Muscle Cells and Structure

Skeletal Muscle Structure Properties of muscular tissue – Contractility- the ability to generate tension while shortening to approximately half its resting length – Elasticity- the ability to be stretched beyond its resting length

Skeletal Muscle Structure Characteristics of a muscle cell – Long, thin strand – Multinucleated Contains several hundred nuclei per inch – Contraction Most of the cell interior is made up of the proteins Actin and Myosin (in bunches called myofibrils) which generate force

Skeletal Muscle Structure Characteristics of a Muscle Cell – Mitochondria The organelle within the cell where most energy production for muscle contraction occurs The majority of ATP is made here – Sarcoplasm The fluid inside the cell where many metabolic enzymes and all structures are found

Muscle Contraction Myofibrils are the long strands of contractile proteins arranged in parallel The Sarcomere is the smallest unit of contraction within the myofibril strand – A thick strand of Myosin is surrounded on both ends by thin strands of Actin – Myosin pulls both ends of Actin towards the center of the sarcomere to generate force

Muscle Contraction Striations – The term used to describe the light and dark bands of muscle tissue – Dark stripes result from an overlap of Myosin and Actin, called the A band – Light stripes are regions of only Actin, and is called the I band

Fiber Type All skeletal muscles have similar quantities of at least 3 different fiber types: – Type I (Slow Twitch) – Type IIa (Intermediate Twitch) – Type IIb/x (Fast Twitch) The term “twitch” refers to the cell’s ability to shorten and generate tension

Fiber Type Type I (Slow Twitch) characteristics: – Very oxidative (uses O 2 to make lots of ATP) – Difficult to fatigue – Does not produce much lactic acid – Produces low tension and force – Has a greater capillary network (more blood supply) – Has a slow contractile speed

Fiber Type Type IIa (Intermediate Twitch): – Has properties of both slow and fast twitch – Produces more tension than type I – Fatigues more quickly than type I Studies have demonstrated that cardio exercise promotes a shift of many fast twitch fibers to type IIa

Fiber Type Type IIb or IIx (Fast Twitch): – Produces the most force – Recruited last and fatigues the quickest – Has very few mitochondria – Has many anaerobic enzymes – Produces much lactic acid

Fiber Arrangement All muscle cells are long and thin, but can be arranged in various ways to facilitate joint action Arrangement types: – Fusiform- tapered on both ends, and the individual cells run the entire length (fig. A) Example: Biceps Brachii – Pennate and Bipennate- shaped like a feather, the cells angle in toward a tendon Example: Gastrocnemius (fig. B & C)

Fiber Arrangement Arrangement types: – Fan shaped- flat muscle whose fibers taper from a wide end to a narrow end Example: Pectoralis Major (fig. D) – Quadrate- four sided flat muscle with parallel fibers Example: Rhomboids (fig. E & F)

Fiber Arrangement Angle of Pennation – This refers to the arrangement of fibers compared to the muscle’s line of action – Generally, the more parallel the arrangement to the line of action, the more velocity of shortening – The more angled the arrangement, the more force per unit can be generated

Muscle Function

Muscle Actions Specific muscles are often named for their primary (or only) action – Flexor, extensor, adductor, abductor, rotator It is important to note that many muscles are capable of multiple actions

Muscle Actions – For example, one portion of the pectoralis adducts the arm when it is low, but abducts that same arm when it is high

Contraction Types It is important to note that a muscle contracts when it is generating force by shortening, staying the same length, or lengthening A common misconception is that a lengthening muscle is relaxing – A muscle only relaxes when it is still and not producing tension of any type

Contraction Types Concentric – The shortening portion of a dynamic (moving) contraction – The sarcomere gets shorter because heads on the myosin protein strand pull the actin strands toward the center – When many or all muscle cells act concentrically, the whole muscle shortens

Contraction Types Eccentric (negative) – The lengthening portion of the dynamic contraction – Force is still directed toward the center of the muscle, but the joint angle is allowed to increase – Myosin allows actin to move to the outside of the sarcomere much like lowering a rope with a weight attached

Contraction Types Isometric – Producing tension without any joint angle change – Pushing against a wall or pulling an immovable object are examples – Typically antagonistic muscles contract equally and the joint remains static

Muscle Roles Agonist (prime mover) – The muscle(s) most directly responsible for any given movement – Examples: the agonists of elbow flexion and knee extension are the biceps brachii and rectus femoris, respectively – There are assistant movers that can help as well in any given movement, such as the brachialis in elbow flexion

Muscle Roles Stabilizers – Muscles that contract statically or isometrically to support some part of the body during movement – Example: the teres major adducts the arm downward and the scapula outward and upward, but the scapula adductors stabilize it so only the arm moves downward

Muscle Roles Antagonists – The muscle that opposes the agonist in any given movement – In a dynamic contraction, the antagonist must relax throughout most of the movement – The antagonist can help as a stabilizer or brake at both ends of a dynamic contraction

Biarticular Muscles While many muscles only cross one joint, there are several that cross two or more The rectus femoris of the thigh crosses both the hip and knee joints – It is a mover of hip flexion and knee extension The biceps femoris (hamstring) also crosses the hip and knee joints, allowing both hip extension and knee flexion

Biarticular Muscles Other examples: – Gastrocnemius Crosses the knee and ankle joints Flexes the knee and extends the foot – Flexors and Extensors of the hand Cross the wrist and several finger joints

Biarticular Muscle Actions Concurrent movements – When 2 joints crossed by one muscle perform opposite actions – Example: flexing the hip and bending the knee, or extending the hip and knee – Tension is retained despite action across two joints

Biarticular Muscle Actions Countercurrent movements – When 2 joints crossed by one muscle perform the same action, and its antagonist performs opposite actions across two joints – Example: flexing the hip and extending the knee with the rectus femoris while the biceps femoris is lengthened across the hip and knee

Biarticular Muscle Actions Countercurrent movements – The maximum activity of a muscle occurs across only one of its 2 joints however – For example, the rectus femoris is recruited maximally as a knee extension (without hip flexion) – For this to occur, other stabilizers must be recruited to hold the hip steady