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Muscle tissue
Introduction


Cells in muscle tissue are differentiated to optimize contractility



Most are mesodermal origin – undergo gradual differentiation by lengthening
and generation of myofibrillar proteins



Three types of muscle tissue: skeletal, cardiac, smooth



Some organelles in muscle cells named slightly differently to other cell types
o
o

Sarcoplasmic reticulum

o
Name

Sarcoplasm
Sarcolemma

Long, cylindrical,

Cross-

Contraction

striations
Skeletal

Cell morphology

type

Yes

Quick,

multinucleated

Control
Voluntary

forceful

cells

Cardiac Elongated,

Yes

branched,

Vigorous,

Involuntary

rhythmic

individual cells,
lie parallel to
each other

Smooth

Fusiform cells

No

Slow

Involuntary

Images taken from Mescher, Junqueira’s Basic Histology: Text and Atlas, Twelfth Edition
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medicalook
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html

Muscle fibers


Myofibrils: which are long filamentous bundles located in the sarcoplasm of
each muscle fibre; they run parallel to the long axis of the fibre
o

contain repetitive units called sarcomeres

o

Sarcomere: basic functional unit extending from Z line to Z line; contain
cross-striations of alternating light and dark bands, due to regular
arrangement of myofilaments


Thick and thin myofilaments



A bands: dark, contain thick myofibrils (mostly myosin)



I bands: light, contain thin myofibrils (mostly actin)



Z lines: dark transverse lines separating the I bands



H zone: lighter area in A band where thin and thick myofibrils
don’t overlap



M line: bisects the H zone, proteins within it make lateral
connections between adjacent thick filaments

Taken from Wikipedia
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wikipedia
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After membrane depolarisation,
Ca2+ is actively transported back into the sarcoplasmic reticulum



Contraction increases the amount of overlap between the thick and thin
filaments (no change in their length)

Taken from The A Level Biologist, The Sliding Filament Theory of Muscle Contraction, available
at http://www
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co
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Acetylcholine vesicles located here



Synaptic cleft: space between neuromuscular junction and the muscle
(contains basal lamina matrix)



Junctional folds: folding of sarcolemma at neuromuscular junction to increase
surface area



Membrane depolarisation
o

Action potential reaches end plate, acetylcholine released from
vesicles into synaptic cleft

o

Acetylcholine binds acetylcholine receptors on sarcolemma, Na2+
channels opened

o

Depolarisation of sarcolemma

o

Acetylcholine broken down by cholinesterase’s in synaptic cleft basal
lamina

o

T tubule system conducts the membrane depolarisation to the
sarcoplasmic reticulum



Motor unit: all the muscle fibers innervated by a single nerve fibre (can be
single or multiple)

Muscle spindles and tendon organs
 Muscle spindles: stretch detectors located within the muscle fascicles
o

connective tissue capsule enclosing a fluid-filled space that
contains a few thin, non-striated muscle fibers densely filled with
nuclei (intrafusal fibers)

o

Sensory nerves innervate the muscle spindle and wrap around the
intrafusal fibers; detect changes in length (stretch) and transmit this
information to the spinal cord



(Golgi) tendon organs: sensory receptor that penetrates the
myotendinous junction

Types of muscle fibre


Classified into three types based on physiological, biochemical and
histochemical characteristics; all three are found within most muscle types



Differentiation is dependent on innervation; fibers of a motor unit are the
same type



Classification is important for diagnosis of muscular diseases
Medical application

 Muscle structure
o There is variation in diameter of skeletal muscle fibers, which is dependent on
which muscle it is, age, sex, nutrition state, physical training
o Exercise increases the size and number of muscle fibers; new myofibrils are
formed (hyperplasia) and individual muscle fibers increase in size (hypertrophy)
 Myasthenia gravis
o Autoimmune disorder, progressive muscular weakness
o Antibodies are generated against the acetylcholine receptors in the
sarcolemma at the neuromuscular junction; reduced functionality
o Body attempts to correct the condition by internalisation and degradation of
affected receptors, and replace with new ones, but the process repeats,
leading to progressive disease

Name

Description

Features

Colour

Energy source

Contraction type

Type 1

Slow, red

Many

Dark red (due to

Aerobic oxidative

Slow, continuous over long

oxidative

mitochondria,

abundant

phosphorylation of fatty acids

periods (e
...
postural muscles in

abundant

myoglobin

myoglobin

binding O2)

Many

Intermediate

Oxidative metabolism and

pinky-red colour

anaerobic glycolysis (produces

Type 2a Fast,

intermediate, mitochondria,
oxidative-

abundant

glycolytic

back)
Rapid, short bursts (e
...
athletics)

lactate)

myoglobin,
considerable
glycogen

Type 2b Fast, white,
glycolytic

Fewer
mitochondria,
less myoglobin,
abundant
glycogen

Pale

Anaerobic glycolysis

Rapid contraction, fatigue easily
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Contraction


Structure and function of contractile proteins in cardiac muscle cells is the
same as in skeletal muscle cells



Organisational differences:
1
...
Cardiac sarcoplasmic reticulum associates with T-tubules to form
dyads not triads
3
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Taken from faculty
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edu, Histology of Muscle, available at
http://faculty
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edu/forsman/histologyofmuscleforweb
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Mescher
...
Twelfth Edition

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