Muscle Part 4 Flashcards
Three types of muscle tissue
- Cardiac
- Skeletal
- Smooth
Smooth Muscle
Found in walls of most hollow organs (except heart)
Usually in two layers (longitudinal and circular)
- Single nucleus
- Mitotic
- Smaller than skeletal muscle fibers
– thick myosin-containing filaments
– thin actin-containing filaments
– tropomyosin
– NO TROPONIN
* The thin filaments are anchored either to the plasma membrane or to dense bodies
-NO SARCOMERES
-Contraction occurs by a sliding-filament mechanism
Smooth Muscle Contraction
Contraction occurs by a sliding-filament mechanism
Hollow structures/organs undergo volume changes during smooth
muscle contraction
Two sources of Ca2+
1. The sarcoplasmic reticulum
2. Extracellular Ca2+ from voltage & ligand gated Ca2+ channels
To relax, the Ca2+ has to be removed either to the SR or back to the extracellular fluid
1) Ca2+ binds to calmodulin in cytosol
2) Ca2+-calmodulin complex binds to myosin light-chain kinase
3) Myosin light-chain kinase uses ATP to phosphorylate myosin cross-bridges
4) Phosphorylated cross-bridges bind to actin filaments
5) Cross-bridge cycle produces tension and shortening
Skeletal Muscle Contraction
1) Ca2+ binds to troponin on thin filaments
2) Conformational change in troponin moves tropomyosin out of blocking position
3) Myosin cross-bridges bind to actin
4) Cross-bridge cycle produces tension and shortening
Smooth Muscle Membrane Activation
Smooth muscle responses can be graded
– more cytosolic calcium= bigger responses
* Input to smooth muscle can be either excitatory or
inhibitory
– Excitatory= more calcium channels open
– Inhibitory= fewer calcium channels open
Innervation of Smooth Muscle
Neurotransmitters released by autonomic neuron endings
Swollen regions known as varicosities
– Contain vesicles with neurotransmitter
– Some are released when AP passes
through
– No motor end plate
Varicosities from a single axon may be located
along several muscle cells
A single muscle cell may be located near varicosities of SNS and PNS axons
Single-unit smooth muscles
– respond to stimuli as a single unit
– cells are connected by gap junctions
– Ex.: intestines, uterus, small-diameter blood vessels
Multi-unit smooth muscles
– contain cells that respond to stimuli
independently
– contain few gap junctions
– Ex.: large airways, large arteries,
attached to hair bulbs
Local factors can also alter smooth muscle tension
– Paracrine signals (Nitric Oxide, NO)
– Acidity
– O2 and CO2 levels
– Osmolarity
– Ion composition of ECF
Cardiac Muscle
Use the sliding filament mechanism to contract
Striated
1-2 central nuclei
Intercalated discs with desmosomes and gap junctions
Auto rhythmicity – spontaneous APs
from SA and AV nodes
The absolute refractory period is about 250 ms. This prevents tetanic contractions in the heart
Excitation-Contraction Coupling in Cardiac Muscle
1) The membrane is depolarized by Na+ entry as an action potential begins
2) Depolarization opens L-type Ca2+ channels in the T-tubules
3) A small amount of Ca2+ enters the cytosol, contributing to cell depolarization. The trigger Ca2+ binds to, and opens, ryanodine receptor Ca2+ channels in the sarcoplasmic reticulum membrane.
4) Ca2+ flows into the cytosol, raising the Ca2+ concentration.
5) Binding of Ca2+ to troponin exposes cross-bridge binding sites on thin filament.
6) Cross-bridge cycling causes force generation and sliding of thick and thin filaments.
7) Ca2+-ATPase pumps returns Ca2+ to the sarcoplasmic reticulum
8) Ca2+-ATPase pumps remove Ca2+ from the cell
9) The membrane is depolarized when K+ exits to end to end the action potential
Skeletal vs. Cardiac Muscle Twitches
Be able to draw a picture
What kind of muscle does not have sarcomeres or transverse tubules?
Smooth muscle
Does skeletal muscle have gap junctions between cells?
No
Where is the site of Ca2+ regulation in smooth muscle? How about skeletal and cardiac muscle?
Myosin
Troponin
The stretch of the cells produces contraction in
Single unit smooth muscle cells
