regarding the contraction of muscles , it says that the sliding filament theory is applicable to skeletal , cardiac and smooth muscle fibres.

later in my buk it also says, skeletal musclea contract only if stimulated. they dont have the property of automaticity inherent like in cardiac and visceral muscles... i undrstand cardiac .. bt visceral muscles? i mean they do need a stimulus ryt? for eg in the alimentary canal , there r nerve plexus ryt? they r involuntary bt how can they not need a stimulus? then what r the nerve plexus for? plz explain quick ..thank u :)

There are two kinds of muscles in our body. Voluntary muscles that are under our direct control. It incudes skeletal muscles of our body. Involuntary muscle that contract without conscious control. This includes cardiac muscles and smooth muscles.

Visceral muscles are the smooth muscles that are present in the internal organs of the body. They are involuntary muscles as they are not under our conscious control. Let us take an example to understand this. You can move your arm in any direction according to your will as it contains skeletal muscles that are under our control but you cannot stop the movement of smooth muscles of visceral organs like lungs, stomach, intestines, blood vessels. These muscles operate without our conscious control. The difference in the involuntary action of cardiac muscles and visceral muscles is that muscles of heart perform rapid action and are auto excitable while as visceral muscles act very slowly and are not auto excitable but need stimulus.

Nerve plexus are the interwoven network of nerves. The main function of the nerve plexus is ensure all the body parts are connected and able to send and receive messages from the peripheral nervous system. 

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