Muscular system human bio 2
All muscles are:
Excitable (able to conduct and generate action potentials)
Contractile (able to shorten)
Extensible (able to stretch)
Elastic (able to return to original length)
There are 3 types of muscle tissue
Skeletal
Responsible for movement and posture
Supports internal organs and soft tissue
Pushes against veins and lymphatic vessels to move blood and lymph around the body
Controls swallowing (only initial stage), peeing and pooping
Generates heat to maintain body temperature
VOLUNTARY
Main focus
Cardiac
Found only in the heart
INVOLUNTARY
Smooth
Found in the GI Tract, hair follicles and eyes
INVOLUNTARY
Tendon
A band of connective tissue that joins muscle to bone
Aponeurosis
Sheet of connective tissue that joins muscle to muscle
Hierarchal Nature of Muscle
Whole muscle
Surrounded by epimysium
Fascicle
Surrounded by perimysium
Number of fascicles depends on the size of the muscle
Muscle fiber (cell)
Surrounded by endomysium (superficial) and sarcolemma (plasma membrane)
Muscle fibers are the largest cells in our body as they span the entire length of the muscle
Skeletal muscle is multi nucleated
Has lots of mitochondria
Has T Tubules, which is the sarcolemma folded inside the cell. This is unique to muscle cells
Myofibril
Surrounded by sarcoplasmic reticulum
Myofilaments
Proteins called actin and myosin
Proteins are arranged in units called sarcomeres
Proteins are responsible for muscle contractions
Structure of a sarcomere
Each myofibril can contain as many as 10,000 sarcomeres
Sarcomeres extend from Z-line to Z-line
Muscle proteins
Contractile – causes muscle to contract
Myosin
Actin
Regulatory – regulates contraction cycle
Troponin
Tropomyosin
Structural – helps to keep muscle cells together and helps with elasticity
Titin
Nebulin
Alpha-actin
Myomesin
Dystrophin
Under a microscope:
Where actin and myosin overlap, it has a dark appearance. This is called the A-band
Where only actin is present, it has a light appearance. This is called the I-band
During muscle contraction, myosin does not move. Actin is pulled towards myosin
BEFORE
AFTER
The Contraction cycle
3 steps to muscle contraction
Action potential travels down the motor neuron and triggers events at the Neuromuscular junction. This creates a muscle action potential
The neuromuscular junction is the same as a synapse
The motor neuron attached to the muscle branches to many different muscle fibers
It is rare to have one motor neuron only branch to one muscle fiber
Muscle fibers have similar structures to motor neurons. Where the neurons have a synaptic end bulb, muscle fibers have motor end plates. This is where the neuromuscular junction is formed.
Arrival of a nerve impulse at the presynaptic neuron (motor neuron) triggers the opening of voltage gated calcium channels
Calcium flows inward down the concentration gradient
The presence of calcium triggers the synaptic vesicles to migrate and fuse to the membrane
The synaptic vesicles contain the neurotransmitter acetylcholine. It is released into the synaptic cleft (neuromuscular junction)
Acetylcholine diffuses across the neuromuscular junction and binds to the receptors on the motor plate. The receptors are ligand gated channels. The channels open which allows sodium to cross the sarcolemma and enter the muscle fiber
Rapid influx of sodium makes the muscle fibers positively charged. This creates a muscle action potential which spreads across the sarcolemma. The action potential travels through the cell thanks to the T tubules, which is the sarcolemma folding inside the cell.
The muscle action potential spreads across the sarcolemma of the muscle fiber.
The end sacs of the sarcoplasmic reticulum are called terminal cisterns. They store large amounts of calcium.
2 terminal cisterns butt against a T tubule which forms a triad.
As the muscle action potential spreads across the sarcolemma and into the T tubules, calcium is released from the sarcoplasmic reticulum which is essential for muscle contraction.
The calcium binds to the Troponin which causes it to change shape. This reveals the active sites on the actin when the tropomyosin is pulled out of the way.
The myosin heads bind to the actin forming a cross bridge.
The myosin heads perform a power stroke which pulls the actin towards the m-line. This is the start of a muscle contraction.
These events will eventually cause the muscle fiber to shorten. This is called the sliding filament theory.
After a powerstroke, the myosin head has exhausted all of its energy.
A molecule of ATP binds to the myosin head to break the cross bridge and detach it from the actin.
The myosin head hydrolyzes the ATP, which breaks it down into ADP and phosphate. This process releases energy
The myosin head reenergizes and reorients itself
The myosin head reattaches itself to the actin, forming another crossbridge. It releases the phosphate to change shape.
The myosin head performs another power stroke, pulling the actin toward the M-line. The ADP molecule is also released.
Steps to stop a contraction
No new action potentials reach the synaptic end bulb. Acetylcholine is broken down and no more action potentials are generated on the sarcolemma.
Pumps in the sarcoplasmic reticulum reabsorb calcium with energy from ATP
Calcium unbinds from troponin. Troponin changes shape blocking the active sites with tropomyosin. With the active sites blocked, myosin heads can’t bind, and a contraction can’t occur.
Motor neuron
A typical skeleton contains thousands of muscle fibers
In areas of fine motor control (eyes and fingers) some motor neurons may control as few as 2 muscle fibers.
In areas where precision is not needed, one motor neuron can control as many as 2000 muscle fibers
The fewer of muscle fibers a motor neuron controls, the greater the precision
A motor unit is a single motor neuron and all the muscle fibers it controls.
In its simplest form, a single motor neuron will control a single muscle fiber. This is extremely rare and may only be found in the tiniest muscles in the eye.
Typically, one motor neuron will innervate several muscle fibers. The motor neuron will divide at its terminal end.
In areas of the body that do not require precision, a single motor neuron can innervate thousands of muscle fibers. The more muscle fibers that are stimulated, the greater the force of contraction.
The muscle fibers from different motor units are intermingled. This ensures that the direction of pulling on a tendon is constant.
Contraction always begins with the smallest motor unit. More units are employed as the force of contraction increases.
Peak tension can only be sustained for 10-15 seconds
Muscle hypertrophy
Cells increase in cross sectional area, not length.
The increase in size is due to growth from adding sarcomeres and non-contractile elements such as sarcoplasmic fluid.
Muscle size increase is due to the increase in size of the individual cells, not the number of cells.
Muscular dystrophy
A group of neuromuscular diseases, which results in weakening and breakdown of skeletal muscle over time
There are 30 different genetic disorders
Most common type is Duchenne muscular dystrophy, which generally affects only boys (on X chromosome) and begins to show signs at the age of 4
By the age of 10, most will need braces to assist with walking and by the age of 12 most are in a wheelchair permanently
Lifespan ranges from 15-45 depending on the severity
There is no known cure
With Duchenne muscular dystrophy, the muscle protein dystrophin is affected. This protein provides strength to sarcomere.
Without dystrophin, muscle tissue is unable to repair itself properly after damage. The muscle tissue is replaced by adipose tissue or scar tissue.
Genetic technologies
Gene therapy
Gene editing
CRISPR – a technology that allows for gene editing
Genetic engineering
All muscles are:
Excitable (able to conduct and generate action potentials)
Contractile (able to shorten)
Extensible (able to stretch)
Elastic (able to return to original length)
There are 3 types of muscle tissue
Skeletal
Responsible for movement and posture
Supports internal organs and soft tissue
Pushes against veins and lymphatic vessels to move blood and lymph around the body
Controls swallowing (only initial stage), peeing and pooping
Generates heat to maintain body temperature
VOLUNTARY
Main focus
Cardiac
Found only in the heart
INVOLUNTARY
Smooth
Found in the GI Tract, hair follicles and eyes
INVOLUNTARY
Tendon
A band of connective tissue that joins muscle to bone
Aponeurosis
Sheet of connective tissue that joins muscle to muscle
Hierarchal Nature of Muscle
Whole muscle
Surrounded by epimysium
Fascicle
Surrounded by perimysium
Number of fascicles depends on the size of the muscle
Muscle fiber (cell)
Surrounded by endomysium (superficial) and sarcolemma (plasma membrane)
Muscle fibers are the largest cells in our body as they span the entire length of the muscle
Skeletal muscle is multi nucleated
Has lots of mitochondria
Has T Tubules, which is the sarcolemma folded inside the cell. This is unique to muscle cells
Myofibril
Surrounded by sarcoplasmic reticulum
Myofilaments
Proteins called actin and myosin
Proteins are arranged in units called sarcomeres
Proteins are responsible for muscle contractions
Structure of a sarcomere
Each myofibril can contain as many as 10,000 sarcomeres
Sarcomeres extend from Z-line to Z-line
Muscle proteins
Contractile – causes muscle to contract
Myosin
Actin
Regulatory – regulates contraction cycle
Troponin
Tropomyosin
Structural – helps to keep muscle cells together and helps with elasticity
Titin
Nebulin
Alpha-actin
Myomesin
Dystrophin
Under a microscope:
Where actin and myosin overlap, it has a dark appearance. This is called the A-band
Where only actin is present, it has a light appearance. This is called the I-band
During muscle contraction, myosin does not move. Actin is pulled towards myosin
BEFORE
AFTER
The Contraction cycle
3 steps to muscle contraction
Action potential travels down the motor neuron and triggers events at the Neuromuscular junction. This creates a muscle action potential
The neuromuscular junction is the same as a synapse
The motor neuron attached to the muscle branches to many different muscle fibers
It is rare to have one motor neuron only branch to one muscle fiber
Muscle fibers have similar structures to motor neurons. Where the neurons have a synaptic end bulb, muscle fibers have motor end plates. This is where the neuromuscular junction is formed.
Arrival of a nerve impulse at the presynaptic neuron (motor neuron) triggers the opening of voltage gated calcium channels
Calcium flows inward down the concentration gradient
The presence of calcium triggers the synaptic vesicles to migrate and fuse to the membrane
The synaptic vesicles contain the neurotransmitter acetylcholine. It is released into the synaptic cleft (neuromuscular junction)
Acetylcholine diffuses across the neuromuscular junction and binds to the receptors on the motor plate. The receptors are ligand gated channels. The channels open which allows sodium to cross the sarcolemma and enter the muscle fiber
Rapid influx of sodium makes the muscle fibers positively charged. This creates a muscle action potential which spreads across the sarcolemma. The action potential travels through the cell thanks to the T tubules, which is the sarcolemma folding inside the cell.
The muscle action potential spreads across the sarcolemma of the muscle fiber.
The end sacs of the sarcoplasmic reticulum are called terminal cisterns. They store large amounts of calcium.
2 terminal cisterns butt against a T tubule which forms a triad.
As the muscle action potential spreads across the sarcolemma and into the T tubules, calcium is released from the sarcoplasmic reticulum which is essential for muscle contraction.
The calcium binds to the Troponin which causes it to change shape. This reveals the active sites on the actin when the tropomyosin is pulled out of the way.
The myosin heads bind to the actin forming a cross bridge.
The myosin heads perform a power stroke which pulls the actin towards the m-line. This is the start of a muscle contraction.
These events will eventually cause the muscle fiber to shorten. This is called the sliding filament theory.
After a powerstroke, the myosin head has exhausted all of its energy.
A molecule of ATP binds to the myosin head to break the cross bridge and detach it from the actin.
The myosin head hydrolyzes the ATP, which breaks it down into ADP and phosphate. This process releases energy
The myosin head reenergizes and reorients itself
The myosin head reattaches itself to the actin, forming another crossbridge. It releases the phosphate to change shape.
The myosin head performs another power stroke, pulling the actin toward the M-line. The ADP molecule is also released.
Steps to stop a contraction
No new action potentials reach the synaptic end bulb. Acetylcholine is broken down and no more action potentials are generated on the sarcolemma.
Pumps in the sarcoplasmic reticulum reabsorb calcium with energy from ATP
Calcium unbinds from troponin. Troponin changes shape blocking the active sites with tropomyosin. With the active sites blocked, myosin heads can’t bind, and a contraction can’t occur.
Motor neuron
A typical skeleton contains thousands of muscle fibers
In areas of fine motor control (eyes and fingers) some motor neurons may control as few as 2 muscle fibers.
In areas where precision is not needed, one motor neuron can control as many as 2000 muscle fibers
The fewer of muscle fibers a motor neuron controls, the greater the precision
A motor unit is a single motor neuron and all the muscle fibers it controls.
In its simplest form, a single motor neuron will control a single muscle fiber. This is extremely rare and may only be found in the tiniest muscles in the eye.
Typically, one motor neuron will innervate several muscle fibers. The motor neuron will divide at its terminal end.
In areas of the body that do not require precision, a single motor neuron can innervate thousands of muscle fibers. The more muscle fibers that are stimulated, the greater the force of contraction.
The muscle fibers from different motor units are intermingled. This ensures that the direction of pulling on a tendon is constant.
Contraction always begins with the smallest motor unit. More units are employed as the force of contraction increases.
Peak tension can only be sustained for 10-15 seconds
Muscle hypertrophy
Cells increase in cross sectional area, not length.
The increase in size is due to growth from adding sarcomeres and non-contractile elements such as sarcoplasmic fluid.
Muscle size increase is due to the increase in size of the individual cells, not the number of cells.
Muscular dystrophy
A group of neuromuscular diseases, which results in weakening and breakdown of skeletal muscle over time
There are 30 different genetic disorders
Most common type is Duchenne muscular dystrophy, which generally affects only boys (on X chromosome) and begins to show signs at the age of 4
By the age of 10, most will need braces to assist with walking and by the age of 12 most are in a wheelchair permanently
Lifespan ranges from 15-45 depending on the severity
There is no known cure
With Duchenne muscular dystrophy, the muscle protein dystrophin is affected. This protein provides strength to sarcomere.
Without dystrophin, muscle tissue is unable to repair itself properly after damage. The muscle tissue is replaced by adipose tissue or scar tissue.
Genetic technologies
Gene therapy
Gene editing
CRISPR – a technology that allows for gene editing
Genetic engineering
