SMOOTH MUSCLE Structure of smooth muscle •     Shape of muscle fiber is spindle shaped                             - 1-5 ...

SMOOTH MUSCLE - CONTRACTION AND RELAXATION |FINDYOURSELF

July 22, 2019 2 Comments

SMOOTH MUSCLE

Structure of smooth muscle

•    Shape of muscle fiber is spindle shaped

                            - 1-5 µm in diameter

                            - 20-500 µm in length

•    A single nucleus present in the central thick portion.

•    Sarcolemma (cell membrane).

•    Cytoplasm appears homogenous without striations.

•    Fewer mitochondria as compared to the skeletal muscle.

•    Metabolism mostly glycolytic.

•    Actin, Myosin & Tropomyosin but NO Troponin

•    Dense bodies present attached to the cell membranes OR dispersed throughout the cell

Dense bodies serve the same purpose as the Z-discs

•    Attached to the dense bodies are numerous numbers of Actin filaments

•    Interspersed between the actin filaments are Myosin filaments  ( their diameter twice as much as actin filaments)

Usually, 5-10 times as many actin filaments as Myosin filaments

•    SIDEPOLAR CROSS-BRIDGES:

1.          Myosin filaments have sidepolar cross-bridges

   2.  Bridges on one side hinge in one direction & on the other side in the opposite direction

 3. Allows myosin to pull an actin filament in one direction while simultaneously pulling it in the other direction on the other side

4. Allow smooth muscle to contract 80% as compared to only 30 % in the skeletal muscle

(Force of contraction in skeletal muscle is limited because of the presence of the z-disc, against which the thick filament against and cannot move any further)

•    Calcium Pump: pumps Ca back into the SR if present for relaxation to take place. But it is very slow so that duration of cont. is longer.

•    Neuromuscular Junction:

Does not occur in smooth muscle. Instead the autonomic nerves make diffuse junctions that secrete NT into the matrix coating of smooth muscle. A few micrometers away from the muscle fiber

Also the axons supplying them do not have terminal buttons but varicosities on their terminal axons that contain the vesicles containing the NT

•    Neurotransmitter:

Apart from Ach, norepinephrine can also be released

Instead of synaptic clefts, smooth muscles have contact junctions

CLASSIFICATION OF SMOOTH MUSCLES

UNITARY/ SINGLE UNIT/SYNCYTIAL/VISCERAL

1.                      Muscles of visceral organs .e.g. GIT, uterus, ureters & some of the smaller blood vessels.

2.                      Form a sheet or bundles of tissue.

3.                      Cell membranes show a gap junction that allows AP to pass rapidly from cell to cell.

4.                      AP spreads rapidly throughout the sheet of cells – cells contract as a single unit.

MULTI-UNIT

1.                      Iris & Ciliary body of the eye, large arteries, Piloerector muscles

2.                      Showing discrete, individual smooth muscle fibers.

3.                      Smooth muscle cells not electrically linked. Each muscle fiber innervated by a single nerve ending. NT itself can spread and lead to an AP.

4.                      Selective activation of each muscle fiber that can then contract independently of each other.

Smooth muscle contraction:

When unitary (visceral) smooth m. is stretched, spontaneous AP is usually generated, because:

1.                      Normal slow potentials caused by stretch

2.                      Overall ↓ in memb. Negativity caused by stretch

SMOOTH MUSCLE CONTRACTION SEQUENCE OF EVENTS:

·    Binding of Ach to the receptors

·    Increased Influx of Ca into the cell from the following sources:

·    ECF thru Ca channels

·    Ca released from SR

·    Stretch-activated Ca channels when membrane is deformed

·    Chemical-gated Ca channels by NT & hormones

·    Ca binds to Calmodulin

·    Ca-Calmodulin activates the enzyme: Myosin light chain kinase MLCK or simply Myosin kinase

·    Phosphorylation of myosin, using energy & Pi from ATP

·    Increased ATPase activity & binding of myosin to actin.

·    Contraction of smooth muscle

SMOOTH MUSCLE RELAXATION
SEQUENCE OF EVENTS:

1. Dephosphorylation of Myosin by myosin phosphatase/ MLCP

2. Decreases its ATP activity

3. Ca removed from cytoplasm using Ca-Na antiport protein & Ca-ATPase

4. Calmodulin releases Ca & uncomplexes from MK

5. MK is phosphorylated by Protein kinase, inactivating it

6. Relaxation OR sustained contraction

Latch system

It is a state in which the dephosphrylated myosin remains attached to actin for prolonged period of time. This produces sustained contraction without consuming ATP & thus enables the smooth muscle to sustain long-term maintenance of tone without fatigue. E.g. urinary bladder full of urine.