Lecture 7: Basic physiology of muscles Flashcards
Describe skeletal muscle
Large fibres, multinucleate cells
How do skeletal muscle cells appear beneath a microscope?
Stiped/striated
Describe cardiac muscle
Striated, smaller, branched, unicnucleate
How are cardiac muscle cells joined?
In series by junctions called intercalated disks
Describe smooth muscle fibers
Small, lack striations, spindle shaped cells
What is skeletal muscle attached to?
Bone
What muscle types are capable of generating spontaenous action potentials?
Smooth and cardiac
What is the diameter of a skeletal muscle fiber?
10-80micrometers
How long are skeletal muscle fibers extended?
The entire length of the muscle
What usually innervates a skeletal muscle fiber?
One nerve ending
What membrane encloses a skeletal muscle fiber?
Sarcolemma
What surrounds the myofibrils?
Sarcoplasm (intracellular fluid)
What does the sarcolemma consist of?
True cell membrane (plasma membrane) and an outer coat
What makes up the outercoat of the sarcolemma?
Thin layer of polysaccharide material containing numerous collagen fibrils
What happens to the sarcollema at the end of a muscle fiber?
The surface layer fuses with a tendon fiber
What do tendon fibers collect into?
Bundles to form muscle tendons
What do muscle tendons connect?
Muscles to bone
What does the sarcoplasm contain large quantities of?
K, Mg, Pi, enzymes
Why are mitochondria abundant in the sarcoplasm?
ATP synthesis
What stores and releases calcium in the sarcoplasm?
Sarcoplasmic reticulum (specialised endoplasmic reticulum of skeletal muscle)
How many myofibrils do muscle fibers contain?
Several hundred to several thousand
What are myofibrils composed of?
1500 adjacent myosin filaments
3000 actin filaments
What are thick and thin filaments composed of?
Thick: myosin
Thin: Actin
What protein attaches myosin to the Z disk?
Titin
What forms between overlapping mysoin and actin filaments?
Cross bridges
A sarcomere stretches from ____ to _____
Z disk
next z disk
The A band consist of…
The entire length of the myosin filament
The I band is…
Light filaments only
How is the sarcoplasmic reticulum arranged?
As a repeating series of networks around the myobrils from one A-I junction to the next
What is the name for where the sarcoplasmic reticulum meets at the A-I junction?
Terminal cisterna
What do cisterna act as reservoirs for?
Ca2+
What are T tubules?
The plasma membrane of the sarcoplasmic reticulum invaginates transverselty to form a tubular system
What is contained in T tubules?
Voltage-sensor proteins
When are voltage-sensor proteins activated?
When the membrane depolarizes inducing the sarcoplasmic reticulum to release Ca2+
What do T tubules and the adjacent cisternae form?
A triad
What else is present in the sarcoplasmic reticulum?
Mitochondria to provide energy
Describe resting stage of the sarcomere
Some interdigitation of thin and thick filaments
The contracted stage with increased interdigitation of thick and thin filaments is also called
Concentric contraction
What name is given to the stretched stage where thin and thick filaments do not interact?
Eccentric contraction
What name is given to the mechanism of contraction of the sarcomere?
Sliding filament mechanism
What does myosin consist of?
6 polypeptide chains: 2 heavy, 4 light (tail and head)
What do thin filaments consist of?
Actin, tropomyosin, troponin
What makes up the backbone of the thin filment?
F-actin (2 in a helix), troponin intermittently attached
What are the loosely bound protein subunits of troponin?
T, I, C
What comprises the motor unit?
A motor neuron and the muscle fibers it innervates
What is between the motor neuron and the muscle fibers it innervates?
The neuromuscular junction
What is the transmitter responsible for generating an end plate potential in muscle fibers?
Acetylcholine
What happens when acetylcholine is released at the neuromuscular junction?
Opens Ach gated cation channels
What happens once Ach gated cation channels open at the neuromuscular junction?
Large quantities of sodium ions diffuse into the interior of the muscle fiber membrane, causing depolarisation
What happens once the action potential has depolarized the muscle membrane?
The sarcoplasmic reticulum releases large amounts of Ca2+
What happens once Ca2+ is released into the sarcoplasm?
It binds to troponin complex, allowing, causing changes in tropomyosin, allowing myosin heads to attach (contraction is initiated)
What is the first stage of the contraction cycle?
Attachment (myosin head is tightly bound to actin molecule) Rigor state
What is the second stage of the contraction cycle, after attachment.
Release
What happens during the release stage of the contraction cycle?
ATP binds to the myosin head, inducing release of actin (muscle is relaxed)
What happens after the release stage of the contraction cycle?
Bending
What happens during the bending stage of the contraction cycle?
ATP causes further changes to myosin head, causing it to bend, this initiates breakdown of ATP to ADP which remain in the myosin head
After the myosin head binds to the new site, the inorganic phosphate is released. What does this do?
Increases binding affinity for the myosin to the actin
Myosin head generates a force to straighten up, power stroke shortens sarcomere
What else happens during the power stroke?
ADP is lost from the myosin head
What happens when ADP is released?
Reattachment of the myosin head to the actin filament, rigor stage is re-established
What happens to Ach during skeletal muscle relaxation?
It is recycled by being reabsorbed into the synaptic knob
What happens to calcium ions to cause relaxation?
Active transport pumps in the SR pump it from the cytosol back into cisternae
What happens when Ca2+ concentration decreases during relaxation?
Ca2+ unbinds from troponin, tropomyosin recovers the binding sites, tension is no longer produced/maintained
What name is given to process by which skeletal muscle contracts after receiving an action potential?
Excitation-contraction coupling
What are the two types of smooth muscle?
Multiunit smooth muscle
Unitary (single unit) smooth muscle
Describe multiunit smooth muscle?
Each fibre behaves as a separate unit
Dense innervation
Describe unitary smooth muscle.
Cells are linked by gap junctions
Chacterized by spontaneous pacemaker activity
What is smooth muscle important in maintaining?
Homeostasis
Where is smooth muscle found?
Blood vessels, GI tract and gallbladder, Ureter and urinary bladder, uterus, respiratory system, eye
What is the diameter of a smooth muscle cell?
5 micrometers
What is the length of a smooth muscle cell?
20-500 micrometers
What surrounds each smooth muscle cell?
Basal lamina, small amount of connective tissue
Why is there some connective tissue between smooth muscle cells?
Allow for passage of nerve tracts and blood vessels
What are the two main types patterns of contraction of smooth muscle?
Phasic and tonic
Give an example of phasic contraction
Oesophagus: contracts only when food enters
When is tonic contraction seen?
Sphincters: relax to open to allow material to pass through
What pattern of contraction is seen in vascular smooth muscle?
Variable tonic contraction
Why does vascular smooth muscle have variable tonic contraction?
To control pressure and quantity of blood in a particular tissue
What is smooth muscle capable to do to a greater degree then skeletal muscle?
Greater degree of contraction
How are contractile units different in smooth muscles vs skeletal?
Less regular in smooth
How else is smooth muscle contraction different to that of skeletal?
Slow onset and relaxation, prolonged contraction and slower cross-bridge cycling
What happens when calcium ions enter a smooth muscle cell?
Induces the release of Calcium ions from the sarcoplasmic reticulum
What does calcium bind to in smooth muscle cells that is different from cardiac muscle?
Calmodulin (CaM)
What once calcium is bound to calmodulin in a smooth muscle cell?
Calmodulin activates myosin light chain kinase (MLCK) resulting in phosphorylation
What happens once the mysosin light chain kinase phosphorylates the myosin?
the head attaches to actin. In the presence of ATP, the myosin head bends to produce contraction
What is required to bring about relaxation in smooth muscle?
A calcium pump
How is the calcium pump in smooth muscle different to that in skeletal muscle?
It acts on a slower timescale
Other than the calcium pump, what else is important in cessation of smooth muscle contraction?
Myosin phosphatase
What determines the time to relaxation in smooth muscle cells?
Myosin phosphatase
What does myosin phosphatase do?
Removes phosphate from the myosin light chain, causing detachment of the myosin head from actin
How are muscle fibres arranged in cardiac muslce?
Spiral arrangement
What is the result of spiral arrangement of cardiac muscle fibres during ventricular contraction?
Upward squeezing of ventricular blood from apex
What is found between myocaridal cells?
Intercalated discs
What do intercalated discs contain?
Desmosomes (formed of protein plaques)
What do desmosomes in the intercalated discs do?
Transfer the force from cell to cell
What allows for the passage of electrical signals in cardiac mucles?
Gap junction in the intercalated dics
What is the size of cardiac muslce?
Smaller than skeletal
10-20 micrometer diameter
50-100 micrometer long
How are the nucleus/’s arranged in cardiac muscle?
Single, centrally placed
How are T tubules different in cardiac muscle compared to skeletal muscle?
Short and broad (larger than skeletal muslce’s)
No triads
How is the sarcoplasmic reticulum of cardiac muscle arranged differently to that of skeletal muscle?
It is organised as an anastomosing network (not expanded cisterna)
Why does cardiac muscle contain large numbers of mitochondria?
It is completely dependent on aerobic metabolism
What else is found in cardiac muscle?
Glycogen and lipid inclusions which store energy, myoglobin stores oxygen
Cardiac muslce contracts without _____ stimulation
Neural
What are pacemakers?
Specialised cardiac muslce cells, more excitable than others and thus contract first
How are pacemaker cells regulated?
Innervated by cells of the nervous system
In cardiac cells, T tubules form…
Diads
How does cardiac muscle resist stretching as the heart fills with blood?
Desmosomes in intercalated discs link adjacent cells forming a physical bond
How does action potential enter a cardiac muscle cell?
From an adjacent cell opening
What happens when calcium ions enter the cardiac muscle cell?
The bind to troponin to initiate contraction
When does relaxation occur in cardiac muslce?
When Ca2+ unbinds from troponin and is pumped back into the sarcoplasmic reticulum
Thin filament attachment occurs where in each muscle type?
Skeletal: Z disc
Cardiac: Z disc
Smooth: Dense bodies
