BIOL 22000 Lecture Notes - Lecture 11: Shoulder Girdle, Epiphyseal Plate, Chemical Synapse
8 Jun 2018
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Chapter 11 The Musculoskeletal System
11.1 The Muscular System
TYPES OF MUSCLE
Skeletal Muscle
• Skeletal muscle is responsible for voluntary movement and is therefore innervated by the
somatic nervous system
• The arrangement of actin and myosin into repeating units called sarcomeres
• There are multiple different types of fibers within skeletal muscle
• Red fibers, also known as slow-twitch fibers, have high myoglobin content and
primarily derive their energy aerobically
o Myoglobin is an oxygen carrier that uses iron in a heme group to bind oxygen,
imparting a red color
o Red fibers contain many mitochondria to carry out oxidative phosphorylation
• White fibers, also known as fast-twitch fibers, contain much less myoglobin
o Because there is less myoglobin, and therefore less iron, the color is lighter
• Muscles that contract slowly, but that can sustain activity (such as the muscles that
support posture), contain a predominance of red fibers
• Muscles that contract rapidly, but fatigue quickly, contain most white fibers
Smooth Muscle
• Smooth muscle is responsible for involuntary action, so is controlled by the autonomic
nervous system
• Smooth muscle cells have a single nucleus located in the center of the cell
• Smooth muscle cells contain actin and myosin, but not organized
• Smooth muscle is capable of more sustained contractions, and a constant state of low-
level contraction is called tonus
• Smooth muscle can contract without nervous system input in what is known as myogenic
activity
o In this case, the muscle cells contract directly in response to stretch or other
stimuli
MCAT EXPERTISE
• Both smooth and cardiac muscle exhibit myogenic activity
• These muscle cells will respond to nervous input, but do not require external signals to
undergo contraction
Cardiac Muscle
• Cardiac muscle has characteristics of both smooth and skeletal muscle types
o Like smooth muscle, cardiac muscle is involuntary and innervated by the
autonomic nervous system
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o Like skeletal muscle, cardiac muscle appears striated
• Cardiac muscle cells are connected by intercalated discs, which contain many gap
junctions
• Cardiac muscle cells can define and maintain their own rhythm through myogenic
activity
o Starting at the sinoatrial (SA) node, depolarization spreads using conduction
pathways to the atrioventricular (AV) node
o From there, the depolarization spreads to the bundle of His and its branches, and
then to the Purkinje fibers
o The gap junctions allow for progressive depolarization to spread via ion flow
across the gap junctions between cells
• One of the ways epinephrine does this is by increasing intracellular calcium levels within
cardiac myocytes
• Cardiac contraction—like that of all types of muscle—relies on calcium
Skeletal Muscle
Cardiac Muscle
Smooth Muscle
Striated
Striated
Nonstriated
Voluntary
Involuntary
Involuntary
Somatic innervation
Autonomic innervation
Autonomic innervation
Many nuclei per cell
1-2 nuclei per cell
1 nucleus per cell
Ca2+ required for contraction
Ca2+ required for contraction
Ca2+ required for contraction
MICROSCOPIC STRUCTURE OF SKELETAL MUSCLE
The Sarcomere
• The sarcomere is the basic contractile unit of skeletal muscle
• Sarcomeres are made of thick and thin filaments
o The thick filaments are organized bundles of myosin
o The thin filaments are made of actin along with two other proteins: troponin and
tropomyosin
• Each sarcomere is divided into different lines,
zones, and bands
• Z-lines define the boundaries of each
sarcomere
• The M-line runs down the center of the
sarcomere, through the middle of the myosin filaments
• The I-band is the region containing exclusively thin filaments
• The H-zone contains only thick filaments
• The A-band contains the thick filaments in their entirety, including any overlap with thin
filaments
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• MNEMONIC
o Z—Z is the end of the alphabet, and the end of the sarcomere
o M—in the middle of the myosin filaments
o I—I is a thin letter (thin filaments only)
o H—H is a thick letter (thick filaments only)
o A—all of the thick filament, whether or not it is overlapping
Gross Structure of Myocytes
• Sarcomeres are attached end-to-end to form myofibrils
• Myofibrils are surrounded by a covered by sarcoplasmic reticulum (SR), a modified
endoplasmic reticulum that contains a high concentration of Ca2+ ions
• The sarcoplasm is a modified cytoplasm located just outside the sarcoplasmic reticulum
• The cell membrane of a myocyte is known as the sarcolemma
• The sarcolemma can propagate and distribute
action potential to all sarcomeres in a muscle
using a system of transverse tubules (T-
tubules)
• Each myocyte, or muscle cell, contains many
myofibrils arranged in parallel and can also be
called a muscle fiber
• The nuclei are usually found at the periphery of
the cell
• KEY CONCEPT
o The sarcoplasmic reticulum is just a
fancy name for the specialized
endoplasmic reticulum in muscle cells
o A myofibril is an arrangement of many
sarcomeres in series
o A muscle fiber (or myocyte, or muscle
cell) contains many myofibrils within it,
arranged in parallel
o A muscle is made up of parallel muscle fibers
MUSCLE CONTRACTION
Initiation
• Contraction starts at the neuromuscular junction, where the nervous system
communicates with muscles via motor (efferent) neurons
• This signal travels down the neuron until it reaches the nerve terminal (synaptic
bouton), where acetylcholine is released into the synapse
• Acetylcholine binds to receptors on the sarcolemma, causing depolarization
• Each nerve terminal controls a group of myocytes; together, the nerve terminal and its
myocytes constitute a motor unit
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Document Summary
Mcat expertise: both smooth and cardiac muscle exhibit myogenic activity, these muscle cells will respond to nervous input, but do not require external signals to undergo contraction. Cardiac muscle: cardiac muscle has characteristics of both smooth and skeletal muscle types, like smooth muscle, cardiac muscle is involuntary and innervated by the autonomic nervous system. Ca2+ required for contraction ca2+ required for contraction ca2+ required for contraction. Adp and pi from myosin that is responsible for the powerstroke, not the binding of atp. Stimulation, summation, and muscle fatigue: for muscle cells to respond, stimuli must reach a threshold value, all-or-nothing response, nerves control overall force by the number of motor units they recruit to respond. Simple twitch: the latent period is the time between reaching threshold and the onset of contraction. It is during this time that the action potential spreads along the muscle and allows for calcium to be released from the sarcoplasmic reticulum.
