Muscular System:Histology and Physiology

Chapter 9

Muscular System Functions

Body movement Maintenance of posture Respiration Production of body heat Communication Constriction of organs and vessels Heart beat

Criteria for Naming Muscles Shape: romboideus, trapezius, biceps Location: pectoralis (chest) intercostal (ribs) Attachment: zygomaticus, sternocleidomastoid Size: maximus, minimus, brevis, longis Orientation of fibers: rectus (straight), oblique

(slanting) Relative position (lateral, medial, internal,

external) Function: adductor, flexor, extensor, pronator

Properties of Muscle

Contractility Ability of a muscle to shorten with force

Excitability Capacity of muscle to respond to a stimulus

Extensibility Muscle can be stretched to its normal

resting length and beyond to a limited degree

Elasticity Ability of muscle to recoil to original resting

length after stretched

Skeletal Muscle

Smooth Muscle

Cardiac Muscle

Features Skeletal Muscle Smooth Muscle Cardiac Muscle

Location Attached to bone walls of hollow organs, blood vessels, eyes, glands and skin

heart

Cell shape very long, cylindrical

Spindle shaped Cylindrical and branched

Nucleus Multiple, peripherally located

Single centrally located

single centrally located

Special features

Gap junctions join visceral smooth muscle

Intercalated disks join cells

Control voluntary and involuntary reflexes

Involuntary Involuntary

Spontaneous contraction

No Yes Yes

Function Body movement Food movement, urinary bladder, blood vessels, glands and duct

pumps blood

Skeletal Muscle Structure

Muscle fibers or cells Develop from

myoblasts Numbers remain

constant Hypertrophy –

increase in the size of each fiber.

Connective tissue Nerve and blood

vessels

Connective Tissue, Nerve, Blood Vessels

Connective tissue External lamina Endomysium Perimysium Fasciculus Epimysium

Fascia Binds adjacent muscles

or overlying skin. Nerve and blood

vessels Abundant

Parts of a Muscle

Structure of Actin and Myosin

Components of Sarcomeres

Sliding Filament Model

Actin myofilaments sliding over myosin to shorten sarcomeres Actin and myosin do not change length Shortening sarcomeres responsible for

skeletal muscle contraction During relaxation, sarcomeres

lengthen

Sarcomere Shortening

Physiology of Skeletal Muscle

Nervous system Controls muscle

contractions through action potentials

Resting membrane potentials

Membrane voltage difference across membranes (polarized)

• Inside cell more negative and more K+

• Outside cell more positive and more Na+

Must exist for action potential to occur

Ion Channels

Types Ligand-gated

• Example: neurotransmitters

Voltage-gated• Open and close in

response to small voltage changes across plasma membrane

Action Potentials Phases

Depolarization• Inside plasma

membrane becomes less negative

Repolarization• Return of resting

membrane potential All-or-none principle

Like camera flash system

Propagate Spread from one

location to another Frequency

Number of action potential produced per unit of time

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Action Potential Propagation

Neuromuscular Junction

Synapse or NMJ Presynaptic terminal Synaptic cleft Postsynaptic membrane or motor end-plate

Synaptic vesicles Acetylcholine: Neurotransmitter Acetylcholinesterase: A degrading enzyme in synaptic cleft

Function of Neuromuscular Junction

Excitation-Contraction Coupling

Mechanism by which an action potential causes muscle fiber contraction

Involves Sarcolemma Transverse or T

tubules Terminal cisternae Sarcoplasmic

reticulum Ca2+

Troponin

Action Potentials and Muscle Contraction

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Muscle Twitch

Muscle contraction in response to a stimulus that causes action potential in one or more muscle fibers

Phases Lag or latent Contraction Relaxation

Stimulus Strength and Muscle Contraction All-or-none law for

muscle fibers A motor unit contracts

with a consistent force in response to each action potential

• Sub-threshold stimulus• Threshold stimulus• Stronger than threshold

Motor units Single motor neuron and

all muscle fibers that it innervates

Graded for whole muscles Strength of contractions

range from weak to strong depending on stimulus strength

Multiple Motor Unit Summation

A whole muscle contracts with a small or large force depending on number of motor units stimulated to contract

Muscle performing delicate and precise movements have motor units with smaller numbers of fibers

Multiple-Wave Summation As frequency of action

potentials increase, frequency of contraction increases

Incomplete tetanus• Muscle fibers partially relax

between contraction Complete tetanus

• No relaxation between contractions

Multiple-wave summation Muscle tension increases

as contraction frequencies increase

Due to increased calcium concentration around myofibrils and more complete stretching of muscle elastic elements

Treppe Increase in the force of

contraction during the first few contractions of a rested muscle.

Occurs in muscle rested for prolonged period

Each subsequent contraction is stronger than previous until all equal after few stimuli

Due to Ca++ ion levels around myofibrils and increased temperature of muscle Enzymes for muscle

contraction respond more effectively at higher temperature.

Types of Muscle Contractions

Isometric: No change in length but tension increases Postural muscles of body

Isotonic: Change in length but tension constant Concentric: Overcomes opposing resistance

and muscle shortens Eccentric: Tension maintained but muscle

lengthens Muscle tone: Constant tension by

muscles for long periods of time

Muscle Length and Tension

Fatigue

Decreased capacity to work and reduced efficiency of performance Usually follows a period of activity

Types Psychological (in CNS)

• Depends on emotional state of individual• Perception that muscle is too tired (

• Home court advantage Muscular

• Results from ATP depletion in muscle Synaptic

• Occurs in NMJ due to lack of acetylcholine

Energy Sources ATP provides immediate energy for muscle

contractions from 3 sources Creatine phosphate

• During resting conditions stores energy to synthesize ATP• Exhausted quickly (10-15 sec.)

Anaerobic respiration• Occurs in absence of oxygen and results in breakdown of

glucose to yield ATP and lactic acid Aerobic respiration

• Requires oxygen and breaks down glucose to produce ATP, carbon dioxide and water

• More efficient than anaerobic

Oxygen Debt After anaerobic respiration, aerobic respiration is higher

than normal to replace creatine phosphate and convert lactic acid to glucose.

Slow and Fast Fibers Slow-twitch or high-oxidative

Contract more slowly, smaller in diameter, well developed blood supply, more mitochondria and high myoglobin content, more fatigue-resistant than fast-twitch

Fast-twitch or low-oxidative Respond rapidly to nervous stimulation, less blood

supply, fewer and smaller mitochondria, lower myoglobin content than slow-twitch, fatigue easily.

Two types:• Fast twitch fatigable fibers• Fast twitch fatigue resistant (highly trained muscle)

Distribution of fast-twitch and slow twitch Most muscles have both but varies for each muscle

Effects of Exercise Training muscle increases muscular size and strength

(Hypertrophy). Aerobic exercise can convert fast-twitch easily fatigued muscle

into fatigue-resistant fast-twitch muscle. • Change in myosin type, increase size and number of mitochondria

and increased blood supply Muscles that are not used Atrophy or decreases in muscle

size. Atrophy or hypertrophy are the result of changes in the size

of individual muscle cells not the number of muscle cells. Number of myofibrils and sacromeres changes. Blood vessels, mitochondria and connective tissues increase.

Trained athletes: Have the ability to recruit large numbers of motor units

simultaneously improving coordination. Have a greater capacity for nutrient uptake and ATP production

(increased metabolism) Have improved circulation and more efficient respiration.

Heat Production Heat is a biproduct of the chemical

reactions that occur in the body. As muscles are worked they produce

excess heat that must be disipated by other body systems (circulatory and integument)

When body temperature drops muscle shiver to generate more heat (up to 18 times that of resting muscle).

Smooth Muscle Characteristics

Spindle shaped Fewer actin and myosin

• Organized in loose bundles.

• Not striated. Dense bodies hold actin

filaments together and are attached to noncontractile intermediate filaments

Ca2+ required to initiate contractions

Sarcoplamic reticulum is not well developed.

Fig. 9.23

Smooth Muscle Contraction

Types of Smooth Muscle Visceral or Unitary Smooth Muscle

Found in digestive, urinary and reproductive tracts.

Contains gap junctions, contracts in waves and often has autorhythmicity.

Multiunit smooth muscle Found in iris, blood vessels, arrector pili. Fewer gap junctions, groups of cells act as

independent units, only contracts when stimulated by nerves or hormones.

Electrical Properties of Smooth Muscle

Functional Properties of Smooth Muscle

Some visceral muscle exhibits autorhythmic contractions

Tends to contract in response to sudden stretch but not to slow increase in length

Exhibits relatively constant tension: Smooth muscle tone

Amplitude of contraction remains constant although muscle length varies

Smooth Muscle Regulation

Innervated by autonomic nervous system

Neurotransmitter are acetylcholine and norepinephrine

Hormones important as epinephrine and oxytocin

Receptors present on plasma membrane which neurotransmitters or hormones bind determines response

Cardiac Muscle

Found only in heart Striated Each cell usually has one nucleus Has intercalated disks and gap junctions Autorhythmic cells Action potentials of longer duration and

longer refractory period Ca2+ regulates contraction

Types of Muscle Contraction Isometric

Increase in tension with no change in length during the contraction process (postural muscles)

Isotonic Tension produced by muscle remains constant while

length changes. Note - Both Isometric and Isotonic contractions are used in

most body movements

Concentric contractions Eccentric contractions

Effects of Aging on Skeletal Muscle

Reduced muscle mass Increased time for muscle to contract in

response to nervous stimuli Reduced stamina Increased recovery time Loss of muscle fibers Decreased density of capillaries in muscle