All muscles share the following properties, which are interrelated and achieve movement
 Contractility (ability to shorten in response to stimuli) Excitability (ability to react to stimulus
Extensibility (ability to undergo stretch
Elasticity (ability to return to original shape and size

The three types of muscle you should be familiar with are: Skeletal muscle

Smooth muscle

Cardiac muscle

Skeletal muscle

Skeletal muscle is that which is attached to the skeleton, contraction of which brings about skeletal movements. It may also be referred to as striated muscle, reflecting the presence of the alternating light and dark striations (thin and thick filaments, respectively) seen under the microscope. Skeletal muscle is under voluntary control, although it is capable of involuntary contraction. It is usually in a state of partial contraction ,(muscle tone). Skeletal muscle makes up 40-50% of the total body weight of an adult male, and 30-40% of an adult female.

Structure of skeletal muscle

Skeletal muscle is composed of cylindrical muscle fibres, many of which run the whole length of the muscle, and these are bound together by connective tissue. Each muscle fibre is enclosed in a cell membrane (sarcolemma) and contains

• Myofibrils (stacked length ways, running the entire length of the fibre; composed of thick and thin filaments called myofilaments)

Endoplasmic reticulum
. Many nuclei
 Cellular cytoplasm (termed sarcoplasm

Each myofibril is made up of arrays of parallel filaments. The thick filaments have a diam­eter of about 15 nm and are composed of the protein myosin the thin filaments have a diameter of about 5 nm and are composed of the protein actin, with smaller amounts of troponin and tropomyosin. A sarcomere is composed of one thick and two thin filaments, which interact to cause muscle contraction, and it is this arrangement that gives skeletal muscle its striated appearance.

Individual muscle fibres are grouped together into long bundles (fasciculi), which in turn are bunched together by connective tissue called perimysium to make up the muscle mass.The entire muscle is then surrounded by epimysium. Although each individual striated fibre can contract individually (eg the ocular muscles), muscle fibres tend to contract in groups

Innervation of skeletal muscle

Motor neurons in peripheral nerves leading to skeletal muscles have branching axons, each of which terminates in a neuromuscular junction with a single muscle fibre. Nerve impulses passing down a single motor neuron will thus trigger contraction in all the muscle fibres at which the branches of that neuron terminate. The nerve together with the muscle fibres it innervates make up a motor unit

The size of the motor unit is small in muscles over which we have precise control. For example, a single motor neuron triggers fewer than 10 fibres in the muscles controlling eye movements. In contrast, a single motor unit for a muscle like the gastrocnemius may include 1000-2000 fibres. Although the response of a motor unit is all or none, the number of motor units activated determines the strength of the response of the entire muscle.

The junction between the motor neuron and the muscle fibre is called the neuromuscular junction. Here the axon terminals of the nerve cross the endomysium of the muscle to contact the corresponding muscle fibre, at a specialised area called the motor end plate. Nerve impulses are transmitted to the muscle by release of acetylcholine, a neurotransmit­ter, the effect of which is to trigger an action potential in the muscle fibres.

Activation of the muscle fibre causes the myosin (in the thick filament) to bind to actin (in the thin filament), which draws the thin filament a short distance ( 10 nm) past the thick filament. These bonds then break (for which ATP is needed) and reform further along the thin filament to repeat the process. As a result, the filaments are pulled past each other in a ratchet-like action, and contraction of the muscle occurs. This is called the sliding filament model of muscle contraction.

Excitation-contraction coupling

Activation of the muscle requires translation of the action potential from the motor neuron into a stimulus which causes the actin and myosin filaments to interact. This is achieved using calcium stores in the muscle sarcoplasmic reticulum. The arrival of the action potential triggers the release of calcium, which diffuses among the thick and thin filaments where it binds to troponin, and this initiates the contraction of the sarcomere. When the process is over, the calcium is pumped  back into the sarcoplasmic reticulum and the process is ready to repeat. ATP fuels muscle contraction. The level of ATP is maintained by creatine phosphate and glycogen.

Types of muscle fibre

Two different types of muscle fibre can be found in most skeletal muscles  type I and type II.

Type I fibres

• Loaded with mitochondria

• Resistant to fatigue

• Rich in myoglobin (red colour

• Activated by slow-conducting motor neurons

• Known as slow-twitch fibres

• Dominant in muscles that depend on tonus (eg posture muscles)

Type II fibres

• Few mitochondria

• Rich in glycogen

• Fatigue easily

• low in myoglobin (whitish in colour

• Activated by fast-conducting motor neurons

• Known as fast-twitch fibres

• Dominant in muscles used for rapid movement

Most skeletal muscles contain some mixture of type I and type II fibres, but a single motor unit always contains one type or the other never both. The ratios of type I and type I fibres can be changed by endurance training (this produces more type I fibres.

TYPES of muscle contraction

There are five types of skeletal muscle contraction:

, Twitch: a transient contraction in response to a short-lived stimulus

Isotonic contraction: muscle becomes shorter and thicker with no change in tension

Isometric contraction: muscle tension increases without change in muscle length

Treppe: repetitive stimuli over a prolonged period causes contractions of increasing strength, followed by levelling off of tension

Tetanus: rapid repeated muscle stimulation that causes a continuous contraction as the muscle cannot relax between the stimuli

Smooth muscle

Smooth muscle is found in the walls of arteries and veins, the respiratory. digestive and urogenital systems. It is composed of single spindle-shaped cells. which. despite the lack of visible microscopic striations, still possess thick and thin filaments, which slide against each other to achieve contraction of cells

Smooth muscle (like cardiac muscle) does not depend on motor neurons to be stimulated; it is innervated by the autonomic nervous system and therefore not under voluntary control. Smooth muscle can also be made to contract by other substances released in the vicinity (paracrine stimulation. eg histamine causes contraction of the smooth muscle lining airways). or by hormones circulating in the blood (eg oxyctocin contracts the uterus to begin childbirth).

Unlike skeletal muscle. smooth muscle is not attached to bone. Compared with skeletal muscle. smooth muscle contractions and relaxation are slower, more rhythmic and sustained. In addition. smooth muscle lacks the calcium-binding protein troponin, instead using calmodulin to mediate contraction.

Cardiac muscle

This is a specialised form of muscle only found in the heart. Cardiac muscle is striated, and each cell contains sarcomeres with sliding filaments of actin and myosin.

Unlike skeletal muscle. the action potential, which triggers cardiac muscle contraction, is generated within the heart itself. Although autonomic fibres pass to the heart, they merely modulate the intrinsic rate and strength of the contractions generated.

n addition, tetany is not possible within cardiac muscle, as the muscle's refractive period
is much longer than the time it takes the muscle to contract and relax.

Arthritis and metabolic bone diseases covered in this section: Arthritis

   rheumatoid arthritis

Metabolic bone diseases


Rickets and osteomalacia


AVN avascular nrcrosis
 Osteochondritis dissecans


0 comment:

Post a Comment