3. Muscular System Flashcards
Functions of muscular system
- Movement - a result of muscular contraction. This relies on the integrated functioning of the muscles, bones and joints
- Maintaining posture - stabilising joints, posture and balance through continued partial muscle contraction
- Heat production (thermogenesis) - helps maintain normal body temperature (36.5 - 37.5°C). Shivering describes involuntary contractions of skeletal muscles.
- Storage of substances - glycogen and oxygen
- Movement of substances:
- the heart muscle pumps blood around the body
- sphincters prevent out-flow from hollow organs
- smooth muscle in blood vessel walls helps control blood flow
- smooth muscle moves food through the digestive tract and urine through the urinary system
- the diaphragm draws air into airways/lungs
Muscle properties
- Contractility - ability to contract (shorten)
- Excitability - ability to conduct an electrical current. nerve impulses cause muscles to contract
- Extensibility - ability to stretch without being damaged
- Elasticity - ability to return to original length and shape after contraction or extension (spring)
Muscles in the body contain cells that are either ___ or ___
striated and non-striated
striated muscles
striated muscles contain cells that are aligned in parallel bundles, so that their different regions form stripes visible with a microscope
non-striated muscles
non-striated muscles contain cells that are randomly arranged (no stripes visable)
what muscle are striated?
skeletal and cardiac muscles
what muscles are non-striated?
smooth muscle
skeletal muscle
- striated
- attaches between bones and creates movement at joins
- voluntary movement
cardiac muscle
- striated
- forms the heart muscle
- involuntary muscle that generates its own rhythmic contraction (autorhythmic)
smooth muscle
- non-striated
- found in the walls of blood vessels, walls of the gut and in the iris
- involuntary muscle
how many skeletal muscles in the body?
640 skeletal muscles in the body, accounting for about 40% of body weight
functions of skeletal muscle
- motion and posture
- speech (larynx, lips, tongue)
- breathing
fascia
skeletal muscle is covered by ‘fascia’
a dense sheet of connective tissue that organises muscle, secures it to skin and provides stability. collagen is a major component.
sarcolemma
the cell membrane of a skeletal muscle fibre
sarcoplasm
the muscle cell cytoplasm
transverse tubules
tubes which extend from the cell membrane into the muscle cells
sarcoplasmic reticulum
A membrane-bound structure found within muscle cells that is similar to the endoplasmic reticulum in other cells. The main function of the SR is to store calcium ions (Ca2+)
Myoglobin
Red coloured, iron and oxygen binding protein
mitochondria are located close by for aerobic respiration
Myocytes (muscle fibre)
Muscle fibres are formed from the fusion of cells called ‘myoblasts’ in the embryo. This is why skeletal muscle cells contain many nuclei.
Once mature muscle cells are formed (becoming ‘myocytes’), they can no longer undergo mitosis.
However, there is limited regenerative capacity by satellite cells.
This means that the number of skeletal muscle fibres each person has is set at birth.
Myofibrils
Cylindrical structures formed of bundles of protein filaments within the muscle fibre. They are contractile threads arranged in a striated pattern:
Each myofibril is surrounded by a network of sarcoplasmic reticulum.
Myofibrils are made up of smaller filaments called myofilaments (two types):
- actin (thin filaments)
- myosin (thick filaments) - shaped like golf clubs - the myosin heads can bind to actin
The myofilaments overlap to form ‘sarcomeres’.
Sarcomeres
A sarcomere is the basic unit of striated muscle and contains the following areas:
- H zone = myosin only
- A band = dark are where actin and myosin overlap
- I band = light area of actin only
- Z discs = filaments of actin that are arranged at 90° angles, where they separate sarcomeres
Actin
Thin filament
Myosin
Thick filament
Connective tissue
Skeletal muscles consist of muscle fibres bound by connective tissue
Collagen fibres in connective tissues assist to tightly intermingle with out structures - connections transfer force better.
Hierarchy of Muscle from smallest
Myocytes contain myofibrils that are made up of smaller myofilaments called actin and myosin.
Individual muscle fibres are surrounded by a thin sheath called the ‘endomysium’.
Bundles of between 10‒100 muscle fibres are bound together to form ‘fascicles’, which are surrounded by the ‘perimysium’.
The entire muscle is surrounded by the ‘epimysium’ that attaches it to fascia and tendons.
Neuromuscular Junction (NMJ)
The ‘neuromuscular junction’ is the meeting point (synapse) where motor neurons meet a muscle fibre.
The neuron ending is the synaptic end bulb, which contains vesicles that store the neurotransmitter ‘acetylcholine’
Acetylcholine diffuses across the gap and causes the nerve impulse to continue along the sarcolemma.
Acetylcholine
Neurotransmitter causing the nerve impulse
motor end plate
Describes the location where motor neurons terminate in tiny pads on the muscle fibre.
The strength of muscle contraction depends on the number of motor neurons that are conducting an electrical impulse at one time, as well as the frequency of impulses.
Sliding Filament: Contraction
- Nerve impulse arrives at the neuromuscular junction.
- The action potential spreads along the sarcolemma and transverse tubules into the muscle cell releasing calcium (Ca2+) from storage in the sarcoplasmic reticulum.
- Calcium and ATP cause the myosin head to bind to the actin filament next to it. As
the actin and myosin bind, this movement causes the filaments to slide over each other, thereby shortening the fibre.
Sliding Filament: Relaxation
- Nerve stimulation stops (no nerve impulse).
- Using magnesium and ATP, calcium is actively transported (pumped back) into storage, breaking the actin and myosin bond.
- Actin and myosin slide back into starting positions, lengthening the fibre again (relaxation).
Magnesiums role in muscle movement
Magnesium makes muscle fibres less excitable and prevents myosin binding with actin.
Muscle lengthening
Muscle relaxation is associated with lengthening of the sarcomere and muscle overall.
Muscle shortening
Muscle contraction is associated with the binding of actin and myosin. This causes the sarcomere (and muscle overall) to shorten.
Muscle growth
Muscle growth is called ‘muscle hypertrophy’.
Calcium, magnesium, sodium, potassium and iron are essential ingredients for effective muscle activity and athletic performance.
The following hormones promote muscle hypertrophy:
- Growth hormone
- Testosterone
- Thyroid hormones
During strength training, individuals experience high levels of muscle tissue breakdown and hence protein is required to support hypertrophy.
The two main pathways for ATP synthesis
- Aerobic respiration (with oxygen present)
- Anaerobic respiration (absence of oxygen)
Aerobic respiration
Requires oxygen to generate ATP
Requires a continual blood supple. The oxygen needed comes from breathing.
Aerobic respiration occurs in the mitochondria
Each reaction produces 38 ATP molecules. However, a the two ATP molecules are used up in the reaction, a net of 36 ATP molecules is produced
Aerobic respiration is used most of the time, as long as oxygen delivery is maintained
Aerobic respiration formula
Oxygen + (glucose) > carbon dioxide + water + energy
Anaerobic respiration
Anaerobic respiration allows cells to produce ATP in the absence of oxygen
Utilised for intensive short-term activity
Takes place in the cytoplasm and occurs via glycolysis (breaking down of glucose). The reaction produces a net of 2 ATP molecules
Also produces lactic acid which lowers the muscle pH and can cause muscle fatigue
Anaerobic respiration formula
Glucose > lactic acid + energy
Creatine phosphate
Protein unique to muscles and is an energy storage form
This is important because muscle cells have
very little energy within them that they can use up immediately.
Creatine phosphate provides a small, but ready source of energy during the first 15 seconds of contraction.
There is three to six times more creatine phosphate in a muscle cell than ATP.
Creatinine
by-product from the breakdown of creatine phosphate.
Based on colour, skeletal muscle fibres can be categorised into two types ___ and ___ ?
Red and white
White muscle fibres
- white due to the low quantity of myoglobin
- large diameter fibres
- anaerobic respiration
- Fast and strenuous work, hence fatigue quickly
Red muscle fibres
- Red due to the large quantity of myoglobin
- small diameter fibres
- aerobic respiration (hence hots of mitochondria)
- designed for sustained activity with no fatigue
Skeletal muscle fibres can be categorised as one of three types
- slow oxidative (SO)
- Fast oxidative-glycolytic (FOG)
- Fast glycolytic (FG)
Slow oxidative (SO)
Dark red
Aerobic
Highest levels of myoglobin and mitochondria
Longest duration
Good for endurance
Smallest diameter
Fast oxidative- glycolytic (FOG)
White-pink
Aerobic / anaerobic
Less myoglobin and mitochondria
Resistant to fatigue
Good for walking / sprinting
Intermediate diameter
Fast glycolytic (FG)
White
Anaerobic
Least amount of myoglobin and mitochondria
Fatigue quickly
Good for weights
Largest diameter
What are the exercises which can cause muscle fibres to change
Endurance athletes often have more SO fibres.
Strength training increases the size
(hypertrophy) and strength of fast glycolytic fibres..
Occipitofrontalis (skeletal face muscle)
Raises eyebrows
Orbicularis oculi (skeletal face muscle)
Closes eyes
Orbicularis oris (skeletal face muscle)
closes / pouts lips
Masseter (skeletal face muscle)
Mastication
Temporalis (skeletal face muscle)
Mastication
Sternocleidomastoid (skeletal neck muscle)
Turns and tilts head
Trapezius (skeletal neck muscle)
Pulls head backwards
Elevates (shrug’s) and retracts shoulder
Supraspinatus (skeletal back muscle)
Initial abduction of shoulder
Psoas (skeletal back muscle)
Hip flexor (pulls thigh towards trunk)
Latissimus dorsi (skeletal back muscle)
Extends, adducts and internally rotates arms
Quadratus lumborum (skeletal back muscle)
Bending backwards or sideways (vertebral extension or lateral flexion)
Erector spinae (skeletal back muscle)
Extension of the vertebral column. Keeps spin upright
Pectoralis major (skeletal chest & abdomen muscle)
Draws arm forward: shoulder flexion and medial rotation
Rectus abdominis (skeletal chest & abdomen muscle)
Vertebral flexion: bending forward (crunches)
Internal and external obliques (skeletal chest & abdomen muscle)
Rotation, bending sideways
Respiratory diaphragm
Attaches to the lower six ribs, sternum and upper lumbar spine.
When contracts, it descends into the abdominal cavity, increasing the space for air to enter the lungs.
Vital structures pass through the diaphragm, which further emphasises its importance in the body. These structures include the aorta, inferior vena cava, vagus nerve and oesophagus.
Deltoid (skeletal arm muscle)
Attaches from the scapula, acromion and clavicle to the humerus
Flexion, abduction and extension of shoulder joint
Biceps brachii (skeletal arm muscle)
Attaches from the scapula to the radius
Stabilises shoulder joint
flexion and supination of forearm
Triceps brachii (skeletal arm muscle)
Arm adduction - extends the elbow
Flexor carpi muscle (skeletal arm muscle)
flexes the hand at the wrist joint
Extensor carpi muscle
Extends the hand at the wrist joint
Gluteus maximus (skeletal leg muscle)
Attaches from the ilium (pelvis) to the femur
External rotation, abduction and extension of the hip joint
Piriformis (skeletal leg muscle)
Attaches from the sacrum to the femur
Externally rotates hip
Hamstring (skeletal leg muscle)
Three separate muscles in the posterior thigh
Bends knee (flexes knee)
Rectus femoris (skeletal leg muscle)
(one of four quadricep muscles) attaches from the pelvis to the tibia
Flexes hip and extends knee (e.g. kicking a football)
Thigh adductors (skeletal leg muscle)
Attach from the pubis to the femur
Squeeze the thighs together
Tibialis anterior (skeletal leg muscle)
‘front of tibia’ attaches from the tibia to the metatarsals
Dorsiflexion and inversion of the foot (and supports medial arch of foot)
Soleus (skeletal leg muscle)
Attaches from the posterior tibia and fibula to the calcaneum (heel bone)
Plantar flexion of the foot at the ankle
Gastrocnemius (skeletal leg muscle)
Attaches from the femur to the calcaneum
Flexes leg at the knee
Plantar flexion of foot
Muscle belly
The fleshy part of a muscle
Tendons
Tendons attach the skeletal muscles to the periosteum of bone
When tendons span across a joint they can produce movement (i.e. flex or extend the joint).
When fibres contract, the muscle becomes thicker and shorter. This exerts a force on the which pull on the bone, producing movement at a joint.
Muscle attachments
The location of muscle attachment points to bone
Sciatic nerve
The sciatic nerve often runs through the belly of the muscle ‘piriformis’. This makes the sciatic nerve particularly vulnerable to compression in this location
Antagonistic pairs
Most skeletal muscles are arranged in antagonistic pairs over a joint e.g. biceps brachii/ triceps brachii.
Depending on the movement, one muscle is the prime mover, whilst the other is the antagonist.
eg. Biceps (prime mover), Tricep (antagonist)
Synergist
A synergist assists the prime mover in its action, e.g. when flexing the elbow, brachialis helps the biceps by pulling the ulna towards the humerus.
Fixator
A fixator is a muscle that keeps the origin bone stable while a bone mover contracts, e.g. in the shoulder
Cardiac muscle
- Cardiac muscle is a specialised muscle that is only found in the heart. It forms the myocardium.
- Cardiac muscle fibres are striated and involuntary
- Cardiac muscle cells are joined end-to-end by specialised structures known as intercalated discs. These are unique to cardiac muscle and allow contraction to spread from cell to cell like a wave.
- Cardiac muscle cells are branching cells, so each cell is in contact with 3-4 other cells. This enables the wave of contraction to spread to more cells.
- Cardiac muscle is autorhythmic (generates its own rhythm of contraction, approx. 75 times per minute at rest).
- Cardiac muscle stays contracted 10-15 times longer than skeletal muscle.
- Cardiac muscle depends highly on aerobic respiration and hence the cells contain lots of mitochondria. Cardiac muscle cells therefore require a constant blood supply and delivery of oxygen and nutrients like glucose.
- Can also use lactic acid to produce ATP.
Smooth muscle
- Smooth muscle is found in the walls of blood vessels, airways, hollow organs (i.e. stomach, bladder), as well as the iris and arrector pili in the skin.
- Used to change diameter, shape or orientation of the tissue.
- Under autonomic nervous system control (involuntary).
- Also contracts in response to hormones, cell-to-cell signalling and local chemical agents.
- Are the smallest type of muscle cell and contain single elongated, central nucleus.
- Non-striated, giving a smooth appearance.
Filaments are attached to structures called dense bodies (similar function to Z-discs in skeletal muscle) that are NOT arranged in lines.
During contraction the dense bodies are pulled closer together by filaments causing the muscle to shorten and twist like a corkscrew.
Smooth muscle properties
- Smooth muscle contraction is slower and longer. It also shortens and stretches more than skeletal muscle.
- Produces ‘stress-relaxation response’ allows organs such as stomach and bladder to expand when filled, causing a contraction in order to carry contents.
- Smooth muscle maintains partial contraction (important for blood pressure regulation).
- Smooth muscle contracts in response to autonomic nervous system, hormones, stretch and blood gases.
Single unit smooth muscle:
- In an organ, smooth muscle fibres function as one unit
(all the fibres relax and contract together). - Found in the walls of vessels and hollow viscera.
- Fibres connected by gap junctions to allow action
potential to spread through the muscle.
Muscle
Multi unit smooth muscle
- Fibres are stimulated individually & operate independently
from each other. They contain no gap junctions. - Found in walls of large arteries and airways; iris and in arrector pili muscles in hair follicles.
Muscle Regeneration: Skeletal
All muscle types can hypertrophy (increase in size).
- Skeletal muscle cells can’t divide.
- Limited regeneration by satellite cells –when damage occurs, they divide slowly & fuse with existing fibres.
Muscle Regeneration: Smooth
- Can increase in number (hyperplasia)–often seen in the uterus and vascular smooth muscle.
- Regeneration can occur from stem cells in blood vessels.
Muscle Regeneration: Cardiac
- Post heart attack, tissue remodeling by fibroblasts (scarring).
- More recent evidence has identified that stem cells in the endothelium can undergo division.
- Hypertrophy can be physiological (i.e. athletes) or pathological (i.e. heart disease).
