Download Chapter 49 and 50 Presentations-Sensory and Motor Mechanisms

“I like nonsense; it wakes up the brain cells.”
--Dr. Seuss
Muscle Cell Components
 The function of muscle cells
relies on microfilaments—the
actin components of the
cytoskeleton.
 Microfilament movement is
governed by chemical energy
transformations that result in
contraction.
Vertebrate Skeletal Muscle
 Skeletal muscle (also called
striated muscle) is attached to
bones and is responsible for
movement.
 It is organized in a hierarchial
scheme of smaller and smaller
units.
 Most skeletal muscles consist
of a bundle of long fibers
consisting of a single,
multinucleated cell.
Vertebrate Skeletal Muscle
 Muscle fibers contain
bundles of small subunits
called myofibrils.
 The myofibrils are
composed of both thin and
thick filaments.
 Thin filaments conatin two
strands of actin and two
strands of tropomyosin
coiled around one another.
 Thick filaments consist of
myosin molecules.
The Sarcomere
 The sarcomere is the
contractile unit of the muscle.
 Borders of each sarcomere line
up with adjacent myofibrils.
 Thin filaments are attached at
the Z lines and extend inward
toward the center of the
contractile unit. Thick
filaments, on the other hand,
are attached to the M lines in
the center of the sarcomere.
The Sarcomere
 In resting muscle fibers, thick and thin fibers only
partially overlap. At the edges of the sarcomere, you can
see only thin filaments, whereas at the center only thick
filaments overlap. Such an arrangement allows for
contraction to occur.
The Sliding-Filament Model
 This model explains muscle contraction and is based
upon the interaction of actin and myosin which
comprise the filaments.
The Sliding-Filament Model
 Neither the thick nor the thin model change length
during contraction.
 Instead, the filaments slide past one another along
their lengths increasing the amount of overlap.
The Sliding-Filament Model
 The myosin molecues are
comprised of a long tail and
a globular head that extend
out to the side of each thick
filament.
 The head is the main region
where the reactions that
power muscle contraction
occur.
The Sliding-Filament Model
 The head binds ATP and
hydrolyzes it into ADP + Pi
thus changing the myosin
head into a high energy
form capable of binding
actin.
The Sliding-Filament Model
 When actin and myosin
bind, a cross-bridge is
formed and the myosin acts
to pull the actin filament
toward the center of the
sarcomere.
The Sliding-Filament Model
 The myosin head ‘lets go’
when a new ATP
molecule binds to its
head.
 These reactions repeat
themselves very rapidly
resulting in contraction.
2+
Ca
and Regulatory Proteins
 The regulatory protein,
tropomyosin, and the
troponin complex
(additional regulatory
proteins) bind to actin
strands and are involved
in muscle contraction
and relaxation.
2+
Ca
and Regulatory Proteins
 At rest, tropomyosin covers
the myosin binding sites
along the actin filament—
this prevents actin-myosin
interactions.
 As Ca2+ accumulates in the
cytosol, it binds to the
troponin complex causing
proteins bound along the
actin strand to move and
expose myosin-binding sites.
2+
Ca
and Regulatory Proteins
 With myosin binding
sites now available on
the actin filaments,
myosin can bind.
 This binding is what
contraction is—the
sliding of actin and
myosin fibers past one
another.
2+
Ca
and Regulatory Proteins
 The increase in Ca2+
concentration results
in contraction.
 As the concentration of
Ca2+ falls, the muscles
relax as the binding
sites are covered again.
Motor Neurons and Muscle
Contaction
 Signals from motor
neurons stimulate the
release of Ca2+ ions into
the cytosol of muscle
cells.
 Ca2+ concentration
within the muscle cell
is a regulated, multistep
process involving
numerous cell
structures.
Motor Neurons and Muscle
Contaction
 The first step in muscle
contraction is the
arrival of an action
potential at the synaptic
terminal of a motor
neuron.
 An action potential
stimulates the release of
acetylcholine which
binds to receptors on
the muscle fiber.
Motor Neurons and Muscle
Contaction
 This leads to a depolarization within plasma membrane
of the muscle fiber and triggers an action potential
within the cell.
 The muscle cell contains transverse (T) tubules, and the
action potential spreads to the interior of the cell
through these structures.
Motor Neurons and Muscle
Contaction
 From the T-tubules, the
action potential spreads to
the sarcoplasmic reticulum
(SR), causing the release of
Ca2+
 The Ca2+ enters the cytosol,
binds to the troponin
complex causing it to move
and open myosin binding
sites stimulating contraction
of the muscle.
Motor Neurons and Muscle
Contaction
 Relaxation of the muscle
occurs when the neural input
stops.
 During this phase, transport
proteins within the SR pump
Ca2+ out of the cytosol
allowing regulatory proteins
to bind to the thin filament
and block the myosin binding
sites.
 Ca2+ accumulates in the SR
and the muscle cell is ready
for the next round of
contraction.
Motor Neurons and Muscle
Contaction
 Obviously during muscle contractions, there is some
control which you have over whole muscles.
 There are two basic ways in which graded muscle
contractions are controlled by the nervous system.
 1. By varying the number of muscle fibers that contract.
 2. By varying the rate at which the fibers are
stimultated.
Motor Neurons and Muscle
Contaction—Varying the Number
 In vertebrate skeletal
muscle, each muscle
fiber is controlled by one
motor neuron, however,
each motor neuron may
form a synapse with
many muscle fibers.
 The motor unit is the
single motor neuron and
all the muscle fibers it
controls.
Motor Neurons and Muscle
Contaction—Varying the Number
 When an action potential is produced by a motor
neuron, all the fibers within the motor unit contract as
a group.
 Motor units contain as many as a few muscle fibers or
as many as a few hundred.
 The strength of the contraction is governed by the
number of muscle fibers the motor neuron controls.
Motor Neurons and Muscle
Contaction—Varying the Number
 The regulation of the strength
of contraction is controlled
by the nervous system. At
any given instant, the nervous
system can select a large or
small numbe of motor
neurons to activate.
 The force of the contraction
developed by the muscle is
increased/decreased as the
number of motor neurons
controlling the motor unit
increases or decreases.
Motor Neurons and Muscle
Contaction—Recruitment
 The process of activating
an increased number of
motor neurons is called
recruitment.
 Depending on the
number of motor
neurons recruited by
your brain, you can lift
something light or
something heavy.
Motor Neurons and Muscle
Contaction—Varying the Rate
 By varying the rate at
which the nervous system
stimulates muscules to
contract, it can vary the
amount of tension
created by the muscle.
 For instance, if a second
action potential arrives at
the muscle fiber before it
has completely relaxed,
the two contractions will
add together and create
greater tension.
Motor Neurons and Muscle
Contaction—Varying the Rate
 This summation will
continue to increase if
the stimuli is frequent
enough so as to not
allow the muscle to
relax.
 When this is the case, a
fused sustained contract
called tetanus results.
Motor Neurons and Muscle
Contaction
 The increase in tension
generated by tetanus
occurs due to the fact
that muscles are
connected to bones.
 The tension due to the
contraction of the
muscle fibers is
transferred to the bones
likely resulting in
movement.
Skeletons
 There are a number of
different skeletons in the
animal world. Three
important ones are:
 1. Hydrostatic skeletons
consist of fluid held under
pressure within a body
compartment.
 Cnidarians, flatworms,
annelids.
http://arnica.csustan.edu/photos/animals/Jelly_Fish1.jpg
Skeletons
 2. Exoskeletons are comprised
of a hard encasement on the
animal’s surface.
 Clams, insects, crustaceans.

Chitin, calcium carbonate
http://blog.friendseat.com/clams-uses-recipes
Skeletons
 3. Endoskeletons
consists of hard
supporting elements like
bones, cartilage, bony
plates.
 These are most familiar
to us.
Joints
 Within our body lies
bones comprised of
calcium carbonate.
 The joints of our skeletons
allow us move and there
are three types.
 1. Ball and socket
joints—allows for
rotation around the joint.
Joints
 The joints of our skeletons allow us move and there are
three types.
 2. Hinge joints—restrict movement to a single plane.
Joints
 The joints of our skeletons
allow us move, and there
are three types.
 3. Pivot joints—allow for
rotation within a joint—
usually a hinge joint.
Think forearm.
Joint Movement
 The movement in your body is
due to the muscles attached to
the bones which articulate about
your joints.
 The muscle always contracts
toward the origin of the fiber.
 For instance, your hamstring
originates on the ischial
tuberosity of your pelvis, and
inserts on the fibula. Contaction
of this muscle puls the lower leg
up.
http://orthoinfo.aaos.org/figures/A00408F01R.jpg