5.5.14 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

• 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 components of an optical microscope are:
  • The eyepiece lens
  • The objective lenses
  • The stage
  • The light source
  • The coarse and fine focus

• Other tools used:
  • Forceps
  • Scissors
  • Scalpel
  • Coverslip
  • Slides
  • Pipette

Image showing all the components of an optical microscope

Method

• Preparing a slide using a liquid specimen:
  • Add a few drops of the sample to the slide using a pipette
  • Cover the liquid/smear with a coverslip and gently press down to remove air bubbles
  • Wear gloves to ensure there is no cross-contamination of foreign cells

• Preparing a slide using a solid specimen:
  • Use scissors to cut a small sample of the tissue
  • Peel away or cut a very thin layer of cells from the tissue sample to be placed on the slide (using a scalpel or forceps)
  • Some tissue samples need be treated with chemicals to kill/make the tissue rigid
  • Gently place a coverslip on top and press down to remove any air bubbles
  • A stain may be required to make the structures visible depending on the type of tissue being examined
  • Take care when using sharp objects and wear gloves to prevent the stain from dying your skin

• When using an optical microscope always start with the low power objective lens:
  • It is easier to find what you are looking for in the field of view
  • This helps to prevent damage to the lens or coverslip incase the stage has been raised too high

• Preventing the dehydration of tissue:
  • The thin layers of material placed on slides can dry up rapidly
  • Adding a drop of water to the specimen (beneath the coverslip) can prevent the cells from being damaged by dehydration

• Unclear or blurry images:
  • Switch to the lower power objective lens and try using the coarse focus to get a clearer image
  • Consider whether the specimen sample is thin enough for light to pass through to see the structures clearly
  • There could be cross-contamination with foreign cells or bodies

• Using a graticule to take measurements of cells:
  • A graticule is a small disc that has an engraved ruler
  • It can be placed into the eyepiece of a microscope to act as a ruler in the field of view
  • As a graticule has no fixed units it must be calibrated for the objective lens that is in use. This is done by using a scale engraved on a microscope slide (a stage micrometer)
  • By using the two scales together the number of micrometers each graticule unit is worth can be worked out
  • After this is known the graticule can be used as a ruler in the field of view

The stage micrometer scale is used to find out how many micrometers each graticule unit represents

Limitations

• The size of cells or structures of tissues may appear inconsistent in different specimen slides
  • Cell structures are 3D and the different tissue samples will have been cut at different planes resulting in this inconsistencies when viewed on a 2D slide

• Optical microscopes do not have the same magnification power as other types of microscopes and so there are some structures that can not be seen
• The treatment of specimens when preparing slides could alter the structure of cells

Skeletal muscle under the microscope

• It can be very difficult to make out the features of skeletal muscle fibres using an optical microscope
• Banding is visible, this is why it is referred to as striated muscle

The dark bands produce a characteristic striped appearance

• Electron microscopes are often used to see muscle fibres in more detail
• They reveal the structure of myofibrils

The detailed structures of the muscle fibres are visible due to the much stronger magnification of the electron microscope