Chapter 12 Muscular System

CHAPTER 12

Muscular System

• Skeletal muscles are composed of individual muscle fibers that contract when stimulated by a somatic motor neuron. Skeletal muscles are usually attack to 

Origins, Insertions and Actions (OIA’s)

• Origins - The stationary bone that a muscle is anchored or attached to

• Insertions - The bone that moves and is usually located on the other side of a  joint.

• Action - The movement caused when the insertion is moved toward the origin

• Flexion of forearm 

• Extension of forearm

Skeletal Muscle Actions

• Exterior - Increase the angle at a joint

• Flexor - Decrease the angle at a joint

• Abductor - Moves limb away

Structure of Skeletal Muscles

• The surface of the whole muscle is covered in an extension of the tendon which is connective tissue. This whitish covering is known as fascia

• The whole muscle is divided into bundles of muscle fibers called fascicles

• Each muscle cell or fiber is surrounded by a specialized plasma membrane referred to as the sarcolemma. The root “sarco” refers to muscle in anatomy

Muscle Fiber

• A skeletal muscle fiber contains the same organelles as any other cell but unlike most cells they are multi nucleated. Because during embryonic development many cells fuse together to form very long muscle fibers.

• Besides being multinucleate and being very long, muscle fibers contain specialized organelles of contraction known as myofibrils which themselves are composed of a functional unit of the muscle known as a sarcomere. 

• The sarcomere is where the physiology of contraction takes place by the interaction of the contractile proteins actin and myosin.

Muscle Organization

• The sarcomere is the functional unit of the muscle. The shortening of millions of sarcomeres all at once is what produces a contraction (shortening) of a muscle.

• Sarcomeres are composed of two interactive proteins called actin and myosin. We are finally done onto the physiology of muscle contraction 

The Neuromuscular Junction

• The neuromuscular junction is the synapse between the nerve fiber and muscle fiber. The motor end plate is the specialized portion of the sarcolemma of a muscle fiber surrounding the axon terminal.

• Notice the slight gap between the membrane of the axon terminal and that of  the muscle fiber. This is referred to as the neuromuscular cleft.

Motor End Plates

• Each muscle fiber receives a single axon terminal from a somatic motor neuron. These terminals secrete ACh onto a specialized region of the sarcolemma known as a motor end plate. ACh binding to its nicotinic receptors generates an action potential along the sarcolemma eventually leading to contraction of skeletal muscle fiber.

Motor Units

• The cell body of a somatic neuron is located in the ventral horn of the gray matter of the spinal cord. Motor neurons travel within spinal nerves to the muscles they innervate.

• Each motor nerve axon however can produce a number of collateral branches (axon terminals ) to innervate an equal number of muscle fibers

• A motor unit describes a motor neuron and all the muscle fibers it innervates. When a motor neuron fires the entire motor unit contracts.

Innervation Ratios

• The number of muscle cells per motor unit is a tradeoff between strength and fine or precise control. Larger motor units generate more force  but offer less control.

• The opposite is true when fine motor control and coordination are needed as in the muscles controlling pupil diameter.

• In the eye a motor unit may consist of only 10 muscles

• In the gastrocnemius muscle where strength is needed, a single motor unit can consist of a 1000 muscle cells.

Force of Contraction 

• The mechanical force that muscles generate when they contract is called muscle tension. How much muscle tension is generated by a muscle contraction depends on two factors.

• The number of muscle cells in each motor unit (larger motor units activated)

• The number of motor units activated at one time. This is known as recruitment.

Mechanism of Contraction 

• Within the sarcomere are arranged the contractile proteins actin & myosin. As these proteins “slide” over one another they overlap and the sarcomere and thus, the muscle, shortens which is referred to as contraction.

Actin & Myosin

• Actin (purple) and myosin (green) are sometimes referred to as the thin and thick filaments based on their relative protein size. A third important protein associated with the sarcomere is called titin . Titin proteins are springy (elastic) they are the largest proteins in the human body and run through the thick filaments connecting both sides of the sarcomere which stabilizes the sarcomere and helps return it to its resting length after contraction.

The Sliding Filament Theory of Contraction

• Sliding of the filaments is produced by the action of numerous cross-bridges  that extend out from the myosin toward the action. The “head” portion of the cross-bridge connects to and pulls on the actin molecule when the muscle is contracting.

• The myosin contains an ATP-binding site closely associated with an actin binding site.

• The myosin head has an actin-binding site and an ATP -binding site which serves as an ATPase. When ATP is hydrolyzed into ADP and Pi the myosin head becomes activated and changes its position.. It is now ready to bind to the actin subunits. At this point the ADP and Pi are still attached to the myosin head.

Cross Bridge Cycle

1.  At rest myosin is not attached to actin 

2.  In the presence of Ca2+ the cross bridge binds to actin 

3.  Pi is released from myosin and causes myosin to change shape. This is the power stroke.

4.  Power stroke causes actin & myosin to slide past each other. 

5. A new ATP binds to myosin head and myosin detaches.

6. ATP is hydrolyzed and myosin is again ready for action.

Regulation of Contraction

• The actin filament is a polymer composed of 300 to 400 globular subunits arranged in a double row and twisted around each other in a helix. A different type of protein known as tropomyosin lies within a groove between the rows of actin. Attached to the tropomyosin is another protein called trophin.

• Troponin and tropomyosin act together on actin to regulate the attachment of cross bridges formed with myosin thus serving as a switch alternating between muscle contraction and relaxation.

• In a relaxed muscle, the position of the tropomyosin on the thin filaments is such that it physically blocks the myosin cross bridges from connecting to specific attachment sites on actin. Thus, in order for the myosin cross bridges to attach to actin, the tropomyosin must be shifted out of the way. This requires the interaction of troponin with Ca2+

The Role of Ca2+ in Muscle Contraction 

• The attachment of Ca2+ to troponin causes movement of the troponin-tropomyosin complex, which exposes binding sites on the actin. The myosin cross bridges can then attach to actin and undergo a power stroke.

The Physiology of Muscle Contraction

• ATP is used to energize the myosin so it can form the cross-bridges and pull  on the actin fibers. Once the pulling has occurred, another molecule of ATP is required to detach the myosin head to start another contraction cycle

The Muscle Fiber

• The figure to the right shows the relationship between myofibrils, the transverse tubules and the sarcoplasmic reticulum in a muscle cell. The sarcoplasmic reticulum is shown in green stores calcium ions and is stimulated to release Ca2+ by the action potentials arriving down the transverse tubules (pink).

Sarcoplasmic Reticulum (SR)

• Is a modified endoplasmic reticulum in muscle cells. In a muscle cell Ca2+ pumps actively transport Ca2+ into the SR a process in physiology known as sequestration. That stored Ca2+ can then be released when needed to drive muscle contraction.

Excitation-Contraction Coupling

1.  ACh released by somatic motor neurons binds to nicotinic ACh receptors in the sarcolemma causing the membrane to depolarize in the region of the motor end plate. 

2. Depolarization stimulates voltage gated Na+ channels to open and an action potential spreads along sarcolemma.

3. Action potentials along the transverse tubules open voltage gated Ca2+ channels which are coupled to calcium release channels in the sarcoplasmic reticulum.

4. Ca2+ then diffuses out of the sarcoplasmic reticulum so that it can bind to troponin and facilitate cross-bridge formation.

• Action potentials along the transverse tubules open voltage gated Ca2+ channels which are coupled to calcium release channels in the sarcoplasmic reticulum.

Contractions of Skeletal Muscles

• Contraction of muscle generates tension which allows muscles to shorten and thereby perform work. The contraction strength of skeletal muscles must be sufficiently great to overcome the load on a muscle in order for that muscle to shorten. Some terms associated with muscle contraction (in vitro) are twitch, summation and tetanus.

Muscle Twitch and Summation

• When a single electrical shock  is applied to a muscle in vitro the contraction strength is recorded which reflects a single contraction/relaxation event known as muscle twitch.

• If a second stimulus is applied before the muscle has had a chance to relax then the contraction strength builds on itself to dramatically increase the strength of the contraction. This is summation.

Tetanus

• Tetanus should not be confused with the disease of the same name. Tetanus in muscle physiology refers to muscle response to continuous stimulation. An incomplete tetanus is a series of stimuli that results in a jerky contraction. A faster frequency of stimulation produces a smooth, sustained contraction known as complete tetanus. After some time in complete tetanus the muscle loses its ability to maintain the contraction and it fatigues.

Energy Requirements of Skeletal Muscles

• Skeletal muscles generate ATP through aerobic cell respiration and through the use of phosphate groups donated by creatine phosphate. Skeletal muscles at rest obtain most of their energy from the aerobic respiration of fatty acids. During exercise muscle glycogen is favored as an energy source

• During heavy exercise muscle glycogen  is the primary source of energy and indeed the glycogen content of muscles helps determine how long heavy exercise can be sustained. One glycogen molecule packages about 55,000 glucose molecules together so there is a lot of energy in glycogen. 

Glucose Uptake in Leg Muscle During Exercise with a Cycle Ergometer

• Note the uptake of blood glucose increases with the intensity of the exercise and with the exercise time. The increased uptake is largely due to the ability of muscle contraction to increase the amount of glucose  carriers in the sarcolemma of muscle fibers. 

• The take home message is that during exercise the muscle makes ATP from glucose released from glycogen stores and by increasing glucose uptake from the blood via GLUT4 transporters. 

Phosphocreatine

• During short intense bouts of exercise ATP may be used faster than it can be regenerated by aerobic respiration. Phosphocreatine serves as a muscle reserve of high-energy phosphate which can be used for the rapid formation of ATP. These reactions are catalyzed by creatine phosphokinase. Creatine is produced in the liver and kidneys and consumed in dietary meat and fish. Creatine monohydrate is a synthetic form taken by some athletes to increase muscle fiber creatine stores

Muscle Spindle Stretch Receptors

• Skeletal muscles contain stretch  receptors called muscle spindles that stimulate the production of impulses in sensory neurons when a muscle is stretched. These activate sensory  neurons that synapse with motor neurons in the spinal cord. This is the arc that stimulates the knee jerk reflex for example.

Neural Control of Skeletal Muscles

• Lower motor neurons (green) are somatic motor neurons with their cell bodies located in the spinal cord and axons that travel in spinal nerves to stimulate skeletal muscle contraction. Their actions are facilitated by sensory feedback from muscle and tendon stretch receptors and excitation and inhibition from upper motor neurons which compose the descending motor tracts (blue).

The Monosynaptic Stretch Reflex

• Reflexive contraction of skeletal muscles occurs in response to sensory input and does not depend on the activation of upper motor neurons. The reflex arc describes the nerve impulse pathway from sensory to motor endings. The knee jerk reflex involves only one synapse within the CNS.

The Monosynaptic (Knee-Jerk) Stretch Reflex

1. Reflex hammer strikes the patellar ligament which stretches the quadriceps muscles.

2. The muscle spindle stretch receptors within the quadriceps muscles are stretched which activates a sensory neuron.

3. Sensory neuron (green) enters the dorsal root of the spinal cord and directly synapses with a motor neuron.

4. Motor neuron (purple) leaves via ventral root of spinal cord.

5.  ACh released from motor neurons contracts the quadriceps muscle group.

Golgi Tendon Organ

• Are stretch receptors found in the tendon. An increase in muscle tension stimulates the activity of sensory nerve endings in the golgi tendon organ. This sensory input stimulates an interneuron which in turn inhibits the activity of a motor neuron innervating that muscle.

Reciprocal Innervation

• In many stretch reflexes the sensory neuron that stimulates the motor neuron of a muscle also stimulates interneurons within the spinal cord via collateral branches. These interneurons inhibit the motor neurons of antagonistic muscles. This dual stimulatory and inhibitory activity is called reciprocal innervation . 

• Muscle that would be inhibited during the knee jerk reflex is hamstring.

Crossed Extensor Reflex 

• The crossed extensor reflex demonstrates double reciprocal innervation. The foot that steps on the tack sends sensory input to the right side of the spinal cord which will contract the flexors (hamstrings) on that side and relax (inhibit) the extensors (quadriceps) on that side. An interneuron also carries that signal to the other side of the spinal cord to innervate the left leg. For the opposite balancing leg the extensors (quadriceps) contract and the flexors (hamstrings) are inhibited. This removes the offended foot from the stimulus and braces the opposite leg for balance. 

• Upper Motor Neuron Damage

1. Hemiplegia - paralysis of upper and lower limbs on one side commonly produced by damage to motor tracts as they pass through internal capsule. Paralysis on one side of the body.

2. Paraplegia - paralysis of the lower limbs on both sides as a result of lower spinal cord damage. Paralysis of legs only both.

3. Quadriplegia - Paralysis of upper and lower limbs on both sides as a result of damage to the upper region of the spinal cord or brain. Paralysis of both arms and legs.

Cardiac and Smooth Muscle

• Cardiac Muscle is found only in the heart and is the hardest working muscle tissue in the body. Smooth muscle is found mostly in the visceral organs especially in the organs of digestion. Both cardiac and smooth muscle are involuntary and are under the control of the autonomic nervous system.

Cardiac Muscle 

• Cardiac muscle, like skeletal muscle, is striated and contains sarcomeres that shorten by the sliding of actin and myosin. However, while skeletal muscle requires stimulation to contract, cardiac muscle can produce impulses and contract spontaneously.

• Myocardial cells are short, branched and interconnected. Each myocardial cell is tubular in structure and joined together by electrical synapses called GAP junctions. The gap junctions are concentrated at the ends of each cell which permits electrical impulses to be conducted primarily from cell to cell. Gap junctions stain darker than the rest of the cell and are referred to as Intercalated Discs.

Control of Heart Rate

• Unlike skeletal muscle which requires voluntary stimulation cardiac muscle tissue is able to produce action potentials intrinsically. In other words cells within the pacemaker self-depolarize producing action potentials by themselves. This is due to these cells being “leaky” to NA+ but we will get to that in chapter 13. However, the rate of this spontaneous depolarization and thus the heart rate is regulated by the autonomic nervous system. 

Smooth Muscle

• Unlike skeletal and smooth muscles which are striated. Smooth muscles lack sarcomeres and thus striations. However smooth muscle cells do contain actin and myosin that overlap to produce contractions.

• Notice how the actin and myosin are arranged in the plasma membrane of the smooth muscle cell. When smooth muscle cells contract they don’t just shorten they sort of bunch up.

• In a single unit smooth muscle (top) the individual smooth muscle cells are electrically joined by gap junctions so that depolarization can spread from one cell to the next with the whole muscle acting as a single unit.

•  Smooth muscle of the uterus or digestive tract would act in this manner. These smooth muscle cells also display intrinsic electrical activity and contraction in response to stretch. This is how food is moved down the digestive tract via peristalsis. 

• In multiunit smooth muscle each smooth muscle cell is controlled individually. There are no gap junctions.

• Ciliary muscles attached to the ends of the eye work this way in order to provide more precise control when shaping the lens.