Introduction to Muscular Tissue

Special Characteristics of Muscle

Recall that there are 4 types of tissues in the body: epithelial, connective, muscular, and nervous.  What sets muscular tissue apart from the other tissues is its ability to contract.  Tiny proteinsLarge molecules made of amino acids with various functions in the body. exist in your muscle cellsThe basic structural and functional units of life.. When they all work together, they produce movementA fundamental property of life involving motion of the body or its parts. and tension.  Muscular tissue has some other characteristics such as excitabilityA neuron’s ability to respond to stimuli by generating an electrical signal..  This means that it is capable of receiving electricity, or what is called an action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission..  The terms extensibility and elasticity both refer to action of whole muscles.  For example, the detrusor muscleThe smooth muscle layer of the bladder that contracts to expel urine. that surrounds the bladderA muscular organ that stores urine before excretion. is able to extend and relax as your bladder incrementally fills.  The muscle has elasticity that allows it to expand and recoil to its original shape.

Functions of Muscle Tissue

Much like the other organ systemsGroups of organs that work together to perform functions. we’ve already looked at, muscular tissue has some intuitive functions. Some other functions might be unexpectedly possible.  I’m sure that many of you have thought about your muscles’ ability to produce movement. You might also have considered how they maintain your posture. But have you ever thought about the support of your squishy organs? Or the entrances and exits that are guarded by smooth muscle? 

We are all familiar with shivering to produce heat. However, muscles can also help store various forms of readily available energyThe capacity to do work or cause change..  There are some strange and wonderful functions of the muscular systemThe organ system responsible for movement and heat production.. These include the complex actions that cause pupil dilation or constriction. Another function is the scattered little muscles that pull your hairs upright when you get goosebumps.

Terminology

There are three types of muscular tissue: smooth, cardiac, and skeletal.  I’ve listed these three cells here. They are a spindle-shaped smooth muscle cell, a branched cardiac muscle cell, and a cylindrical shaped skeletal muscle cell.  For the rest of this chapter, we focus on skeletal muscle as an example tissue.  And, I have to stop calling them muscle cells, we call them muscle fibers.  This is because one of these muscle fibers is born from the fusing of precursor cells. 

Muscle fibers all have a specialized cell membrane that we call the sarcolemma.  Sarco- is a prefix we commonly use for muscle, along with myso.  The sarcoplasmic reticulum sounds like it is completely a made-up word, but it is not.  This IS an endoplasmic reticulum, but in muscle fibers, it is specialized to intake and release calcium cations.  The sarcoplasm is just the cytoplasmThe gel-like substance within a cell that contains organelles and cytosol., again laden with calcium. 

And finally, just as skinThe body’s largest organ, providing protection and regulation. cells called melanocytesCells in the stratum basale that produce melanin, the pigment responsible for skin color. make the protein melaninA brown-black pigment made by melanocytes in the stratum basal and given to keratinocytes as melanos, muscle fibers make these proteins called myofibrils.  Unfortunately, a myofibrilCylindrical structures within muscle fibers that contain myofilaments. is cylinder-shaped just like a muscle fiber is cylinder shaped.  This can lead to a lot of misunderstand and frustration with diagrams.  Always, always, always ask if you are looking at a muscle fiber. The muscle fiber is pictured at left with the multiple nucleiClusters of neurons in the CNS responsible for processing information. and striations. Determine if it is a myofibril. It is pictured on the right with actin and myosin visible. The sarcoplasmic reticulum covers it.  It is these myofibrils that fill up over 80% of the sarcoplasm of a muscle fiber.