Muscle Physiology Supplements

Muscle Anatomy:
The scale and arrangement of muscles goes from muscle organ, fascicle, fiber, myofibril, to myofilaments within repeating sarcomere units (Fig. 1)

1. A muscle organ = whole muscle group, made of a bunch of fascicles (ex. biceps brachii).
2. muscle fascicle = bundle of muscle muscle fibers.
3. muscle fiber = a single muscle cell, which is innervated by a motor neuron in the neuromuscular junction (the junction between a motor neuron and muscle fiber). A muscle fiber consists of a bundle of muscle myofibrils.
4. muscle myofibril = contains thousands of sarcomeres - sometimes as many as 10,000 sarcomere units end-to-end.
5. sarcomere = is the functional unit of a muscle contraction because it is this structure that actually shortens during a contraction. Sarcomeres are made up of myofilaments (actin and myosin) and various "lines" (see below).



The Neuromuscular Junction:
A somatic motor neuron synapses onto one or many muscle fibers in a motor unit. At the neuromuscular junction, action potentials (APs) in the motor neuron end at the syanaptic knobs and the following steps occur to cause an action potential within the muscle fiber.

1. Secretory vesicles in the motor neuron contain the neurotransmitter acetylcholine (ACh).
2. When the APs reach the synaptic knobs it causes release of ACh from the vesicles into the synaptic cleft.
3. The ACh crosses the cleft and
4. binds to nicotinic (ion-gated channel) cholinergic receptors on the muscle fiber.
5. Receptor binding with ACh opens Na+ ion-gated channels to open. Na+ floods into the cell leading to cell membrane depolarization - a new AP is generated. Now the AP travels down transverse tubules (T-tubules) and stimulates the sarcoplasmic reticulum to release calcium (Ca+2) into the muscle fiber. Calcium will play an important role within the sarcomere in initiating a muscle contraction (Fig. 2)
6. When a motor neuron is done stimulating a muscle fiber, the ACh released within the synapse is degraded by acetylcholinesterase - an enzyme that breaks ACh into its compenent parts of acetate and choline. This prevents the ACh from remaining in the synapse and continuing to stimulate a muscle fiber.



Components of a Sarcomere: (Fig. 3)
Myofilaments (actin & myosin)
A) Actin = the thin filament having active sites and the proteins troponin and tropomyosin. Troponin is a protein that is in close association with tropomyosin - a long fiber-like protein that wraps around actin and actively blocks its "active sites".
B) Myosin =the thick filament having globular heads that can form crossbridges with actin by binding to active sites. Myosin grabs and pulls actin causing the myofilaments to slide past each other during a muscle contraction; thus the phrase "sliding filament" theory of muscle contraction.

Lines of a Sarcomere:
> Z-lines
= the endpoints of a each sarcomere, which move closer together durin g acontraction.
> M-line = the middle of sarcomere.



The Sliding Filament Theory of Muscle Contraction:
(Fig. 4)


1. A motor neuron stimulates a muscle fiber by ACh binding to nicotinic cholinergic receptors on the muscle fiber.
2. Binding of the nicotinic cholinergic receptors causes formation of an action potential (AP) in the muscle cell.
3. The AP travels through the muscle fiber down Transverse tubules (T-Tubules) and stimulates Ca+2 to be released from the Sarcoplasmic Reticulum.
4. The Ca+2 binds to troponin, causing tropomyosin to change shape and lift off the active sites on actin.
5. The myosin heads contain the high energy molecule ADP (adenosine diphosphate), which dephosphorylates (loses a phosphate group) in order to "grip" onto the active sites on acting forming crossbridges. The ADP molecule splits away from the myosin head providing energy for the "power stroke" in which myosin head bends and pulls on actin, sliding it inward towards the M-line and bringing the Z-lines of the sarcomere closer together.
6. In order for myosin to "re-grip" and continue pulling on actin it must first let go or break the crossbridges. ATP is required to break the crossbridge between myosin and actin.
Immediately after breaking the crossbridges ATP dephosphorylates into ADP, allowing the myosin head to reposition and be ready to bind to actin again.
7. The myosin, through this process of "grip and regrip" pulls on actin until the Z-lines of the sarcomere move inwards toward the M-line. In this process the sarcomere shortens. When the sarcomeres shorten the muscle fiber then shortens. When the muscle fibers shorten the entire muscle organ itself shortens producing an "isotonic muscle contraction".



Click HERE to view a video demonstrating the Sliding Filament Theory of Muscle Contraction

Click HERE to view a video (shown in class) demonstrating the use of ADP, for forming crossbridges and the "power stroke", and use of ATP to break crossbridges in preparation for myosin to pull against actin again.



Types of Muscle Tissue:
There are 3 categories of muscle tissue: skeletal, cardiac, and smooth. Each type of muscle tissue has some similarities and differences among them - as in indicated in the following table.
Skeletal Muscle Cardiac Muscle Smooth Muscle
Is under voluntary (somatic motor) control Is under involuntary (autonomic motor) control by the medulla oblongata cardiac center. Is under involuntary (autonomic motor) control by the medulla oblongata vasomotor center, and other CNS regulatory centers.
Are the muscles of body movement Are the muscles of the heart Are the muscles lining arterioles, bronchioles, gastrointestinal tract, urinary system, and reproductive system.
Requires stimulation by a motor neuron to contract.
(is not autorhythmic)
Does not require stimulation by a motor neuron to contract because the stimulus (pacemaker potential) originates from within (is "autorhythmic") Does not require stimulation by a motor neuron to contract because the stimulus (pacemaker potential) originates from within (is "autorhythmic")
Stimulation of contraction by neurotransmitter ACh binding to nicotinic cholinergic receptors. Stimulation of contraction by norepinephrine & epinephrine binding to Beta-1 adrenergic receptors. Stimulation of contraction by norepinephrine & epinphrine (to alpha and Beta adrenergic receptors) as well as other hormones (e.g. prostaglandin).
Contraction occurs through a Ca+2 and troponin mechanism. Ca+2 is stored in the sarcoplasmic reticulum. Contraction occurs through a Ca+2 and troponin mechanism. Ca+2 is stored in the sarcoplasmic reticulum. Contraction occurs through a Ca+2 and calmodulin mechanism. There is no sarcoplasmic reticulum.
Has striated muscle fibers but no gap junctions or intercalated discs. Has striated muscle fibers with gap junctions as intercalated discs, along which action potentials are trasmitted. Has unstriated or smooth muscle fibers, with gap junctions but no intercalated discs.
Has the fastest contraction speed. Has an intermediate contraction speed. Has the slowest contraction speed.
Functions by both aerobic & anaerobic metabolism. Functions primarily by aerobic metabolism. Functions primarily by aerobic metabolism.
Is prone to fatigue and buildup of lactic acid. Is not prone to fatique. Is not prone to fatigue.

Factors that Influence Muscle Contractile Force:
Muscle contractile force depends on a variety of factors including the strength of a motor stimulus, arrangement of muscle fibers in a motor unit, the number of motor units that respond to a stimulus, and also the resistance (or mass) of the object against which muscles are contracting.

Power versus Precision Principle:
In general there is a physiological tradeoff with muscle arrangement and contractile force produced. Postural muscles, or those that function primarily in support and movement of the body's torso, tend to function more heavily with power than precision. Muscles of the back, legs, and arms are designed for power to move us around and to move objects around in our environment. However, these muscles lack fine and precise movements. Conversely, muscles of our hands tend to function with more precision rather than power. Precision gives muscles an ability for fine, controlled movements that typically lack power. Power is accomplished when a motor unit synapses onto many muscle fibers. The cummulative force produced by all the muscle fibers, and their corresponding sarcomere units, is much greater than if fewer fibers were responding to a motor stimulus. Precision, however, is accomplished when a motor neuron synapses onto few muscle fibers to produce less force but having finer control.

Contractile Force Corresponds to the Number of Muscle Fibers Responding: (Fig. 5)
As shown in Fig. 5, motor unit 1 synapses onto only 2 muscle fibers and produces a low amount of contractile force. Motor unit 2 synapses onto 3 muscle fibers and is capable of producing more contractile force than motor unit 1.

Contractile Force Corresponds to the Number of Motor Units Involved ("Recruitment"): (Fig. 5)
As shown in Fig. 5, if the motor stimulus is strong enough it will cause multiple motor units to respond. The combined contractile force produced by motor unit 1 and 2, and their combined 5 muscle fibers, is much greater than the force produced by either motor unit alone. When multiple motor units respond to a stimulus it is called "recruitment".




Isotonic Versus Isometric Muscle Contraction:
As mentioned earlier, muscles contract to produce force in order to move something - either the body, body parts, or objects.
The mass of an object provides a force of resistance to the force produced by the contraction of your muscles. If the muscle contractile force produced is greater than the mass of the object being moved then the muscle organ will shorten - known as "isotonic contraction". Whenever you move or lift an object (Fig. 6) your muscles are engaging in isotonic contractions. If the muscle contractile force produced is less than the mass of the object then the muscle organ will have tone but will not contract (not shorten). This is called an isometric contraction ("iso" = same, "metric" = measure). With an isometric contraction the muscle doesn't change shape and the object will not be moved. This is like trying to lift a 2,000 lb boulder (Fig. 6).



Check this page for supplements on muscular physiology including the sliding filament theory and neuromuscular functioning.