Functions and Types of Muscles
Contractile cells, called fibers (well, really skeletal muscle normally)
Spindle shaped cells, usually found in parallel lines forming sheets
Found in walls of hollow organs (also called visceral)
Slow contractions but can sustain – doesn’t fatigue easily
Auto-rhythmic – doesn’t require nervous system stimulation to contract
Cylindrical and branched cells (fibers)
Joined by intercalated disks – allow passage of electrical activity throughout muscle
Relax completely between contractions, avoids fatigue
Cylindrical fibers are long – run the length of the muscle
Can contract multiple times between relaxations, can fatigue (use up all the available ATP)
Connective Tissue Coverings
Epimysium – outer covering of muscle (covers bundles of fasicles), extends beyond muscle to become tendon
Perimysium – surrounds bundles of muscle fibers called fasicles
Endomysium – covers individual fibers (cells)
Functions of Skeletal Muscles
Stabilize joints – tendons may extend across joints
Maintains posture, which also takes strain off bones and joints
“Opposes the force of gravity and keeps us upright”
“Muscle contraction refinements allow us to assume different positions”
Contraction produces heat
Microscopic Anatomy and Contraction of Skeletal Muscle Structure
Myofibrils and Sarcomeres
Muscle fibers contain bundles of myofibrils, the contractile elements of the cell, which run the length of the fiber
Myofibrils contain actin and myosin myofilaments arranged in sarcomeres (sarcomeres can be said to be the contractile elements of the myofibrils).
Myosin filaments are thick, consist of two molecules with entertwined tails and two heads that bind to actin.
Actin filaments consist of two entertwined chains of actin monomers with myosin head binding sites. The myosin binding sites are blocked by a linear molecule of tropomyosin, which runs the length of the thin filament in the groove between the two chains. A regulatory molecule, troponin, is bound to the actin and the tropomyosin.
The myofilaments are anchored to Z-discs at each end of the sarcomere.
The sarcolemma (plasma membrane) forms tubes that dip into cell –T-tubules (transverse) - over each z-disc.
T-tubules come in contact with sarcoplasmic reticulum (ER) at sites of calcium storage (calcium storage sacs, or terminal cisternae of the SR)
Skeletal Muscle Contraction
Neuromuscular junction – site of interaction between neuron and muscle fiber
A nerve impulse travels from neuromuscular junction, along sarcolemma, down T-tubules, stimulates calcium release from sarcoplasmic reticulum, which allows the thin filaments to slide to the center of the sarcomere, causing it to shorten and the muscle to contract.
The Role of Actin and Myosin
Sliding Filament Mechanism of Contraction:
Calcium binds to troponin.
Troponin changes shape and pulls tropomyosin out of the myosin head binding sites.
The interaction between myosin and actin pulls the actin filaments toward the center of the sarcomere.
The myosin head has an ATP binding site. When ATP binds the myosin head hydrolyzes ATP to ADP + P and "cocks" into its high energy conformation, pointing away from the center of the sarcomere and toward the z-discs.
If calcium is present and the myosin head binding sites are unblocked the myosin heads form crossbridges with the actin filaments.
The myosin heads relax into their low energy conformation, pulling the actin filaments toward the center of the sarcomere (causing all the sarcomeres along the myofibril to shorten, causing the myofibril to shorten, causing the muscle to shorten - or contract).
At this point ADP and P are released and ATP can bind to the myosin head. ATP binding causes the myosin head to relase from the actin filament and the cycle starts all over.
Contraction of Smooth Muscle
Thick and thin filaments are present but there are no myofibrils, no sarcomeres, no striations.
Thin filaments are anchored to proteins in the sarcoplasm called dense bodies or directly to the sarcolemma.
Contraction cause fibers to shorten in all directions, cells go from spindle shaped to more round.
Contraction of smooth muscle is much slower than skeletal muscle but smooth muscle is more fatigue resistant than skeletal muscle and can maintain contractions for far longer.
Energy for Muscle Contration
ATP required for contraction and is generated by 3 mechanisms.
Under anaerobic conditions only about 4-6 seconds worth of ATP present.
Creatine Phosphate Breakdown
Creatine phosphate quickly regenerates ATP by directly donating a phosphate (good for about 15 seconds, first ATP regeneration mechanism to kick in under anaerobic conditions)
Anaerobic production of ATP by glycolysis occurs during strenuous exercise after CP stores are exhausted.
Very inefficient, good for only 1-2 minutes before muscle fatigue occurs.
The end product of glycolysis, pyruvic acid, is converted to lactic acid if it isn't metabolized by cellular respiration fast enough to meet ATP needs of the muscle. Lactic acid builds up, the pH drops, muscles fatigue and ache.
ATP is made in mitochondria by aerobic means - requires oxygen. Pyruvic acid is transported into mitochondria and oxidized to carbon dioxide and water.
As long as demand is below anaerobic threshold ATP made by this mechanism will fuel muscle activity - much more efficient than glycolysis alone.
Lactic acid is transported to the liver where it is converted back to pyruvic acid, which is then metabolized by cellular respiration, which of course requires oxygen.
The amount of oxygen required to finish metabolism of lactic acid is the oxygen debt - that’s the wind-sucking you do after going anaerobic.
In the Laboratory
When stimulated enough to contract a muscle fiber (cell) completely contracts
A muscle (a bunch of fasicles, which is a bunch of fibers) doesn’t follow the All-or-None Law; how much the muscle contracts depends on how many fibers are stimulated to contract
Muscle Twitch, Summation, and Tetanus
Single stimulus, muscle contracts and relaxes
Stimulation in succession before the muscle can relax results in summation of tension – tension gets greater with same level of stimulation because it doesn’t get back to “0” tension before stimulated to contract again
Tetanus is maximal sustained contraction – muscle can’t shorten anymore despite continued stimulation
Fatigue – muscle runs out of ATP, can’t maintain contraction in spite of continued stimulation, lactic acid builds up and pH drops, motor nerves run out of neurotransmitter. Usually the voice in your head talks you into quitting before you actually experience fatigue.
In the Body
Motor unit – motor neuron from spinal cord branches and innervates several muscle fibers.
Each motor unit obeys the all-or-none law, so the amount of force a muscle contracts with depends on recruitment of motor units in that muscle.
Muscles that perform fine, delicate tasks and require great precision have motor units with few fibers per motor neuron (as few as 4), muscles that do the heavy lifting have motor units with many fibers per motor neuron (may be thousands).
Muscle Tone – partial contraction of muscles, maintains posture
Muscle spindles are sensors that send information to the CNS to allow partial contraction or tone to be maintained
Isotonic Vs. Isometric Contraction
Isotonic – muscle contracts, shortens, movement occurs (tension remains constant)
Isometric - muscle contracts, can’t move load, tension increases but length of muscle remains constant
Exercise and Size of Muscles
Hypertrophy – increase in size due to repeated forceful contraction (exercise)
Requires 75% maximum effort to stimulate hypertrophy
Number of muscle fibers may also increase (but this is due to splitting of fibers; basically ripping them into shreds and allowing them to heal, not hyperplasia)
Atrophy – disuse or very low intensity use causes muscles to shrink – fibers shorten and are replaced with fat and CT
Slow Twitch and Fast-Twitch Muscle Fibers
Skeletal Muscles of the Body
Muscles connect two bones, when they contract one bone moves and one remains stationary
Origin – where the muscle attaches to the stationary bone
Insertion – attachment site on the bone that moves
Prime mover – muscle that does most of the work
Synergists – assisting muscles
Antagonist – muscle that opposes the action of the prime mover
Muscles can only move bones in one direction since they shorten when they contract –don’t cause movement by lengthening
Have to have an opposing muscle to move the bone back in the opposite direction
Size – gluteus maximus, the largest buttock muscle
Shape – deltoid, shaped like a delta
Direction of Fibers – rectus abdominus – longitudinal abdominal muscle (rectus means straight)
Location – frontalis overlies the frontal bone
Number of Attachments – biceps, triceps
Action – extensor digitorum, flexors, adductors
Skeletal Muscle Groups
Superficial Skeletal Muscles - Anterior View
Superficial Skeletal Muscles - Posterior View
Muscles of the Head
Muscles of Facial Expression
Frontalis – corn row muscle
Orbicularis oculi – blinking muscle, crow’s feet
Orbicularis oris – pucker muscle
Buccinator – in cheek, compresses cheek
Zygomaticus – cheekbone to corners of mouth, smiley muscle
Muscles of Mastication
Masseter – chewing muscle
Muscles of the Neck
Muscles That Move the Head
Sternocleidomastoids (2) – sternum to mastoid process, both: head to chest (flex neck); one: head turns
Trapezius – shoulder shrug, neck extension
Muscles of the Trunk
Muscles of the Thoracic Wall
External intercostals – ribs up and out
Internal intercostals – ribs down and in
Diaphragm - separates thoracic cavity from abdominal cavity, assists inspiration
Muscles of the Abdominal Wall
External oblique – abdominal wall, lateral rotation
Internal oblique – same
Transversus abdominis – abdominal wall
Rectus abdominis – flexes vertebral column
Muscles of the Shoulder
Muscles That Move the Scapula
Trapezius – shoulder shrug, neck extension
Serratus anterior – pulls scapula down and forward (pushing)
Muscles That Move the Arm
Deltoid – abducts the arm
Pectoralis major – flexes and adducts the arm (pulls across chest)
Latissimus dorsi – extends and adducts the arm
Muscles of the Arm
Biceps brachii – flexes forearm and supinates hand
Triceps brachii – extends forearm
Brachialis – flexes forearem
Muscles of the Forearm
Extensor and flexor carpi – move wrist and hand
Extensor and flexor digitorum – move fingers
Muscles of the Hip and Lower Limb (Leg)
Muscles That Move the Thigh
Iliopsoas – flexes thigh
Gluteus maximus – extends thigh
Gluteus medius – abducts thigh
Adductor group - adducts thigh
Muscles That Move the Lower Limb (Leg)
Quadriceps femoris group – extends lower leg
Hamstring group – flexes lower leg and extends hip
Sartorius – flexes, abducts, and rotates leg
Muscles That Move the Ankle and Foot
Gastrocnemius – plantar flexion and eversion of foot
Tibialis anterior – dorsiflexion and inversion of foot
Peroneus group – plantar flexion and eversion of foot
Soleus – plantar flexes foot
Flexor and extensor digitorum longus – moves toes
Effects of Aging
Mass and Strength - tend to decrease
Endurance – tends to decrease
Exercise – anti-aging effect, combats loss in mass, strength and endurance
Medical Focus - Muscular Dystrophy
A group of muscular disorders characterized by muscle degeneration.
Symptoms result from progressive skeletal muscle weakness
Due to degeneration of the fibers
Several types, all inherited, cause unknown in most
Duchenne muscular dystropy
Passed from the mother and nearly always only seen in male offspring
Muscle weakness, difficulty in walking, curvature of the spine
Gradual wasting of muscle, replacement with fat and CT
Confinement to a wheelchair by about 12 an death by 20 or so
Recently discovered that the cause is lack of a protein that maintains the integrity of the sarcolemma (dystrophin)
Myoblast transfer therapy – inject with health myoblast cells that fuse with health ones, this provides the normal gene and allows the fibers to produce normal dystrophin
Insertion of plasmids with the correct genes also being tried
Myotonic muscular dystrophy
Failure of muscles to relax after contraction
Inherited from either parent
Face and neck affected first
Lifting and turning head
Abnormal heart rhythms
Sometimes leads to confinement to bed or wheelchair
MedAlert – Benefits of Exercise
The force a muscle can exert against resistance in one maximal effort.
The size of the muscle and the number of myofibrils and even fibers will increase as the strength increases
Capillaries and CT will increase also, including CT of tendons and ligaments
Strength training (resistance) benefits all adults (remember it also increases bone density)
Ability of the muscle to contract repeatedly or to sustain a contraction.
Range of motion around a joint
Any level of exercise can improve health:
Lowers risk of heart attack
Increases HDL levels
Lowers resting heart rate, lowers blood pressure
Reduces pain and swelling in arthritis patients
Also reduces fatigue and depression associated with the disease
May be beneficial in a variety of other long-term diseases that include fatigue and depression as part of their secondary symptoms
Stimulates osteoblast activity, helps prevent osteoporosis
Linked to decreases in liklihood of developing colon, breast, cervical, uterine, and ovarian cancers
Increases intestinal motility and fat mobilization (and utilization)