5-5-14-mammalian-muscle-structure

Mammalian Muscle Structure

Types of muscle

• There are three types of muscle found within mammals

  • Skeletal muscle (also called striated or voluntary muscle)

  • Smooth muscle (also called involuntary muscle)

  • Cardiac muscle

• Skeletal muscles are responsible for moving the rigid skeleton of mammals

  • These muscles have a complicated, unique structure

Skeletal muscle

• Striated muscle makes up the muscles in the body that are attached to the skeleton

• Striated muscle is made up of muscle fibres

• A muscle fibre is a highly specialised cell-like unit:

  • Each muscle fibre contains an organised arrangement of contractile proteins in the cytoplasm

  • Each muscle fibre is surrounded by a cell surface membrane

  • Each muscle fibre contains many nuclei – this is why muscle fibres are not usually referred to as cells

• The different parts of a muscle fibre have different names to the equivalent parts of a normal cell:

  • Cell surface membrane = sarcolemma

  • Cytoplasm = sarcoplasm

  • Endoplasmic reticulum = sarcoplasmic reticulum (SR)

• The sarcolemma has many deep tube-like projections that fold in from its outer surface:

  • These are known as transverse system tubules or T-tubules

  • These run close to the SR

• The sarcoplasm contains mitochondria and myofibrils

  • The mitochondria carry out aerobic respiration to generate the ATP required for muscle contraction

  • Myofibrils are bundles of actin and myosin filaments, which slide past each other during muscle contraction

• The membranes of the SR contain protein pumps that transport calcium ions into the lumen of the SR

Skeletal muscle structure diagram

Striated muscle tissue is made up of muscle fibres, which contain many myofibrils

Myofibrils

• Myofibrils are located in the sarcoplasm

• Each myofibril is made up of two types of protein filament:

  • Thick filaments made of myosin

  • Thin filaments made of actin

• These two types of filament are arranged in a particular order, creating different types of bands and lines

Myofibril structure table

Sarcomere structure diagram

Sarcomeres are the contractile units of myofibrils

Smooth (involuntary) muscle

• Smooth muscle is vital for the unconscious control of many body parts

• Similar to skeletal muscle it contains both actin and myosin filaments however it does not have any banding or striation

• Several internal organs (e.g. the gut) contain smooth muscle within their walls

  • For example, the walls of blood vessels have a layer of smooth muscle that allows for the narrowing of arteries to control blood flow

• The structure of smooth muscle is relatively simple

  • It consists of small elongated cells/spindle-shaped fibres that contain one nucleus

Smooth muscle diagram

Smooth muscles cells are substantially smaller than skeletal muscle cells and have a spindle-like shape

Cardiac muscle

• Cardiac muscle is only present within the heart

• It is a type of specialised striated muscle with the following properties:

  • It is myogenic, meaning that it can contract without external stimulation via nerves or hormones. This allows the heart to beat at its own regular intervals (the length of the intervals can be regulated by the nervous system and endocrine system)

  • It does not tire or fatigue so it can contract (beat) continuously throughout an individuals life

  • The cardiac muscle fibres form a network that spreads through the walls of the atria and ventricles

  • Cardiac muscle fibres are connected to each other via specialised connections called intercalated discs

  • There is a large number of mitochondria present in the muscle fibres. These are needed to provide the large quantity of ATP needed for continual contraction

Cardiac muscle diagram

Cardiac muscle contains only one nucleus per cell

Mammalian Muscle Under The Microscope

• Many biological structures are too small to be seen by the naked eye

• Optical microscopes are an invaluable tool for scientists as they allow for tissues, cells and organelles to be seen and studied

• For example, the movement of chromosomes during mitosis can be observed using a microscope

How optical microscopes work

• Light is directed through the thin layer of biological material that is supported on a glass slide

• This light is focused through several lenses so that an image is visible through the eyepiece

• The magnifying power of the microscope can be increased by rotating the higher power objective lens into place

Apparatus

• The key compon