Lecture 7.3 Smooth Muscle Types: Multi-Unit and Unitary

• Many principles of skeletal muscle contraction also apply to smooth muscle
• Smooth muscle contraction involves attractive forces between myosin and actin
• The main difference between the two lies in the internal physical arrangement

Types of Smooth Muscle

• Smooth muscle is divided into multi-unit and unitary types

Multi-Unit Smooth Muscle

• Composed of discrete, separate, smooth muscle fibers that operate independently
• Often innervated by single nerve endings
• Covered by a basement membrane, which helps insulate the fibers
• Controlled mainly by nerve signals
• Examples include ciliary muscles of the eye and piloerector muscles

Unitary Smooth Muscle

• Also called visceral smooth muscle
• Consists of hundreds to thousands of smooth muscle fibers that contract together as a single unit
• Fibers are arranged in sheets or bundles
• Joined by gap junctions that allow ions to flow freely between cells
• Action potentials easily travel from one fiber to the next
• Found in the walls of most viscera, including the GI tract, bile duct, uterus, and blood vessels

Smooth Muscle Contraction

• Contains actin and myosin, but with different physical organization compared to skeletal muscle
• Differences include excitation contraction coupling, control by calcium ions, duration of contraction, and energy requirements

Actin and Dense Bodies

• Large numbers of actin filaments are attached to dense bodies
• Some dense bodies are attached to the cell membrane, others are dispersed inside the cell
• Dense bodies can be bonded to adjacent cells by intracellular protein bridges
• These bridges transmit the force of contraction from one cell to the next

Myosin

• Myosin filaments are interspersed among the actin filaments
• Myosin has a diameter 5 to 10 times that of actin
• The contractile unit (actin and myosin) lacks the regularity of skeletal muscle structure

Characteristics

• Most smooth muscle contraction is a prolonged tonic contraction, which can last for hours or even days
• Cycling of myosin cross-bridges (attachment to and release from actin) is much slower
• The fraction of time the cross bridges remain attached is greatly increased, causing a stronger contraction
• Causes low energy requirements due to the slow cycling of cross bridges, as only one ATP molecule is required per cycle
• Contraction begins 50 to 100 milliseconds after excitement, reaches full contraction in 0.5 seconds, and declines in force for 1 to 2 seconds
• Total contraction time is 1 to 3 seconds, about 30 times longer than a single skeletal muscle contraction
• The maximum force of contraction of smooth muscle is often greater than skeletal muscle, due to the prolonged attachment of myosin cross-bridges

Latch Mechanism and Stress Relaxation

• Latch mechanism refers to the prolonged attachment of myosin to actin filaments
• It requires much less energy and can be maintained for extended periods with little excitatory signal
• Stress relaxation enables smooth muscle to return to its original force of contraction during elongation or shortening
• When pressure increases, the muscle relaxes quickly to maintain the same pressure (e.g., urinary bladder)
• Reverse stress relaxation occurs with less volume, causing constriction of the smooth muscle

Calcium's Role

• Stimulus for smooth muscle contraction is calcium ion concentration
• Can be caused by nerve stimulation, hormonal stimulation, stretch, or changes in the chemical environment
• Smooth muscle does not contain troponin
• Calcium combines with calmodulin to initiate contraction

Contraction Process

• Increased calcium concentration in the cytosol occurs due to influx through calcium channels or release from the sarcoplasmic reticulum
• Calcium binds to calmodulin
• The calcium-calmodulin complex activates myosin light chain kinase
• Active myosin light chain kinase causes the attachment of the myosin head to the actin filament and contraction

Additional Info

• Sarcoplasmic reticulum is less developed than in skeletal muscle
• Most calcium ions come from the extracellular fluid
• Causes a delay of 2 to 300 milliseconds for contraction (latent period)
• Contraction is dependent on extracellular calcium ion concentrations
• The more extensive the sarcoplasmic reticulum, the more rapidly the muscle contracts

Smooth Muscle Relaxation

• Calcium must be removed from intracellular fluids to cause relaxation
• Calcium pumps move calcium ions back into the extracellular fluid or sarcoplasmic reticulum
• This pump is much slower compared to skeletal muscle, causing longer contraction times
• Depletion of calcium stops all processes except the phosphorylation of the myosin head
• Myosin phosphatase causes the myosin head to stop cycling, ceasing contraction
• Without myosin phosphatase, contraction would not stop

Neurotransmitters & Hormones

• Smooth muscle contains many types of receptors that can be stimulated or inhibited by the nervous system, hormones, or stretch
• Most of these receptors are secondary messenger receptors
• Autonomic nerve fibers branch diffusely on top of the muscle sheet
• Nerve fibers often innervate only the outer layer, with excitation traveling to inner layers by action potential conduction or diffusion
• Nerve varicosities contain acetylcholine, norepinephrine, or other substances
• Acetylcholine and norepinephrine are important neurotransmitters with different effects depending on the receptor type

Neuromuscular Junctions

• Instead of diffusion through action potential conduction
• The innovation is different from the motor end plate on skeletal muscle fibers
• They are very closely in the nerve.

Summary

• These neurotransmitters can stimulate a receptor which causes of an effect
• The effect is truly dependent upon the receptor and that their effects can be different
• In some muscles, acetylcholine is inhibitory, while in others, it's excitatory
• Acetylcholine and norepinephrine typically oppose each other and are not released by the same nerve fiber

Local Tissue Chemical Factors

• Changes in blood flow can be due to contraction and dilation of pre capillary sphincters
• Increased hydrogen ion concentration, lack of oxygen, or chemicals like adenosine can cause increased blood flow

Circulating Hormones

• Various circulating hormones affect smooth muscle contraction like norepinephrine, epinephrine, angiotensin II, endothelin, vasopressin, oxytocin, serotonin, and histamine
• Receptors for these hormones are second messengers
• The action of a hormone or neurotransmitter varies in different tissues based on the receptor type

Smooth muscle contraction involves myosin and actin, differing from skeletal muscle in arrangement. Smooth muscle is divided into multi-unit and unitary types. Multi-unit muscles are discrete fibers controlled by nerve signals, while unitary muscles contract as a single unit via gap junctions.