Download Topic 4.1 Neuromuscular Function Student Exercise Book Answers

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Topic Four: The Neuromuscular Junction
Quick Review
Answers – B, F, C, A, G, D, E
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Investigating The Effects of Temperature on Muscle Function
Materials: Ice/ Pen or pencil
1. Write your signature 3 times under the column labelled “Normal”.
2. Obtain a handful of ice and hold it in your writing hand (over a sink!)
2. Write your signature 3 times under the column labelled “Cold”.
3. Place your hands under warm running water for a few minutes and massage your
hands.
4. Write your signature 3 times under the column labelled “Warm”.
Normal
Cold
Warm
Analysis
1. What effect did the changes in temperature have on your hand muscles?
Answer:
2. How could you explain this effect?
Answer:
3. Why do you think dancers wear leg warmers and baseball pitchers wear jackets before
pitching?
Answer:
To keep the leg muscles warm and to prevent cramping or other muscle injuries.
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You got Nerve...
The nervous system is divided into two parts: the central nervous system and the
peripheral nervous system. While the brain and the spinal cord constitute the central
nervous system, the so-called cranial and spinal nerves form parts of the peripheral
system. The peripheral nerves connect the central nervous system with the sense organs,
i.e. the organs for vision, hearing, smell, taste and perceptional touch, and other effector
organs like muscles and glands. Together, the CNS and PNS work as an electrochemical
communication system that:
o receives sensory signals from the external environment (sensory neurons)
o organizes and integrates information (interneurons)
o interprets information and initiates an appropriate response (motor neurons)
Your brain is made of approximately 100 billion specialized cells called neurons. Neurons
have the amazing ability to gather and transmit electrochemical signals -- they are
something like the gates and wires in a computer. Neurons share the same characteristics
and have the same organelles as other cells, but the electrochemical aspect lets them
transmit signals over long distances (up to several feet or a few meters) and pass
messages to each other.
The Human Nervous System
Central Nervous System (CNS)
Peripheral Nervous System (PNS)
Brain and Spinal Cord
Sensory and Motor Neurons
Somatic Nervous System
Autonomic Nervous System
motor neurons to skeletal muscle
nerves from internal receptors
sensory neurons from receptor sense organs
nerves to smooth muscle
_______________________________________________________________________________________
Remember the Ice Bucket Challenge….
Neuromuscular disorders affect the nerves that control your voluntary muscles.
Amyotrophic lateral sclerosis (ALS) is a nervous system disease that attacks nerve
cells called neurons in your brain and spinal cord. These neurons transmit messages
from your brain and spinal cord to your voluntary muscles. At first, this causes mild
muscle problems. Eventually, you lose your strength and cannot move. When muscles
in your chest fail, you cannot breathe. A breathing machine can help, but most people
with ALS die from respiratory failure.
There is no cure.
Source ~ National Institute of Health, 2014
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Topic Four: The Neuromuscular Junction
Skeletal muscles are told what to do by the nervous system. They only contract when
told to do so. A single motor unit consists of one motor neuron and all of the muscle
fibers it innervates. The neuromuscular junction is the term for the connection
between the nervous system and the muscle fiber.
How do we control and move our muscular system?
4.1.1
Label a diagram of a motor unit
Figure 1
Label:
dendrite, cell body, nucleus, axon, motor end plate, synapse & muscle.
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Motor unit
Answers – B, C, D, E, A
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Topic Four: The Neuromuscular Junction
Motor Unit Questions
Figure 3
A- A motor unit. B- A motor neurone
1. Name the different regions of a motor
unit. See Figure 3.
Answer:
 Cell body.
 Dendrites.
 Axon.
2. Figure 4 has been created from a slide of
skeletal tissue as seen with a light
microscope at a magnification of 800
times. It shows part of two motor units.
Use evidence from the drawing:
Figure 4 Two motor units in skeletal tissue
(a) What is a motor unit?
Answer:
A motor unit consists of a motor neurone
that connects. To a group of muscle fibres
by specialised structures called motor
end-plates.
(b)
Why all the muscle fibres shown will not necessarily contract at the
same time.
Answer:
Motor nerve fibres have different lengths, hence shorter one will contract earlier.
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Investigation 1 – The Reflex Arc
Task One
Figure 2 Mechanism of knee jerk reflex
Work in pairs. One of you sit crossed
legged in a relaxed position. Your partner
firmly taps your patella tendon using a
patella hammer or ruler (the patella
tendon is positioned just below the knee
cap). Describe and record the response of
the knee to the tap. How can you tell that
this response was a reflex action?
Notes:
 Refer students to the diagram of the simple reflex arc - the patella tendon has nerve
endings which send sensory information via afferent sensory nerve fibres which enter
the spinal cord via the posterior root ganglion directly to the motor neurone which
sends out an immediate motor response to the striped muscle tissue.
Answer :

Before striking the patella tendon the lower leg hangs free.

Once the patella tendon is struck contraction of the quadriceps tendon causes the foot
to be kicked forward.

The flexors of the knee are inhibited to allow this phasic contraction.

The involuntary reflex jerk is a very rapid motor response to a sensory stimulus and
therefore is not under the control of the voluntary cerebrum.
Task Two
Give an example of a skill you have learned which has become an automatic
response to a stimulus.
Answer:
 Catching a ball, reflex strike of a ball, blinking.
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Task Three
Research- Distinguish between an automatic (reflex response) and fast reactions.
Use examples from sporting situations to illustrate your answer.

Automatic reactions differ from fast reactions since they are predictable automatic
responses to a specific stimulus that have been thoroughly learnt and regulated to the
reflex arc of the spinal cord as described in Task One.

Whereas fast reactions involve the voluntary motor cortex.
There is an elapse of time between the stimulus and response often involving other factors
such as signal detection, channelling and interpretation of information in the cerebrum of the
brain
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Motor Unit Questions Part B
(c)
Briefly describe the sequence of events at the muscle end plate which
leads to an action potential passing along the muscle fibre.
Answer:
Notes :
Acetylcholine must be mentioned in this answer since it provides the chemical stimulus
that activates muscles.
Answer :
Nerve impulse arrives at motor end plate.




Ca++ triggers release of Acetylcholine from synaptic end bulb.
Acetylcholine diffuses across synaptic cleft to post synaptic receptors on muscle
sarcolemma.
This causes membrane to become depolarised.
Thus nerve action potential is followed by muscle action potential, hence impulse begins to
spread over muscle tissue.
3. Figure 5 shows the pathways of transmission of impulses through the central
nervous system during the knee jerk reflex.
Figure 5: Knee jerk reflex
(a)
Give the names of the five
structures lettered P-T.
Answer:
 P = muscle spindle receptor.
 Q = sensory/afferent neurone.
 R = synapse.
 S = motor/efferent neurone.
 T = effector muscle/motor end plate.
(b)








With reference to figure 5, briefly describe the sequence of events in a
knee jerk reflex.
Answer:
Hammer stimulates receptors in patella tendon.
Causing impulse to travel up sensory neurone to spinal cord.
Stimulating motor neurone (via a synapse).
Impulse travels down motor neurone to quadriceps femoris muscle.
Which contracts causing knee jerk reflex.
(c)
Describe briefly the structure and function of a synapse.
Answer:
A small gap (synaptic cleft) which separates two excitable cells.
For example between two neurones or between a neurone and skeletal muscle fibre.
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(d)



Topic Four: The Neuromuscular Junction
Explain why the release of a transmitter substance, such as
acetylcholine, into a neuromuscular junction does not stimulate the
muscle to go into prolonged contraction.
Answer:
Acetylcholine is only released for a the period of the action potential, then:
The enzyme cholinesterase breaks down acetylcholine.
Thus clearing the gap for the arrival of the next impulse.
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4.1.2
Topic Four: The Neuromuscular Junction
Explain the role of neurotransmitters in stimulating skeletal
muscle contraction.
1. Watch… Events at the Neuromuscular Junction:

http://www.as.wvu.edu/~sraylman/physiology/fig_7_6_WITH_LOADING_SCR.html

http://www.dnatube.com/video/5034/Contraction-of-muscle-function-ofneuromuscular-junction
Explain the following terms:

Action potential -An action potential occurs when a neuron sends information
down an axon, away from the cell body.
https://faculty.washington.edu/chudler/ap.html

Vesicle - Vesicles are miniscule membrane-enclosed sacs within the cell organelles of
eukaryotic cells. These sacs help transport or absorb proteins, enzymes and other cell
necessities.
http://www.wisegeek.com/in-cell-biology-what-are-vesicles.htm#didyouknowout

Neurotransmitter - A substance in the body that carries a signal from one nerve cell
to another. They transmit signals across a chemical synapse, such as in a
neuromuscular junction, from one neuron (nerve cell) to another "target" neuron,
muscle cell, or gland cell.
http://www.merriam-webster.com/dictionary/neurotransmitter
https://en.wikipedia.org/wiki/Neurotransmitter
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
Acetylcholine (Ach) - Acetylcholine triggers an action potential in the post synaptic
membrane by opening sodium channels. Action potentials stop as the acetylcholine is
broken down by the enzyme cholinesterase.

Cholinesterase – Cholinesterase is an enzyme, which breaks down acetylcholine.
This prevents the post synaptic membrane from being continually depolarized and
therefore continually generating an action potential.
Summarize the events that occur at the neuromuscular junction by sequencing the
following steps:
1. A motor neuron receives a stimulus and it gets ‘excited’.
2. The neuron initiates an action potential down its axon to the axon
terminal/motor end plate.
3. The action potential opens Ca+2 gates and calcium ions enter and interact with
vesicles in the terminal ends of the neuron.
4. Vesicles release neurotransmitters that travel to specialized neurotransmitter
receptors on the muscle cell.
5. Once the neurotransmitters bind to the specialized receptors, a muscle
contraction is initiated.
6. ACh is released from receptors and broken down by specialized enzymes in the
synaptic cleft.
See: https://quizlet.com/16523318/exam-3-anatomy-and-physiology-flashcards/
Identify the key components from the process that ends in a muscle contraction:
o The electrical trigger for a muscle contraction is called an: Electrical impulse
o The binding of ACh triggers the exchange of which two ions? Sodium and Calcium
o Ca+2 triggers the release of this neurotransmitter: Acetlycholine
o ACh binds to special receptors on which structure? Skeletal muscle fiber
o The enzyme that breaks down Ach is called:
Cholinesterase
o Tiny little sacs that store neurotransmitters are called: Vesicles
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Topic Four: The Neuromuscular Junction
Neatly label the following components in figure 6 below. Neatness counts!
o vesicles, neurotransmitters, muscle, neuron, specialized receptors
Figure 6
The role of neurotransmitters
1. In your own words review and explain the role of acetylcholine and
cholinesterase in the stimulation of skeletal muscle contraction.
Answer:
Acetylcholine triggers an action potential in the post synaptic membrane by opening
sodium channels. Action potentials stop as the acetylcholine is broken down by the enzyme
cholinesterase.
Cholinesterase is an enzyme which breaks down acetylcholine. This prevents the post
synaptic membrane from being continually depolarized and therefore continually generating
an action potential.
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Sarcomeres
Myofibrils are the contractile units of the muscle and are organized into sarcomeres.
It’s the sarcomere that contracts, pulling and shortening the entire muscle fiber.
Sarcomeres are defined at each end by a Z-band or Z-line. This is the boundary line
for each sarcomere; one unit is bound on both sides by a Z-line. The action filaments
are attached. Additional areas or distances along the filament can be identified. These
include the I-band, A-band and a middle H-band. Actin filaments attach to the Zbands. I-bands are regions that contain only actin. A-bands are regions that consist of
overlapping actin and myosin. The H-band is a region that contains only myosin.
The banding patterns arise from the organization within the sarcomere of two major
protein filaments: thick myosin filaments surrounded by thin actin filaments. It is the
interaction of these filaments that results in contraction of the sarcomeres.
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In the images of a sarcomere below, clearly label the following:
o The Z bands, the I band, the A band and the location of actin and myosin
1. When the sarcomere shortens, which band(s) will change length?
Answer:
The I band contains the portion of the thin filaments that does not overlap with thick
filaments. When a muscle fiber contracts, thin filaments are pulled over the thick
filaments, so there is more overlap of thin and thick filaments and a shorter I band.
2. When the sarcomere shortens, which band(s) will not change lengths?
Answer:
By definition, the A band is the length of the thick filaments; since the filament length
does not change, the A band length does not change.
https://quizlet.com/30007197/exercise-12-skeletal-muscle-structure-flash-cards/
3. Which protein filament is attached to the Z line?
Answer:
Actin
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Structure of a sarcomere
Match the terms:
B, C, A, D, F, G, H, E
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The contraction, or shortening, of a muscle powers movement.
The basic contractile unit of a muscle is the sarcomere.
Figure 7: Diagrammatic detail of muscle sarcomere
1. When the muscle contracts, do the actin and myosin filaments shorten?
Answer:
 Actin and myosin filaments do not change length during contraction of skeletal muscle.
During muscle contraction, the actin and myosin filaments slide past one another,
resulting in a shortening of the sarcomere
2. Explain how the sarcomere shortens when the parts that make it up don’t
shorten.
Answer:
In a relaxed sarcomere:
o The I bands, consisting of thin actin filaments, extend from the Z line to
the ends of the thick myosin filaments.
o The A bands extend the length of the thick myosin filaments within the
sarcomere.
o In relaxed muscle there is a small overlap of thin actin and thick myosin
filaments at both ends of the A bands.
o In the H zone, only thick myosin filaments are present.
o In a relaxed sarcomere, all the bands are visible.
In a contracted sarcomere:
 The I bands and H zone narrow and then disappear due to the thin actin filaments
being drawn further towards and between the thick myosin filaments.
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4.1.3
Topic Four: The Neuromuscular Junction
Explain how skeletal muscle contracts by the sliding
filament theory.
Let's take a look at what occurs within a skeletal muscle, from excitation to contraction to
relaxation:

An electrical signal (action potential) travels down a motor neuron, causing it to
release the chemical acetylcholine - ACh (neurotransmitter) into a small gap between
the nerve cell and muscle cell. This gap is called the synapse.

The neurotransmitter crosses the gap, binds to a protein (specialized receptor) on the
muscle-cell membrane and causes an action potential in the muscle cell.

The action potential rapidly spreads along the muscle cell and enters the cell through
tubules.

The action potential opens gates in the muscle's calcium store (sarcoplasmic
reticulum).

Calcium ions flow into the cytoplasm, where the actin and myosin filaments are.

Calcium ions bind to troponin molecules located in the grooves of the actin filaments.

Upon binding, troponin changes shape and slides a molecule, tropomyosin out of its
groove, exposing an actin-myosin binding sites. Normally, the rod-like tropomyosin
molecule covers the sites on actin where myosin can form cross-bridges.

The myosin heads then interact with actin by reaching up and pulling on the actin. The
muscle thereby creates force, and shortens. This is the contraction.

After the action potential has passed, the calcium gates close, and calcium pumps
located on the sarcoplasmic reticulum remove calcium from the cytoplasm.

As the calcium gets pumped back into the sarcoplasmic reticulum, calcium ions are
released from the troponin.

The troponin then returns to its normal shape, allowing the tropomyosin to once again
cover the actin-myosin binding sites on the actin filament.

Because no binding sites are available now, no crossbridges can form, and the muscle
relaxes back to its normal length.
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Topic Four: The Neuromuscular Junction
As you can see, a muscle contraction is regulated by the level of calcium ions in the
cytoplasm. In skeletal muscle, calcium ions work at the level of actin (actin-regulated
contraction). They move the troponin-tropomyosin complex off the binding sites, allowing
actin and myosin to interact.
All of this activity requires energy in the form of ATP. The energy released by breaking the
covalent bond holding the last phosphate group is used to reset the myosin cross-bridge
head and release the actin filament. To generate a supply of ATP, the muscle can
complete the following:
o Break down creatine phosphate, adding the phosphate to ADP to create ATP
o Carry out anaerobic respiration, by which glucose is broken down to lactic acid
and a small amount of ATP is formed
o Carry out aerobic respiration, by which glucose (and sometimes glycogen, fats
and amino acids) is broken down in the presence of oxygen to produce large
quantities of ATP.
o Review the following…

http://highered.mcgraw-hill.com/olc/dl/120104/bio_b.swf

https://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/animation__myofilament_contraction.html

https://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/animation__sarcomere_contraction.html

http://www.wellcome.ac.uk/Education-resources/Education-and-learning/Big-Picture/All-issues/Exercise-energy-andmovement/WTDV033020.htm
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Role of the various components involved in a Muscle Contraction
Myosin is a motor protein that uses the energy from ATP hydrolysis to move along actin filaments.
Myosin
A protein that converts chemical energy in the form of ATP to mechanical energy, thus generating
force and movement.
http://www.mechanobio.info/modules/go-0008570
Actin is a protein that functions in the contractile system of skeletal muscle. It allows cells to move
Actin
and function.
http://www.wisegeek.org/what-is-actin.htm
The myosin head is pushed back into its high-energy state using energy from the ATP that just
Cross
bridge
bound to the myosin (hydrolysis). Myosin can now attach to actin and form the attached state once
again. The cross-bridge will continue to cycle and cause contraction as long as the muscle is
stimulated.
study.com/academy/.../muscular-contraction-cross-bridge-formation.html
Troponin
Tropomyosin and troponin are two other proteins found in small quantities in muscle. They help
regulate muscle contraction. Troponin is associated with the thin filaments and can bind to the actin
molecules. There is usually one troponin per 6-8 actin molecules.
Muscle Contraction lecture notes- see Haiku
Tropomyosin
Tropomyosin is a long thin protein that extends between, and binds to, the troponin molecules.
When troponin is bound to actin, the tropomyosin is positioned so it prevents the myosin heads
from contacting actin, thus preventing contraction.
Muscle Contraction lecture notes- see Haiku
Sarcoplasmic
reticulum
The sarcoplasmic reticulum releases calcium ions during muscle contraction and absorbs them
during relaxation.
http://www.biology-online.org/dictionary/Sarcoplasmic_reticulum
https://www.khanacademy.org/science/biology/human-biology/muscles/v/role-of-the-sarcoplasmicreticulum-in-muscle-cells
choline
Acetylcholine normally triggers an action potential in the post synaptic membrane by opening
sodium channels. Action potentials stop as the acetylcholine is broken down by the enzyme
cholinesterase.
Calcium
The sarcoplasmic reticulum stores calcium and it is the regluation of calcium release that causes
muscular contraction. During rest most of the calcium resides in the sarcoplasmic reticulum and and
very few myosin cross bridges are attached to actin filaments - nor can they flex.
Acetyl-
http://www.sport-fitness-advisor.com/sliding-filament-theory.htm
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ATP is often referred to as the energy currency of life. ATP is a high-energy molecule found in
every cell. Its job is to store and supply the cell with needed energy. ATP is found in the cytoplasm
of the cell. ATP gets its energy by breaking down food. It is created in the mitochondria, the
powerhouse factory organelle, during the process of cellular respiration.
ATP
http://www.sports-training-adviser.com/what-is-atp.html
https://www.youtube.com/watch?v=bbtqF9q_pFw
http://www.biologyinmotion.com/atp/
http://study.com/academy/lesson/atp-definition-molecules-quiz.html
1. Describe the action between myosin and actin during contraction.
Answer:
Actin and myosin are the major proteins found in muscle. The interaction of these two
proteins, within each sarcomere causes the sarcomere to shorten.
During contraction, myosin heads in the thick filaments (which stick out towards the thin
filaments) bind actin in the thin filaments and pull the thin filaments in towards the center.
There are many sites at which myosin binds actin, running the entire length of the thin
filament/thick filament overlap.
This process is similar to pulling on a rope, with each pull (or stroke of the myosin heads),
the rope (or thin filament) is pulled inward more.
Note that during contraction, the ends of the
sarcomeres are pulled closer together, thus shortening the length of the muscle fiber.
Great Websites

http://www.wwnorton.com/college/biology/discoverbio3/full/content/ch27/animations.asp

http://www.getbodysmart.com/ap2/muscletissue/contraction/propagation/tutorial.html

http://faculty.massasoit.mass.edu/whanna/201/201_content/topicdir/muscle/muscle_media/muscle_VD/
page143/page143.html

http://www.wellcome.ac.uk/Education-resources/Education-and-learning/Big-Picture/All-issues/Exerciseenergy-and-movement/WTDV033020.htm
_______________________________________________________________________
What’s rigor mortis?
After death, calcium levels inside the muscle cells rise and the body's level of ATP
naturally drops. Inside the muscles, myosin binds to actin and the muscles contract.
However, with no ATP to reset the cross-bridges and release the myosin, all of
the muscles remain in contraction and are stiff; this state is called rigor mortis.
____________________________________________________________________
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Topic Four: The Neuromuscular Junction
1. In your own words, explain the major events that occur during muscle contraction to
your partner.
Check your understanding
Motor Unit
1. The single functioning unit for muscle contraction
Sarcomere
2. The structural and contractile unit of a skeletal muscle
Sarcoplasmic reticulum
3. The specialized organelle that stores calcium
Acetycholine
4. The neurotransmitter released at a muscular junction
I Band
5. The thin protein band found in a sarcomere
A Band
6. The thick protein band found in a sarcomere
Troponin
7. The protein filament that holds tropomysium in position
covering myosin binding sites on actin proteins.
Calcium Ions
8. The ion that reacts with troponin during muscle
contraction. These ions bind to troponin, changing the
shape of the troponin-tropomyosin complex and therefore
uncovering the myosin-binding sites on the protein actin.
Myosin
9. a protein with ‘heads’ that interacts to form cross bridges
Myofibrils
10. a long strand of repeating sarcomeres found in a
muscle fiber
https://www.boundless.com/biology/textbooks/boundless-biology-textbook/themusculoskeletal-system-38/muscle-contraction-and-locomotion-218/skeletal-musclefiber-structure-824-12067/
https://quizlet.com/1688264/ap-structure-of-a-muscle-fiber-flash-cards/
http://www.colorado.edu/Outreach/BSI/pdfs/muscleContraction.pdf
https://www.boundless.com/biology/textbooks/boundless-biology-textbook/themusculoskeletal-system-38/muscle-contraction-and-locomotion-218/regulatoryproteins-827-12070/
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Topic Four: The Neuromuscular Junction
Complete the flow chart below
A nerve impulse is sent from the
brain through motor neurons or
nerves to stimulate muscle
contraction
The nerve impulse travels down
the motor neuron, generating an
action potential which causes
calcium ions to be released from
the sarcoplasmic reticulum.
Ca+ ions diffuse into the sarcomere
and attach to troponin Which
changes shape.
As troponin changes shape it pulls
tropomyosin away from the
myosin binding sites on the actin –
which are now exposed!
When the nerve impulse stops,
the calcium gates close, Ca+ ions
are removed via the sarcoplasmic
reticulum and troponin returns
to normal shape
Tropomyosin covers the myosin
binding sites and the muscle
relaxes.
Myosin heads use ATP to pull
themselves along the actin
molecule, forming cross bridges
at each binding site before
breaking and Power stroking to
the next one.
The sarcomere shortens – Z lines
moves closer together – the muscle
is contracting
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Topic Four: The Neuromuscular Junction
What is the structure of our muscles?
4.1.4
Explain how slow and fast twitch fibre types differ in
structure and function

Type I Fibres – Slow Oxidative
These fibres, also called slow twitch or slow oxidative fibres, contain large amounts
of myoglobin, a high number of mitochondria and a well developed capillary system.
Type I fibres appear red (due to the high concentration of myoglobin and blood flow),
split ATP at a relatively slow rate, have a slow contraction velocity, are very resistant
to fatigue and have a high capacity to generate ATP by oxidative (aerobic)
metabolic processes. Relative to fast twitch fibres, they have a smaller muscle
diameter. These types of fibres are found in large numbers in the postural muscles of
the neck due to the necessity for endurance.

Type II A Fibres – Fast Oxidative
These fibres, also called fast twitch or fast oxidative fibres, are infrequently found in
humans. They contain high concentrations of myoglobin, mitochondria and a rich
blood supply. Type II A fibres are red, have a very high capacity for generating ATP by
oxidative metabolic processes, split ATP at a very rapid rate, have a fast contraction
velocity and are resistant to fatigue.

Type II B Fibres – Fast Glycolytic
These fibres, also called fast twitch or fast glycolytic fibres, contain a low concentration
of myoglobin, relatively few mitochondria, have a limited blood supply but large
amounts glycogen. Type II B fibres are white, geared to generate ATP by anaerobic
metabolic processes, not able to supply skeletal muscle fibres continuously with
sufficient ATP, fatigue easily, split ATP at a fast rate and have a fast contraction
velocity. Such fibres are found in large numbers in the muscles of the arms.
adapted from

see also
http://www.brianmac.co.uk/muscle.htm
http://courses.washington.edu/conj/bess/types/fibertypes.html
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Investigation- To consider some of the characteristics of fibre types
Training or genetics?
1. Discuss the role of genetics in determining the proportions of muscle fibre
types and the potential for success in selected activities.
Answer: Muscle fiber characteristics determined early in life; Alpha-motor neuron
determines the muscle fiber type by innervating that muscle fiber; Type I fibers are
more suited for prolonged endurance activities and Type II are better suited for highintensity,short,explosive activities; Intermediate fibers are have the ability to shift to
Type I/II based on training; Muscles lose type II motor units as we age.
https://quizlet.com/67860125/chapter-one-study-questions-flash-cards/
Find out…
1. What is myoglobin?
Answer:
Myoglobin is a protein in heart and skeletal muscles. When you exercise, your muscles
use up available oxygen. Myoglobin has oxygen attached to it, which provides extra
oxygen for the muscles to keep at a high level of activity for a longer period.
https://www.nlm.nih.gov/medlineplus/ency/article/003663.htm
2. What is glycogen?
Answer:
Most carbohydrates are broken down in the body to a type of sugar called glucose,
which is the main source of fuel for our cells. When the body has extra glucose, it
stores it in the liver and muscles. This stored form of glucose is called glycogen.
Glycogen is like your backup fuel. It releases glucose into the bloodstream when the
body needs a quick energy boost or when a person's blood glucose level drops.
http://kidshealth.org/kid/word/g/word_glycogen.html
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Topic Four: The Neuromuscular Junction
3. For each muscle fiber type, list three identifying characteristics:
Slow Twitch (Type 1)
Answer:
o Slow-twitch fibers have a very good blood supply -- so much so that they are
often referred to as red fibers.
o This plentiful supply of blood ensures that slow-twitch fibers receive a large
amount of oxygen, which allows them to work for a long time before becoming
fatigued.
o Slow-twitch muscle fibers rely on oxygen as their main energy source.
Fast Twitch (Type 2a)
Answer:
o Fast-twitch fibers have a relatively poor blood supply and are subsequently
referred to as being white in colour.
o The lack of blood results in relative oxygen restriction, so fast-twitch fibers tend
to fatigue much faster than the better oxygenated slow-twitch fibers.
o Fast-twitch muscle fibers rely on ATP and glycogen as their main energy source.
o As ATP sources can be rapidly depleted, lactic acid is a by product of the
breakdown of glycogen, fast-twitch fibers can only be active for a short period
of time.
o Type IIA- These fast twitch muscle fibers are also known as intermediate fasttwitch fibers. They can use both aerobic and anaerobic metabolism almost
equally to create energy. In this way, they are a combination of Type I and
Type II muscle fibers.
Fast Twitch (Type 2b)
Answer:
o Glycolytic fibers rely on glycolysis to fuel muscle contractions and consist of
fast-twitch (Type II) fibers, which are characterized by fast muscle contractions
of short duration. Fast-twitch fibers are constituents of white muscles and have
less myoglobin due to their primary reliance on glycolysis (anaerobic
respiration) to fuel muscle contractions. Although glycolysis is very quick, it is
also inefficient at producing ATP. Glycolysis produces lactic acid as a byproduct,
which leads to fatigue. The use of the glycogen cycle is the reason why fasttwitch muscles tire out quickly.

Type IIa and IIb are high in glycogen content depending on training status.
https://www.boundless.com/physiology/textbooks/boundless-anatomy-and-physiologytextbook/muscle-tissue-9/skeletal-muscle-96/types-of-skeletal-muscle-fibers-538-5126/
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Topic Four: The Neuromuscular Junction
Characteristics of Muscle Types
Fibre Type
Type I fibres
Type II A fibres
Type II B fibres
Contraction time
Slow
Fast
Very Fast
Size of motor neuron
Small
Large
Very Large
Resistance to fatigue
High
Intermediate
Low
Activity Used for
Aerobic
Long term anaerobic
Short term anaerobic
Force production
Low
High
Very High
Mitochondrial density
High
High
Low
Capillary density
High
Intermediate
Low
Oxidative capacity
High
High
Low
Glycolytic capacity
Low
High
High
Major storage fuel
Triglycerides
CP, Glycogen
CP, Glycogen
http://www.brianmac.co.uk/muscle.htm
4. Outline different types of activities that would rely more heavily on each muscle type:
o Type I (slow twitch oxidative)
Answer:
Type I fibers are used in lower-intensity exercises such as very light resistance
work aimed at muscular endurance and long-duration aerobic activities such as 5K
and 10K runs.
o Type II (fast twitch)
Answer:
Are recruited for very short-duration high-intensity bursts of power such as maximal
and near-maximal lifts and short sprints.
http://www.flashcardmachine.com/exercise-physiology19.html
http://athletics.wikia.com/wiki/Types_of_Muscle_Fiber
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Topic Four: The Neuromuscular Junction
Read and review the following:
The rate of fatigue
Fast glycolytic fibers fatigue rapidly, while slow oxidative fibers are highly resistant to
fatigue. Muscles that need to be active continuously, such as weight-supporting
postural muscles, contain a higher percentage of fatigue-resistant slow oxidative
muscle fibers.
Size
Slow oxidative muscle fibers have the smallest diameter, fast oxidative fibers are
intermediate in size, and fast glycolytic fibers are the largest. Consequently, the fast
glycolytic fibers produce the most force, since they contain the most myofibrils. Fast
glycolytic motor units also produce more force because they tend to have more muscle
fibers in each motor unit.
Another effect of size relates to recruitment. Recruitment refers to the process of
increasing activation of motor units to increase the force produced by a whole muscle.
The slow oxidative motor units are innervated by somatic efferent neurons with the
smallest cell bodies.
It turns out that the smallest neurons are the easiest to excite. So slow oxidative
motor units are recruited first, for low-intensity activity such as standing. As activity
intensifies and excitatory drive increases, larger and larger somatic efferent neurons
will be excited and so fast oxidative, and finally fast glycolytic motor units will be
recruited. This orderly recruitment of motor units according to size of somatic efferent
neurons is known as the size principle.
http://courses.washington.edu/conj/bess/types/fibertypes.html
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Topic Four: The Neuromuscular Junction
Summary Comparison of Muscle Fiber Types
Characteristics
Type II b (Fast Twitch
Glycolytic)
Type I (Slow Twitch
Oxidative)
Relative speed of
contraction
For strength or
endurance?
Fatigue resistance
(high or low?)
Fast
Slow
Strength
Endurance
Low
High
Main type of
respiration used
Anaerobic Glycolysis
Relative supply
of blood vessels
Relative concentration
of myoglobin
Relative concentration
of mitochondria
Advantageous for
these types of activity
…
Examples of this type …
Relies on oxygen from the
blood for the supply of
energy.
Low
High
Low
High
Low
High
Type IIB fibers are
characterized by high
force/power/speed
production and low
endurance
Recruited for very shortduration high-intensity
bursts of power such as
maximal and near-maximal
lifts and short sprints.
Type I fibers are
characterized by low
force/power/speed
production and high
endurance
5km/ 10km run

Think of the aerobic system as the big diesel bus with a massive fuel tank as
opposed to the V8 car of the ATP-PC system and the V6 car of the anaerobic
glycolytic system.

Anaerobic Glycolysis is the transformation of glucose to lactate when limited
amounts of oxygen (O2) are available.
http://www.ptdirect.com/training-design/anatomy-and-physiology/the-aerobic-system
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Topic Four: The Neuromuscular Junction
Refer to the readings and additional resources for help
Type II muscle fiber is also known as fast twitch muscle fiber. Muscle fiber types can
be broken down into two main types: slow twitch (Type I) muscle fibers and fast
twitch (Type II) muscle fibers. These fast twitch fibers can be further categorized into
Type IIa and Type IIb fibers, which are also known as "fast twitch oxidative" and "fast
twitch glycolytic," respectively.
Type I fibers are characterized by low force/power/speed production and high
endurance, Type IIB fibers are characterized by high force/power/speed production
and low endurance, while Type IIA fall in between the two.
It is possible that a fibre might be transformed from Type IIB to Type IIAB to Type IIA
with exercise training.
There are significant benefits to working to the point of temporary fatigue—and
therefore making sure fast-twitch fibers have been recruited. For instance, if you're
looking to increase muscle mass, and improve strength, using fast-twitch fibers is the
only way to do it.
On the other hand, aerobic exercises, those that mainly use slow-twitch fibers, can
increase stamina and the oxygen capacity of your muscles, allowing the body to burn
energy for longer periods of time. A high proportion of slow-twitch fibers has also been
associated with low blood pressure. Previous research has also shown that women
may have a greater distribution of type I muscle fibers and lower distribution of type II
muscle fibers than men.

http://athletics.wikia.com/wiki/Type_II_Muscle_Fiber

http://www.builtlean.com/2012/09/10/muscle-fiber-types/

http://greatist.com/fitness/what-are-fast-and-slow-twitch-muscles

http://ccahill.hubpages.com/hub/Fast-Twitch-vs-Slow-Twitch-Muscle-Fibres-Endurance-orStrength

http://www.ptdirect.com/training-design/anatomy-and-physiology/the-anaerobic- glycolytic-systemfast-glycolysis
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Topic Four: The Neuromuscular Junction
Investigation 1
Table 1 shows the percentage composition of slow twitch fibres found in the
leg muscles of male athletes specialising at different distances of athletic
event.
1. Briefly discuss the relationship between the percentage of slow twitch
fibres and race distance, as suggested by the data in Table 1
Table 1: Percentage composition of slow-twitch fibres in male leg muscles
Range of %
Mean % slow
slow twitch
twitch fibres
Event
fibres
Marathon runners
85
50-95
800 m runners
55
50-80
100/200 m
35
20-55
sprinters

Answer:
The longer the race distance, the greater the mean percentage of slow twitch fibres.


2. Which group of athletes is the most specialised in terms of slow twitch
muscle fibre composition? Explain your choice by reference to Table 1.
Answer:
800m.
As they have the smallest range of ST fibre composition.






3. Arrange the group of runners in rank order according to how closely their
slow twitch muscle fibre composition matches the 'ideal' for their distance.
Answer:
800m runners need a balance (50/50) between ST and FT.
At a mean ST fibres of 55% that are closest to the ideal.
Marathon runners need a high proportion of ST fibres as possible ideally their upper
limit is 95%.
At a mean of 85% they are close to this ideal, but not as close as the 800m runners,
whilst being closer to their ideal than the sprinters.
Sprinters need as high a proportion of FT fibres as possible, therefore their proportion
of ST should be as near the 20% low limit as possible.
At a mean of 35% they are further away then the other two.
1. List three features of slow twitch fibres that contribute to their greater
aerobic capacity.
Answer:
 Large number of mitochondria.
 High myoglobin content.
 High capillary density.
 High level of oxidative enzymes
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Topic Four: The Neuromuscular Junction
Investigation 2: To consider some of the characteristics of fibre types
1. The effects of specialised training can alter the metabolic functioning of
fast twitch type IIb fibres so that they take on some of the characteristics
of type 1 fibres and become type IIa fibres. Describe the ways in which
metabolic functioning of type IIa fibres will change as a result of specialist
aerobic training.
Answer:

It is thought that fast twitch type11a fibres adapt so that their metabolic functioning
becomes aerobic in nature: for example, myoglobin content, capillary and
mitochondrial density, and myosin ATPase activity all improve. This means that the
provision of energy is provided by the aerobic system as opposed to the anaerobic
system (refer to page 94 onwards of the main text 4th edition). The former results
from the complete breakdown of glucose to carbon dioxide and water.

Therefore, as a result of specialised training, it is possible to convert fast twitch
glycolytic fibres into fast twitch oxidative fibres, thereby improving aerobic capacity.
2. In which sporting activities would the adaptation of fast twitch (type IIb)
fibres to type IIa fibres be relevant to a sportsperson?
Answer:
 Obviously endurance based events such as middle distance running.
3. What types of training would cause the adaptation of fast twitch fibres to
type IIa fibres?
Answer:

Low intensity interval training such as 4 x 2000 metres run.

4. Using the information in Figure 8, describe the order in which fibre types
are recruited as the number of motor units increase.
Answer:
Different types of muscle fibres are recruited during physical activity with increasing
resistance.

When little force is needed in light work type 1 fibres are called upon first, before the
fast twitch type 11a FOG become active during medium work intensity.

As the load progressively gets harder type
11b (FG) fibres are used until all the
energy supply has been used up. At this
point the work must stop owing to
exhaustion.
Figure 8: Recruitment of fibre types
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Topic Four: The Neuromuscular Junction
Review: To consider some of the characteristics of fibre types
Another interesting study is the relationship between distribution of fibre type and
different sporting activities as illustrated in Table 9. Basically, the more explosive and
intense the demands of a sport, the more likely it is that successful sportspeople will
have a higher proportion of fast twitch muscle fibres in their muscles.
Table 9; Percentages of slow-twitch (type I) and fast-twitch (type II) fibres in males and
females (found in leg muscles) compared to sporting activity (based on references 1922).
Event
Distance runners
Cross-country
skiers
Cyclists
800 m runners
Javelin throwers
Shot putters
Sprinters
Untrained





% Type I
Males
Females
79
64
60
48
50
38
24
45
69
59
52
61
43
50
29
55
% Type II
Males
Females
21
36
40
52
50
62
76
55
31
41
48
39
57
50
71
45
1. Using the information from this table, comment on the distribution of
fibre type (for males and females) with respect to different sporting
activities.
Answer:
In general endurance athletes whether male or female have a predominance of slow
twitch fibres and the explosive athletes have a predominance of fast twitch fibres. This
has been found to be the case by a series of experiments with various sportspersons
in which a muscle biopsy (removal of small amounts of muscle tissue from relevant
muscles followed by microscopic analysis of the sample) has indicated the proportions
of slow twitch and fast twitch fibres.
2. Compare and account for differences in percentage fibre distribution with
respect to males and females.
Answer:
Males and females have similar percentage fibre distribution, but males have slightly
greater extremes.
For example, male distance runners with 79% type I and male sprinters with 76% type
II, as opposed to 69% and 71% for female counterparts.
In throwing events, the female table is less differentiating possibly due to a smaller
sample or lack of event specialisation or training.
Since fibre distribution is genetically determined, there could be inherited differences
between males and females.
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

Topic Four: The Neuromuscular Junction
3. Compare and account for differences in percentage fibre distribution
between trained and untrained performers.
Answer:
There is very little differentiating of type I and II fibres in the untrained male and female
categories.
Whereas the effects of aerobic and anaerobic exercise / training stimulates the
development and adaptation of slow twitch and fast twitch fibres respectively.
Exam Questions
Q.1
Explain the process of contraction once a muscle fibre has been stimulated by a
neurotransmitter. (8)
Impulse travels along sarcoplasmic reticulum;
Release of calcium;
Calcium attaches to tropomyosin;
Allows myosin to attach to actin / actmyosin complex / forming a cross bridge;
The heads of the myosin release ATPase;
ATPase splits ATP into ADP and Pi / releasing energy;
Cross bridge formation / swivel / ratchet mechanism / power stroke;
Individual sarcomeres shorten / fibres shorten / Z-lines come closer together;
H-zone shortens;
A-band stays the same / constant;
This is an energy requiring process;
Q.2
Explain the role of acetylcholine in muscle contraction. (2)
Acetylcholine is a chemical / transmitter between nerve ending and muscle / diffuses
across a synapse;
Initiates an action potential / contraction of the muscle fibre;
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Q.3
Topic Four: The Neuromuscular Junction
Explain how the function of type I muscle fibres differs from the function of type
IIb. (3)
Type I aerobic, type IIb anaerobic;
Type I slow twitch, type IIb fast twitch;
Type I slow contraction time, type IIb anaerobic fast contraction time;
Type I low power, type IIb high power;
Type I high fatigue resistance, type IIb low fatigue resistance;
Type I low glycolytic capacity, type IIb high glycolytic capacity;
Example of activity of both e.g. long distance running and sprinting