Muscular System

Week 13

What are the functions of the muscular system | 4

• Producing body movement (all controlled by muscles)

• Stabilizing body positions (posture, standing/sitting)

• Storing/moving substances (sphincter control, pumping blood, digestion)

• Generating heat (thermogenesis, regulation of body temp, shivering)

What are the different types of muscular tissue | 3

• Skeletal muscle

• Smooth muscle

• Cardiac muscle

What does skeletal muscle do

Move bones of skeleton - produce body movement

What does smooth muscle do

Walls of hollow organs, BVs + digestive tract

What does cardiac muscle do

Contraction of heart chambers

What are the properties of muscular tissue | 4

• Electrical excitability (production n propagation of action potentials)

• Contractility (contract n relax to produce force/movement)

• Extensibility (ability to stretch within limits)

• Elasticity (ability to return to its original length following stretch)

What is skeletal muscle composed of | 4

Muscle fibres (myocytes - muscle cells), connective tissue, BVs + nerves

What about that subcutaneous layer of skin (hypodermis) | 3

• What separates muscle from skin

• Composed of adipose (fat) tissue + connective tissue

• Provides pathway for nerves, BVs + lymph vessels in/out of muscle tissue

What does adipose tissue do | 3

Stores energy, insulates from heat loss + reduces physical trauma

What is the fascia

Irregular thick connective tissue that lines body walls + limbs

What does fascia do | 3

• Supports/surrounds muscles + organs

• Holds muscles with similar functions together

• 3 layers of connective tissue extend from fascia to protect + strengthen skeletal muscle

What is the epimysium 

Outer layer

What is the perimysium

Middle layer - Surrounds groups of muscle fibres + bundles them into fascicles

What is the endomysium

Inner layer

What are tendons | 2

• Epimysium, perimysium + endomysium band together n form a rope band of tissue

• Connects skeletal muscle to bone

What about that nerve n blood supply | 2

• 1 artery n 2 veins usually accompany every nerve that penetrates skeletal muscle

• Rich capillary bed in muscle tissue due to high metabolic demand

What are somatic motor neurons

Neurons that stimulate skeletal muscles

What is a sarcolemma

Plasma membrane of muscle fibre

What are transverse tubules (t-tubules)

Tunnels in sarcolemma filled with interstitial fluid

What is important to note about t-tubules

Myocyte action potentials travel along the sarcolemma through t-tubules ensuring excitation/contraction of entire muscle fibre happen simultaneously

What is the sarcoplasm and what does it contain | 2

• Cytoplasm of a myocyte

• Contains glycogen + myoglobin

What is glycogen

Convert to glucose (energy) when needed

What is myoglobin

Protein that binds to n release O2 when needed

What is myofibrils | 2

• Contractile organelles within sarcoplasm

• Extend entire length of muscle fibre n give cell striped (straited) look

What is the sarcoplasmic reticulum (SR) | 3

• Encircles each myofibril

• Kinda looks like endoplasmic reticulum in non-muscular cells

• Stores n releases Ca+ to trigger muscle contraction

What are filaments (myofilaments)

Small protein structures within myofibrils that are responsible for contractile process

What are thick filaments composed of

Myosin (protein)

What are thin filaments composed of

Actin (protein)

What are sarcomeres | 2

• Compartments made of 2 thin filaments n 1 thick filament

• Separated from next sarcomere w/Z-disc

What are the different muscle proteins | 3

Contractile, regulatory n structural proteins within skeletal myocytes

What are the primary contractile proteins

Actin n myosin

What is troponin

Regulatory protein that aids in muscle contraction

What is important to note about troponin | 2

• Troponin in heart n different parts of body show differently on blood tests

• When muscle is damaged, troponin leaks out of cell into circulation showing high levels of troponin in blood via blood test (MI in heart muscle)

What is important to note about skeletal muscle contraction | 5

• The size n length of filaments DONT change

• Thin n thick filaments slide past each other pulling on z-disc

• This shortens the length of the sarcomere

• Which shortens the muscle fibre

• This leads to shortening of entire muscle 

When does muscle contraction/relaxation occur | 2

• Contraction due to increase of Ca+ in sarcoplasm

• Relaxation due to decrease of Ca+ in sarcoplas

What is step 1 in muscle contraction

Muscle action potentials travelling along sarcolemma n through t-tubules stimulate release of Ca+ from SR (voltage gated Ca+ channels)

What is step 2 in muscle contraction

Ca+ binds to troponin causing cascading reaction leading to muscle contraction

What is important to note after step 2

• When stimulated, Ca2+ is continually pumped back into SR via Ca2+-ATPase pumps

• If Ca2+ channels stay open (action potentials still propagating), Ca2+ flows out of SR faster then Ca2-ATPase pumps it back into SR

What is step 3 of muscle contraction

When action potential stops, voltage gated Ca2+ channels close n remaining Ca2+ in sarcoplasm is pumped back into SR

What about rigor mortis (rigidity of death)

• After death, membranes become leaky, Ca2+ leaks out SR

• W/o ATP Ca2+ isn’t pumped back in SR

What is the neuromuscular junction (NMJ) | 3

• Connects neuron to muscle

• Origin of muscle cell action potential

• Synapse between somatic motor neuron n skeletal muscle fibre

What is cholinergic neurotransmitter

Axon terminal of a motor neuron contains thousands of acetylcholine (ACh) molecules

Where are the millions of acetylcholine receptors

Region of sarcolemma opposite the motor neuron

What are the different steps of nerve impulse creating muscle action potential | 4

• Release of ACh

• Activation of ACh receptors

• Production of muscle action potential

• Termination of ACh activity

What is release of ACh

Nerve impulse stimulates release of ACh across synapse

What is activation of ACh receptors

Na+ n other ions flow across cell membrane

What is production of muscle action potential | 2

• Inflow of Na+ makes inside of muscle fibre positively charged

• Change in charge triggers action potential

What is termination of ACh activity

Acetylcholinesterase (AChE) breaks down any ACh remaining in synaptic cleft

What are the 4 types of muscle metabolism

• ATP stored in muscle fibres last a few secs of contraction

• Creatine phosphate - unique to muscle fibres

• Anaerobic glycolysis

• Aerobic respiration - most efficient 

What is creatine phosphate | 3

• Excess ATP is converted n stored as creatine phosphate

• During muscle contraction, enzyme creatine kinase (CK) converts creatine back into ATP

• Energy stores last 15 secs during maximal muscle contraction

What is anaerobic glycolysis | 3

• Pyruvic acid is converted into lactic acid n ATP (lactic acid fermentation)

• Provides 2 mins during maximal muscle activity

• Faster process than aerobic respiration but produce less ATP

What is aerobic respiration | 4

• Pyruvic acid is converted into ATP in mitochondria of cell

• O2 is obtained via diffusion from blood n release of myoglobin

• Provides energy for light/moderate exercise from mins/hrs

• Requires O2, glucose, fatty acids n proteins

What is muscle fatigue

Inability to maintain force of contraction after prolonged activity

What is muscle fatigue caused by | 5

• Inadequate release of Ca2+ from SR

• Depletion of creatine phosphate

• Insufficient O2

• Depletion of glucose

• Buildup of lactic acid

What is O2 debt (recovery O2 uptake) | 3

• Additional O2 required above resting O2 consumption following exercise

• Extra O2 is used to aid in restoration of homeostasis

• Convert lactic acid into glycogen, resynthesize creatine phosphate, replace O2 released from myoglobin

What is motor unit | 2

• Single somatic motor neuron synapses with average 150 skeletal muscle fibres

• All muscle fibres in motor unit contract in unison

What is muscle tone | 3

• While resting, skeletal muscles still exhibit muscle tone

• Small amount of tension due to weak, involuntary contractions of motor unit

• Very important in smooth muscle (digestion, BP)

What is flaccid | 2

• A state of limpness in which muscle tone is lost

• When motor neuron becomes damaged effecting muscle becomes flaccid

What is isotonic contraction | 2

• Force of contraction (tension) developed in muscle remains almost constant while muscle changes its length

• Body movements n moving objects

What is isometric contraction | 3

• Tension generated is not enough to exceed the resistance of the object to be moved

• Holding object steady with outstretched arm

• Body posture, stabilizing joints n extremities

What does cardiac muscle tissue do different to skeletal muscle | 5

• Contain intercalated discs

• Remain contracted longer than skeletal muscle

• Contain plateau period during action potential

• Contracts when stimulated by its own autorhythmic muscle fibres

• Dependant on aerobic respiration

What are intercalated discs

Irregular transverse thickening of sarcolemma that connect 1 cardiac myocyte to the next

What does smooth muscle do different than skeletal muscle | 5

• Filaments not arranged in orderly sarcomeres (not striated hence smooth look)

• Lack transverse tubules n have little SR

• Slower n longer lasting contractions

• Can shorten n stretch to greater extents (more elastin)

• Sustains muscle tone long term

How do skeletal muscle produce movements | 2

• By exerting force on tendons which pull on bones/other structures like skin

• Bones act as levers n joints act like fulcrums

What is a lever

A rigid structure that can move around a fixed point

What is a fulcrum

The fixed point

How is a lever acted on | 3

• Acted on by 2 different points by 2 different forces

• The effort - causes the movement

• The load - opposes the movement (resistance)

What is the sternocleidomastoid

Muscle on sides of neck

What is the scalene

Group of 3 anterior neck muscles on each side that make a triangle

What are all the upper body muscles | 6

• Trapezius

• Latissimus dorsi (sides of back)

• Deltoid

• Biceps brachii

• Triceps brachii

• Rectus abdominis

What are the muscles in lower body | 2

• Quadriceps group

• Gluteus maximus

What is important to note about deltoid | 2

• Anterior, lateral, posterior

• Main site for prehospital IM injection

What is important to note about the quadriceps | 4

• Rectus femoris

• Vastus lateralis

• Vastus medialis

• Vastus intermedius

What about them neck muscles (accessory respiratory muscles) | 3

Sternocleidomastoid, scalene n trapezius

What about building muscle | 3

• Mature skeletal muscle fibres cant do cell division

• Growth of skeletal muscle after birth is due to hypertrophy (enlargement of existing cells)

• Exercise causes skeletal muscle hypertrophy

What about losing muscle | 2

• Humans (30-50yrs) undergo progressive skeletal muscle mass loss (atrophy) that is replaced w/fibrous connective tissue n adipose tissue

• Accompanying this is decrease in maximal strength, slowing of muscle reflexes n loss of flexibility