Chapter 8: Muscular System

Muscle Overview

The three types of muscle tissue are skeletal, cardiac, and smooth

These types differ in structure, location, function, and means of activation

(Skeletal – voluntary & striated, Cardiac – involuntary & striated, Smooth – involuntary & non-striated)

Muscle Similarities

Skeletal and smooth muscle cells are elongated and are called muscle fibers (muscle cell = muscle fiber)

Muscle contraction depends on two kinds of myofilaments – actin & myosin (proteins)

Muscle terminology is similarSarcolemma – muscle plasma membrane

Sarcoplasm – cytoplasm of a muscle cell

Prefixes – myo, mys, and sarco all refer to muscles (Ex sarcoplasmic reticulum – ER in muscle cell)

Skeletal Muscle Tissue

Packaged in skeletal muscles that attach to and cover the bony skeleton

Has obvious stripes called striations

Is controlled voluntarily (i.e., by conscious control)

Contracts rapidly but tires easily

Is responsible for overall body motility

Is extremely adaptable and can exert forces ranging from a fraction of an ounce to over 70 pounds

Cardiac Muscle Tissue

Occurs only in the heart

Is striated like skeletal muscle but is not voluntary

Contracts at a fairly steady rate set by the heart’s pacemaker

Neural controls allow the heart to respond to changes in bodily needs

Smooth Muscle Tissue

Found in the walls of hollow visceral organs, such as the stomach, urinary bladder, and digestive organs

Forces food and other substances through internal body channels

It is not striated and is involuntary

Functional Characteristics of Muscle Tissue

Excitability or irritability – the ability to receive and respond to stimuli

Contractility – the ability to shorten forcibly

Extensibility – the ability to be stretched or extended

Elasticity – the ability to recoil and resume the original resting length

Muscle Function

Skeletal muscles are responsible for all locomotion

Cardiac muscle is responsible for coursing the blood through the body

Smooth muscle helps maintain blood pressure and squeezes or propels substances (food & feces) through organs

Muscles also maintain posture, stabilize joints, and generate heat

Skeletal Muscle

Each muscle is a discrete organ composed of muscle tissue, blood vessels, nerve fibers, and connective tissue

Skeletal Muscle

The three connective tissue sheaths are:Endomysium – fine sheath of connective tissue composed of reticular fibers surrounding each muscle fiber

Perimysium – fibrous connective tissue that surrounds groups of muscle fibers called fascicles (bundle of muscle cells)

Epimysium – an overcoat of dense regular connective tissue that surrounds the entire muscle

These three membranes merge to make up a tendon

Skeletal Muscle: Nerve & Blood Supply

Each muscle is served by one nerve, an artery, and one or more veins

Each skeletal muscle fiber is supplied with a nerve ending that controls contraction

Contracting fibers require continuous delivery of oxygen and nutrients via arteries

Wastes must be removed via veins

Skeletal Muscle: Attachments

Most skeletal muscles span joints and are attached to bone in at least two places

When muscles contract the movable bone, the muscle’s insertion moves toward the immovable bone, the muscle’s origin

Skeletal Muscle: Attachments

Muscles attach:Directly – epimysium of the muscle is fused to the periosteum of a bone

Indirectly – connective tissue wrappings extend beyond the muscle as a tendon or aponeurosis

Microscopic Anatomy of a Skeletal Muscle Fiber

Each fiber is a long, cylindrical cell with multiple nuclei just beneath the sarcolemma

Fibers are 10 to 100 micrometers in diameter and up to hundreds of centimeters long

Each cell is a syncytium produced by fusion of embryonic cells

Skeletal Muscle Contraction

In order to contract, a skeletal muscle must:Be stimulated by a nerve ending

Propagate an electrical current or action potential along its sarcolemma

Have a rise in intracellular Ca2+ levels, the final trigger for contraction

Linking the electrical signal to the contraction is excitation-contraction coupling

Nerve Stimulus of Skeletal Muscle

Skeletal muscles are stimulated by motor neurons of the somatic nervous system

Axons of these neurons travel in nerves to muscle cells

Axons of motor neurons branch profusely as they enter muscles

Each axonal branch forms a neuromuscular junction with a single muscle fiber

Neuromuscular Junction

The neuromuscular junction is formed from:Axonal endings which have small membranous sacs (synaptic vesicles) that contain the neurotransmitter acetylcholine (ACh)

The motor end plate of a muscle, which is a specific part of the sarcolemma that contains ACh receptors and helps form the neuromuscular junction

Neuromuscular Junction

Though exceedingly close, axonal ends and muscle fibers are always separated by a space called the synaptic cleft

Energy Sources for Muscle Contraction

ATP molecules supply the energy for muscle fiber contraction

Initial source of energy available to a contracting muscle comes from existing ATP molecules in the cell

When initial ATP is gone:Creatine phosphate is used to transform ADP to ATP

Muscle fibers depend on cellular respiration of glucose as an energy source for synthesizing ATP

Oxygen Debt

Muscle fibers must depend increasingly on anaerobic respiration for energy

In anaerobic respiration:Glucose molecules are broken down by glycolysis to yield pyruvic acid

When oxygen supply is low, the pyruvic acid reacts to produce lactic acid which can accumulate in the muscles

Lactic acid enters blood stream and travels to the liver where glucose is made from the lactic acid

Oxygen Debt

During strenuous exercise:Available oxygen is used primarily to synthesize ATP the muscle fiber requires to contract

This causes the lactic acid to accumulate in the muscles which causes an oxygen debt

Equals the amount of oxygen liver cells require to convert the accumulated lactic acid into glucose plus the amount muscle cells require to restore ATP & creatine phosphate to their original concentrations

Muscle Fatigue

A muscle exercised strenuously for a prolonged period may lose its ability to contract - fatigue

Can happen from an interruption in the muscle’s blood supply or lack of acetylcholine in motor nerve fibers (rare)

Most likely to arise from the accumulation of lactic acid in the muscle as a result of anaerobic respiration

Lactic acid buildup lowers pH levels which causes the muscle fibers to no longer respond to stimulation

Muscle Fatigue

Muscles can cramp which happens when the muscle undergoes a sustained involuntary contraction

Cramps are thought to occur when changes in the extracellular fluid surrounding the muscle fibers and their motor neurons trigger an uncontrolled stimulation

Muscular ResponsesThreshold stimulus – the minimal strength required to cause a contraction

All-or-none response – if muscles contract, it contracts completely; muscles do not contract partially

Twitch – when a muscle is exposed to a single stimulus that causes it to contract and relax; last a fraction of a second; produces a myogram (recording of a muscle contraction)

Latent period – the delay between the time the stimulus was applied and the time the muscle responded

Period of contraction – when the muscle pulls at its attachments

Period of relaxation – when muscle returns to its former length

Muscular Responses

Summation – increased force of contraction by a skeletal muscle fiber when twitches occur so rapidly that the next twitch occurs before the previous twitch relaxes

Tetanic contraction – continuous, forceful muscular contraction without relaxation

Recruitment – increase in the number of motor units activated as stimulation intensity increases

Muscular Responses

Sustained contraction – the combination of summation and recruitment; Ex) lifting weights or walking; response to a rapid series of stimuli transmitted from the brain and spinal cord on motor neurons

Muscle tone (tonus) – a response to nerve impulses that originate repeatedly from the spinal cord and stimulate a few muscle fibers

Important in maintaining posture

If muscle tone is suddenly lost the body collapses (when a person loses consciousness)

Smooth Muscle2 major types:

Multiunit smooth muscle – muscle fibers are separate rather than organized into sheets; found in irises of the eyes & in walls of blood vessels

Visceral smooth muscle – composed of sheets of spindle-shaped cells in close contact with one another; more common; found in walls of hollow organs (stomach, intestines, bladder,& uterus)

Contraction:Resembles skeletal muscle contraction in a # of ways

Both include reactions of actin & myosin

Both triggered by membrane impulses

Both use ATP for energy

Smooth Muscle

Contraction differences:Two neurotransmitters affect smooth muscle: acetylcholine & norepinephrine

Hormones affect smooth muscle stimulates contractions and alters the degree of response to neurotransmitters in some cases

Smooth muscle is slower to contract & relax than skeletal muscle

Can maintain a forceful contraction longer with a given amount of ATP

Fibers can change length without changing tautness

Cardiac Muscle

Only found in the heart

Main function is the pumping action of the heart

Well-developed transverse tubule system

Has intercalated disks that separate adjacent cells

Moves involuntarily and has striations

Contraction characteristics:Network of fibers contracts as a unit

Self-exciting

Rhythmic

General Overview

3 Major Muscle Functions:Heat production

Movement

Posture

Four Major Actions of Muscles:Flexor

Extensor

Abductor

Adductor

General Overview

Muscle Fibers Run 3 Ways:Rectus – parallel

Transverse – perpendicular (across)

Oblique - diagonal

There are 5 Shapes/Sizes of Muscles:Maximus

Minimus

Longus

Deltoid

Trapezius

Muscle DiseasesFibrosis – degenerative disease in which fibrous connective tissue replaces skeletal muscle tissue

Fibrositis – inflammation of fibrous connective tissues, especially in the muscle fascia (muscle rheumatism)

Muscular dystrophies – group of inherited disorders in which deficiency of cytoskeletal protein (glycoprotein) collapses muscle cells, leading to progressive loss of function

Myalgia – pain from any muscular disease or disorder

Torticollis – condition where the neck muscles contract involuntarily (wryneck)

Muscle DiseasesMyasthenia gravis – chronic disease in which muscles are weak and easily fatigued because of malfunctioning neuromuscular junctions

Myokymia – persistent quivering of a muscle

Myoma – tumor composed of muscle tissue

Myopathy – any muscular disease

Myositis – inflammation of skeletal muscle tissue

Myotonia – prolonged muscular spasm

Shin splints – soreness on the front of the leg due to straining the flexor digitorum longus, often results from walking up and down hills