Physiology of Muscles

PHYSIOLOGY I (PHL 215)

Dr. Gamal Gabr

Muscular System Functions

Body movement (Locomotion) Maintenance of posture Respiration

*Diaphragm and intercostals contractions Communication (Verbal and Facial) Constriction of organs and vessels Heart beat Production of body heat (Thermogenesis)

Typical cells Muscle cell=fiber

Plasma membrane Sarcolemma

Cytoplasm Sarcoplasm

Endoplasmic reticulum

Sarcoplasmic reticulum (SR)

Many mitochondria

Multiple nuclei

Muscle cell structures not found in other cells

Myofibrils: bundles of very fine fibers Thick and thin myofilaments: very fine fibers that make

up myofibrils Sarcomere: segment of myofibril between two Z lines;

contractile unit T tubules: transmit electrical impulses through cell

Myofilaments

4 protein molecules that make up myofilaments: Myosin, actin, tropomyosin, troponin

Thin filaments: actin, tropomyosin, troponin

Thick filaments: mostly myosin

Properties of Muscle

Excitability: Capacity of muscle to respond to a stimulus

Contractility: Ability of a muscle to shorten and generate pulling force

Extensibility: Ability stretches when pulled

Elasticity: Ability to return to original shape and length after contraction or extension

Muscle structure

Connective Tissue Sheaths

Connective Tissue (CT) of a Muscle

Epimysium: Dense regular CT surrounding entire muscle

*Separates muscle from surrounding tissues and organs

Perimysium: Collagen and elastic fibers surrounding a group of muscle fibers called a fascicle

*Contains blood vessels and nerves

Endomysium: Loose CT that surrounds individual muscle fibers

*Also contains blood vessels and nerves

*Collagen fibers of all 3 layers come together at each end of muscle to form a tendon or aponeurosis.

Motor neurons

*Stimulate muscle fibers to contract

*Neuron axons branch so that each muscle fiber (muscle cell) is innervated

*Form a neuromuscular junction

Capillary beds surround muscle fibers

*Muscles require large amount of energy

*Extensive vascular network delivers necessary oxygen and nutrients and carries away metabolic waste produced by muscle fibers

Energy for Muscle Contractions

*ATP: adenosine triphosphate

*CP: creatine phosphate

Glucose & Oxygen

*Glucose stored in form of glycogen in muscle

*Excess oxygen molecules in sarcoplasm bound to myoglobin

Anaerobic respiration

Allows body to avoid use of oxygen in short termProduces lactic acidAccumulation of lactic acid in muscles causes burning sensation

Types of MuscleSkeletal

*Attached to bones

*Makes up 40% of body weight (Women’s skeletal muscle makes up 36% of their body mass, Men’s skeletal muscle makes up 42% of their body mass)

*Responsible for locomotion, facial expressions, posture, respiratory movements, other types of body movement

*Voluntary in action; controlled by somatic motor neurons

Smooth

*In the walls of hollow organs, blood vessels, eye, glands, uterus, skin

*Functions: propel urine, mix food in digestive tract, regulating blood flow

*Controlled involuntarily by endocrine and autonomic nervous systems

Cardiac

*Heart: major source of movement of blood

Basic Features of a Skeletal Muscle

Muscle attachments*Most skeletal muscles run from one bone to another

*One bone will move – other bone remains fixed

*Origin – less movable attachment

*Insertion – more movable attachment

*Muscles attach to origins and insertions by connective tissue

Fleshy attachments – connective tissue fibers are short

Indirect attachments – connective tissue forms a tendon

Skeletal Muscle Structure

*Composed of muscle cells (fibers), connective tissue, blood vessels, nerves

*Fibers are long, cylindrical, and multinucleated

*Tend to be smaller diameter in small muscles and larger in large muscles. 1 mm - 4 cm in length

*Striated appearance

*Nuclei are peripherally located

Muscle Fiber Anatomy

Sarcolemma - cell membrane

* Surrounds the Sarcoplasm (cytoplasm of fiber)

* Punctuated by openings called the transverse tubules (T-tubules)

Myofibrils -cylindrical structures within muscle fiber

* Are bundles of protein filaments (=myofilaments)

* Two types of myofilaments

1. Actin filaments (thin filaments)

2.Myosin filaments (thick filaments)

– When myofibril shortens, muscle shortens (contracts)

Sarcoplasmic Reticulum (SR)

SR is an elaborate, smooth endoplasmic reticulum Runs longitudinally and surrounds each myofibrilSR stores Ca++ when muscle not contractingWhen stimulated, calcium released into sarcoplasm SR membrane has Ca++ pumps that function to pump Ca++ out of the sarcoplasm back into the SR after contraction

Smooth Muscle

*Cells are not striated

*Fibers smaller than those in skeletal muscle

*Spindle-shaped; single, central nucleus

*More actin than myosin

*No sarcomeres

*Not arranged as symmetrically as in skeletal muscle, thus no striations.

Smooth Muscle

• Grouped into sheets in walls of hollow organs• Longitudinal layer: muscle fibers run parallel to organ’s long

axis• Circular layer: muscle fibers run around circumference of the

organ• Both layers participate in peristalsis

Cardiac Muscle Found only in heart where it forms a thick layer called the

myocardium

Striated fibers that branch

Each cell usually has one centrally-located nucleus

Fibers joined by intercalated disks

Disorders of Muscle Tissue

*Muscle Fatigue

*Lack of oxygen causes ATP deficit

*Lactic acid builds up from anaerobic respiration

*Muscle Atrophy

*a decrease in the mass of the muscle

*Weakening and shrinking of a muscle

*May be caused

*Immobilization

*Loss of neural stimulation

Disorders of Muscle Tissue

*Muscle Hypertrophy

*Enlargement of a muscle

*More capillaries

*More mitochondria

*Caused by*Strenuous exercise

*Steroid hormones

*Muscle Tonus

*Tightness of a muscle

*Some fibers always contracted