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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

MCB 246: Human Anatomy and Physiology II Attachment site© fUonri vtehriscitky foifl aIlmlinoeins tBsoard of Trustees

 

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