Muscle Types

Structure and function of muscle

Muscle tissue (contractile tissue) can contract and relax. It can only pull, not push.

Muscle is classified by the following three types:

Striated Muscle

Also referred to as voluntary or skeletal muscle, it is under conscious control.

The muscle fibres are made up of connective tissue which in turn are made up of fibrils.

Non-striated Muscle

Also referred to as smooth or involuntary muscle, it is not under conscious control but rather the autonomic system.

Cardiac Muscle

Cardiac muscle is only found in the heart.

Characteristics

Muscle tissue has four principle features:

• It is excitable, i.e. it will respond to stimuli (either internal or external)
  
  
• It is contractible, i.e. can shorten and thicken (pull) in response to a stimulus
  
  
• It is extensible, i.e. is able to stretch
  
  
• It is elastic, i.e. it will return to its original length after contracting or extending
  
  

Muscle cannot push and must in nearly all cases operate in pairs.

Function

Muscle tissue performs four distinct functions:

• Motion - this includes locomotion, skeletal movement & internal motion, e.g. gut
  
  
• Sphincters control - closes orifices to restrict movement or increase pressure
  
  
• Maintenance of posture - Antagonistic muscle contraction leads to muscle tone
  
  
• Heat production - muscles are only approximately 30% efficient, the remainder of the energy is released as heat
  
  

There are three types of muscle:

	Appearance	Location	Innervation	Control
Skeletal	Striated	Skeleton	Voluntary
Smooth	Unstriated	Viscera/Organs	Involuntary
Cardiac	Striated	Heart	Myogenic	Involuntary

note: myogenic ≡ self-stimulating

The Skeletal Muscle System

Skeletal muscles are under conscious control and are able to bring about movement across a joint. It therefore follows that they must be attached firmly to the skeleton over each side of the joint. These are known as:

• Origin - a firm, non-movable part of the skeleton
  
  
• Insertion - a freely movable part of the skeleton which moves towards the origin
  
  

Attachment is through a tendon which is connective tissue containing an abundance of tough collagen fibres. This is continuous with the muscle sheath (outer covering of the muscle) and the Periosteum (outer covering of the bone).

Muscles are only capable of exerting a force by contracting. They are unable to push and must therefore be arranged in most cases in pairs called antagonistic pairs. In many cases, these pairs serve to operate across a single joint. Where muscles act as antagonistic pairs, any given movement requires an initial contraction by the prime mover (antagonist) accompanied by a corresponding lengthening by the antagonist.

The antagonistic pairs then reverse roles as the joint returns to its original position.