Chapter 9 - Muscular Movement and Control


Muscle Contraction
     Muscles exert force by shortening. This contraction creates tension that exerts a pulling force on the point of attachment. The mechanism of this contraction is well explained by the sliding filament theory. 
  Sliding Filament Theory  Tutorial
     The sliding filament theory explains the following observations:
1. The H band and I band gets smaller.
2. The zone of overlap gets larger.
3. The Z lines move closer.
4. The A band remains constant. 
     The mechanism:
1. Contraction begins when the cross-bridges of myosin filaments bind to active sites on the actin filaments.
2. The myosin heads pivot toward the M line pulling the thin filament with it toward the center.
3. The cross-bridges detach.
4. The cross-bridges returns to their original positions. 
     This cycle repeats itself and in the process the thin filaments slide toward the center and the sarcomere shortens. 
  The Start of Contraction Role of Calcium
     The cross-bridge cycle begins when the active sites on actin are uncovered. The calcium ion (Ca2+) is key to this uncovering. Ca2+ concentration is normally low in the sarcoplasm surrounding the filaments and high in the sarcoplasmic reticulum. When an electrical impulse travels along the sarcolemma it enters the interior of the cell along the T-tubules. This electrical event causes the membrane of the terminal cisternae to become more permeable to Ca2+. Ca2+  rushes out and Ca2+ binds to troponin. This causes the tropomyosin to move out of position and uncover the binding sites on actin. As long as binding sites are uncovered the cross-bridge cycling occurs.  
  The End of Contraction
     The contraction ends when electrical impulses end. The sarcoplasmic reticulum becomes less permeable to Ca2+ and actively reabsorbs Ca2+. With the drop in Ca2+ concentration Ca2+ binding to troponin decreases and tropomyosin recovers the binding sites. 
     Contraction also requires ATP. During cross-bridge cycling, the cross-bridges detaches when ATP binds to it and the cross-bridge returns to its unpivoted state (becomes "cocked"). Energy is required for this and comes from the breakdown of ATP to ADP and a phosphate group. 
     Contracted muscles return to their original length through passive (not requiring energy) processes. 
Neural Control of Muscle Fiber Contraction
     Each individual skeletal muscle cell or fiber is controlled by a motor neuron (nerve cell). A process of the neuron, the axon, reaches the muscle fiber and forms a connection called the neuromuscular junction. At the neuromuscular junction the axons branches and the tips of these branches expand into what are called synaptic end bulbs or synaptic knobs. Within the synaptic end bulbs there are synaptic vesicles that contain the neurotransmitter acetylcholine (ACh). 
     The motor neuron initiates a contraction when an electrical impulse travels along the axon and reaches the synaptic end bulb. The impulse arriving at the synaptic end bulb causes the synaptic vesicles to fuse with the neuron's cell membrane and release ACh into a space between the neuron cell membrane and the sarcolemma called the synaptic cleft. ACh diffuses across the synaptic cleft to the motor end plate, a part of the sarcolemma that is highly folded and contains receptor molecules for ACh. 
     The binding of ACh to its receptors on the motor end plate triggers an electrical impulse called an action potential, that sweeps along the sarcolemma and T- tubules. Neural stimulation ends when the enzyme in the synaptic cleft, acetylcholinesterase, breaks down the ACh.  
Motor Units and Muscle Control
     All the muscle fibers controlled by a motor neuron constitutes a motor unit. Motor units vary in size according to the number of fibers controlled. In muscles where precise and fine movements are required (e.g. eye muscles) the motor units are small with a motor neuron controlling a few fibers. In muscles which require powerful contractions (e.g. leg muscles) a motor neuron may control thousands of fibers.
     The amount of tension generated by a contracting muscle depends on:
1. the frequency of stimulation; and
2. the number of motor units stimulated.
  Muscle Hypertrophy
     Muscular hypertrophy occurs in muscles repeated stimulated to near-maximal tension. Hypertrophy results in the increase in the diameter of the muscle fiber due to an increase in the number and size (diameter) of myofibrils.
  Muscle Atrophy
     Lack of neural stimulation of skeletal muscle causes the muscle to loose mass and tone. The overall loss in muscle size is called atrophy.
Muscle Terminology
     Each muscle has its attachment points and produces an action when it contracts. The attachment points are called origins and insertions.
  Origins and Insertions
     Typically, the attachment of the muscle that remains stationary is called its origin and the attachment that moves is the insertion. The origin is usually proximal to its insertion.
     The various kinds of actions that muscles cause were covered under articulations. There are two ways that actions are described with respect to the skeletal system:
1. The effect on the region of the body is described (e.g. flexes the forearm).
2. The effect at the joint is described (e.g. flexes at the elbow joint).
     When a movement occurs the skeletal muscles involved with the movement can be placed one of three categories:
1. Prime mover or Agonist
     The agonist is a muscle that is chiefly responsible for producing the particular movement.
2. Synergist
     Synergists are muscles that assist the prime mover in performing the action. Synergists may provide additional pull at the insertion or stabilize the movement at the origin of the agonist. Muscles that stabilize joints at the origin are called fixators.
3. Antagonist
     An antagonist produces actions that oppose the actions of the agonist. Normally, antagonists work with agonists to control the speed and smoothness of a movement.
     The movement created by muscle contraction is modified in force, speed and direction by the way that bones that the muscles move act as levers. A lever is a rigid structure that moves on a fixed point called the fulcrum. In the body the joint is the fulcrum, the muscle exerts an applied force, and this force is opposed by resistance.
     There are three classes of levers in the body:
1. First-Class Lever (RFA)
     This class of lever has the fulcrum in between  the applied force and the resistance as in a seesaw. There are not many first-class levers in the body. The neck muscles that extend the head at the atlanto-occipital joint is an example.
2. Second-Class Lever (FRA)
     In this lever the resistance is located between the fulcrum and the applied force. The wheelbarrow offers a real world example. Plantar flexion at the ankle joint is an example of this lever in the body.
3. Third-Class Lever (FAR)
     In this lever the applied force is between the fulcrum and the resistance. This is the most common lever in the body.