Functional Human Physiology
for the Exercise and Sport Sciences
Muscle Physiology
Jennifer L. Doherty, MS, ATC
Department of Health, Physical Education, and
Recreation
Florida International University
Types of Muscle Tissue - Classified by
location, appearance, and by the type of nervous system control or
innervation.

Skeletal muscle
1) Located throughout the body connected to bones and joints
2) Striated in appearance
3) Under voluntary nervous control.

Smooth or visceral muscle
1) Located in the walls of organs
2) No striations
3) Under involuntary or unconscious nervous control.

Cardiac muscle
1) Located only in the heart
2) Striated in appearance
3) Under involuntary or unconscious nervous control.
Skeletal Muscle
 Most skeletal muscles are connected to at
least two bones
 Muscles attach directly to bone
 Or muscles attach indirectly to bone through tendons
 Muscles produce movement by producing tension
between its ends
 Skeletal Muscle Structure
 Cellular Level
 Molecular Level
Skeletal Muscle Structure – Cellular
Level
 A Skeletal muscle fiber is an individual
muscle cell
1) Muscle fibers are long and narrow in shape
 Sarcolemma
1) The plasma membrane of the muscle cell
2) Surrounds the sarcoplasm
 Many nuclei (multi-nucleated)
1) Located in the periphery of the muscle cell just beneath
the sarcolemma
Skeletal Muscle Structure – Cellular
Level
 Each muscle fiber contains various
organelles specifically designed to meet
the needs of the contractile skeletal muscle
fiber
 Abundant mitochondria
1) High demand for energy (ATP) required for muscle
contraction
 Myoglobin
1) Protein with a high affinity for oxygen
2) Transfers oxygen from the blood to the mitochondria of
the muscle cell
Skeletal Muscle Structure – Cellular
Level
 Each muscle fiber contains:
1) Myofibrils – a cylindrical bundle of contractile proteins,
which are called Myofilaments, within a muscle fiber
 Located in the sarcoplasm of the muscle cell
2) Myofilaments – the contractile protein filaments that
make up the Myofibrils
 Actin – thin filament
 Myosin – thick filament
Skeletal Muscle Structure – Cellular
Level

Sarcoplasmic reticulum (SR)
 Saclike membranous network of tubules
1) Elaborate form of smooth endoplasmic reticulum
 Surrounds each myofibril
 Contains terminal cisternae
1) Located where the SR ends, which is near the area where
actin and myosin overlap
 The SR tubules and terminal cisternae store high
concentrations of calcium, which is important in the
process of skeletal muscle contraction
Skeletal Muscle Structure – Cellular
Level

Transverse tubules (T-tubules)
 Closely associated with SR
 Connected to the sarcolemma
 Penetrate the sarcolemma into the interior of the
muscle cell (invaginations)
 Bring extracellular materials into close proximity of
the deeper parts of the muscle fiber
 SR and T-tubules Function
 Activate skeletal muscle contraction when the muscle
cell is stimulated by a nerve impulse
 Transmit nerve impulses from the sarcolemma to the
myofibirls
Skeletal Muscle Structure –
Molecular Level
 Sarcomere
 Smallest contractile unit of the muscle fiber
 Arrangement of Myofilaments
1) Alternating bands of light and dark areas
2) Due to the organization of the actin and myosin
 Striated appearance
Sarcomere Components
 Z-lines = borders of the sarcomere
 Perpendicular to long axis of the muscle fiber
 Anchor thin myofilaments (actin)
 M-lines
 Perpendicular to long axis of the muscle fiber
 Anchor thick myofilaments (myosin)
Sarcomere Components
 A-Bands
1) Dark area where actin and myosin overlap
2) Equal to the length of the thick myofilaments (myosin)
3) Contains the H-Zone
 Lighter area within the A-Band that contains only myosin
 The M-Line is located with the H-zone
 I-Bands
1) Light area composed of actin only
2) Contains the Z line, which is the boarder of the
sarcomere
 Actin is directly attached the Z-Line
 Appears as a darker line through the I-Band.
Skeletal Muscle Structure –
Molecular Level

Actin
 G-actin (globular actin) = the basic component of
each actin myofilament
1) Contains myosin binding sites
 The actin myofilament consists of two strands of Gactin molecules
1) The two strands of G-action molecules are twisted together
with two regulatory proteins:
 tropomyosin
 troponin
Skeletal Muscle Structure –
Molecular Level

Tropomyosin
 Rod-shaped protein that occupies the groove
between the twisted strand of actin molecules
 Blocks the myosin binding sites on the G-actin
molecules
 Troponin
 A complex of three globular proteins.
1) One is attached to the actin molecule
2) One is attached to tropomyosin
3) One contains a binding site for calcium
Skeletal Muscle Structure –
Molecular Level
 Myosin
 Crossbridges
1) Composed of a rod-like tail and two globular heads
 The tails form the central portion of the myosin
myofilament
 The two globular headsface outward and in opposite
directions
2) Interact with actin during contraction.
3) Contain binding sites for both actin and ATP
 The enzyme ATP-ase is located at the ATP binding site for
hydrolysis of ATP
Skeletal Muscle Structure –
Molecular Level
 Titin
 Connects myosin to the Z-lines in the sarcomere
 It is very elastic
1) Able to stretch up to 3 times its resting length
 Important molecule because it is responsible for
muscle flexibility
Skeletal Muscle Contraction
 The chemical components and reactions that
occur when a muscle is stimulated by a motor
nerve result in the sliding of the myofibrils past
one another.
 The sliding of each myofibril within a muscle
fiber cause the muscle fiber to shorten.
 When many muscle fibers shorten, the result is
contraction of the skeletal muscle.
Skeletal Muscle Contraction
 Role of Actin and Myosin
 These myofilaments are responsible for muscle
contractility
 Arrangement of actin and myosin
 Cross bridges are oriented around the myosin
myofilament in rows so that they may interact
with actin molecules
 The purpose of this complex structure is the
production of tension (pulling force) within the
muscle causing the muscle to shorten, thus
causing movement
Skeletal Muscle Contraction –
Force Generation
 Chemical or heat energy in the body is
converted to mechanical work or movement.
 A nerve impulse arrives at the neuromuscular
junction (NMJ) and stimulates the beginning of
the contraction process
 NMJ = synapse between a motor neuron and a
skeletal muscle cell
 Stimulation of the skeletal muscle cell triggers
the release of calcium ions from the terminal
cisternae of the sarcoplasmic reticulum
 Calcium catalyzes the contraction process
Skeletal Muscle Contraction –
Force Generation

Calcium ions bind to troponin causing a conformational
change


Troponin then pushes tropomyosin away thus exposing the
active site that it is covering on actin
Myosin crossbridges have a strong affinity for the
exposed active site on the actin molecule
 Myosin binds to the exposed active site

Myosin crossbridges pull on the actin myofilament
pulling it toward the center of the sarcomere
 This motion physically shortens the sarcomere, the myofibril,
and the muscle fiber.
Skeletal Muscle Contraction –
Force Generation

After the sarcomere is shortened, the calcium ions
are pumped back into the sarcoplasmic reticulum
 Calcium ions are stored until another nerve stimulus arrives
at the NMJ

Tropomyosin moves back to its original position of
covering the active site
 This causes the myosin crossbridges to release their hold on
the actin myofilament

The actin myofilaments slide back to their original
position
The Sliding-Filament Model

A muscle contracts because the myosin and actin
myofilaments slide past each other
Myosin cross bridges attach and pull, release, reattach and
pull, sliding the actin toward the center of the sarcomere



Results in shortening of the I-band and the H-zone
Neither actin nor myosin actually change length even
though the sarcomere is shortened in the contraction
process


The A-band remains the same length (length of myosin)
A single attachment of the cross bridge results in about a
1% shortening of the total muscle

Muscles normally shorten 35 to 50% of their total resting length
The Sliding-Filament Model

Each myosin cross bridge must attach and reattach many
times during a single contraction
 Called crossbridge cycling
 Power Stroke - Attachment of the myosin cross bridge to actin
requires energy

Breakdown of ATP into ADP and P provides the energy required
for pulling on the actin myofilament
 ATP-ase catalyzes the breakdown of ATP
 Rigor – low-energy, strong bond between myosin and actin
 ADP and P are released from the myosin head thus breaking
the bond between the myosin crossbridge and actin
 Now the muscle is in a state of relaxation
 Cocking - Upon completion of the pulling mechanism, another
ATP attaches to the myosin crossbridge
 Preparation for another crossbridge cycle
Excitation-Contraction Coupling
 Sequence of events that links the nerve
impulse and skeletal muscle contraction
 Motor Neurons – stimulates skeletal
muscles
 Excitatory effect
 When a skeletal muscle cell receives input
from a motor neuron, it depolarizes
 Depolarization causes the muscle cell to fire an
action potential
Excitation-Contraction Coupling
 Action Potentials
 Large changes in cell membrane potential
(charge)
 Inside of the cell becomes more positive relative
to the outside of the cell
 Function to transmit information over long
distances
Excitation-Contraction Coupling
 Neuromuscular Junction (NMJ)
 The synapse between the motor neuron and the
muscle cell
 Synaptic Cleft
 The extra-cellular space between the motor
neuron and the muscle cell
Excitation-Contraction Coupling
 The NMJ releases a neurotransmitter from the
motor neuron into the synaptic cleft
1) The neurotransmitter is acetylcholine (ACh)
2) This neurotransmitter is synthesized by the nerve cell and
stored in synaptic vesicles
3) When an nerve impulse reaches the NMJ, the synaptic
vesicles release acetylcholine into the synaptic cleft.
Excitation-Contraction Coupling
4) Acetylcholine rapidly diffuses across the synaptic cleft to
combine with receptors on muscle cell membrane
(sarcolemma)
 The muscle cell is also called the motor end plate membrane
5) ACh causes depolarization of the muscle cell membrane
 Generates an action potential
6) Acetylcholine bound to the receptor is rapidly decomposed by
acetylcholinesterase (enzyme) preventing continuous
stimulation of the muscle fiber.
Excitation-Contraction Coupling
 Stimulation of Contraction
 Action potential propogates along the
sarcolemma and down the T-tubules to reach
the sarcoplasmic reticulum
 Sarcoplasmic reticulum releases calcium
1) Calcium is actively pumped into and stored in the
SR leaving a small concentration of calcium ions
in the sarcoplasm
 The action potential causes the calcium ions
to be released from the SR into the
sarcoplasm
Excitation-Contraction Coupling
 When released from the SR, calcium travels
toward the myofilaments
 Calcium binds with troponin on the actin myofilament
causing a conformational change, which results in
moving tropomyosin off the active site
 Myosin heads are then able to bind to the G-actin on
the active sites
 This begins the contraction process of crossbridge
cycling
Excitation-Contraction Coupling

Crossbridge cycling continues as long as there is an
adequate supply of ATP and if there is stimulation from
a motor neuron
Crossbridge cycling stops if there is an inadequate
supply of ATP or if the motor neuron impulse stops





When the motor neuron impulse stops, calcium ions are rapidly
pumped back into the sarcoplasmic reticulum for storage
The calcium ion concentration in the sarcoplasm decreases
Tropomyosin returns to its original position blocking the myosin
binding site on actin
The muscle cell relaxes
Muscle Cell Metabolism
 How Muscle Cells Provide ATP to Drive the
Crossbridge Cycle…
 The sources of ATP:
1) Available ATP in the sarcoplasm
2) Creatine phosphate
3) Glucose
Muscle Cell Metabolism
 Available ATP
 There is a limited supply of readily available ATP
 A small amount of ATP is stored in the myosin
crossbridges immediately available when the muscle
begins to contract.
 Contraction uses up this source of ATP in about 6
seconds making it necessary to have other sources of
ATP available
Muscle Cell Metabolism
 Creatine Phosphate (CP)
 When the ATP stores in the myosin crossbridges are
exhausted, ADP and CP are used to regenerate ATP.
1) CP + ADP = ATP + Creatine.
 The energy available from stored ATP and from the
reaction of joining ADP with CP provides only about
20 seconds worth of energy
1) The muscles could contract only long enough to run a 100 m
dash on the energy from these sources
Muscle Cell Metabolism
 Glucose
 Cellular respiration of glucose is an energy
source utilized to generate ATP
 Muscle contractions that are longer than 15 - 20
seconds depend on cellular respiration of
glucose as a source of ATP
Muscle Cell Metabolism


Recall
Cells store glucose in the sarcoplasm in the form of glycogen
 The cell must break apart the glycogen molecules to release the
individual glucose molecules – this is called glycogenolysis
 The breakdown of glucose, called glycolysis, occurs in the
sarcoplasm of the muscle cell and does not require oxygen, it
is anaerobic
1) Glycolysis produces pyruvic acid, and a small amount of ATP.

The majority of the ATP used by muscles is formed by
aerobic processes in the mitochondria.

At low intensities, the muscle cell depends on aerobic glycolysis
during which oxidative phosphorylation becomes more
important
Muscle Cell Metabolism – Changes
with Exercise Intensity
 Anerobic Metabolism
 Oxygen is not readily available
 During intense exercise, when the supply of oxygen
cannot keep up with metabolic demand of the cells,
pyruvic acid produced during glycolysis is converted
to lactic acid.
1) Lactic acid accumulates in the muscle resulting in the burning
sensation during short duration, high intensity muscular
exercise such as lifting weights
2) Lactic acid is quickly removed from the muscle and taken to
the liver where it is converted to glucose
Muscle Cell Metabolism – Changes
with Exercise Intensity
 Aerobic Metabolism
 Oxygen is readily available
 During prolonged, low-intensity exercise, the
muscles are supplied with adequate oxygen by the
protein myoglobin
 Myoglobin
1) Similar to hemoglobin (oxygen binding protein in the blood)
2) Myoglobin has a high affinity for oxygen and binds to it
loosely inside muscle cells
 Myoglobin brings oxygen into the muscle cell and stores it
temporarily
 This provides a continuous supply of oxygen even when blood
flow to the muscle is reduced
Muscle Cell Metabolism – Changes
with Exercise Intensity
 When exercise stops, the body's need for oxygen
continues for a period of time
 The body responds to this need by continuing to
breathing heavily until all the sources of ATP have
been replenished
 Oxygen Debt
 The amount of oxygen necessary to restore the resting
metabolic state of the body
 A better, and more currently accepted, term to
describe the events following exercise is recovery
oxygen consumption
Muscle Cell Metabolism – Changes
with Exercise Intensity
 Recovery oxygen consumption
 Includes the oxygen needed to:
1) Restore muscles to their resting metabolic condition
2) Convert lactic acid to pyruvic acid in the liver
3) Replenish cellular stores of glycogen, creatine phosphate,
and ATP
4) Return resting body temperature to normal
5) Return the heart muscle and the muscles of respiration to
normal, which need repair from the minor tissue damage that
occurs due to exercise
 The amount of oxygen needed to meet recovery
oxygen consumption demands depends on an
individual's physical condition and the duration and
intensity of the exercise session.
Types of Skeletal Muscle Fibers


Not all muscle fibers are the same physiologically
Muscles vary depending on:

The predominant pathway utilized to synthesize ATP
1) Oxidative fibers - predominantly aerobic pathways
 Oxidative phosphorylation in the mitochondria
 Fatigue-resistant fibers
2) Glycolytic fibers – predominantly anaerobic pathways
 Glycolysis in the sarcoplasm
 Fatigable fibers

The amount of myoglobin
1) Red fibers - high amounts of myoglobin
2) White fibers - small amounts of myoglobin

Efficiency of ATPase
1) Fast twitch fibers - decompose ATP rapidly
2) Slow twitch fibers - decompose ATP slowly
Types of Skeletal Muscle Fibers
 Slow-twitch fatigue-resistant fibers




Slow oxidative fibers, or red muscle fibers.
Contain abundant myoglobin giving them their red color.
Slow acting ATPase enzymes
Abundant mitochondria
1) Depend upon aerobic pathways for production of ATP
 Endurance type muscles
1) Able to deliver strong, prolonged contractions.
2) Examples:
 Postural muscles - spinal extensors
 Anti-gravity muscles - calf muscle
Types of Skeletal Muscle Fibers

Fast-twitch fatigable fibers



Fast glycolytic fibers, or white muscle fibers.
Contain small amounts of myoglobin
Fast acting ATPase enzymes
1) Allows the muscle fiber to contract rapidly

Few mitochondria
1) Contract for limited periods of time because fatigue rapidly

Plenty of glycogen
1) Depends on anaerobic metabolism

Extensive sarcoplasmic reticulum
1) Rapidly releases and stores calcium ions contributing to rapid
contractions

Best suited for short duration, high intensity contractions
Types of Skeletal Muscle Fibers
 Intermediate Fibers
 Fast-twitch fatigue-resistant fibers
1) Fast glycolytic fibers
2) Pale muscle fibers
 Characteristics lie between the red and white
fibers
Types of Skeletal Muscle Fibers
 Most of the body's muscles contain a mixture of
fiber types.
 It is the motor nerve that innervates the muscle cell
that determines its type
 Therefore, all of the muscle cells in a single motor unit are
of the same type
 Motor Unit – a motor neuron and all of the muscle fibers
it innervates
 Examples:
 Running – the motor nerve stimulates the motor units
containing fast-twitch fibers.
 Posture – the motor nerve stimulates the motor units
containing slow-twitch fibers.
Types of Skeletal Muscle Fibers
 Slow twitch fibers are recruited first
 This is because they are found in small motor
units
 Fast twitch fibers are recruited last
 This is because they are found in large motor
units
Types of Skeletal Muscle Fibers
 People are genetically predisposed to have
relatively more of one fiber type than
another
 People who excel at marathon running have
higher percentages of slow twitch fatigue
resistant muscle fibers
 People who excel at sprinting have higher
percentages of fast twitch fatigable fibers
Other Muscle Types: Smooth Muscle
 In comparison to skeletal muscle fibers
 Smooth muscle fibers are shorter and thinner
 They have a single, centrally located nucleus
 Lack striations
1) Although smooth muscle fibers do contain actin and myosin,
the filaments are thin and randomly arranged so that it lacks
striations
 No T-tubules
 A poorly developed sarcoplasmic reticulum
Other Muscle Types: Smooth Muscle

Smooth muscle fibers contract in a similar manner to
skeletal muscles with a few important functional
similarities and differences.
 Similarities
1) Both contractile mechanisms depend on the action of actin
and myosin;
2) Both are triggered by membrane impulses and the release of
calcium ions; and
3) Both require ATP.
Other Muscle Types: Smooth Muscle
 Differences in smooth muscle include
1) Actin has no troponin, the protein that binds to myosin in
skeletal muscle. Rather smooth muscle has a calcium binding
protein called calmodulin. This protein activities the actin and
myosin crossbridge formation.
2) Most of the calcium required for contraction comes into the
cell by diffusion from the extracellular fluid.
3) Smooth muscle is more resistant to fatigue and produces a
slower, longer lasting contraction than skeletal muscle.
4) It is more energy efficient than skeletal muscle in that it can
maintain a more forceful contraction for a longer period of
time with the same amount of ATP.
Other Muscle Types: Smooth Muscle
 Autonomic nervous system control
 Unconscious control of smooth muscle contraction
 Nuerotransmitters
1) Acetylcholine (as in skeletal muscle)
2) Norepinephrine.
3) Neurotransmitters for smooth muscle can be either excitatory (cause
muscle contraction), or inhibitory (prevent muscle contraction)
depending on the receptor on the smooth muscle cell membrane.
Whereas, the neurotransmitter for skeletal muscle is always
excitatory.

Smooth muscle is also stimulated by certain hormones such as
oxytocin, which stimulates smooth muscle contraction in the
walls of the uterus during childbirth.
Other Muscle Types: Smooth Muscle
 Multiunit smooth muscle
 Fibers are not very well organized
1) Occur as separate fibers scattered throughout the
sarcoplasm rather than in sheets.
 Requires stimulation by a motor nerve impulse
from the autonomic nervous system.
 This type of smooth muscle is found in the irises
of the eyes, arrector pili muscles, blood vessels,
and large airways of the lungs
Other Muscle Types: Smooth Muscle

Single Unit Smooth Muscle




Also called Visceral Smooth Muscle because it is found in the
walls of the hollow visceral organs such as the stomach,
intestines, urinary bladder and uterus.
More common of the two types of smooth muscle.
The muscle fibers are organized into sheets of cells held in
close contact by gap junctions.
Organized into two layers:
1) Longitudinal layer
 Outer layer directed longitudinally along the length of the structure.
 Contraction of this layer causes the structure to dilate and shorten
2) Circular layer
 Inner layer arranged circularly around the structure.
 Contraction of this layer causes the structure to constrict and elongate.
Other Muscle Types: Smooth Muscle
 Intrinsic Control of Smooth Muscle Contraction
 Myogenic Response
1) Smooth muscle is stimulated to contract when it is stretched
2) Smooth muscle is able to distend, or stretch, without great
increases in tension or tightness
 Allows hollow organs to be filled
3) When the smooth muscle reaches is stretching capacity, it will
contract and force the contents out

Such as occurs in the intestines or urinary bladder.
Other Muscle Types: Cardiac Muscle


Found only in the heart
Composed of interconnecting, branching fibers that are
striated
Each cell has a single nucleus similar to skeletal
muscle
Contains actin and myosin similar to smooth muscle.
Abundant mitochondria



 Depends on aerobic metabolism
 It cannot sustain an oxygen debt and still function efficiently

No motor units

Not every cardiac muscle cell is innervated by a nerve in order
to stimulate contraction
Other Muscle Types: Cardiac Muscle

Extensive system of T-tubules
 Release large quantities of calcium ions

Well developed sarcoplasmic reticulum
 Terminal cisternae contain less calcium than in skeletal
muscle
 Strength of the cardiac muscle contraction depends largely on
the influx of calcium from the extracellular space in addition to
that released from the T-tubules and sarcoplasmic reticulum

Contains intercalated disks


Membrane junctions that hold adjacent cells together and
transmit the contraction force to each cell
Gap Juntions

Most important intercellular junction that allow interchange and
communication between the sarcoplasm of connected cardiac
muscle cells
Other Muscle Types: Cardiac Muscle
 Communication between Cardiac Muscle Cells
is important to allow the nerve impulse to
rapidly travel from cell to cell to stimulate
contraction
 Stimulation of part of the cardiac muscle cell results
in impulses sent across the entire area of the heart
muscle tissue
 All-or-none Response
 The entire heart muscle contracts as a unit, or in
syncytium
Other Muscle Types: Cardiac Muscle
 Two syncytium are in heart:
 The atrial syncytium and the ventricular syncytium
 They are almost completely separated from each
other by fibrous tissue
 The all-or-none response applies to the entire
syncytium
1) Either both atria contact, or both do not contract at all
2) Either both ventricles contact, or both do not contract at all
Other Muscle Types: Cardiac Muscle
 Cardiac Muscle Contraction

Plateau Phase
 The prolonged depolarization in cardiac muscle due to Calcium
influx from the extra-cellular fluid
 The prolonged plateau phase prevents tetany, or prolonged
contractions, that would interfere with the pumping ability of the
heart

Refractory Period

Due to the calcium influx in cardiac muscle, there is a prolonged
absolute refractory period of cardiac muscle lasting about 250
msec.
1) Much longer than skeletal muscle which lasts about 1-2 msec.

Repolarization
 Calcium is pumped back into sarcoplasmic reticulum and out of
cell to the extracellular space.
Other Muscle Types: Cardiac Muscle
 Cardiac muscle is self-exciting
 It is able to stimulate itself to contract
 Cardiac muscle is autorhythmic
 It contracts in a periodic manner
 Autorhythmicity causes the automatic
contraction and relaxation of the heart
 Known as the heartbeat.
Other Muscle Types: Cardiac Muscle

Autorhythmicity


Ability of cardiac muscle to repeatedly and rhythmically contract
without external stimulation
Due to the presence of Pacemaker Cells in the heart
 Specialized smooth muscle cells that depolarize
spontaneously at regular intervals causing excitation of the
muscle cells without nervous system stimulation
 The spontaneous impulses travel into the surrounding
muscle tissue through gap junctions that connect the cell
membranes of adjacent muscle fibers, thus allowing the
heart to contract as a coordinated unit