mass of hundreds to thousands of fibers that act together
unitary smooth muscle is arranged in
sheets or bundles, and their cell membranes are connected so force is transmitted over all of them
what specific complex does skeletal muscle possess that smooth muscle does not have?
troponin complexes.
what visible structural quality immediately distinguishes smooth muscle from skeletal muscle?
it doesnt have striations found in the actin/myosin arrangement of skeletal muscle
actin filaments attach to DENSE bodies instead
Dense bodies
actin fibers in smooth muscle attach to “Dense bodies” which themselves attach to membrane, while others are dispersed inside the cell
ratio of actin to myosin in smooth muscles
5-10 times as many actin filaments as myosin filaments
dense bodies = _____ of smooth muscles
Z discs
troponin is to skeletal muscles as _____ is to smooth muscle
calmodulin
sequence of events leading to smooth muscle contraction
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”
where does the majority of the Ca2+ in smooth muscle come from?
comes from the extracellular matrix, only a small amount comes from the sarcoplasmic reticulum
the time required for calcium diffusion into smooth muscle is called the
latent period
happens before contractions begins
latent period is 50 x as great for smooth muscle as for skeletal muscle
How does smooth muscle cause relaxation?
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
when Ca falls below a certain level, what happens in the cell
- Ca is released from calmodulin
- myosin phosphatase removes phosphate from myosin light chain –> causes detachment of myosin head from actin
3.
Myosin Phosphatase
important in cessation of contraction
sequence leading to smooth muscle relaxation
- calcium is pumped out of cytosol by ATP-Ca pump (pumps Ca into SR and ECM)
- Ca falls to critical level, Calmodulin releases Ca
- Myosin phosphatase removes Pi from regulatory light chain
- cycling stops, contraction ceases
what two big factors determine time of relaxtion?
calcium concentration in the muscle and amount of myosin phosphatase
