22. Characteristics of smooth muscle Flashcards
What should be mentioned?
Smooth Muscles
Microscopic structure & morphologic basis of contraction (single unit)
Molecular Structure of Smooth Muscle Sarcomer
Myosin in the smooth muscle
MLCK / MP balance
Characteristics of Smooth Muscle Contraction
Varicosities
Types of Smooth Muscles:
Multy Unit
Single Unit
Special Structure of „SR”
Types of AP in Smooth Muscle
Local Chemical Factors Influencing Smooth Muscle Contraction
Myogen Response of Single-Unit Muscles
Spontaneous Generation of Smooth Muscle AP (Pacemaker or Slow Wave Potential)
Smooth Muscles
Smooth muscles play an essential role in the movement and in the maintenance of the form of visceral organs. The arrangement of actin and myosin filaments is less organized then in skeletal muscles. Smooth muscles can be functionally allotted into two major groups:
- Multi-unit smooth muscles. Here, individual fibres are not connected with gap junctions. Single firers or small groups of fibres are under direct neural control: this kind of smooth muscle is capable of fast and accurate movements (it can be found inside the eye or in piloerector muscles).
- A single unit smooth muscle consists of many hundreds of fibres connected with gap junctions and forming a functional syntytium. These fibers are innervated by varicosities. Smooth muscles of vessels or luminal organs are innervated like this.
Microscopic structure & morphologic basis of contraction (single unit)
The dense bodies and intermediate filaments are networked through the sarcoplasm, which cause the muscle fiber to contract.
Microscopic structure & morphologic basis of contraction (single unit)
Contraction of smooth muscles is substantially different from that of skeletal muscles. Steps:
In smooth muscles, activation of sliding filaments is triggered by MLCK (myosin light chain kinase) enzyme, if intracellular calcium level is appropriately high.
Actomyosin complex formation.
- Contraction stays continuous until relaxation is triggered by another enzyme, MP (myosin phosphatase). If this enzyme is active, a phosphate group is hydrolyzed from actomyosin, myosin and actin separates, muscle relaxes.
- This process (unlike in skeletal muscle) is not based on the “all- or-none” law. Most of the smooth muscles are in weak, but continuous contraction. The measure of this contraction depends on processes which may set the IC calcium level with changing the activity ratio of MLCK / MP enzymes.
Smooth Muscle: basics
Poor blood supply
• Small, spindle shaped cells
• Non-striated
• Non-regular myofilaments
• Intermediate filaments and dense bodies
• No transversal tubulary system
• Small sarcoplasmic reticulum
Molecular Structure of Smooth Muscle Sarcomer (picture)
Myosin in the smooth muscle (picture)
Characteristics of Smooth Muscle Contraction
Prolonged tonic contraction (for hours, days).
Lower frequency of cross-bridge cycles ( 1/10 – 1/300 of sceletal muscles).
reason: lower ATPase activity – slower ATP catabolism – slower Myosin head power-stroke.
Energetically economic (only 1 ATP/cross bridge cycle; irrespective of the length of contraction;
provides permanent tonicity of the visceral organs with low energy.
Length of contraction (1-3 sec) 30x longer than the sceletal muscle.
Max contraction Tension = 6kg/cm2 (2x higher than in sceletal muscles). Reason: prolonged cross-bridge – actin contact.
Max Contraction (length): 66% or 1/3-of resting length (60% in sceletal muscles) Reason: the Myosin can pull the individual „dense bodies” closer, because Actin molecules are longer.
Maximal (prolonged) contraction – is maintained by „Latch” mechanism. (contraction can be maintained for hours (latched) with minimum (1/300) energy) – only MP can release the contraction.
Stress-Relaxation (plasticity). It is characteristic to visceral organs (i.e. urinary bladder)
mechanism: Increase of urine volume creates stretch – creates increased tension – MP release (extension of muscle to the new volume) – Tension is reduced – so volume increases again – Tension increases again, etc…….
Varicosities
Types of Smooth Muscles: Multy Unit
Characteristics:
- Individual Fibers
- No GAP junctions
- Individual Innervation (1/1)
- Contraction is independent in each fiber, and stimulated by nerves – but not through AP. (because the fibers are too small to generate AP (normally 30-40 fibers are needed for AP).
Transmitters cause local depolarization.
Multiunit smooth muscle tissues innervate individual cells; as such, they allow for fine control and gradual responses, much like motor unit recruitment in skeletal muscle.
Spontaneous contraction is rare.
Fibers are insulated by collagen and glycoprotein.
Types of Smooth Muscles: Multy Unit (picture)
Types of Smooth Muscles: Single Unit (picture)
Types of Smooth Muscles: Single Unit
Hundreds and millions of fibers contracting all at once, creating a functional syncitium.
lots of Gap-Junctions
fast penetration of ions
fast spreading of AP.
Within single-unit cells, the whole bundle or sheet contracts as a syncytium (i.e. a multinucleate mass of cytoplasm that is not separated into cells).
Spontaneous contraction may happen.
Types of Smooth Muscles (picture)
Types of AP in Smooth Muscle
-AP occurs exclusively in Single-Unit (visceral) smooth muscle. Resting membrane potential (treshold): only -50, -60 mV
(in striated muscles: – 90 mV)
Forms of AP:
a) Typical peak potential (as in sceletal muscle)
however it can be initiated:
- by electric stimuli
- by hormones
- can be generated spontaneously. (by pacemaker activity)
Relevance:
Prolonged contraction (i.e. myocardium, uterus, urether, etc.) never makes tetanic contraction.
Types of AP in Smooth Muscle (picture1)
Factors causing contraction:
Spreading of sympathetic (alpha1) or parasympathetic (mACh) neural AP to the myolemma
and its effect on voltage gated ion channel (result: calcium influx).
Binding of chemical ligands to ligand gated calcium channels.
Intracellular IP3 release and G-protein or phospholipase C (PLC) mediated, ligand gated,
calcium influx from sarcoplasmic reticulum or from a calcium sequester vesicle.
Types of AP in Smooth Muscle (picture 2)
Relaxation is enhanced by every stimulus which can increase IC cAMP or cGMP level (blue parts): these also decrease IC calcium concentration. Phosphorylation of MLCK or enhanced MP activity also facilitate relaxation.
The most important signals are sympathetic beta2 receptor agonists (epinephrine, VIP) also the NO (direct, or diffusible from capillary endothelia) and, presumably, ATP (which acts on purinergic receptors).
