Muscles Flashcards
What are the types of muscle
Skeletal
Cardiac
Smooth
How is muscle classified
Striated or not
Voluntary or not
Classify skeletal muscle
Multinucleated
Striated
Long stacked in parallel
Classify cardiac muscle
Uninucleated
Striated
Stacked end to end intercalated disk
Classify smooth muscle
Uninucleated
Not striated
Sheets or tubes
What does controlled muscle contraction allow?
Movement of joints limbs and whole body
Propulsion of contents through various hollow internal organs
Emptying of contents of certain organs to external environment
Describe skeletal muscle
Controlled by the CNS
Neurons with cell body in the motor cortex synapse on motor neurons in the SC
Motor neurons with cell body in spinal cord send axons to synapse on muscle cells
Nerve muscle synapses is called
Neuromuscular junctions
The group of muscles cells controlled by a motor neuron is called a
Motor unit
Explain the motor unit
Each muscle is composed of a large number of muscle cells. In mammals each muscle cell receives ONLY ONE synapse
The motor unit consists of
One motorneuron
And all the muscle cells it innervates
The NMJ is ___ compared to a central synapse
HUGE
How is the NMJ special
It’s huge
The postsynaptic membrane is folded and has a high density of nAChR
A single AP in a motor neuron will always cause an AP in the postsynaptic muscle cell
Explain how there is no summation of EPSP
So much ACh released so many receptors the EPSP brings muscle cell to threshold
High number of voltage gated Na+ channels at the synapse
What is a single skeletal muscle called
A muscle fibre
Fibres usually extend entire length of muscle
What is a muscle fascicle
Bundle of fibres
What are the structures of the myofibral
Actin myosin and Titin
What are thick filaments
Myosin
Describe thick filaments (myosin)
Protein molecule consisting of two identical subunits shaped like a golf club
Tail ends are intertwined with each other
Heads project out
What do myosin heads form
Cross bridges between thick and thin filaments
What are the two important sites of a myosin head
An actin binding site
A myosin ATPase
What are thin filaments
Actin Titin Tropomyosin Troponin Nebulin
Describe actin
Primary structural component of thin filaments
G-actin monomers are spherical but assemble into long chains
Each actin molecule has a special binding site for attachment with a myosin head
The binding of actin with a myosin head results in what
Contraction of a muscle fibre
Describe tryopomyosin and tropinin
Regulatory proteins
Tropomyosin are thread like molecules that lie alongside the grove of the actin spiral and covers myosin binding sites
Describe tropinin
Made of three polypeptide units
- one binds with tropomyosin
- one binds with actin
- one can bind with Ca
When not bound to Ca tropinin stabilizes tropomyosin in blocking position over actins cross bridge binding sites
When Ca binds to tropinin tropomyosin moves away from the blocking position this allows actin to bind to myosin
Describe Titin
Giant elastic protein
Joins M lines to Z lines at opposite ends of sarcomere
Two important roles
- helps stabilize position of thick filaments in relation to thin filaments
- improves muscle elasticity
Describe nebulin
Aligns actin filaments
How does a muscle shorten
When actin and myosin fibres slide past each other
In general explain muscle contraction (4 steps)
Binding = myosin binds to actin
Power stroke = cross bridge bends pulling thin myofilaments in
Detachment = cross bridge detaches at end of power stroke and returns to original form
Binding = cross bridge binds to more distal actin and cycle repeats
Myosin is properly called a ____
Motor protein
Define a motor protein
A protein that hydrolyzes ATP to convert chemical energy to carry out mechanical work
Muscle cells have extensive network of endoplasmic reticulum called
Sarcoplasmic reticulum (SR)
Describe the SR
Has a very high Ca concentration
Has a powerful Ca ATPase transporter
- uses ATP to pump Ca from cytoplasm into SR
Also has a Ca binding protein called Calquestrin which helps maintain Ca concentration
Describe T-tubules
Run perpendicular from surface of muscle cell membrane into central portions of the muscle fibre
T-tubules aligned on the edges of the A band
Are continuous with surface membrane - action potential on surface membrane also invade T-tubules
The spread of action potential down a T tubule triggers release of _____ from ___ to ___
Ca
Sarcoplasmic reticulum
Cytosol
What is the A band made up of
Thick filaments
Myosin
The voltage gated Ca channel has two purposes
1) to act as a voltage sensor that can open the Ryanodine receptor
2) lets in small amounts of Ca and contributes to the opening of RyR
What is rigor mortis
3-4 hours after death (peak at 12 hours)
After death intracellular Ca rises and leaks out of SR
Rigor mortis subsides when __
Enzymes start to break down myosin heads
Ca allows myosin to bind to actin BUT when dead (rigor mortis) ..
ATP is needed to separate myosin from actin
Dead cells can’t produce more ATP
So once bound cross bridges can’t detach
Describe the relaxation of muscles
Action potentials stop arriving at NMJ
ACh dissociates from AChR gets degraded
Ca dissociates from tropinin pumped back into SR
Tropomyosin moves back into position blocking cross bridge binding site
Muscle ceases to maintain tension
Actin and myosin slip past each other pulled by Titin and antagonistic muscles
What is myasthenia Gravis
Common symptoms
- drooping eyelid
- blurred or double vision
- slurred speech
- difficulty chewing and swallowing
- weakness in arms and legs
- chronic muscle fatigue
- difficulty breathing
Muscle Weakness
Myasthenia Gravis treatment
Anticholinesterase treatment
- drugs that inhibit ACh esterass within the NMJ
- allow ACh to remain in the NMJ longer
Contraction-relaxation steps requiring ATP
Splitting of ATP by myosin ATPase for power stroke
Active transport of Ca back into SR
Na/K ATPase
Energy sources for contraction
1) creatine phosphate
- first energy storehouse tapped at onset of contractile activity
2) oxidative phosphorylation
- takes place within muscle mitochondria if sufficient O2 is present
3) glycolysis
- supports anaerobic or high intensity exercise
Creatine phosphate is used when
During times of rest when ATP demand is low muscle stores energy in the form of creatine phosphate
First store of energy tapped to fuel muscle contraction
Provide 4-5 times the energy of stored ATP
Limited supply (only a few min)
Oxidative phosphorylation is used when
The process that provides energy during light to moderate exercise
Uses stores of glycogen in muscle (30min)
Good yield of ATP
Aerobic exercise
Adequate supply of oxygen
Oxidative phosphorylation maintains adequate oxygen by
Increase ventilation
Increase heart rate and force of contraction
Dilate skeletal blood vessels
Anaerobic glycolysis is used when
Primary source of ATP when oxygen supply is limited
During intense exercise
Rapid supply of ATP only a few enzymes involved
Very low ATP yield
- only two per glucose molecule
- lactic acid acidifies muscle and contributes to fatigue
What causes muscle fatigue
Central fatigue
- CNS
- psychological
Peripheral fatigue
- decrease in ACh
- receptor desensitization
- changes in RMP
- impaired Ca release
- pH
- lack of ATP is not thought to be a factor
Explain generation of tension
Takes several AP to cause generation of maximal tension
Two therories:
-intracellular Ca reaches its Maximum after first AP
- takes several AP to increase intracellular Ca enough to saturate actins myosin binding sites
Summation and tetanus develop because ____ and allows greater exposure of actin binding sites and therefore maximizes interaction with myosin
Sustained elevation of increases Ca
Types of motor units
Turkey= fibre types grouped together
Mammals= interspersed most have 3 types of Motor units
- slow twitch
- Fast twitch (x2)
Slow twitch = __
Fast twitch = ___
Oxidative which is red muscle
Fast twitch oxidative-glycolytic which is red muscle
Fast twitch glycolytic which is white muscle
Explain slow twitch oxidative
Slow fatigue resistant
Small amounts of tension for long periods of time without running down energy stores
Lots of mitochondria
Small fibres
Well vascularized myoglobin
Explain fast twitch oxidative glycolytic
Fast fatigue resistant
Generate lots of tension moderately fast
Somewhat resistant to fatigue
Moderate # of mitochondria
Fibres are larger then slow twitch
Explain fast twitch glycolytic
Fast fatiguable
White muscle
Generate most tension
Fatigue rapidly
Few mitochondria (anaerobic)
Fibres are larger then slow twitch
First motor units recruited
Next recruited
Last recruited
Slow twitch fatigue resistant
Each MU has only a few fibres and small motorneuron
Motor units that include fast fatigue resistant fibres and these motor neurons are slightly larger
Fast fatigue and the largest MN including the most fibres
Cardiac muscle cells are
Interconnected by intercalated discs and form functional syncytia
Within intercalated discs two kinds of membrane junctions
- desmosomes
- gap junctions
What are the 3 types of cardiac muscle cells
Myocardial auto rhythmic cells
Myocardial contractile cells
Conducting cells
Describe myocardial autorhythmic cells
Initiate and maintain electrical activity in the heart
Do not contract
Describe myocardial contractile cells
Working cells
99% of cardiac muscle cells
Contractile muscle part of the heart does the mechanical work of pumping
Joined electrically by gap junctions
Describe conducting cells
Carry electrical signals from the pacemakers to the contractile cells
Where do cardiac impulses originate
At the SA node
Where do AP spread in the heart
Throughout the right and left atria
The impulse passes from where to where in the heart
From atria into ventricles through AV node
What is the only point of electrical contact between chambers in the heart
AV node
The action potential is briefly delayed at the AV node because
Ensures atrial contraction precedes ventricular contraction to allow complete ventricular filling
After the AP is delayed at the AV node where does it travel
Rapidly down interventricular septum by means of bundle of His
The impulse eventually rapidly disappears throughout the myocardium by means of what
Purkinje fibres
Rest of ventricular cells activated by cell to cell spread of impulse through ___
Gap junctions
Describe intrinsic conduction system
Autorhythmic cells initiate AP
No real resting membrane potential
IF : a Na current
ICaT : fast calcium current
ICaL : slow calcium current
Use calcium influx rather then VG sodium for rising phase of AP
Pacemaker potential =
Membrane slowly depolarizers “drifts” to threshold, initiates AP and the membrane repolarizes to -60 mV
Ap of contractile cells
Rapid depolarization
Rapid partial early repolarization, prolonged period of slow repolarization (plateau phase)
Rapid final (repolarization phase)
Why is the AP of contractile cells so long?
Plateau primarily due to activation of slow L-type Ca channels
Ensures adequate ejection of blood
How does the long AP contractile cells avoid tetanus
Long AP causes long refractory period and long contraction
Excitation contraction coupling in cardiac contractile cells
Ca entry through ca channels in T tubules triggers massive release of Ca from SR RyR
Ca induced Ca release leads to cross bridge cycling and contraction
Key role for secondary active transport
Describes smooth muscle
Highly variable
Must operate over a range of lengths
Layers may run in several directions
Small spindle shaped cells with one nucleus
Contracts and relaxes much more slowly
Uses less energy
Sustains contractions for extended periods
Classify smooth muscle
Location
- vascular gastrointestinal, urinary, respiratory, reproductive, ocular
Contraction pattern
- phasic or tonic
Communication
- single unit smooth muscle or visceral smooth muscle
- multi unit smooth muscle
What’s the difference between single unit smooth muscle cells and multi unit
Single
- are connected by gap junctions and the cells contract as a single unit
Multi
- are not electrically linked and each cell muscle must be stimulated independently
Describe smooth muscle structure and function
Not arranged in sarcomeres
Contraction initiated by electrical or chemical signals or both
Controlled by the autonomic nervous system
Lacks specialized receptor regions
Ca is from where in smooth muscle
From the extracellular fluid and sarcoplasmic reticulum
Ca initiates a cascade ending with phophorylation of myosin light chain and activation of myosin ATPase
Describe the structure of the smooth muscle cytoskeleton
Intermediate filaments and protein dense bodies form a cytoskeleton
Actin attaches to the dense bodies
Each myosin molecule is surrounded by actin filaments
Smooth muscle SR and t tubules
Amount of SR varies and is less organized
No t tubules but caveolae
- small invaginations in cell
Describe the protein filaments of smooth muscle
Actin is more plentiful then in striated
Lack troponin
Less myosin, but are longer and the entire surface is covered in myosin heads
Additional cytoskeleton with intermediate filaments and dense bodies
Smooth muscle myosin has ____ all along its length
Hinged heads
Describe sarcoplasmic calcium release in smooth muscle
Ryanodine receptor (RyR) calcium release channel
Calcium induced calcium release (CICR)
IP3 receptor channel
Store operated Ca channels
Describe smooth muscle cell membrane calcium entry
Voltage gated Ca channels
Ligand gated Ca channels or receptor operated calcium channels (ROCC)
Stretch-activated calcium channels
- open when pressure or other force distorts cell membrane
- known as myogenic contraction (arteries)
describe smooth muscle regulation
Many controlled by both sympathetic and parasympathetic neurons
Hormones and paracrines also control smooth muscle contraction
- histamine constricts smooth muscle of airways
- nitric oxide relaxes smooth muscles of blood vessels (mall sex response)
Describe smooth muscle contraction
Electromechanical coupling
-contraction caused by electrical signaling or mechanical signalling and contraction dependent on changes in membrane potential
Pharmacomechanical coupling
- contraction caused by chemical signalling , no changes in membrane potential required
