Smooth Muscle: Structure and Types | Biology

In this article we will discuss about:- 1. Introduction and Structure of Smooth Muscle 2. Types of Smooth Muscle 3. Properties 4. Characteristics 5. Contraction.

Introduction and Structure of Smooth Muscle:

Smooth muscle cells are spindle-shaped (2 to 10 m diameter and 50 to 400 m in length), have single nucleus and are capable of cell division. They lack cross striations and hence the name smooth muscle.

They contain actin and myosin filament and contract by sliding filament mechanism. The actin filaments take their origin from dense bodies in the cytoplasm which are functionally similar to Z lines in skeletal muscle. Actin filaments contain actin and tropomyosin, but troponin is absent. Sarcoplasmic reticulum is present but poorly developed. T-tubules are absent, and hence, triads are absent.

Contraction of smooth muscle occurs by sliding mechanism. Ca2+ ions are responsible for excitation- contraction coupling. Calcium binds to calmodulin, a calcium binding protein present in the cytoplasm. This calcium-calmodulin complex binds to another cytosolic protein, myosin light-chain kinase, thereby activating the enzyme for cross-bridge formation. In smooth muscle cross-bridge cycling is controlled by calcium-regulated enzyme that phosphorylates myosin, instead of actin filaments that regulates cross-bridge cycling in skeletal muscle.

Types of Smooth Muscle:

1. Single-Unit Smooth Muscle:

It is also called as visceral smooth muscle, since they are present in the walls of hollow viscera such as gastrointestinal tract, uterus, urinary bladder and respiratory tract.

Features:

i. Has a low resistance bridge (gap-junctions) between individual muscle cells and function in a syncytial fashion.

ii. Have own rhythmic contractility myogenic tone that is independent of nerve supply.

iii. Contraction is also stimulated by stretching.

iv. Contractile activity is also controlled by hormones and local tissue factors.

2. Multi-Unit Smooth Muscle:

It is made up of multiple individual units without interconnecting bridges, i.e. non-syncytial in nature. These are located in most blood vessels, epididymis, vas deferens, iris, ciliary body and piloerector muscles.

Features:

i. Multiple individual units of muscle fibers each innervated by a single nerve ending.

ii. No spontaneous contractions, i.e. no pacemaker activity.

iii. Gap junctions are absent; hence the excitation remains localized within the motor unit.

iv. Does not respond to stretching.

Properties of Smooth Muscle:

1. RMP:

It ranges from -50 to -70 mV. The peculiarity of this RMP is it is highly unstable.

2. Sinusoidal Wave:

The instability of RMP can be recorded from the longitudinal muscles of stomach and intestine. This is known basic electrical rhythm (BER).

3. Action Potential (AP):

Three types of AP occur in visceral smooth muscle:

a. Spike potential

b. Spike potential initiated by slow wave rhythm

c. Action potential with plateau

d. Tonic contraction of the muscle without any AP.

Characteristics of Smooth Muscles:

Some of the characteristics of smooth muscles are:

i. It is Nonstriated

ii. Actin and myosin not organized into sarcomeres

iii. Gap junctions

iv. Electrical syncytium

v. Calmodulin to bind calcium

vi. Low ATPase activity (slow muscle)

vii. Limited sarcoplasmic reticulum

viii. Lack T-tubules

ix. Autonomic nerve supply

x. Pacemaker potential- Absent in multi- unit and present in single-unit type smooth muscle

Smooth Muscle Contraction:

Increase cytosolic Ca²⁺ → Ca²⁺ binds to calmodulin → Ca²⁺ – calmodulin complex binds to myosin light-chain kinase → Myosin light-chain kinase uses ATP to phosphorylate myosin cross-bridges → Phosphorylated cross-bridges bind to actin filaments → Cross-bridge cycling produces tension and shortening of smooth muscle.

Factors Influencing Smooth Muscle Contractile Activity:

i. Spontaneous electrical activity in plasma mem­brane of smooth muscle cell

ii. Neurotransmitters released by autonomic nervous system

iii. Hormones

iv. Stretch

v. Changes in chemical composition of ECF surroun­ding the cell (paracrine agents, acidity, oxygen, osmolarity and ion concentration).