[englishgent (OP)]

As i understand it we have three types of muscle in our body, cardic, skeletal (voluntary), viseral (involuntary).
I was wondering which type of muscle controls the eye lids becuase the movement there can be voluntary or involuntary? Also are the muscles that control the movement striated ?

[chris]

Great question.

There are indeed 3 types of muscles - striated, smooth and cardiac, but to classify them as voluntary and involuntary can be a bit misleading.

Striated muscle is a syncitium. The muscle cells (myotubes) merge producing a giant 'cell' containing lots of nuclei congregated in the centre. This muscle responds very rapidly to electrical stimuli meaning that the whole 'cell' (syncitium) contracts at once producing large aaccurate movements very quickly.

Voluntary movements are produced by striated muscles, but so are some involuntary responses ! A reflex - such as tapping a knee with a hammer to make a 'knee jerk reflex' - is an involuntary movement. Ditto when you inadvertently touch something hot and flinch.

Smooth muscle, on the other hand, is a functional, but not a true, syncitium. Smooth muscle cells exist as independent units that are 'wired' together by connections (gap junctions) between their cell membranes. This means that when you activate a group of smooth muscle cells the signal has to spread slowly from cell to cell in an expanding wave, making the contraction much slower - but this kind of response is ideal for situations where gross slower tonic control of a structure is necessary such as in blood vessels, the uterus, or the GI tract. These responses are usually under involuntary control - but beware, the oesophagus contains lots of striated muscle, but you never have to think about forcing food down into your gullet - it's involuntary !

Now returning to your example of the eyelid. You don't have to think about opening and closing your eyes - it's automatic (involuntary) but, like breathing, you can superimpose voluntary control on top. The eyelid contains striated muscles - you need them in order to generate sufficiently fast movements to enable you to blink. But these muscles are also driven by the sympathetic branch of the 'autonomic' (automatic) nervous system - these are the nerves that switch on during 'fight or flight' situations. They open your eyes wide and dilate pupils for distance vision. That's where we get the expression 'wide eyed with fright' and why you can tell if someone is interested in you / what you are saying because their eyes usually widen (open wider) and their pupils dilate. In an exam situation when people are really nervous they occasionally find that they can't read the exam questions clearly because their pupils are wide (for distance vision).

Anyway, i hope that answers your question. It's not as straight forward as it might first seem.

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[englishgent (OP)]

Yes thats great i've been wondering about it for ages thanks a lot mate