Time to Move!

Have you ever stopped yourself during something as simple as playing catch or simply walking to really think about what is happening to your body? How you can manipulate a structure made up of bones and muscles to perform the simplest or even the most complex of tasks?

Take a moment and stand up. What did you have to "tell" your body to do in order to accomplish this? Did you have to tell it to produce force? Did your body react quickly? Slowly?

Now carefully sit back down. What did you "hear" from your body as feedback so that you didn’t fall on the floor? What did you do with this information? Which did you find harder to perform, sitting down or standing up? Everything you just did was in a controlled, precise movement involving a large number of bones and muscles. The muscles involved all required the same information, to contract or not, and regardless of size, they all went through the same processes in order to successfully contract.

So how does all this happen? From the moment you tell yourself to move until you actually do, what are the steps that need to happen?

What are the steps involved in movements that are even more complex?

What is this skateboarder "telling" his body to do? How do we accomplish such complex physical tasks?

Now it’s time to take a closer look at what is happening when you finally make the decision to contract a muscle. Keep in mind, this entire process happens every time you blink, take a breath, scratch your nose, let alone perform complex tasks such as riding a bike, throwing a ball or performing a trick on a snowboard. The only difference between any contraction to make it a "stronger" or more "controlled" contraction is the number of muscle fibers that the brain recruits as a response to the task to be performed.

The Central Nervous System (CNS),
Peripheral Nervous System (PNS) and a Motor Unit.

Let’s piece together the story of the muscle contraction.

A Time to Contract

"The Command Centre" - The Nervous System

The story begins when you are ready to move a muscle (ie. blink, throw a ball, jump, etc.). Contraction “commands” are sent from the brain or spinal cord to the muscle and eventually cause a chemical reaction to take place in the muscle(s) you have ordered to move.

Our ability to move faster or slower is all a result of being able to predict and react to stimuli being presented; we call this "Reaction Time". No matter how fast or slow you can react, this seemingly long process, involving the brain, many nerves and muscles, is a choreographed electrical and chemical "dance" that takes milliseconds to orchestrate and perform.

There are 2 main components of the nervous system:

Central Nervous System (CNS)

Includes the main control centre (the brain) and the highway of nerves running down your vertebral column, the spinal cord.

Peripheral Nervous System (PNS)

The PNS is a collection of all the nerves outside the brain and spinal cord. The main function of the PNS is to connect the central nervous system (CNS) to the limbs and organs, essentially serving as a communication relay going back and forth between the brain and spinal cord with the rest of the body.

The following video let's you see the Central and Peripheral nervous sytem in action:

"Subway Surfer" - The Motor Unit

We use the term "motor" to make reference to movement. So when you hear the term "motor neuron", you should make the connection that this is a nerve responsible for sending the signal to the muscle to move. These neurons form a network or pathway that an impulse must follow to reach a muscle telling it to contract, much like subway tunnels carry trains from station to station eventually ending up at a main, central station.

Making up only one part of the bigger motor unit, motor neurons help deliver the impulse to control muscular contraction to follow the “all-or-none” principle, which stipulates that once the impulse is sent down the axon that all the muscle fibres connected to that neuron will fully contract or not at all. Thus, the level of control for a muscle is determined by the number of muscle fibres each motor neuron innervates (controls).

Therefore, a motor unit is comprised of the following 3 items:

• The motor cell body
• The axon (pathway)
• The muscle fibres it stimulates

What exactly is the "go message" to contract? Is it chemical? Is it electrical? The answer is quite simple: it’s both.

Let’s examine how the electrical impulse travels in one direction from the neural cell body down the axon to the axon terminals where they will then cause chemicals to cross the neuromuscular junction to the muscle side of the unit. Hover over the titles to find out what each term means. Imagine you were in a big circle of friends holding hands. Where your hands are connected are the Nodes of Ranvier (see diagram) with your body acting as the axon and your clothes as the myelin sheath.

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From Neural Cell Body to Terminal Branch:

1. Electrical impulses are collected from other neurons and centralized to follow down the axon to the muscle.
2. Travelling down the axon, electrical impulse moves very rapidly through the myelinated section only to slow down and concentrate between each section at the Nodes of Ranvier before moving once again down the axon.
3. Once the electrical impulse reaches the ends of the axon, the terminal knobs, it causes vesicles filled with the neurotransmitter Acetylcholine (Ach) to move closer to the outer membrane of the knob.
4. Attaching to the membrane, the vesicle is ready to release the Ach molecules into the synaptic cleft, from the nervous system, into the space across to the muscular system.

When looking at the movement of an electrical impulse from the central nervous system down to the awaiting muscle, one could use the analogy of a subway ride to see a Toronto Raptors basketball game at the Scotibank Arena in downtown Toronto. Can you match the “Subway trip” description to the actual neuromuscular sequence of events?

Let's see the motor neuron in action:

Jumping the Gap - The Neuromuscular Junction

Eventually the message to contract needs to transfer from the nervous system to the muscular system. However, this cannot be done by an impulse of electricity crossing through space from one side to another. In order for the command to "Jump the Gap", it must go from electrical, in the form of impulses moving down the axon (presynaptic membrane), to chemical as molecules of Acetylcholine move across the space (synaptic cleft), and then back to electrical once the Ach binds to receptors on the muscle side (postsynaptic membrane).

To control the amount of Ach in the synaptic cleft (space), Acetylcholinesterase is released. Acetylcholinesterase is an enzyme that breaks down Ach into acetate and choline to be reabsorbed and reused by the presynaptic terminal knob. This transition point between the nervous and muscular system is called the NEUROMUSCULAR JUNCTION (NMJ).

Let's see the neurumuscular junction in action.

The Final Movement - The Muscle Contraction

Finally the command to contract has made it to the muscular system. But first, we need to better understand a few things. What exactly is muscle? What is it made of? What the are characteristics of muscle that allow it to function the way it does?

Here are some interesting facts, some of which you will need to know for the next activity:

• When we eat meat, aside from organs like the liver, we are eating muscle.
• The colour of the meat can tell you the type of activity that muscle is best at:
• Dark meat: contains a protein in it (myoglobin) that carries oxygen into the muscle for long duration, low force producing (aerobic) activities.
• Light meat: does not contain oxygen carrying capabilities and thus is good for short burst, high intensity movements. Skeletal muscles contract only if stimulated to do so.
• Skeletal muscles contract only if stimulated to do so.
• Movement is produced by pulling on bones.
• Bones serve as levers, whereas joints serve as fulcrums on these levers.
• Muscles that move a segment do not usually lie over that part (e.g. bicep flexes elbow but is in upper arm).
• Every muscle in the body must have the following characteristics:
• Contractility: the capacity of muscle to contract or shorten forcefully.
• Excitability: muscle responds to stimulation by nerves and hormones, making it possible for the nervous system to regulate muscle activity.
• Extensibility: muscles can be stretched to their normal resting length and beyond to a limited degree.
• Elasticity: if muscles are stretched, they recoil to their original resting length.
• Skeletal muscles almost always act in groups, based on their behaviours, rather than individually in order to perform movements.
• Prime Movers/Agonists
• Muscles considered primarily responsible for generating a specific movement.
• In flexion of the arm at the elbow (bicep curl) the primary mover or agonists is the bicep.
• In extension of the leg at the knee (kicking a soccer ball) the primary mover is the quadriceps.
• Antagonists
• Muscle responsible for movement opposite to the desired movement. It is important to exercise both the agonist and antagonist in order to have proper balance across a joint.
• In flexion of the arm at the elbow (bicep curl) the agonist is the tricep.
• In extension of the leg at the knee (kicking a soccer ball) the hamstrings are the antagonists.
• Stabilizers/fixators
• Acts to stabilize one part of the body during movement of another part.
• When doing a push-up or a bench press, your deltoids act as stabilizers to support the movement.
• Synergists
• Perform, or help perform, the same set of joint motion as the agonists.
• In a push-up or bench press, the triceps help the agonist, the pectoral muscles, in the movement and thus are the synergists to this movement.

Time to complete our story of "A Time to Contract"...

As we last left off, Ach molecules have now crossed the synaptic cleft and bound to receptor molecules on the postsynaptic side of the neuromuscular junction. This combination causes a depolarization of the muscle membrane which now causes an electrical impulse to spread across the Sarcolemma (outer membrane of muscle tissue). But how does an electrical impulse spread across and deep into a muscle to get the entire muscle to contract?

It’s time to assess your understanding of the sliding filament theory of muscle contraction before you complete the final assignment. Use the following learning tools to drag and drop the appropriate words in the spaces to complete the story of muscle contraction.

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