Fast-Twitch and Slow-Twitch Fibers
In addition to skeletal muscle, vertebrates have cardiac muscle and smooth muscle.
Cardiac muscle, found only in the heart, consists of striated cells electrically connected by intercalated disks.
Cardiac muscle can generate action potentials without neural input.
In smooth muscle, found mainly in walls of hollow organs, contractions are relatively slow and may be initiated by the muscles themselves.
Contractions may also be caused by stimulation from neurons in the autonomic nervous system.
Skeletal muscles are attached in antagonistic pairs, with each member of the pair working against the other
The skeleton provides a rigid structure to which muscles attach.
Skeletons function in support, protection, and movement.
The interaction of antagonistic muscles and skeletons in movement
Flexor muscle relaxes
Flexor muscle contracts
Extensor muscle contracts
Extensor muscle relaxes
The three main types of skeletons are:
Hydrostatic skeletons (lack hard parts)
Exoskeletons (external hard parts)
Endoskeletons (internal hard parts)
A hydrostatic skeleton consists of fluid held under pressure in a closed body compartment
This is the main type of skeleton in most cnidarians, flatworms, nematodes, and annelids.
Annelids use their hydrostatic skeleton for peristalsis, a type of movement on land produced by rhythic waves of muscle contractions.
Crawling by peristalsis
Circular muscle contracted
Circular muscle relaxed
Longitudinal muscle relaxed (extended)
Longitudinal muscle contracted
An exoskeleton is a hard encasement deposited on the surface of an animal.
Exoskeletons are found in most molluscs and arthropods.