Physiology of Skeletal Muscle
The material contained in these slides corresponds to your assigned readings found in
Chapter 10 of our text.
MCB 246: Human Anatomy and Physiology II © University of Illinois Board of Trustees
Introduction to Skeletal Muscle
•Learning Objectives:
1. Be familiar and understand the five general functions of skeletal
muscle.
2. Know the five characteristics of skeletal muscle tissue.
MCB 246: Human Anatomy and Physiology II © University of Illinois Board of Trustees
Functions of Skeletal Muscle
• Movement (body) • Regulating elimination of
• Move bones, speak, breathe, materials
swallow
• Circular sphincters control
• Maintenance of posture passage of material at orifices
• Stabilize joints, allows us to (digestive system)
maintain body position • Heat production
• Protection and support • Help maintain body temperature
• Package internal organs and (e.g. shivering thermogenesis)
hold them in place
MCB 246: Human Anatomy and Physiology II © University of Illinois Board of Trustees
Characteristics of Skeletal Muscle Tissue
•Excitability: can respond stimuli •Elasticity: ability to return to
(neurotransmitters) by changing original length following a
electrical membrane potential (and lengthening or shortening
producing action potentials)
•Extensible: ability to be
•Conductivity: transmit/propagate stretched
action potentials along the
sarcolemma (similar to AP
propagation along an axon)
•Contractility: allows for muscle
fibers/cells (and whole muscles) to
shorten (exhibited when filaments
slide past each other)
MCB 246: Human Anatomy and Physiology II © University of Illinois Board of Trustees
Anatomy of Skeletal Muscle
•Learning Objectives:
1.Identify and describe the three CT layers associated with a muscle.
2.Describe the structure and function of a tendon and an aponeurosis.
3.Explain the function of blood vessels and nerves serving a muscle.
4.Explain how a skeletal muscle fiber becomes multinucleated.
5.Describe the sarcolemma, T-tubules, and sarcoplasmic reticulum of a skeletal
muscle fiber.
6.Distinguish between thick and thin filaments.
MCB 246: Human Anatomy and Physiology II © University of Illinois Board of Trustees
Anatomy of Skeletal Muscle con’t
•Learning Objectives:
7.Understand the structural organization of myofibrils, myofilaments, and
sarcomeres.
8.List and describe the structures associated with energy production within skeletal
muscle fibers.
9.Define and know the components of a motor unit. Describe its distribution in a
muscle, why it varies in size and how that affects muscle tension.
10.Be familiar with the three components of a neuromuscular junction.
11.Describe a skeletal muscle fiber at rest.
MCB 246: Human Anatomy and Physiology II © University of Illinois Board of Trustees
Gross Anatomy of Skeletal Muscle
• What is the hierarchy of structures
in a muscle?
• A whole muscle contains many
fascicles
• A fascicle consists of many
muscle fibers
• A muscle fiber is a muscle
cell
• In addition to the muscle cells, a
skeletal muscle contains nerves,
blood vessels, and connective
tissue
Figure 10.1
MCB 246: Human Anatomy and Physiology II © University of Illinois Board of Trustees
Copyright © McGraw-Hill Education. Permission required for reproduction or display.
Gross Anatomy of Skeletal Muscle
Tendon: cordlike structure of Deep fascia
dense regular connective Dense irregular connective tissue
tissue external to epimysium
Tendon
(attaches muscle to bone); Separates different muscles while binding
aponeurosis attaches muscle them together; contains nerves, blood
to muscle vessels, and lymph vessels
Whole Skeletal
muscle
Epimysium – dense irregular
Epimysium CT (covers entire muscle)
Artery
Vein
Perimysium Perimysium – dense irregular CT
Nerve
(covers fascicles); contains nerves
and blood vessels (arteries &
Fascicles veins)
Endomysium Endomysium – areolar CT
(covers individual muscle
fibers); provides capillary
Muscle fibers
support to muscle fiber cells
Microscopic Anatomy of Skeletal Muscle
Development of Skeletal Muscle
Skeletal muscles are unique in
that they are one of the few types
of cells in our body which is
multinucleated
Single muscle fibers are formed
from the fusion of embryonic
myoblasts cells. Each myoblast
retains its nucleus during fusion
leading to mature muscle fibers
with multiple nuclei.
Figure 10.2
MCB 246: Human Anatomy and Physiology II © University of Illinois Board of Trustees
Microscopic Anatomy of Skeletal Muscle
Development of Skeletal Muscle
When muscle cells are injured,
unfused embryonic cells ‘satellite’
(myosatellite) cells will fuse and
attempt to repair damaged muscle
fiber cells.
Figure 10.2
MCB 246: Human Anatomy and Physiology II © University of Illinois Board of Trustees
Copyright © McGraw-Hill Education. Permission required for reproduction or display.
Microscopic Anatomy of Skeletal Muscle
Skeletal muscle
Fascicle Sarcolemma (plasma membrane)
Muscle fiber Has T-tubules (transverse tubules) that
Sarcoplasmic reticulum extend deep into the cell; sarcolemma and its T- Figure 10.3c
tubules contain voltage-gated ion channels (see
Sarcolemma Fig 10.3c inset) that allow for conduction of
(plasma memberlaencet)rical signals
Nucleus
Myofibrils (bundle of myofilaments)
Openings into Nucleus
T-tubules
Mitochondrion
Sarcoplasm
Sarcoplasm (cytoplasm)
Has typical organelles (e.g.
mitochondria) plus contractile proteins
From Figure
10.3
Copyright © McGraw-Hill Education. Permission required for reproduction or display.
Microscopic Anatomy of Skeletal Muscle
Sarcoplasmic Myofibrils (hundreds to
reticulum thousands per cell)
Bundles of myofilaments
Sarcolemma
(plasma membrane) (contractile proteins) enclosed in
sarcoplasmic reticulum; comprise
most of the cell’s volume
Myofibrils (bundle of myofilaments)
(a) Skeletal muscle fiber
Mitochondrion
Sarcoplasmic
Triad Sarcoplasm
reticulum
From Figure T-tubule Term (stores Ca2+
inal )
Myofilaments
cisternae Sarcomere
10.3 (protein filaments)
(b) Myofibril
Sarcoplasmic reticulum (SR)
Internal membrane complex similar to smooth endoplasmic
reticulum; contains
Terminal cisternae: blind sacs of sarcoplasmic reticulum
Stores calcium ions until muscle fiber cells is stimulated;
arranged in groups of two which border a T-tubule to form a
Triad
SR also contains channels which allow for calcium diffusion
when a muscle fiber is stimulated and calcium pumps (SR
Ca2+ ATPase) which actively transport calcium from the
sarcoplasm to the SR.
Microscopic Anatomy of Skeletal Muscle
• Myofibrils contain thick and
thin filaments
• Thick filaments
(myosin – contractile protein)
• Consist of bundles of many myosin
protein molecules
– Each myosin molecule has two heads
and two intertwined tails
– Heads have binding site for actin of thin
filaments and ATPase site
– Heads point toward ends of the filament
• Thin filaments
(actin – contractile protein)
• Consist fibrous actin (F-actin)
• Each strand (of F-actin composed of actin globules (G-actin) Figure 10.4
• Each G-actin has a myosin binding site to which myosin heads attach during contraction
MCB 246: Human Anatomy and Physiology II © University of Illinois Board of Trustees
Microscopic Anatomy of Skeletal Muscle
From Figure 10.4
• Myofibrils also contain regulatory proteins
• Troponin and Tropomyosin
(regulatory proteins)
– Tropomyosin: twisted stringlike protein covering actin in a noncontracting
muscle
– Troponin: globular protein attached to tropomyosin
– When Ca2+ binds to troponin it pulls tropomyosin off actin allowing
contraction
MCB 246: Human Anatomy and Physiology II © University of Illinois Board of Trustees
Microscopic Anatomy of Skeletal Muscle
• Organization of a sarcomere
• Myofilaments arranged in repeating units, sarcomeres ‘functional
units’
• Composed of overlapping thick and thin filaments
• Separated at both ends by Z discs which anchor thin filaments
• Specialized proteins perpendicular to myofilaments
• Anchors for thin filaments
• The positions of thin and thick filaments give rise to alternating I-
bands and A-bands
Figure 10.5a
MCB 246: Human Anatomy and Physiology II © University of Illinois Board of Trustees
Microscopic Anatomy of Skeletal Muscle
Figure 10.5 b
I bands A band
Light-appearing regions that Dark-appearing region that contains thick filaments and
contain only thin filaments overlapping thin filaments
Bisected by Z disc Contains H zone and M line
Get smaller when muscle Makes up central region of sarcomere
contracts (can disappear with • H zone: central portion of A band
maximal contraction)
Only thick filaments present; no thin filament
overlap
Disappears with maximal muscle contraction
• M line: middle of H zone
Protein meshwork structure
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Microscopic Anatomy of Skeletal Muscle
The interactions of the contractile
overlap in a hexagonal pattern.
Depending on the location one views
the sarcomere, the presence of
contractile and regulatory proteins will
vary.
Figure 10.5 b
Figure 10.5 c
MCB 246: Human Anatomy and Physiology II © University of Illinois Board of Trustees
Microscopic Anatomy of Skeletal Muscle
• Other structural and functional
proteins Sarcomere
Z disc Thick filament Z disc
• Connectin (Titin) Connectin Thin filament Thin filament
– Stabilizes thick filaments and
has “springlike” properties
(passive tension)
• Dystrophin
– Anchors some myofibrils to
sarcolemma proteins
I band A band I band
– Abnormalities of this protein (b)
cause muscular dystrophy
MCB 246: Human Anatomy and Physiology II © University of Illinois Board of Trustees
Microscopic Anatomy of Skeletal Muscle
• Mitochondria and other structures
associated with energy
production
• Muscle fibers have abundant
mitochondria for aerobic ATP
production
• Myoglobin within cells allows
storage of oxygen used for
aerobic ATP production
• Glycogen is stored for when
fuel is needed quickly
• Creatinine phosphate can
quickly give up its phosphate
group to help replenish ATP
supply
MCB 246: Human Anatomy and Physiology II © University of Illinois Board of Trustees
Innervation of Skeletal Muscle Fibers
•Motor unit: a motor neuron and all the muscle fibers it
controls
Figure 10.6a
MCB 246: Human Anatomy and Physiology II © University of Illinois Board of Trustees
Innervation of Skeletal Muscle Fibers
•Motor unit: a motor neuron and all the muscle fibers it
controls
• Motor unit
• Axons of motor neurons from spinal
cord (or brain) innervate numerous
muscle fibers
• The number of fibers a neuron
innervates varies
• Small motor units have less than five
muscle fibers (allows for precise
control)
• Large motor units have thousands of
muscle fibers (allows for large forces
but not precise control)
• Fibers of a motor unit are dispersed
throughout the muscle (not just in
one clustered compartment)
Figure 10.6a
MCB 246: Human Anatomy and Physiology II © University of Illinois Board of Trustees
Innervation of Skeletal Muscle Fibers
•Neuromuscular junction
• Location where motor neuron innervates muscle
• Usually mid-region of muscle fiber
• Has synaptic knob, synaptic cleft, motor end plate
Figure 2.7a
MCB 246: Human Anatomy and Physiology II © University of Illinois Board of Trustees
Innervation of Skeletal Muscle Fibers
Synaptic knob
Expanded tip of the motor neuron axon that
contains:
• synaptic vesicles containing acetylcholine
(ACh)
• Ca2+ pumps in plasma membrane
(establishes Ca2+gradient)
• voltage-gated Ca2+ channels in membrane
Synaptic cleft
Narrow fluid-filled space
Separates synaptic knob from motor end plate
Acetylcholinesterase resides here
Enzyme that breaks down ACh molecules
Motor end plate
Specialized region of sarcolemma with
numerous folds containing ACh
receptors
Figure 2.7b
MCB 246: Human Anatomy and Physiology II © University of Illinois Board of Trustees
Skeletal Muscle Fibers at Rest
• Muscle fibers exhibit resting membrane potential (RMP)
• Fluid inside cell is negative compared to fluid outside cell
• RMP of muscle cell is about –90 mV
• RMP set by leak channels and Na+/K+ pumps (not shown). Also
present are voltage-gated channels are present (see inset) which
play a role in action potential propagation.
Figure 10.8
MCB 246: Human Anatomy and Physiology II © University of Illinois Board of Trustees