11.2 – Muscles and Movement

11.2 – Muscles and Movement


Hey everybody and welcome to topic 11.2, Muscles
and Movements. This is a very interesting topic for athletes, sports enthusiasts and
in general people who enjoy exercising. Like with many of the other topics in Human Health
and Physiology, if you pay close attention you can almost visualize, in your own body,
all of the concepts we’re going to discuss here. Our nervous, skeletal and transport systems
all work in combination with our muscles to promote movement. You can think of movement
as an emergent property – one that only arises from the combined effort of different
tissues. In order for us to understand the specifics
of muscular contraction, we’ll look at some definitions and then zoom into an elbow joint
to see what it looks like. – Muscles are tissues that specialize in contraction.
They are made of fibers that can bring their edges close together, resulting in a muscular
contraction. Ligaments connect a bone to another bone.
An example is the anterior cruciate ligament (or ACL), one of the four major ligaments
in the knee, which famously produces some of the nastiest injuries in sports and in
the past has forced players to end their careers. Nowadays, ACL reconstruction can even use
stem cells to aid regeneration of the ligament and generally speaking, players are able to
return to play after a 6-month recovery period. Nerves will stimulate the contraction of muscle
cells by the transmission of electrochemical impulses via a motor neuron, as you’ve seen
in our study of the nervous system. Bones are a mixture of living tissue and calcified
material used as leverage and anchorage for the muscles. They’re the structures the
muscular system attaches to, and as such, are often studied together as one system called
the human musculoskeletal system. The tendons are similar to ligaments in the
sense that they are made of connective tissue (as opposed to contractile, muscular tissue).
However, they act as springs by complementing the action of the muscles. A great example
is the Achilles tendon, which attaches the heel bone to the calf muscle. The name comes
from a very cool story from Greek mythology that I won’t get into.. I’ll just have
you consider this: if Achilles in fact existed, and if he in fact were shot by an arrow in
his Achilles tendon, which movement would he not be able to accomplish? – Joints are another important component of
our musculoskeletal system. Their primary purpose is to increase the variety in the
direction of movement and allow flexibility to the body as a whole. In addition, it also
reduces friction between bones during movement. In order to reduce friction, joints consist
of a cartilage (a tissue much softer than bone!), synovial fluid (a water-based lubricant)
and a joint capsule, which keeps the synovial fluid. Two important types of joints in our body
are the hinge joint (such as the knees and elbows) and the ball-and-socket joints (such
as the hip and shoulder). Compare these two types of joints: which one is multidimensional
and which is one-dimensional? Also, compare the forces that the muscles next to each joint
are able to exert. – This is a diagram of the elbow joint, which
you are expected to know how to label, including the names of the bones and muscles. Using
the information in the previous slide, you should be able to annotate a diagram like
with the function of each of the structures. Note the antagonistic (opposing) functions
of the biceps and triceps and the supporting bones (radius, ulna and humerus). – This picture is on page 1106 on Campbell.
A single muscle fiber is a cell that has differentiated to promote contraction. These cells produce
large amounts of proteins called myosin and actin, the structures ultimately responsible
for contraction. Within each cell, you’ll find repeating units of proteins called sarcomeres,
which altogether form a myofibril (comes from latin for ‘muscle fibre’). Many fibres
group to form a bundle of fibres, and many bundles form the muscle itself. Many parts
of our body actually contain muscle groups, composed of many individual muscles, such
as the pectoral and abdominal muscles. When someone works out at the gym, the tissue damage
promotes cell division in these muscle fibres, leading to an increase in muscular mass. The
use of steroids actually makes these cells bigger by increase water uptake, and not increasing
the number of muscle fibres. This is Egyptian bodybuilder Moustafa Ismail, who has the Guinness
Book of Records people conducting independent research to find out if he has indeed the
world’s largest biceps without the use of artificial methods, at almost 78cm in diameter.
Consider this, though: what would be the consequences for people who abuse steroids in order to
achieve a certain look? – In these pictures you can see muscle fibres
in an electron micrograph. The term ‘striated’ refers to the lines that form across the fibre
and here’s why they’re there. – Take a look at this diagram, also on page
1106 on Campbell. You can see the actin and myosin filaments alternating within the sarcomere,
the basic unit of the muscle fibre. The Z-line is the boundary of each individual sarcomere.
The myosin filaments contain a region called the ‘head’, which attaches to the actin
filament and slides it, causing the contraction. When contraction happens, myosin and actin
filaments are going to slide against each other, bring the two ends of the sarcomere
closer together. It’s important to note that neither filament gets smaller! They simply
slide to reduce the space between them. That’s why you can see a much smaller light area
around the M-line in a micrograph of a contracted muscle fibre. Another interesting aspect of these sarcomere
is that you can see darker and ligther areas. In the region where the actin and myosin filaments
are stacked on top of each other, that creates a darker image in a micrograph. In the areas
of the sarcomere where there’s a space above and below the actin filament, more light can
go through and the image on the micrograph is ligther. Make sure you can draw this diagram in any
of these three stages and label it. – During contraction, a motor neuron delivers
a signal that triggers the release of calcium ions from the rough endoplasmic reticulum
surrounding muscle fibres. In muscle cells, the rER is known as the sarcoplasmic reticulum.
The calcium ions will expose the myosin heads (high energy configuration), which bind to
the actin filaments forming a cross-bridge. The formation of this bond makes the myosin
head return to its low energy state and slide the actin filament in the process. ATP is
then used to break the cross-bridge and allows for another cross-bridge to be formed further
down the filament (increasing the intensity of the contraction).

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