VT 111 Lec. 9 Muscles

How Muscles Work

• Muscles are engines

• They burn fuel (glucose)

• They produce mechanical movement

• They move loads

Definitions

• Muscles act on the principle of leverage

• The bone is a lever

• The joint is the fulcrum

• The muscle is the motor

• The load is the body part or anything that is attached

First Class Lever System

Second Class Lever System

Third Class Lever System

Movements of Muscles

• A muscle that is the major mover of something is a prime mover (agonist)

• A muscle that acts against the prime mover is an antagonist

• A muscle that helps another muscle is a synergist

Skeletal Muscle Action

• Agonist = prime mover

  • Directly produces a desired movement

• Antagonist

  • Directly opposes the action of an agonist

• Synergist

  • Contracts at same time as agonist to assist its action

• Fixator

  • Stabilizes joints to allow other movements

Naming Muscles 

• Location:  “brachialis” (upper arm)

• Shape:  “teres” (round shaped)

• Size:  “maximus” (greatest/largest)

• Direction of fibers: “rectus” (straight/longitudinal)

• Attachments: “brachioradialis” (humerus → radius)

• Number of origins: “triceps” (3)

• Action: “flexor” (closes joint angle)

Movement in Animals: Basic Principles

• Muscle contraction is based in contractile proteins

• Muscles only do work when they contract

• Vertebrates have opposing muscle in the same plane.

• Muscles connect to the skeleton through tendons

Types of Muscles

• Skeletal: voluntary, striated, multinucleate

• Cardiac: involuntary, striated, usually mononucleated but sometimes multinucleate, intercalated discs

• Smooth: involuntary, non striated, mononucleate, spindle shaped

Skeletal Muscle

• Voluntary striated muscle

• Well-defined group of cells surrounded by fibrous connective sheath = epimysium

Gross Structure of Muscle

Gross Structure Continued…

Cutaneous Muscles 

• Thin, broad superficial muscles

• Found in connective tissue just beneath skin

• Little or no attachment to bones

Head & Neck Skeletal Muscles

• Functions

  • Control facial expressions

  • Enable mastication

  • Move sensory structures

  • Support the head

  • Raise the head and neck

  • Move the head laterally

  • Close the jaw

  • Extend the head and neck and pull the front leg forward

  • Flex the head and neck

Abdominal Skeletal Muscles 

• Functions

  • Support abdominal organs

  • Help flex the back

  • Participate in defecation, urination, parturition, vomiting, and regurgitation

  • Have a role in respiration

• Arranged in layers

  • External abdominal oblique muscle

  • Internal abdominal oblique muscle

  • Rectus abdominis

  • Transversus abdominis

• Left and right parts come together at linea alba

Thoracic Limb Skeletal Muscles

• Functions mainly for locomotion

• Superficial muscles of the brachium

  • Adductor muscles –latissimus dorsi, pectoral muscles

  • Abductor muscle—deltoid muscle

• Brachial muscles

  • Flexor and extensor muscles—biceps brachii, triceps brachii

• Carpal and digital muscles

  • Flexor and extensor muscles—extensor carpi radialis, deep digital flexor

Pelvic Limb Skeletal Muscles 

• Functions mainly for locomotion

• Muscles of the hip joint

  • Extensor muscles

    • Luteal muscles and hamstring muscle group

• Muscles of the stifle joint

  • Extensor muscles

    • Quadriceps femoris

• Muscles of the tarsus and digits

  • Flexors and extensors

    • Gastrocnemius muscle

      • Achilles tendon

Skeletal Muscles of Respiration

• Function to increase and decrease size of thoracic cavity

  • Inspiratory muscles

    • Diaphragm

    • External intercostal muscles

• Expiratory muscles

  • Internal intercostal muscles

  • Abdominal muscles

Skeletal Muscle Cell = Muscle Fiber

• Very large, quite long and thin

• Multinucleate

• Myofibrils form interior of muscle fiber

• Network of sarcoplasmic reticulum

• System of T tubules (transverse tubules)

• Sarcomere = series of protein filaments that make up contractile units of muscle cells

• Many sarcomeres lined up end to end = one myofibril

Sarcomere

• 2 primary protein filaments responsible for contraction

  • Thick, dark myosin

  • Thin, light actin

• Filaments comprise various visible bands

  • A band

  • H band

  • I band

  • Z line

Thin Filament Structure

• Actin

• Tropomyosin

• Troponin

Thick Filament Structure

• Myosin subunits

  • Heads

  • Tail region

Filament Structure

Structure of Sarcomere

Electron Microscope: Sarcomere

Neuromuscular Junction

• Site where ends of motor nerve fibers connect to muscle fibers

• Motor nerve

  • One nerve fiber and all the muscle fibers it innervates

  • Few muscle fibers per motor unit

    • Small, delicate movement of muscles

  • Huge numbers of muscle fibers per motor unit

    • Large, powerful movement of muscles

• Synaptic vesicles at end of nerve fiber contain neurotransmitter acetylcholine

Neuromuscular Junction Diagram

Sliding Filament Theory I

• Action potential travels down axon and reaches axon terminal.

• Synapse between axon and muscle cell is called the myoneural junction or the neuromuscular junction.

• Neurotransmitter of the somatic nervous system is acetylcholine

Sliding Filament Theory II

• Nerve impulse travels over muscle fiber sarcolemma and into T tubules

Sliding Filament Theory III

• Nerve impulse triggers Ca release from S/R (Sarcoplasmic reticulum)

• Ca floods cytoplasm and attaches to troponin

• Troponin moves and pulls tropomyosin with it.

• Myosin binding site on actin is exposed

T Tubule/Myofibril System 

Sliding Filament Theory IV

• Myosin heads function as ATPase. This cleaves ATP to ADP.

• Myosin head can bind only if ADP is formed

• Once binding takes place, ADP and Phosphate group is released.

• Release of ADP causes the cross bridge to change orientation, resulting in the power stroke of the myosin head

Sliding Filament Theory V

• At the end of the power stroke, Myosin head binds new ATP

• Binding causes myosin cross bridge to break bond with actin.

• Myosin ATPase cleaves ATP to ADP in preparation for a new cycle.

• If no further action potentials arrive, Ca is taken back into S/R by active transport pumps

Sarcomere Animation

Refer to PowerPoint

Muscle (Contraction) & Relaxation

• Nerve impulse comes down motor nerve fiber

• Impulse reaches end bulb of nerve fiber

• Acetylcholine released into synaptic space and binds to receptors on sarcolemma surface

• Impulse travels along sarcolemma and through T tubules to interior of the cell

• Impulse reaches sarcoplasmic reticulum

  • Ca⁺⁺ ions released into sarcoplasm

  • Ca⁺⁺ diffuses into myofibrils and starts contraction

  • Energy supplied by ATP

Muscle (Relaxation) & Contraction Continued…

• Sarcoplasmic reticulum begins pumping ca⁺⁺ back in again

  • Ca⁺⁺ is pulled out of myofibrils

  • Energy supplied by ATP

• Contraction stops

• Muscle returns to original length

Mechanics of Muscle Contraction

• Muscle fibers in relaxed state

  • Actin and myosin filaments slightly overlap

• Muscle fibers stimulated to contract

  • Cross-bridges ratchet back and forth

  • Actin filaments pulled toward center of myosin filaments

  • Sarcomere is shortened

• Muscle contraction = shortening of all sarcomeres in a muscle fiber

Characteristics of Muscle Contractions

• All-or-nothing principle

  • When stimulated, individual muscle fiber contracts completely—or not at all

  • Nervous system controls number of muscle fibers stimulated

• Twitch contraction

  • A single muscle fiber contraction

    • Latent phase

    • Contracting phase

    • Relaxation phase

Muscle Contraction

• Maximum contraction efficiency

  • When nerve impulses arrive 0.1 second apart

  • Result is series of complete muscle fiber twitches

• Smooth, sustained muscle contractions

  • Average out activity of all muscle fibers

• Twitches

  • Contractions out of sync with each other

Chemistry of Muscle Contraction

• ATP provides energy to allow sliding of actin and myosin filaments

• CP (creatine phosphate) coverts ADP back to ATP

• Catabolism of glucose and oxygen help to produce ATP and CP

  • Glucose stored in muscle as glycogen

  • Oxygen stored as myoglobin

• Aerobic metabolism

  • Adequate oxygen supply for energy needs of muscle fiber

  • Maximum energy extracted from each glucose molecule

• Anaerobic metabolism

  • Need for oxygen exceeds available supply

  • Lactic acid formed in incomplete glucose breakdown

    • Lactic acid is what makes muscles burn when working out

Heat Production 

• Muscle activity generates heat

• Mechanisms to eliminate excess heat

  • Panting or sweating

• Spasmodic muscle contractions that increase heat production

  • Shivering

Cardiac Muscle = Striated Involuntary Muscle

• Found in only the heart

• Small cells with single nucleus

  • Longer than wide, with multiple branches

• Intercalated disks fasten cells together

Physiology & Anatomy of Cardiac Muscle

• Cells contract with no external stimulation

• Groups of cells contract at the rate of the most rapid cell in the group

• Contractions are rapid and wavelike

• Microscopic anatomy

  • Striated like skeletal muscle cells with many of the same organelles and intracellular structures

  • Much smaller than muscle cells

  • Single nucleus per cell

  • Longer than they are wide with multiple branches

Cardiac Conduction System

• Sinoatrial (SA) node

  • Located in wall of right atrium

  • Generates impulse to start each heartbeat

• Impulse follows controlled path through the heart

• Structures in heart transmit, delay, and redirect

  • Walls of heart chambers contract in coordinated manner

Nerve Supply to Cardiac Muscle

• Not needed to initiate contractions

• Heart innervated from 2 systems

  • Sympathetic system

    • Stimulates heart in fight-or-flight response

• Parasympathetic system

  • Inhibits cardiac function

Smooth Muscle Anatomy

• Gross anatomy

  • 2 main forms

    • Visceral smooth muscle

      • Large sheets of cells in walls of some hollow organs

  • Multiunit smooth muscle

    • Small, discrete groups of cells -> ex. iris

• Microscopic anatomy

  • Small and spindle shaped with single nucleus in the center

  • Smooth, homogeneous appearance

  • Actin and myosin filaments crisscross at various angles

Smooth Muscle = Involuntary Muscle

• Cells not under conscious control

• Cells small and spindle shaped

• Single nucleus in center

• Smooth, homogeneous appearance

• Cell balls up as it contracts

Microscopic Anatomy of Smooth Muscle Cells

• Actin and myosin filaments arranged as small contractile units that crisscross the cell

• Dense bodies at each end correspond to Z lines of skeletal muscle

Visceral Smooth Muscle

• Found in walls of many soft internal organs

  • Stomach, intestines, uterus, urinary bladder

• Contracts in large, rhythmic waves

• Contracts without external stimulation

  • Reacts to stretching

• Innervated from 2 systems

  • Sympathetic system decreases activity

  • Parasympathetic system increases activity

Multiunit Smooth Muscle 

• Individual smooth muscle cells or small groups of cells

• Found where small, delicate contractions are needed

  • Iris and ciliary body of eye, walls of small blood vessels, and small air passageways in lungs

• Contraction requires impulses from autonomic nervous system