Muscle Contraction and Motor Unit Recruitment

Structure of Skeletal Muscle

• Skeletal muscle is attached to the skeleton using tendons.
• Tendons link two bones and are key to movement.
• There are two types of skeletal muscle: slow twitch (red) and fast twitch (white).
• Slow twitch muscle is used for duration events.
• Fast twitch muscle is used for speed and power events.
• Muscle belly is surrounded by an epimysium.
• Muscle fascicles are bundles that make up the muscle belly.
• Each fascicle contains individual muscle fibres surrounded by an endomysium.
• The muscle fibre contains myofibrils running parallel to each other along the muscle fibre length.
• Myofibrils contain sarcomeres, which are composed of actin and myosin filaments responsible for muscle contraction.
• Muscle contraction occurs when a muscle pulls on one of the bones it connects to.
• When a muscle changes length movement is created.
• Muscle length changes when the myofibril length also changes.
• Myofibrils are made from many sarcomeres joined end to end.
• A sarcomere is the functional until of a muscle fibre, and is found between two z-lines.

Actin

• The thin protein filament that attaches to the z-line are called Actin.
• When stimulated by the release of calcium, cross bridges on the myosin attach to the actin.

Myosin

• The thick protein filament containing cross bridges are called Myosin.
• When stimulated by calcium, the myosin cross bridges attach to the actin.

Z-line

• Z-lines are found on the end of the sarcomere.
• In concentric contractions they come closer and expand during eccentric contractions.

Cross Bridges

• Cross bridges are tiny projections from myosin filaments that briefly attach to actin filaments which pull the actin forward.

H-zone

• H-zone is the space between the actin filaments, which get longer or shorter as the sarcomere changes length.

I-band

• The light band that contains thin actin filament is called the I-band.
• Thin filaments do not completely overlap thick myosin filaments in a relaxed muscle.

A-band

• A-band contains both thick and thin filaments and is the centre of the sarcomere that spans the h-zone.

Sliding Filament Theory

• The Sliding Filament Theory explains muscle contraction based on the interaction of actin and myosin filaments to generate movement.
• Neuromechanical stimulation releases calcium from the sarcoplasmic reticulum into the sarcomere.
• Calcium causes the actin filaments to reveal a binding site for the myosin head to connect.
• Myosin head then brings the actin filaments together, creating a cross bridge.
• Breakdown of ATP releases energy to stimulate the myosin cross bridges to pull the actin filaments towards the midline of the sarcomere.
• This results in the shortening of the sarcomere as the actin and myosin filaments slide over each other, causing the z-lines to come closer together and the h-zone to shorten.
• Shortening each sarcomere shortens the myofibril, resulting in the shortening of the muscle fibres and movement occurs
• Cross bridges attach and reattach at different times to create movement and maintain tension
• The neural impulse remains present or the muscle relaxes in response to the end of the neural impulse
• One drawn the sliding filament theory at rest is long, and when contracted its short.

Contractions

• Contraction is initiated by the release of calcium into the muscle
• During eccentric contraction the distance between z-lines increases
• During concentric contraction the distance between z-lines decreases
• During eccentric contraction the size of h-zone increases
• During concentric contraction the size of the h-zone decreases
• During eccentric contraction the I-band increases
• During concentric contraction the I-band decreases
• A-band stays the same during both contractions
• The sarcomere length increases during eccentric contraction
• The sarcomere length decreases during concentric contraction
• The Myofibril length increases during eccentric contraction
• The Myofibril length decreases during concentric contraction
• The Muscle fibre length increases during eccentric contraction
• The Muscle fibre length decreases during concentric contraction
• The Muscle length increases during eccentric contraction
• The Muscle length decreases during concentric contraction

Nervous control of the Muscular system

• Before muscle contraction the muscle fibres have to be stimulated by an nerve sent from the brain.
• The brain sends a message in the form of an action potential to the spinal cord.
• The spinal cord is responsible for the transmission of the message between the brain and the muscle, and muscle to the brain
• The motor neuron receives the message / action potential form the spinal cord and delivers it to the targeted muscle and where movement occurs
• The sensory neuron sends the messages send messages back to the brain via the spinal cord
• The brain analyses the information delivered by the spinal cord to determine the next action then the process repeats
• The three key functions of the nervous system are as follows: through sense organs and sensory nerves, it receives information about changes in the body and the environment, it sends this information to the brain, the brain determines a suitable response, the brain sends commands to muscles to carry out the selected response

CNS and PNS

• The nervous system is split up into two parts, the central and peripheral nervous system
• Brain and spinal cord are the main parts of the CNS
• The brain is connected to the body and the information goes both ways via the spinal cord
• The brain analyses a message after receiving messages from the sensory neurons
• The brain determines the most suitable response, then sends a message to the targeted muscle via the spinal cord and peripheral nerves to contract, creating movement
• The peripheral nervous system is remainder of the nervous system, it includes sensory neurons and motor neurons, which transmit messages to and from the CNS
• The PNS is made up of the sensory division and the motor division
• Sensory neurons send messages to the brain
• Motor neurons send messages to the muscles
• Information received by the environment by the eyes, ears, taste and smell goes strait to the brain bypassing the spinal cord

Motor Neurons

• Motor Neurons are made of three parts including Dendrites
• Dendrites act as an antenna to detect the impulse from the sensory receptors and deliver it to the cell body
• The cell body contains the nucleus which directs the neurons activities and sends the messages to the axon
• The axon transmits the message away from the cell body to the muscle
• Motor neurons attach to muscles at the motor end plates
• The neural impulse is transmitted to the muscle, initiating the process of muscle contraction

Motor Units

• The motor neuron and the fibres it activates are called a motor unit
• A whole muscle contains many different motor units, which allows the muscle to generate different amounts of force
• Muscle fibres need to be stimulated by nerves or electrical impulses sent via motor neurons
• Messages are sent in the form of an action potential down the spinal cord
• The action potential is detected by the dendrites of motor neurons, which send the information to the body
• The cell body directs this information down the axon to the motor end plate
• The action potential is delivered to the targeted muscle, which is innervated as long as the signal is strong enough
• The number of muscle fibres within each motor unit an vary, some have small ammounts where as some have thousends
• The smaller a motor unit the more precise the action of the muscle
• The larger a motor unit requires a larger action potential and results in the creation of a gross motor skill
• A motor unit requires a action potential to reach a certain threshold before it will activate
• If it does not reach the required thereshold the muscle will not contract
• If the impulse reaches the required threshold, all the muscle fibres in the motor unit will contract at 100% of its capacity.
• Decreasing weight/ size will affect how many motor units are required
• This is called the all or none law
• Motor unit requitement refers to the increasing number of motor units firing to increase the force being generated.
• Motor units are requited in order depending on exercise intensity
• Slow twitch (type 1) have low activation levels, and are requited first for light to moderate activity
• Higher threshold motor units (type 2a) are requited as exercise intensity increases, the slow twitch fibres are still activated
• When generating peak force, motor units that generate the greatest forces (type 2b) and have the highest stimulus threshold are requited
• Slow twitch and 2a motor units are still being activated due to the size of the stimulus.
• Low intensity stimulus of 15k run will innervate motor units containing type 1 units, the stimulus is not strong enough to activate type 2 muscle fibres
• 800m - a sustained high intensity event where type 1 fibres are unable to provide enough force, the brain sends a higher intensity signal to the muscles and will innervate type 2a fibres, the signal is not intense enough to innervate a large % of type 2b fibres
• 100m - maximum force required over a short distance, brain sends an even larger stimulus which innervates motor units containing type 2b fibres to generate maximum force, type 1 and 2a are also activated due to the size of the stimulus
• Force from a muscle can be increases in two ways: bigger stimulus or more frequent stimuluses

Muscle Fibre Type

• Type 1 muscle fibres are 'slow twitch', red coloured, purely aerobic and are suited to events which require continuous activity
• They use slow contraction speed, used for endurance exercises, use the aerobic system and are fatigue resistant with a small diameter.
• Type 2a muscle fibres are 'Fast twitch', white coloured, partially aerobic and are suited to events that require both aerobic and anaerobic qualities
• They have an Intermediate speed of contraction, moderate force of contraction.
• Type 2b muscle fibres are 'Fast Twitch', white coloured, purely anaerobic and are suited to events that require explosive movements
• They are used for speed, strength and power-based activities with a large diameter, requiring a large neuron to stimulate it

Genetics

• People are given a specific amount of red and white fibres that are genetically determined and cannot change
• An athlete with more fast twitch fibres will be better at explosive events
• An athlete with more slow twitch fibres will be better at endurance events

Fibre Types

• Type 1 muscle fibres are red, Type 2a muscle fibres are white and Type 2b are also white
• Type 1 muscle fibres have a slow contraction time while type 2a are fast, And type 2b are verry fast
• Type 1 muscle fibres have a small force production while type 2a are high, And type 2b are verry high
• Type 1 muscle fibres have are a small diameter while type 2a are intermediate, And type 2b are a large diameter
• Type 1 muscle fibres have a high resistance to fatigue while type 2a have a medium resistance, And type 2b have a low resistance
• Type 1 muscle fibres have a aerobic activity type while type 2a are long term anaerobic, And type 2b are short term anaerobic
• Type 1 muscle fibres have a small sise of motor neuron, Type 2a have a large sise and Type 2b have a verry large
• Type 1 muscle fibres have a high Capillary density, while Type 2a have a intermediate Capillary density and Type 2b have a low
• Type 1 muscle fibres have a high Mitochondrial density, while Type 2a have a intremediate Mitochondrial density and Type 2b have a low Mitochondrial density
• Type 1 muscle fibres have a high Oxidative density, while Type 2a have a moderate Oxidative density and Type 2b have a low Oxidative density
• Type 1 muscle fibres have a low Glycolic capacity, while Type 2a have a high Glycolic capacity and Type 2b have a high
• Type 1 muscle fibres have a Triglycerides and glycogen Major fuel source, while Type 2a have a Creatine phosphate and glycogen and Type 2b have a Creatine phosphate and glycogen

Force Velocity

• Force velocity is the relationship between force production and the velocity of the movement
• The muscle can create a large force with a decrease in the velocity of the concentric contraction
• More force is generated during the concentric contraction going slowly as it is able to recruit more motor units, giving the sarcomere more time to fully contract

Force Length

• Force length is the amount of muscle force that can be produced at varying muscle lengths
• The length of a muscle will affect the amount of force it can generate, muscles will generate less force when contracted and lengthend past their optimal lengths
• The greater amount of cross bridges that can be formed the greater force can be generated

Explore how motor units are recruited during muscle contraction. This includes the factors determining muscle fiber contraction, the order of recruitment during exercise, and the impact of weight or resistance. We also cover ATP's role, changes in muscle structure during contraction, and the effects of neural impulses.