Anatomy and Physiology 7th Edition Chapter 7 the Muscular System
The M uscular System is anorgan system consisting ofskeletal,smooth andcardiacmuscles. It permits movement of the body, maintains posture, and circulates blood throughout the body. The muscular system invertebrates is controlled through thenervous system, although some muscles (such as thecardiac muscle) can be completely autonomous. Together with theskeletal system it forms themusculoskeletal system, which is responsible for movement of thehuman body
Muscles
There are three distinct types of muscles: skeletal muscles, cardiac or heart muscles, and smooth (non-striated) muscles. Muscles provide strength, balance, posture, movement and heat for the body to keep warm. Upon stimulation by an action potential, skeletal muscles perform a coordinated contraction by shortening each sarcomere. The best proposed model for understanding contraction is the sliding filament model of muscle contraction. Actin and myosin fibers overlap in a contractile motion towards each other. Myosin filaments have club-shaped heads that project toward the actin filaments. Larger structures along the myosin filament called myosin heads are used to provide attachment points on binding sites for the actin filaments. The myosin heads move in a coordinated style, they swivel toward the center of the sarcomere, detach and then reattach to the nearest active site of the actin filament. This is called a rachet type drive system. This process consumes large amounts of adenosine triphosphate (ATP). Energy for this comes fromATP, the energy source of the cell. ATP binds to the cross bridges between myosin heads and actin filaments. The release of energy powers the swiveling of the myosin head. Muscles store little ATP and so must continuously recycle the discharged adenosine diphosphate molecule (ADP) into ATP rapidly. Muscle tissue also contains a stored supply of a fast acting recharge chemical, creatine phosphate which can assist initially producing the rapid regeneration of ADP into ATP. Calcium ions are required for each cycle of the sarcomere. Calcium is released from the sarcoplasmic reticulum into the sarcomere when a muscle is stimulated to contract. This calcium uncovers the actin binding sites. When the muscle no longer needs to contract, the calcium ions are pumped from the sarcomere and back into storage in the sarcoplasmic reticulum.
Types of Muscles
3 types of muscles:
1) Smooth muscle - controlled by the autonomic nervous system; may either be generally inactive and then respond to neural stimulation or hormones or may be rhythmic
2) Cardiac muscle - found in the heart, acts like rhythmic smooth muscle, modulated by neural activity and hormones
3) Skeletal muscle - move us around and responsible for most of our behavior; most attached to bones at each end via tendons
movement:
- flexion - contraction of flexor muscles, drawing in of a limb
- extension - opposite of flexion, produced by contraction of extensor muscles (antigravity)
anatomy:
- extrafusal muscle fibers - served by axons of the alpha motor neurons (serve multiple muscle fibers); contraction of these muscles provides movement - extrafusal muscle fibers and associated alpha motor neurons are called a motor unit
- intrafusal muscle fibers - specialized sensory organs served by 2 axons, one sensory and one motor - also called muscle spindles
- gamma motor neuron - efferent axon causes the intrafusal muscle fiber to contract, but contributes little force; serves to modify the sensitivity of the fiber's afferent axon to force
- myofibrils contain actin & myosin - proteins that provide the physical basis for muscular contraction myosin attaches to actin, lets go, then reattaches lower on the strand, etc. - rowing motion produces muscular contraction
Muscular contraction:
- neuromuscular junction - synapse between efferent terminal button and the membrane of a muscle fiber
- motor endplates (postsynaptic membrane)
- acetylcholine - released by efferent axon's terminal buttons, result in depolarization at endplate; endplate potential all or none (no threshold), produces contraction or "twitch" of the muscle fiber (calcium channels open - trigger rowing action)
- single impulse produces single twitch, need series of action potentials to produce a sustained contraction of the muscle fiber
Sensory feedback from muscles:
- intrafusal muscle fibers - stretch when the muscle lengthens and relaxed when it shortens - detect muscle length
- Golgi tendon organ - stretch receptors located within the tendons, detecting the amount of stretch exerted by the muscles on the bones to which they are attached; encode degree of stretch by the rate of firing; don't respond to length, but to how hard it is pulling
- passive movement - someone lowering your relaxed arm while he holds it - muscles lengthen passively - effect on Golgi tendon organ?
- arm dropped quickly - effect on Golgi tendon organ?
- weight dropped into hand held parallel to the floor - effect on Golgi tendon organ?
Monosynaptic stretch reflex:
- stimulating patellar tendon causes knee to kick - occurs in 50 milliseconds, too fast for brain to be involved
- if asked to move leg when touched on knee, would be slower
Weight placed in a person's hand -
1) afferent impulses from the muscle spindle are conducted to the terminal buttons in the gray matter of the spinal cord
2) terminal buttons synapse onto an alpha motor neuron
3) alpha motor neuron synapses on motor endplate on the extrafusal muscle fibers of the same muscle
4) if arm starts to drop, then muscle spindle afferent neurons start to fire as they detect muscle lengthening, they then synapse on alpha motor neurons and rate of firing increases, and then muscle contraction increases
example: posture - if pushed forward, muscles in back of calves stretch, causing contractions in the toes
Gamma motor system:
- allows for adjustment of sensitivity of muscle spindles to muscle length
- when muscle spindles are relaxed, they are relatively insensitive to stretch; but when already taught, they feel stretch quicker
- gamma motor neurons contract muscle spindles, making them more sensitive
Brain sends message for movement:
- alpha motor neuron and gamma motor neurons activated
- alpha motor neurons start the muscle contracting
- if no resistance, both extrafusal and intrafusal muscle fibers contract at the same rate, sending little info from muscle spindles
- if resistance, then extrafusal muscle fibers are halted, but intrafusal continue to contract, as told to by the gamma motor neuron
- then, sensory info from the intrafusal fibers goes to the spinal cord, where there is a synapse onto the alpha motor neuron, which then increase muscular contraction
Polysynaptic reflexes:
Golgi tendon organs have 2 kinds of receptors:
- more sensitive - tell how hard the muscle is pulling
- less sensitive - their terminal buttons synapse onto an interneuron in the spinal cord gray matter which then synapse onto the relevant alpha motor neuron, producing inhibitory (glycine) potentials
- decreases muscular contraction, prevents injury
Agonist-antagonist muscle groups
Muscle spindles send terminal buttons to:
- alpha motor neurons
- the brain
- inhibitory interneurons
Organization of the motor cortex:
- homunculus - disproportionate amount of motor cortex devoted to fingers and speech muscles
- primary motor cortex - stimulation produces movement; connected to primary sensory cortex
- monkeys respond faster when trained to pull a lever following a stimulus to the hand, rather than sight or sound
- frontal association cortex - stimulates primary motor cortex; receives input from association areas of the occipital (visual), parietal (spatial), and temporal (auditory) lobes
Pathways that originate in the cortex:
1) corticospinal tract - axons terminate in gray matter of spinal cord, mostly originating in primary motor cortex, through pyramidal tracts, then at the end of the medulla they cross and descend through the contralateral spinal cord, forming the lateral corticospinal tract (control distal part of limbs); the remaining fibers stay on the same side and form the ventral corticospinal tract (control upper legs and trunk)
2) corticobulbar tract - projects to the medulla, ending at cranial nerves which control movements of the face and tongue
Pathways that originate in the brainstem:
1) rubrospinal tract - originates in the red nucleus, which receives info from motor cortex and cerebellum; axons terminate on motor neurons in the spinal cord (control arms and legs, but not fingers)
2) ventromedial pathways - terminate in gray matter of spinal cord; include vestibulospinal, tectospinal, and reticulospinal tracts (control movement of the truck and proximal limb muscles, such as walking, head turning, autonomic functions)
apraxia - inability to properly execute a learned skilled movement
1. limb apraxia - moving wrong part of limb, moving correct part in the wrong way, or correct movements in the wrong sequence (assessed by pantomiming)
- callosal apraxia - apraxia of the left limb caused by damage to the anterior corpus callosum (think about pathway from hearing speech to following command)
- sympathetic apraxia - apraxia of left hand due to damage to the anterior left hemisphere (can't communicate to right if can't process info from verbal channels); why was it called sympathetic?
- left parietal apraxia - apraxia of both limbs due to lesions of the posterior left parietal lobe (left verbal area sends info to left parietal, which gets info re environment from right parietal, and then calculates movement; also, acalculia)
2. constructional apraxia - caused by lesions of the right parietal lobe
Basal ganglia - know Parkinson's & Huntington's
Cerebellum - know definitions of terms in bold and section about lesions of the lateral zone
Reticular formation - controls activity of the gamma motor system - regulates muscle tone (remember association with arousal and autonomic functioning)
Muscles and Movements
HOW MUSCLES WORK
A voluntary muscles usually works across a joint. It is attached to both the bones by strong cords called tendons.
When the muscles contracts, usually just one bone moves.
For example when the biceps in the arm contracts, the radius moves but the scapula does not.
ORIGIN AND INSERTION
When a muscle contracts, usually just one bone moves. The other is stationary. The origin is where the muscle joins the stationary bone. The insertion is where it joins the moving bone. When a muscle contracts, the insertion moves towards the origin.
TENDONS
Tendons are the cords and straps that connect muscles to bones. At the bone, the fibres of the tendon are embedded in the periosteum of the bone. This anchors the tendon strongly and spreads the force of the contraction, so the tendon won't tear away easily.
MUSCLE WORKING IN PAIRS
Muscles usually work in pairs or groups, e.g. the biceps flexes the elbow and the triceps extends it.
This is called antagonistic muscle action. The working muscle is called the prime mover or agonist. (it's in agony!) The relaxing muscle is the antagonist. The other main pair of muscle that work together are the quadriceps and hamstrings.
The prime mover is helped by other muscles called synergists. These contract at the same time as the prime mover. They hold the body in position so that the prime mover can work smoothly.
When muscles cause a limb to move through the joint's range of motion, they usually act in the following cooperating groups:
agonists
These muscles cause the movement to occur. They create the normal range of movement in a joint by contracting. Agonists are also referred to asprime movers since they are the muscles that are primarily responsible for generating the movement.
antagonists
These muscles act in opposition to the movement generated by the agonists and are responsible for returning a limb to its initial position.
synergists
These muscles perform, or assist in performing, the same set of joint motion as the agonists. Synergists are sometimes referred to asneutralizers because they help cancel out, or neutralize, extra motion from the agonists to make sure that the force generated works within the desired plane of motion.
fixators
These muscles provide the necessary support to assist in holding the rest of the body in place while the movement occurs. Fixators are also sometimes calledstabilizers.
TYPES OF CONTRACTION
The contraction of a muscle does not necessarily imply that the muscle shortens; it only means that tension has been generated. Muscles can contract in the following ways:
isometric contraction
This is a contraction in which no movement takes place, because the load on the muscle exceeds the tension generated by the contracting muscle. This occurs when a muscle attempts to push or pull an immovable object.
isotonic contraction
This is a contraction in which movementdoes take place, because the tension generated by the contracting muscle exceeds the load on the muscle. This occurs when you use your muscles to successfully push or pull an object.
Isotonic contractions are further divided into two types:
concentric contraction
This is a contraction in which the muscle decreases in length (shortens) against an opposing load, such as lifting a weight up.
eccentric contraction
This is a contraction in which the muscle increases in length (lengthens) as it resists a load, such as lowering a weight down in a slow, controlled fashion.
During a concentric contraction, the muscles that are shortening serve as the agonists and hence do all of the work. During an eccentric contraction the muscles that are lengthening serve as the agonists (and do all of the work).
Energy and Muscle Contraction
Metabolism
Metabolism is a sum of events which are carried out in the human body to create energy and other substances necessary for its activities. In our organism there are catabolic and anabolic processes.
Catabolism is a process during which organic matter is broken down and the energy is simultaneously released. It is characterized by missing reserves of glycogen and mobilisation of non-saccharide sources of energy – fats and proteins. Catabolism takes place during increased movement activity and is necessary to sustain life functions.
Anabolism, on the other hand, is a energy-consuming process during which substances are created. The substrate supply exceeds the immediate need. The organism creates energy reserves, tissues are created and renewed. Anabolic processes are prevalent in situations of reduced physical activity.
The basic nutrients (carbohydrates, lipids, proteins) are present in food we eat. Those are transformed and absorbed through the digestive system. Carbohydrates break down into individual carbohydrates (monosaccharides) where the glucose ranks among the most important ones. Lipids break down into free fatty acids and glycerol. Proteins break down into amino acids. These simple agents can then become involved in more complicated processes.
Carbohydrates are used in both anaerobic and aerobic activities. ATP resynthesizes from glycogen (muscle glycogen, liver glycogen) which transforms into glucose. Supplies of glycogen in the human body are restricted. Lipids are used in endurance-based movement activity of low intensity. While the use of proteins in the ATP resynthesis is very limited, free fatty acids are used to a large extent. Glucose is generated through gluconeogenesis.
Muscle metabolism
Muscles need energy to produce contractions (Fig. 6). The energy is derived fromadenosine triphosphate (ATP) present in muscles. Muscles tend to contain only limited quantities of ATP. When depleted, ATP needs to be resynthesized from other sources, namelycreatine phosphate (CP) andmuscle glycogen. Other supplies of glycogen are stored in the liver and the human body is also able to resynthesize ATP from lipids, i.e. free fatty acids. Different modes of energy coverage are used depending on intensity and duration of the workload put on the organism.
Figure 6 Energy for muscles
The ATP-CP system
The above mentioned ATP and CP are the energy sources of muscle contraction (Fig. 7, 8, 9). The production of energy used in muscle contraction takes place through the anaerobic way (without oxygen).
Figure 7 ATP molecule
Figure 8 ATPase (ATP breakdown and energy production for muscle contraction)
Figure 9 ATP resynthesis from CP
Anaerobic glycolysis
It is a chemical process during which ATP gets renewed from glycogen, i.e. glucose in an anaerobic way (without access to oxygen). In these processes lactate, i.e. salt of the lactic acid is generated in muscles. This energy system produces 2 molecules of ATP. Glycolysis - transformation of glucose into 2 molecules of the pyruvate generating the net yield from ATP molecules and 2 NADH molecules (anaerobic breakdown of glucose into pyruvate and lactate) – see. Fig. 10.
Oxydative system
This is a chemical process during which the ATP resynthesis takes place through an aerobic way (with access to oxygen). Both glycogen or glucose and free fatty acids act here as sources of energy.
Aerobic glycolysis takes place in the cytoplasm of the cell where 34 ATP molecules are generated from the glycogen, i.e. glucose with oxygen present (Fig. 10).
Figure 10 Anaerobic and aerobic glycolysis
Free fatty acids present in mitochondria of muscle fibres transformed into acetyl CoA are used in the ATP resynthesis. Acetyl CoA enters the Krebs cycle and thus ATP molecules are generated.
Individual energy systems get involved according to the intensity of a movement activity carried out. If the performance is conducted at the maximum level, there is a gradual involvement of all the systems (Fig. 11, 12).
Figure 11 Energy coverage under maximum workload
Figure 12 Energy coverage under maximum workload
Types of muscle fibres
Human muscle fibres have distinct qualities. Although nowadays almost 30 types of muscle fibres are known to be present in the human body, we tend to work only with the following three types:
Slow red muscle fibre I (SO - slow oxidative fibres)
The slow red muscle fibre is typified by a high aerobic capacity and resistance to fatigue. As their anaerobic capacity is slow, they are not able to show great muscle strength. Muscle contraction tends to be slow – 110 ms/muscle contraction. One motoric unit contains about 10-180 muscle fibres.
Fast red muscle fibre IIa (FOG – fast oxidative glycolytic fibres)
The fast red muscle fibre shares some of qualities with a slow fibre or a fibre of IIx type. This fibre is typified by medium aerobic capacity and resistance to fatigue. It also shows high anaerobic capacity and is able to display great muscle strength. The speed of contraction is 50 ms/muscle contraction. One motoric unit contains about 300-800 fibres.
Fast white fibre IIx (FG – fast glycolytic fibre)
Unlike the previously mentioned types the fast white fibre is characterized by low aerobic capacity and tendency to fast fatigue. On the other hand, it has the greatest anaerobic capacity and is able to display considerable muscle strength. The speed of contraction is 50 ms/muscle contraction. One motor unit contains about 300-800 fibres.
The volume of this type muscle fibres is genetically given (up to 90 %) (Jančík et al., 2007) and varies in individual persons. In the average population the ratio of slow to fast fibres is 1:1. The following Figure (Fig. 13) shows the ratio of slow to fast fibres in athletes engaged in different disciplines.
Figure 13 Ratio of fast (type FG and FOG) to slow (type SO) fibres in different type athletes
In muscle contraction individual types of muscle fibres get activated in accordance with the intensity of muscle movement. During low intensity exercise slow fibres are primarily recruited. However, with increasing intensity of exercise fast fibres get activated. It is important to note here that the fibre ratio differs in different muscles of the human body. For example, postural muscles tend to contain more slow fibres.
Muscle Disorders
Muscle disease , any of the diseases and disorders that affect the human muscle system . Diseases and disorders that result from direct abnormalities of the muscles are called primary muscle diseases; those that can be traced as symptoms or manifestations of disorders of nerves or other systems are not properly classified as primary muscle diseases. Because muscles and nerves (neurons) supplying muscle operate as a functional unit, disease of both systems results in muscular atrophy (wasting) and paralysis.
Indications of muscle disease
Muscular atrophy and weakness are among the most common indications of muscular disease (see below Muscle weakness). Though the degree of weakness is not necessarily proportional to the amount of wasting, it usually is so if there is specific involvement of nerve or muscle. Persistent weakness exacerbated by exercise is the primary characteristic of myasthenia gravis.
Pain may be present in muscle disease because of defects in blood circulation, injury, or inflammation of the muscle. Pain is rare, except as a result of abnormal posture or fatigue inmuscular dystrophy—a hereditary disease characterized by progressive wasting of the muscles.Cramps may occur with disease of the motor or sensory neurons, with certain biochemical disorders (e.g., hypocalcemia, a condition in which the blood level of calcium is abnormally low), when the muscle tissues are affected by some form of poisoning, with disease of the blood vessels, and with exercise, particularly when cold.
Muscle enlargement (muscular hypertrophy) occurs naturally in athletes. Hypertrophy not associated with exercise occurs in an unusual form of muscular dystrophy known as myotonia congenita, which combines increased muscle size with strength and stiffness.Pseudohypertrophy, muscular enlargement through deposition of fat rather than muscle fibre, occurs in other forms of muscular dystrophy, particularly the Duchenne type.
Tetany is the occurrence of intermittent spasms, or involuntary contractions, of muscles, particularly in the arms and legs and in the larynx, or voice box; it results from low levels ofcalcium in the blood and from alkalosis, an increased alkalinity of the blood and tissues.Tetanus, also called lockjaw, is a state of continued muscle spasm, particularly of the jaw muscles, caused by toxins produced by the bacillus Clostridium tetani .
The twitching of muscle fibres controlled by a single motor nerve cell, called fasciculation, may occur in a healthy person, but it usually indicates that the muscular atrophy is due to disease of motor nerve cells in the spinal cord. Fasciculation is seen most clearly in muscles close to the surface of the skin.
Glycogen is a storage form of carbohydrate, and its breakdown is a source of energy. Muscle weakness is found in a rare group of hereditary diseases, the glycogen-storage diseases, in which various enzyme defects prevent the release of energy by the normal breakdown of glycogen in muscles. As a result, abnormal amounts of glycogen are stored in the muscles and other organs. The best-known glycogen-storage disease affecting muscles is McArdle disease, in which the muscles are unable to degrade glycogen to lactic acid on exertion because of the absence of the enzyme phosphorylase. Abnormal accumulations of glycogen are distributed within muscle cells. Symptoms of the condition include pain, stiffness, and weakness in the muscles on exertion. McArdle disease usually begins in childhood. No specific treatment is available, and persons affected are usually required to restrict exertion to tolerable limits. The condition does not appear to become steadily worse, but serious complications may occur when the muscle protein myoglobin is excreted in the urine. Other glycogen-storage diseases result from deficiency of the enzymes phosphofructokinase or acid maltase. With acid maltase deficiency, both heart and voluntary muscles are affected, and death usually occurs within a year of birth.
Muscle weakness
Signs and symptoms
Weakness is a failure of the muscle to develop an expected force. Weakness may affect all muscles or only a few, and the pattern of muscle weakness is an indication of the type of muscle disease. Often associated with muscle weakness is the wasting of affected muscle groups. A muscle may not be fully activated in weakness because of a less than maximal voluntary effort; a disease of the brain, spinal cord, or peripheral nerves that interferes with proper electrical stimulation of the muscle fibres; or a defect in the muscle itself. Only when all causes have been considered can weakness be attributed to failure of the contractile machinery (i.e., the anatomy) of the muscle cell.
The effect of weakness in a particular muscle group depends on the normal functional role of the muscle and the degree to which force fails to develop. A weakness in muscles that are near the ends of the limbs usually results in a tendency to drop things if the upper limb is affected or in "foot drop" if the lower limbs are affected. The overall disability is not as great as weakness of more proximal (closer to the body) muscles controlling the pelvic or shoulder girdles, which hold large components of the total body mass against the force of gravity. Weakness of the proximal muscles that control the shoulder blade (scapula), for example, results in "winging" (i.e., when the sharp inner border protrudes backward) as the arms are held outstretched. If the weakness is severe, the arms cannot be raised at all.
Assessment
Muscle disease may be detected by assessing whether the muscle groups can withhold or overcome the efforts of the physician to pull or push or by observing the individual carrying out isolated voluntary movements against gravity or more complex and integrated activities, such as walking. The weakness of individual muscles or groups of muscles can be quantified by using amyometer, which measures force based on a hydraulic or electronic principle. Recordings of contraction force over a period of time are valuable in determining whether the weakness is improving or worsening.
The assessment of muscle weakness (and wasting) is directed toward discovering evidence of muscle inflammation or damage. These changes are discerned by blood tests or by measuring alterations of the electrical properties of contracting muscles. Another investigative tool is the muscle biopsy, which provides muscle specimens for pathological diagnosis and biochemical analysis. Muscle biopsies can be taken with a needle or during a surgical procedure.
Anatomy and Physiology 7th Edition Chapter 7 the Muscular System
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