Types, Properties, Contraction & Relaxation Smooth Muscle: Types, Properties, Contraction & Relaxation

Smooth Muscle - Types Unitary / Visceral Smooth Muscle Multi-Unit Smooth Muscle

Unitary Smooth Muscle A group of smooth muscle fibers that contract together as a single unit. The fibers usually are arranged in sheets or bundles, and their cell membranes are adherent to one another at multiple points so that force generated in one muscle fiber is transmitted to the next. In addition, the cell membranes are joined by many gap junctions through which ions can flow freely from one muscle cell to the next.

Unitary /Visceral Smooth Muscle Found primarily in the walls of viscera e.g. the musculature of the intestine, the uterus, and the ureters.

Multi unit Smooth Muscle Composed of discrete, separate smooth muscle fibers. Each fiber is innervated by a single nerve ending. The outer surfaces of these fibers are covered by a mixture of fine collagen and glycoprotein basement membrane that helps insulate the fibers from one another. The most important characteristic: each fiber can contract independently of the others, and their control is exerted mainly by nerve signals. In contrast, to unitary smooth muscle where control is exerted by non-nervous stimuli.

Multi-Unit Smooth Muscle Found in structures in which fine, graded contractions occur e.g. of multi-unit smooth muscle : the ciliary muscle of the eye, the iris muscle of the eye, the piloerector muscles that cause erection of the hairs when stimulated by the sympathetic N.

Most smooth muscle contraction is prolonged tonic contraction, sometimes lasting hours or even days.

Physical Organization of Smooth Muscle E M studies show large numbers of actin filaments attached to dense bodies. Some of these are attached to the cell membrane. Others are dispersed inside the cell. Interspersed among the actin filaments are myosin filaments. These have a diameter more than twice that of the actin filaments. Actin filaments are usually 5 to 10 times myosin filaments in number.

Syncytial Arrangement Some of the membrane dense bodies of adjacent cells are bonded together by intercellular protein bridges through which the force of contraction is transmitted from one cell to the next cell. (Smooth muscles do not have striated arrangement of actin and myosin filaments).

Contractile Unit An individual contractile unit within a smooth muscle cell shows large numbers of actin filaments radiating from dense bodies; the ends of these filaments overlap a myosin filament located midway between the dense bodies. (The dense bodies of smooth muscle serve the same role as the Z discs in skeletal muscle).

The actin –myosin organization of smooth muscle is such that it allows smooth muscle cells to contract as much as 80 per cent of their length.

Slow Contraction & Relaxation A typical smooth muscle begins to contract 50 to 100 msecs after it is excited, reaches full contraction in about 0.5 second & declines in contractile force in 1 to 2 secs, giving a total contraction time of 1 to 3 Secs. Some types of smooth muscle contraction can be as short as 0.2 second or as long as 30 seconds. (May be due to the slowness of attachment and detachment of the cross-bridges with the actin filaments. & the Initiation of contraction in response to calcium ions is also much slower).

Force of Contraction The maximum force of contraction of smooth muscle is often greater than that of skeletal. This great force of smooth muscle contraction results from the prolonged period of attachment of the myosin crossbridges to the actin filaments.

Prolonged Contraction : Mechanism “Latch” Mechanism for Prolonged Contractions of Smooth Muscle: Once smooth muscle has developed full contraction, even if the excitation is reduced yet the muscle maintains its full force of contraction. Further, the energy consumed to maintain contraction is often v. little

The importance of the latch mechanism is that it can maintain prolonged tonic contraction in smooth muscle for hours with little use of energy. Little continued excitatory signal is required from nerve fibers or hormonal sources

Role of Ca in Contraction The initiating stimulus for most smooth muscle contraction is an increase in intracellular calcium ions. Caused by: Neural or hormonal stimulation of the smooth muscle fiber, stretch of the fiber, change in the chemical environment of the fiber.

Role of Ca- Calmodulin Combination of Calcium Ions with Calmodulin—Activation of Myosin Kinase and Phosphorylation of the Myosin Head.

Sequence of events in Contraction Calmodulin activates the myosin cross-bridges leading to contraction through the following sequence: 1. The calcium ions bind with calmodulin. 2. The calmodulin-calcium combination joins with and activates myosin kinase, a phosphorylating enzyme. 3. One of the light chains of each myosin head, called the regulatory chain, becomes phosphorylated in response to this myosin kinase. 4. The head binds repetitively with the actin filament and proceeds through the entire cycling process of intermittent “pulls,” causing muscle contraction.

Relaxation With fall in the calcium ion concentration below a critical level: Reversal of phosphorylation of the myosin occurs through the enzyme, myosin phosphatase, located in the fluids of the smooth muscle cell, which splits the phosphate from the regulatory light chain. Then the cycling stops and contraction ceases. The time required for relaxation of muscle contraction, therefore, is determined to a great extent by the amount of active myosin phosphatase in the cell.

Smooth Muscle Neuromuscular Junctions The autonomic nerve fibers that innervate smooth muscle form diffuse junctions that secrete their transmitter substance into the matrix of the smooth muscle often a few nanometers to a few micrometers away from the muscle cells; the transmitter substance then diffuses to the cells.

The axons that innervate smooth muscle fibers at terminal axons have multiple varicosities. At these points the Schwann cells that envelop the axons are interrupted so that transmitter substance can be secreted through the walls of the varicosities. In the varicosities are vesicles that contain transmitter substance, the vesicles of the autonomic nerve fiber endings contain acetylcholine in some fibers and norepinephrine in others—and occasionally other substances as well.

Membrane Potentials in Smooth Muscle RMP:In the normal resting state, the intracellular potential is usually about -50 to -60 millivolts Spike Potentials. Typical spike action potentials, such as those seen in skeletal muscle, occur in most types of unitary smooth muscle. The duration of this type of action potential is 10 to 50 milliseconds, as shown in Action Potentials with Plateaus. The onset of the action potential is similar to that of the typical spike potential, instead of rapid repolarization of the muscle fiber membrane, the repolarization is delayed for several hundred to as muchas 1000 milliseconds (1 second). RMP about 30millivolts less negative than in skeletal muscle.

Action Potentials in Unitary Smooth Muscle. Action potentials occur in unitary smooth muscle (such as visceralmuscle) in the same way that they occur in skeletal muscle. They do not normally occur in many, multi-unit types of smooth muscle. The action potentials of visceral smooth muscle occur in one of two forms: (1) spike potentials or (2) action potentials with plateaus.

Action Potentials in Smooth Muscles

Calcium Channels in Generating the Smooth Muscle Action Potential The smooth muscle cell membrane has more voltage-gated calcium channels but few voltage gated sodium channels. Flow of calcium ions to the interior of the fiber is mainly responsible for the action potential. The Ca channels open slowly and they also remain open much longer. (Responsible for the prolonged plateau AP of some smooth muscle fibers). And calcium ion entry into the cells during the AP also act directly on the smooth muscle contractile mechanism to cause contraction. Thus, the calcium performs two tasks at once. Sodium participates little in the generation of the action potential in most smooth muscle.

Slow Wave Potentials Some of the smooth muscles are self-excitatory. AP arise within the muscle cells themselves without an extrinsic stimulus. This causes a basic slow wave rhythm of the membrane potential. A typical slow wave in a visceral smooth muscle is seen in the gut. The slow wave itself is not the AP, it is a local property of the smooth muscle fibers that make up the muscle mass.

Cause of Sow wave rhythm Unknown. One suggestion is that the slow waves are caused by waxing and waning of the pumping of positive ions (presumably sodium ions) outward through the muscle fiber membrane; Another suggestion is that the conductances of the ion channels increase and decrease rhythmically

The importance of the slow waves The slow waves cannot cause muscle contraction, but when the peak of the negative slow wave potential inside the cell membrane rises in the positive direction from -60 to about -35 mvs (approx threshold for eliciting AP), an AP develops and spreads over the muscle mass leading to contraction. These repetitive sequences of APs elicit rhythmical contractions. Therefore, the slow waves are called pacemaker waves.

Depolarization of Multi-Unit Smooth Muscle Without Action Potentials Muscle fibers of multi-unit smooth muscle normally contract in response to nerve stimuli. The nerve endings secrete acetylcholine in the case of some multi-unit smooth muscles and norepinephrine in the case of others. The transmitter substances cause depolarization of the smooth muscle membrane, and this in turn elicits contraction. AP usually do not develop.

Source of Calcium Ions Ca ions that cause contraction enter muscle cell from the ECF at the time of the AP. The conc. of Ca ions in the ECF is greater in comparison with inside the smooth muscle cell; this causes rapid diffusion of the Ca ions into the cell from the ECF when the calcium pores open. The tm required for diffusion avg 200 to 300 msecs ,so latent period of contraction is more for smooth muscle

Learning Outcomes State the types of smooth muscle give an example of each. Explain the electrical properties of smooth muscle with reference to RMP, slow wave potential, spike potential Outline the molecular basis of smooth muscle contraction & relaxation

Learning Outcomes ( Cont. ) Explain the mechanism of excitation contraction coupling in smooth muscle Outline the autonomic innervation of smooth muscles

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