Musculoskeletal System

Characteristics of muscles

Excitability = can send an action potential done the length of its membrane

Contractility = ability to contract - requires myosin, actin and calcium

Elasticity = can return to original length when relaxed

Extensibility = can be stretched

Function of smooth muscle

Moves fluids/solids along digestive tract and other internal organs, control diameter of blood vessels

Location of smooth muscle

In walls of hollow internal structures

Blood vessels - changes diameter

Stomach, intestine, bladder, uterus, airway to lungs

How is smooth muscle controlled

Involuntary, spontaneous rhythmic cycles (pacesetter cells) or influenced by hormones, stretching, ANS or all three

Smooth muscle histology

No tendons, central uni-nucleated, spindle shaped, non-striated, autorhythmic (cells located close together so action potential spreads quickly across cells - contract in unison), can divide/regenerate

Function of cardiac muscle

Pumps blood through cardiovascular system

Located of cardiac muscle

Only in the heart

How is cardiac muscle controlled

Involuntary (pacesetter cells), controls its own contractions, influenced by ANS and hormones

Cardiac muscle histology

Central uni-nucleated, striated, branched, autorhythmic, intercalated discs (action potentials can spread quickly - contract in unison), cannot divide/regenerate

Function of skeletal muscle

Pulls on bones to produce movement

How is skeletal muscle controlled

Voluntary control, controlled by nerves of CNS, can be influenced by hormones

Skeletal muscle histology

Very long, multinucleated (squashed, outside muscle fibres), cylindrical, striated (organised myosin and actin fibres), myoglobin stores O2, cannot divide by can repair due to stem cells

Specific functions of skeletal muscle

Fine and gross motor skills, stabilises body positions (posture, balance, walking, sitting upright), proprioception, support soft tissue, storing and moving substances within the body (protein reserves, sphincter muscles), generates heat

Muscle growth and regeneration

Cardiac = cannot regenerate, heart muscle replaced by scar tissue when injured (cannot contract)

Smooth = greatest capacity to regenerate

Skeletal = can grow/develop over time (increase or decrease in size)

Hypertrophy

Increased use leads to increased tissue size due to increase in cell size (skeletal muscle)

Hyperplasia

Increase in tissue size due to increased number of cells (smooth muscle)

Atrophy

Decrease in use leads to decrease in tissue size due to decrease in cell size (skeletal muscle)

Describe epimysium

Surrounds the muscle organ, maintains structural integrity when contracting, separates muscle from other organs/tissues in the area

Describe perimysium

Surrounds fascicles

Fascicle organisation allows nervous tissue to trigger a specific subset of muscle fibres within a fascicle

Describe endomysium

Surrounds each muscle fibre

Contains ECF and nutrients for muscle fibre

Define fascicle

Functional unit which muscle fibres work together

Define muscle fibre

Individual cells (myocytes)

Define satellite cell

Stem cells that allow growth of skeletal muscle, under epimysium but outside of muscle

Define myofibril

Contractile cells (long, run parallel)

Define myofilament

Thin actin, thick myosin, elastic titin filaments, long tubular structures within myofibrils

How are epimysium, endomysium and perimysium connected

Epimysium, perimysium and endomysium intertwine at muscle tendon, fused to peristeum of bone shaft

Define skeletal muscle fibres

Long chains of myoblast cells fused together to form one, long multinucleated cells, lots of copies of genes for the production of proteins/enzymes needed for muscle contraction

Name structures in skeletal muscle fibres

Sarcolemma, sarcoplasm, transverse tubules, sarcoplasmic reticulum, sarcomere

Define sarcolemma

Cell membrane around myocyte

Define sarcoplasm

Cytoplasm in muscle cells, surrounds myofibrils and other structures

Define transverse tubules

Invaginations where the cell membrane is pushed down into the interior of the cell, run along the length of muscle cells, have ECF, ensure action potential gets deep into the muscle

Define sarcoplasmic reticulum

Highly specialised ER that stores, releases and retrieves Ca2+ ions

Define sarcomere

Functional unit of skeletal muscle where contraction happens, highly organised arrangement of actin/myosin, runs from one z line to another z line

Myofibril structure

Bundles of myofilaments responsible for muscle contraction

Striated structure, myofilaments anchored to cell membrane of muscle cell - contraction causes whole cell to shorten

Describe sarcomeres

Actin is attached to z line, myosin attached to m line

When myosin grips onto actin, it pulls actin towards the m line (on left and right), causes sarcomere to shorten

Describe the process of muscle contraction

Action potential flows down axon of motor neuron, neuron synpases with sarcolemma of muscle fibre, triggers vesicles containing acetylcholine, ACh binds to nicotinic receptors on sarcolemma, opens sodium gated channels and sodium enters sarcolemma cells which triggers AP down length of muscle fibre, AP travels down t-tubes which triggers sarcoplasmic reticulum to release calcium, calcium binds to troponin on actin, causes tropomyosin to roll off actin and expose active sites, myosin binds to actin and contracts, action potential stops which stops contraction

Describe contraction cycle

Myosin head already energised by ADP+P attached, calcium ion binds to troponin (calcium-troponin-tropomyosin complex rolls of binding sites), myosin forms crossbridges with actin, ADP+P released, ATP attaches to myosin head and releases it from actin to expose active sites, myosin becomes reenergised and forms another crossbridge

Define muscle tone

Certain degree of contraction or undertone of contraction that occurs in muscles while at rest

Maintains posture, stabilises bone and joints, ready response state

Define hypotonia

Absence or lower level of contractions for muscle tone due to damage to CNS or lack of innervation

Causes floppy limbs, instability and difficulty standing

Define hypertonia

Higher level of contractions for muscle tone

Causes stiff limbs, difficulty moving and muscle spasms

Define neuromuscular junction

Synapse between motor neuron and muscle cell

Neuromuscular junction and muscle contraction process

Action potential arrives at axon terminal, stimulation of calcium ion voltage gated channels, calcium ions enter axon terminal, exocytosis of acetycholine, ACh binds to nicotinic receptors on sarcolemma of muscle, sodium enters cell which causes depolarisation which is transferred along sarcolemma, reaches t-tubes, sarcoplasmic reticulum releases calcium ionc into sarcoplasm, calcium attaches to troponin